Handbook of curatives and crosslinkers 9781523124541, 1523124547, 9781927885482, 1927885485, 9781927885475

Table of contents :
Content: Front Cover
Handbook of Curatives and Crosslinkers
Copyright Page
Table of Contents
Chapter 1. Introduction
Chapter 2. Crosslinkers
2.1 CHEMICAL COMPOSITION AND PROPERTIES
2.2 POLYMERS AND THEIR CROSSLINKERS
2.3 PARAMETERS OF CROSSLINKING
2.4 EFFECT OF CROSSLINKERS ON PROPERTIES
Chapter 3. Curatives
3.1 CHEMICAL COMPOSITION AND PROPERTIES
3.2 POLYMERS AND THEIR CURATIVES
3.3 PARAMETERS OF CURING
3.4 EFFECT OF CURATIVES ON PROPERTIES
Index

Citation preview

Handbook of

Curatives and Crosslinkers

George Wypych

Toronto 2019

Published by ChemTec Publishing 38 Earswick Drive, Toronto, Ontario M1E 1C6, Canada © ChemTec Publishing, 2018 ISBN 978-1-927885-47-5 (hard cover); ISBN 978-1-927885-48-2 (epub) Cover design: Anita Wypych

All rights reserved. No part of this publication may be reproduced, stored or transmitted in any form or by any means without written permission of copyright owner. No responsibility is assumed by the Author and the Publisher for any injury or/and damage to persons or properties as a matter of products liability, negligence, use, or operation of any methods, product ideas, or instructions published or suggested in this book. Library and Archives Canada Cataloguing in Publication Wypych, George, author Handbook of curatives and crosslinkers / George Wypych. -- 1st edition. Includes bibliographical references and index. Issued in print and electronic formats. ISBN 978-1-927885-47-5 (hardcover).--ISBN 978-1-927885-48-2 (PDF) 1. Polymers--Additives--Handbooks, manuals, etc. 2. Curing--Equipment and supplies--Handbooks, manuals, etc. 3. Crosslinked polymers-Handbooks, manuals, etc. 4. Curing. 5. Crosslinking (Polymerization). I. Title. TP1142.W953 2019

668.9

C2018-904007-6 C2018-904008-4

Printed in Australia, United Kingdom and United States of America

1

Introduction According to the standard definition, the crosslinker is an additive which links two polymer chains by the covalent or ionic bond. In polymer chemistry, the curing reaction changes mechanical properties or viscosity by increasing molecular weight of a polymer by reacting two different components. Components can be polymers, prepolymers, oligomers, or monomers. If the components to be linked are polymers, there is no visible difference between curing or crosslinking. For this reason, both terms are frequently interchangeable in meaning and application. If the components have lower molecular weight (e.g., monomer, oligomer) then two different kinds of curatives are typically used, namely chain extenders and/or crosslinkers. The chain extender has two functional groups divided by a spacer which may be used to regulate hardness of the resultant material, and the crosslinker has the functionality of three or more by which it can react with more than two monomers or prepolymers thus forming a crosslink. Polyurethane chemistry provides still another useful example. Typically, hydroxyl or amine groups react with the isocyanate groups to build a high molecular weight polymer. But in this case, either group may become crosslinker (either isocyanate, or amine, or hydroxyl containing moiety may have more than two functional groups thus playing a role of crosslinker). In the area of commercial curatives and crosslinkers, the chemical composition of the product is usually unknown as well as the curing mechanism is frequently not disclosed, therefore distinguishing between curatives and crosslinkers is generally difficult if possible and often does not provide any benefits for the selection of these additives. The commercial additives are targeted by their manufacturers to perform under specific compositional constraints. Mainly, the reactivity of components (the material to be cured and the curing moiety) has the primary importance in their selection. For this reason, throughout Databook of Curatives and Crosslinkers, which is a companion book containing information on the commercial products used to increase the molecular weight of polymers, the terms curative and crosslinker are used interchangeably mostly to reflect the naming system adopted by their manufacturers. The curatives/crosslinkers were organized in Databook of Curatives and Crosslinkers according to the alphabetical order of their commercial or chemical names. In this book, the goal is to discuss the scientific background of curing and crosslinking reactions, therefore, both types of reactions will be discussed in separate chapters including their chemical compositions and properties based on data provided by manufacturers and presented by the authors of scientific publications, their applications to different polymers and the best selection for particular polymer, conditions of reaction and their

2

Introduction

effect on final product, effect of various additives on the properties of different polymers, and applications in different manufactured products. In spite of the fact that most scientists agree that the results of crosslinking and curing are similar in the sense that molecular weight of polymer increases and a network is formed, authors of the majority of scientific papers distinguish between both processes and indicate their naming preference. In this book, we will follow nomenclature suggestions of authors of published articles, even though it may sometimes lead to overlaps between applications as curatives or crosslinkers. In this book, we will attempt to present objective scientific information on the mechanisms characterizing typical crosslinking and curing reactions and list a variety of available options for the modification of different resins showing potential benefits of the process and the effect of conditions under which reactions are performed.

2

Crosslinkers 2.1 CHEMICAL COMPOSITION AND PROPERTIES Table 2.1 shows the averages of the typical properties of crosslinkers based on data given by their manufacturers and presented in scientific publications. Table 2.1. Typical properties of crosslinkers. GENERAL INFORMATION Chemical components: aziridine, carboxylic acids, dicumyl peroxide, fatty acid trimer, hydrogenpolysiloxane, isocyanate, modified inorganic fillers, N,N’-methylenebisacrylamide, organic peroxide, oxime silane, pentaerythritol derivative, peroxyketal, polyamide-epichlorohydrin, polyaziridine, polycarbodiimide, polydimethylsiloxane, polyrotaxane, polythiol, silane, sodium tetraborate, titanate, triallyl cyanurate, triallyl isocyanurate, triallyl trimellitate, trismaleimide, zirconate Molecular weight: 148-1394

Average functionality: 2.5-3.3 Solids content, wt%: 12-100

Active oxygen content, %: 2.06-11.17

NCO content, %: 30.5-33.5

Ti content, %: 3.3-20.5 PHYSICAL DATA Color: colorless, white, yellow, brown, blue, red

State: liquid, paste, or solid

Odor: odorless, amine, aromatic, ether, menthol, solvent Acid number, mg KOH/g: 1-25

pH: 2-11.5 Density, kg/m3: 800-1630

Activation energy, kcal/mol: 36.8-38.1 o

o

Boiling point, C: 17->390

Melting/freezing point, C: 235 to -70 o

Glass transition temperature, C: -5 to -10

Vapor density: 1-11.7

Half life: 1 min/140-193oC to10 h/99-131oC Refractive index: 1.384-1.5675 Solubility: acetone, benzene, butanol, chloroform, diethyl ether, dioxane, DMF, DSMO, ethanol, ethyl acetate, hydrocarbons, isopropanol, methanol, methyl ethyl ketone, n-heptane, toluene, water, xylene Vapor pressure, kPa @20oC: 1.7E-07 to 3.86 Viscosity, mPa s @20oC: 5.7 to 9000 HEALTH & SAFETY Autoignition temperature, oC: 220-600 HMIS: Fire: 0-4; Health: 0-3; Reactivity: 0-3 NFPA: Flammability: 0-3; Health: 0-3; Reactivity: 0-2

Flash point, oC: 6-290

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Crosslinkers

Table 2.1. Typical properties of crosslinkers. Carcinogenicity: IARC, NTP, OSHA: No component of this product present at level greater than or equal to 0.1% is identified as probable, possible, or confirmed human carcinogen; to suspected carcinogen Mutagenicity: none to suspected to category 1B Teratogenicity: none to category 1A Explosive concentration, %: LEL: 1.4-2; UEL: 7-19.9 LC50: Inhalation-rat, ppm: >5->800; Dermal-rabbit, mg/kg: >1000-12800; Oral-rat, mg/kg: 390>7000 UN/NA class: 1219, 1263, 1760, 1824, 1933, 3077, 3082, 3101, 3103, 3105, 3106, 3107, 3108, 3110, 3335 UN risk phrases: R10,R20,R21/22,R22,R36/38,R38, R41,R43,R52/53,R68 UN safety phrases: S7,S17,S26,S27,S35, S36/37/38,S44,S47 ECOLOGICAL PROPERTIES Aquatic toxicity, EC50, mg/l: Algae: 0.8->100/72H, Bluegill sunfish: >603->1000; Daphnia magna: 0.31->1000/48H; Fathead minnow: 6.32-40; Zebra fish: 1.6->1000 Bioaccumulation: not expected to low Bioconcentration factor, BCF: 6.49-839 Partition coefficient, log Kow: 2.2-7.3 APPLICATIONS Recommended for resins: ABS, acrylamide, acrylics, alkyd, biopolymers, bromobutyl rubber, butyl rubber, carboxymethyl cellulose, cellulose acetate butyrate, cellulose acetate propionate, chlorinated polyethylene, chloroprene, cyanoacrylate, epoxidized natural rubber, EPDM, epoxy, EVA, fluoroelastomers, guar, high impact polystyrene, hydrogenated nitrile rubber, natural rubber, nitrile rubber, novolac, polyamide, poly(butylene terephthalate), polycarbonate, polyetheretherketone, polyetherketoneketone, polyethylene, polyimide, polymethylmethacrylate, polyphenylene sulfide, polyphthalamide, polypropylene, polysulfone, polyurethanes, polyvinylalcohol, polyvinylbutyrate, polyvinylchloride, proteins, plastics-fillers coupling, resorcinol, silicones, sodium carboxymethylcellulose, starch, styrene butadiene rubber latex, sulfonated polyetheretherketone, thermoplastic elastomer, unsaturated polyester, vinyl ester resin Recommended for products: adhesives, automotive, barrier finishes, bushings, coatings, concrete sealers, conductive membranes, dental, electrophoresis, foams, footwear, foundry, fracking fluids, furniture, gaskets, grout, inks, medical, paper release, paints, pharmaceutical, pipes, profiles, sealants, tank lining, tapes, textile finishing, top-coats, tubes, varnish, wire & cable Crosslinking target: amine, carboxyl, double bonds, epoxy, hydroxyl, ionic, isocyanate, thiol, triethoxysilyl, and vinyl functionality Reaction temperature range, oC; room temperature-220 Maximum compounding temperature, oC: 85-0.1 μm and/or thermally.8 The crosslinking agents have allophanate, biuret, uretdione, uretoneimine, bridged carbamate, carbodiimide, isocyanurate, iminooxadiazindione, or oxadiazintrione structural elements.9 These crosslinking agents are suitable for crosslinking nitrile rubber and hydrated nitrile rubber.9 The crosslinking improved compression set of rotomolded compounds.9

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Crosslinkers

References 1 2 3 4 5 6 7 8 9

Cong, C; Liu, Q; Li, J; Meng, X; Zhou, Q, Polym. Deg. Stab., in press, 2019. Choi, S-S; Kim, J-C, J. Ind. Eng. Chem., 18, 3, 2012. Liu, J; Li, X; Xu, L; Zhang, P, Polym. Testing, 54, 59-66, 2016. Liu, X; Zhao, J; Yang, R; Iervolino, R; Barbera, S, Polym. Deg. Stab., 151, 136-43, 2018. Xiong, Y; Chen, G; Guo, S; Li, G, J. Ind. Eng. Chem., 19, 5, 1611-16, 2013. Ahmed, FS; Shafy, M; El-megeed, AAA; Hegazi, EM, Mater. Design, 36, 823-8, 2012. Nagamori, H; Umetsu, K, US Patent 9163132, Oct. 20, 2015. Ziser, T; Kleinknecht, H; Wawrzinski, L, US20160075867A1, Lanxess Deutschland GmbH, Mar. 17, 2016. Brandau, S; Klimpel, M; Magg, H, WO2011141275A3, Lanxess Deutschland GmbH, Apr. 18, 2013.

2.2.4 Agar

13

2.2.4 AGAR Agar-based bioplastic was prepared by chemical crosslinking with diisocyanates (4,4diphenyl and 1,6-hexamethylene diisocyanates).1 Crosslinked agar films showed improved tensile strength, water resistance, were hemocompatible and non-toxic to cell proliferation.1 The agar-based hybrid biosorbents were synthesized by the free radical copolymerization of acrylamide, N,N′-methylenebisacrylamide on the agar backbone in the presence of a free radical initiator (ceric ammonium nitrate) via oxidation, grafting, and crosslinking reactions.2 The hybrid materials are useful in environmental remediation from the textile effluents and heavy metal ion-contaminated water bodies.2 The following order Fe+3 > Mn+2 > Ni+2 > Cr+3 of remediation effectiveness was observed and explained by linking it to their radius sizes and reactivity.2 The wet-spinning process was used to fabricate agar fibers by the gelation process.3 The addition of barium chloride into the coagulation process improved the mechanical properties of fibers.3 Further improvement of properties was by immersion of agar fibers in aminosilicone solution.3 Figure 2.6 shows the mechanism of crosslinking by barium chloride and EDS spectra of fiber surface and cross-section.3

Figure 2.6. The crosslinking between agar molecule and barium chloride in the processing of wet spinning; B: EDS spectra of agar fiber: (a) cross-section; (b) surface, where agar fiber shows characteristic peaks for barium and sulfur. [Adapted, by permission, from Liu, J; Xue, Z; Zhang, W; Yan, M; Xia, Y, Carbohydrate Polym., 181, 760-7, 2018.]

A method of crosslinking agarose or agar suspended and then reacted with a bi-functional compound has been invented.4 The bifunctional compound contains the following functional groups: −COCl, −SO2Cl, and −N=C=S.4 References 1 2 3 4

Sonker, AK; Belay, M; Rathore, K; Jahan, K; Verma, V, Carbohydrate Polym., 202, 454-60, 2018. Rani, GU; Konreddy, AK; Mishra, S, Int. J. Biol. Macromol., 117, 902-10, 2018. Liu, J; Xue, Z; Zhang, W; Yan, M; Xia, Y, Carbohydrate Polym., 181, 760-7, 2018. Honkanen, EJ; Teppo, AM, US3860573A, Orion-Yhtyma Oy, Jan, 14, 1975.

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Crosslinkers

2.2.5 ALKYD RESIN The primary driers (Co, Mn, or Fe) are used in combination with secondary driers (e.g. Ca, Zr) to enhance drying of the alkyd film.1 Coating formulators aim at a high drying speed and hardness development.1 Each combination of primary and secondary driers leads to a different oxidative drying pattern resulting in variations in the crosslink density.1 Calcium promoted the front speed, and zirconium increased the crosslink density.1 Catalytic activity of four primary driers was investigated in high-solid alkyd binder modified with tall oil fatty acids.2 Front-forming drying occurs when the autoxidation is fast and air-oxygen diffusion is slower than its consumption.2 In extreme case, impermeable skin is formed and drying-through does not happen for a very long time.2 Homogeneous drying is observed when autoxidation is slow and oxygen consumption is not fast enough to establish an oxygen gradient in the coating (Figure 2.7).2

Figure 2.7. Schematic view of film-formation process. [Adapted, by permission, from Charamzová, I; Vinklárek, J; Honzíček, J, Prog. Org. Coat., 125, 177-85, 2018.]

Cobalt-based drier is the example of catalyst causing permanent skin formation, manganese drier or high concentrations of iron-based drier are the examples of frontforming drier, and lowering the levels of iron-based drier to 0.0005 wt% decelerates the autoxidation process to cause homogeneous drying process.2 The linseed oil-based alkyd resin was modified with glycidyl polyhedral oligomeric silsesquioxanes to obtain a hybrid material.3 Simultaneous reactions between hydroxyl groups coming from ring-opened epoxides of silsesquioxanes and partial glycerides and carboxylic acid groups of phthalic anhydride occurred.3 The silsesquioxanes molecules were linked to the resin matrix through the reaction of phthalic anhydride with the additional crosslinking.3 The material containing 5% silsesquioxanes showed better film properties, such as flexibility, adhesion, drying time, resistance to alkali, acid, and water.3 By changing the nature of monomers and the length of the oil’s unsaturated fatty acid side groups for crosslinking, the alkyd resins become suitable for a wide range of applications, in particular as metal paints.4

2.2.5 Alkyd resin

15

A conformal series of castor oil-based hyperbranched alkyd/Fe3O4@SiO2 nanocomposites was developed as a coating material.5 Magnetite-coated silica (Fe3O4@SiO2) particles with 60-70 nm average diameter were prepared by in situ method that binds magnetite nanoparticles to silica nanospheres.5 The higher chemical stability of 0.5% nanofiller loading was attributed to good dispersion of the nanofillers enhancing crosslinking and leading to the protection of the ester groups against hydrolysis (Figure 2.8).5

Figure 2.8. Solution casting method for the preparation of a series of dehydrated castor oil, DCO, fatty acid based hyperbranched alkyd/Fe3O4@SiO2 nanocomposites with various nanofiller loadings and alkyd auto-oxidation curing by cobalt, calcium and zirconium driers. [Adapted, by permission, from Selim, MS; Feng Q. Wang, FQ; Yang, H; Huang, Y; Kuga, S, Mater. Design, 135, 173-83, 2017.]

The α-eleosterate pendent fatty acid of a tung oil-based alkyd was functionalized via a Diels-Alder reaction with three different acrylate compounds: (1) 2,2,2-trifluoroethyl methacrylate, (2) 3-methacryloxypropyl trimethoxysilane, and (3) triallyl ether acrylate.6

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Crosslinkers

The alkyd modified with siloxane and triallyl group had a faster drying time, a higher crosslink density, and a higher glass transition temperature compared to the unmodified alkyd.6 A coating composition was produced by forming an aqueous dispersion of a crosslinking agent, and a resin that comprised the reaction product of an alkyd polymer, an optional surfalkyd, diacetone acrylamide, and another acrylate monomer.7 The crosslinking agent was adipic dihydrazide.7 Self-crosslinking aqueous coating compositions comprised a water-dispersible alkyd having both sulfonate and acetoacetate functionality, a suitable bi- or poly-functional primary amine, and a mono-functional primary or cycloaliphatic secondary amine.8 The crosslinking of the composition includes oxidative curing via the fatty acid segments of the alkyd, and crosslinking via reaction between the acetoacetate groups of the alkyd and the primary amine groups of the polyfunctional amine.8 The aqueous alkyd systems disclosed do not require the presence of amines to disperse the alkyds, while exhibiting superior tack-free and through-dry times when used in coating compositions.8 Hydrogels useful for delivery of sensitive drugs such as proteins and oligonucleotides, cell encapsulation and delivery, coatings on medical devices, wound dressings, and films can be produced from alkyd-type polyesters prepared by the polycondensation of a polyol, polyacid, and fatty acid.9 Crosslinking may be initiated among crosslinkable regions by a light-activated free-radical polymerization initiator such as 2,2-dimethoxy-2phenyl acetophenone, yielding crosslinked hydrogel.9 References 1 2 3 4 5 6 7 8 9

Erich, SJF; Gezici-Koç, O; Michel, M-EB; Thomas, CAAM; Adan, OCG, Polymer, 121, 262-73, 2017. Charamzová, I; Vinklárek, J; Honzíček, J, Prog. Org. Coat., 125, 177-85, 2018. Sogukkanli, S; Yilmazoglu, M; Tasdelen, MA; Erciyes, AT, Prog. Org. Coat., 124, 175-84, 2018. Pierlot, C; Ontiveros, JF; Royer, M; Catté, M; Salager, J-L, Colloids Surf. A: Physicochem. Eng. Aspects, 536, 113-124, 2018. Selim, MS; Feng Q. Wang, FQ; Yang, H; Huang, Y; Kuga, S, Mater. Design, 135, 173-83, 2017. Thanamongkollit, N; Soucek, MD, Prog. Org. Coat., 73, 4, 382-91, 2012. Julien, T; Ryer, D; Willhite, J, US7220802B2, Cook Composites and Polymers Inc, May 22, 2007. Schick, MF; Bolton, AL; Kuo, T, EP1572818B1, Eastman Chemical Co, Jun. 7, 2006. Nathan, A, EP1430916A1, Ethicon Inc, Jun. 23, 2004.

2.2.6 Biopolymers

17

2.2.6 BIOPOLYMERS Wilson's disease is a genetic disorder causing accumulation of copper in the body, resulting in toxic damage to the liver and nervous system.1 The chemically modified biopolymer carrier based on microcrystalline cellulose and chitosan, containing the highly specific copper chelator 8-hydroxyquinoline, can be used to prevent effects of Wilson's disease.1 The chelator scavenges copper ions from food and copper ions present in secretions in the gastrointestinal tract.1 The chelator is covalently bound to indigestible biopolymer carriers (crosslinked chitosan and modified cellulose); therefore, it is not taken up by the gastrointestinal tract, and it can be eliminated.1 Silane-crosslinked hydrogels were prepared from an eco-friendly biodegradable chitosan/guar gum and used for controlled drug release.2 The crosslinked hydrogels revealed more swelling, but with further increase in the amount of crosslinker, swelling was decreased.2 The hydrogels showed low swelling at basic and neutral pH while maximum swelling was observed at acidic pH which was used for control of drug release.2 Crosslinked multilayer biopolymer coatings were formed around lipid droplets.3 Emulsion gastrointestinal stability depended on coating composition, and the crosslinking.3 Primary fish gelatin-stabilized oil-in-water emulsion was prepared by microfluidization.3 Secondary emulsion was prepared by electrostatically depositing sugar beet pectin on the gelatin-coated droplets.3 Laccase (a polysaccharide crosslinking enzyme) was then added to the double-layered emulsions to promote crosslinking of the adsorbed pectin molecules.3 The natural biopolymers require chemical modification by crosslinking to improve their mechanical properties.4 The feasibility of using the dialdehyde carboxymethyl cellulose as a crosslinking reagent has been studied.4 It was found to be non-cytotoxic, biodegradable and biocompatible in the applications such as a crosslinking agent of carboxymethyl chitosan.4 The electrospun gelatin nanofibers have been used as a drug carrier, and effect of crosslinking on sustained release has been studied.5 The layer-by-layer crosslinking provided improved structural integrity, the thermal and chemical stability of the drug, as well as yielded a tight control over sustained drug release for up to 8 h.5 Microbial transglutaminase catalyzed the crosslinking of bulk and interfacial proteins, influencing the physical properties of food dispersions.6 The spatial make-up of biopolymers in the gelled matrices played a key role in the enzyme accessibility.6 The enzyme-crosslinking was influenced by both the pore size and the number of crosslinks within the network.6 Mixing different types of biopolymers (e.g., polysaccharides and proteins) influenced the internal morphology of the network and the pore size and number of junction zones affecting diffusion.6 One of the limitations of electrospun collagen as the bone-like fibrous structure is the potential collagen triple helix denaturation in the fiber state causing inadequate wet stability also after crosslinking.7 This can be prevented by diacid-based crosslinking of collagen triple helices.7 The crosslinked fibers were coated with carbonated hydroxyapatite through biomimetic precipitation, resulting in an attractive biomaterial for guided bone regeneration.7 The friction and wear behavior of soybean oil-based polymers prepared by cationic polymerization of low saturated soybean oil with divinylbenzene and polystyrene were

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Crosslinkers

evaluated as a function of crosslink density.8 Higher crosslink density resulted in lower adhesive wear.8 Increased abrasive wear was observed for the lowest and the highest crosslink densities.8 Large number of review papers, books, and encyclopedia help in further understanding the possibilities provided by the crosslinked biopolymers.9-15 Boronated biopolymer crosslinking agents, useful in producing viscosified treatment fluids, include an aqueous fluid, a base polymer, and the boronated biopolymer crosslinking agent.16 The boronated biopolymer crosslinking agent comprises a biopolymer derivatized with a boronic acid, a boronate ester, or both.16 The viscosified treatment fluids are useful in fracturing operations, gravel packing operations, drilling operations, and so on.16 Gelatin is chemically modified with the use of a low-volatile chemical substance.17 The crosslinking method produced a biopolymer having high strength (a high degree of crosslinking).17 The glutaraldehyde was used as a crosslinking agent.17 References 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

Vetrik, M; Mattova, J; Mackova, H; Kucka, J; Hruby, M, J. Controlled Release, 273, 131-8, 2018. Paraskevopoulou, P; Gurikov, P; Raptopoulos, G; Chriti, D; Argyraki, A, Polyhedron, 154, 209-16, 2018. Zeeb, B; Lopez-Pena, CL; Weiss, J; McClements, DJ, Food Hydrocolloids, 46, 125-33, 2015. Jiang, X; Yang, Z; Peng, Y; Han, B; Liu, W, Carbohydrate Polym., 137, 632-41, 2016. Laha, A; Sharma, CS; Majumdar, S, Mater. Today: Proc., 3, 10, 3484-91, 2016. Grossmann, L; Wefers, D; Bunzel, M; Weiss, J; Zeeb, B, LTW, 75, 271-8, 2017. Arafat, MT; Tronci, G; Yin, Wood, DJ; Russell, SJ, Polymer, 77, 102-12, 2015. Bhuyan, S; Holden, LS; Sundararajan, S; Andjelkovic, D; Larock, R, Wear, 263, 7-12, 965-73, 2007. Reddy, N; Reddy, R; Jiang, Q, Trends Biotechnol., 33, 6, 362-9, 2015. Garavand, F; Rouhi, M; Razavi, SH; Cacciotti, I; Mohammadi, R, Int. J. Biol. Macromol., 104A, 687-707, 2017. Ling, S; Chen, W; Fan, Y; Zheng, K; Kaplan, DL, Prog. Polym. Sci., 85, 1-56, 2018. Galiano, F; Briceño, K; Marino, T; Molino, A; Figoli, A, J. Membrane Sci., 564, 562-86, 2018. Jacob, J; Haponiuk, JT; Thomas, S; Gopi, S, Mater. Today Chem., 9, 43-55, 2018. Sudhakar, YN; Selvakumar, M; Bhat, DK, An introduction of Biopolymer Electrolytes. Biopolymer Electrolytes. Elsevier, 1-34, 2018. Davidenko, N; Cameron, R; Best, S, Natural Biopolymers for Biomedical Applications. Encyclopedia of Biomedical Engineering, Elsevier, 162-76, 2019. Singh, D; Holtsclaw, J; Reddy, BR, US9688906B2, Halliburton Energy Services Inc., Jun. 27, 2017. Ooya, S; Hirato, T, US8268968B2, Fujifilm Corp, Sep. 18, 2012.

2.2.7 Bromobutyl rubber

19

2.2.7 BROMOBUTYL RUBBER Thermally-stable bromobutyl rubber having a high crosslink density was prepared using a 4,40-bismaleimidodiphenylmethane as a curing agent.1 Zinc oxide accelerated crosslinking reaction. The dicumyl peroxide was used as the reaction accelerator.1 The resultant rubber had low compression set at an elevated temperature and excellent thermal stability.1 High-performance graphene/bromobutyl rubber nanocomposites were developed for tire inner liner application by grafting of bromobutyl rubber on simultaneously functionalized and reduced graphene oxide surface. Figure 2.9 shows the effect of graphene on crosslinking bromobutyl rubber.

Figure 2.9. Schematic diagram for the tethering of bromobutyl rubber on (a) phenyl amine functionalized graphene and (b) graphene. [Adapted, by permission, from Kotal, M; Banerjee, SS; Bhowmick, AK, Polymer, 82, 121-32, 2016.]

The rubber materials modified by crosslinking have an excellent balance of cure rate, cure state, and scorch safety.3 They have improved tire carcass adhesion and impermeability to gases, and they are useful in inner tire tubes and inner liners.3 The crosslinking composition consists of a sulfur curative system (i) sulfur, (ii) an accelerator, and (iii) a zinc oxide promoter, and a non-sulfur curative system ((i) a non-sulfur curative selected from the group consisting of di- and tri-functional mercapto compounds and their derivatives and (ii) a metal compound promoter chosen from the group consisting of oxides, hydroxides, and carbonates of metals in Groups Ia and IIa).3 The multifunctional phosphine, (2-diphenylphosphinophenyl)ether, the crosslinking agent, has been used for crosslinking bromobutyl rubber.4 This invention provides a sulfur-free and ZnO-free crosslinking composition.4 References 1 2 3 4

Sathi, SG; Jang, JY; Jeong, K-U; Nah, C, J. Appl. Polym. Sci., 44092, 2016. Kotal, M; Banerjee, SS; Bhowmick, AK, Polymer, 82, 121-32, 2016. Berta, DA, US4591617A, Hercules Inc, May, 27, 1986. Nguyen, P; Gilles Arsenault, G, US10040873B2, Lanxess Inc, Aug, 7, 2018.

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Crosslinkers

2.2.8 BUTYL RUBBER Butyl rubber, an isobutylene/isoprene copolymer, has low permeability to gases, good thermal stability, and high resistance to oxygen and ozone action.1 The ionizing radiation causes chain scission accompanied by a significant reduction in molar mass.1 Doses higher than 150 kGy destroy its properties, irrespective of the vulcanization system used; however, compounds cured with phenolic resin showed a decrease in properties proportional to the dose.1 The crosslink density of butyl rubber exposed to γ-radiation in cobalt bomb was estimated using the Flory-Rehner equation.2 Crosslinking and chain scission reactions occurred under irradiation as estimated by the Charlesby-Pinner equation.2 At 25°C, the chain crosslinking process predominated over the chain scission, and both phenomena competed at a temperature of 70°C.2 The rubber properties change followed plastic to brittle transition.2 Hydrosilylation crosslinking of acrylic modified butyl rubber resulted in improved compression set.3 Crosslinking butyl rubber comprised addition to 100 wt% of an isoprene-isobutylene rubber, 5 to 25 wt% of an alkylphenol-formaldehyde resin and 0.1 to 5 wt% of a hydrazide compound, or 5 to 25 wt% of an alkylphenol-formaldehyde resin, 0.1 to 5 wt% of a hydrazide compound and 0.3 to 10 wt% of an epoxy compound.4 Superabsorbent materials in the form of polymer membranes, rods or beads were based on butyl rubber which was crosslinked using sulfur monochloride.5 Butyl rubber was crosslinked using stable nitroxide radical, such as 4-hydroxy-2,2,6,6-tetramethylpiperidinyl-1-oxyl.6 References 1 2 3 4 5 6

Scagliusi, SR; Cardoso, ECL; Lugao, AB, Radiat. Phys. Chem., 81, 8, 991-4, 2012. Smith, M; Berlioz, S; Chailan, JF, Polym. Deg. Stab., 98, 2, 682-90, 2013. Medsker, RE; Patel, R; Wang, D, US6084031A, Advanced Elastomer Systems LP, Jul. 4, 2000. Onizawa, M, US6388008B2, May 14, 2002. Erman, B; Okay, O; Durmaz, S, WO2001048042A1, Sabanci Universitesi Tubitak Marmara Arastirma Merkezi, Jul. 5, 2001. Ashiura, M; Kawazura, T, US7956134B2, Yokohama Rubber Co Ltd, Jun. 7, 2011.

2.2.9 Cellulose acetate butyrate

21

2.2.9 CELLULOSE ACETATE BUTYRATE Moisture sensor was made from cellulose acetate butyrate crosslinked using tolylenediisocyanate, 1.3-butadienediepoxide, terephthalic acid, or maleic anhydride.1 The sensor detected variation of humidity in the atmosphere by changes in the dielectric constant of the crosslinked polymer film.1 The compositions used in polarizing optical devices comprised of cellulosic material and a crosslinking agent.2 Preferred cellulosic materials included cellulose esters such as cellulose acetates, cellulose triacetates, cellulose acetate phthalates, and cellulose acetate butyrates.2 Preferred crosslinking agents were triazines such as those derived from melamine and benzoguanamine.2 Crosslinking cellulose acetate butyrate was conducted in the presence of atmospheric oxygen and one of the following crosslinking agents: trimethylolpropane trimethacrylate, trimethylolpropane triacrylate, a poly(ethylene acrylate) copolymer, triallyl cyanurate, or triallyl isocyanurate.3 References 1 2 3

Uda, K; Hijikigawa, M, US4773935A, Sharp Corp, Sep. 27, 1988. McCreight, KW; Hoffman, DC; Hale, WR, US20120222793A1, Eastman Chemical Co, Sep. 6, 2012. Chang, F-J; Pavlek, WP; Palys, LH; Dluzneski, PR; Despotopoulou, M; Defrancisci, A; Brennan, JM; Abrams, MP; Tartarin, I, EP3230360A4, Arkema Inc, Jul. 11, 2018.

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Crosslinkers

2.2.10 CELLULOSE ACETATE PROPIONATE Cellulose acetate propionate has been functionalized with 1,6-hexamethylene diisocyanate and subsequently dispersed in castor oil to obtain chemical oleogel, which can be used as lubricating grease based on renewable resources.1 The presence of non-polar groups reduced cellulose polarity and, therefore, increased the affinity to the oil medium.1 Cellulose acetate propionate was crosslinked by hexamethoxymethylmelamine catalyzed by ρ-toluenesulfonic acid.2 The material was used for protective cover layers to protect graphic art images.2 Cellulose acetate propionate film with excellent mechanical strength and dimensional stability was developed for application as an optical compensation sheet, a polarizing plate, and a liquid crystal display device. The cellulose acetate propionate film was free of crosslinks. References 1 2 3

Gallego, R; Arteaga, JF; Valencia, C; Franco, JM, Chem. Eng. Sci., 134, 260-8, 2015. Cahill, DA; Himmelwright, RS; Kearney, FJ; Roach, JM, US5397634A, Rexam Graphics Inc, Mar. 14, 1995. Watano, A; Kawanishi, H, US20080227881A1, Fujifilm Corp, Sep. 18, 2008.

2.2.11 Chitosan

23

2.2.11 CHITOSAN The application of biodegradable chitosan fiber for healthy and hygienic textiles is limited due to its poor acid resistance in wet processing and poor antioxidant activity.1 To improve this, chitosan fiber was crosslinked using a water-soluble aziridine crosslinker, and then dyed with natural lac dye consisting of polyphenolic anthraquinone compounds.1 The fiber had significantly reduced weight loss in acidic solution and excellent acid resistance.1 Lac dyeing imparted good antioxidant activity to chitosan fiber.1 Fiber retained its good antibacterial activity after crosslinking and dying.1 Formaldehyde and sodium tripolyphosphate were used crosslinkers for fabrication of chitosan hybrid scaffolds using bovine serum albumin as a growth factor by direct incorporation and microencapsulation methods.2 The scaffolds were designed for biomedical applications.2 Tripolyphosphate-crosslinked scaffolds retained their shape after 14 days of biodegradation while the formaldehyde-crosslinked scaffolds did not, and they were disintegrated.2 Also, formaldehyde-crosslinked scaffolds were toxic, and had the poor mechanical strength and burst release.2 The tripolyphosphate-crosslinked scaffolds had good mechanical strength with the slow release of bovine serum albumin and were the bestsuited combination for fabricating hybrid scaffolds for tissue engineering.2 Genipin crosslinked chitosan hydrogel was used for the controlled release of tetracycline.3 The genipin amount modulated the physical properties of the hydrogel.3 The hydrogel was able to release tetracycline in a controlled manner and keep its bioactivity for a long time.3 The tetracycline-loaded chitosan hydrogel showed good antibacterial effect

Figure 2.10. Optical micrographs of chitosan droplets stabilized by different concentrations of functionalized reduced graphene oxide: (a) 0.5 mg/mL, (b) 0.75 mg/mL, (c) 1.0 mg/mL, (d) 1.25 mg/mL, (e) 1.5 mg/mL. Chitosan concentration: 10 mg/mL, water/oil ratio 1/5. [Adapted, by permission, from Zhang, Y; Wang, X; Xu, C; Yan, W; Tian, Q; Sun, Z; Yao, H; Gao, J, Carbohydrate Polym., 186, 1-8, 2018.]

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Crosslinkers

Figure 2.11. Schematic representation of ionic crosslinks in chitosan membranes modified with pentasodium tripolyphosphate (DDA – degree of deacetylation). [Adapted, by permission, from Gierszewska, M; OstrowskaCzubenko, J, Carbohydrate Polym., 153, 501-11, 2016.]

even under alkaline environment.3 The chitosan hydrogel exhibited lower cytotoxicity than tetracycline alone.3 Colloidal emulsions stabilized by solid particles are called Pickering emulsions.4 They have important applications in food, cosmetics, and pharmaceutical fields.4 Hydrophobic and dispersible functionalized reduced graphene oxide was employed to prepare chitosan gel droplets by crosslinking an inverse Pickering emulsion.4 Drug loaded chitosan gel droplets (microgels) were prepared (Figure 2.10).4 Pickering emulsification was utilized to make gentamycin loaded chitosan gel droplets by in situ crosslinking for controllable drug release.4 Addition of citric acid as a biocompatible crosslinker and treatment with alkali produced chitosan films produced excellent tensile strength and aqueous stability.5 The alkali treatment decreased water sorption to 100-250% and made the films resistant to boiling water.5

2.2.11 Chitosan

25

Figure 2.12. Methods of ionic and covalent crosslinking of chitosan. [Adapted, by permission, from Jóźwiak, T; Filipkowska, U; Szymczyk, P; Rodziewicz, J; Mielcarek, A, Reactive Funct. Polym., 114, 58-74, 2017.]

Chitosan-based membranes having different ionic crosslink density were developed for pharmaceutical and industrial applications.6 The membranes were ionically crosslinked with pentasodium tripolyphosphate.6 Figure 2.11 shows the structure of ionically crosslinked chitosan.6 The hydrogel chitosan sorbents were crosslinked with eight agents (four ionic ones: sodium citrate, sodium tripolyphosphate, sulfosuccinic acid, and oxalic acid and four covalent ones: glutaraldehyde, epichlorohydrin, trimethylpropane triglycidyl ether, and ethylene glycol diglycidyl ether) (Figure 2.12).7 The process of ionic crosslinking was the most effective at the pH value below which hydrogel chitosan sorbent began to dissolve (pH 4).7 Higher temperature during ionic crosslinking slightly decreased sorption effectiveness of hydrogel.7 The sorbents crosslinked with covalent agents were harder and more fragile.7 The chitosan in the aqueous solution was chemically modified or crosslinked by a selective substitution on the amino group of chitosan for solid, semi-solid, or solidifying implants, which were implanted or injected into body tissue.8

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Crosslinkers

Chitosan is a basic component of some diet supplements, possibly in association with other compounds, used to reduce cholesterol and lipid intake as well as to reduce sugar absorption and body weight.9 The crosslinked chitosan cannot be attacked or degraded by digestive enzymes and/or the physiological conditions of the gastrointestinal tract.9 The aliphatic aldehyde (formaldehyde and glutaraldehyde) was used as crosslinker.9 A smoking article filter having a porous resin with a high surface area to mass ratio comprised of a chitosan derivative.10 Chitosan was crosslinked with glutaraldehyde and glyoxal.10 The chitosan derivative provided the selective filtration of cigarette smoke, particularly for the removal of aldehydes, hydrogen cyanide, heavy metals and carbonyls.10 References 1 2 3 4 5 6 7 8 9 10

Li, X-Q; Tang, R-C, Polymers, 6, 8, 119, 2016. Singh, BK; Sirohi, R; Archana, D; Jain, A; Dutta, PK, Int. J. Polym. Mater. Polym. Biomater., 64, 242-52, 2015. Zhang, W; Ren, G; Xu, H; Zhang, J; Lin, H; Mu, S; Cai, Z; Wu, T, J. Polym. Res., 23, 156, 2016. Zhang, Y; Wang, X; Xu, C; Yan, W; Tian, Q; Sun, Z; Yao, H; Gao, J, Carbohydrate Polym., 186, 1-8, 2018. Nataraj, D; Sakkara, S; Meghwal, M; Reddy, N, Int. J. Biol. Macromol., 120A, 1256-64, 2018. Gierszewska, M; Ostrowska-Czubenko, J, Carbohydrate Polym., 153, 501-11, 2016. Jóźwiak, T; Filipkowska, U; Szymczyk, P; Rodziewicz, J; Mielcarek, A, Reactive Funct. Polym., 114, 58-74, 2017. Chenite, A; Berrada, M; Chaput, C; Dabbarh, F; Selmani, A, WO2003042250A1, Biosyntech Canada Inc., May 22, 2003. Mezzina, C; Scapagnini, G; Volpato, I; Bizzini, B; Franchi, G, WO2008007321A3, Sirc Spa Natural & Dietetic Foods, Mar. 20, 2008. Caraway, JW; Jackson, TJ, EP1746906A1, Brown and Williamson Holdings Inc, Jan. 31, 2007.

2.2.12 Chlorinated polyethylene

27

2.2.12 CHLORINATED POLYETHYLENE Microcellular chlorinated polyethylene foams were prepared using nitrogen as a blowing agent.1 Dicumyl peroxide was used for crosslinking. Some level of crosslink density must be attained for effective foaming.1 The average cell sizes of the foams decreased, and cell density increased when crosslink density was increased.1 Arkema has introduced the latest innovation in its Vul-Cup® line of crosslinking peroxide products − Vul-Cup 40C-SP2 is suitable for crosslinking of chlorinated polyethylene.2 A chlorinated polyolefin composition was found suitable for use in the manufacture of crosslinked, thermoset, flame-retardant articles such as jackets for electrical or fiber optic cables, insulating coatings or layers for electrical conductors, and heat-shrinkable tubing, sheets or other shapes for protection of cable connectors and splices.3 The hydrolyzable silane groups formed silane crosslinks between the molecules of the silane-grafted polyethylene upon exposure to moisture, and the degree of crosslinking was sufficient to provide the article with thermoset properties.3 Foamed chlorinated polyethylene was manufactured using phenylmaleimide and ketal peroxide.4 The foam was design for thermal insulation.4 The polymercapto crosslinking agent (2-mercapto-1,3,4-thiadiazole-5-thiobenzoate) was selected for curing chlorosulfonated polyethylene.5 References 1 2 3 4 5

Lang, X-h; Wang, D; Prakashan, K; Zhang, X; Zhang, Z-X, J. Polym. Res., 24, 175, 2017. Addit. Polym., 2015, 12, 2, 2015. Jackson, P; Prema, JR, US20120128906A1, Shawcor Ltd, May, 24, 2012. Rametsteiner, K, DE19943547A1, Kelit Kunststoffwerk GmbH, Mar. 16, 2000. Laakso, RL; Marchand, GR, US7964110B2, Dow Global Technologies LLC, Jun. 21, 2011.

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Crosslinkers

2.2.13 CHLOROPRENE Thiophosphoryl disulfide was found to be suitable crosslinking agents for chloroprene rubber.1 A latex of a benzene-soluble chloroprene polymer was subjected to a crosslinking post-treatment.2 The dialkoxyxanthogen disulfides were used as crosslinking agents.2 References 1 2

Das, A; Naskar, N; Basu, DK, J. Appl. Polym. Sci., 91, 1913, 2004. Mayer-Mader, R; Boldt, J, US4000222A, Bayer AG, Dec. 28, 1976.

2.2.14 Cyanoacrylate

29

2.2.14 CYANOACRYLATE Polyethylene glycol 400 biscyanoacrylate was used as a crosslinker of 2-octyl cyanoacrylate for potential use as bioadhesive.1 PEG400 biscyanoacrylate was synthesized by esterification of anthracenyl cyanoacrylic acid in which the anthracene unit served as a vinylprotecting group.1 The octyl cyanoacrylate chains were covalently crosslinked and prevented the structure from falling apart.1 The crosslinking prevented interactions between the octyl cyanoacrylate chains, resulting in increased plasticity.1 Polyhedral oligomeric silsesquioxane (10 wt%) was used as reinforcing/crosslinking nanofiller in octyl cyanoacrylate adhesive to increase the microshear bond strength to dentin.2 The adhesives also exhibited lower water sorption and higher hydrolytic stability.2 To improve the cohesive strength of the α-cyanoacrylate, difunctional monomeric crosslinking agents can be added.3 Examples of suitable crosslinking agents include alkyl bis(2-cyanoacrylates), triallyl isocyanurates, alkylene diacrylates, alkylene dimethacrylates, trimethylolpropane triacrylate, and alkyl bis(2-cyanoacrylates).3 References 1 2 3

Basu, A; Veprinsky-Zuzulia, I; Levinman, M; Barkan, Y; Golenser, J; Domb, AJ, Macromol. Rapid Commun., 37, 253-6, 2016. Fadaie, P; Atai, M; Imani, M; Karkhaneh, A; Ghasaban, S, Dental Mater., 29, 6, e61-9, 2013. Liu, H, US7238828B2, Ethicon Inc, Jul. 3, 2007.

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Crosslinkers

2.2.15 EPOXIDIZED NATURAL RUBBER Biological phytic acid was applied as a multifunctional curing agent for epoxidized natural rubber used for skin-touchable and flame retardant electronic sensors (wearable electronics).1 An electronic sensor was prepared by depositing a nanostructured conductive layer on the elastomer substrate for human-motion monitoring (Figure 2.13).1 The electronic sensor is capable of self-extinguishing after ignition owing to the presence of phosphorus-rich groups.1

Figure 2.13. Schematic illustration of the preparation of the skin-touchable and flame retardant electronic sensor with a nanostructured design. PA − phytic acid, ENR − epoxidized natural rubber, CNT − multiwalled carbon nanotube, CNC − cellulose nanocrystal. [Adapted, by permission, from Guo, Q; Cao, J; Han, Y; Tang, Y; Zhang, X; Lu, C, Green Chem., 19, 3418-27, 2017.]

Epoxidized natural rubber (ENR)-silica hybrids can be cured with silica as a crosslinking and reinforcing agent.2 13C-NMR spectroscopy confirmed the presence of covalent and hydrogen bonds between the epoxy group and Si− OH.2 Modulus was increased by 35 times, tensile strength by 20 times, and tear strength by 12 times because of crosslinking.2 Figure 2.14 shows the proposed model of interactions and bonding between silanol groups on the surface and epoxidized natural rubber, ENR, molecules in the hybrid materials.2 Disulfide groups were introFigure 2.14. Proposed model of interactions and bonding. duced into the epoxidized natural [Adapted, by permission, from Xu, T; Jia, S; Wang, S; Chen, Y; rubber using dithiodibutyric acid as a Luo, Y; Jia, D; Peng, Z, J. Appl. Polym. Sci., 134, 44605, 2017.]

2.2.15 Epoxidized natural rubber

31

crosslinker which gave it strong elastomeric properties.3 The material behaved like a standard natural rubber up to 100°C and became reprocessable above 150°C, owing to disulfide rearrangement.3 Good adhesion of this rubber permitted repairing of cracks by heating and thus led to fatigue improvement.3 The crosslinking of epoxidized natural rubber with dodecanedioic acid was accelerated by 1,2-dimethylimidazole (Figure 2.15).4 The activation of the crosslinking agent was enabled by the synergistic association of 1,2-dimethylimidazole and diacid forming a soluble species in the rubber matrix, which enabled the efficient crosslinking of functionalized natural rubber without the use of sulfur or peroxides.4

Figure 2.15. Crosslinking of epoxidized natural rubber by dodecanedioic acid in the presence of 1,2-dimethylimidazole. The epoxidation ratio m was equal to 0.1 or 0.25. [Adapted, by permission, from Pire, M; Norvez, S; Iliopoulos, I; Le Rossignol, B; Leibler, L, Polymer, 52, 23, 5243-9, 2011.]

Semimetallic friction composites consisting of epoxidized natural rubber, alumina nanoparticles, steel wool, graphite, and benzoxazine were vulcanized using sulfur and electron-beam crosslinking system.5 The friction coefficients at normal and hot conditions, as well as the hardness and density of the irradiated composites, were higher than those of the sulfur-vulcanized samples at all applied doses.5 The specific wear rates of the irradiated samples were lower than those of the sulfur-vulcanized samples at all applied doses.5 The sample crosslinked via electron beam irradiation at 150 kGy exhibited the greater tribological property compared to the sulfur-vulcanized composite.5 The effect of pre-irradiation of epoxidized natural rubber on the properties of 50/50 poly(vinyl chloride)/epoxidized natural rubber blend was investigated.6 The irradiation dose had a significant influence on the blend homogeneity, as well as it improved tensile strength, modulus, hardness, impact strength, and gel fraction of the blend.6 Crosslinking the rubber phase in PVC/ENR blend played a significant role in resisting crack propagation during the impact tests.6 The epoxidized natural rubber composition with a chlorosulfonated polyethylene rubber composition was formed by mixing and kneading, and then it was heated, and selfcrosslinked.7 Proper crosslinking depended on the proportion of both polymers.7 Crosslinking was carried out in a composition comprising (a) elastomeric polymer containing carboxylic groups; (b) a liquid organic compound containing epoxide groups located internally on the molecule, and (c) an oxide or an inorganic salt of a metal selected from Fe, Cu, Sn, Mo and Ni.8 Upon heating, the elastomeric material reached a high

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Crosslinkers

degree of crosslinking without the addition of conventional crosslinking agents, with crosslinking times maintained within limits which were acceptable for an industrial use.8 These compositions are particularly suitable for producing tread bands of tires.8 The epoxidized natural rubber, Lewis acid, and diene were subjected to a ring-opening reaction in a mixer.9 The epoxidized natural rubber after crosslinking became thermally-reversible crosslinked elastomer.9 Polycarboxylic acid was used as a crosslinking agent and 1,2-dimethylimidazole as a vulcanization accelerator for crosslinking epoxidized natural rubber.10 References 1 2 3 4 5 6 7 8 9 10

Guo, Q; Cao, J; Han, Y; Tang, Y; Zhang, X; Lu, C, Green Chem., 19, 3418-27, 2017. Xu, T; Jia, S; Wang, S; Chen, Y; Luo, Y; Jia, D; Peng, Z, J. Appl. Polym. Sci., 134, 44605, 2017. Imbernon, L; Oikonomou, EK; Norvez, S; Leibler, L, Polym. Chem., 6, 4271-8, 2015. Pire, M; Norvez, S; Iliopoulos, I; Le Rossignol, B; Leibler, L, Polymer, 52, 23, 5243-9, 2011. Almaslow, A; Ratnam, CT; Ghazali, MJ; Talib, RJ; Azhari, CH, Compos. Part B: Eng., 54, 377-82, 2013. Ratnam, CT; Kamaruddin, S; Sivachalam, Y; Talib, M; Yahya, N, Polym. Testing, 25, 4, 475-80, 2006. Mitsunari, K, JP3611505B2, Kurashiki Kako Co Ltd, Jan. 19, 2005. Nahmias, NM; Serra, A, EP1231079B1, Pirelli Pneumatici SpA, Jul. 28, 2004. CN106554429B, Mar. 30, 2018. Schnell, B; Fleury, E, EP3157973B1, Compagnie Generale des Establissements Michelin et Cie, Apr. 25, 2018.

2.2.16 EPDM

33

2.2.16 EPDM A green process of plasticization and crosslinking of an EPDM was based on a thiofuran coupling agent.1 Tung oil, composed of around 80 wt% of α-eleostearic acid, was used as both plasticizer and thermal crosslinker.1 The 2-furanmethanethiol was used as a difunctional compatibilizing agent.1 EPDM matrix, and the tung oil were crosslinked by DielsAlder reaction (Figure 2.16).1

Figure 2.16. Theoretical structure after thermal curing at 250°C of the system EPDM/2-furanmethanethiol and tung oil. [Adapted, by permission, from Bétron, C; Cassagnau, P; Bounor-Legaré, V, Mater. Chem. Phys., 211, 361-74, 2018.]

Mica, treated by three types of coupling agents, isopropyl trioleic titanate, 3-aminopropyltriethoxysilane, and vinyltrimethoxysiloxane homopolymer, was utilized to improve the properties of ethylene propylene diene monomer.2 The increased crosslink density caused by coupling agents increased the volume resistivity of EPDM composites.2 Sorbic acid was used as a crosslinker to enhance properties of ethylene propylene diene monomer rubber using γ-radiation.3 The largest improvement was achieved by using sorbic acid concentration of 10 phr and a dose of 100 kGy of γ-irradiation.3 The incorporation of triallyl isocyanate and peroxide (bis(t-butylperoxy-isopropyl) benzene) into the network enhanced the network heterogeneity.4 The glass transition temperature shifted towards higher temperature and the peak values of the loss factor gradually decreased, both of which were caused by the restricted segmental mobility due to the increase in crosslink density.4 The crosslinking efficiency of the peroxide was 0.77–0.95 for the EPDM, which contained 4.3 wt% unsaturations, and the efficiency of the triallyl isocyanate was estimated to be about 33%.4 Crosslink network evolution of brominated butyl rubber/ethylene propylene dienemonomer rubber blends during peroxide vulcanization was studied at a mesoscale level.5 EPDM was added to increase the crosslink density of brominated butyl rubber vulcaniza-

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Crosslinkers

tion.5 It was suggested that the addition of EPDM improved crosslinking of brominated butyl rubber by repairing the damaged crosslink network resulting from the degradation of brominated butyl rubber.5 The irradiation stability of ethylene propylene diene terpolymer/polypropylene blends was studied in an attempt to develop radiation compatible blend suitable for medical applications.6 The irradiation-induced crosslinking of blend increased with the increase of the irradiation dose and the EPDM content in the blend.6 The thermal stability of the blend did not show any significant changes upon irradiation.6 The EPDM-rich blend had higher compatibility than PP dominant blends.6 Ethylene propylene diene monomer rubber having good flame-retardance and γ-ray radiation resistance were prepared by adding complex flame retardants and phenanthrene.7 The γ-ray radiation resistance was enhanced by adding phenanthrene.7 The variation of crosslink density, the spatial distribution of the crosslinks in the network, and the presence of network defects in EPDM rubber were investigated on a molecular level using proton low-field solid-state double-quantum NMR spectroscopy.8 The type and concentration of peroxides did not affect the overall inhomogeneous spatial distribution of the crosslinks but did affect significantly the average crosslink density.8 Luperox F (di(t-butylperoxy)diisopropylbenzene) gave the highest crosslink density, followed by dicumyl peroxide, and by Luperox 101 (2,5-dimethyl-2,5-di(t-butylperoxy)hexane).8 References 1 2 3 4 5 6 7 8

Bétron, C; Cassagnau, P; Bounor-Legaré, V, Mater. Chem. Phys., 211, 361-74, 2018. Su, J; Zhang, J; J. Appl. Polym. Sci., 134, 44833, 2017. El-Nemr, K; Mohamed, RM, J. Macromol. Sci. Part A: Pure Appl. Chem., 54, 10, 711-9, 2017. Wang, H; Zhao, S-G; Wrana, C, J. Macromol. Sci. Part B: Phys., 56, 1, 39-52, 2017. Wang, J; Pan, S; Zhang, Y; Guo, S, Polym. Testing, 59, 253-61, 2017. Balaji, AB; Ratnam, CT; Khalid, M; Walvekar, R, Radiat. Phys. Chem., 141, 179-89, 2017. Chen, J; Huang, W; Jiang, S-B; Li, X-Y; Chen, H-B, Radiat. Phys. Chem., 130, 400-5, 2017. Saleesung, T; Reichert, D; Saalwächter, K; Sirisinha, C, Polymer, 56, 309-17, 2015.

2.2.17 Epoxy resin

35

2.2.17 EPOXY RESIN The crosslinkable epoxy resins containing azobenzene or/and phenylacetylene groups have been synthesized.1 Incorporation of crosslinkable groups significantly improved flame retardance of epoxy resins.1 Formation of compact char as a result of the crosslinking reactions improved flame retardance of epoxy resin without the presence of flame retardant.1 The difference in heat capacity of the liquid state and the glassy state at the glass transition temperature has been determined for an epoxy resin having different crosslink density.2 As crosslinking density increased, the ΔCp(Tg) decreased which suggested a resistance of the glassy structure to the breakdown due to a “strengthening” effect of the crosslinked structure.2 Thermal oxidation of three epoxy resins (prepolymer: bisphenol A diglycidyl ether and 1,4-butanediol diglycidyl ether and hardener: isophorone diamine and 4,7,10-trioxa1,13-tridecanediamine) was monitored by changes in glass transition temperature.3 Predominance of crosslinking over chain scission was noted for materials having linear aliphatic segments.3 The presence of unreacted epoxy groups is more detrimental for the thermalmechanical properties than the excess of unreacted amine groups.4 Several factors may contribute, as follows: (1) the presence and size of pendant mono-reacted epoxy chainends carrying an excess of free volume, (2) a significant decrease in crosslink density and (3) the plasticizing effect of the soluble fraction.4 The increase in crosslink density results in a decrease of the mobility of the network, which can only relax at higher temperatures. The crosslink density has a strong influence on glass transition temperature.4 An epoxy-functional (meth)acryloyl copolymer and epoxy resin were crosslinked using an ionic photoacid generator to form a pressure-sensitive adhesive.5 The ionic photoacid generator was selected from iodonium salts, sulfonium salts, sulfoxonium salts, selenonium salts, phosphonium salts, and arsonium salts.5 The ionic photoacid generator was used in the amount not greater than 1 phr.5 Epoxy resin, a curing agent (4,4′-diaminodiphenyl methane), and a modification agent (9,10-dihydro-9-oxa-10-phosphaphenanthrene 10-oxide) were used to produce epoxy resin with flame retardant properties.6 Crosslinking agents for epoxy resin compositions were based on an isomeric mixture of tris(2-hydroxyphenyl)-phosphine oxide compounds.7 Epoxy resins cured or crosslinked with this crosslinking agent gave non-halogenated, ignition-resistant epoxy resin formulations.7 The ignition-resistant epoxy resin formulations can be used for laminates for printed wiring boards and composite materials.7 A composition contained a hydrogenated bisglycidyl ether and a crosslinking agent. The hydrogenated bisglycidyl ether has been developed for the production of scratchresistant articles such as car windows, encapsulants for solar cells, claddings for motorcycles, mopeds, and scooters.8 The multi-functional epoxy resin has three epoxy groups.9 It was used for coating a face of the subterranean formation (wells) with the consolidation composition.9 The resin was cured with a polyamine.9

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Crosslinkers

References 1 2 3 4 5 6 7 8 9

Liu, B-W; Zhao, H-B; Tan, Y; Chen, L; Wang, Y-Z, Polym. Deg. Stab., 122, 66-76, 2015. Montserrat, S, Polymer, 36, 2, 435-6, 12995. Ernault, E; Richaud, E; Fayolle, B, Polym. Deg. Stab., 138, 82-90, 2017. Fernández-Francos, X; Ramis, X, Eur. Polym. J., 70, 286-305, 2015. Mahoney, WS; Weikel, AL; Krepski, LR; Gaddam, BN, EP2723812B1, 3M Innovative Properties Co, Apr. 13, 2016. Su, W-C; Liu, C-K; Jeng, R-J; Dai, SA; Lin, C-H, US7897702B2, National Chung Shan Institute of Science and Technology, Mar. 1, 2011. Brennan, DJ; Everett, JP; Nader, BS, US6403220B1, Dow Chemical Co, Jun. 11, 2002. Klopsch, R; Becker, M; Arndt, J-D; Van, LF, EP1844085A1, BASF SE, Oct. 17, 2007. Wadekar, SD; Agashe, SS; Kshirsagar, SG, US9567511B2, Halliburton Energy Services Inc, Feb. 14, 2017.

2.2.18 Ethylene-vinyl acetate copolymer

37

2.2.18 ETHYLENE-VINYL ACETATE COPOLYMER The crosslinking using dicumyl peroxide affected the complex viscosity of ethylene-vinyl acetate copolymer.1 The values of the power law parameter (n) of the EVA without dicumyl peroxide ranged from 0.39 to 0.50, and the EVA with dicumyl peroxide ranged from 0.03 to 0.12.1 Ethylene-vinyl acetate copolymers were crosslinked in the melt with tetrapropoxysilane in the presence of dibutyl tin oxide as a catalyst.2 The excess of propoxy groups slowed down the reaction kinetics.2 The effects of crosslinking and foaming agents on EVA density and mechanical properties of the cured foams with two curing systems (peroxide and sulfur-peroxide) for the potential use in automotive applications, were studied.3 The density of foam was reduced with an increase in vinyl acetate content.3 The foams cured by peroxide system had higher densities than the foams cured with sulfur-peroxide.3 Ethylene-vinyl acetate dominates the encapsulation of solar cells.4 The crosslinking optimization may lead to shortening of the photovoltaic module lamination time.4 A sufficient level of crosslinking occurred after 5 min which corresponded to 50% crosslinking.4 EVA not fully cured during the lamination undergoes post-lamination crosslinking, but the remaining active crosslinker causes discoloration at soldering ribbons after accelerated aging.4 Ethylene vinyl acetate compositions that have been partially crosslinked using organic peroxides had low melt index values and increased tensile strength, leading to better flexibility and heat resistance.5 Maleic anhydride was used as a crosslinker.5 Peroxide mixtures were used for the accelerated crosslinking of ethylene vinyl acetate.6 A peroxide mixture was composed of 0.25% t-butyl-per-2-ethylhexanoate and 1% tert-butylperoxy(2-ethylhexyl)carbonate of copolymer.6 References 1 2 3 4 5 6

Sung, YT; Kum, CK; Lee, HS; Kim, JS; Kim, WN, Polymer, 46, 25, 11844-8, 2005. Goutille, Y; Carrot, C; Majeste, J-C; Prochazka, F, Polymer, 44, 11, 3165-71, 2003. Abady, ARN; Movahed, SO, J. Appl. Polym. Sci., 134, 45357, 2017. Oreski, G; Rauschenbach, A; Hirschl, C; Kraft, M; Eder, GC; Pinter, G, J. Appl. Polym. Sci., 134, 44912, 2017. Allermann, GA; Fruitwala, HA, US7939607B2, ExxonMobil Chemical Patents Inc, May 10, 2011. Kunz, M; Seitz, K; Nagl, I, US9382405B2, United Initiators GmbH and Co KG, Jul. 5, 2016.

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Crosslinkers

2.2.19 FLUOROELASTOMER Fluoroelastomer was reinforced by incorporation of carboxylic-functionalized multiwalled carbon nanotubes which were modified by ethylenediamine.1 Participation of modified nanotubes formed a dual crosslinking network.1 The silicone terpolymers bearing Si–H reactive groups were used for membrane crosslinking and fluoroalkyl side-chains as network filling polymer.2 The degree of crosslinking of the silicone segments forming the polymer matrix and the fluoroalkyl chain content in the material influenced vapor permeation.2 Fibrous sepiolite modified silane coupling agent/fluororubber nanocomposite was prepared by heat molding.3 The apparent crosslink densities, curing speed, and tensile strength of fluororubber composites were increased by the addition of sepiolite.3 Graphene nanoribbons are attractive materials for polymer nanocomposites due to their high interfacial area.4 Adding nanofillers affects crosslink density of elastomers.4 The concentration and type of the surface functional groups were the main contributors in determining the cure behavior of the nanocomposites.4 A crosslinked fluoroelastomer was obtained by heat treatment of a mixture of a fluoroelastomer and an organic peroxide.5 Useful organic peroxides include dicumyl peroxide, tertiary butyl cumyl peroxide, 1,3-bis(tert-butyl peroxy isopropyl) benzene, and/or ditertiary butyl peroxide.5 Heat resistance, chemical resistance, weather resistance, and mold release characteristics were improved by crosslinking.5 A fluoroelastomer was crosslinked under an inert gas atmosphere (nitrogen) in the presence of an initiator (azo compound).6 The 1,6-divinylperfluorohexane was used as the crosslinker.6 The crosslinked fluoroelastomer can be used as a sealing material for which vapor resistance is required.6 References 1 2 3 4 5 6

Gao, W; Guo, J; Xiong, J; Smith, AT; Sun, A, Compos. Sci. Technol., 162, 49-57, 2018. Guizard, C; Boutevin, B; Guida, F; Ratsimihety, A; Naiglin, S, Separation Purification Technol., 22-23, 23-30, 2001. Yan, P; Wang, Y; Wang, M; Lu, J; Han, F, Polym. Compos., 38, E208-13, 2017. Khajehpour, M; Sadeghi, S; Yazdi, AZ; Sundararaj, U, Polymer, 55, 24, 6293-6302, 2014. Yodogawa, M; Saito, M, US8044145B2, AGC Inc, Oct. 25, 2011. Shimizu, T; Maezawa, A; Sekimoto, Y; Kuzawa, N, US20160032039A1, Nichias Corp, Feb. 4, 2016.

2.2.20 Gelatin

39

2.2.20 GELATIN The gelatin hydrogel was crosslinked by oxidized carboxymethylcellulose intended for biomedical purposes.1 The covalent crosslinking facilitated the interfacial interaction between gelatin and oxidized carboxymethylcellulose which beyond 30% oxidized carboxymethylcellulose in composition was impeded due to depletion (microdomain formation leading Figure 2.17. Representative model of the interaction between gela- to phase separation) (Figure tin and oxidized carboxymethylcellulose with increasing oxidized 2.17).1 The gelatin hydrogel was carboxymethylcellulose content in the gelatin hydrogel. [Adapted, crosslinked by oxidized carby permission, from Joy, J; Gupta, A; Jahnavi, S; Verma, RS; Ray, boxymethylcellulose in applicaA; Gupta, B, Polym. Int., 65, 181-91, 2016.] tions intended for biomedical purposes.1 The crosslinked hydrogel generated favorable conditions for cells to adhere and proliferate showing potential for biomedical applications.1 Composite films based on salmon (Salmo salar) skin gelatin and zein were prepared by crosslinking using glutaraldehyde.2 The optimum concentrations (g/mL) to maximize tensile strength and elongation and to minimize water absorption were 3% zein and 0.02% glutaraldehyde.2 Addition of glutaraldehyde caused the formation of crosslinks between proteins and a denser packing of protein chains.2 The adhesive hydrogels were fabricated by the site-directed coupling of tyramineconjugated hyaluronic acid (1 wt%) and gelatin (3 wt%) using tyrosinase derived from Streptomyces avermitilis.3 The enzyme-based crosslinking was fast (less than 50 s for complete gelation), effectively enhancing adhesive strength of the hydrogel.3 The enzymebased crosslinking hydrogel has a potential application in tissue engineering and regenerative medicine.3 The gelatins extracted from carp and tilapia skins were chemically crosslinked by electrolytes (NaCl and MgSO4) and nonelectrolytes (gallic acid and citric acid).4 All crosslinked films showed a significant increase in tensile strength and a slight reduction in elongation.4 Electrospun gelatin fibers were prepared from both types A and B gelatin solutions.5 The concentration of gelatin solution at 20-40 wt% was found to be optimal for production of the gelatin fibers with a smooth surface throughout the fiber length.5 Physical crosslinkings from dehydrothermal treatment, plasma treatment, and their combinations resulted in low crosslinking of fiber mats because crosslinking occurred only at the surface of the material.5 Combination of dehydrothermal and chemical crosslinking using 1-ethyl-3-(3dimethylamino propyl) carbodiimide hydrochloride and glutaraldehyde resulted in higher crosslinking since both the surface and the bulk were crosslinked.5

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Crosslinkers

The enzymatic crosslinking process was applied to obtain gelatin with special properties.6 A polyphenol oxidase was used as the crosslinking enzyme. Reversible nature of a gel on the basis of gelatin renders gelatin particularly suitable for desserts because the gelled material melts in the mouth.6 A therapeutic composition comprising gelatin and a crosslinking agent has been developed for use in biological regenerative methods.7 The composition can be administered to a target area of the body while ensuring that the suspended cells and/or the growth factors remain in the target area of the body.7 Transglutaminase was the crosslinking agent.7 Hydrogels based on gelatin crosslinked with N,N'-methylenebisacrylamide are useful for cell culture growth and proliferation, for the controlled-release of bioactive molecules, and to promote tissue regeneration.8 References 1 2 3 4 5 6 7 8

Joy, J; Gupta, A; Jahnavi, S; Verma, RS; Ray, A; Gupta, B, Polym. Int., 65, 181-91, 2016. Fan, HY; Duquette, D; Dumont, M-J; Simpson, BK, Int. J. Biol. Macromol., 107A, 678-88, 2018. Kim, S-H; Lee, S-H; Lee, J-E; Park, SJ; Kim, B-G, Biomaterials, 178, 401-12, 2018. Santos, JP; Esquerdo, VM; Moura, CM; Pinto, LAA, Colloids Surf.A: Physicochem. Eng. Aspects, 539, 184-91, 2018. Ratanavaraporn, J; Rangkupan, R; Jeeratawatchai, H; Kanokpanont, S; Damrongsakkul, S, Int. J. Biol. ‘Macromol., 47, 4, 431-8, 2010. Hubertus, JGA, WO2003079805A1, Nederlandse Organisatie Voor Toegepast- Natuurwetenschappelijk Onderzoek Tno, Oct. 2, 2003. Gaissmaier, C; Ahlers, M, US8637081B2, Tetec Tissue Engr Tech AG, Jan. 28, 2014. Tanzi, MC; Fare, S; Gerges, I, WO2012164032A1, Politecnico Di Milano, Dec. 6, 2012.

2.2.21 Guar gum

41

2.2.21 GUAR GUM Guar galactomannan was crosslinked using enzymatic oxidation to form a hydrogel.1 Nanofibrillated cellulose was used as reinforcement prior to the crosslinking.1 Galactose oxidase was used for crosslinking.1 Galactose oxidase catalyzed the oxidation of the primary hydroxyl groups at C-6 of the terminal galactosyl units in guar galactomannan in aldehyde groups, which enabled crosslinking through hemiacetal bonds and thus the formation of a hydrogel.1 Partially carboxymethylated guar gum was crosslinked in situ by Ca2+ ions.2 The concentration of Ca2+ ions increased the viscosity of the gel layer and reduced the water penetration into the matrix as well as swelling of the tablets and drug release.2 Nonionic gels were prepared from hydroxypropyl guar gum having different substitution degrees by crosslinking with ethylene glycol diglycidyl ether.3 Hydrogels were prepared from guar gum via esterification with 1,2,3,4-butanetetracarboxylic dianhydride.4 An increase in the crosslinker amount led to increase in the degree of crosslinking, which affected the swelling behavior and rheological properties of the hydrogels.4 The hydrogels adsorbed bovine serum albumin and hen egg white lysozyme through electrostatic and hydrophobic interactions.4 Hydrogel was capable of slow release of protein over 24 h period.4 Hydraulic fracturing enables the production of oil and gas from conventional and unconventional hydrocarbon reservoirs.5 Fluids are injected under high pressure to initiate fracture and then the proppant loaded slurry is injected to uphold the created fracture to allow oil and gas to flow into the wellbore from the formation.5 The crosslinked gel has been synthesized from gum karaya and zirconium crosslinker as an alternative to guarbased fracturing fluids to increase the proppant pack conductivity with competitive rheo-

Figure 2.18. The orientation of polymer chains in a) sol (blue dots: physical hydrogen bonds) b) crosslinked gel (Red X: crosslink junctions). [Adapted, by permission, from Chauhan, G; Verma, A; Doley, A; Ojha, K, J. Petroleum Sci. Eng., 172, 327-39, 2019.]

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Crosslinkers

logical performance.5 Zirconium crosslinked gels are viscoelastic (as compared to the borate crosslinked gels) due to the presence of both hydrogen and covalent bonds.5 When shear or heat is applied, the crosslink junctions demote the growth of hole or crack and promote rapid stress/heat dissipation.5 The crosslinking results in a three-dimensional structure with localized regions of crosslink junctions of enhanced strength (Figure 2.18).5 A self-hydrating, self-crosslinking dry composition is used to prepare a hydrated, crosslinked fracturing fluid upon addition of water to the composition comprising (A) guar powder or a guar derivative powder; (B) crosslinker selected from the group consisting of boric acid, borax, borate ore, boron ore, antimony compounds, aluminum compounds, zirconium compounds, and titanium compounds; and (C) slow dissolving alkaline buffer.6 Fracturing an oil or gas containing subterranean formation follows the preparation of a hydrated, crosslinked fracturing fluid by adding water or completion brine to the composition without the use of a hydrating tank, adding proppants, and introducing the resultant hydrated, crosslinked fluid into an oil or gas well.6 The method for crosslinking a polysaccharide (galactomannans such as guar gum, locust bean gum, and tara gum), includes the step of contacting particles of the polysaccharide with a titanium compound (e.g., diisopropyl di-triethanolamine titanate or diisopropyl di-triethanolamine titanate) in an aqueous medium under conditions appropriate to crosslink the discrete particles.7 The resultant product is used in personal care compositions.7 References 1 2 3 4 5 6 7

Ghafar, A; Gurikov, P; Subrahmanyam, R; Parikka, K; Mikkonen, KS, Compos. Part A: Appl. Sci. Manuf., 94, 93-103, 2017. Singh, R; Maity, S; Sa, B, Carbohydrate Polym., 106, 414-21, 2014. Kono, H; Hara, H; Hashimoto, H; Shimizu, Y, Carbohydrate Polym., 117, 636-43, 2015. Kono, H; Otaka, F; Ozaki, M, Carbohydrate Polym., 111, 830-40, 2014. Chauhan, G; Verma, A; Doley, A; Ojha, K, J. Petroleum Sci. Eng., 172, 327-39, 2019. Neyraval, P; Boukhelifa, A; Kesavan, S, WO2007143524A1, Rhodia Inc., Dec. 13, 2007. Luczak, K; Mabille, C, CA2720582A1, Rhodia Operations Sas, Oct. 15, 2009.

2.2.22 Hydrogenated nitrile rubber

43

2.2.22 HYDROGENATED NITRILE RUBBER The crosslinking dominated degradation of hydrogenated nitrile rubber during the most of aging process and the chain scission was more prevalent at the elevated temperature in the middle and later stages.1 Formation of the hardened, brittle outer layer, voids, and agglomerates was observed.1 Struktol has launched processing additive Struktol® HT 750, the first in a new series of additives for diamine crosslinked polymers such as ethylene acrylate, polyacrylate elastomers, and hydrogenated nitrile rubber.2 The hydrogenated nitrile rubber composition was subjected to peroxide crosslinking, using an organic peroxide (e.g., Percumyl D).3 References 1 2 3

Lou, W; Zhang, W; Liu, X; Dai, W; Xu, D, Polym. Deg. Stab., 144, 464-72, 2017. Addit. Polym., 2016, 4, 5, 2016. Tokumitsu, H; Yamanaka, T, US7173087B2, NOK Corp, Feb. 6, 2007.

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Crosslinkers

2.2.23 HYPERBRANCHED POLYMER The hyperbranched polymers were photocured by irradiation at 365 nm in the presence of 2,2-dimethoxy-2-phenylacetophenone as a photoradical initiator.1 The hyperbranched polymers can also be photocured by irradiation at 254 nm due to the homolysis of the disulfide groups with subsequent initiation of the radical polymerization.1 The hyperbranched polymer end-capped with 4-methylcoumarin group was prepared via thiol-ene addition reaction of thiol-modified hyperbranched polyester with a vinyl monomer (Figure 2.19). The average doses of UVA irradiation for the maximum degree of photo-crosslinking were 22.08 for polymer end-capped with 4-methylcoumarin group and 28.29 J cm-2 for polyester with a vinyl monomer. The average UVC doses of complete photo-cleavage were 9.44 J cm-2 for polymer end-capped with 4-methylcoumarin group and 9.58 J cm-2 for polyester with a vinyl monomer.

Figure 2.19. Synthetic routes for hyperbranched polyester with a vinyl monomer, VBMC, and polymer endcapped with 4-methylcoumarin group, MCTH40. TAH40 − fully thioglycolic acetate of Boltorn™ H40, DPTS − 4-dimethylaminopyridine p-toluenesulfonate. [Adapted, by permission, from Fu, Q; Cheng, L; Zhang, Y; Shi, W, Polymer, 49, 23, 4981-8, 2008.]

The injectable hydrogel with desirable biocompatibility and tunable properties can improve the efficacy of stem cell-based therapy.2 The injectable hydrogel system was fabricated from hyperbranched multi-acrylated poly(ethylene glycol) macromers and thiolated hyaluronic acid for use as a stem cell delivery and retention platform.2 The hydrogel was synthesized via in situ reversible addition fragmentation chain transfer polymerization.2 Hydrogels encapsulating adipose-derived stem cells (ADSCs) demonstrated promising regenerative capabilities and were tested in the treatment of a diabetic wound in a diabetic murine animal model, showing enhanced wound healing (Figure 2.20).3

2.2.23 Hyperbranched polymer

45

Figure 2.20. Schematic concepts of the generation and application of injectable hyperbranched multi-acrylated poly(ethylene glycol) macromers, HP-PEG,-based hydrogel. Scheme illustrates the synthesis of the HP-PEG polymers by in situ reversible addition fragmentation chain transfer, RAFT, polymerization using 2,2′-azobis(2methylpropionitrile), AIBN, as the initiator and Disulfiram, DS, as the precursor of RAFT agent. The hydrogel is in situ formed via thiol-ene reaction by mixing HP-PEG with hyaluronic acid, HA-SH. Adipose-derived stem cells, ADSCs, are encapsulated in the hydrogel, and the hydrogel system is applied onto a humanized diabetic wound model. [Adapted, by permission, from Xu, Q; Sigen, A; Gao, Y; Guo, L; Wang, W, Acta Biomater., 75, 63-74, 2018.]

The stability of membrane is a key issue for pervaporation separation of aromatic/ aliphatic hydrocarbon mixtures.4 The hydroxyl and carboxyl groups reacted during the thermal crosslinking process, and then hyperbranched polymers were assembled forming morphology and structure of “pore-filling” composite membrane.4 The “pore-filling” membrane showed stable separation performance, due to its excellent anti-swelling properties.4

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Crosslinkers

The copper-free thermal Huisgen 1,3-dipolar crosslinking reaction was applied to a high glass transition temperature, hyperbranched polyimide polymer to stabilize its electro-optic activity.5 Two functional groups of the hyperbranched polymer are reactive under different conditions.6 This allows the first type of functional group to be used for crosslinking the hyperbranched polymer molecules to form a network structure, and the second type of functional group to create a nanoscopic domain which can act as a particle-like reinforcing agent within the hyperbranched polymer network.6 The crosslinking of the hyperbranched polymer was achieved by the hydrolysis of functional groups that were uniformly distributed at the terminals of the hyperbranched polymer molecules.6 References 1 2 3 4 5 6

Sato, E, Yamashita, Y; Nishiyama, T; Horibe, H, J. Photopolym., Sci. Technol., 30, 2, 241-6, 2017. Fu, Q; Cheng, L; Zhang, Y; Shi, W, Polymer, 49, 23, 4981-8, 2008. Xu, Q; Sigen, A; Gao, Y; Guo, L; Wang, W, Acta Biomater., 75, 63-74, 2018. Wang, N; Wang, L; Zhang, R; Li, J; Ji, S, J. Membrane Sci., 474, 263-72, 2015. Cabanetos, C; Blart, E; Pellegrin, Y; Montembault, V; Odobe, F, Eur. Polym. J., 48, 1, 116-26, 2012. Dvornic, PR; Hu, J; Meier, DJ; Nowak, RM, US6995215B2, Michigan Molecular Institute, Feb. 7, 2006.

2.2.24 N-isopropylacrylamide

47

2.2.24 N-ISOPROPYLACRYLAMIDE The composite hydrogel was fabricated using surface-modified cellulose nanofiber and Nisopropylacrylamide as a multifunctional crosslinker and monomer, respectively.1 The vinyl groups on the surface of cellulose nanofiber reacted by condensation with the hydroxyl groups and 3-(trimethoxysilyl)propylmethacrylate (Figure 2.21).1 The gels can be stretched by more than 700 times of their original lengths and recover shape with a small permanent deformation.1

Figure 2.21. Surface modification of cellulose nanofiber and hydrogel formation. [Adapted, by permission, from Kobe, R; Yoshitani, K; Teramoto, Y, J. Appl. Polym. Sci., 133, 42906, 2016.]

The effect of gelation solvent and temperature on poly(N-isopropylacrylamide) gel synthesized by post-polymerization crosslinking was studied.2 First, radical copolymerization of poly(N-isopropylacrylamide) and activated ester monomer (N-(acryloyloxy)succinimide) followed by crosslinking reaction of the obtained prepolymer and diamine crosslinker (ethylenediamine).2 The gelation time was strongly dependent on solvent polarity.2 The turbidity resulted from polymer aggregation and entanglement fixed by chemical crosslinking at elevated temperatures.2 Glycerol dimethacrylate, pentaerythritol triacrylate, and pentaerythritol propoxylate triacrylate were used in dispersion polymerization N-isopropylacrylamide as crosslinking agents with different hydrophilicities.3 The crosslink density exhibited a quadratic decrease with the increasing radial distance in the spherical microgel particles.3 The microgel obtained by the most hydrophilic crosslinking agent (glycerol dimethacrylate) exhibited highest hydrodynamic diameters in the fully swollen form.3 The reactions between the OH, H and eaq− transients of water radiolysis and poly(Nisopropylacrylamide) were responsible for crosslink formation.4 The isopropyl-centered radical forms mainly in OH radical attack on the polymer.4 Gels prepared by irradiation have a swelling degree up to four times higher than the gels prepared by chemical crosslinking, and their shrinking velocity is up to twenty times higher for the gels prepared by

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chemical crosslinking.5 The gels prepared from monomers by chemical crosslinking show polyelectrolyte behavior, whereas gel prepared by e-beam irradiation of polymer solutions show amphoteric behavior.5 A preparation of graphene oxide/poly(N-isopropylacrylamide) composite hydrogel involves placing the graphene oxide in water and ultrasonic dispersion of the graphene oxide for 30-60 min to obtain a graphene oxide colloidal solution, followed by adding Nisopropylacrylamide, a crosslinking agent (N,N'-methylene-bis-acrylamide), and the first initiator (sodium sulfide or bisulfide).6 The second initiator (potassium persulfate) is added to the deoxidized solution, which after reaction leads to the formation of a composite hydrogel.6 The hydrogel is suitable for tissue engineering and drug controlled release.6 The mesoscopically periodic materials that combine crystalline colloidal array selfassembly with the temperature induced volume phase transitions of poly(N-isopropylacrylamide) are useful in optical switches, optical limiters, optical filters, display devices, membrane filters, and processing elements.7 They are tunable in response to temperature.7 References 1 2 3 4 5 6 7

Kobe, R; Yoshitani, K; Teramoto, Y, J. Appl. Polym. Sci., 133, 42906, 2016. Ida, S; Katsurada, A; Yoshida, R; Hirokawa, Y, Reactive Funct. Polym., 115, 73-80, 2017. Elmas, B; Tuncel, M; Şenel, S; Patir, S; Tuncel, A, J. Colloid Interface Sci., 313, 1, 174-83, 2007. Sáfrány, A; Wojnárovits, L, Radiat. Phys. Chem., 69, 4, 289-93, 2004. Lugo-Medina, E; Licea-Claveríe, A; Cornejo-Bravo, JM; Arndt, KF, Reactive Funct. Polym., 67, 1, 67-80, 2007. CN102580633B, Apr. 16, 2014. Asher, SA; Weissman, JM; Sunkara, HB, US6165389A, University of Pittsburgh, Dec. 26, 2000.

2.2.25 Liquid crystalline elastomers

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Figure 2.22. Schematic drawing of an azo-liquid crystalline elastomer’s photoactuation with an azo and a nonazo crosslinker. Compared to the latter, the azo crosslinker can contribute to the chemical modification of the liquid crystalline polymer. [Adapted, by permission, from Braun, LB; Linder, TG; Hessberger, T; Zentel, R, Polymers, 8, 435, 2016.]

2.2.25 LIQUID CRYSTALLINE ELASTOMERS Photoactuating liquid crystalline elastomers are promising candidates for artificial muscles in microdevices.1 By optimizing the illumination conditions and the mixture of azo monomer and azo crosslinker, thick films of an allazo liquid crystalline elastomer can be produced.1 A substantial length change can be obtained without bending during photoactuation with white light (about 440 nm), whose absorption is low, leading to a significant penetration depth.1 Addition of an azo crosslinker resulted in a stronger photoactuation at lower operational temperature. Figure 2.22 shows photoactuation mechanism.1 The epoxy-based shapememory liquid crystalline lightly crosslinked networks were synthesized and characterized for development of two-way autonomous shape-memory actuators.2 Figure 2.23. Scheme of epoxidation of 4,4’-dihydroxybiphenyl and Carboxylic acids of different aliphatic chain lengths were used as curing of the diepoxy monomer with dicarboxylic acids C4-10. [Adapted, by permission, from Belmonte, A; Lama, GC; Gentile, curing agents for a rigid-rod G; Fernandez-Francos, X; De la Flor, S; Cerruti, P; Ambrogi, V, epoxy-based mesogen (Figure J. Phys. Chem., 121C, 22403-14, 2017.

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Crosslinkers

2.23).2 The liquid crystalline lightly crosslinked networks with isotropization temperatures above 100°C, controlled degree of liquid crystallinity, and high actuation stress and strain can be obtained by varying the aliphatic chain length of the curing agent.2 These materials are capable of producing very high actuation stress values during the shapememory effect.2 A high-performance halloysite nanotubes-doped liquid crystalline bionanocomposite ionogels have been synthesized by in situ crosslinking of cellulose/ionic liquid solutions using bisphenol A epoxy resin via ring opening reactions with cerium ammonium nitrate, and halloysite nanotubes as the ionic conducting promoter.3 The ionogel was used as a flexible gel electrolyte for supercapacitor device.3 Its specific capacitance was maintained for up to 5000 charge-discharge cycles.3 Main-chain smectic-C liquid crystalline elastomers having varying crosslink densities were prepared by a two-stage hydrosilylation in which diene mesogens were polymerized with hydride-terminated polydimethylsiloxane and subsequently crosslinked by a tetravinyl molecule, permitting control over network crosslink density.4 The thermal cycling around the isotropization transition under tensile load leads to reversible extension (cooling) and contraction (heating), called two-way shape memory or “actuation”.4 Elastomer strained to the same level (by creep or actuation) had different orientation levels which indicated that cooling to the liquid crystalline phase under stress was important to achieving large strains associated with actuation.4 The actuation resulted in more oriented material than when prepared by creep.4 The molecularly imprinted polymer nanoparticles were prepared by a low level Figure 2.24. Schematic representation of lamellar crosslinking using liquid crystalline monostructure containing liquid crystalline mesogenic units mer as a physical crosslinker.5 The particles and ionic clusters. [Adapted, by permission, from Meng, F; Zhang, B; Liu, L; Zang, B, Polymer, 44, 14, were used in capillary chromatography.5 3935-43, 2003.] Siloxane-based liquid-crystalline elastomers were synthesized using crosslinking agents containing sulfonic acid groups.6 The ion aggregated in domains forced the siloxane chains to fold and form the irregular lamellar structure (Figure 2.24).6 The monotropic nematic diglycidylether of 4-hydroxyphenyl-4'-hydroxybenzoate was crosslinked in the isotropic and in the nematic states using α,α'-dimethoxydeoxybenzoin and diphenyliodoniumhexafluorophosphate as the photoinitiator system.7 The glass transition temperature of networks synthesized in the liquid-crystalline state was higher than in the isotropic state.7

2.2.25 Liquid crystalline elastomers

51

High crystallinity copolymer was synthesized by copolymerization of 2,5-bis(3-bromododecylthiophen-2-yl)thieno[3,2-b]thiophene monomer with thiophene and thieno [3,2-b]thiophene via Stille reaction.8 The bimolecular crystal morphology can be frozen at the state of optimal morphology and preserved by UV crosslinking.8 A known way of fixing liquid-crystalline properties is the binding of the liquid crystals into polymer networks (e.g., subsequent crosslinking of aligned liquid-crystalline side-chain polymers).9 Organosiloxane containing two mesogenic side groups were used for the production of optically anisotropic layers.9 References 1 2 3 4 5 6 7 8 9

Braun, LB; Linder, TG; Hessberger, T; Zentel, R, Polymers, 8, 435, 2016. Belmonte, A; Lama, GC; Gentile, G; Fernandez-Francos, X; De la Flor, S; Cerruti, P; Ambrogi, V, J. Phys. Chem., 121C, 22403-14, 2017. Guo, S; Zhao, K; Feng, Z; Hou, Y; Li, H; Zhao, J; Tian, Y, Song, H, Appl. Surf. Sci., 455, 599-607, 2018. Burke, KA; Rousseau, IA; Mather, PT, Polymer, 55, 23, 5897-5907, 2014. Liu, X; Zong, H-Y; Huang, Y-P; Liu, S-Z, J. Chromatography, 1309A, 84-9, 2013. Meng, F; Zhang, B; Liu, L; Zang, B, Polymer, 44, 14, 3935-43. Strehmel, V; Strehmel, B, Thin Solid Films, 284-285, 317-20, 1996. Yao, K; Chen, L; Hu, T; Chen, Y, Org. Electronics, 13, 8, 1443-55, 2012. Hanelt, E; Sandmeyer, F; Häberle, N, US6300454B1, Wacker Chemie AG, Oct. 9, 2001.

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2.2.26 NATURAL RUBBER The crosslink density increased while the chain entanglement density decreased when natural rubber was crosslinked with increasing dicumyl peroxide concentration.1 The relative distribution of the molar mass between crosslinks did not change with peroxide level.1 The molar mass distribution between crosslinks substantially broadened due to heat aging.1 The crosslink density of carbon black-reinforced natural rubber was correlated with 1 H chemical shift by liquid-state 1H NMR spectroscopy.2 The chemical shift difference was directly proportional to the apparent crosslink density.2 The natural rubber latex particles containing acetoacetoxy groups crosslinked at ambient temperature by reaction with glutaraldehyde.3 Figure 2.25 shows mechanism of crosslinking.3

Figure 2.25. Crosslinking reaction between the acetoacetoxy groups on the natural rubber latex grafted with poly(acetoacetoxyethyl methacrylate) and glutaraldehyde. [Adapted, by permission, from Thongnuanchan, B; Ninjan, R; Nakason, C, Iranian Polym. J., 26, 41-53, 2017.]

The natural rubber latex particles bearing diacetone acrylamide functional groups was crosslinked using adipic acid dihydrazide during film formation via keto-hydrazide reaction, which is a nitrosamine precursor-free and zinc-free vulcanization system.4 References 1 2 3 4

Howse, S; Porter, P; Mengistu, T; Pazur, RJ, Polym. Testing, 70, 263-74, 2018. Chae, YK; Kang, WY; Jang, J-H; Choi, S-S, Polym. Testing, 29, 8, 953-7, 2010. Thongnuanchan, B; Ninjan, R; Nakason, C, Iranian Polym. J., 26, 41-53, 2017. Thongnuanchan, B; Ninjan, R; Kaesaman, A; Nakason, CC, J. Polym. Res., 22,115, 2015.

2.2.27 Phenolic resin

53

2.2.27 PHENOLIC RESIN Phenolic fibers were prepared by the crosslinking of heat-meltable spun filaments derived from melt-spinning of a novolac resin in a combined solution of formaldehyde and hydrochloric acid.1 With a formaldehyde concentration of 18.5%, a hydrochloric acid concentration of 12%, and a heating rate of 15.4°C h-1 homogeneous highly crosslinked phenolic fibers with the maximum tensile strength of 260 MPa were obtained.1 The porous epoxy phenolic novolac resin microcapsules have nearly spherical shapes with sizes in the range of 7-30 μm.2 The release rate of pendimethalin decreased with an increase of epoxy value, and microcapsules had a lower herbicidal control efficacy.2 An increase in the number of epoxy groups caused a higher crosslink density.2 The extent of cure utilizing the formaldehyde/phenol ratio, Figure 2.26. Structure of a novolac resin. o/o, o/p, p/p indicate the position of the methylene bridges. [Adapted, by permission, from the degree of conversion, and the Ottenbourgs, B; Adriaensens, P; Carleer, R; Vanderzande, D; Gelan, characterization of the final strucJ, Polymer, 39, 22, 5293-5300, 1998.] ture after curing was determined using quantitative 13C solid-state nuclear magnetic resonance with cross-polarization and magic angle spinning (Figure 2.26).3 Thermoplastic vulcanizates based on high-density polyethylene/ground tire rubber with phenolic resin and dicumyl peroxide as vulcanizing agents were prepared through dynamic vulcanization.4 The mechanical properties of blends were significantly improved by the addition of 4 phr phenolic resin and 0.3 phr dicumyl peroxide.4 Microwave promoted rapid crosslinking in phenolic fibers.5 At the heating rate of 1.2°C min-1, the homogeneous highly crosslinked phenolic fiber was obtained with the maximum tensile strength.5 The microwave irradiation promoted diffusion of −CH2OH from the skin into the inner layer of the fiber and their reaction with the phenolic ring.5 A phenol-formaldehyde novolac resin having a low concentration of free phenol were found suitable for the production of resin-coated molding sands for shell molding and sand cores, as well as for the production of resin coated proppants for use in oil and gas recovery operations.6 The novolac resins are thermoplastic, i.e., they are not selfcrosslinkable.6 The novolac resins are converted to cured resins by, for example, reacting them under heat with a crosslinking agent, such as hexamine (hexamethylenetetramine), or by mixing them with a solid acid catalyst and paraformaldehyde and reacting them under heat.6 Novolac resins may also be cured with other crosslinkers such as resoles and epoxies.6 The coated particulate matter contains a combination of phenolic/furan resin, furan resin, or phenolic-furan-formaldehyde terpolymer, on a proppant such as sand. They are

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Crosslinkers

coated on the surface with a novolac resin containing curative for use as a proppant, gravel pack, or for sand control.7 The phenolic novolacs do not harden upon heating but remain soluble and fusible unless a hardener (crosslinking agent) is present.7 Appropriate crosslinking agents include hexamethylenetetramine, paraformaldehyde, oxazolidines, melamine resin, or other aldehyde donors and/or phenol-aldehyde resole polymers.7 The crosslinkers can be used alone or in combinations.7 References 1 2 3 4 5 6 7

Liu, C-L; Guo, Q-G; Shi, J-L; Liu, L, Mater. Chem. Phys., 90, 2-3, 315-21, 2005. Zhang, X-p; Luo, J; Jing, T-f; Zhang, D-x; Liu, F, Colloids Surf. B: Biointerfaces, 165, 165-71, 2018. Ottenbourgs, B; Adriaensens, P; Carleer, R; Vanderzande, D; Gelan, J, Polymer, 39, 22, 5293-5300, 1998. He, M; Li, Y; Qiao, B; Ma, X; Song, X; Song, J; Wang, M, Polym. Compos., 36, 1907-16, 2015. Liu, C-L; Ying, Y-G; Feng, H-L; Dong, W-S, Polym. Deg. Stab., 93, 2, 507-12, 2008. Rediger, R; Lucas, E, US9458349B2, Georgia-Pacific Chemicals LLC, Oct. 4, 2016. Anderson, RW; Diep, T; McCrary, AL, US7153575B2, Hexion Inc, Dec. 26, 2006.

2.2.28 Poly(2-oxazoline)

55

2.2.28 POLY(2-OXAZOLINE) Crosslinked hydrophilic poly(2-oxazoline)-based nanofibers were fabricated via in situ photoinitiated radical thiol-ene crosslinking during electrospinning.1 The crosslinked nanofibers were multi-functionalizable as they contain two functional handles, being the alkene moieties from the parent copolymer and the residual thiol groups from the tetrathiol-based crosslinker.1 The sensitivity of biocompatible polymers to ionizing radiation is essential for the construction of polymer radiopharmaceutics, simultaneous radiotherapy and therapy with polymeric drugs, and the radiation sterilization of polymeric materials.2 Poly[N-(2hydroxypropyl)methacrylamide] and poly(N-vinyl-2-pyrrolidone) were more resistant against ionizing radiation than poly(ethylene oxide) or poly(2-ethyl-2-oxazoline), which were rapidly crosslinked.2 The pH-responsive layer-by-layer assemblies of partially hydrolyzed poly(2-ethyl-2oxazoline) and poly(acrylic acid) were prepared for the effective prevention of protein, cell, and bacteria surface attachment (Figure 2.27).3 Coatings exhibited thickness dependent resistance to protein adsorption.3 Crosslinked multilayers of ~220 nm were highly effective in suppressing surface adsorption of bovine serum albumin, while thinner or thicker layers were increasingly susceptible to its adsorption.3 The coatings of ~220 nm and above were effective at preventing surface attachment of fibroblasts, gram-positive (S. aureus) and gram-negative (E. coli) bacteria.3

Figure 2.27. General chemistry of poly(2-oxazoline)s (POX), poly(2-methyl-2-oxazoline) (PMOX) and poly(2ethyl-2-oxazoline) (PEOX), as contrasted against poly(ethylene glycol) (PEG) and polypeptides. Similarities between POX and polypeptides are circled by dotted lines. [Adapted, by permission, from He, T; Jańczewski, D; Guo, S; Man, SM; Tan, WS, Colloids Surf. B: Biointerfaces, 161, 269-78, 2018.]

The difunctional 2-oxazolines were prepared from the thiol-ene reaction of glycol dimercaptoacetate or 2,2′-(ethylenedioxy)diethanethiol and 2-but-3′-enyl-2-oxazoline or

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Crosslinkers

2-dec-9′-enyl-2-oxazoline.4 The gels that did not contain any 2-ethyl-2-oxazoline acted as lipogels, whereas the gels that did not contain any 2-nonyl-2-oxazoline acted as hydrogels; all other gels may be classified as amphigels.4 The maximum swelling degrees (in water) were observed for the gels with the lowest degrees of crosslinking. The synthesis of crosslinked networks had been achieved by performing the polymer analogous thiol-ene reaction of copoly(2-oxazoline)s containing olefinic side-chains and glycol dimercaptoacetate.4 The methods of synthesis are illustrated in Figure 2.28.4

Figure 2.28. Route 1 (top) and Route 2 (bottom) for the synthesis of crosslinked networks. 2-But-3′-enyl-2-oxazoline (m = 3) and 2-Dec-9′-enyl-2-oxazoline (m = 9) were used as olefinic monomers. [Adapted, by permission, from Luef, KP; Petit, C; Ottersböck, B; Oreski, G; Wiesbrock, F, Eur. Polym. J., 88, 701-12, 2017.]

A biocompatible medical product contained a covalently crosslinked polymer that was obtained by reacting a nucleophilically activated polyoxazoline with an electrophilic crosslinking agent.5 The electrophilic crosslinking agent comprised electrophilic groups, some of which were capable of forming covalent bonds.5 The electrophilic groups of the crosslinking agent provided cohesion, and the excess of electrophilic groups enabled the crosslinked polymer to create links to tissue.5 References 1 2 3 4 5

Kalaoglu-Altan, OJ; Verbraeken, B; Lava, K; Gevrek, TN; Sanyal, R; Dargaville, T; De Clerck, K; Hoogenboom, R; Sanyal, A, ACS Macro Lett., 5, 676-81, 2016. Sedlacek, O; Kucka, J; Monnery, BD; Slouf, M; Hruby, M, Polym. Deg. Stab., 137, 1-10, 2017. He, T; Jańczewski, D; Guo, S; Man, SM; Tan, WS, Colloids Surf. B: Biointerfaces, 161, 269-78, 2018. Luef, KP; Petit, C; Ottersböck, B; Oreski, G; Wiesbrock, F, Eur. Polym. J., 88, 701-12, 2017. Mathias, JC; Bender, E; Hoogenboom, R; Cornelis, J; Van Hest, M; Van Goor, H, US9416228B2, Bender Analytical Holding BV, Aug. 16, 2016.

2.2.29 Polyamide

57

Figure 2.29. Schematic illustration of the synthesis of the polyamide-graphene oxide membrane by intra- and inter-crosslinking graphene oxide nanosheets. [Adapted, by permission, from Jin, L; Wang, Z; Zheng, S; Mi, B, J. Membrane Sci., 545, 11-8, 2018.]

2.2.29 POLYAMIDE Crosslinking of polyamide-6 with and without triallyl cyanurate as a crosslinking coagent was initiated by proton beam irradiation.1 The gel point was found to be 144 and 40 kGy for virgin PA6 and for PA6 with 1 wt% of triallyl cyanurate.1 The ratio between crosslinking and scission of macroradicals formed by irradiation was found to be around 0.65 regardless of presence or absence of triallyl cyanurate and its concentration.1 The irradiation resulted in a significant increase in glass transition temperature with rising absorbed dose.1 The effect was substantially magnified by triallyl cyanurate presence, and it increased when its concentration was risen.1 Crosslinking treatment of the commercially available aromatic polyamide reverse osmosis membrane was carried out to improve its chlorine resistance.2 The crosslinking agents included 1,6-hexanediol diglycidyl ether, adipoyl dichloride, and hexamethylene diisocyanate ester.2 The crosslinking agents reacted with amine and amide II groups.2 The crosslinking treatment decreased membrane hydrophilicity by introducing methylene groups to the membrane surface.2 The gel formation in polyamide 610 by γ-ray irradiation depended on the content of polyfunctional monomer and nucleating agent.3 The crosslinking by γ-irradiation enhanced the mechanical properties of PA610.3

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Polyamide-graphene oxide membrane was synthesized on polyethersulfone support by first intra-crosslinking graphene oxide aggregates by m-xylylenediamine and then inter-crosslinking using trimethylene chloride (Figure 2.29).4 The oxygen-containing groups on the edge and basal planes of graphene oxide make it dispersible in water and offer reactive sites for crosslinking to increase the stability of graphene oxide membranes against swelling.4 A tubing includes a layer of crosslinked polyamide.5 The polyamide layer can include a crosslinking aid, such as triallyl isocyanurate, to assist in crosslinking the layer.5 When the polyamide layer is exposed to high-level radiation, the polyamide layer crosslinks which gives high temperature and glycol resistance.5 References 1 2 3 4 5

Porubská, M; Szöllös, O; Janigová, I; Jomová, K; Chodák, I, Radiat. Phys. Chem., 133, 52-7, 2017. Wei, X; Wang, Z; Xu, J; Wang, J; Wang, S, Chinese J. Chem. Eng., 21, 5, 473-84, 2013. Feng, W; Hu, FM; Yuan, LH; Zhou, Y; Zhou, YY, Radiat. Phys. Chem., 63, 3-6, 493-6, 2002. Jin, L; Wang, Z; Zheng, S; Mi, B, J. Membrane Sci., 545, 11-8, 2018. Roloff, D, US20050005989A1, Cooper Technology Services LLC, Jan. 13, 2005.

2.2.30 Polybenzimidazole

59

2.2.30 POLYBENZIMIDAZOLE Polybenzimidazole membranes were crosslinked using a solution containing trimesoyl chloride and environmentally benign 2-methyl tetrahydrofuran (Figure 2.30).1 The crosslinked membrane had a rejection of 99.6% of remazol brilliant blue R and permeated acetonitrile, acetone, ethanol, and isopropanol with the rates of 40.7, 29.0, 13.8, and 5.8 lm2h/ bar at 10 bar, respectively.1

Figure 2.30. Chemical mechanism of crosslinking polybenzimidazole with trimesoyl chloride in 2-methyl tetrahydrofuran. [Adapted, by permission, from Farahani, MHDA; Chung, T-S, Separation Purification Technol., 209, 182-92, 2019.]

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Figure 2.31. The crosslinking reaction and the structure of crosslinked polybenzimidazole. [Adapted, by permission, from Su, J; Pu, H; Chang, Z; Wan, D, Polymer, 53, 16, 3587-93, 2012.]

The ethyl phosphoric acid-grafted polybenzimidazole/polybenzimidazole blend was crosslinked with epoxy resin.2 The crosslinked blend membranes had better fuel cell performance than the neat polybenzimidazole and epoxide-crosslinked polybenzimidazole membranes.2 The polybenzimidazole membranes crosslinked with dibromoxylene retained molecular separation performance under harsh conditions characteristic of organic solvent nanofiltration.3 High gain in weight and high bromine content were typical of membranes having high crosslink degree.3 Polybenzimidazole was crosslinked with sulfonyl azide groups for proton conducting membranes.4 Upon heating, polybenzimidazole containing sulfonyl azide lost nitrogen and formed nitrene, which reacted with CH-bond of the backbone of another chain of polybenzimidazole via reactions of hydrogen abstraction, recombination, or CH-bond insertion (Figure 2.31).4 The crosslinked membranes exhibited improved tensile strength, migration stability of phosphoric acid, dimensional stability, and oxidative chemical stability.4

2.2.30 Polybenzimidazole

61

A crosslinked, supported polybenzimidazole membrane for gas separation was prepared by layering a solution of polybenzimidazole and α,α′-dibromo-p-xylene onto porous support and evaporating solvent.5 A supported membrane of crosslinked poly-2,2′-(mphenylene)-5,5′-bibenzimidazole exhibited an enhanced gas permeability compared to the non-crosslinked analog at temperatures over 265°C.5 An electrolyte membrane and an electrode for a fuel cell contain the double crosslinked phosphoric acid-doped polybenzimidazole.6 The crosslinking reactions occurred between carboxyl- and amino-groups.6 Oxazine-based monomer also participated in the crosslinking reaction.6 References 1 2 3 4 5 6

Farahani, MHDA; Chung, T-S, Separation Purification Technol., 209, 182-92, 2019. Ngamsantivongsa, P; Lin, H-L; Yu1, TL, J. Polym. Res., 23, 22, 2016. Valtcheva, IB; Marchetti, P; Livingston, AG, J. Membrane Sci., 493, 568-79, 2015. Su, J; Pu, H; Chang, Z; Wan, D, Polymer, 53, 16, 3587-93, 2012. Jorgensen, BS; Young, JS; Espinoza, BF, US6946015B2, Los Alamos National Security LLC, Sep. 20, 2005. Choi, S-W; Lee, J-C; Park, J-o; Kim, S-K; Jung, J-w, US20110189581A1, Samsung Electronics Co LtdSeoul National University R&DB Foundation, Aug. 4, 2011.

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2.2.31 POLY(BUTYLENE SUCCINATE-CO-BUTYLENE FUMARATE) Crystallization and photo-curing kinetics of biodegradable poly(butylene succinate-cobutylene fumarate) short-segmented block copolyester were studied.1 Crosslinked copolyesters showed a lower crystallization rate and degree of crystallinity while the crystallization temperature shifted to higher temperatures as compared with uncrosslinked copolyesters because of the formation of nucleating agents by crosslinkages.1 The degree of crystallinity of the copolyesters decreased slightly with increasing unsaturation, but the biodegradation was not enhanced suggesting that in addition to the chemical structure and molecular stiffness also the morphology of the spherulites had an influence on biodegradation.2 The highest biodegradability was observed for the copolyesters containing 5 and 10 mol% of fumarate units.2 References 1 2

Sheikholeslami, SN; Rafizadeh, M; Taromi, FA; Shirali, H, Polym. Int., 66, 289-99, 2017. Nikolic, MS; Poleti, D; Djonlagic, J, Eur. Polym. J., 39, 11, 2183-92, 2003.

2.2.32 Poly(butylene terephthalate)

63

2.2.32 POLY(BUTYLENE TEREPHTHALATE) Addition the flame retardants (melamine and aluminum phosphate) and crosslinking by electron beam radiation in the presence of triallyl cyanurate made poly(butylene terephthalate) self-extinguishing.1 A dose of 200-400 kGy leads to the highly crosslinked structure.1 The radiation crosslinking lowered its dielectric loss coefficient by ten times as compared non-irradiated polymer but did not affect its dielectric constant.1 The irradiated samples burned with lower speed and less dripping.1 Crosslinkable, flame retardant polymer compositions contain polyester, a flame retardant system, filler, and a crosslinking agent.2 The crosslinking agent comprised of two or more groups capable of forming free radicals under beta or gamma radiation, namely, a polyallylic compound or a polyol poly(meth)acrylate prepared from an aliphatic diol, triol, or tetraol.2 The fibrous filler is glass fiber and the low-aspect ratio filler is zinc borate.2 The retarding system contains melamine or its derivatives and metal phosphinate salt.2 An electrochemical battery cell of a lithium ion battery has a physically crosslinked gel electrolyte situated between a negative electrode and a positive electrode.3 The gel electrolyte included a block copolymer host and a liquid electrolyte, which could transport lithium ions, absorbed onto the block copolymer host.3 The block copolymer host included poly(terephthalate)ester (e.g., poly(butylene terephthalate)) block units and physically crosslinkable block units.3 The physical crosslinks provided the block copolymer host with enhanced electrochemical, thermodynamic, and mechanical properties.3 The manufacture of the thermally crosslinked polyester composition (e.g., poly(butylene terephthalate)) involved contacting the polyester and the carboxy-reactive material at a temperature and for a time sufficient to crosslink the polyester with the carboxy-reactive material.4 Useful polyfunctional non-polymeric carboxy-reactive material comprises a combination of epoxy and silane functional groups, and in particular terminal cycloaliphatic epoxy groups and terminal silane groups.4 Other examples include novolac epoxy resins, styrene-acrylic copolymers, and oligomers containing glycidyl groups incorporated as side chains, or poly(ethylene-glycidyl methacrylate-co-methacrylate).4 References 1 2 3 4

Hooshangi, Z; Feghhi, SAH; Sheikh, N, Radiat. Phys. Chem., 108, 54-9, 2015. Cartier, H; Chopin, A; Perego, C, US7423080B2, General Electric Co, Sep. 9, 2008. Huang, X, US9350046B2, GM Global Technology Operations LLC, May 24, 2016. Hein, CL; Kannan, G; Karanam, S; Yu, CQ, US8114515B2, General Electric Co, Feb. 14, 2012.

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2.2.33 POLYCAPROLACTONE Based on Diels-Alder reaction, a furyl-telechelic semicrystalline polycaprolactone was crosslinked by a trismaleimide crosslinker.1 The resultant material showed mendability of scratches under thermal treatment.1 The mending process was a combination of the shape recovery effect favoring scratch closure and the re-crosslinking of the cleaved Diels-Alder bonds at temperatures slightly above the melting transition of polycaprolactone chains.1 The crystallization of γ-radiation crosslinked polycaprolactone was governed by heterogeneous nucleation and single-dimension growth. The crystal fraction and rates of crystallization were related to the radiation dose and the degree of crosslinking. The relationship between relative crystallinity and time followed the Ozawa equation: the higher the degree of crosslinking, the lower the crystal velocity constant. The activation energy of crystallization for irradiated polycaprolactone was between 65 and 54 kJ/mol. Polycaprolactone/collagen composite nanofibers were a crosslinked in the presence of radiation (dose 15-35 kGy). No chemical was required for crosslinking. Optimal polycaprolactone/collagen ratio by weight was 13:3. References 1 2 3

Nguyen, L-TT; Nguyen, HT; Truong, TT, J. Polym. Res., 22, 186, 2015. Zhu, G; Xu, Q; Qin, R; Yan, H; Liang, G, Radiat. Phys. Chem., 74, 1, 42-50, 2005. KR101882477B1, Jul. 27, 2018.

2.2.34 Polycarbonate

65

2.2.34 POLYCARBONATE Cinnamoyl and coumarin groups can be used to introduce photo-response.1 Coumarin functionalized polycarbonates crosslink faster at 365 nm than polymers with cinnamoyl functionality.1 Crosslinks can be reversed at 254 nm in the dilute solutions.1 Figure 2.32 shows the mechanism of crosslinking, and their reverse reaction.1

Figure 2.32. Reversibility of polycarbonate crosslinking by coumarin. [Adapted, by permission, from Chesterman, JP; Hughes, TC; Amsden, BG, Eur. Polym. J., 105, 186-93, 2018.]

The relative effects of crosslinking and chain scission during the photothermal aging of polycarbonates depends on their chemical structure.2 The presence of the four ortho-methyl substitutes on the aromatic rings accounts for the differences in the photothermal aging of tetramethyl bisphenol-A polycarbonate as compared with Figure 2.33. SEM image showing detail of (a) a craze as the precursor tip of a crack and (b) of micro-fibrillation at higher magnifi- bisphenol-A polycarbonate.2 cation. [Adapted, by permission, from Silva, PPJ; Araújo, PLB; Tetramethyl substitution inhibits da Silveira, LBB; Araújo, ES, Radiat. Phys. Chem., 130, 123-32, photo-Fries rearrangements in 2017.] tetramethyl bisphenol-A polycarbonate.2 These methyl groups also involve crosslinks under irradiation and are the source of a higher oxidizability of tetramethyl bisphenol-A polycarbonate than bisphenol-A polycarbonate.2 Ion-irradiation of polycarbonate improved its thermal and mechanical properties.3 Elongation was decreased and tensile stress was increased when ion fluence was increased indicating an increase in crosslinking over chain scission.3

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Polycarbonate is used in medical devices, which are frequently exposed to gamma radiosterilization and chemical agents.4 Environmental stress cracking was observed in γirradiated polycarbonate (Figure 2.33).4 Branching sites in polycarbonate cause resin chains to join each other to form partially or fully crosslinked resin networks which will no longer be thermoplastic in nature and which are expected to exhibit enhancements, over corresponding linear resins (e.g., exposure to organic solvents).5 A variety of methods have been employed to crosslink polycarbonate resin, such as the incorporation of a suitably reactive chemical group either into the resin chain at its time of manufacture or as an additive to the resin after manufacture, or both.5 The multi-functional phenolic or carboxylic branching agent is selected from the group consisting of 1,1,1-tri(4-hydroxy-phenyl) ethane, 2,2'5,5'-tetra(4-hydroxyphenyl) hexane, trimellitic anhydride, trimellitic acid, trimellitoyl trichloride, 4-chloroformyl phthalic anhydride, pyromellitic acid, pyromellitic dianhydride, mellitic acid, mellitic anhydride, trimesic acid, benzophenonetetracarboxylic acid and benzophenonetetracarboxylic anhydride.5 The crosslinked polycarbonate has been derived from a polycarbonate having endcapping groups, such as a monohydroxybenzophenone (e.g. 4-hydroxybenzophenone).6 The polycarbonate of this composition can achieve a UL94 5VA rating which is required for thin-wall products.6 The crosslinked unsaturated polycarbonate resins had an improved flexibility compared to the corresponding polyester resins.7 The vinyl monomer crosslinking agent was selected from a group consisting of substituted styrenes, acrylates, vinyl ethers, and vinyl esters.7 References 1 2 3 4 5 6 7

Chesterman, JP; Hughes, TC; Amsden, BG, Eur. Polym. J., 105, 186-93, 2018. Rivaton, A; Mailhot, B; Soulestin, J; Varghese, H; Gardette, J-L, Eur. Polym. J., 38, 7, 1349-63, 2002. Reheem, AMA; Atta, A; Maksoud, MIAA, Radiat. Phys. Chem., 127, 269-75, 2016. Silva, PPJ; Araújo, PLB; da Silveira, LBB; Araújo, ES, Radiat. Phys. Chem., 130, 123-32, 2017. Hoeks, TL; Kusters, AAM; Lin, Y-G; McCloskey, PJ; Mestanza, R; Wu, P-p, US6087468A, General Electric Co, Jul. 11, 2000. Morizur, J-F; Sybert, PD; Hoover, JF; Lake, D, US9481761B2, SABIC Global Technologies BV, Nov. 1, 2016. Costa, VVI; Nohales, PA, EP20160733004, Ube Corp Europe S A U, Dec. 21, 2016.

2.2.35 Polydimethylsiloxane

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2.2.35 POLYDIMETHYLSILOXANE Varying the degree of crosslinking in the polydimethylsiloxane network allows tuning of its mechanical properties.1 The elastic modulus was studied over a wide range of the crosslinking weight ratios in which the adhesive interactions were represented by an interaction potential and the surface deformations.1 The decomposition of di-t-butyl peroxide and diaroyl peroxide in permethylated silicone oil resulted in the production of high molecular weight compounds, which were different for both peroxides.2 The homolysis of the peroxide generated silylalkyl radicals by abstraction of hydrogen. The radicals re-combine to produce the dehydrodimer.2 The drug-free and drug-loaded delivery devices were made out of polydimethylsiloxane using the 3D printing technique.3 Sufficient crosslinking yield and mechanical strength was obtained with minimum three minutes of UV exposure.3 3D printing technique in combination with UV-LED crosslinking is an applicable method in the production of prednisolone containing polydimethylsiloxane.3 3D printed oral dosage form has got FDA approval and has entered the market.3 The crosslinked polydimethylsiloxane nano-membranes with elastic moduli of only a few hundred kPa are similar to human soft tissues.4 The crosslinking of the vinyl-terminated polydimethylsiloxane was conducted using in situ ultraviolet irradiation from deuterium lamp through a CaF2-window.4 The UV spectrum of the deuterium lamp had a peak intensity at a wavelength of approximately 190 nm.4 The preferential pathway of vinylgroup radicalization is accompanied by methyl side group radicalization via UV irradiation for wavelengths below 170 nm.4 CH and even SiC bonds are radicalized and form linking sites for the three-dimensional crosslinking between the polydimethylsiloxane chains.4 An elastomeric vitrimer was Figure 2.34. Top: Compounds and polymers used for network formation: amino-functionalized PDMS (PDMS-NH2), mono- (1) and synthesized via the crosslinking of bis- (2) vinylogous urethane compounds, and non-dynamic diacry- a polydimethylsiloxane bearing late crosslinker (3). Bottom: Topological rearrangement of the vitrimer network via transamination of vinylogous urethane crosslinks pendant amino functions with a by pendant amines. [Adapted, by permission, from Stukenbroeker, bis-vinylogous urethane crossT; Wang, W; Winne, JM; Du Prez, FE; Nicolaÿ, R; Leibler, L, linker (Figure 2.34).5 At room Polym. Chem., 8, 6590-3, 2017. temperature, the material has tunable elastic properties.5 At elevated temperatures, samples were reshapable and recyclable.5 Crosslinker and catalyst concentrations have been varied to prepare different hydroxy-functional polydimethylsiloxane network compositions.6 The different geome-

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tries of the tear specimens, trouser, Graves, and crescent, gave different values of tearing strength.6 The decrease in trouser tear strength with increase in the concentration of crosslinker and catalyst was observed unlike in the Graves and crescent-shaped specimens in which the trend had a reverse order (tear strength increased with increase in crosslinker concentration).6 Using polydimethylsiloxane samples with different degrees of crosslinking, a polydimethylsiloxane-polydimethylsiloxane pattern transfer was achieved, and the microhole and micropillar arrays were obtained from a single master substrate.7 The surface geometries of these materials were characterized by scanning electron microscopy and laser scanning microscopy.7 The polydimethylsiloxane micropillar and microhole arrays were effective in changing the contact angles of liquids having different surface tensions (water, ethylene glycol, olive oil, and hexadecane) (Figure 2.35).7

Figure 2.35. The soft lithography process used for fabricating PDMS microhole and micropillar arrays from a single master substrate (left). Contact angle results (right). [Adapted, by permission, from Kameya, Y, Mater. Lett., 196, 320-3, 2017.]

Gelation of polydimethylsiloxane irradiated with 256 MeV Ar ion resulted in the weight of the insoluble residue to be proportional to the number of irradiated ions.8 The gel string was generated in each ion track.8 The molecular weight of polydimethylsiloxane precursors and the content of the tetraethylorthosilicate crosslinker on the degree of swelling in ethanol and ethanol contact angle were reported.9 The degree of equilibrium swelling of the networks was strongly influenced by the tensile modulus and the content of the uncrosslinked precursors.9 Polymethylmethoxysiloxane with dense pendant Si-bound methoxy groups was synthesized by ring-opening polymerization and dehydrocoupling reaction.10 Polymethylmethoxysiloxane was incorporated with polydimethylsiloxane via hydrolytic condensation to prepare polymethylmethoxysiloxane crosslinked polydimethylsiloxane.10 The dense polymethylmethoxysiloxane phases reduced the pyrolysis of polydimethylsiloxane at elevated temperature.10 Polydimethylsiloxane was crosslinked by a chemical reaction with pentaerythritol.11 The chemical reaction included a condensation cure reaction using dibutyltin dilaurate as a catalyst.11

2.2.35 Polydimethylsiloxane

69

Hydroxyl-terminated polydimethylsiloxane was used as cure control additive for the silane crosslinking of polyolefins to minimize scorch of a silane-functionalized polyolefin during melt mixing with a flame retardant.12 Polydimethylsiloxane used for the purpose had number average molecular weight ≥ 4,000 g/mol, viscosity ≥ 90 cP, and hydroxyl group content ≤ 0.9 wt%.12 The silicone polymer was crosslinked by contacting it with a crosslinking agent with a hydrolytic enzyme.13 The enzymes silicatein and trypsin have the ability to catalyze, hydrolyze, and subsequently cause condensation of tetramethoxy- and tetraethoxysilanes.13 For this reaction pepsin, α-chymostrypsin, bromelain, and trypsin were selected for crosslinking of polydimethylsiloxane.13 References 1 2 3 4 5 6 7 8 9 10 11 12 13

Jin, C; Wang, Z; Volinsky, AA; Sharfeddin, A; Gallant, ND, Polym. Testing, 56, 329-36, 2016. Baquey, G; Moine, L; Babot, O; Degueil, M; Maillard, B, Polymer, 46, 17, 6283-92, 2005. Holländer, J; Hakala, R; Suominen, J; Moritz, N; Sandler, N, Int. J. Pharm., 544, 2, 433-42, 2018. Liu, J; Cheng, Y; Xu, K; An, L; Zhang, Z, Compos. Sci. Technol., 167, 355-63, 2018. Stukenbroeker, T; Wang, W; Winne, JM; Du Prez, FE; Nicolaÿ, R; Leibler, L, Polym. Chem., 8, 6590-3, 2017. Shah, GB, J. Appl. Polym. Sci., 133, 43115, 2016. Kameya, Y, Mater. Lett., 196, 320-3, 2017. Koizumi, H; Taguchi, M; Kobayashi, Y; Ichikawa, T, Nuclear Instr. Meth. Phys Res. Sect. B: Beam Interact. Mater. Atoms, 179, 4, 530-5, 2001. Zhang, W; Su, X; Shi, B; Wang, L; Li, S, Reactive Funct. Polym., 86, 264-8, 2015. Han, Y; Zhang, J; Shi, L; Qi, S; Jin, R, Polym. Deg. Stab., 93, 1, 242-51, 2008. Kobilka, BM; Kuczynski, J; Mann, PV; Wertz, JT, US20170121469A1, International Business Machines Corp, May 4, 2017. Chaudhary, B; Waxman, A; Dreux, PC; Bolz, KK, WO2016176034A1, Dow Global Technologies Llc, Nov., 3, 2016. Zelisko, PM; Arnelien, K; Frampton, M, US8383755B2, Brock University, Feb. 26, 2013.

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2.2.36 POLYETHERETHERKETONE An aromatic hydrocarbon polymer electrolyte membrane was prepared by modification of polyetheretherketone film by the radiation-induced crosslinking and grafting.1 The crosslinking structure induced by the electron-beam irradiation enhanced the insolubility of the polyetheretherketone base film.1 Radiation-induced crosslinking of polyetheretherketone film was carried out by 40 MGy dose of electron-beam irradiation.1 The gel fraction of the crosslinked film reached a high level of 93%.1 The radiation crosslinking of a polyetheretherketone film was done to prevent dissolution and deformation of the original film in sulfonating solutions.2 The films crosslinked with doses of more than 33 MGy can be effectively sulfonated in a chlorosulfonic solution, resulting in a crosslinked sulfonated polyetheretherketone electrolyte membrane with high proton conductivity comparable to Nafion.2 Polyetheretherketone chains may be crosslinked using two or more Schiff base linkages.3 Polyetheretherketone crosslinking methods also include ion or electron beam irradiation, use of elemental sulfur as a crosslinker, and a diamine as a crosslinker to crosslink PEEK.3 References 1 2 3

Chen, J; Li, D; Koshikawa, H; Asano, M; Maekawa, Y, J. Membrane Sci., 362, 488-94, 2010. Chen, J; Maekawa, Y; Asano, M; Yoshida, M, Polymer, 48, 20, 6002-9, 2007. Tu, H; Robisson, A, CA2731798C, Schlumberger Canada Ltd, Jul. 21, 2015.

2.2.37 Polyetherketoneketone

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2.2.37 POLYETHERKETONEKETONE A polymer electrolyte that may be used in a fuel cell includes sulfonated polyetherketoneketone and a crosslinking agent.1 The crosslinking agent was selected from the group consisting of polyethyleneglycol diacrylate, polyethyleneglycol dimethacrylate, polyethyleneglycol dimethyl ether, and polyethyleneglycol methacrylate.1 The polymer electrolyte maintained a high ionic conductivity even at a relative humidity of close to 0% by preventing the leakage of phosphoric acid impregnated in its polymer matrix.1 References 1

Lee, M-J; Cho, M-D; Sun, H-y, US7850873B2, Samsung SDI Co Ltd, Apr. 1, 2010.

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2.2.38 POLYETHERIMIDE Polyetherimide-based carbon membranes were obtained by chemical crosslinking on the support of phenolic resin sheets.1 The chemical crosslinking was more beneficial than the popular thermal crosslinking.1 Ethylene glycol was used at 10 wt%.1 Polyetherimide fibrous membrane for lithium battery separator was fabricated with crosslinking network morphologies via electrospinning and in situ micro-melting techniques.2 The in situ micro-melting process converted the loose and weak polyetherimide nonwoven membrane to the compact and robust crosslinked fibrous membrane (Figure 2.36).2 It also improved the tensile strength of the membrane from 4 to 21 MPa.2 The crosslinking structure significantly reduced the risk of fibrous membrane disassembling into the loose nonwovens during long-term battery cycling.2

Figure 2.36. The procedure for preparing the polyetherimide fibrous membrane with crosslinked topographies through electrospinning and in situ micro-melting crosslinking technique. [Adapted, by permission, from Kong, L; Liu, B; Ding, J; Yan, X; Tian, G; Qi, S; Wu, D, J. Membrane Sci., 549, 244-50, 2018.]

Crosslinking agents were added to the polyimide polymer prior to formation of the membrane.3 Many crosslinking agents can be used, including polyamines, diamines, and compounds having dual alcohol or thiol functionalities.3 The surface of micronized particles made out of polyetherimide was chemically crosslinked.4 The surface crosslinking provided better chemical resistance properties without compromising thermal stability and provided better barrier properties of coatings manufactured from the micronized particles.4 Solvent stability of the polyimide membranes to solvents or solvent mixtures that would dissolve polyimide under the conditions applied during filtration have been improved by crosslinking.5 The crosslinking procedure involves immersion of a polyimide membrane in a solution comprising an amino-compound, such as a solution of p-xylenediamine in methanol.5 References 1 2 3 4 5

Zhang, X; Zhang, B; Wu, Y; Wang, D; Wang, T, J. Appl. Polym. Sci., 134, 44889, 2017. Kong, L; Liu, B; Ding, J; Yan, X; Tian, G; Qi, S; Wu, D, J. Membrane Sci., 549, 244-50, 2018. Cano, OA; Koeckelberghs, G; Vanherck, K; Vankelecom, I, WO2010111755A3, Katholieke Universiteit Leuven - K.U.Leuven R & D, Jan. 13, 2011. Kalyanaraman, V, WO2014151481A1, Sabic Innovative Plastics Ip B.V., Sep. 25, 2014. Vandezande, P; Vanherck, K; Vankelecom, I, EP2164615A1, Evonik Fibers GmbH, Mar. 24, 2010.

2.2.39 Polyethylene

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2.2.39 POLYETHYLENE The peroxide-crosslinking process was adopted to obtain a different crosslink degree by controlling the content of dicumyl peroxide in low-density polyethylene.1 The crosslinked polyethylene had great anti-water-treeing performance compared to LDPE.1 Although crystal growth was inhibited due to the crosslinking reaction, the density of tie molecular chains greatly increased in the amorphous region and exhibited significantly tighter lamellar stacking, which was the reason that the water tree growth was restrained with increasing crosslinking degree.1 Wear and polyethylene damage continue to be important factors affecting outcomes of unicompartmental knee arthroplasty.2 The polyethylene crosslinking reduced wear by 68% while backside wear comprised 46% of the total wear in the mobile bearing.2 High-density polyethylene was chemically crosslinked with various amounts of ditert butyl cumyl peroxide.3 Crosslinking caused a decrease in glass transition temperature explained by the reduction of crystallinity and increase in free volume as a result of a restriction in chain packing.3 The chemical crosslinking had no significant effect on the thermal stability.3 The crosslinked HDPE showed a decrease in creep strain and an increase in creep modulus with an increase in di-tert butyl cumyl peroxide.3 The crosslinking of thick layers of polyolefin is used for the production of electrical insulation material (HV DC cable insulation).4 High voltage (HV) cables are used for electric power transmission at high voltage.4 Low-density polyethylene is widely used as insulation material in power devices.4 In many electrical applications, such as medium (MV) and high voltage (HV) cables, crosslinked polyethylene (XLPE or PEX) is applied.4 The primary purpose of LDPE crosslinking is the improvement of dimensional and therefore thermo-mechanical stability while retaining a high flexibility.4 This permits higher conductor operating temperatures and reduces the level of short circuit and overload protection required.4 The radiation used for crosslinking is X-ray radiation or gamma radiation at the overall radiation dose from 150 to 350 kGy.4 The annealing is performed at a temperatures from 120 to 140°C.4 Ultrahigh molecular weight polyethylene has anthocyanin dispersely imbedded in the polyethylene. The implant is exposed to γ-ray or electron beam irradiation in dose of at least 2.5 Mrad followed by a heat treatment to prevent the implant from becoming brittle in the long-term as well as to improve strength and wear. Under these conditions, polyethylene was crosslinked in the presence of anthocyanin which reduced the presence of free radicals and improved wear. The doped UHMWPE particles were compressed into blocks at temperatures in the range of 135-250°C and pressures in the range of 2-70 MPa. Medical implants were made from the blocks. References 1 2 3 4 5

Chen, J; Zhao, H; Xu, Z; Zhang, C; Lei, J, Polym. Testing, 56, 83-90, 2016. Netter, J; Hermida, JC; D’Alessio, J; Kester, M; D’Lima, DD, J. Arthroplasty, 30, 8, 1430-3, 2015. Khonakdar, HA; Morshedian, J; Wagenknecht, U; Jafari, SH, Polymer, 44, 15, 4301-9, 2003. Ho, C-H; Kornmann, X; Krivda, A, WO2014075727A1, Abb Research Ltd, May, 22, 2014. He, S; Yau, S-S; Wang, A; Lawrynowicz, DE, US8133436B2, Howmedica Osteonics Corp, Mar. 13, 2012.

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2.2.40 POLY(HYDROXYETHYL METHACRYLATE) Delayed release systems find applications in chronotherapeutics and colon-specific delivery.1 The delayed release of proxyphylline from poly(2-hydroxyethyl methacrylate) hydrogels can be achieved by surface crosslinking.1 Ammonium peroxydisulfate and sodium metabisulfite were used as redox initiators and glutaraldehyde was used as a crosslinker.1 The highly crosslinked surface layers and the ruptures occurring in these layers during swelling were likely responsible for the delayed release.1 The directional freezing and radiation-induced polymerization and crosslinking were used to fabricate anisotropic hydrogels from poly(2-hydroxyethyl methacrylate) (Figure 2.37).2 The hydrogels show significant anisotropy in microstructure and mechanical properties. The aligned porous structure was observed along the freezing direction and an irregular porous structure in the vertical direction.2

Figure 2.37. The process of the directional freezing and radiation-induced polymerization and crosslinking: (a) The monomer solution; (b) and (c) directional freezing; (d) the radiation polymerization and crosslinking of monomeric molecules and (e) the thawing of solvent crystal. [Adapted, by permission, from Chen, M; Zhu, J; Qi, G; He, C; Wang, H, Mater. Lett., 89, 104-7, 2012.]

Covalent crosslinking of weak polyelectrolyte brushes widens the tuning potential for their swelling, nanomechanical, and nanotribological properties.3 Poly(hydroxyethyl methacrylate) brushes and brush hydrogels, and their ionizable, succinate-modified derivatives were covalently crosslinked with different amounts of di(ethylene glycol) dimethacrylate during surface-initiated atom transfer radical polymerization.3 References 1 2 3

Wu, L; Brazel, CS, Int. J. Pharmaceutics, 349, 1-2, 1-10, 2008. Chen, M; Zhu, J; Qi, G; He, C; Wang, H, Mater. Lett., 89, 104-7, 2012. Dehghani, ES; Ramakrishna, SN; Spencer, ND; Benetti, EM, Macromolecules, 50, 2932-41, 2017.

2.2.41 Polyimide

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2.2.41 POLYIMIDE Polyimide aerogels were crosslinked by cyclic ladder-like, and cage polyamine functionalized polysilsesquioxanes.1 Polyimide polyhedral oligomeric silsesquioxane aerogels gave stable and strong structures. These aerogels may find application in thermal protection against high temperature.1 Diamine crosslinked polyimide membranes are well-known for their excellent performance in pervaporation, gas separation, and solvent resistant nanofiltration.2 The commonly used post-synthesis crosslinking method involves a second processing step.2 In situ crosslinking method avoids the use of methanol by introducing the diamine crosslinker as an additive in the coagulation bath containing water.2 The optimal membranes show ten times higher fluxes than the commercial membranes.2 The polyimide membrane was crosslinked by oxygenated hydrocarbon.3 Crosslinking the polyimide with the paraformaldehyde solution improved the solvent resistance of the polyimide.3 References 1 2 3

Wu, Y-W; Ye, M-F; Zhang, W-C; Yang, R-J, J. Appl. Polym. Sci., 134, 45296, 2017. Hendrix, K; Vanherck, K; Vankelecom, IFJ, J. Membrane Sci., 421-2, 15-24, 2012. Yeager, GW, US7339009B2, General Electric Co, Mar. 4, 2008.

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2.2.42 POLYMETHYLMETHACRYLATE The X-ray fluence 4.48 mJ-cm-2 was the optimal fluence for crosslinking of polymethylmethacrylate that improved the crystallization of the films and caused the generation of new phases.1 The film irradiated with 2.56 mJ-cm-2 had amorphous behavior; 3.20 and 3.84 mJ-cm-2 X-rays fluences produced a material with a high level of disorder.1 The crosslinking at fluence 4.48 mJ-cm-2 improved crystallinity by promoting a structural rearrangement of polymer (Figure 2.38).1

Figure 2.38. XRD pattern of PMMA thin film deposited at room temperature after irradiation with X-rays fluence ranging from 2.56 to 5.76 mJ-cm-2. Inset is a close-up image of the peaks indicating the presence of C60 and C70. [Adapted, by permission, from Iqbal, S; Rafique, MS; Iqbal, N; Bashir, S; Ahmad, R, Prog. Org. Coat., 111, 202-9, 2017.]

Reversible crosslinks were prepared in polymethylmethacrylate by Diels-Alder reaction using multi-furan and multi-imide precursors.2 Furan functionalized PMMA was obtained by reactive extrusion (transesterification) between a commercial PMMA and furfuryl alcohol using tin(II)2-ethylhexanoate (Sn(oct)2) or 1,5,7-triazabicyclo[4.4.0]dec-5ene catalysts.2 The hemispherical polymethylmethacrylate beads were produced by polymerization of mixture containing an acrylic monomer (methyl(meth)acrylate and trimethylolmethanetriacrylate), a crosslinking agent (e.g., trimethylolmethane tetracrylate, trimethylolmethane triacrylate, trimethylolbutane triacrylate, ethyleneglycol diglycidyl methacrylate, and

2.2.42 Polymethylmethacrylate

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divinylbenzene), an initiator (azobisisobutyronitrile), stabilizer (polyvinylalcohol), a cosolvent (cyclohexane), and a solvent.3 A composition comprising a silicone oil and a porous crosslinked polymethylmethacrylate bead was designed to boost the silicone feel of a cosmetic composition.4 References 1 2 3 4

Iqbal, S; Rafique, MS; Iqbal, N; Bashir, S; Ahmad, R, Prog. Org. Coat., 111, 202-9, 2017. Okhay, N; Jegat, C; Mignard, N; Taha, M, Reactive Funct. Polym., 73, 5, 745-55, 2013. Lee, SH; Park, JH; Kim, SU; Yoon, YB; Nam, H, US9487651B2, Sunjin Chemical Co Ltd, Nov. 8, 2016. Brock, A; Gstoettmayr, C; Hueber, A; Mesaros, S; Vollhardt, JH, WO2014184315A1, Dsm Ip Assets B.V., Nov. 20, 2014.

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2.2.43 POLY(METHYLMETHACRYLATE-CO-HYDROXYETHYL ACRYLATE) Self-healing polymer based on a photo-active reversible addition-fragmentation chain transfer agent acted as a crosslinker in crosslinking poly(methylmethacrylate-cohydroxyethyl acrylate) due to interactions of benzophenone moieties of trithiocarbonate and hydroxyl groups of copolymer on exposure to UV irradiation.1 References 1

Cheng, C; Bai, X; Zhang, X; Li, H; Huang, Q; Tu, Y, J. Polym. Res., 22, 46, 2015.

2.2.44 Poly(N-isopropylacrylamide)

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2.2.44 POLY(N-ISOPROPYLACRYLAMIDE) Telechelic poly(N-isopropylacrylamide) was used as homogeneous and self-healable gel by end-crosslinking.1 Its end groups were quantitatively converted into activated esters as reactive sites for gelation, and they were employed for an end-crosslinking reaction with trifunctional amine crosslinker.1 The gelation was closely related to the molecular weight and the concentration of the telechelic prepolymers.1 Azido-poly(N-isopropylacrylamide) was prepared by reversible addition-fragmentation chain transfer polymerization of N-isopropylacrylamide and glycidyl methacrylate followed by the azidization of pendant epoxy groups.2 The polymer was crosslinked via click chemistry by bis- or tetra-alkynyl terminated compounds.2 The decrease in crosslinker dosage or increase in chain length enhanced swelling of hydrogels because of larger free volume.2 Diamine crosslinker was used for post-polymerization of poly(N-isopropylacrylamide).3 Unlike in the case of conventional divinyl crosslinking, the reaction could be performed in both water and solvents.3 The swelling behavior of gels was independent of the gelation solvents.3 References 1 2 3

Ida, S; Kimura, R; Tanimoto, S; Hirokawa, Y, Polym. J., 49, 237-43, 2017. Wang, J; Zhang, Z; Liu, Y; Lv, Y; Shao, Z, Int. J. Polym. Mater. Polym. Biomater., 64, 104-10, 2015. Ida, S; Katsurada, A; Yoshida, R; Hirokawa, Y, Reactive Funct. Polym., 115, 73-80, 2017.

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2.2.45 POLY(PHENYLENE SULFIDE) Heating the poly(phenylene sulfide) and the polyphenylsulfone in the presence of a crosslinking agent leads to the crosslinked blend.1 The crosslinking agent is a peroxide, sulfur, metal oxide, or sulfur donor agent.1 The degree of crosslinking can be regulated by controlling crosslinking temperature and time.1 The elastomers are used in applications, such as packer elements, blow out preventer elements, O-rings, and gaskets.1 The crosslinked poly(phenylene sulfide) and/or semi-crosslinked poly(phenylene sulfide) were used in the non-contact data receiver/transmitter with excellent heat resistance and weather resistance (degradation in communication occurring in the antenna was prevented).2 A composition contains a crosslinked product of a polyarylene, a crosslinked product of a substituted polyphenylene, a crosslinked product of poly(phenylene sulfide) and polyphenylsulfone.3 The composition has high-temperature elastomeric properties and excellent mechanical strength.3 The compositions are useful in oil and gas downhole applications.3 References 1 2 3

Gerrard, DP; Ren, J; Duan, P, US8604157B2, Baker Hughes Inc, Dec. 10, 2013. WO2009119816A1, Oct. 1, 2009. Roy, S; Richard, BM; Potts, JR; Sadana, AK, US9303150B2, Baker Hughes Inc, Apr. 5, 2016.

2.2.46 Polypropylene

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2.2.46 POLYPROPYLENE The melt-state crosslinking of polypropylene used a small amount of 4-vinylbenzoic-2,2,6,6tetramethylpiperidin-N-oxyl to trap peroxide-derived alkyl radical intermediates during the early stages of the modification process, yielding a macromonomer derivative that crosslinks in the post-quenching phase through side-chain oligomerization (Figure 2.39).1 The process produced thermosets of a PP homopolymer with gel contents in excess of 90%, while providing the induction period needed to shape the formulation before the melt viscosity increased.1 The complete conversion of nitroxyl functionality to alkoxyamines marks the end of the induction period.1 During phase 2 of the process, residual peroxide initiates monomer oligomerization and chain scission concurrently, the balance of which dictates crosslink density.1 In phase 3 of the process, cure reversion involves unfettered PP degraThe alkoxyamine dation.1 functionality introduced by PP Figure 2.39. Simplified reaction mechanism to account for polypro- macroradical trapping by nitroxyl pylene crosslinking dynamics. [Adapted, by permission, from improves the material's perforOzols, KE; Molloy, BM; Parent, JS, Polymer, 123, 211-8, 2017.] mance in an accelerated oxidation test.1 iPP was oriented via solid-state stretching at elevated temperature to various draw ratios and, then, γ-irradiated in air.2 The radiation-induced changes in the degree of crystallinity of iPP resulted from the combination of two opposing effects, i.e., from an increase caused by chain scission and a decrease induced by crosslinking on the lamellae surfaces.2 The reduced chain mobility in the oriented structure enabled recombination and formation of crosslinks.2 With the increase in draw ratio, the mobility of tie-chains decreased, thus reducing the crosslinking efficiency.2 Since the greatest concentration of alkyl radicals is at the lamellae surfaces, the gel content is greatly influenced by interlamellar contact.2 The crosslinking behavior of drawn iPP cannot be described by chemical structural parameters or by morphological changes, but it strongly correlates with both.2

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Peroxide-induced crosslinking of isotactic and syndiotactic polypropylene has been investigated.3 Both polymers showed similar crosslinking behavior, with iPP having a higher crosslinking efficiency.3 The attack of peroxide radicals on iPP and sPP chains took place at the same position, giving rise to the similar spectral hyperfine structures.3 A lower radical concentration was observed in sPP than in iPP with the same peroxide concentration and temperature which might have been caused by steric hindrance of the hydrogen abstraction of peroxide radicals.3 The crosslinking was controlled by temperature, peroxide type, and its concentration.3 At high peroxide concentrations, significant β-scission and other side reactions occurred simultaneously, introducing carbonyl and unsaturated groups to the structure of the polymeric networks.3 Isotactic polymer can be crosslinked with sulfur or peroxide (e.g., 2-tert-butylperoxydopropyl) benzene) in the presence of accelerator, such as tetramethyl thiuram monosulfide or tetramethyl thiuram disulfide, and vegetable oil.4 References 1 2 3 4

Ozols, KE; Molloy, BM; Parent, JS, Polymer, 123, 211-8, 2017. Suljovrujic, E, Eur. Polym. J., 45, 7, 2068-78, 2009. Yu, Q; Zhu, S, Polymer, 40, 11, 2961-8, 1999. Bouhelal, S, US6987149B2, Jan. 17, 2006.

2.2.47 Polystyrene

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2.2.47 POLYSTYRENE A series of crosslinked, comb-shaped polystyrene anion exchange membranes with C-16 alkyl side-chain were synthesized by “click chemistry”, and subsequently azide-assisted self-crosslinked, and intended for alkaline fuel cell application.1 The crosslinked membranes retained high ion conductivity even in 10 M NaOH at 80°C for 400 h.1 The covalent crosslinking was reported as an effective method to stabilize high ion exchange capacity in anion exchange membranes against high water swelling.1 Post-consumer waste polystyrene was converted to a conjugated microporous polymer and activated into a sulfonic-group carrying resin (Figure 2.40).2 The surface chemistry confirmed successful crosslinking.2 The resultant polymers were thermally stable (decomposition temperatures above 300°C).2 They had surface heterogeneity, and BET surface areas from 510 to 752 m2/g.2

Figure 2.40. Synthetic pathways for conjugated microporous polymer and sulfonic-group carrying resin synthesis from waste polystyrene. [Adapted, by permission, from Chaukura, N; Mamba, BB; Mishra, SB, J. Environ. Management, 193, 280-9, 2017.]

The materials had maximum adsorption capacities of Congo Red from 357 to 500 mg/g.2 The polystyrene-type beads having different degrees of crosslinking by divinylbenzene were studied for the effect of crosslinking on their stability.3 The thermal stability has been greatly improved by crosslinking.3 With less than 30% of crosslinking degree, the

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glass transition temperature was increased along with crosslinking, and glass transition temperature disappeared above 30% crosslinking.3 The melting point could only be measured for materials having a crosslinking degree below 15%.3 Many factors during polymerization of high impact polystyrene can affect its properties.4 The degree of crosslinking in the rubber phase is one of such factors, which may result in decreased impact resistance and environmental stress cracking resistance.4 At the same time, some crosslinking helps to stabilize the rubber particle morphology through the devolatilization process, but excessive crosslinking alters the elasticity of elastomer phase.4 Examples of crosslinking agents useful in the production of HIPS include a polyfunctional (meth)acrylic monomer and a metal salt of an unsaturated monocarboxylic acid.4 A partly sulfonated styrenic polymer was crosslinked in the presence of polyphosphoric acid.5 Crosslinking was used to suppress swelling degree and to improve stability.5 References 1 2 3 4 5

Liu, W; Liu, L; Liao, J; Wang, L; Li, N, J. Membrane Sci., 536, 133-40, 2017. Chaukura, N; Mamba, BB; Mishra, SB, J. Environ. Management, 193, 280-9, 2017. Li, Y; Fan, Y; Ma, J, Polym. Deg. Stab., 73, 1, 163-7, 2001. Wang, W; Knoeppel, D; Bluhm, M, US9593186B2, Fina Technology Inc, Mar. 14, 2017. Xia, Z; Fang, J; Macdonald, RJ; Lu, S; Yang, H; Barber, JH, WO2012094632A3, General Electric Company, Nov. 1, 2012.

2.2.48 Polystyrene-co-poly(N-isopropylacrylamide)

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2.2.48 POLYSTYRENE-CO-POLY(N-ISOPROPYLACRYLAMIDE) Swelling of stimuli-responsive materials is one of the factors used to control the uptake of nanotherapeutics.1 Polystyrene-co-poly(N-isopropylacrylamide) microgels were crosslinked with various amounts of crosslinker N,N’-methylenebisacrylamide.1 A minor increase in the swelling ratio was observed with increasing N,N’-methylenebisacrylamide.1 This increased swelling of polystyrene-co-poly(N-isopropylacrylamide) resulted from the shifting density gradient away from the surface toward the particle interior (core had higher crosslink density) which was consistent with the mechanical data reported.1 Even though the N,N’-methylenebisacrylamide crosslinking was increased, the density gradient at the particle surface was reduced and thus swelling could increase only in a limited fashion because the overall tendency of increased crosslinking to oppose swelling.1 References 1

Mohapatra, H; Kruger, TM; Lansakara, TI; Tivanski, AV; Stevens, LL, Soft Matter, 13, 5684-95, 2017.

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2.2.49 POLY(SULFOBETAINE METHACRYLATE) Integration of antifouling and bactericidal moieties was used for optimizing the efficacy of antibacterial coatings.1 Poly(sulfobetaine methacrylate) was used as antifouling and N[(2-hydroxy-3-trimethylammonium)propyl] chitosan chloride as a bactericidal polymer. Poly(sulfobetaine methacrylate) was grafted onto an aminolized silicone surface using genipin-induced crosslinking.1 The bifunctional coating inhibited bacterial colonization by approximately two orders of magnitude.1 Poly(sulfobetaine methacrylate) can be utilized in filtration and wound dressing applications.2 The zwitterionic poly(sulfobetaine methacrylate) did not completely dissolve in water at room temperature due to the formation of the ionically crosslinked structures formed by the intragroup, intrachain, and interchain interactions of the positive and negative charges on the pendant groups.2 The osmotic force which draws solvent into the network of polymer chains is not sufficient to break this ionic crosslinking thus prevents solvation of the polymers.2 A photo-responsive bio-inspired adhesive consisting of a zwitterionic polymer, poly(sulfobetaine methacrylate), 3,4-dihydroxyphenylalanine, and a photocleavable nitrobenzyloxycarbonyl containing crosslinker was synthesized by thermally-initiated free radical polymerization.3 The 3,4-dihydroxyphenylalanine functionality is known to be the major component of the adhesion properties in mussels even in the presence of water.3 The adhesion properties of the terpolymer can be precisely controlled by UV irradiation.3 References 1 2 3

Wang, R; Neoh, KG; Kang, E-T, J. Colloid Interface Sci., 438, 138-48, 2015. Lalani, R; Liu, L; Polymer, 52, 23, 5344-54, 2011. Kim, M; Chung, H, Polym. Chem., 8, 6300-8, 2017.

2.2.50 Polysulfone

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2.2.50 POLYSULFONE A covalently crosslinked structure was formed between reduced graphene oxide and quaternized polyether sulfone by nitrene addition reaction (Figure 2.41).1 The excessive swelling of the membrane was improved, and water uptake was decreased by crosslinking which were important properties for high-performance anion exchange membranes.1 The degree of crosslinking was controlled by changing the amount of azide groups.1

Figure 2.41. The methods of preparation of crosslinked polysulfone membrane. [Adapted, by permission, from Hu, B; Miao, L; Zhao, Y; Lü, C, J. Membrane Sci., 530, 84-94, 2017.]

Polysulfones with –O–(CH2)3–SO3H side chains were crosslinked with 1,4-benzenedimethanol for use as polymer electrolytes.2 The crosslinked film showed proton conductivity of 0.26 S cm-1 at 80°C and 90% humidity.2 Polysulfone is an amorphous thermoplastic polymer which is frequently used in automotive, electrical, electronic, and medical parts because of its excellent mechanical properties and radiation resistance.3 Its heat stability and shrinkage behavior can be improved by electron beam irradiation.3 Crosslinking in molten state enhanced the thermo-mechanical properties of polysulfone.3 For the best results, an optimum between irradiation temperature and dose had to be used.3 Polysulfones with benzoxazine end-groups were obtained from the phenol-terminated polysulfone, aniline, and paraformaldehyde as starting materials.4 Thermally activated crosslinking resulted in tough films with good thermal stability.4 The crosslinked microporous polysulfone and polysulfone copolymer were used as battery electrode separator membranes.5 They swell in the electrolyte at the elevated temperature instead of dissolving.5 The crosslinker was chosen from dicyandiamide, benzoyl peroxide, di-glycidyl ethers, and tri-glycidyl ethers.5 Thermally crosslinked polysulfone was made from linear polysulfone, such as polyethersulfone, in a powder form blended with a powdered inorganic peroxide such as magnesium peroxide or another oxygen source, to form a mixture followed by compression inside a mold.6 The mixture was cured at an elevated temperature (above 325oC).6

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References 1 2 3 4 5 6

Hu, B; Miao, L; Zhao, Y; Lü, C, J. Membrane Sci., 530, 84-94, 2017. Kojima, T; Koizumi, T-a; Sei, Y; Shiozaki, S; Yamamoto, T, Eur. Polym. J., 55, 179-85, 2014. Stephan, M; Pospiech, D; Heidel, R; Hoffmann, T; Dorschner, H, Polym. Deg. Stab., 90, 2, 379-85, 2005. Ates, S; Dizman, C; Aydogan, B; Kiskan, B; Yagci, Y, Polymer, 52, 7, 1504-9, 2011. Hauser, RL; US20170331095A1, Samsung Electronics Co Ltd, Nov. 16, 2017. Duan, P; Agrawal, G; Walls, C, WO2012166234A1, Baker Hughes Incorporated, Dec. 6, 2012.

2.2.51 Polyurethane

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2.2.51 POLYURETHANE Polyurethanes were synthesized using a mixture of neat polyol and glycerol, in different ratios, with a 4,4-diphenylmethane diisocyanate.1 The prepolymer was subsequently extended using 1,4-butanediol.1 Glycerol caused crosslinking.1 As its content increased, the swelling ratio decreased and crosslink density increased, resulting in the increase of glass transition temperature and storage modulus at room temperature.1 Thermomechanical stability, tensile strength, elastic modulus, and hardness of PUs increased with the incorporation of glycerol.1 Polyurethane phase change materials were synthesized from polyethylene glycol, hexamethylene diisocyanate, and halloysite nanotubes which crosslinked and reinforced the system (Figure 2.42).2

Figure 2.42. (a) Illustration of halloysite (b) and (c) entrapping of poly(ethylene glycol) and hexamethylene diisocyanate, respectively (d) and (e) crosslinking network with small and large amount of halloysite (f) crosslinking network in halloysite. [Adapted, by permission, from Zhou, Y; Sheng, D; Liu, X; Lin, C; Yang, Y, Solar Energy Mater. Solar Cells, 174, 84-93, 2018.]

The UV-crosslinked waterborne siloxane–polyurethane was synthesized from isophorone diisocyanate, glycidyl azide polymer, polytetramethylene glycol, dihydroxybutyl terminated polydimethylsiloxane, and dimethylol propionic acid.3 The polymer cured by UV showed good thermal stability, surface properties, and water resistance.3 The incorporation of azide group into waterborne polyurethane can be used in organic coatings.3 The presence of water during fabrication severely affects final properties (increased polymer mobility, reduced mechanical stiffness, etc.) by reacting with isocyanate (forming primary amines and urea) and thereby lowering crosslink density, even under seemingly uncritical humidity conditions (Figure 2.43).4 Various chemical side reactions (involving allophanate, carbodiimide, uretdione) are potential sources of maturation/property variations.4 Water also plays a big role in polymer mobility which is extremely sensitive to variations in moisture.4 The impact of moisture in terms of water diffusion during cure can

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be reduced by fast polyaddition kinetics (catalysts) and/or curing regime (e.g., temperature).4

Figure 2.43. Some examples of water impact on polyurethane curing. [Adapted, by permission, from Zimmer, B; Nies, C; Schmitt, C; Paulo, C; Possart, W, Polymer, 149, 238-52, 2018.]

Mimicking the mechanical properties of native tissues is a critical criterion for an ideal tissue engineering scaffold.5 Most biodegradable synthetic materials, including polyester-based polyurethanes, consist of rigid polyester chains and have high crystallinity lacking the elasticity of human tissues.5 Biodegradable PU with excellent elasticity was synthesized using the controlled crosslinking of poly(ester ether) triblock copolymer diols and polycaprolactone triols linked by urethane linkages.5 Three-dimensional porous scaffolds of defined geometry, tunable microstructure, and adjustable mechanical properties were synthesized in situ from an isocyanate-ended copolymer, a tri-armed polycaprolactone, and a chain extender (Figure 2.44).5 These scaffolds, although mostly made of rigid polycaprolactone chains, had remarkable elasticity and cyclical properties with maximum recovery rate of 99.8%.5 The copolymer provided the molecular flexibility while the long chain crosslinking of polycaprolactone triol hindered crystallization, thus making the polyurethane behaving like an amorphous elastic material, which was also biocompatible.5 These attributes made polyurethane scaffolds suitable for the regeneration of tissues that experience dynamic loading.5

2.2.51 Polyurethane

91

Figure 2.44. The synthesis route of the controlled crosslinking of polyurethanes by varying the A to B ratio. [Adapted, by permission, from Mi, H-Y; Jing, X; Yilmaz, G; Hagerty, BS; Turng, L-S, Chem. Eng. J., 348, 78698, 2018.]

Waterborne polyurethanes with different crosslinking densities were synthesized by varying the amount of trifunctional crosslinker trimethylolpropane.6 With the increase in trimethylolpropane content, the interface adhesion strength decreased due to the reduced mobility of polyurethane molecular chains, the cohesive strength of waterborne polyurethane films increased because of the increase in the crosslinking density, and the T-peel strength initially increased and then decreased with the change of debonding failure from a cohesive failure to an interface failure.6 The thermoplastic elastomers have physical crosslinks formed by hard domains embedded in a soft matrix material.7 These domains serve not only as crosslinks, but also function as fillers.7 Acting as fillers, the domains provide a high modulus of elasticity.7 Once the domains take the load, they lose their filler function and the material gains in extensibility.7 UV-curable acrylic end-capped polyurethane dispersions were studied for the crosslinking degree using heat capacity measurements.8 The reduction of the heat capacity was an indirect measurement of crosslinking degree.8 The glass transition temperature being a direct measurement of molecular mobility depended on crosslinking degree.8 The photoinitiator concentration increase caused an increase in the termination reactions, reducing the crosslink degree.8 The final conversion did not depend on the photoinitiator concentration.8 The crosslink density was larger with lower molecular weight polyols and higher curing temperature.8

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Figure 2.45. The reaction scheme of the synthesis of modified blocked polyisocyanates. CAP − ε-caprolactam, IPDI − isophorone diisocyanate, PF-636 or PF-6520 − fluorinated polyols of different molecular weights. [Adapted, by permission, from Pilch-Pitera, B; Byczyński, Ł; Myśliwiec, B, Prog. Org. Coat., 113, 82-9, 2017.]

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The increasing length of the diol sequences between neighboring isocyanate units acting as crosslinkers caused a decrease of the calorimetric glass transition temperature and the softening temperatures which was explained by an internal plasticization of the polymeric network by long, highly mobile diol units.9 Blocked polyisocyanate crosslinkers for powder coatings were synthesized from isophorone diisocyanate, formic acid, and ε-caprolactam (Figure 2.45).10 Dibutyltin dilaurate and dibutyl phosphate were used as catalysts of their reaction with fluorinated polyols.10 The resultant powder coatings had improved thermal stability and surface properties (lower roughness and surface free energy, and higher hydrophobicity, oleophobicity, gloss, scratch, and abrasion resistance).10 The self-crosslinking of polysiloxane-modified polyhydroxy polyurethane resin was derived from the reaction of a 5-membered cyclic carbonate polysiloxane compound (reaction product of an epoxy-modified polysiloxane compound and carbon dioxide) and an amine compound having polysiloxane segments and masked isocyanate groups.11 The product had excellent lubricity, abrasion resistance, chemical resistance, non-tackiness, and heat resistance.11 The self-crosslinking aqueous polyurethane dispersions contained a crosslinking agent selected from diamines and dihydrazides.12 The compositions dried more quickly and exhibited an improved chemical resistance.12 Two-component polyurethane composition with delayed crosslinking contains polyols, aromatic (MDI) and aliphatic isocyanates (HDI), and retarders.13 Retarders are substances which slow down the reaction between OH and NCO groups (organic or inorganic acids, acid chlorides, acidic inorganic salts or other acidic organic compounds in amounts of 0.05 to 3.0 wt%).13 Suitable organic acids are those which have a pKa range from 2.8 to 4.5 (e.g., phthalic acid, isophthalic acid, terephthalic acid, ascorbic acid, benzoic acid, ohydroxybenzoic acid, p-hydroxybenzoic acid, etc.).13 Potlife of the composition was 60 min.13 The above examples are not exhaustive. They were selected from a large number of studies available on polyurethane crosslinking to show some recent trends and solutions. Selection of crosslinking reaction for polyurethane is the primary task which determines their properties and performance. References 1 2 3 4 5 6 7 8 9 10 11 12 13

Trzebiatowska, PJ; Echart, AS; Correas, TC; Eceiza, A; Datta, J, Prog. Org. Coat., 115, 41-8, 2018. Zhou, Y; Sheng, D; Liu, X; Lin, C; Yang, Y, Solar Energy Mater. Solar Cells, 174, 84-93, 2018. Ge, Z; Huang, C; Zhou, C; Luo, Y, Prog. Org. Coat., 90, 304-8, 2016. Zimmer, B; Nies, C; Schmitt, C; Paulo, C; Possart, W, Polymer, 149, 238-52, 2018. Mi, H-Y; Jing, X; Yilmaz, G; Hagerty, BS; Turng, L-S, Chem. Eng. J., 348, 786-98, 2018. Lei, L; Xia, Z; Ou, C; Zhang, L; Zhong, L, Prog. Org. Coat., 88, 155-63, 2015. Stribeck, A; Pöselt, E; Eling, B; Jokari-Sheshdeh, F; Hoell, A, Eur. Polym. J., 94, 340-53, 2017. Llorente, O; Fernández-Berridi, MJ; González, A; Irusta, L, Prog. Org. Coat., 99, 437-42, 2016. Zajac, M; Kahl, H; Schade, B; Rödel, T; Beiner, M, Polymer, 111, 83-90, 2017. Pilch-Pitera, B; Byczyński, Ł; Myśliwiec, B, Prog. Org. Coat., 113, 82-9, 2017. Hanada, K; Kimura, K; Takahashi, K; Kawakami, O; Uruno, M, US10000609B2, Dainichiseika Color and Chemicals Manufacturing Co Ltd, Jun. 19, 2018. Schafheutle, M; Arzt, A; Burkl, J; Garber, G; Gsöll, H; Jedlicka, R; Neumayer, S; Petritsch, G; Pittermann, R; Wango, J, US7393894B2, Cytec Surface Specialties Austria GmbH, Jul. 1, 2008. Thiele, L; Jüttner, W; Okamoto, O-K, EP2665760B1, Henkel AG and Co KGaA, Oct. 12, 2016.

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2.2.52 POLYVINYLALCOHOL Graphene oxide composite of polyvinylalcohol a bioplastic material was fabricated by chemical crosslinking (dicarboxylic acids, such as succinic and adipic acids).1 Crosslinking neat PVA with dicarboxylic acids resulted in enhanced swelling resistance, increased tensile strength (over 200%), and thermal stability.1 The covalently crosslinked smart hydrogel based on polyvinylalcohol and methacrylic acid was synthesized by free radical polymerization using glyoxal (40% water solution) as crosslinker.2 The increase in the concentration of methacrylic acid caused an increase in swelling just opposite to the effect of increased crosslink density.2 The release of metoprolol tartrate decreased with the increased concentration of glyoxal, but increased with increase in concentration of methacrylic acid.2 Carboxymethylcellulose reinforced polyvinylalcohol was crosslinked using trimethylol melamine.3 The compression strength of crosslinked hydrogel was 15 times higher than the hydrogel from non-crosslinked polyvinylalcohol.3 The hydrogel has potential applications as wound dressings, facial masks, and skin-protection layers.3 Polyvinylalcohol membrane was crosslinked with L-maleic acid.4 The swelling and hygroscopic performance of the PVA membranes were decreased with increasing heat treatment temperature.4 The swelling decreased along with increased crosslink density. The content of hydrophilic groups decreased and increased with increasing heat-treatment temperature and crosslinker concentration, respectively.4 Graphene oxide reinforced PVA was crosslinked with boric acid to improve thermal and mechanical properties.5 Five wt% boric acid increased the tensile strength by a factor of three and the incorporation of 0.2 wt% graphene oxide increased tensile strength by 30%.5 The thermal stability was increased by both crosslinking and reinforcement.5 PVA hydrogels are promising implants due to their similar mechanical properties to soft tissue.6 The addition of hydroxyapatite-modified hydrogel promoted crosslinking and stability.6 The mechanical properties (compression, tension, and nanoindentation) of the hydrogels were improved by the addition of hydroxyapatite.6 A mathematical model describing the release kinetics of antimicrobial agents from crosslinked polyvinylalcohol into water is based on the diffusion of water molecules into the polymeric film, the counter-diffusion of the incorporated antimicrobial agent from the film into the water, and the polymeric matrix swelling kinetics.7 The amount of water sorbed at equilibrium increased when the degree of crosslink decreased.7 Also, the value of the active compound diffusion coefficient always increased as the degree of crosslink decreased.7 Polyvinylalcohol substituted with different alkyl chains (iodododecane, bromotetradecane) was crosslinked with bis-chloro-ethoxy-ethane as an injectable drug carrier.8 βcarotene was used as a lipophilic model drug.8 PVA substituted with iodododecane and crosslinked with bis-chloro-ethoxy-ethane improved drug release.8 The crosslinked polyvinylalcohol-extracellular matrix composite was obtained by contacting a polyvinylalcohol solution and an extracellular matrix to obtain their mixture; freezing and thawing the mixture to obtain a gelled composite; and contacting the gelled composite with a poly(ethylene glycol) solution to obtain a crosslinked composite.9 The composite had excellent elongation, and it was suitable for patch types used in regenerative medicine.9

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A gel produced by dissolving a polyvinylalcohol powder in an aqueous solution and crosslinking by adding a crosslinking agent.10 The crosslinking agent is an organic titanium compound or sodium tetraborate.10 The gel is used in tire treads of the pneumatic tire.10 Gas separation membrane has been made from hydrophylic polymer (e.g., polyvinylalcohol), amino compound (polyethylenimine), an amine cation (aminoisobutyric acid-K salt).11 References 1 2 3 4 5 6 7 8 9 10 11

Belay, M; Sonker, AK; Nagarale, RK; Verma, V, Polym. Int., 66, 1737-46, 2017. Khan, JA; Pervaiz, F; Ranjha, NM; Naeem, M; Khalid, N; Javaid, Z, J. Polym. Environ., 25, 556-68, 2017. Gao, Z; Yu, Z; Huang, C; Duan, L; Gao, GH, J. Appl. Polym. Sci., 134, 44590, 2017. Gao, Y; Ye, H; Wang, L; Liu, M, J. Appl. Polym. Sci., 134, 44481, 2017. Chen, J; Li, Y; Zhang, Y; Zhu, Y, J. Appl. Polym. Sci., 132, 42000, 2015. Gonzalez, JS; Alvarez, VA, J. Mech. Behav. Biomed. Mater., 34, 47-56, 2014. Buonocore, GG; Del Nobile, MA; Panizza, A; Corbo, MR; Nicolais, L, J. Controlled Release, 90, 1, 97-107, 2003. Cerchiara, T; Luppi, B; Bigucci, F; Orienti, I; Zecchi, V, Eur. J. Pharm. Biopharm., 56, 3, 2003. Park, KD; Kim, IG; Kim, SH; Lee, KW; Hwang, MTP, WO2016186482A1, Korea Institute of Science and Technology, Nov. 24, 2016. Akahori, Y, US9353248B2, Yokohama Rubber Co Ltd, May, 31, 2016. Ho, WSW, US8277932B2, Ohio State University Research Foundation, Oct. 2, 2012.

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2.2.53 PROTEIN A green composite with good mechanical properties and acceptable biodegradability was developed using wood flour and soybean protein that was modified by chemical crosslinking with glyoxal and polyisocyanate.1 The chemical crosslinking improved the mechanical properties and water resistance of composite but decreased its biodegradability.1 Both crosslinkers in mixture gave the best improvement of properties.1 Oxidized sucrose, a biobased crosslinker, was proven as effective as glutaraldehyde in improving water stability of ultrafine fibrous proteins without causing cytotoxicity.2 Sucrose was oxidized into polar polyaldehydes to crosslink ultrafine fibrous scaffolds from corn protein (Figure 2.46).2

Figure 2.46. a) Oxidation of sucrose by sodium periodate; b) reaction between oxidized sucrose and amine groups; c) reaction between oxidized sucrose and hydroxyl groups. [Adapted, by permission, from Liu, P; Xu, H; Mi, X; Xu, L; Yang, Y, Macromol. Mater. Eng., 300, 414-22, 2015.]

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Environmentally-friendly packaging based on whey protein has the potential to replace petroleum-based oxygen barrier materials.3 Effective crosslinking of the whey protein is a prerequisite for this application.3 The effect of storage time was studied based on the post-crosslinking of whey protein.3 An increase in storage time led to decreased swelling of whey protein isolate based films.3 The crosslinking was increased by a factor of 3 during 21 days of storage.3 Laccases, tyrosinases, and peroxidases are useful crosslinking agents for generating new textures or for modulating the product formulation.4 The enzymatic treatment improved properties of protein-stabilized emulsions and gels.4 Proteins (keratin) were crosslinked using citric acid as crosslinker and polyols as a crosslinking extender.5 Extenders containing both primary and secondary hydroxyl groups were more effective in enhancing crosslinking efficiency.5 The α-helix in the secondary structure of hair keratin was partially disrupted, while the amounts of β-sheet and random coil increased after crosslinking.5 The method comprising the step of irradiating a photo-activable metal-ligand complex and an electron acceptor in the presence of the protein or peptide initiated a crosslinking reaction to form a 3-dimensional matrix of the biomaterial.6 A crosslinking agent comprising an oxidized sugar having at least two aldehyde groups was used for crosslinking of protein (keratin-containing fibers).7 Fibers included sheep, vicuna, alpaca, llama, muskox, goats, bison, camel, yak, horse, chinchilla, and rabbit.7 Sugar was selected from a group consisting of galactose, sucrose, maltose, lactose, raffinose, and stachyose.7 The oxidizing agent was hydrogen peroxide.7 References 1 2 3 4 5 6 7

Zhang, Y-H; Zhu, W-Q; Gao, Z-H; Gu, J-Y, J. Appl. Polym. Sci., 132, 41387, 2015. Liu, P; Xu, H; Mi, X; Xu, L; Yang, Y, Macromol. Mater. Eng., 300, 414-22, 2015. Schmid, M; Merzbacher, S; Müller, K, Mater. Lett., 215, 8-10, 2018. Isaschar-Ovdat, S; Fishman, A, Trends Food Sci. Technol., 72, 134-43, 2018. Song, K; Xu, H; Mu, B; Xie, K; Yang, Y, J. Cleaner Production, 150, 214-23, 2017. Brownlee, AG; Elvin, CM; Werkmeister, JA; Ramshaw, JAM; Lindall, CM, US9216235B2, Cook Medical Technologies LLC, Dec., 22, 2015. Netravali, A; Zhong, Z, EP3137536A4, Cornell University, Jan. 24, 2018.

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2.2.54 SILICONE RUBBER The crosslinking mechanism of silicone elastomer with dicumyl peroxide is illustrated in Figure 2.47.1 The radical crosslinking reactions are controlled by the radical diffusion depending on viscosity.1 Vinyl groups may be linked increasing viscosity which will lower the diffusion rate.1 During crosslinking reaction, silicone chain mobility is greatly reduced leading to enhanced moduli.1

Figure 2.47. Dicumyl peroxide decomposition and free radical crosslinking mechanisms of poly(methylvinyldimethyl)siloxane. [Adapted, by permission, from Métivier, T; Beyou, E; Cassagnau, P, Eur. Polym. J., 101, 37-45, 2018.]

Silicone rubber (hydroxy-terminated polydimethylsiloxane) was crosslinked by rosin modified by aminopropyltriethoxysilane at room temperature in the presence of an organotin catalyst.2 Compared to the silicone rubber using tetraethoxysilane as crosslinker, the modified rosin-crosslinked silicone rubber exhibited a significant enhancement

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in the thermal stability and mechanical properties because of a strong rigidity and polar hydrogenated phenanthrene ring structure of rosin and its uniform distribution in silicone rubber.2 Figure 2.48 shows the mechanism of crosslinking.2

Figure 2.48. Synthetic route of RTV silicone rubber. Rosin (RO), glycidyl ester of rosin acid (ER), and rosin modified aminopropyltriethoxysilane (RA). [Adapted, by permission, from Li, Q; Huang, X; Liu, H; Shang, S; Song, Z; Song, J, ACS Sustainable Chem. Eng., 6, 10002-10, 2017.]

The rheological behavior of lightly crosslinked vinyl-terminated polydimethylsiloxane was discussed according to the polymer tube model theory and an extension at nanoscale of the Einstein equation of viscosity.3 The isolated crosslinked nanodomains were formed, and the longer chains were preferentially bonded which caused a decrease in the entanglement density in a polymer medium.3 On the nanodomain surface, the dangling chains were not sufficiently long to give stable entanglements (lower chain length than entanglement critical length).3 Two commercially available silicone elastomers (high and room temperature vulcanizates, HTV and RTV) were filled with barium titanate having different particle sizes.4 The silicone elastomers formed crosslinks with the barium titanate particles which acted as an active filler (no significant influence of the particle size was detected).4 A filling degree exists at which the mechanical properties are maximized (in the studied case, the optimum filling degree was determined by the molar mass between crosslinks which was Mc≈ 20 kg/mol regardless of the particle size).4 Polydimethylsiloxane chains were grafted on the silica surface via covalent and hydrogen bonding forming an interpenetrating network (Figure 2.49).5 The interpenetrating network and short polydimethylsiloxane chains provided more crosslinking sites, leading to low viscosity and high mechanical properties of composites.5 The silicapolydimethylsiloxane elastomers exhibited shear thinning behavior.5 The active −OH

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group of nanosilica reacted with the terminal −OH group of polydimethylsiloxane chain.5 The steric effect of nanosilica influenced the polymerization process by stopping the growth of polydimethylsiloxane chains.5 It can be inferred that nanosilica might have acted as an end-capping agent in the inhibition of the grafting process of nanosilica.5

Figure 2.49. Condensation mechanism between nano-silica and PDMS chains. [Adapted, by permission, from Liu, J; Cheng, Y; Xu, K; An, L; Zhang, Z, Compos. Sci. Technol., 167, 355-63, 2018.]

Elastomeric silicone foams are frequently produced by the generation of hydrogen through reaction of Si−H groups with active hydrogen compounds (water and alcohols), in a process catalyzed by platinum or tin complexes.6 It is difficult to control the rate and magnitude of bubble formation.6 Piers-Rubinsztajn reaction offers an alternative process in which α,ω-hydride-terminated polydimethylsiloxane reacts with an alkoxysilane crosslinker such as tetraethyl orthosilicate in the presence of catalyst B(C6F5)3 (Figure 2.50).6 The reaction leads to both crosslinking and bubble formation.6 The reaction was not significantly impacted by humidity.6 The foam was generated by the release of alkane gases derived from the alkoxysilane crosslinker, typically methane or ethane, rather than hydrogen.6 The crosslinker reactivity and concentration, and silicone molecular weight can be used to control bubble nucleation, coalescence, viscosity, and, therefore, final foam density and structure (the formation of open or closed cell foams).6 Better quality resulted when hexane, which acted as a blowing agent, was added to the pre-foam mixture.6

Figure 2.50. Piers-Rubinsztajn reaction. [Adapted, by permission, from Grande, JB; Fawcett, AS; McLaughlin, AJ; Gonzaga, F; Brook, MA, Polymer, 53, 15, 3135-42, 2012.]

Platinum catalyst and nitrogen-containing silane were introduced to improve the thermal stability of silicone rubber.7 A significant synergism was found between the two resulting in increased temperature to 10 and 20% weight loss by 36 and 119oC, respectively.7 It could be argued that the nitrogen atom coordinated with platinum and improved

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its catalytic efficiency.7 The combination of the platinum catalyst and nitrogen-containing silane efficiently catalyzed thermal crosslinking and suppressed degradation of silicone chains from oxidation (Figure 2.51).7

Figure 2.51. Effect of platinum catalyst, Pt/nitrogen-containing silane, NS, on the degradation mechanism of silicone rubbers. [Adapted, by permission, from Chen, W; Zeng, X; Lai, X; Li, H; Liu, T, Thermochim. Acta, 632, 1-9, 2016.]

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Electron irradiation in the air with doses ranging from 25 to 500 kGy have led to additional crosslinking of silicone rubber making it more rigid and brittle.8 The apparent crosslink density was higher for filled elastomers because additional links at the polymersilica interfaces were created as a result of irradiation.8 A further crosslink density enhancement was observed for the surface-treated silica fillers.8 Crystallization was limited by the formation of a higher number of crosslinks which was amplified for filled polydimethylsiloxane.8 A silicone composition having SiH/SiOH groups was polymerized/crosslinked by a dehydrocondensation reaction in the presence of a catalyst (iron-based complex or salt) requiring a low activation temperature.9 Silicone elastomers were crosslinked in the presence of at least one rhodium or iridium catalyst or mixtures of both catalyst types.10 The resultant products were transparent and colorless and were well suited for their use as food and baking molds in the food industry.10 The catalyzed mixture displayed excellent potlife and good high temperature cure.10 Platinum, ruthenium, and rhodium catalysts were used in crosslinking silicone rubber products which had good adhesion to the substrates but also were easy to de-mold.11 Silicone compositions having hydrophilic surface and a high capacity for reversible absorption of moisture have been produced from polyorganosiloxane having at least two alkenyl groups per molecule, SiH functional crosslinker (vinyl or allyl), hydrosilylation catalyst (platinum and its compounds), hydratable salt (anhydrous sodium or magnesium sulfate), and filler.12 The reactive composition contains an amino-modified organopolysiloxane and an epoxy-modified organopolysiloxane.13 The composition cures slowly at room temperature or may be rapidly cured at or above room temperature in the presence of a carboxylic acid.13 Silicone rubber shields of composite insulators were economically produced by coating a support with a crosslinkable silicone rubber composition containing a light-activated hydrosilation catalyst (a platinum catalyst activated by light of from 200 to 500 nm, e.g., MeCp(PtMe3)).14 References 1 2 3 4 5 6 7 8 9 10 11 12 13 14

Métivier, T; Beyou, E; Cassagnau, P, Eur. Polym. J., 101, 37-45, 2018. Li, Q; Huang, X; Liu, H; Shang, S; Song, Z; Song, J, ACS Sustainable Chem. Eng., 6, 10002-10, 2017. Villani, V; Lavallata, V, Macromol. Chem. Phys., 218, 1700037, 2017. Ziegmann, A; Schubert, DW, Mater. Today Commun., 14, 90-8, 2018. Liu, J; Cheng, Y; Xu, K; An, L; Zhang, Z, Compos. Sci. Technol., 167, 355-63, 2018. Grande, JB; Fawcett, AS; McLaughlin, AJ; Gonzaga, F; Brook, MA, Polymer, 53, 15, 3135-42, 2012. Chen, W; Zeng, X; Lai, X; Li, H; Liu, T, Thermochim. Acta, 632, 1-9, 2016. Stevenson, I; David, L; Gauthier, C; Arambourg, L; Vigier, G, Polymer, 42, 22, 9287-92, 2001. Maliverney, C, US8470951B2, Bluestar Silicones France Sas, Jun. 25, 2013. Fehn, A; Weidinger, J, US7129309B2, Wacker Chemie AG, Oct. 31, 2006. Bosshammer, S, US7288322B2, GE Bayer Silicones GmbH and Co KG, Oct. 30, 2007. Funk, E; Gottschalk-Gaudig, T, EP3110889A1, Wacker Chemie AG, Jan. 4, 2017. Czech, AM; Creamer, CE, US6515094B2, Lanxess Solutions US Inc, Feb. 4, 2003. Lambrecht, J; Simson, G; Winter, H-J, US9236164B2, Wacker Chemie AG, Jan. 12, 2016.

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2.2.55 STYRENE-BUTADIENE RUBBER The crosslink density of electron beam-irradiated styrene-butadiene rubber with 1,1,1trimethylolpropane triacrylate (irradiation sensitizer) increased loading was increased up to a certain level (stronger influence of crosslinking than chain scission), and then decreased with the irradiation dose ranging from 50 to 200 kGy.1 As the crosslink density increased, the thermal stability of irradiated SBR increased.1 Three coupling agents (triethoxy(octyl)silane, bis[3-(triethoxysilyl)propyl]disulfide, and bis[3-(triethoxysilyl) propyl]tetrasulfide) were used to foster the filler-rubber interaction, and change crosslink density and crosslink structure of the silica-filled solution styrene-butadiene rubber vulcanizates.2 Triethoxy(octyl)silane did not contribute to an increase in filler-rubber interaction.2 More sulfur groups in silane resulted in a better interaction and higher crosslink density.2 Styrene-butadiene rubber was cured with different sulfur content.3 On increasing the crosslink density, the glass transition temperature increased and the fractional free volume decreased.3 References 1 2 3

Wang, Q; Wang, F; Cheng, K, Radiat. Phys. Chem., 78, 11, 1001-5, 2009. Lee, J-Y; Park, N; Lim, S; Ahn, B; Kim, W; Moon, H; Paik, H-j, Kim, W, Compos. Interfaces, 24, 7, 711-27, 2017. Salgueiro, W; Marzocca, A: Somoza, A; Consolati, G; Goyanes, S, Polymer, 45, 17, 6037-44, 2004.

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2.2.56 SULFONATED POLYETHERETHERKETONE The major drawbacks of the highly sulfonated polyetheretherketone proton exchange membranes for fuel cells are high water uptake and dissolution properties at elevated temperatures.1 Formation of crosslinks by accommodation of the strontium via ionic bonding with sulfonic groups of the sulfonated polyetheretherketone helps in improvement of the membrane.1 Addition of a small amount of Sr (2-5 wt%) considerably decreased water uptake and increased the mechanical strength in water at 80°C.1 Multicomponent membranes prepared using layer-by-layer self-assembly technique (100 layers deposited) was constructed from sulfonated polyetheretherketone as polyanion, polyurethane (polymer), ionic liquid polycation of 1-butyl-3-methylimidazolium, and phosphoric acid doping (Figure 2.52).2 Membranes were suitable for membrane electrolytes working in proton exchange membrane fuel cells.2 The membranes could reach 1.03×10-1 S/cm at 160°C and tensile strength of 2.38 MPa.2

Figure 2.52. The diagram of the interaction force between sulfonated polyetheretherketone, SPEEK, polyurethane, PU, and sulfonated polyetheretherketone with ionic liquid polycation of 1-butyl-3-methylimidazolium, bmim+, in one layer of the layer-by-layer membrane. [Adapted, by permission, from Che, Q; Fan, H; Duan, X; Feng, F; Han, X, J. Mol. Liquids, 269, 666-74, 2018.]

An increase in the sulfonation degree increased the glass transition temperature and enhanced the thermal stability of the sulfonated polyetheretherketone membrane.3 The room-temperature ion conductivity of the homogeneously sulfonated PEEK membrane with 68% sulfonation degree was higher than that of Nafion® 117, while its methanol permeability was lower (0.5×10-6 vs. 1.55×10-6 cm2 s-1).3 The cell performance of the homogeneously sulfonated PEEK membrane was much better than that of the heterogeneously sulfonated PEEK and Nafion® 117 membranes.3 The degree of sulfonation of polyaryletherketones is important since polyaryletherketones having a very high degree of sulfonation are water-soluble whereas polyaryletherketones having a very low degree of sulfonation are poor ion conductors (preferable degree of sulfonation was 55-90%).4 Suitable crosslinking reagents were epoxide crosslinking agents, for example, the commercially available Denacole.4 Sulfonated polyetheretherketone nanocomposite film containing silsesquioxane exhibits excellent proton conductivity and mechanical strength.5 Sulfonated polyetheretherketone membrane having a sulfone group was mixed with a polyhedral oligomeric silsesquioxane having a sulfonic acid group.5 The proton conductive polymer

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membrane is electrically isolated but acts as a medium that transfers hydrogen ions from the anode to the cathode during cell operation and simultaneously separates liquid or gas fuel from the oxidant gas.5 The POSS used had a size of 1-2 nm, and thus hardly obstructed the migration of protons in the ion channel in the polymer membrane, thereby realizing excellent proton conductivity.5 References 1 2 3 4 5

Lu, DX; Kim, D, Solid State Ionics, 192, 1, 627-31, 2011. Che, Q; Fan, H; Duan, X; Feng, F; Han, X, J. Mol. Liquids, 269, 666-74, 2018. Do, KNT; Kim, D, J. Power Sources, 185, 1, 63-9, 2008. Mohwald, H; Fischer, A; Frambach, K; Hennig, I; Thate, S, US20070117958A1, BASF SE, May 24, 2007. Rhee, H; Kim, S; Youn, T, US20170200962A1, Sogang University Research Foundation, Jul. 13, 2017.

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2.2.57 SULFONATED POLYSULFONE Poly(ethylene glycol) crosslinked sulfonated polysulfone composite membranes were developed for forward osmosis.1 The crosslinked membrane had a hydrophilic skin with high hydrophilicity, a low contact angle of 15.5o, and a high water flux of 15.2 L m-2 h-1.1 Crosslinking decreased the water contact angle and the fouling trend.1 The water permeation flux decreased with increasing crosslinking density.1 A crosslinked composite membrane having a thin chitosan layer on microporous sulfonated polysulfone substrate was synthesized for application in fuel cells.2 The interaction between polysulfone support and chitosan was Figure 2.53. Reaction scheme. Ding, X; Liu, Z; Hua, M; Kang, T; improved by surface modificaLi, X; Zhang, Y, J. Appl. Polym. Sci., 133, 43941, 2016.] tion of polysulfone (sulfonation) (Figure 2.54).2 The composite had high ion exchange capacity and proton conductivity higher than Nafion 117 at a temperature above 30oC.2 A poor wettability of membrane substrate was improved by its immersion in sulfuric acid solution before casting the uppermost chitosan layer.2 Ion beam irradiation was used to modify the surface of a sulfonated polysulfone water treatment membrane.3 The sulfonic and C−H bonds were broken and new C−S bonds (crosslinking) were formed after irradiation.3 A significant increase in flux without changing selectivity after ion beam irradiation was observed.3 The hydrophobicity and the pore size distribution of the membrane were not affected by ion beam irradiation.3 The sulfonated polysulfone membrane was used for dehydrating water/ethanol mixture by pervaporation.4 Sulfonation improved the pervaporation performance of polysulfone membrane.4 A crosslinked microporous polysulfone or polysulfone copolymer battery electrode separator membrane was developed.5 These membranes without crosslinking are soluble above a particularly high temperature in the selected battery electrolyte systems.5 When even partially crosslinked, they swell in the electrolyte instead of dissolving.5 If the membrane separators are restrained between solid electrodes in a battery, the separator cannot increase in bulk volume, and the swelling occurs within the pores with the pore volume decreasing from its original bulk volume.5 The drop in pore volume causes the battery current density to drop, thereby reducing the heat generation within the hot area of the battery.5 The crosslinker was chosen from dicyandiamide, benzoyl peroxide, di-glycidyl ethers, and tri-glycidyl ethers.5 Batteries made with crosslinked separators are expected to

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Figure 2.54. Ionic interaction between chitosan and sulfonated polysulfone. [Adapted, by permission, from Smitha, B; Devi, DA; Sridhar, S, Int. J. Hydrogen Energy, 33, 15, 4138-46, 2008.]

provide a significant measure of safety against overheating and fires and to allow for continued use of batteries that experience only local overheating, by reducing the effects of local dendrites or particulate contaminants that may cause hot spots in the battery.5 References 1 2 3 4 5

Ding, X; Liu, Z; Hua, M; Kang, T; Li, X; Zhang, Y, J. Appl. Polym. Sci., 133, 43941, 2016. Smitha, B; Devi, DA; Sridhar, S, Int. J. Hydrogen Energy, 33, 15, 4138-46, 2008. Chennamsetty, R; Escobar, I; Xu, X, J. Membrane Sci., 280, 1-2, 253-60, 2006. Chen, S-H; Yu, K-C; Lin, S-S; Chang, D-J; Liou, RM, J. Membrane Sci., 183, 1, 29-36, 2001. Hauser, RL, US20170331095A1, Samsung Electronics Co Ltd, Nov. 16, 2017.

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2.3 PARAMETERS OF CROSSLINKING This section shortly reviews the effect of conditions under which crosslinking reactions are conducted on their outcomes. Parameters included in this discussion were already mentioned but here their influences on the properties of the crosslinked materials are generalized. The most essential parameters of crosslinking reaction include activation energy, concentration, conversion degree, glass transition temperature, melting point, radiation dose, solubility, temperature, thickness of a part, time, and viscosity. 2.3.1 ACTIVATION ENERGY The activation energy of the crosslinking process of ethylene-vinyl acetate initiated by dicumyl peroxide was in the range of 87-100 kJ mol-1 depending on conversion.1 At conversion 0.10.9, the temperature increases and the EVA crosslinking follows homogeneous kinetics.1 The activation energy values for EVA crosslinking with dicumyl peroxide were in the range of 91-107 kJ/mol, increasing with crosslinking degree.5 In bulk process such as the crosslinking of ethylene-vinyl acetate, the viscosity increased during the course of the reaction with a consequent decrease in the macromolecular radical mobility leading to the higher values of activation energy.5 Activation energies of bismaleimide crosslinking were ~100 kJ/mol for isothermal curing, ~92 kJ/mol when determined from gelation time and ~96 kJ/mol when calculated from DSC data.2 The low temperature crosslinking of protein by citric acid was pseudo-second-order reaction with activation energy of ~25 kJ/mol, indicating easy occurrence of the reaction.3 Activation energy of radical crosslinking of polydimethylsiloxane the presence of 2,2,6,6-tetramethylpiperidinyloxyl nitroxide, derived from the anisothermal DSC results according to the Kissinger method, was 87 kJ/mol.4 Addition of graphene decreased the activation energy of the chemical reaction controlling stage, while it had an opposite effect on the activation energy of the diffusion controlling stage.6 2.3.2 CONCENTRATION OF CROSSLINKER Artificial extracellular matrix protein was crosslinked with hexamethylene diisocyanate in dimethylsulfoxide.7 The hydrogel films were transparent, uniform, and highly extensible.7 The increase in the crosslinker concentration caused stiffening the film and reduced elongation.7 The increase in peroxide (bis(t-butyl peroxy-isopropyl) benzene) and co-agent (triallyl isocyanurate) concentrations in peroxide-cured ethylene propylene diene rubber, EPDM, increased the total crosslink density.8 The incorporation of triallyl isocyanurate into the network enhanced the network heterogeneity.8 The glass transition temperature shifted towards higher temperature, and the peak values of the loss factor gradually decreased as a result of the restricted segmental mobility caused by the increase in crosslink density.8 Photoactuating liquid crystalline elastomers were crosslinked by azo crosslinker (1,3,3,10,30,30-hexamethyl-11-chloro-10,12-propylene-tricarbocyanine iodide).9 The

2.3.3 Conversion degree

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crosslinker concentration increase caused an increase in actuation properties up to a certain point from which the actuation was decreased by a high stiffness.9 Aminated starch coated iron oxide magnetic nanoparticles loaded with curcumin were crosslinked by glutaraldehyde, genipin and citric acid.10 Crosslinking enhanced drug loading and encapsulation efficiency.10 Both drug loading and encapsulation efficiency increased with the increase in genipin (1-5%; the best crosslinker) concentration.10 Polydimethylsiloxane was crosslinked with quantum dot nanocrystals.11 The higher the concentration of crosslinkers, the faster the formation of the network, and the greater the likelihood of preventing the particles from segregating.11 Polyurethane based pressure-sensitive adhesives were studied for the effect of the concentration of crosslinkers (methylene diphenyl di-isocynate, isophorone di-isocynate, and triallyl isocynurate).12 With increasing dose of e-beam irradiation and concentration of crosslinker, all the adhesion properties, such as peel adhesion, shear adhesion strength, and initial tack were increased and reached a plateau followed by a decline with the further increase of the dose and the crosslinkers concentration.12 2.3.3 CONVERSION DEGREE The activation energy varies with changing temperature and conversion.13 The conversion dependence of the effective activation energy of the process permits understanding the process kinetics without knowing the real rates of the reaction steps.13 For a single step process, activation energy is independent of the degree of conversion, but in the multi-step processes, the activation energy depends on the degree of conversion.13 The rate of conversion slows down significantly at 60-70% conversion as the mean separation between reactive sites increases and molecular mobility decreases due to the formation of the network.14 As the viscosity of the system increases to a very high level, the diffusion ability of crosslinkers is also restricted, which causes a decrease in the conversion rate.15 A thermoset polymer with a higher conversion degree has a lower cohesive energy density and solubility parameter.16 The cohesive energy density shows a linearly decreasing relationship with the crosslink density.16 The cohesive energy density decreases with increasing conversion degree in the beginning with a modest rate but dramatically at around 60% conversion, which is about the gelation point.16 2.3.4 GLASS TRANSITION TEMPERATURE Chemical crosslinking of polyurea spray coatings by long-chain trifunctional amine led to the restraining of the segmental motions which resulted in increased glass transition temperature.17 The transition from glass to rubber occurs over a wide temperature range (ΔT>80°C) − a property unique to polyurea.17 Crosslinking of polyamide-6 initiated by the proton beam irradiation caused a decrease of both lamellar thickness and crystalline portion, and an increase of glass transition temperature.18 The presence of crosslinking coagent triallyl cyanurate resulted in significant increase of crosslinked portion at lower irradiation doses and the irradiation resulted in a significant increase of glass transition temperature with rising absorbed dose.18

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The epoxy resin (diglycidyl ether of bisphenol A) was crosslinked with polyetheramine. The glass transition temperature continued to increase with consumption of epoxy functionalities.19 The glass transition temperature and elastic properties of crosslinked epoxy systems increased with increasing levels of crosslink density, and the thermal expansion coefficient decreased with crosslink density, both above and below the glass transition temperature.20 The increased crosslink densities (from 318 to 6028 mol/m3) of thermoset obtained by adding norbornenyl-functionalized castor oil alcohol caused a linear increase in glass transition temperature from -17.1 to 65.4°C.21 The thermosets with higher crosslink densities had better thermal stabilities, and the mechanical properties, namely Young’s modulus, tensile strength, and toughness, were also improved.21 A considerable increase in glass transition temperature upon e-beam irradiation of unplasticized polyvinylchloride in the presence of trimethylolpropane triacrylate was recorded.22 The irradiation of unplasticized polyvinylchloride alone showed a negligible effect on its glass transition temperature.22 2.3.5 MELTING TEMPERATURE The melting point dropped by ~10°C after crosslinking because the crystalline domains were interfered and suppressed by the crosslinks.23 The melting temperature of polyamide-6 decreased with increasing the dose of ebeam radiation and the triallyl cyanurate level.24 Upon irradiation up to 150 kGy, the crystalline region in the pure polyamide-6 samples without the triallyl cyanurate remained intact (the melting temperature did not show a significant change with dose increase).24 The samples containing triallyl cyanurate showed a reduction of the melting point at 40 kGy and then exhibited only a slight change with a further increase in the absorbed doses.24 The melting temperature depression may be attributed to the reduction of crystal size.24 The irradiation below the melting point caused crosslinking reactions in the amorphous part of the polymer.24 The melting temperature of the crosslinked polytetrafluoroethylene decreased with the increase in radiation dose.25 The crystallization of the crosslinking part was disrupted, resulting in a low degree of crystallinity and small crystals.25 2.3.6 RADIATION DOSE Sorbic acid was used as a crosslinker together with gamma radiation to improve the properties of ethylene propylene diene monomer rubber, EPDM.26 Tensile strength was increased consistently with the content of sorbic acid up 10 phr and irradiation dose 100 kGy.26 Tensile and impact strengths of polylactide/trimethylolpropane trimethacrylate samples decreased with increasing e-beam radiation dose while both were enhanced when polylactide/triallyl isocyanurate samples were irradiated.27 The lifetime of prostheses utilizing ultrahigh-molecular-weight polyethylene has been determined by their wear resistance.28 It has been confirmed that radiation crosslinking of ultrahigh-molecular-weight polyethylene can substantially increase its wear resistance.28 It is also well recognized that there is a radiation-dose-dependent decrease in several important mechanical properties of ultrahigh-molecular-weight polyethylene, such

2.3.7 Temperature

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as fracture toughness and resistance to fatigue crack propagation.28 It was found that the use of an alumina counterface would circumvent the need to use a high radiation dose without compromising wear resistance.28 Study of e-beam irradiation of styrene-butadiene rubber showed that crosslinking predominated over chain scission, and constituted approx. 81% of the reactions causing an increase in the crosslink density along with the increase of dose ranging from 25 to 200 kGy.29 In styrenic block copolymers, the crosslinking reaction was found to be the predominant reaction at radiation dose up to 240 kGy.30 At higher radiation doses (>190 kGy), the chain scission reaction took place and the lower molecular weight polystyrene and polydiene chains were formed.30 2.3.7 TEMPERATURE Di-tert-butyl peroxide was selected for dynamic vulcanization of styrene-ethylene-butadiene-styrene block copolymer/polypropylene blends because of its higher decomposition temperature than the conventional dicumyl peroxide.31 The crosslinking process starts at temperatures between 120 and 130oC.32 It is initiated by decomposition of peroxides participating in a radical crosslinking reaction.32 The temperature is a significant parameter to control the degree of crosslinking.32 To avoid a premature crosslinking during film processing, the curing agent has to be dispersed at relatively low temperatures.32 The crosslinking temperature of gelatin hydrogel by oxidized carboxymethylcellulose influenced equilibrium swelling.33 The higher the temperature the higher the volume of the swollen gel.33 When the cure temperature of the chlorinated polyethylene vulcanizate was 140°C, the chlorinated polyethylene foam had relatively small sized cells, fewer in number and widely dispersed in the matrix indicating poor foaming of the matrix.34 With the cure temperature of the chlorinated polyethylene increased to 150°C, the average cell of these foams gradually increased as did uniformity of cell distribution.34 Permeabilities of crosslinked polyimides gradually increased with the crosslinking temperature increasing.35 The polyimides completed their crosslinking reactions at a temperature reaching 425°C.35 Polydimethylsiloxane gas permeable membranes were synthesized at different crosslinking temperatures.36 The crosslinking temperature altered the polymeric structure of PDMS membrane, resulting in different permeability.36 The maximum gas permeability was achieved with crosslinking at a temperature of 75°C.36 The properties of acrylic latex coatings crosslinked with cycloaliphatic diepoxide, such as water resistance, solvent resistance, pencil hardness, and pull-off adhesion improved with the crosslinking temperature, time, and the amount of the crosslinker.37 2.3.8 THICKNESS OF A PART The rate of crosslinking by UV of aliphatic polycarbonates functionalized with coumarin decreased when the thickness of the film was increased.38 Nanocomposite materials made of a multifunctional acrylic resin containing graphite, oxidized graphite or acetylene black were crosslinked by UV irradiation.39 The carbon particles were screening the UV-radiation depending on sample thickness and the filler

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content.39 A redox initiator consisting of benzoyl peroxide and a tertiary amine was used to achieve a deep through-cure of thick samples.39 UV-curable polymers (e.g., acrylates) are commonly used in the stereolithography.42 The mechanical properties of the 3D-printed product were affected by the intrinsic material heterogeneity along the sample thickness.42 The thickness of sample may also affect properties due to the diffusion process of oxygen.40 The diffusion control of oxidation led to the development of a profile of Young's modulus within the sample thickness of thermally degraded poly(ether imide).40 The Young’s modulus was increased in oxidized (and crosslinked) surface layers.40 Crosslinking rate depended on the thickness of high-density polyethylene sheets after γ-ray irradiation in the presence of air.41 The polymer chain crosslinking rate depended on the sample thickness when the material was irradiated with doses of up to 200 kGy.41 For irradiation doses above 600 kGy, the polymer chain scission was the dominant phenomenon which was independent of the sample thickness.41 The sample thickness influenced the uniformity of heating and vulcanization during compression molding.43 The changes of the material properties in the natural rubber compound were detected by the ultrasonic online control.43 The time to reach a sound velocity comparable to the final value after cure increased from 15 s for a 1 mm thick sample to more than 45 s for a 3 mm thick sample.43 Irradiation of polyethylene generates P. radicals within the whole thickness of sam44 . ple. P radicals are scavenged by O2 to yield POO. radicals in the surface layer.44 P. radicals are present in bulk and can either combine to yield crosslinking (which is used to obtain a wear resistant UHMWPE) or react with Vitamin E (it can react with both types of radicals).44 Vitamin E induced a lower level of crosslink density.44 The 2,6-di-tert-butylphenol (another antioxidant) reacted only with POO. radicals.44 Cyanine dye-borate complex, 1,3,3,1′,3′,3′-hexamethyl-11-chloro-10,12-propylenetricarbocyanine triphenylbutyl borate, was employed as the photoinitiator, and a nearinfrared laser diode emitting at 796 nm was used as the irradiation source of photopolymerization of acrylate monomers.45 The double bond conversion decreased dramatically when the sample thickness increased from 20 to 60 μm due to the significant filter effect.45 2.3.9 TIME The crosslinking reaction time of dynamic vulcanization of SEBS/PP blends decreased with the increasing loading of trimethylolpropane trimethacrylate (crosslinker).46 The thickness and crosslinking density of the surface crosslinked layer of polyvinylalcohol were highly dependent on the surface crosslinking conditions, i.e., exposure time and glutaraldehyde concentration.47 The initial burst release of proxyphylline observed for untreated PVA hydrogels was almost entirely eliminated from the release profile of surface crosslinked PVA hydrogels.47 The photo-crosslinking of coumarin moieties was measured at different exposure times.48 When irradiation time was increased, the maximum absorption intensity of coumarin groups observed at λ = 321 nm decreased, indicating formation of cyclobutane ring formed in photocycloaddition reaction.48 Surface morphology of modified polyethylene depended on the plasma discharge power and exposure time.49 Maximum microhardness and elastic modulus were achieved

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after 240 s exposure which was sufficient for PE crosslinking initiated by plasma discharge.49 2.3.10 VISCOSITY Viscosity measurement is frequently used to quantify progress of crosslinking reaction since crosslinking causes viscosity increase. At the same time, viscosity increases as the reaction progresses reducing the mobility of molecules which participate in the crosslinking reactions and slows down reaction rate. Please refer to Section 2.3.3 which gives some examples. The gel time of liquid rubber-modified epoxy mixtures increased with the increase in the rubber concentration.50 This delay was attributed to a lower reactivity of the modified epoxy produced as a result of chain extension and an increase in viscosity of the medium due to the addition of rubber.50 The radical crosslinking process of poly(ethylene-octene) copolymer has been conducted using dicumyl peroxide trapped in fumed silica.51 The formation of a less perfect network (the elastic modulus was 1.5 times lower) in comparison with the crosslinking reaction under homogeneous conditions.51 This may suggest that crosslinking at higher conversion rates (more obstructions to diffusion of crosslinker) may also produce less perfect network.51 Low concentrations of polymer favor intramolecular crosslinking leading to decrease in polymer viscosity.52 Higher polymer concentrations promote intermolecular crosslinking and gel rigidity.52 References 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29

Bianchi, O; Martins, JDN; Fiorio, R; Oliveira, RVB; Canto, LB, Polym. Testing, 30, 6, 616-24, 2011. Park, JO; Yoon, B-J; Srinivasarao, M, J. Non-Newtonian Fluid Mech., 166, 16, 925-31, 2011. Xu, H; Shen, L; Xu, L; Yang, Y, Ind. Crops Prod., 74, 234-40, 2015. Mani, S; Cassagnau, P; Bousmina, M; Chaumont, P, Polymer, 51, 17, 3918-25, 2010. Bianchi, O; R. Oliveira, RVB; Fiorio, R; Martins, JDN; Canto, LB, Polym. Testing, 27, 6, 722-9, 2008. Wu, J; Xing, W; Huang, G; Li, H; Liu, Y, Polymer, 54, 13, 3314-23, 2013. Nowatzki, PJ; Tirrell, DA, Biomaterials, 25, 7-8, 1261-7, 2004. Wang, H; Zhao, S-G; Wrana, C, J. Macromol. Sci., Part B Phys., 56, 1, 39-52, 2017. Braun, LB; Linder, TG; Hessberger, T; Zentel, R, Polymers, 8, 435, 2016. Saikia, C; Das, MK; Ramteke, A; Maji, TK, Int. J. Biol. Macromol., 93A, 1121-32, 2016. Shojaei-Zadeh, S; Morris, JF; Couzis, A; Maldarelli, C, J. Colloid Interface Sci., 363, 1, 25-33, 2011. Singh, AK; Mehra, DS; Niyogi, UK; Sabharwal, S; Khandal, RK, Int. J. Adhesion Adhesives, 41, 73-9, 2013. Giménez, V; Reina, JA; Mantecón, A; Cádiz, V, Polymer, 40, 10, 2759-67, 1999. Li, C; Alejandro Strachan, A, Polymer, 51, 25, 6058-70, 2010. Wu, J; Zhao, Z; Hamel, CM; Mu, X; Qi, HJ, J. Mech. Phys. Solids, 112, 25-49, 2018. Li, C; Strachan, A, Polymer, 135, 162-70, 2018. Iqbal, N; Tripathi, M; Parthasarathy, S; Kumar, D; Roy, PK, Prog. Org. Coat., 123, 201-8, 2018. Porubská, M; Szöllös, O; Janigová, I; Jomová, K; Chodák, I, Radiat. Phys. Chem., 133, 52-7, 2017. Gauthier, C; Galy, J; El-Kettani, MEC; Leduc, D; Izbicki, J-L, Int. J. Adhesion Adhesives, 80, 1-6, 2018. Bandyopadhyay, A; Valavala, PK; Clancy, TC; Wise, KE; Odegard, GM, Polymer, 52, 11, 2445-52, 2011. Xia, Y; Larock, RC, Polymer, 51, 12, 2508-14, 2010. Ratnam, CT; Nasir, M; Baharin, A, Polym. Testing, 20, 5, 485-90, 2001. Cai, L; Wang, S, Biomaterials, 31, 29, 7423-34, 2010. Dadbin, S; Frounchi, M; Goudarzi, D, Polym. Deg. Stab., 89, 3, 436-41, 2005. Tang, Z; Wang, M; Tian, F; Xu, L; Wu, G, Eur. Polym. J., 59, 156-60, 2014. El-Nemr, KF; Mohamed, RM, J. Macromol. Sci. Part A Pure Appl. Chem., 54, 10, 711-9, 2017. Rytlewski, P; Malinowski, R; Moraczewski, K; Żenkiewicz, M, Radiat. Phys. Chem., 79, 10, 1052-7, 2010. Bistolfi, A; Bellare, A; Acta Biomaterialia, 7, 9, 3398-403, 2011. Bandzierz, KS; Reuvekamp, LAEM; Przybytniak, G; K. Dierkes, WK; Bieliński, DM, Radiat. Phys.

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Chem., 149, 14-25, 2018. Wu, J; Soucek, MD; Cakmak, M, Prog. Org. Coat., 100, 141-52, 2016. Wu, Y; Shentu, B; Weng, Z, J. Appl. Polym. Sci., 134, 44392, 2017. Oreski, G; Rauschenbach, A; Hirschl, C; Kraft, M; Eder, GC; Pinter, G, J. Appl. Polym. Sci., 134, 44912, 2017. Joy, J; Gupta, A; Jahnavi, S; Verma, RS; Rayb, AR; Gupta, B, Polym. Int., 65, 181-91, 2016. Lang, X-h; Wang, D; Prakashan, K; Zhang, X; Zhang, Z-X, J. Polym. Res., 24, 175, 2017. Zhang, C; Li, P; Cao, B, J. Membrane Sci., 528, 206-16, 2017. Berean, K; Ou, JZ; Nour, M; Latham, K; Kalantar-Zadeh, K, Separation Purification Technol., 122, 96-104, 2014. Wu, S; Soucek, MD, Polymer, 41, 6, 2017-28, 2000. Chesterman, JP; Hughes, TC; Amsden, BG, Eur. Polym. J., 105, 186-93, 2018. Salmi, A; Benfarhi, S; Donnet, JB; Decker, C, Eur. Polym. J., 42, 9, 1966-74, 2006. Courvoisier, E; Bicaba, Y; Colin, X, Polym. Deg. Stab., 151, 65-79, 2018. Shaban, AM; Kinawy, N, Polymer, 36, 25, 4767-70, 1995. Anastasio, R; Maassen, EEL; Cardinaels, R; Peters, GWM; van Breemen, LCA, Polymer, 150, 84-94, 2018. Jaunich, M; Stark, W, Polym. Testing, 28, 8, 901-6, 2009. Richaud, E, Radiat. Phys. Chem., 103, 158-66, 2014. Zhang, S; Li, B; Tang, L; Wang, X; Zhou, Q, Polymer, 42, 18, 7575-82, 2001. Wu, Y; Shentu, B; Weng, Z, J. Appl. Polym. Sci., 134, 44392, 2017. Wu, L; Brazel, CS, Int. J. Pharm., 349, 1-2, 144-51, 2008. Rahimi, S; Khoee, S; Ghandi, M, Carbohydrate Polym., 201, 236-45, 2018. Švorčík, V; Kotál, V; Slepička, P; Bláhová, O; Hnatowicz, V, Nucl. Instruments Methods Phys. Res. Section B: Beam Interactions Mater. Atoms, 244, 2, 365-72, 2006. Ratna, D, Polymer, 42, 9, 4209-18, 2001. Akbar, S; Beyou, E; Chaumont, P; Cassagnau, P, Mater. Chem. Phys., 117, 2-3, 482-8, 2009. Shu, P; Schmitt, KD, Colloids Surf. A: Physicochem. Eng. Aspects, 110, 3, 273-85, 1996.

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2.4 EFFECT OF CROSSLINKERS ON PROPERTIES The discussion in this chapter summarizes the main changes of properties of materials to provide expectation guidelines. More than 30 physical-mechanical properties are discussed in the alphabetical order below. 2.4.1 ADHESION The acrylic pressure-sensitive adhesives were crosslinked with isocyanate.1 When the content of isocyanate increased, the peel strength and tack of the synthesized PSAs decreased.1 Also, the maximum stress and shear strain decreased with an increase in the crosslinking agent concentration.1 Figure 2.55 generalizes the effect of crosslinking on cohesion, adhesion, and tack in pressureFigure 2.55. Crosslink density effect on tack, peel adhesion, and sensitive adhesives.5 cohesion. [Adapted, by permission, from Czech, Z, Int. J. Adhesion A photo-responsive bioAdhesives, 27, 1, 49-58, 2007.] inspired adhesive contained poly(sulfobetaine methacrylate), 3,4-dihydroxyphenylalanine (a major component of the adhesion properties in mussels), and a photocleavable nitrobenzyloxycarbonyl crosslinker.2 Photocleavage of the o-nitrobenzyl ester occurs rapidly for 30 minutes, then gradually continues for another 3 hours (the same performance was observed in the adhesive).2 Crosslinks impose spatial constraints on the polymer segmental dynamics by introduced covalent bonds.3 Properties of epoxy nanocomposites, such as adhesion characteristics depend on the crosslink ratio.3 Waterborne polyurethanes with different crosslinking densities were synthesized by varying the amount of trifunctional crosslinker trimethylolpropane.4 The interface adhesion strength between polyurethane ink and the substrate (poly(ethylene glycol terephthalate) film) showed a decrease in the interface adhesion strength due to the decreased mobility of PU molecular chains.4 T-peel strength initially increased (with crosslink density increase) and then decreased accompanied by change of debonding failure from a cohesive failure to an interface failure.4 Control of the degree of elastomer crosslinking in the layer bordering with the substrate was studied.8 The maximum adhesion strength could be achieved with the elastomer crosslinked incompletely.8 Such a situation corresponds to a transient stage between the viscous-flow (plastic) and highly elastic states, irrespective of the crosslinking mode.8 Formation of silane bridges between wood flour particles and polyethylene improved the interfacial adhesion.6 The composite was produced by a reactive extrusion process in which vinyl-trimethoxy silane and dicumyl peroxide were used to produce crosslinks.6 The organic functional groups on silica surfaces modified by silane surface treatment led to an increase of the adhesion at interfaces between silica and the rubber matrix.7 The

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increased crosslink density improved mechanical properties, mainly due to increased adhesion at interfaces between silica and rubber matrix.7 When a bead of moisture-cured alkoxysilicone sealant having the diameter of 5-50 mm is placed on a sheet of glass, allowed to cure in ambient air, and then scraped from the glass with a metal edge, two zones of adhesive behavior can be seen.9 In the outer zone, the failure is cohesive, but in the center, there is an interfacial failure between sealant and glass.9 The same effect also occurs on copper, brass and mill finished aluminum, but not on anodized aluminum.9 This behavior is known as “channel adhesion”.9 The cure process involves water diffusing into the sealant from the outer faces, to reach reactive groups in the polydimethylsiloxane and the crosslinking agents/adhesion promoters.9 However, once the adhesion promoters are consumed there will be a flux of the crosslinking agents/ adhesion promoters in the opposite direction to replace them.9 This flux meets the water molecules and reacts with them.9 Another consequence is that the concentration of crosslinking agents/adhesion promoters will be greater towards exposed surfaces.9 The “channel adhesion” is caused by these diffusion processes and resulting distribution of adhesion promoter.9 Typical organosilanes which are used as adhesion promoters or crosslinkers are mercaptosilanes or aminosilanes.10 However, these have disadvantages.10 Mercaptosilanes have an unpleasant odor and, with isocyanates, form thiourethanes that are not very thermally stable and are readily redissociatable at elevated temperature.10 Aminosilanes are basic and very reactive, which limits their use as adhesion promoters.10 The hydroxysilane has a long shelf-life and exhibits an excellent action as an adhesion promoter and/or crosslinking agent.10 2.4.2 ANTIBACTERIAL PROPERTIES The acrylic acid-grafted and chitosan/collagen-immobilized polypropylene nonwoven fabric was produced.11 The antibacterial properties of the wound dressing fabrics were excellent.11 Phenolic compounds, present in many fruits and vegetables, are known group of secondary metabolites with a wide range of pharmacological activities.12 Cotton fabrics were treated with several phenolic compounds.12 The treatment was conducted in two steps; a crosslinker was incorporated onto cotton cellulose, and the phenolic compound was bound to the crosslinker already anchored onto the cotton fabrics.12 The treated cotton fabrics had >99.9% antibacterial ability and >80% antioxidant ability, even at low concentrations of crosslinker and phenolic compounds.12 The application of chitosan fiber for healthy and hygienic textiles is limited by its poor acid resistance and poor antioxidant activity.13 Chitosan fiber with good acid resistance and high antioxidant activity can be prepared by crosslinking with a water-soluble aziridine and dyeing with natural lac dye consisting of polyphenolic anthraquinone compounds.13 The crosslinked fiber exhibited greatly reduced weight loss in acidic solution, and possessed excellent acid resistance.13 Crosslinking and dyeing had no impact on the good inherent antibacterial activity of chitosan fiber.13 The highly-porous structures made of hyaluronic acid were modified with bioactive compounds (nanoadditives and antibiotics).14 Introduction of a small amount of zinc and zinc oxide nanoadditives to the hyaluronic acid matrix (3 phr) resulted in material with strong antimicrobial properties.14 Hyaluronic acid provided favorable conditions for

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wound healing and treatment. A small dose of antibiotic from the cephalosporin group did not result in increased antibacterial activity.14 Keratin fibers (wool) were modified by thiol-ene click reaction.15 First, tris(2-carboxyethyl) phosphine hydrochloride was applied on wool samples to generate thiols by controlled reduction of cystine disulfide bonds in keratin followed by grafting thiol-acrylate quaternary ammonium salt with divinyl terminated by click chemistry combining with a step-growth dithiol-diacrylate reaction.15 The treatment resulted in a wool fabric with good antibacterial, antistatic properties, and hydrophilicity.15 Polycarbonate films were irradiated by Fe heavy ions resulting in a decrease of crystallinity and molecular weight.16 Modification in their surfaces prevented biofilm formation of the human pathogen, Salmonella typhi.16 The silane-based interfacial crosslinking technique was developed to endow polyvinylidenefluoride membrane with superwettability, excellent protein desorption, and antibacterial properties.17 Antibacterial properties are of great significance for application in medical and drinking water treatment membranes.17 2.4.3 BIOCOMPATIBILITY Gelatin hydrogel was crosslinked by oxidized carboxymethylcellulose for use in the biomedical field.18 The crosslinked hydrogel generated favorable conditions for cells to adhere and proliferate.18 Bioprosthetic heart valves derived from glutaraldehyde-treated xenogenetic tissues undergo structural degeneration and calcification sometimes even in less than ten years.19 The radical polymerization-crosslinking was developed to improve extracellular matrix stability, prevent calcification, and reduce inflammatory response in bioprosthetic heart valves.19 The porcine pericardium tissue was decellularized, functionalized with methacryloyl groups, and subsequently crosslinked by radical polymerization which improved valve biocompatibility.19 Cellulose nanocrystals/hydroxyapatite composite has been investigated as a bone substitute.20 The nanocomposite was crosslinked and then freeze-casted into a porous scaffold.20 Hydroxyapatite helped to reduce the denaturation of protein in the surroundings.20 Crosslinking with poly(methyl vinyl ether-alt-maleic acid) and poly(ethylene glycol) enhanced water stability and mechanical properties of the composite.20 Collagen scaffolds are frequently crosslinked with 1-ethyl-3-(3-dimethylaminopropyl-carbodiimide hydrochloride) in the presence of N-hydroxy-succinimide.21 Carbodiimide crosslinking of collagen inhibited native-like, while increasing non-native-like, cellular interactions.21 Most biodegradable synthetic materials, including polyester-based polyurethanes, consist of rigid polyester chains and have high crystallinity, lacking the elasticity of most human tissues.22 The controlled crosslinking of poly(ester ether) triblock copolymer diols and polycaprolactone triols by urethane linkages produced three-dimensional porous scaffolds with remarkable elasticity and cyclical properties.22 The long chain crosslinking of polycaprolactone triol hindered crystallization, making the polyurethane to become an amorphous elastic and biocompatible material.22 Much other research works can be quoted which show that the controlled crosslinking have been effectively used for the synthesis of biocompatible materials.

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2.4.4 CELL SIZE The density of the poly(styrene-b-butadiene-b-styrene) triblock copolymer/polystyrene/), poly(styrene-b-butadiene) diblock copolymer foams increased with increasing the content of dicumyl peroxide.23 The high density of polymeric foams improved mechanical properties such as hardness, shrinkage, tensile strength, tear strength, elongation at break, and compression set.23 Microcellular chlorinated polyethylene foams were prepared using nitrogen as blowing agent.24 There was a minimum crosslink density required for effective foaming. When the crosslink density was decreased beyond the required crosslink density, the average cell size decreased, and cell density increased.24 Expansion of the foams reduced their mechanical properties whereas an increase in crosslink density of the matrix improved the mechanical properties.24 The presence of carbon black caused a decrease in the average cell size and increase in cell density.24 Biobased crosslinker 3-hydroxy-N,N-bis(2-hydroxyethyl)butanamide, HBHPA, was compared with diethanolamine regarding the properties of polyurethane foam.25 The HBHBA crosslinker produced hard domains having better order.25 It acted as a chain extender because of the presence of a low reactivity secondary hydroxyl, reducing the crosslink density of the foam and favoring larger cell sizes.25 Microcellular polymethylvinylsiloxane/carbon nanotubes foam was prepared by supercritical CO2.26 Carbon nanotubes were uniformly dispersed because of their interaction with molecular chains.26 Nanotubes acting as heterogeneous nucleating agent decreased the nucleation energy barrier, resulting in the uniform cell morphology of the foam having smaller cell size and higher cell density.26 The heterogeneous nucleation effect of nanographite improved cell morphology of silicone rubber foams, producing foams having the highest cell density, and the smallest cell sizes (decreased average cell diameter from 4.97 to 1.12 μm and an increased cell density from 8.87×108 to 1.25×1010 cells/cm3). The peak of the cell size distribution curve shifted to the smaller cell size and the cell size distribution became narrower with increasing nanographite content (Figure 2.56).

Figure 2.56. Cell morphology of silicone rubber foam saturated at 50°C, 2 h containing different nanographite contents. [Adapted, by permission, from Bai, J; Liao, X; Huang, E; Luo, Y; Li, G, Mater. Design, 133, 288-98, 2017.]

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The epoxide-based chain-extender in poly(lactic acid) foams produced by chemical foaming extrusion with 4 wt% chemical foaming agent decreased the void fraction, the cell size, and the open cell content, and caused an increase in the cell density.27 The tensile mechanical properties such as yield stress and elongation at break of cellular materials increased.27 Rigid polyurethane foam is widely used in appliance and building industries due to its excellent thermal insulation.28 The rigid PU foam had a number average cell size of less than 10 μm, a crosslink density from 1.0 to 3.0, and a weight average molecular weight between crosslinks from 300 to 900.28 The foam was produced by expansion of carbon dioxide.28 Minimizing the number of collisions between gas molecules in the cells can effectively reduce gas conductivity without the use of HCFC, HFC, HFOs, or hydrocarbons which can be achieved by selection of the size of the cells of the rigid PU foam to be close to or smaller than the mean free path of gas molecules between collisions.28 This is known as the “Knudsen effect” and can be achieved either by reducing the size of the cells, by reducing the gas pressure inside the cells, or both.28 2.4.5 COMPRESSION SET The compression set of the vulcanized carbon black reinforced EPDM solely depended on overall crosslink density, resulting from selection of crosslinking system and vulcanization time.29 The crosslink distribution: the ratios of mono- to di- and polysulfidic crosslinks, had only a minor effect on compression set.29 The efficiently cured compound had a relatively low crosslink density and a high amount of monosulfidic bridges, resulting in high tear strength, elongation at break, good compression set, and low tensile strength.29 Compression set is a measure of the ability of a given material to recover after being compressed for a specific time at a specified temperature.30 The higher the degree of crosslinking, the lower the value of the compression set.30 Vinyltrimethoxysilane concentration and dicumyl peroxide affected compression set of ethylene propylene diene monomer/polypropylene thermoplastic vulcanizates.30 With increased addition of vinyltrimethoxysilane the crosslink density increased, and compression set decreased.30 The 4,40-bismaleimidodiphenylmethane was used as a crosslinking agent to produce thermally stable brominated isobutylene-isoprene rubber with a high crosslinking density.31 Zinc oxide significantly accelerated crosslinking reaction rate.31 The brominated isobutylene-isoprene rubber exhibited a high crosslink density with a low compression set at elevated temperatures and an excellent thermal stability.31 By increasing crosslink density, hardness, and modulus of nitrile rubber vulcanizates, lower compression set, and higher tensile strength were achieved.32 The nitrile rubber containing a carboxyl group was crosslinked with diurethane compound to lower compression set in the case of hose production.34 A two-component moisture curable system was used for producing a crosslinked silylated polymer having improved compression set.33 The process of crosslinking ethylene-propylene and ethylene-propylene-diene terpolymer elastomers typically uses either sulfur or peroxides.35 Peroxide crosslinking generally provides better thermal properties because of the higher thermal stability of the formed C−C bonds versus S−S bonds but has one significant disadvantage: oxygen inhibition leaves a tacky surface.35 The way to improve the crosslinking is by the use of both sulfur and peroxide as crosslinking aids which is called hybrid cure that leads to better tear

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strength compared to peroxide cure and better temperature resistance compared to sulfur cure.35 The sulfur crosslinking was improved using two accelerators: n-cyclohexyl-2-benzothiazole sulfenamide and zinc dibenzyldithiocarbamate.35 2.4.6 COMPRESSIVE STRENGTH Fiber crosslinking with polycarboxylic acids can be used to improve certain properties of paper products, including wet tensile and compressive strength.36 Under cyclic humidity and static compressive loading, citric acid-treated corrugated boxes showed a greater than three-fold increase in resistance to compressive creep.36 Carboxymethyl cellulose-reinforced polyvinylalcohol was prepared with trimethylol melamine as a chemical crosslinker.37 The compression strength of the modified hydrogels was 15 times larger than that of the polyvinylalcohol hydrogels.37 The unsaturated polyester resin was used as a crosslinker for polymethylmethacrylate used as a binder in mortars.38 The compressive strength of the mortar increased with the increase of the unsaturated polyester resin content up to a maximum of 5%.38 Hydrazide-modified hyaluronic acid reacted with diketone to produce a hydrogel.39 Compressive strength and elastic modulus increased with the degree of crosslinking.39 The hydrogels are expected to find biomedical applications such as cell delivery and tissue regeneration.39 Temporary dental cement compositions are a two-part system.40 The first part includes an amine activator and zinc oxide reactive filler.40 The second part consists of a polymerization initiator (e.g., benzoyl peroxide) and a polymerizable component (acrylate or methacrylate group at one end and a carboxylic acid group at another end).40 A temporary cement composition has sufficient compressive strength to provide adequate strength to facilitate normal use of the teeth while the provisional appliance is in use, as well as facilitates easy removal.40 The silicone composition contains an organocyclosiloxane; an organosilane having two crosslinking groups; an organosilicone resin having two crosslinking groups.41 The silicone composition was used to impart moisture resistance and improve its compression strength.41 2.4.7 CONTACT ANGLE AND SURFACE ENERGY Fluorinated polysiloxanes having different grafting densities of fluorinated groups were synthesized via hydrosilylation reaction between polymethylhydrosiloxane and tridecafluorooctyl methacrylate which was followed by the introduction of vinyltriethoxysilane.42 The resultant polymer underwent self-crosslinking to generate icephobic coatings.42 Supercooled water droplets easily slipped away from the tilted coating surface at -15°C due to a high receding contact angle.42 In situ crosslinking of surface-initiated ring-opening metathesis polymerization of polynorbornene caused an increase in contact angle hysteresis when compared to noncrosslinked coatings.43 The decrease in the contact angle was attributed to the introduction of hydrophilic poly(ethylene glycol) chains in crosslinker.43 The crosslinking treatment decreased hydrophilicity of aromatic polyamide reverse osmosis membrane by introducing methylene groups to membrane surface.44 With increasing amount of crosslinker molecules, the hydrolysis of unconnected functional groups of crosslinking agent produced more polar groups and increased membrane hydro-

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philicity.44 The highly crosslinked membranes showed higher salt rejections and lower water fluxes.44 A thermal crosslinking process was used for hydrophilic surface modification of polyethersulfone porous membranes.45 Difunctional poly(ethylene glycol) diacrylate was used as the main crosslinking modifier with the addition of trifunctional trimethylolpropane trimethylacrylate which accelerated the crosslinking progress.45 The original contact angle of 89o was reduced to 6o with mass gain of 345.6 μg/cm2.45 When a moderate mass gain was obtained (about 150 μg/cm2), both the permeability and antifouling ability of PES membrane were optimized.45 2.4.8 CROSSLINK DENSITY Here are a few examples demonstrating the effect of crosslink density on properties of crosslinked materials. The epoxy foams based on epoxy/2-ethyl-4-methylimidazol system were prepared with different concentrations of curing agent.46 The reduced crosslink density improved foamability of epoxy resin.46 The microcellular epoxy foams have been obtained with a moderate crosslink density, controlled by varying 2-ethyl-4-methylimidazol content.46 In the case of the completely cured epoxy, the crosslink density of epoxy resin was 232.40 mol m-3 (crosslinker content 35 mol%) or lower.46 The microcellular structure was adjusted by the foaming conditions.46 Microcellular chlorinated polyethylene foams were prepared by using nitrogen as blowing agent.55 There was a minimum crosslink density needed for effective foaming.55 When crosslink density was increased above the sufficient crosslink density level, the average cell sizes of the foams decreased, and cell density increased.55 An acrylonitrile-butadiene rubber was crosslinked by methylene diphenyl isocyanate.47 It acted as a co-vulcanizing agent along with the sulfur crosslinking.47 The 10 phr MDI increased the crosslink density and decreased the swelling of NBR. MDI reacted with NBR producing strong urea (i.e., NH−CO−NH) and thiourethane groups (S−CO− NH).47 Epoxidized natural rubber-silica hybrids were cured with silica as a crosslinking and reinforcing agent.48 The 100% modulus, tensile stress, tear strength, and Shore A hardness increased with increasing silica contents, but the elongation at break and glass transition temperature declined.48 Silanes influenced the crosslink density and crosslink structure of silica-filled styrene-butadiene rubber.49 The tensile strength of composite increased and elongation decreased.49 The peroxide crosslinking and post-crosslinking of ethylene vinyl acetate in photovoltaic modules has been studied.50 A degree of crosslinking higher than 70%, as measured by Soxhlet extraction, is considered standard.50 Thermomechanical properties of the investigated EVA films demonstrated a sufficient state of crosslinking when a Soxhlet value was around 50%.50 At this stage, a substantial amount of peroxide was active which was then used in post-lamination crosslinking.50 For this reason, the regime of crosslinking is significant for the properties of solar cells.50 Thermally stable bromobutyl rubber was produced with a high crosslink density obtained with 4,40-bismaleimidodiphenylmethane.51 Introduction of zinc oxide accelerated cure rate, and the resulting vulcanizate had a higher crosslink density with excellent

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overcure reversion stability even at a temperature of 190oC for two hours.51 Compared with the conventional sulfur cure, the crosslinked rubber had a higher crosslink density, superior compression set, and excellent thermal stability.51 The increase in peroxide and co-agent (triallyl isocyanurate) concentrations resulted in an increase in total crosslink density of ethylene propylene diene rubber.52 The glass transition temperature shifted towards higher temperatures, and the peak values of the loss factor gradually decreased because of restricted segmental mobility due to the increase in crosslink density.52 Di-tert-butyl peroxide was used as the curing agent in combination with trimethylolpropane trimethacrylate and poly(styrene-b-butadiene-b-styrene) as the coagents of the curing process, resulting in the synergistic effect producing better solvent resistance and excellent mechanical properties.53 Ethylene propylene diene monomer/barium titanate (or silane)/mica system was studied.54 The addition of untreated mica increased the complex viscosity, while the 3aminopropyltriethoxysilane modified mica reduced the complex viscosity.54 The increased crosslink density caused by coupling agents increased the volume resistivity of EPDM composites.54 Ethylene propylene diene monomer/polypropylene was cured with dicumyl peroxide in the presence of vinyltrimethoxysilane.56 The crosslink density, gel content, tensile strength, Young’s modulus, elongation at break, and viscosity increased; whereas the compression set, melting temperature, enthalpy of melting, crystallinity, and damping factor decreased with increased addition of vinyltrimethoxysilane.56 This was attributed to the physical crosslinking caused by vinyltrimethoxysilane grafting on EPDM and chemical crosslinking induced by vinyltrimethoxysilane between PP and EPDM.56 2.4.9 CROSSLINKING KINETICS Ethylene-vinyl acetate was crosslinked with dicumyl peroxide in a differential scanning calorimeter, under non-isothermal conditions.57 Crosslinking degree as a function of temperature (or time) was calculated from the crosslinking enthalpies.57 The Avrami exponents (n) were found to remain practically constant (2.08–2.32) during the reaction and with varying dicumyl peroxide content, suggesting that the growth of crosslinks is sporadic and spherical, and occurs from nuclei.57 The Avrami, Ozawa, Avrami-Ozawa and Flynn-Wall-Ozawa models successfully describe the EVA crosslinking kinetics.57 The crosslinking behavior of short-segmented block copolymer of poly(butylene succinate-co-butylene fumarate) was evaluated based on the rate constant (k) and the orders of the initiation and propagation reactions (m and n, respectively).58 More crosslinking occurred in the amorphous phase.58 The crosslinking kinetics of peroxide crosslinking of isotactic and syndiotactic polypropylene showed that the initial gelation rate increased linearly with the peroxide concentration and temperature.59 At high peroxide concentration levels, significant β-scission and other side reactions occurred simultaneously, introducing carbonyl and unsaturated groups to the structure of the polymeric networks.59 The iPP and sPP showed similar crosslinking behavior.59 The lower crosslinking efficiency of sPP was attributed to the lower concentration of polymer radicals, caused by the steric hindrance of the sPP chain to hydrogen abstraction.59

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2.4.10 CRYSTALLIZATION TEMPERATURE Increasing crosslinking degree of polylactide resulted in increase of onset crystallization temperature.60 The perfection of crystal lamellas with a higher crosslinking degree was inferior.60 The changes in onset crystallization temperature and crystallization enthalpy were attributed to the breakage of chains induced by dicumyl peroxide, and these shortened chains stacked in lamellas (Figure 2.57).60 The low or moderate crosslinking bundles (not complete polymer crosslinking network) better promoted the crystallization of PLA samples.60

Figure 2.57. Mechanism of crosslinking bundles-promoted crystallization. [Adapted, by permission, from Zhang, Y; Wang, C; Du, H; Li, X; Zhang, J, Mater. Lett., 117, 171-4, 2014.]

Short-segmented block copolymers of poly(butylene succinate-co-butylene fumarate) showed a lower crystallization rate and degree of crystallinity when the crystallization temperature shifted to higher temperatures compared with uncrosslinked copolyesters because of the formation of nucleating agents by crosslinkages.58 Seven ethylene-octene copolymers with a wide range of octene content (17-45 wt%) were crosslinked by e-beam radiation (in the range of 30-120 kGy).61 Increased octene content lowered melting point, crystallization temperature, crystallinity, glass transition temperature, and storage modulus.61 Branching and crosslinking of polyhydroxybutyrate was accomplished by reactive modification in the melt state, using dicumyl peroxide as the free-radical initiator and a trifunctional coagent, triallyl trimesate.62 In addition to the significant increase in viscosity due to branching/crosslinking, the coagent modified samples exhibited a substantial increase in the crystallization temperature (nucleating effect) accompanied by the decrease of crystal spherulite sizes.62 In situ crosslinking and foaming process of ethylene-vinyl alcohol copolymers by propylene carbonate increased the gel content and decomposition temperature with increasing propylene carbonate content.63 The melting point, crystallization temperature, and crystallinity decreased with the increase in crosslink density.63

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2.4.11 CRYSTALLINE STRUCTURE Relationship between crosslink densities of peroxide cured poly(ethylene-co-vinyl acetate) was studied to discover the formation of semi-crosslink points by crystalline structure of the ethylene sequences in EVA and long crosslink points formed by the cocrosslinker and VA moiety (Figure 2.58).64 Crystalline region of polymer may play a role of crosslink point similar to the ionic bond.64 Figure 2.58. Schematic diagram of various EVA networks including chemical crosslinks and semi-crosslink points of crystalline Long crosslinks increase the region of the ethylene sequences. TAC − triallyl isocyanurate. [Adapted, by permission, from Choi, S-S; Chung, YY, Polym. Test- swelling ratio, whereas semicrosslink points by the ethylene ing, 66, 312-8, 2018.] crystalline structure decrease the swelling ratio.64 Radiation-induced (γ-rays at room temperature at 50-500 kGy in a vacuum and then thermal oxidation in air) crosslinking of polyacrylonitrile fibers had little effect on their crystalline structure and orientation.65 Electron beam induced modifications in the crystalline structure of polyvinylidenefluoride/nanoclay composites.66 The α form was the dominant crystalline structure, but the β crystalline form peak at 20.7° became prominent in the nanocomposites and the intensity of the peaks belonging to the α crystalline form diminished on exposure to radiation.66 The crosslinks impaired crystalline regions and created defects within the crystalline structure (lowering the melting point).66 The crystallinity level was affected by the amount of crosslink density.66 2.4.12 CRYSTALLINITY The kinetics of the surface photo-crosslinking and chemical crosslinking treatments were evaluated for carboxymethylcellulose film modified by UV induced photo-crosslinking and sodium benzoate and glutaraldehyde chemical crosslinking.67 Fast crystallization occurred after modifying carboxymethyl cellulose by photo-crosslinking and chemical crosslinking.67 Crystallinity degree of both crosslinked films was increased.67 Low-density polyethylene was crosslinked by dicumyl peroxide.68 The three-dimensional network restrained crystal growth and the degree of crystallinity and thickness of the lamellae decreased.68 Many tie molecule chains were introduced in the amorphous region because of the crosslinking of LDPE molecules.68 They acted as the barrier to the invasion of water molecules.68 The lamellar arrangement became extremely compacted due to the formation of cross-bonds; therefore the amorphous region among lamellae was compressed.68 Di-tert-butyl peroxide was used as the curing agent in combination with trimethylolpropane trimethacrylate and poly(styrene-b-butadiene-b-styrene) as the coagents in the curing process of dynamically vulcanized styrene-ethylene-butadiene-styrene block copo-

2.4.13 Cytotoxicity

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lymer/polypropylene blends.69 Crystallinity was decreased when the degree of crosslinking was increased.69 Biodegradable poly(butylene succinate-co-butylene fumarate) short-segmented block copolyester was crosslinked using bis(2,4,6-trimethylbenzoyl) phenyl phosphine oxide photoinitiator.58 Crosslinked copolyesters had a lower crystallization rate and degree of crystallinity.58 Their crystallization temperature was shifted to higher temperatures as compared to uncrosslinked copolyesters which was caused by the formation of nucleating agents by crosslinkages.58 Ethylene vinyl acetate photovoltaic module encapsulants were thermally crosslinked in the presence of initiator during lamination.70 The degree of crystallinity changed with crosslinking, but the effect was weak (crystallinity of approximately 9% for the uncured EVA and of around 7% for the most strongly crosslinked EVA samples).70 2.4.13 CYTOTOXICITY DNA crosslinking agents make up a broad class of chemotherapy agents that target rapidly dividing cancer cells by disrupting DNA synthesis.71 Crosslinking agents can produce a variety of DNA lesions in cells, but cytotoxicity is often attributed to the formation of inter-strand crosslinks.71 Inter-strand crosslinks are considered to be the most cytotoxic lesions, creating a covalent roadblock to replication and transcription.71 Crosslinked pectin nanofibers had no apparent cytotoxicity, and both adipic acid dihydrazide crosslinking and a high degree of methoxylation facilitated cell adhesion and proliferation on pectin nanofiber mats.72 The acidic glutaraldehyde crosslinking process caused cytotoxicity of pectin nanofibers at high concentrations.72 Crosslinking improves the properties of biomaterials, but most crosslinkers either cause undesirable changes to the functionality of the biopolymers or result in cytotoxicity.73 Glutaraldehyde is difficult to handle and contradictory views have been presented on the cytotoxicity of glutaraldehyde-crosslinked materials.73 Cytotoxicity of glutaraldehyde depends on the concentration used (up to 8% glutaraldehyde is considered to be non-cytotoxic).73 Porous scaffolds were fabricated using decellularized meniscus and 1-ethyl-3-3dimethyl aminopropyl carbodiimide and glutaraldehyde as crosslinkers.74 Scaffolds crosslinked with 1.0% and 2.5% glutaraldehyde were toxic to chondrocytes and MC3T3 cells, while carbodiimide groups showed no cytotoxicity.74 pH-degradable polyvinylalcohol nanogels, prepared by photo-crosslinking of thermo-preinduced PVA nanoaggregates in water without any surfactants or toxic organic solvents, were used for intracellular paclitaxel release and anticancer treatment.75 Smaller nanogels exhibited enhanced cellular uptake and more significant tumorous cytotoxicity.75 Mussel adhesive moiety, catechol, has been utilized to design a wide variety of biomaterials. H2O2 generated during catechol crosslinking exhibited localized cytotoxicity in culture and upregulated the expression of an antioxidant enzyme, peroxiredoxin 2, in primary dermal and tendon fibroblasts.76 The thermosensitive chitosan-gelatin hydrogel was developed to improve the bioavailability of ophthalmic eye drops.77 Chitosan and gelatin showed low cellular cytotoxicity.77 The cell viability increased with the increase of genipin concentration, which might have been caused by genipin crosslinking inhibition of the biodegradation of the

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hydrogels.77 Genipin crosslinking reduced the drug release rate and had low cytotoxicity to V79 cells.77 2.4.14 FOAM MORPHOLOGY Polyurethane foams containing biobased crosslinker 3-hydroxy-N,N-bis(2hydroxyethyl)butanamide were compared with PU foams containing the conventional crosslinker diethanolamine.78 The lower reactivity of biobased crosslinker favored larger cell sizes and more complete cell opening as compared to the more reactive diethanolamine (Figure 2.59).78 This behavior may be related to the onset of phase separation and the rate of viscosity build-up.78 Biodegradable microcellular foams with high thermal insulation were manufactured from poly(butylene succinate) urethane ionomer foamed by supercritical carbon dioxide.79 The foam morphology was sensitive to the urethane ionic group content affecting its mechanical and thermal insulation properties.79 The foams with 3-5 wt% urethane ionic group content exhibited elliptical shape and stretched in Figure 2.59. Scanning electron micrographs of samples the foam-mold height direction.79 Their of a) diethanolamine foam and b) 3-hydroxy-N,N-bis(2cell size and opening ratio were smaller hydroxyethyl)butanamide foam. {Adapted, by permission, from Lan, Z; Daga, R; Whitehouse, R; McCarthy, than 7.0 μm and 13%, while their cell denS; Schmidt, D, Polymer, 55, 11, 2635-44, 2014.] sities were higher than 4.7×109 cells/cm3.79 2.4.15 FRICTION The friction and wear behavior of soybean oil-based polymers prepared by cationic polymerization of low saturated soybean oil with divinylbenzene (crosslinker) and polystyrene were evaluated as a function of crosslink density.80 A higher crosslink density resulted in lower adhesive wear. Increased abrasive wear was observed for the lowest and highest crosslinking densities. Crosslinking reduced the coefficient of friction up to 15 wt% of divinylbenzene.80 The friction and the wear of norbornene polymers prepared by ring-opening metathesis polymerization were evaluated as a function of the crosslink density.81 Crosslinking with a concentration of crosslinker up to 10 wt% reduced coefficient of friction and wear of uncrosslinked polymer (higher concentrations of crosslinker increased coefficient of friction).81 Trimethylolpropane trimethacrylate and tripropylene glycol diacrylate were used as crosslinking agents for ultra-high molecular weight polyethylene.82 Both crosslinkers can

2.4.16 Gel content

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increase the gel fraction and crystallinity with the increasing dose which is beneficial for improvement of the tribological properties of ultra-high molecular weight polyethylene, but they also have the plasticizing effect.82 Semimetallic friction composites consist of epoxidized natural rubber (50 mol% epoxidation), alumina nanoparticles, steel wool, graphite, and benzoxazine.83 The composites were vulcanized using sulfur and electron-beam crosslinking systems.83 The friction coefficients under normal and hot conditions were higher than those of the sulfurvulcanized samples at all applied doses.83 The specific wear rates of the irradiated samples were lower than those of the sulfur-vulcanized samples at all applied doses.83 A low friction coating having also a low sliding coefficient of friction with plastic or metal working part includes a waterborne polyisocyanate, hydrogel, and crosslinkers such as aziridines or carbodiimides.84 Addition of low-viscosity silicone oils having a partial incompatibility to silicone elastomers is known to reduce the friction coefficient of silicone elastomers.85 As a rule, phenyl-silicone oils with a viscosity of 5-1000 mPas are added to the non-crosslinked addition-silicone composition.85 These compositions, although capable of lowering friction coefficient, change properties because silicone oil becomes depleted.85 By using the alkenyl polydiorganosiloxane in combination with the non-functionalized polydiorganosiloxane, silicone elastomers have a reduced friction coefficient because they form an oil film on the silicone surface which is composed of addition-crosslinkable silicone.85 2.4.16 GEL CONTENT The crosslinking kinetics of nitroxide-mediated radical polymerization of styrene in the presence of a small amount of divinylbenzene showed that the gel content increased very rapidly after the gelation point and the swelling index decreased from a maximum at the gelation point to a plateau of lower value at high conversions.86 The swelling index decrease suggested that the polymer network was loose at the onset of the gelation point and it became more compact, as the polymerization proceeded.86 The gel content of SEBS/PP, crosslinked using di-tert-butyl peroxide as a crosslinker in combination with trimethylolpropane trimethacrylate and poly(styrene-b-butadiene-bstyrene) as the coagents of the curing process, was in the range of 40-70%.69 For weakly crosslinked EVA samples (30–75% gel content), the standard deviation was only 35-308

Melting/freezing point, C: -113-243 o

Gel time, @23oC, min.: 1-600

Glass transition temperature, C: 42-166 o

Refractive index: 1.462-1.512 Viscosity, mPa s @25 C: 5-50,000 Vapor pressure, kPa @20oC: 0.013-12.8

Vapor density: 1

HEALTH & SAFETY Autoignition temperature, oC: 240-385

Flash point, oC: 7-275

HMIS: Fire: 1-3; Health: 0-2; Reactivity: 0-1 NFPA: Flammability: 0-3; Health: 1-3; Reactivity: 0-1 Carcinogenicity: IARC, NTP, OSHA: No component of this product present at levels greater than or equal to 0.1% is identified as probable, possible or confirmed human carcinogen Mutagenicity: negative Teratogenicity: No teratogenicity effect observed Explosive concentration, %: LEL: 1.4-1.8; UEL: 13 LC50: Dermal-rabbit, mg/kg: 270-2,000; Oral-rat, mg/kg: 200->5,000

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TLV, ppm: ACGIH; 20-25; OSHA: 0.75-50 UN/NA class: 1866 UN safety phrases: S26;S36/37/39 ECOLOGICAL PROPERTIES Aquatic toxicity, EC50, mg/l: Algae: >500-3230; Bluegill sunfish: 1.51-10; Daphnia magna: 12.5813; Fathead minnow: 12.6-460; Rainbow trout: 1.41-5.8; Zebra fish: 30-85 Bioaccumulation: “Not expected to significantly bioaccumulate” to “Has the potential to bioaccumulate” APPLICATIONS Recommended for resins: acrylic, alginate, bromobutyl rubber, cyanate resin, epoxy, non-isocyanate polyhydroxyurethane, novolac, PA, polybutadiene, poly(phthalazione ether sulfone ketone), polyimide, polyurethane, protein, silicone, styrene-butadiene rubber Recommended for products: adhesives, automotive topcoats, casting, coatings, composites, concrete repair, corrosion and wear protection, crack bridging, crack injection, electrical encapsulants, enamels, filament winding, flooring, foams, food contact, gelcoats, grouts, high-solids coatings, hot-melt adhesives, inks, laminating binders, marine paints, masonry, mortars, patching compounds, potting, primers, sealers, sealants, tank lining, textiles Dose, phr: 0.5-222 Spacer arm length, Å: 8-17.8

3.2.1 Acrylics

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3.2 POLYMERS AND THEIR CURATIVES 3.2.1 ACRYLICS A 100% solid, curable liquid encapsulant for photovoltaic modules based on acrylic/urethane hybrid chemistry was cured with polyisocyanates to form rubbery material having excellent optical properties and UV stability of over 5000 h of accelerated aging.1 The pot life and curing profile were adjusted by catalyst (e.g., dibutyltin diacetate) as well as temperature.1 Waterborne acrylic resin modified with glycidyl methacrylate was cured by aminopropyltriethoxysilane.2 Curing with aminopropyltriethoxysilane increased crosslink density of resin and improved adhesion to metal preventing flash corrosion.2 Hybrid waterborne alkyd-acrylic dispersions were synthesized by a melt co-condensation reaction between an acrylic prepolymer bearing carboxylic groups and a long-oil alkyd resin (Figure 3.1). Spontaneous emulsification of the ensuing hybrid resin was achieved by the addition of an aqueous ammonia solution that neutralized the carboxylic functions. This is an example of oxidative cured hybrid for high quality zero VOC coatings.

Figure 3.1. Synthesis of the hybrid alkyd/acrylic resin. [Adapted, by permission, from Elrebii, M; Mabrouk, AB; Boufi, S, Prog. Org. Coat., 77, 4, 757-64, 2014.]

The metal chelate aluminum acetylacetonate and zirconium acetylacetonate were used as curing agents to obtain optically clear acrylic pressure-sensitive adhesives.4 Figure 3.2 shows a chemical reaction.4

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Figure 3.2. Reaction between acrylic pressure sensitive adhesives with carboxylic acid and aluminum acetylacetonate. [Adapted, by permission, from Lee, S-W; Park, J-W; Park, C-H; Kim, H-J; Kim, E-A; Woo, H-S, Int. J. Adhesion Adhesives, 47, 21-5, 2013.]

Comparison of information in this section and section 2.2 shows that crosslinking reactions are more universally used in the processing of acrylic resins, but some curing reactions are also suitable for their processing. References 1 2 3 4

Einsla, ML; Teich, CI; Bender, MT; Ottinger, JA; Greer, EC, Solar Ener. Mater. Solar Cells, 165, 103-110, 2017. Guo, X; Ge, S; Wang, J; Zhang, X; Guo, Z, Polymer, 143, 155-63, 2018. Elrebii, M; Mabrouk, AB; Boufi, S, Prog. Org. Coat., 77, 4, 757-64, 2014. Lee, S-W; Park, J-W; Park, C-H; Kim, H-J; Kim, E-A; Woo, H-S, Int. J. Adhesion Adhesives, 47, 21-5, 2013.

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3.2.2 ALGINATES Sodium alginate is a polysaccharide gum produced from brown seaweed extract.1 Sodiumalginate can be solidified by multivalent cations, such as calcium cations.1 Calcium-alginate spherical gel beads can be formed using external or internal gelation.1 Bioactive hyaluronic acid/sodium alginate-based films were prepared by crosslinking with Ca2+, Zn2+, or Cu2+ metal cations.2 They were found to be suitable for wound dressing due to their bacterial properties and mechanical performance.2 Sugar concentrations above 15 wt% reduced extensibility of alginate molecules and led to a more open or less connected gel network with aggregated alginate strands which impacted the swelling-deswelling ability of calcium alginate gels.3 Figure 3.3 shows the mechanism of the formation of alginate gel by calcium cations.4

Figure 3.3. Formation of an alginate gel by calcium cations, resulting in calcium linked junctions. [Adapted, by permission, from Li, J; Jinmei He, J; Yudong Huang, Y, Int. J. Biol. Molec., 94A, 466-73, 2017.]

References 1 2 3 4

Lee, B-B; Bhandari, BR; Howes, T, Chem. Eng. Sci., 183, 1-12, 2018. Abou-Okeil, A; Fahmy, HM; El-Bisi, MK; Ahmed-Farid, OA, Eur. Polym. J., 109, 101-9, 2018. Lopez-Sanchez, P; Fredriksson, N; Larsson, A; Altskär, A; Ström, A, Food Hydrocolloids, 84, 26-33, 2018. Li, J; Jinmei He, J; Yudong Huang, Y, Int. J. Biol. Molec., 94A, 466-73, 2017.

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3.2.3 BROMOBUTYL RUBBER Poly(isobutylene-co-isoprene) − a synthetic elastomer commonly known as butyl rubber has been produced by the random cationic copolymerization of isobutylene with small amounts of isoprene (usually not more than 2.5 mol%).1 Halogenation of butyl rubber produced reactive allylic halide functionality within the elastomer.1 For most applications, halobutyl rubber must be compounded and vulcanized (chemically crosslinked) to yield useful, durable end-use products.1 The cure of bromobutyl rubber is typically accomplished by the use of sulfur, and zinc derivatives as curing agents.1 High extractable levels of sulfur and zinc oxides are unacceptable for various pharmaceutical applications.1 Bromobutyl rubber which does not contain these impurities can be obtained with the use of multifunctional phosphine curing agent, such as bis(2-diphenylphosphinophenyl)ether.1 References 1

Nguyen, P; Arsenault, G, WO2014100890A1, Lanxess Inc., Jul. 3, 2014.

3.2.4 Cyanate resin

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3.2.4 CYANATE RESIN The high curing temperature of cyanate ester results in higher internal stress and formation of defects. The triazine ring and the rigid structure of cyanate ester make the condensates brittle, leading to the low interlaminar shear strength of the composite material.1 A highperformance hybrid material was prepared by melt blending of glycidyl polyhedral oligomeric silsesquioxane and bisphenol-A cyanate ester and using triethylamine as the curing agent.1 Figure 3.4 shows the reactions mechanisms.1

Figure 3.4. The reaction equation of curing glycidyl polyhedral oligomeric silsesquioxane and cyanate ester. [Adapted, by permission, from Jiao, J; Zhao, L-z; Xia, Y; Wang, L, High Performance Polym., 29, 4, 458-66, 2017.]

The primary reaction pathways of curing cyanates is the cyclotrimerization of cyanate groups to form triazines or reaction with various hydroxyl groups to form imidocarbonates.2 During commercial processing of cyanate resins, coordination metals are usually

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used as catalysts and phenols as co-catalysts.2 In the presence of a metal catalyst, the overall reaction obeys a second-order rate law with respect to cyanate concentration while, in the absence of metal catalysts, the reaction is autocatalytic.2 The cyanate conversion significantly increased after the first addition of the metal catalyst (Co(AcAc)2) and the thermal stability improved.3 The activation energy Ea = 106.7 kJ/mol in the presence of nonylphenol was slightly higher than the value obtained in the presence of the metal catalyst (around 92 kJ/mol).3 References 1 2 3

Jiao, J; Zhao, L-z; Xia, Y; Wang, L, High Performance Polym., 29, 4, 458-66, 2017. Deng, Y; Martin, GC, Polymer, 37, 16, 3593-3601, 1996. Gómez, CM; Recalde, IB; Mondragon, I, Eur. Poly. J., 41, 11, 2734-41, 2005.

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3.2.5 EPOXY RESINS Curing reactions are almost synonymous with epoxy resins which are very typical twocomponent systems, performing according to expectations only after they have reacted with one of many available curatives. The major curatives used to cure epoxy resins belong to a group of amines (aliphatic, aromatic and cycloaliphatic) and amidoamines in the form of low molecular compounds and polymers but there is a large number of less known curatives which are discussed below in alphabetical order (the names of the discussed curatives are given in italics for easier spotting). Amine compounds are classified into primary, secondary, and tertiary amines depending on the number of hydrogen atoms in ammonia (NH3) which have been substituted for hydrocarbon. Depending on the number of the amine groups in a molecule, amines are named as the monoamine, diamine, triamine, or polyamine. Amines are classified into aliphatic, aromatic, and cycloaliphatic concerning the type of hydrocarbon substituent. The cure rate of individual amines mostly depends on the type of amine, the type of epoxy resin, system viscosity, and temperature. Aliphatic amines rapidly react with epoxy resin. They are used as room-temperature curing agents. They generate a large quantity of heat and their mixtures with epoxy resins have a short pot life. Resins cured with aliphatic amines are strong and have excellent adhesion. Aliphatic amines as strong bases are irritant to the skin and toxic. The higher the molecular weight, the lower the toxicity as a general rule. The reaction of a primary amine with an epoxy group produces both a secondary amine and secondary alcohol (1). The secondary amine can react with another epoxy group producing tertiary amine (2). Also, hydroxyl groups are able to react with the epoxy groups (3) if there is an excess of the epoxy groups, the system is catalyzed and/or temperature is elevated.

(1) RNH2 + RCH2 O

RNCH2CHCH2R H

OH OH CH2CHCH2R

(2) RNCH2CHCH 2R + RCH2 H

RN CH2CHCH2R

OH

OH OH (3) RN

CH2CHCH 2R CH2CHCH 2R OH

OH + RCH2

CH2CHCH 2R RN CH2CHCH 2R O

OH

CH2CHCH2R

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Figure 3.5 gives examples of structures of epoxy resin and amines which have two, three, and four functional groups.1

Figure 3.5. Molecular structure of diglycidyl ether of bisphenol A, DGEBA, N,N,N′,N′-tetra(3-aminopropyl)1,3-propanediamine, TAPA, propanediamine, PDA and Jeffamine T-403. [Adapted, by permission, from Wan, J; Li, C; Bu, Z-Y; Xu, C-J; Fan, H, Chem. Eng. J., 188, 160-72, 2012.]

Based on the studies by differential scanning calorimetry N,N,N′,N′-tetra(3-aminopropyl)-1,3-propanediamine and propanediamine have similar reactivities (much higher than that of commercial Jeffamine T-403).1 The curing kinetic analysis shows that N,N,N′,N′-tetra(3-aminopropyl)-1,3-propanediamine causes the higher isothermal conversion at a lower temperature (e.g., 40°C).1 The N,N,N′,N′-tetra(3-aminopropyl)-1,3-propanediamine has lower effective activation energy because of catalytic activity of tertiary amino groups.1 Fully cured network with aliphatic amine absorbed slightly more water and at a higher rate than a not completely cured network (conversion 0.98).2 At a conversion rate of 0.90, additional crosslinking was formed during water immersion, even at 20°C.2 Cure reactions of the stoichiometric mixtures of diglycidyl ether of bisphenol A, and two aliphatic polyether diamines (polyoxypropylenediamine, Mw = 230 and polyoxyethylenediamine, Mw = 148) were studied by using fluorescence and mid- and near-IR spectroscopic techniques.3 The primary amine groups were gradually converted to the secondary and the tertiary amines.3 The polyoxyethylenediamine caused faster reduction of fluorescence intensity (by 40%) than polyoxypropylenediamine.3 A method for utilizing the exothermic energy generated by a low-temperature cure reaction to access a high-temperature cure reaction was used to achieve a cured resin matrix similar to the produced by high-temperature cure reactions but delivered via a short cure time and low cure temperature.4 The composition contains a combination of amines and accelerator, including isophorone diamine, 3,3'-diaminodiphenylsulfone, and imidazole as curing accelerator.4 The formulation was cured in less than five min.4 Composition for forming epoxy resin system contained a combination of amines including dodecyldimethylamine and a mixture of isophorone diamine and the polyetheramine.5 This formulation was developed for the fabrication of windmill blades to provide reduced exothermic heat release during manufacture.5 Triethylenetetramine and an aminated polydimethylsiloxane were used as curing agent for diglycidyl ether of bisphenol A.6 The curing agents were encapsulated using poly(urea-formaldehyde) as microcapsule wall.6 The system had self-healing abilities but

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it required heat (40oC in the case of triethylenetetramine) to initiate healing (activation energy of 87.2 kJ/g).6 Aliphatic dicarboxylic acid curing agent containing a disulfide bond was used to cure biphenyl-based epoxy monomer to produce a liquid crystalline network with exchangeable disulfide bonds.7 Figure 3.6 shows the chemical composition of the system.7

Figure 3.6. (a) Chemical structures of the epoxy monomer and curing agents and (b) schematic illustration of the preparation of an liquid crystalline epoxy network with exchangeable disulfide bonds. [Adapted, by permission, from Li, Y; Zhang, Y; Rios, O; Keum, JK; Kessler, MR, Soft Matter, 13, 5021-7, 2017.]

After optimizing the molar ratio of the two curing agents, the resulting liquid crystalline epoxy network exhibits improved reprocessability and recyclability because of the disulfide exchange reactions, while preserving liquid crystalline properties, such as the reversible liquid crystalline phase transition and macroscopic liquid crystalline orientation, for shape memory applications.7 Amidoamines are suitable for curing under ambient temperature applications. They have a low viscosity, long pot life, good adhesion especially to concrete, corrosion resistance, lower odor, and good performance under humid conditions. Amidoamines are products of reaction of diamine (e.g., tetraethylenepentamine) with fatty acid. They are useful for high solids coatings. Young’s modulus of epoxy resins cured with amidoamine having variable pendant alkyl chain length, n, decreased with the increase of n.8 The Lennard-Jones and covalent bond interactions were responsible for this relationship, with Coulombic interactions playing no significant role.8 The hyperbranched polyamidoamine was grafted onto a colloidal silica surface (Figure 3.7) by repetition of two steps: (1) Figure 3.7. Hyperbranched poly(amidoamine) grafted Michael addition of methyl acrylate to onto a colloidal silica. [Adapted, by permission, from amino groups on the surface and (2) amidaKaneko, Y; Imai, Y; Shirai, K; Yamauchi, T; Tsubokawa, N, Colloids Surf. A: Physicochem. tion of the resulting terminal ester groups Eng. Aspects, 289, 1-3, 212-8, 2006.]

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with ethylenediamine.9 The terminal amino groups of hyperbranched polyamidoaminegrafted silica were effective in curing the epoxy resin.9 Amidoamines made from triethylenetetraamine as the only amine component undergo partial to complete crystallization which can be prevented by reaction with a significant amount of dimeric fatty acid and/or formation of high levels of imidazoline rings. High level of dimer acid can unacceptably increase viscosity whereas a high level of imidazoline rings slows their reactivity.10 This can be prevented by including in the reacting mixture aliphatic monobasic carboxylic acid, triethylenetetraamine, and an amine selected from the group consisting of homologs of polyethylenepolyamines.10 Amino-functionalized carbon nanotubes were used as a curative and reinforcing agent in epoxy resin.11 The glass transition temperature of reinforced composites increased because of the decreased mobility of polymer chains, held firmly by amino carbon nanotubes.11 The tensile strength and modulus were increased by 30 and 48%, respectively.11 The carboxyl-functionalized carbon nanotubes re-aggregated in epoxy-amine matrix whereas the amino-functionalized carbon nanotubes (Figure 3.8) participated in interface strengthening of carbon fiber/epoxy composites resulting in the improved mechanical performance.

Figure 3.8. Functionalization of carbon nanotube. [Adapted, by permission, from Zhang, Q; Wu, J; Gao, L; Liu, T; Yang, X, Mater. Design, 94, 392-402, 2016.]

The interlaminar mechanical properties, interfacial properties, and glass transition temperature of carbon fiber/epoxy composites were significantly improved by the addition of amine-modified carbon nanotube.12 Anhydrides are widely used as curatives of epoxy resins. Their reactivity is much slower when compared with amines, and they usually require catalyst and/or elevated temperature. The reaction mechanism of anhydrides is complex and subject to many competing reactions including reaction with secondary hydroxyl groups in epoxy backbone and formation of esters or diesters with an epoxy group. Use of anhydrides improves UV and thermal stability and glass transition temperature, lowers cure exotherm and shrinkage, as well as provides for an increased ratio of curative to the resin. Two significant drawbacks of anhydrides are moisture sensitivity of cure and cured product (ester linkages can be hydrolyzed). Bisphenol epoxy resin was cured with methyltetrahydrophthalic, methylhexahydrophthalic, and methylnadic anhydrides.13 The type of curing agent influenced pot life.13 The methylnadic anhydride was selected as the most suitable curative.13 The 3,4-epoxycyclohexyl-methyl-3,4-epoxycyclohexanecarboxylate mixed with different types of anhydride curing agents such as 4-methylcyclohexane-1,2-dicarboxylic anhydride and hexahydrophthalic anhydride were used to prepare conductive adhesive for

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LED devices.14 The hexahydrophthalic anhydride gave the best properties, such as the lowest resistivity and the best thermal conductivity.14 The cardanol-based di-anhydride curing agent was synthesized (cardanol and glycidyl methacrylate were first reacted and subsequently maleinized using maleic anhydride to yield di-anhydride compound) and used in the preparation of anticorrosive epoxy coatings.15

Figure 3.9. Anhydride curing agent for epoxy based on cardanol. [Adapted, by permission, from Wazarkar, K; Sabnis, A, Prog. Org. Coat., 118, 9-21, 2018.]

The presence of C15 aliphatic chain, two anhydride functionalities, and the aromatic ring could be the factors responsible for excellent mechanical and chemical performance.15 The long-term behavior during water uptake of an epoxy-anhydride network has been studied.16 A four-stage mechanism includes diffusion (water uptake reaches a pseudo-equilibrium at 1.5% mass increase), hydrolysis occurs in the second (the increase in network mobility and hydrophilicity) and third (glass transition disappearance) stages, and in the last stage mass decrease is observed. Degraded samples showed two glass transitions suggesting a heterogeneous process.16 Hydrolysis undergoes at preferential sites due to the presence of acid products.16 Anhydride-cured epoxy resins used as solid insulation medium in much electrical equipments decompose due to the high temperature caused by partial discharge and the presence of oxygen.17 The oxygen affected the main chain of the epoxy resin by introducing a carbon-oxygen double bond to the tertiary carbon atom attached to an oxygen atom and reduced generation time of degradation products such as CO2, H2O, and C2 and C3 products (Figure 3.10).17

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Figure 3.10. Products of epoxy decomposition. [Adapted, by permission, from Zhang, X; Wu, Y; Wen, H; Hu, G; Tao, J, Polym. Deg. Stab., 156, 125-31, 2018.]

Aromatic amines have lower basicity than aliphatic amines. Their cure rate is much slower at room temperature because of steric hindrance of the aromatic ring. The curing virtually stops and the polymer fully cures because of the large difference between the reaction rates of primary and secondary amines. Curing with aromatic amines typically requires two-step heating. The first step is carried out at a low temperature (around 80°C) to decrease heat generation. The second step is carried out at a higher temperature (150 to 170°C). Aromatic amines provide cured resins with excellent heat resistance, HDT of 150°C to 160°C, and excellent mechanical properties. Also, good electrical properties and excellent chemical resistance to alkali and solvents are typical of these resins. These resins have a long history of successful applications in aerospace. The major disadvantage is potential crystallization of curative, high viscosity, and inferior color stability. The 4,4′-diaminodiphenyl methane, m-phenylene diamine, and 4,4′-diaminodiphenylsulfone are principal commercial aromatic amines used as curing agents.18 They have low molecular weight resulting in the formation of highly crosslinked materials, which are usually brittle.18 The increase of the molecular weight of curing agent causes reduction of crosslink density which improves the network toughness, but heat resistance is also decreased.18 Figure 3.11. Chemical structure of the epoxy and the Diglycidylether of bisphenol A has been curing agents (a) TMBP, (b) BAT, (c) BAMT. (d) PBAB, and (e) ODA. [Adapted, by permission, from cured with an aromatic diamine containing Kim, H, Yeo, H; Goh, M; Ahn, S; Jang, SG; Hahn, JR; phthalide structure.18 The autocatalytic You, N-H, Macromol. Res., 25, 7, 763-8, 2017.] model was suitable to describe the cure mechanism.18 The rate of thermal decomposition was similar to the commercial 4,4′diaminodiphenylsulfone.18

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The epoxy resin composition for the encapsulation of electronic devices was cured with either 3,3′-diethyl-4,4′-diaminodiphenylmethane and diethyltoluenediamine.19 The epoxy resin composition had moisture-resistant adhesion and low residual stress.19 Aromatic ester amines were compared with 4,4'-oxydianiline, ODA, for their curing performance of 4,4'-diglycidyl (3,3',5,5'-tetramethylbiphenyl) epoxy (TMBP), which is widely used because of its thermal stability, mechanical strength, and corrosion protection properties (Figure 3.11).20 The glass transition temperature and thermal degradation of the epoxies derived from the ester-type diamines were higher than obtained with 4,4'-oxydianiline because of intramolecular hydrogen bonding and the rigidity of the ester-type diamine, which suppressed a thermal change in the cured epoxies.20 the TMBP/BAMT system showed approximately 20% higher thermal conductivity compared with TMBP/ ODA, because of the suppression of phonon scattering by the highly ordered structure.20 The rigid-rod-like structure of the ester type diamine increased the inter-chain packing densities.20

Figure 3.12. Schematic overview of the furfuryl amine, diglycidyl ether of bisphenol A bulk pre-polymerization, followed by network formation using bismaleimide as a thermo-reversible crosslinker. The two-step process is followed to avoid bismaleimide side reactions including homopolymerization (extrusion allows to keep the residence time). [Adapted, by permission, from Turkenburg, DH; Fischer, HR, Polymer, 79, 187-94, 2015.]

Thermoreversible crosslinked epoxy resin based on Diels-Alder reaction was produced in a two-step process including synthesis of a side-chain pendant furan-functionalized linear epoxy resin followed by crosslinking with bismaleimide of the pendant furan functional groups in the side chain.21 The temperature of the thermal transition resulting from the synergistic effect of glass transition and retro-Diels-Alder reaction increased with the mole ratio of maleic imide in bismaleimide to furan groups.21 The reversible crosslinking network by the Diels-Alder reaction enhanced the heat resistance of the material.21 Figure 3.12 shows the chemical reactions leading to the formation of thermoreversible crosslinking resin which can undergo at least five breaking and crosslinking cycles.22 The maleimide to furfuryl ratio of 0.95 gave the highest crosslink density, resulting with the product being least susceptible towards swelling and extraction when submerged in dichloromethane.22

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The cellulose nanocrystals contain hydroxyl groups, which have been used to thermally crosslink an epoxy resin.23 These rod-like nanoparticles contain many hydroxyl groups, turning them into reactively functionalized building blocks covalently-bound to the epoxy structure.23 They provide epoxy resins with mechanical reinforcement and thermal stability enhancement.23 Cellulose nanocrystals are suggested to be used at a high concentration of 75 wt%.23 In a further development, cellulose nanocrystals were functionalized with an aminotrimethoxysilane which permitted an additional reaction with a biobased epoxy resin.24 The storage modulus at 160°C increased from 19.5 to 151.5 MPa.24 Some papers provide comparative information on performance of different groups in curatives.25-27 Lignin-based epoxy resins were cured with 4,4-diaminodiphenylmethane (aromatic amine) and diethylenetriamine (aliphatic amine) to generate a 3-dimensional crosslinked structure.25 The activation energy of curing with aliphatic amine was larger than in the case of aromatic amine.25 The activation energy of resin cured with aromatic amine was increasing when cure extent was increased which was opposite to the reaction with aliphatic amine present.25 The hydroxyl groups in the epoxy resin promoted the curing process. Resins cured with aromatic amine had better thermal stability.25 The cycloaliphatic amine, polyamine adduct, modified aliphatic ketimine, and phenalkamide have been used to cure diglicidyl ether of bisphenol-A under four environmental conditions, i.e., at 10 and 30oC and 60 and 90% relative humidity.26 The cure temperature had a stronger effect than relative humidity in surface drying rate for all the crosslinking agents.26 Polyamine adduct and phenalkamine gave the best mechanical performance (tensile strength and scratch resistance).26 Aliphatic ketimine cured epoxy resin had the lowest glass transition temperature and crosslink density as well as water uptake.26 Phenalkamine gave the best overall performance.26 The aromatic, cycloaliphatic, and aliphatic amines were reacted with a bisphenol-A epoxide.27 Resins cured with aromatic and cycloaliphatic amines had more local heterogeneities than epoxy cured with aliphatic amine.27 Similar to previously reported data, the glass transition temperature of epoxy cured with aliphatic amine was the lowest.27 Crown ethers are macrocycles that have the intrinsic ability to complex ions or small organic molecules.28 The macrocycles can be used as crosslinking agents for curing the bisphenol A diglycidyl ether epoxy resin.28 Four crown ethers were used to cure bisphenol A diglycidyl ether (Figure 3.13).28

Figure 3.13. Structure of the crown ethers used as curing agents of the epoxy resin. [Adapted, by permission, from López, FF; Vázquez Barreiro, EC; Jover, A; Manuel, J; Ageitos, M; Rodríguez, E; Vázquez Tato, J, Polym. Int., 66, 1928-34, 2017.]

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The molecules having amine groups can catalyze etherification reactions that confer a high degree of three-dimensional crosslinking.28 The only molecule in this group having a different behavior was H2ODDA because in its structure there were two amide groups (instead of amine groups) which would not catalyze the etherification reaction.28 The reaction between epoxy resin with crown ethers occurred slowly because it had to overcome a high energy barrier of 114 kJ mol-1.28 All systems improve the thermal stability of the epoxy resin.28 Cycloaliphatic amines are colorless liquids which provide epoxy resins with good chemical resistance, color stability, electrical properties, excellent mechanical properties, and moderately high heat distortion temperature, and low viscosity. The 1-(2-aminoethyl)-piperazine epoxy adhesive and piperidine epoxy adhesive presented the Figure 3.14. Initial stages of the amine-epoxy reaction, showing best adhesive strength and the one half of each reagent. [Adapted, by permission, from Hamerton, I; Tang, W; Anguita, JV; Silva, RP, Reactive Functional Polym., 74, largest impact energy.29 The dura1-15, 2014.] bility in water caused less damage to piperidine epoxy networks.29 The diglycidyl ether of bisphenol A was cured with p-3,3′-dimethylcyclohexylamine to form a dielectric polymer suitable for microelectronic applications.30 The amine-epoxy cure involved nucleophilic attack of the exposed electron lone pair of the nitrogen on the strained epoxy ring, leading to its opening (Figure 3.14).30 Mortars were produced from two types of resin (diglycidyl ether of bisphenol A and F), and two aliphatic, two aromatic and one cycloaliphatic curatives were used to analyze the effect of reactive diluents on workability of compositions (Figure 3.15).31 The addition of 5% diluent caused a decrease in viscosities from 52 to 72% with glycidylether C12-C14 alcohol being the most effective.31 The hardener type of epoxy mortar may increase the flow rate by up to 2.8 times.31 The viscosity of the hardener affected the flow of epoxy mortars. The higher the hardener functionality, the shorter the pot life of mixtures.31 Triethylene tetraamine gave the shortest pot life because it was an aliphatic amine with func-

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Figure 3.15. Molecular structures of epoxy, hardeners, accelerators and diluents used. [Adapted, by permission, from Ozgul, EO; Ozkul, MH, Constr. Build. Mater., 158, 369-77, 2018.]

tionality of six.31 Aliphatic amines generally gave shorter pot lifes than cycloaliphatic amines.31 The meta-xylenediamine, modified cycloaliphatic amine, and isophorone diamine were used in the study of the effect of the curing temperature on the compressive strength, flexural strength, and modulus of elasticity of concrete made with bisphenol F-type epoxy resin as a binder.32 The increase in curing temperature increased modulus of elasticity and ultimate strain and strength.32 The compressive and flexural strengths were decreasing in the following order meta-xylenediamine, modified cycloaliphatic amine, and isophorone diamine, respectively.32 Hardening of the polymer concrete depended on the cure kinetics of the polymer matrix.32

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The cycloaliphatic epoxy cured with cycloaliphatic amine or anhydride reinforced with mono- or octa-functional POSS or nanosilica were used to improve their environmental stability in low Earth orbit.33 Silicon reacted with oxygen forming protective silica layer, which slowed down the erosion rate.33 Anhydride substantially increased glass transition temperature as compared with an aliphatic amine.33 The lower glass transition temperature gave greater mobility of chain segments making resin more flexible and ready to accommodate dimensional changes.33 Diglycidyl ether of bisphenol-A cured with cycloaliphatic amine (3-aminomethyl3,5,5-trimethylcyclohexylamine) was studied for the water diffusion behavior.34 The tensile strength decreased when the water content increased, but the elastic modulus was unaffected.34 The fracture toughness increased by a factor of three when water content was in the range of 0.6-1.8% (as compared to the dry specimen), but then it decreased when the water content was above this range.34 Diamine-functionalized graphene oxide was used as a co-curing agent to improve properties of the epoxy resin.35 Diaminodiphenyl sulfone or hexamethylenediamine was reacted with the carboxylic acid groups on graphene oxide to form amide bonds.35 When 1.0 wt% of diamine-functionalized graphene was added, the crosslink density was increased from 0.028 to 0.069 mol cm-3 followed by increase of glass transition temperature and tensile strength from 160.7 to 183.4°C and from 87.4 to 110.3 MPa, respectively.35 Graphene oxide was functionalized with 4-nitroaniline and used in epoxy composite for coating on the mild steel substrate to study the corrosion protection performance and barrier protection properties.36 Incorporation of 0.5 wt% of functionalized graphene enhanced the corrosion resistance of coating as compared to the pure epoxy coating.36 Graphene oxide was functionalized by assembling a supermolecular aggregate of piperazine and phytic acid (Figure 3.16).37 The addition of 3 wt% to epoxy resin resulted in composite characterized with notable suppression of the fire risk (heat release rate

Figure 3.16. Modification of graphene oxide. [Adapted, by permission, from Fang, F; Song, P; Ran, S; Guo, Z; Fang, Z, Compos. Commun., 10, 97-102, 2018.]

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decreased from 727.4 to 367.5 kW/m2 (49%), and the peak smoke production rate decreased from 0.2316 to 0.1379 g/s (40%)).37 Graphene nanoplatelets were functionalized by deposition of polydopamine coating followed by modification with amine groups using polyetheramine grafting (Figure 3.17).38 Addition of 0.1 wt% of modified graphene significantly improved the cryogenic tensile strength and impact strength of the epoxy nanocomposites by 34.5% and 64.5%, respectively, showing greater reinforcing effect than the pristine graphene nanoplatelets (12.6 and 19.1%) and polydopamine coated graphene nanoplatelets (26.3 and 50.1%).38

Figure 3.17. Graphene functionalization. [Adapted, by permission, from Wu, Y; Chen, M; Chen, M; Ran, Z; Liao, H, Polym. Testing., 58, 262-9, 2017.]

A halogen-free flame retardant of 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10oxide-containing H-benzimidazole was synthesized and subsequently used as a co-curing agent of 4,4-diamino-diphenylmethane for diglycidyl ether of bisphenol-A.39 The epoxy resin had a UL 94 V-0 rate and a limiting oxygen index of 35.6%.39 The glass transition temperature was increased by 6.9oC, and a decrement of dielectric constant by 0.3.39 Diamino-di-siloxanes (3,30-(1,3-dimethyl-1,3-diphenyl-1,3-disiloxanediyl)bis(benzenamine) (C1) and 4,40-(1,3-dimethyl-1,3-diphenyl-1,3-disiloxanediyl)bis(benzenamine) (C2)) (Figure 3.18) have been used to cure epoxy resin based on diglycidyl ether of bisphenol-A.40 The two systems have the similar activation energy.40 The reactivity of C1 was higher than that of C2.40 The reaction orders of C1 and C2 were 0.88 and 0.87, respectively.40 The epoxy resins cured with C1 and C2 curatives had improved thermal stability and higher char residue than the epoxy resin cured with 4,40-diaminodiphenylmethane.40

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.

Figure 3.18. Synthesis of 3,30-(1,3-dimethyl-1,3-diphenyl-1,3-disiloxanediyl)bis(benzenamine) (C1) and 4,40(1,3-dimethyl-1,3-diphenyl-1,3-disiloxanediyl)bis(benzenamine) (C2). [Adapted, by permission, from Shi, X; Lin, X; Xu, C; Cui, M, J. Appl. Polym. Sci., 132, 42385, 2015.]

Dicyandiamide is a latent curing agent that forms crystals having a high melting point of 207-210°C. It has a pot life of 24 hours when it is dissolved in epoxy resin using a solvent. When it is dispersed in a form of powder, the pot life is increased to 6-12 months. It cures at temperature of 160-180°C in 20-60 minutes in highly exothermic reaction. It typically requires an accelerator, such as tertiary amine, imidazole, and aromatic amine. Dicyanoamide is used only in thin films (because of exothermic reaction) such as paints, adhesives, and laminates. Positron annihilation lifetime spectroscopy was used to study the curing behavior of epoxy resins with dicyandiamide.41 The kinetic parameters were derived from experimental data which permitted to extrapolate the shelf life of one-component epoxy systems to room temperature or any other temperature.41 Liquefied dicyandiamide solubility and dispersibility of curative in epoxy resin were enhanced which helped to obtain better transparency and surface quality of cured epoxy resin films.42 The hygrothermal performance of cured epoxy based prepregs was improved.42 Diphenyl-(1, 2-dicarboxylethyl)-phosphine oxide curative has been developed to improve the flame retardancy, thermal degradation and moisture resistance of epoxy resins.43 It was used for curing diglycidyl ether of bisphenol-A together with phthalic anhydride.43 The resin cured with curative containing 20% phosphine oxide passed UL-94 V-0 flammability rating and had the LOI value of 33.2%.43 It reduced the combustion parameters of the epoxy resin thermosets, such as heat release rate and total heat release.43 The glass transition temperature decreased with the increase of phosphine oxide content.43 The curative promoted the decomposition of epoxy resin and led to a higher char yield and thermal stability at high temperatures.43 The moisture absorption was also reduced because of the existence of the P–C bonds and the rigid aromatic hydrophobic structure.43 Vitrimers, which are covalently crosslinked networks that can relax stress at elevated temperatures due to thermoreversible bond-exchange reactions, were composed of diglyc-

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idyl ether of bisphenol A, commercial mixture of mono-, di-, and trifunctional fatty acids (Pripol), and catalyst (zinc acetate dihydrate).44 The curing kinetics followed the Arrhenius law and the catalyst accelerated the reactions.44 A self-crosslinking comFigure 3.19. Self-crosslinking. [Adapted, by permission, from pound having epoxy groups and Huang, K; Liu, Z; Zhang, J; Li, S; Zhou, Y, Eur. Polym. J., 70, anhydride groups was synthe45-54, 2015.] sized from tung oil fatty acid by reacting with maleic anhydride via the Diels-Alder reaction.45 The compound had excellent storage stability and was cured with trace amounts of tertiary amine (Figure 3.19).45 Water-based epoxy ester resin was synthesized by partial reaction of bisphenol Abased epoxy resin with different fatty acids (linseed oil, tung oil, and dehydrated castor oil) followed by reaction with diethanolamine.46 The phosphoric acid neutralized tung oil fatty acid-based epoxy ester had the best mechanical and anticorrosive properties of all acid cured resins under the studies.46 Structural adhesives containing a modified epoxy resin, an elastomeric toughener (containing urethane and/or urea groups, and which has capped terminal isocyanate groups), and a curing agent including an epoxy-functional fatty acid oligomer and a polyol which increase the storage stability of the structural adhesive.47 The epoxy resin adhesives are used in many bonding applications, including metal-metal bonding in the frame and other structures in automobiles.47

Figure 3.20. Curative synthesis. [Adapted, by permission, from, Zarybnicka, L; Bacovska, R; Vecera, M; Snuparek, J; Alberti, M; Rychly, J; Kalenda, P, J. Appl. Polym.Sci., 133, 42917, 2016.]

The foundry binder system, which cures in the presence of sulfur dioxide and an oxidizing agent, comprising (a) an epoxy resin; (b) an ester of a fatty acid; (c) a fluorinated acid, preferably hydrofluoric acid; (d) an oxidizing agent (cumene hydroperoxide).48

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A self-emulsifiable waterborne amine-terminated curing agent for epoxy resin was synthesized on the base of the glycidyl tertiary carboxylic ester.49 The cured resin had good thermal resistance, hardness, toughness, adhesion, and corrosion resistance.49 Epoxy resin was cured with halogenophosphazene.50 The hardener was prepared from hexachloro-cyclo-triphosphazene by nucleophilic substitution with isophorone diamine (Figure 3.20).50 The curative improved mechanical properties and the flame resistance, and lowered total heat release, amount of smoke released, and oxygen consumed.50 Polyphosphazene containing active amine groups was used as a reactive flame-retardant additive in epoxy resin.51 It was synthesized from N-aminoethylpiperazine and hexachlorocyclotriphosphazene (Figure 3.21).51 The resistance to fire was improved and smoke generation suppressed.51 The cured resin passed the vertical burning tests V-0 rating.51 The peak heat release rate and total heat release were decreased by 46.7% and 29.3%, respectively.51 The total smoke release was decreased by 48.0%.51

Figure 3.21. Synthesis of curative. [Adapted, by permission, from Yang, G; Wu, W-H; Wang, Y-H; Jiao, Y-H; Qin, X-Y, J. Hazardous Mater., 366, 78-87, 2019.]

The bridged-cyclotriphosphazene flame retardant, named bisphenol-S bridged penta(anilino)cyclotriphosphazene was used to cure and flame retard epoxy resin.52 The thermal stability was improved; the limiting oxygen index increased to 29.7% with 9 wt% of curative.52 The peak of heat release rate, total heat release, and total smoke production declined.52 Organic-acid hydrazide is a powder of a high melting point obtained from carboxylate ester and hydrazine. When dispersed in epoxy resin it works as a latent curing agent. The organic-acid hydrazide has a pot life of 4 to 6 months. It cures at ~150°C during 1-2 hours. It has a lower cure temperature, better water resistance, and adhesiveness than dicyandiamide. It is typically used in powder paints and one-part adhesives. A hydrazide compound having excellent curing properties and moisture resistance was used for curing

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acrylate epoxy resins.53 It was used as adhesive for electronic components, liquid crystal display cell, and sealing material.53 Curable resin compositions comprising an epoxy resin and a hydrazide curing agent was used in cementing and/or remedial operations in subterranean formations (cementing casings into boreholes, plugging (e.g., killing) wells containing organic binders).54 Imidazoles have a relatively long pot life, produce resins with a high heat deformation curing at medium temperatures (80-120°C). They can be used as a curing accelerator or co-curing agent for organic-acid anhydrides, dicyandiamides, polyhydric phenols, and aromatic amines. Anisotropic conductive films are essential components of liquid crystal displays.55 The imidazole-epoxy resin system having a latent behavior is generally used as adhesive for these films.55 The methacrylic acid-dodecyl methacrylate copolymer was used to encapsulate an imidazole curing agent.55 The encapsulated imidazole was mixed with epoxy resin.55 The silver adhesives applied to a light-emitting diode as die-attach materials consisted of silver particles, on epoxy resin (3,4-epoxycyclohexyl-methyl-3,4-epoxycyclohexanecarboxylate), curing agents (2-ethyl-4-methyl-1Himidazole-1-propanenitrile, 2phenylimidazole, 2-methylimidazole, 2-phenyl-2-imidazoline, and 1,2-dimethylimidazole), and accelerators for complete curing Figure 3.22. The curing mechanism of epoxy cured by EMI and its at 150oC for 30 min.56 The silver derivatives. [Adapted, by permission, from Lei, D; Ma, W; Wang, adhesive containing a 100 wt% of L; Zhang, D, J. Appl. Polym. Sci., 132, 42563, 2015.] epoxy resin mixed with 85 wt% of hexahydrophthalic anhydride, 1.0 wt% of 2-ethyl-4-methyl-1H-imidazole-1-propanenitrile, and 80 wt% of hybrid silver particles gave the best performance.56 Three 2-ethyl-4-methylimidazole (EMI) derivatives (N-acetyl EMI, N-benzoyl EMI, and N-benzenesulfonyl EMI) were synthesized reacting EMI with acetyl chloride, benzoyl chloride, and benzenesulfonyl chloride, respectively.57 The diglycidyl ether of bisphenol A epoxy resin was cured with these curatives at a temperature of 110-160oC (activation energy of 71-86 kJ/mol) (Figure 3.22).57 The encapsulation enhanced the storage stability of the curing system.57 To improve the shelf-life of epoxy cured with imidazole derivatives, a 2-(2-hydroxyphenyl)imidazole derivative, having an intramolecular hydrogen bond between the pheno-

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lic hydroxyl group and the nitrogen atom of the imidazole ring, was developed leading to suppression of its reactivity toward epoxy resins at room temperature (Figure 3.23).58 High reactivity with epoxy resins at 150°C was due to breakage of the intramolecular hydrogen, whereas the epoxy resin composition had Figure 3.23. The mechanism of hindrance and curing. [Adapted, by long-term storage stability at room permission, from Kudo, K; Furutani, M; Arimitsu, K, ACS Macro 58 temperature. Lett., 4, 1085-8, 2015.] The 2-ethyl-4-methylimidazol in different concentration was used to cure epoxy resin (10-50 mol%).59 The reduced crosslink density improved the foamability of cured epoxy resin.59 The microcellular epoxy foams were obtained at a moderate crosslink density.59 The introduction of microcellular structure in the epoxy matrix was conducive to the improvement of the ductility of epoxy foams.59

Figure 3.24. Synthesis of curative. [Adapted, by permission, from Huo, S; Wang, J; Yang, S; Li, C; Cai, H, Polym. Deg. Stab., 159, 79-89, 2019.]

The retardant imidazole curing agent containing phosphaphenanthrene group was synthesized and used as a co-curing agent of 4,4‘-diaminediphenyl sulfone to prepare a reactive flame-retarded epoxy thermoset.60 Figure 3.24 shows the method of synthesis of curative.60 The LOI value and UL94 rating of cured resin reached 36.8% and V-0, respectively.60

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A series of maleimide-modified imidazole derivatives were synthesized through the addition reaction between N-(4-hydroxyphenyl) maleimide and imidazole compounds with 1-position N−H bond.61 The maleimide-modified imidazole derivatives were blended with epoxy resin to evaluate their reactivity and thermal latency.61 The exothermic interval of curing the epoxy systems containing maleimide-modified imidazole derivatives shifted to the higher temperature regions.61 The modified imidazole derivatives had much longer pot life at room temperature.61 The enhanced latency was attributed to the strong electron withdrawing effect of the maleimide group, which reduced the nucleophilicity of imidazole moiety.61

Figure 3.25. Storage stability of epoxy systems. 1M − imidazole, 2MI − 2-methylimidazole, 2EI − 2-ethylimidazole, EMI − 2-ethyl-4-methylimidazole. [Adapted, by permission, from Yang, S; Zhang, Q; Hu, Y; Ding, G; Wang, J, Mater. Lett., 234, 379-83, 2019.]

A latent epoxy resin curing agent exhibiting excellent low-temperature fast-curing contains an imidazole-based compound as a main component.62 The particulate imidazole-based compound was coated with an ethyl cellulose film.62 Ionic amines (1-(3-aminopropyl)-3-butylimidazolium bis(trifluoromethylsulfonyl)imide([apbim][NTf2]) and the tetrabutylammonium leucine ([N4444][Leu])) were used with bisphenol-A diglycidyl ether.63 The kinetic data of curing followed exponential first-order kinetics unlike in the case of ordinary amines which usually follow the autocatalytic model.63 The conducting amine changes the mechanism of polymerization acting as both the substrate and the catalyst of this specific chemical conversion. The ionic conductivity of cured samples was significant at the glass transition temperature as compared with other polymerized ionic liquids.63 The mechanical properties of epoxy composite were improved by incorporation of the silica supported by ionic liquid.64 It resulted in improved interfacial interaction and reinforcement of cured epoxy composite.64 Figure 3.26 shows the chemical mechanisms of action of silica which are credited with improvement of mechanical properties of epoxy composite.64 Formation of covalent bonds improved interfacial adhesion and resulted in uniform dispersion of reinforcing filler in the polymer matrix of the composite.64

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Figure 3.26. Schematic of crosslinking reactions between epoxy resin and silica supported by ionic liquid. [Adapted, by permission, from Zhang, C; Mi, X; Tian, J; Zhang, J; Xu, Polymers, 9, 478, 2017.]

Isosorbide-based amine and epoxide resin were used together to produce a bio-based resin.65 The cured resin had good shape memory and recovery and thermal stability despite the presence of heteroatoms.65 Isosorbide-based epoxy resin was synthesized in the one-step reaction from 1,4:3,6-dianhydro-d-glucitol (isosorbide) and epichlorohydrin in the presence of concentrated aqueous NaOH.66 The properties of cured resin suggest that it is a good candidate to replace bisphenol-A-based resins.66 Anti-fogging films derived from isosorbide-based epoxy polymer were fabricated by film formation and subsequent thermal polymerization.67 Isosorbide di(meth)acrylate was synthesized as a photo-curable reactive diluent.68 The reactive diluent was mixed with polyurethane acrylic and epoxy acrylic to prepare a series of UV curable coatings.68 The addition of 30 wt%

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reduced viscosity by three orders of magnitude.68 The coatings that contained reactive diluent had a higher storage modulus and good impact properties.68 Isosorbide-derived epoxies were attached to glycidyl ether to make crosslinkable epoxy resin monomers.69 Adding the hydrophobic functional group into the backbone of isosorbide epoxy or adjusting the amount and type of crosslinker can modify the mechanical properties and water uptake ratio of the isosorbide-derived epoxies.69 High water uptake epoxies with controllable biodegradation rate are suitable for drug delivery systems or extracellular matrices for biomedical applications, while low water uptake epoxies with robust mechanical properties may be used for can coatings, bone cements, and industrial additives and adhesives.69 Ketimines have very low cure rates in epoxy resin. They need to absorb moisture from the air, unblock amines to cure at room temperature. Many amines can be used for the production of Figure 3.27. (A) Regeneration of amine by imine hydrolysis after exposure to atmospheric humidity. The released amine can react ketimines with methylethyl or with epoxy. (B) Tautomeric equilibrium between the imine and the isobutyl ketones. In high solids more basic enamine. [Adapted, by permission, from Vidil, T; paints, ketimine cure takes about 8 Tournilhac, F; Musso, S; Robisson, A; Leibler, L, Prog. Polym. Sci., 62, 126-79, 2016.] hours. The resultant properties are the same as if cured with original amines, but they are applied only to very thin films because of slow moisture diffusion and need to evaporate ketones. In storage conditions, ketimine's nucleophilicity is low enough to obtain mixtures with long shelf-life.70 When used at normal atmosphere as coating agent for painting, the system absorbs moisture that regenerates the amine by imine hydrolysis (Figure 3.27A).70 The shelf-life is not infinite because ketimines are not completely nonreactive toward epoxies due to the enamine-imine tautomerism presented in Figure 3.27B.70 A one-pack moisture-curing epoxy resin composition which can be cured at room temperatures and has improved storage stability without impairing the quickness of curing used ketimine as the curing agent.71 The silyl compound (present in the composition) reacts with a small amount of water entering the composition system during storage before the ketimine compound may react with the water.71 The silyl compound prevents hydrolysis of the ketimine compound and improves storage stability.71 The hydroxyl groups of lignin were converted to the tosyl groups as leaving groups followed by primary amination with an ammonia solution at high pressure and tempera-

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ture, in the presence of a nano-alumina-based catalyst.72 The product of this reaction was used for curing the diglycidyl ether of bisphenol A.72 The mechanical properties of the aminated lignin-epoxy system exhibited performance, which was competitive with the epoxy systems cured by commercial aromatic curing agents.72 There are numerous publications in which epoxy resin is based on lignin or lignin is incorporated into the structure of the epoxy monomer. A high bio-based content was attained by curing epoxidized soybean oil with a green curing agent maleopimaric acid catalyzed by 2-ethyl-4-methylimidazole.73 Activation energy decreased from 82.70 to 80.17 kJ/mol when the amount of catalyst was increased from 0.5 to 1.5 phr.73 The curing exotherms shifted to a higher temperature as the heating rate increased.73 The total heat release for the ring-opening polymerization of epoxidized soybean oil and maleopimaric acid was only 31.7 kJ/mol epoxy group.74 The tensile properties and glass transition temperature of the thermosets cured by maleopimaric acid were close to products cured with a cycloaliphatic ring.74 The products cured with maleopimaric acid had excellent thermal stability (10%-weight-loss at temperatures up to 349°C).74 Rosin-derived anhydride (methyl maleopimarate) and acid-anhydride (maleopimaric acid) exhibit similar curing reactivity to that of their commercial counterparts 1,2-cyclohexanedicarboxylic anhydride and 1,2,4-benzenetricarboxylic anhydride, respectively, and the cured epoxy resins display comparable mechanical and dynamic mechanical properties.75 The epoxy resin was cured with rosin-sourced anhydride (maleopimaric acid) in the presence of imidazole type latent catalyst.76 The result was compared with a resin cured with petroleum-sourced hardener methyl hexahydrophthalic anhydride.76 The bending strength, bending modulus, and the glass transition temperature of biobased cured resin were increased by 44%, 73%, and 70oC as compared with petroleum-based curative.76 The renewable bis-epoxide 2,2′-diglycidyl ether-3,3′-dimethoxy-5,5′-diallydiphenylmethane (BEF-EP) and its hardener 3-methoxy-4-hydroxy-phenylbenzimidazole (VBZMI) were prepared from 1-allyl-3-methoxy-4-hydroxybenzene (eugenol) and 2-methoxy-4-formylphenol (vanillin), respectively (Figure 3.28).77 The biobased bisphenol monomer 2,2′dihydroxy-3,3′-dimethoxy-5,5′-diallydiphenylmethane (BEF) as the precursor of BEF-EP showed substantially lower estrogenic activity than the commercial resin based on bisphenol-A.77 Curing the biobased epoxy resin with 3-methoxy-4-hydroxy-phenylbenzimidazole gave higher thermal stability, and higher glass transition temperature and contact angle than resin cured with benzimidazole.77 This result can be attributed to the presence of hydroxyl groups in the side chain of 3-methoxy-4-hydroxy-phenylbenzimidazole, which enhanced the crosslink density and improved the rigidity of epoxy material.77 Epoxidized sucrose soyate a bio-based epoxy resin derived from sucrose and soybean oil fatty acids, which contained an average of 12 epoxy functional groups per molecule was cured in the presence of a zinc-complex catalyst with methyl hexahydrophthalic anhydride to form thermoset with high crosslink density.78 The molecular networks of the thermosets are complex due to the competing reactions between catalyst-initiated epoxyanhydride, hydroxyl-initiated epoxy-anhydride, and epoxy homopolymerization.78

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Figure 3.28. Synthesis of epoxy resin and curative. [Adapted, by permission, from Jiang, H; Sun, L; Zhang, Y; Liu, Q; Zhao, C, Polym. Deg. Stab., 160, 45-52, 2019.]

Cycloaliphatic epoxies cured with acid anhydride hardeners are used for outdoor electrical applications as they enable high resistance to UV weathering.79 A self-healing hydrophobic epoxy resin composition was developed with methyl hexahydrophthalic anhydride.79 Methyl hexahydrophthalic anhydride and hexahydrophthalic anhydride are widely used as the main curing agents for cycloaliphatic outdoor epoxy resins used in electrical insulation applications, but there is a future need for alternative solutions due to the devel-

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opment of regulatory framework for chemicals which is expected to restrict the use of anhydrides in epoxy resins because of their R42 label as respiratory sensitizer.80 Four cardanol-based phenalkamines synthesized from ethylenediamine, diethylenetriamine, triethylenetetramine, and pentaethylenehexamine were used as curing agents in diglycidyl ether of bisphenol A epoxy systems.81 All four phenalkamines cure epoxy resins according to a similar curing mechanism.81 The diffusion of reactive groups plays an important role in the curing kinetics.81 High molecular weight phenalkamines resulted in faster surface drying due to rapid molecular weight build-up.82 The anticorrosive performance improved as indicated by higher modulus and electrochemical potential values.82

Figure 3.29. Synthesis route of phosphorus-containing dicyclopentadiene novolac curing agent. [Adapted, by permission, from Yang, J-W; Wang, Z-Z; Liu, L, J. Appl. Polym. Sci., 134, 44599, 2017.]

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Phenalkamine blended with a salted polyamine, or a salted polyamine-epoxy adduct was used as a curing agent for an epoxy resin.83 They are suitable for curing epoxy resins under low temperature curing conditions.83 A curing agent composition for an epoxy compound includes (a) phenalkamine, and (b) isocyanate.84 Phenalkamines are moisture insensitive and enable curing even underwater.84 Phenalkamines provide several benefits, such as rapid cure within a workable pot life, which is comparable to many aliphatic amines, good chemical resistance, such as for example passing a 4,000 hour salt spray test; low temperature cure such as curing at temperatures as low as -5°C.84 A phenalkamine used alone as a curing agent will bring brittleness to epoxy resin system.84 A combination of a phenalkamine and an isocyanate is used to flexibilize an epoxy resin.84 A phosphorus-containing dicyclopentadiene novolac curing agent for epoxy resins was prepared from 9,10-dihydro-oxa-10-phosphaphenanthrene-10-oxide and n-butylated dicyclopentadiene phenolic resin (Figure 3.29).85 The cured epoxy resin had LOI value of 31.6% and UL 94 V-0 rating.85 Its glass transition temperature was lower (133oC) than cured with novolac resin.85 The 4,4’-[1,3-phenyl-bis(9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-yl)dimethyneimino)]diphenol was used to cure epoxy resin having single glass transition temperature.86 Good thermal stability and flame retardance were characteristics of this resin (LOI = 37.5%).86 Poly-(meta-xylylenediamine spirocyclic pentaerythritol bisphosphonate) is an intumescent, flame-retardant curing agent suitable for halogen-free flame-retarded epoxy resins.87 The flame-retarded composites containing 3.01% phosphorus had the best combination of properties, including a higher glass transition temperature (147oC), good thermal stability, an initial weight loss temperature at 269oC, LOI of 31.2%, UL94 V-0 level, the tensile strength of 51 MPa, and impact strength of 4.8 kJ/m2.87 Figure 3.30 shows the synthesis route of curative.87

Figure 3.30. Synthesis route of poly-(meta-xylylenediamine spirocyclic pentaerythritol bisphosphonate). [Adapted, by permission, from Liang, B; Wang, G; Hong, X; Long, J; Tsubaki, N, High Performance Polym., 28, 1, 110-8, 2016.]

Color-stable epoxy resin was obtained with poly(propylene glycol)bis(2-aminopropyl ether) curative.88 Various sizes of TiO2 nanoparticles (5-10, 21, 200-400 nm) were used to study the photocatalytic effect on removal of pollutants deposited on the surface of stones. The best results were obtained with the largest particles.88

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Polyamide resin contains reactive primary and secondary amines which can be used to cure the epoxy resin. It is formed by the condensation of dimer acid and polyamine. Polyamide typically cures at normal temperature (or even below) with moderate heat generation. It has a long pot life. Epoxy resins cured by polyamide are highly plasticized rigid thermosetting polymers having high tensile, compression, and bending strengths, which are stiff, strong, and have excellent shock resistance. Polyamide curatives of different molecular weight were made in a two-step process of functionalization of cardanol by maleic anhydride followed by its condensation with diethylenetriamine in the second step (Figure 3.31).89 The performance properties of cured resins improved with an increase in amine value of polyamides because of increased crosslink density as did the anticorrosive performance.89

Figure 3.31. Synthesis of reactive polyamide from functionalized cardanol. [Adapted, by permission, from Balgude, D; Sabnis, A; Ghosh, SK, Prog. Org. Coat., 104, 250-62, 2017.]

The epoxy composites were cured and reinforced by graphene oxide functionalized with hyperbranched polyamide.90 Uniform dispersion of hyperbranched polyamide functionalized graphene oxide in the epoxy matrix was achieved.90 Fully biobased, high performance epoxy thermosets were synthesized by curing of epoxidized soybean oil with crystalline diamine-terminated polyamide 1010 oligomers (Figure 3.32).91 The curing rate decreased with increasing molecular weight of the polyamide oligomers.91 The glass transition and melting temperatures were increased with

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increasing polyamide oligomer chain length.91 The tensile strength, Young's modulus, storage modulus, and elongation at break considerably increased with increasing polyamide oligomer chain length because of the gradually enhanced network stiffness due to crystallization and reduced crosslink density and increased chain length between crosslinking sites.91 The biobased epoxy thermosets showed excellent thermal stability (onset decomposition temperature higher than 330°C).91

Figure 3.32. Synthesis of diamine-terminated polyamide 1010 oligomer. [Adapted, by permission, from Li, Y-D; Jian, X-Y; Zhu, J; Du, A-K; Zeng, J-B, Polym. Testing, 72, 140-6, 2018.]

An epoxy resin/polyamidoamine hardener system was followed in real-time by magnetic resonance.92 The effects of crosslinking have been manifested by changes in signal amplitude and decay time constant.92 Two epoxy networks based on diglycidyl ether of bisphenol A cured by polyoxypropylene or polyamidoamine were compared.93 Thermal oxidation led to carbonyl and amide formation in both systems. Polyoxypropylene systems were more sensitive to oxidation than polyamidoamine cured resin.93 Thermal oxidation resulted in chain scission as evidenced by the decrease in glass transition temperatures.93 The reactive diluent was used to reduce the viscosity of high viscosity polyamidoamines prepared from fatty dimer acids and polyethyleneamines.94 The reactive diluent comprised the amide and/or imidazoline reaction products of low molecular weight carboxylic acids, such as acetic acid, and polyalkyleneamines, such as 2-aminoethylpiperazine and triethylenetetramine.94 The epoxy-amine composition having low mix viscosity and a fast drying property at temperatures below freezing has been found useful for high solids coating applications, adhesive and membrane applications, and preparing impregnated substrates.95 The curative was a liquid polyamidoamine prepared by reacting a long-chain carboxylic acid and an amine.95 Epoxy resins were cured with polyetheramines.96 Reaction rate increased with decreasing molecular weight of curing agents.96 Addition of 3 wt% of curing agent of higher molecular weight produced 270% improvement in storage modulus.96 Fiber-reinforced plastics are used as materials of construction for motor vehicles, aircrafts, ships and boats, for sport articles, and for rotor blades of wind turbines.97 The production of large components imposes particular requirements on the hardener or hardener mixture since during the processing life the viscosity must not rise so sharply that either the fibers are not adequately wetted or the mold is not completely filled before the epoxy resin becomes no longer processable.97 The long pot life can be achieved through the use of polyetheramines of low reactivity.97 The disadvantage of exclusive use of poly-

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Figure 3.33. The mechanism of the color transition. [Adapted, by permission, from Lee, TH; Park, YI; Noh, SM; Kim, JC, Prog. Org. Coat., 104, 20-7, 2017.]

etheramines of low reactivity in infusion technology lies in the extremely long cure times at elevated temperature.97 A combination with isophorone diamine helps in balancing both requirements. A combination of 30% of polyetheramine (Jeffamine D230), isophorone diamine, and 10 wt% of tetramethylguanidine increased pot life by 380%.97 A mixture of cycloaliphatic amine and polyetheramine was used for curing epoxy advantageous for producing a tunable, quick-curing epoxy resin composition that is useful in filament winding applications.98 Polymercaptan, which cures at 0°C to -20°C, is a low-temperature curing agent. Tertiary amine is usually added as an accelerator. Polymercaptan has a pot life of 2 to 10 minutes at normal temperature. It rapidly cures reaching practical strength in 10 to 30 minutes. Similar to liquid polymercaptans, the polysulfide resins have mercaptan groups at its terminals, but they do not have low-temperature and fast-curing properties and are mainly used as the curing agents and a flexibilizers. Polysulfide resins are used in combination with a tertiary amines or polyamines. At a loading of 50 to 100 wt%, the cured resin

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increases flexibility, shock resistance, and permittivity, and decreases in curing shrinkage. It has good water resistance. The polysulfide resins have been used in adhesives, sealing agents, and casting materials. Thioether glycidyl resin bis[3-(2,3-epoxypropylthio)phenyl]-sulfone having high refractive index was synthesized by condensation of bis(3-mercaptophenyl)sulfone with epichlorohydrin.99 Trimercaptothioethylamine was used as curing agent to cure the epoxy resin. The cured resins had the highest refractive index of 1.67.99 The kinetics of the crosslinking of model thiol-epoxy polymer networks was followed by tracking their pH variations.100 The color transitions of the networks were monitored in real-time, and the results were correlated with Fourier transform infrared spectroscopy, isothermal differential scanning calorimetry, and oscillational rheology.100 Figure 3.33 shows the mechanisms of activation, propagation, and termination and the reasons for color changes of added pH indicator.100 Two-component epoxy resin which had excellent adhesion properties to glazed surfaces and a short tack free time at ambient and low-temperature conditions.101 A mixture of polymercaptanes (e.g., Aramine 39-730) and aminophenols such as 2,4,6-tris(diethylaminomethyl)phenol and/or 2,4,6-tris(dimethylamnimethyl)phenol were used as curing agents.101 Curable coating compositions for preventing biofouling included alkylene polyamine, polyalkylene polyamine, or polymercaptan epoxy curing agent providing 0.75 to 1.5 equivalents of amine nitrogen atoms and/or thiol groups per equivalent of epoxy groups.102 The coating exhibits a water contact angle of at least 100°.102 A two part epoxy adhesive system made of a composition (A) including 15 to 50% of a curable bisphenol A epoxy resin; 0.5 to 40% of an alkaline earth metal oxide or hydroxide; and 30 to 80% of an inert filler; and a composition (B) including: 15 to 50% of a polymercaptan obtained by the esterification of mercaptoacetic or mercaptopropionic acid with a polyol including 3 to 6 carbon atoms, and/or a polymercaptan obtained by the effect of hydrogen sulfide on a polyoxyalkylene polyglycidyl ether.103 This system had improved open time and was used as glue for attachment or repair.103 Bis(3-aminopropyl)-terminated polydimethysiloxane has been used as a curing agent of biobased epoxy resin extracted from waste streams of industrial processes, such as biofuel and wood pulp production.104 The epoxy was a mixture of epoxidized pine oils, bisphenol A/F type, and benzyl alcohol.104 The use of curative decreased impact strength.104 The 1,3-bis(2-aminoethylaminomethyl) tetramethyldisiloxane was employed to cure bisphenol A epoxy resin.105

Figure 3.34. Formation of nodes. [Adapted, by permission, from Torrico, RFAO; Harb, SV; Trentin, A; Uvida, MC; Hammer, P, J. Colloid Interface Sci., 513, 617-28, 2018.]

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Epoxy-siloxane-silica hybrids were obtained from poly(bisphenol A-co-epichlorohydrin) cured with diethyltriamine and (3-glycidoxypropyl)methyltriethoxysilane, followed by hydrolytic condensation of tetraethoxysilane and siloxane.106 Highly condensed silicasiloxane domains covalently bonded to the embedding epoxy phase forming the quasispherical sub-nonmetric silica-siloxane nodes (Figure 3.34).106 This dense nanostructure gives high thermal stability (>300°C), strong adhesion to the steel substrate, and excellent barrier property in saline solution, with corrosion resistance in the GΩ cm2 range.106 Coating systems comprising an epoxy-siloxane topcoat have offered better corrosion protection of the steel substrate than its counterpart with a polyurethane topcoat.107 The coating system led to the highest impedance module (GΩ cm2) after more than a year of constant immersion in 3 wt% NaCl solution.107 Epoxy-polysiloxane-based coating and flooring compositions had improved flexibility, and excellent weatherability and corrosion resistance.108 It was prepared by a combination of a polysiloxane, an epoxide resin, and a cure system including a blend of trialkoxy functional aminosilane and amino-functional polysiloxane resin.108 The epoxy-based composition included a polysiloxane flexibilizer and amino-functional alkoxysilane, which provided flexibility, hardness, and gloss to compositions which were found useful as coatings, adhesives, sealants, and composites.109

Figure 3.35. Synthesis of hybrid epoxy. [Adapted, by permission, from Ni, C; Ni, G; Zhang, L; Mi, J; Zhu, C, J. Colloid Interface Sci., 362, 1, 94-9, 2011.]

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Figure 3.36. Curing epoxy with triazine derivative and its controlled degradation in the acidic environment. [Adapted, by permission, from You, S; Ma, S; Dai, J; Jia, Z; Liu, X; Zhu, J, ACS Sustainable Chem. Eng., 5, 4683, 9, 2017.]

Inorganic/organic hybrid material containing silsesquioxane was prepared by the reaction of caged octa (aminopropyl silsesquioxane) with n-butyl glycidyl ether (nBGE) and 1,4-butanediol diglycidyl ether (Figure 3.35).110 The toughening and the thermal properties of the cured epoxy resin were improved by the addition of the hybrid.110 The enhancement resulted from the nano-scale effect of the silsesquioxane structure and the formation of anchor structure in the cured network.110 Silyl ether amine curing agents obtained by selective substitution reactions of chloroalkylsilanes or the transesterification of alkoxysilanes were used with a stoichiometric ratio of bisphenol A diglycidyl.111 The network had the onset of thermal degradation, glass transition temperatures, and storage moduli of 350oC, 70-108oC, and 5-25 MPa, respectively.111 These values were affected by the functionality of the amine curing agents and the number of hydrolyzable silyl ether bonds present per mole of the curing agent.111 The 4,4’,4”(1,3,5-hexahydro-s-triazine-1,3,5-triyl) tris(N-(2-aminoethyl) benzamide bearing three amino groups was synthesized as a curing agent to prepare the aciddegradable epoxy resin.112 The cured epoxy can be controllably degraded in the presence of strong acid (Figure 3.36).112 High frequency printed circuit boards were made using triazine hardener (curing agent).113 The use of this hardener lowers relative permittivity of cured epoxy resin.113

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Jul. 21, 2016. Shin, YJ; Shin, MJ; Shin, JS, Polym. Int., 66, 795-802, 2017. Chiang, TH; Lin, Y-C; Chen, Y-F; Chen, E-Y, J. Appl. Polym. Sci., 133, 43587, 2016. Lei, D; Ma, W; Wang, L; Zhang, D, J. Appl. Polym. Sci., 132, 42563, 2015. Kudo, K; Furutani, M; Arimitsu, K, ACS Macro Lett., 4, 1085-8, 2015. Li, J; Zhang, G; Fan, X; Fan, X; Zhou, L; Li, J; Shi, X; Zhang, H, J. Cellular Plastics, 53, 65, 663-81, 2017. Huo, S; Wang, J; Yang, S; Li, C; Cai, H, Polym. Deg. Stab., 159, 79-89, 2019. Yang, S; Zhang, Q; Hu, Y; Ding, G; Wang, J, Mater. Lett., 234, 379-83, 2019. Masuko, D; Komuro, K; Ito, M; Kawashima, T, US8128837B2, Dexerials Corp Sony Corp, Mar. 6, 2012. Maksym, P; Tarnacka, M; Dzienia, A; Matuszek, K; Chrobok, A; Kaminski, K; Paluch, M, Macromolecules, 50, 3262-72, 2017. Zhang, C; Mi, X; Tian, J; Zhang, J; Xu, Polymers, 9, 478, 2017. Li, C; Dai, J; Liu, X; Jiang, Y; Ma, S; Zhu, J, Macromol. Chem. Phys., 217, 1439-47, 2016. Łukaszczyk, J; Bartosz Janicki, B; Marcin Kaczmarek, M, Eur. Polym. J., 47, 8, 1601-6, 2011. Park, S; Park, S; Jang, DH; Lee, HS; Park, CH, Mater. Lett., 180, 81-4, 2016. Wei, G; Xu, H; Chen, L; Li, Z; Liu, R, Prog. Org. Coat., 126, 162-7, 2019. Hammond, W; East, A; Jaffe, M; Feng, X, US9605108B2, New Jersey Institute of Technology, Mar. 28, 2017. Vidil, T; Tournilhac, F; Musso, S; Robisson, A; Leibler, L, Prog. Polym. Sci., 62, 126-79, 2016. Endo, T; Sanda, F; Horii, H; Suzuki, K; Matsuura, N, EP1362877B1, Konishi Co Ltd, Nov. 22, 2006. Nikafshar, S; Zabihi, O; Moradi, Y; Ahmadi, M; Amiri, S; Naebe, M, Polymers, 9, 266, 2017. Chen, Y; Xi, Z; Zhao, L, AIChE J., 63, 1, 147-53, 2017. Chen, Y; Xi, Z; Zhao, L, Eur. Polym. J., 84, 435-47, 2016. Zhang, J; Zhang, P, US20150344816A1, Washington State University, Dec. 3, 2015. Zhang, XF; Wu, YQQG; Wei, JH; Tong, JF; Yi, XS, Sci. China Technol. Sci., 60, 1318-31, 2017. Jiang, H; Sun, L; Zhang, Y; Liu, Q; Zhao, C, Polym. Deg. Stab., in press, 2019. Paramarta, A; Webster, DC, Reactive Functional Polym., 105, 140-9, 2016. Dubey, PK; Dixit, A; Verma, V, US20160229950A1, Aditya Birla Chemicals (Thailand) Ltd, Aug. 11, 2016. Beisele, C; Liu, Z; Hishikawa, S, WO2017001182A1, Huntsman Advanced Materials (Switzerland) GmbH, Jan. 5; 2017. Zhang, J; Xu, S, Iran, Polym. J., 26, 499-509, 2017. Kathalewar, M; Sabnis, A, Prog. Org. Coat., 84, 79-88, 2015. Sato, S; Shah, SC; Bueno, RC; Moon, RM; Ferreira, A, US8293132B2, Cognis IP Management GmbH, Oct. 23, 2012. Yan, L; Zhang, Y; Zhou, W, EP2914661A4, Dow Global Technologies LLC, Jun. 8, 2016. Yang, J-W; Wang, Z-Z; Liu, L, J. Appl. Polym. Sci., 134, 44599, 2017. Xie, C; Du, J; Dong, Z; Sun, S; Zhao, L; Dai, L, Polym. Eng. Sci., 56, 441-7, 2016. Liang, B; Wang, G; Hong, X; Long, J; Tsubaki, N, High Performance Polym., 28, 1, 110-8, 2016. Xu, F; Li, D, J. Polym. Environ., 25, 1304-12, 2017. Balgude, D; Sabnis, A; Ghosh, SK, Prog. Org. Coat., 104, 250-62, 2017. Qi, Z; Tan, Y; Gao, L; Zhang, C; Xiao, C, Polym. Testing, 71, 145-55, 2018. Li, Y-D; Jian, X-Y; Zhu, J; Du, A-K; Zeng, J-B, Polym. Testing, 72, 140-6, 2018. LaPlante, G; García-Naranjo, JC; Balcom, BJ, NDT & E Int., 44, 3, 329-34, 2011. Zahra, Y; Djouani, F; Fayolle, B; Kuntz, M; Verdu, J, Prog. Org. Coat., 77, 2, 380-7, 2014. Starner, WE; Myers, RS; Smith, AK, US6046282A, Air Products and Chemicals Inc, Apr. 4, 2000. De Cock, CJC; Bouuaert, PC; Kincaid, DS; Van Poppel, K; Vandenberghe, DE; Wang, PC, US7358312B2, Hexion Inc, Apr. 15, 2008. Jagtap, SB; Ratna, D, J. Appl. Polym. Sci., 134, 44595, 2017. Wittenbecher, L; Henningsen, M; Daun, G; Flick, D; Geisler, J-P; Schillgalies, J; Jacobi, E, US9012020B2, BASF SE, Apr. 21, 2015. Meyer, KJ; Hunter, GA; Potts, DL; Ritter, WL, US20170267809A1, Dow Global Technologies Llc, Sep. 21, 2017. Cui, Z; Lü, C; Yang, B; Shen, J; Yang, H, Polymer, 42, 26, 10095-100, 2001. Lee, TH; Park, YI; Noh, SM; Kim, JC, Prog. Org. Coat., 104, 20-7, 2017. Bumbu, GG, US9441071B2, Inovachem Engineering AG, Sep. 13, 2016. Huang, Y; Chen, H; Cao, X; Zhang, Y; Popa, PJ; Lin, Y; Roper, JA; Vandezande, GA, WO2017113149A1, Dow Global Technologies Llc, Rohm and Haas Company, Jul. 6, 2017. Nery, L; Guerineau, Q, EP2456805B1, Bostik SA, Aug. 21, 2013. Abdelwahab, MA; Misra, M; Mohanty, AK, J. Appl. Polym. Sci., 132, 42451, 2015.

3.2.5 Epoxy resins

105 106 107 108 109 110 111 112 113

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Li, C; Fan, H; Hu, J; Li, B, Thermochim. Acta, 549, 132-9, 2012. Torrico, RFAO; Harb, SV; Trentin, A; Uvida, MC; Hammer, P, J. Colloid Interface Sci., 513, 617-28, 2018. Echeverría, M; Abreu, CM; Lau, K; Echeverría, CA, Prog. Org. Coat., 92, 29-43, 2016. Mowrer, NR, US8846827B2, PPG Industries Ohio Inc, Sep. 30, 2014. Geismann, C; Kumar, V; Kondos, C, WO2014042944A3, Momentive Performance Materials Inc., May 30, 2014. Ni, C; Ni, G; Zhang, L; Mi, J; Zhu, C, J. Colloid Interface Sci., 362, 1, 94-9, 2011. Bassampour, ZS; Budy, SM; Son, DY, J. Appl. Polym. Sci., 134, 44620, 2017. You, S; Ma, S; Dai, J; Jia, Z; Liu, X; Zhu, J, ACS Sustainable Chem. Eng., 5, 4683, 9, 2017. Wang, S-S; Kang, H-C; Ma, J-H; Yin, M-S; Hong, S-T; Hsu, K-Y; Cheng, K-L, US6225378B1, Industrial Technology Research Institute, May 1, 2001.

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Curatives

3.2.6 EPOXY-NOVOLAC Flexible epoxy-novolac coatings were obtained by reacting an epoxy-novolac resin, poly[(phenylglycidyl ether)-co-formaldehyde] with an amine curing agent, 4,40-diamino3,30-dimethyldicyclohexyl methane, in the presence of a cardanol-based reactive diluent and cardanol-based flexibilizer.1 Addition of flexibilizer slightly increased glass transition temperature due to crosslinking and toughening of the coating composition.1 The bio-based novolac resin hardeners were synthesized using cardanol-formaldehyde condensation reaction.2 Cardanol present in novolac resins decreased the glass transition temperature due to its plasticizing effect.2 The curative also decreased water absorption of the cured resin.2 Cardanol was condensed with formaldehyde at the ortho- and para-positions of the phenolic ring to yield a series of polymers of novolac- or resol-type phenolic resins which were further modified by epoxidation with epichlorohydrin and cured with carboxyl terminated poly(butadiene-co-acrylonitrile) by the reaction between oxirane group of epoxy resin and the carboxyl group of curative.3 The thermal stability of the cardanol-based epoxy resin was increased with the addition of 15 wt% of curative.3 The ortho-cresol novolac oligomers were crosslinked with epoxies to form tough, flame retardant networks with reduced moisture uptake (due to the presence of the additional methyl group on each repeat unit).4 Novolac epoxy resins, with the high glass transition temperature and excellent mechanical strength, are widely applied in the production of electronic devices.5 Weather exposure and environmental elements affect their durability.5 The moisture absorption increased linearly with the square root of aging time following the Fick's second law.5 The plasticization and deterioration of epoxy resin were attributed to the moisture ingress.5 Phosphorus-containing phenol novolac resin was used as a halogen-free flame retardant epoxy hardener.6 The printed circuit boards having the copper clad laminate or the prepreg and the resin coated copper should be imparted with flame retardancy to prevent ignition upon firing.6 The standard for flame retardancy typically required is a UL-94 V-0 rating as provided by the use of this hardener.6 Pre-impregnated composite material (prepreg) that can be cured/molded to form aerospace composite parts.7 The prepreg includes carbon reinforcing fibers and an uncured resin matrix.7 The resin matrix includes an epoxy component that is a combination of a hydrocarbon epoxy novolac resin, a trifunctional epoxy resin, and optionally a tetrafunctional epoxy resin.7 The resin matrix includes polyethersulfone as a toughening agent and a thermoplastic particle component.7 The system was cured by aromatic amine such as 4,4′-diaminodiphenyl sulfone.7 References 1 2 3 4 5 6 7

Gour, RS; Raut, KG; Badiger, MV, J. Appl. Polym. Sci., 134, 44920, 2017. Liu, Z; Huo, J; Yu, Y, Mater. Today Commun., 10, 80-94, 2017. Srivastava, K; Rathore, AK; Srivastava, D, Spectrochim. Acta Part A: Molec. Biomolec. Spectr., 188, 99-105, 2018. Lin-Gibson, S; Baranauskas, V; Riffle, JS; Sorathia, U, Polymer, 43, 26, 7389-98, 2002. Schmidt, C; Ciesielski, M; Greiner, L; Döring, M, Polym. Deg. Stab., 158, 190-201, 2018. Kong, JW; Lee, SM; Sung, IK, US8648154B2, Kolon Industries Inc, Feb. 11, 2014. Zhu, Y; Emmerson, G; Wang, Y-S; Boyle, M; Leandro, J, US10106661B2, Hexcel Corp, Oct. 23, 2018.

3.2.7 Hydroxyl terminated azido polymer

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3.2.7 HYDROXYL TERMINATED AZIDO POLYMER Bis-propargylhydroquinone is an alkyne functionalized isocyanate-free curing agent for hydroxyl terminated azido polymers (typically glycidyl azide polymers are cured by isocyanate-based curatives).1 The reaction between hydroxyl and isocyanate is highly sensitive to moisture causing voids in the propellant, leading to poor mechanical properties.1 Triazole crosslinked polymer system was a better choice for energetic binder systems of explosives and propellants having improved performance.1 The glycidyl azido polymer was cured with various isocyanate curatives.2 The activation energy of the curing process was 34.21 kJ/mol.2 The delocalization of conjugate electrons between isocyanate groups and benzene rings enhanced the electropositivity of carbonyl carbon, which affected the reactivity of the electrophilic esterification of the hydroxyl groups.2 References 1 2

Sonawane, S; Anniyappan, M; Athar, J; Singh, A; Talawar, MB; Sinha, RK; Banerjee, S; Sikder, AK, Propelants Explos. Pyrotech., 42, 386-93, 2017. Tao, J; Jin, B; Peng, R; Chu, S, 71, 231-7, Polym. Testing, 71, 231-7, 2018.

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3.2.8 NONISOCYANATE POLYHYDROXYURETHANE Crystallization of a long n-alkyl side chains renders polyhydroxyurethane thermoresponsive.1 It enables thermomechanical programming of temperature-induced shape changes.1 Key intermediates of shape memory polyhydroxyurethane are highly branched, semicrystalline polyamidoamine curing agents tailored by amidation of a polyamine-terminated hyperbranched polyethylenimine with semicrystalline long chain behenic acid (Figure 3.37).1 Cure temperature and content of n-alkyl side chains govern crystallization behavior, phase separation, and mechanical properties of semicrystalline polyhydroxyurethane networks obtained by curing pentaerythritol-based polyfunctional cyclic carbonates with hyperbranched, semicrystalline polyamidoamines.1 Unlike conventional polyurethanes, triple-shape memory polyhydroxyurethane requires neither the use of isocyanates nor phosgene.1

Figure 3.37. Semi-crystalline polyamidoamine curing agents PEI-BA prepared by amidation of polyethyleneimine (PEI) with behenic acid (BA) and (b) Preparation of semicrystalline polyhydroxyurethane thermosets with fatty acid side chains. [Adapted, by permission, from Schimpf, V; Heck, B; Reiter, G; Mulhaupt, R, Macromolecules, 50, 3598-3506, 2017.]

A linear hybrid epoxy-amine hydroxyurethane-grafted polymer has been developed for manufacturing of liquid leather materials.2 The networks have been produced by the reaction of oligomers comprising terminal cyclocarbonate groups and oligomers containing terminal primary amine groups which are the products of epoxy resins reacting with carbon dioxide in the presence of a catalyst.2 References 1 2

Schimpf, V; Heck, B; Reiter, G; Mulhaupt, R, Macromolecules, 50, 3598-3506, 2017. Birukov, O; Figovsky, O; Leykin, A; Shapovalov, L, US20160244563A1, Polymate Ltd, Nanotech Industries Inc, Aug. 25, 2016.

3.2.9 Phthalonitrile resin

191

3.2.9 PHTHALONITRILE RESIN Phthalonitrile resins have excellent thermal and thermo-oxidative stability which makes them useful for aerospace and marine applications.1 Difficulties in the curing process prevent their industrial realization.1 Use of aromatic diamines such as 4,4’-diaminodiphenylsulfone, 1,3-bis(3-aminophenoxy)benzene and bis[4-(4-aminophenoxy)phenyl] sulfone can effectively lower the curing temperature of phthalonitrile resin.1 But, thermal decomposition of aromatic diamines does not permit to achieve desirable crosslink density. The mixed curing agents (CuCl/4,4’-diaminodiphenylsulfone (DDS) and ZnCl2/DDS) were developed to reduce temperature and curing time of phthalonitrile resin.1 The phthalonitrile resin cured with mixed curing agents exhibited excellent thermal stability and reasonable thermo-oxidative stability under a milder curing procedure (curing temperature 220oC and post-curing 290oC for 4 h).1 The studies done at the Naval Research Laboratory have shown that the use of aromatic amines as curing agents raises the problem of volatility and may affect the processability of the resin, which can be overcome to a large extent by employing highmolecular-weight, sulfone-containing amines such as meta- or para-derivatives of bis[(amino phenoxy) phenyl] sulfone.2 Phosphazenes (bis-, tris-, and tetrakis-maleimidophenoxy-triphenoxycyclotriphosphazene resins) enhance the speed of cure and add flame retardancy to the cured polymers.2 The phthalonitrile resin system was cured with 4,4'-(l,3-phenyleneoxy)aniline (2.1-4 wt%) at 200oC for 5 h under inert atmosphere to be used in diverse applications from building and construction, electronics packaging, energy and power generation, to transportation.3 References 1 2 3

Wu, Z; Han, J; Li, N; Weng, Z; Wang, J; Jian, X, Polym. Int., 66, 876-81, 2017. Augustine, D; Chandran, MS; Mathew, D; Nair, CPR, Chapter 18: Polyphthalonitrile resins and their high-end applications. Thermosets, 2nd Ed. Elsevier, 2018, pp.577-619. Anderson, BJ; Lomeda, JR; Thompson, WL; Higgins, JM; Patel, AJ, WO2017173195A1, 3M Innovative Properties Company, Oct. 5, 2017.

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3.2.10 POLYIMIDE The molecular weight-controlled aromatic poly(amic acid) resins functionalized with phenylethynyl end-groups were prepared via polycondensation of 3,3’,4,4’-biphenyltetracarboxylic dianhydride, para-phenylenediamine, and 4-phenylethynyl phthalic anhydride, and were thermally converted to the fully-cured polyimide films (Figure 3.38).1 The mechanical property, dimensional stability, and heat resistance of the fully-cured polyimide films with poly(amic acid) having Mn > 20x103 g/mol were better than that of their unreactive phthalic end-capped counterparts.1 The covalent incorporation of chain-extension structures in the backbones, induced by thermal curing of phenylethynyl groups, facilitated a higher degree of polymer chain order and improved resistance strength and elongation at break.1

Figure 3.38. Preparation of polyimide films from the respective series of molecular weight-controlled poly(amic acid) resins. [Adapted, by permission, from Yuan, L; Ji, M; Yang, S, J. Appl. Polym. Sci., 134, 45168, 2017.]

3.2.10 Polyimide

193

To lower the imidization temperature of poly(amic acid), the catalytic activities of the curing agents, such as p-hydroxybenzoic acid, quinoline, benzimidazole, benzotriazole, triethylamine, and 1, 8-diazabicyclo [5.4.0]undec-7-ene, were investigated.2 Because of its moderate base strength, low steric crowding effect, and moderate boiling point, quinoline accelerated poly(amic acid) to achieve complete imidization at 180oC and maintained the mechanical properties and thermal stability of the ordinary PI film.2 References 1 2

Yuan, L; Ji, M; Yang, S, J. Appl. Polym. Sci., 134, 45168, 2017. Xu, Y; Zhao, A; Wang, X; Xue, H; Liu, F, J. Wuhan Uni. Technol. Mater. Sci. Ed., 31, 5, 1137-43, 2016.

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Curatives

3.2.11 POLYSILOXANE Polydimethylsiloxane was cured with dimethyl-methyl hydrogen-siloxane.1 Both the resin and the curing agent had a Si−O−Si structure with CH3 groups which were polymerized according to reaction activated by a Pt-containing catalyst (Figure 3.39).1

Figure 3.39. Chemical structure and curing process of PDMS. [Adapted, by permission, from Lee, MW; Yoon, SS; Yarin, AL, ACS Appl. Mater. Interfaces, 9, 17449-55, 2017.]

After the release of self-healing components, the polymerized domains formed a system of pillars, which spanned the crack banks on the opposite side.1 This “stitching” phenomenon prevented further propagation of the crack.1 The methacrylate based poly(siloxane-silsesquioxane) for optoelectronic application was cured by phenyltris(hydrogendimethylsiloxy)silane.2 Excellent thermal stability, improved glass transition temperature, and lowered coefficient of thermal expansion were observed with the increasing POSS content. Excellent transparency and improved thermal discoloration resistance were also achieved.2 Siliconized epoxy matrix resin was obtained by reacting diglycidyl ethers of bisphenol A with hydroxyl terminated polydimethylsiloxane modifier, using γ-aminopropyltriethoxysilane curative and dibutyltindilaurate catalyst.3 It was then cured with 4,4diaminodiphenylmethane, 1,6-hexanediamine, and bis (4-aminophenyl) phenylphosphate.3 The thermal stability and flame-retardant properties were improved by the incorporation of both silicone and phosphorus moieties.3 Curable coating compositions for preventing biofouling included a) epoxy resin; b) amino-functional poly(dialkylsiloxane) polymer and c) alkylene polyamine, polyalkylene polyamine, or polymercaptan.4 When cured to form a coating, it had a water contact angle of at least 100°, adhered well to many substrates, and provided good anticorrosion protection.4 References 1 2 3 4

Lee, MW; Yoon, SS; Yarin, AL, ACS Appl. Mater. Interfaces, 9, 17449-55, 2017. Loh, TC; Ng, CM; Kumar, RN; Ismail, H; Ahmad, Z, J. Appl. Polym. Sci., 134, 45285, 2017. Kumar, SA; Denchev, Z; Alagar, M, Eur. Polym. J., 42, 10, 2419-29, 2006. Huang, Y; Chen, H; Cao, X; Zhang, Y; Popa, PJ; Lin, Y; Roper, JA; Vandezande, GA, WO2017113149A1, Dow Global Technologies Llc, Rohm and Haas Company, Jul. 6, 2017.

3.2.12 Polyurethane

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3.2.12 POLYURETHANE Polyurethane crosslinking was discussed in section 2.2.51. Similarly, it belongs to this section with an explanation. In terms of the chemical crosslinking, polyurethanes are crosslinked when either tri- or more-functional isocyanate or curing agent (e.g., alcohol or amine) are used. Crosslinks are also formed with biuret or allophanate formation which is most likely result of side reactions. Therefore, in technical terms, the reaction of bifunctional isocyanate with bifunctional, say, amine-cured polyurethane is curing or chain extension. At the same time, we know that polyurethanes have a segmented structure composed of hard and soft blocks, both of which are incompatible therefore they undergo phase separation. Both blocks are responsible for different elements of performance: soft blocks are responsible for polyurethane flexibility, and hard blocks are nodes of structure reinforcement similar to chemical crosslinks. In fact, they are crosslinks formed by hydrogen bonding which are frequently referred to as physical crosslinking because of their low activation energy of formation and degradation. These crosslinks are very reinforcing because they are typically formed by a pair of neighboring hydrogen bonds. It is well acknowledged that both covalent and hydrogen bonded crosslinks frequently interfere with each other leading to complex relationships. Presence and influence of two types of crosslinks lead us to believe that all polyurethanes are crosslinked, even systems which are formed from seemingly chain extending monomers. Having this in mind, we will further illustrate polyurethane curatives with examples which may add information to the already discussed in section 2.2.51. Amino-silica microcapsules containing encapsulated glycerol were used as effective curing agents for polyurethane foams.1 The microcapsules were intended to burst and release their content at the spraying process, to promote accelerated curing of the polyurethane foam.1 Microcapsules also contained multi-functional reactive isocyanate-terminated polyurethane prepolymer as a healing agent.2 The healing agent was prepared through the reaction of an excess amount of isophorone diisocyanate with 2-ethyl-2-hydroxymethyl-1,3propanediol and was encapsulated with a polyurethane shell via oil-in-water emulsion polymerization.2 The microcapsules were very stable after 10 months, and they just lost less than 7 wt% of their loaded isocyanate molecules.2 Microcapsules with isophorone diisocyanate core and polyurethane shell were prepared for self-healing coatings.3 They were dispersed in an acrylic-melamine clearcoat to be used for the development of scratch resistant coating.3 The hydrophilic curing agent was prepared from hexamethylene diisocyanate trimer, polyethylene glycol monomethyl ether, and 2-((2-aminoethyl)amino)-ethanesulfonic acid monosodium salt to be used in flame-retardant, two-component, waterborne polyurethanes.4 Polyurethane ester foams are found in 20th-century museum collections in a variety of artworks and objects.5 Unfortunately, polyurethane ester foams are prone to degradation by hydrolysis leading to yellowing and embrittlement.5 Different binary mixtures of n-octyltriethoxysilane, hydrophobic and trifunctional and 3-aminopropylmethyldiethoxysilane, hydrophilic and bifunctional were tested in application to the consolidation of artworks.5 At least 25% of hydrophobic n-octyltriethoxysilane in the mixture was needed to

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decrease the wettability of the samples.5 Objects were reinforced and their visual appearance was improved.5 The composition useful for rotational casting of cylindrical parts contained an isocyanate-terminated polyurethane prepolymer and a curative agent including a polyaspartic ester.6 Abrasion and tear resistance, good load bearing characteristics, high hardness, solvent resistance, and good flex fatigue resistance are required in this application.6 Alkanol amine ligands reacted with bismuth carboxylates have unique curability properties in application to isocyanates and polyols in the production of polyurethane, especially polyurethane spray-foam.7 The amino-alcohol ligand, when associated with bismuth neodecanoate, offered improved moisture and solvent resistance during polyol storage with cure rates analogous to the tin-based curatives.7 References 1 2 3 4 5 6

Loureiro, MV; Lourenc, MJ; De Schrijver, A; Santos, LF; Bordado, JC; Marques, AC, J. Mater. Sci., 52, 5380-9, 2017. Haghayegh, M; Mirabedini, SM; Yeganeh, H, J. Mater. Sci., 51, 3056-68, 2016. Kardar, P, Pigment Resin Technol., 45, 2, 73-8, 2016. Yin, X; Dong, C; Luo, Y, Colloid Polym. Sci., 295, 2423-31, 2017. Daher, C; Fabre-Francke, I; Balcar, N; Barabant, G; Lattuati Derieux, A, Polym. Deg. Stab., 158, 92-101, 2018. Gajewski, V, US6747117B2, Lanxess Solutions US Inc, Jun. 8, 2004.

3.2.13 Resorcinol

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3.2.13 RESORCINOL The resorcinol-formaldehyde prepolymer, hydroxymethylated resorcinol improves the bond strength of epoxy and polyurethane adhesives to wood because hydroxymethylated resorcinol plasticizes lignin components and stabilizes stress fractures through reactions with lignin subunits and hemicelluloses in wood.1 Resorcinol (1,3-dihydroxybenzene) is more reactive with formaldehyde than phenol itself, the two hydroxyl groups reinforce each other in activating the o- and p-positions. Resorcinol-formaldehyde resins find use as reactive adhesives. References 1

Yelle, DJ, J. Appl. Polym. Sci., 134, 45398, 2017.

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3.3 PARAMETERS OF CURING The essential parameters of curing reaction include activation energy, component ratio, conversion degree, glass transition temperature, melting point, temperature, the thickness of a part, time, and viscosity. 3.3.1 ACTIVATION ENERGY The activation energy was higher in the case of the room temperature cured epoxy resins as compared to the thermally cured samples.1 The activation energy decreased with an increase in the concentration of carbon black in the composite.1 It may be due to an increase of polarization energy and/or charge carrier density leading to a decrease of the domain boundary potential of carbon black aggregates in the epoxy matrix.1 The isothermal curing kinetic constant and activation energy of diglycidyl ether of bisphenol A and m-xylylenediamine was calculated using Kamal autocatalytic model.2 The auto-catalyzed rate constants and activation energy containing 0.1 MPa CO2 were 0.7139 min-1 and 50.83 kJ/mol at 393 K, while those containing 18 MPa CO2 were 1.0928 min-1 and 36.36 kJ/mol, respectively.2 The increased kinetic constant and reduced activation energy suggested that the plasticization effect of CO2 eased the chain movement, promoting the curing of the epoxy resin, and increased the final conversion at relatively low temperature.2 The presence of a solvent in commercial coatings based on epoxy, amine hardener, and organic solvent lowered the curing degree, the tensile strength, and modulus of elasticity, whereas the flexibility was improved by a higher concentration of solvent.3 The activation energy of curing reaction did not change with concentration of solvent, but it increased with the rising conversion which revealed that the curing process was diffusioncontrolled.3 The 4,4′-diglycidyloxybiphenyl epoxy cured with 4,4′-diaminodiphenylsulfone exhibited a high thermal conductivity of 0.34 W/mK.4 Activation energy and pre-exponential factor were as follows Ea1 = 60.76 kJ/mol, Ea2 = 57.20 kJ/mol, A1 = 4.08×104 min-1, and A2 = 101.22×104 min-1 (Figure 3.40).4 In the curing system with amine hardeners, the epoxide ring was opened by attacking nitrogen of amine via nucleophilic substitution reaction.3 If a primary diamine was used, the amine was converted into a secondary amine by first attacking the epoxide group followed by a secondary attack on an epoxide group, forming a network structure consisting of a tertiary amine.4 These bimolecular nucleophilic substitution reactions (SN2) converted the epoxide groups to the hydroxyl groups, which accelerated the next reaction of amine moiety by the interaction between hydroxyl and epoxide groups.4 The suggested reaction mechanisms are described in Figure 3.40.4 The kinetics of curing reaction cannot be explained by a simple nth order kinetic expression.4 It is described by the combined form of nth order and autocatalytic reaction model.4 The subscripts 1 and 2 represent the nth order and autocatalytic reaction, respectively.4 Ea1 was slightly larger than Ea2, and k1 and A1 were faster than those of the autocatalytic reaction.4

3.3.2 Component ratio

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Figure 3.40. Curing kinetics. [Adapted, by permission, from Yeo, H, Polymer, 159, 6-11, 2018.]

Siloxane-containing benzoxazines were prepared from phenol, paraformaldehyde, and bis(p-aminophenoxy)siloxane.5 The activation energy shifted to a higher value with the increase in benzene ring content in benzoxazines, and the siloxane-containing benzoxazines followed an autocatalytic curing model based on Friedman method.5 3.3.2 COMPONENT RATIO The fracture energy of epoxy resins strongly depended on the functionality of epoxy resin and the component ratio between the curing agent (amine) and epoxide.6 The glass transition temperature was the highest at the stoichiometric ratio of both components.6 The fracture energy of the epoxy increased with increasing ratio of curing agent.6 3.3.3 CONVERSION DEGREE For the tetrafunctional epoxy resins, the fracture toughness reached a peak value at a moderate conversion degree (between 65% and 95%) which was attributed to the combined effects of structural rigidity and post-yield deformability.6 The glass transition temperature increased with the increase in conversion degree.6 The rate conversion during the dynamic curing of a ferrocene-functionalized polyurethane system increased rapidly at the start of the reaction to a maximum and then decreased exponentially.7 Higher values of activation energy were observed at lower conversion because the non-auto-catalyzed reaction was occurring during the curing reaction of a diglycidyl ether of bisphenol A with a modified polyamine.8 At higher conversions, the value of activation energy decreased because of the effect of auto-catalyzed reaction.8

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3.3.4 GLASS TRANSITION TEMPERATURE The glass transition temperature depends on the conversion of the curing reaction of thermosetting systems.9 With increasing molecular weight, the glass transition temperature (vitrification temperature) increases.9 The time at which the glass transition temperature equals the reaction temperature is defined as the vitrification time.9 After reaching the vitrification time, the reaction rate (and the change of the glass transition) slows down.9 The epoxy resins exhibit no detectable stress during the curing reaction, nor during cooling to the glass transition temperature, but develop stress on cooling below glass transition temperature.10 The acrylate, by contrast, generates considerable stress throughout the reaction and cooling, with the major part of the stress originating above the glass transition temperature.10 The glass transition temperatures of epoxy materials strongly depended on epoxy resin/ionic liquid ratio and the chemical composition of the ionic liquid.11 The systems with 1-butyl-3-methylimidazolium thiocyanate had higher glass transition temperature.11 3.3.5 MELTING POINT The high melting point of curing agent results in high curing temperature.12 Ionic liquid promoted the curing rate and reduced the curing temperature of phthalonitrile resin.12 At the eutectic point, maleic anhydride/phthalic anhydride mixture used as a curative for epoxidized linseed oil had a lower melting point than the individual anhydrides, which decreased the curing temperature.13 3.3.6 TEMPERATURE Selection of the optimal mass ratio of glycidyl tertiary carboxylic ester/triethylene tetramine system of 3 to 1 permitted the use of a relatively lower curing temperature.14 Polymerization of a liquid epoxy resin containing imidazoles proceeds gradually even at room temperature which affects its storage stability.15 A 2-(2-hydroxyphenyl)imidazole derivative has an intramolecular hydrogen bond between the phenolic hydroxyl group and the nitrogen atom of the imidazole ring, leading to suppression of its reactivity toward epoxy resins at room temperature.15 It requires a temperature of 150°C to break the intramolecular hydrogen bonding and cause curing.15 3.3.7 THICKNESS Processing fast-cure resins is accompanied with strong exothermic reaction during cure.16 This may result in a significant temperature overshoot and large temperature gradients over the thickness.16 The initial film thickness of one layer of an industrial thermoset coating typically varies from about 100 to about 300 μm.17 The drying time increases when the film thickness is increased.17 The increased initial film thickness may lead to higher surface conversion values, but also higher solvent retention in the final film.17 3.3.8 TIME The use of high temperature reduces the curing time but also increases the void formation.18 The overall processing time was reduced from four hours to 30 min with a cure cycle based on a dual step heating process (lower temperature until 80% conversion, followed by higher temperature).18

3.3.9 Viscosity

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Prolonged curing time increased the performance and stability of dental adhesive systems.19 The composite restorations have an average lifetime of 6-7.2 years.19 The use of a light source with adequate radiant energy and exposure times exceeding those recommended by manufacturers has been shown to improve the degree of conversion and reduce the permeability of adhesives, contributing to the enhancement of their in vitro performance.19 Manufacturers recommend air-drying times of 5-10 s, not because it assures the most optimal conversion, but because they want to offer user-friendly products that are considerate of the practitioner’s time.19 3.3.9 VISCOSITY The viscosity of polyurethanes based on hydroxyl-terminated polybutadiene increased with curing time in an exponential way.20 Viscosity control during processing of thermosets is particularly critical because the viscosity varies with temperature, flow conditions, and time during the polymerization reaction.21 Viscosity growth of epoxy acrylic resin during curing was described by an exponential equation.21 The systematic increase in activation energy values showed that the increase in bulk viscosity during the reaction alters the resin curing process by reducing the diffusion of molecules and hindering network growth.22 As the reaction proceeds, viscosity increases, and the reaction rate becomes slower.22 This effect has been caused by the reduction in molecular mobility that strongly contributed to the reduction in collisions between reactive sites.22 References 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22

Trihotri, M; Dwivedi, UK; Khan, FH; Malik, MM; Qureshi, MS, J. Non-crystalline Solids, 421, 1-13, 2015. Hu, D-d; Lyu, J-x; Liu, T; Lang, M-d; Zhao, L, Chem. Eng. Processing Process Intensification, 127, 159-67, 2018. Yi, C; Rostron, P; Vahdati, N; Gunister, E; Alfantazi, A, Prog. Org. Coat., 124, 165-74, 2018. Yeo, H, Polymer, 159, 6-11, 2018. Xu, J; Li, H; Zeng, K; Li, G; Zhao, C, Thermochim. Acta, 671, 119-26, 2019. Meng, Z; Bessa, MA; Xia, W; Liu, WK; Keten, S, Macromolecules, 49, 9474-83, 2016. Lucio, B; de la Fuente, JL, Reactive Funct. Polym., 107, 60-8, 2016. Catalani, A; Bonicelli, MG, Thermochim. Acta, 438, 1-2, 126-9, 2005. Schawe, JEK, Thermochim. Acta, 391, 1-2, 279-95, 2002. Lange, J; Toll, S; Månson, JAE; Hult, A, Polymer, 36, 16, 3135-41, 1995. Mąka, H; Spychaj, T; Zenker, M, J. Ind. Eng. Chem., 31, 192-8, 2015. Weng, Z-H; Qi, Y; Zong, L-S; Liu, C; Jian, X-G, Chinese Chem. Lett., 28, 5, 1069-73, 2017. Ding, C; Tian, G; Matharu, A, Mater. Today Commun., 7, 51-8, 2016. Zhang, K; Huang, C; Fang, Q; Lu, Q, J. Appl. Polym. Sci., 134, 44246, 2017. Kudo, K; Furutani, M; Arimitsu, K, ACS Macro Lett., 4, 1085-8, 2015. Keller, A; Dransfeld, C; Masania, K, Composites Part A: Eng., 153, 168-75, 2018. Kiil, Prog. Org. Coat., 70, 4, 192-8, 2011. Sánchez Cebrián, A; Basler, R; Klunker, F; Zogg, M, Int. J. Adhesion Adhesives, 48, 51-8, 2014. Cadenaro, M; Maravic, T; Comba, A; Mazzoni, A; Breschi, L, Dental Mater., in press, 2019. Lucio, B; de la Fuente, JL, Eur. Polym. J., 50, 117-26, 2014. Cañamero-Martínez, P; Fernández-García, M; de la Fuente, JL, Reactive Functional Polym., 70, 10, 761-6, 2010. Artmann, A; Bianchi, O; Soares, MR; Nunes, RCR, Mater. Sci. Eng.: C, 30, 8, 1245-51, 2010.

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3.4 EFFECT OF CURATIVES ON PROPERTIES The discussion in this chapter summarizes the main changes in properties of materials to provide expectation guidelines. The 25 physical-mechanical properties are discussed in the alphabetical order below. A similar section is included with crosslinkers which should also be consulted. 3.4.1 ACID RAIN Acid rain protection and removal of pollutants deposited on stone surface was based on application of epoxy resin containing poly(propylene glycol)bis(2-aminopropyl ether) as curing agent for hydrogenated bisphenol A epoxy, containing TiO2 nanoparticles.1 Protective preparation was applied on limestone substrates and tested by exposure to simulated acid rain having pH=4.1 The effect of acid rain was evaluated by measuring the change of Ca2+ and pH in run-off solutions.1 The pH value and calcium ion concentration were higher for the untreated sample than for the treated samples.1 3.4.2 ADHESION The adhesion strength of the epoxy adhesives after the introduction of a proper amount of dopamine was 7 MPa even after the sample was immersed in water for over 230 days.2 The adhesion can be further increased by complexation of a small quantity of ferric ions with dopamine.2 The adhesives have higher cohesion strength after complexation.2 Adhesion to the liner is a very crucial factor for the development of solid composite propellants.3 Unusual combustion properties or a sudden explosion in the solid composite propellant during missile or rocket flight are mainly caused by poor adhesion between the propellant and the liner.3 Solid propellant prepared with the triazole curing system had poor adhesion properties because of incompatibility between the dipolarophiles used in the triazole curing system and the isocyanate compounds present in the urethane curing system applied to the liner preparation.3 The adhesion properties of the glycidyl azide polymer-based solid propellants prepared by using the urethane curing system or the dual curing system were excellent.3 An adhesion promoting curative and stabilizer system for an elastomer (epichlorohydrin copolymer) comprises: a) an organic peroxide; b) a diazabicycloamine (diazobicyclo(5.4.0)undec-7-ene phenol); c) an isocyanurate (triallyl isocyanurate); d) a layered organically modified inorganic nanofiller (montmorillonite modified with dimethyl, benzyl, hydrogenated tallow quaternary ammonium salt).4 The moisture-curable, silane-functional polymers having good adhesion properties, suitable as elastic adhesives, sealants or coatings are free from isocyanate groups.5 The polyol was reacted with isocyanatomethyl-methyldimethoxysilane and cured with a derivative of 3-aminopropyl-trimethoxysilane or a few other silanes.5 3.4.3 CELL MORPHOLOGY The epoxy resin system consisting of diglycidylether of bisphenol A and m-xylylenediamine was foamed by carbon dioxide using two-step batch process.6 The material was precured to a different degree and subsequently foamed via temperature-rising foaming process.6 The pre-curing degree in the range from 37.7 to 46.3% was the proper for foaming.6 If the pre-curing degree was increased from 37.7% to 51.6%, viscosity and elasticity of pre-cured resins increased, and correspondingly, the average cell size of epoxy foams

3.4.4 Diffusion

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decreased from 329.8 to 60.8 μm while cell density increased from 1.4x105 to 8.6x105 cells/cm3, respectively.6 Lower pre-curing degree produced coalescing and cracked cells because viscosity was too low to prevent cell breaking.6 With higher pre-curing degree material could not be foamed because of high mass transfer resistance.6 Figure 3.41 Figure 3.41. Different pre-curing degrees: (a) 31.7%; (b) 37.7%; shows the morphology of cells (c) 42.1%; (d) 46.3%; (e) 51.6%; and (f) 60.2%. [Adapted, by formed after different pre-curing permission, from Lyu, J; Liu, T; Xi, Z; Zhao, L, J. Cellular Plast., degree.6 53, 2, 181-97, 2017.] 3.4.4 DIFFUSION When the conversion is increased, the cured part of the polyurethane is transformed into the glassy state, which decelerates the curing reaction due to the restriction of reactant diffusion.7 The cure mechanisms of diglycidyl ether of bisphenol A epoxy with diethanolamine has been related to curative diffusion.8 When the glass transition temperature of the material approaches the reaction temperature, diffusion further slows down the reaction.8 The change in viscosity of the interpenetrating network due to the reaction of one component may influence the curing behavior of the second component because of an effect on the diffusion rate.9 Also, the developing skeletal structure of one network might exert a topological restraint on the development of the other network reducing the rate and final extent of reaction of the other component.9 3.4.5 ELECTRICAL RESISTIVITY The cured silver adhesive containing hexahydrophthalic anhydride had lower electrical resistivity than that of containing methylcyclohexane-1,2-dicarboxylic anhydride.10 The enthalpy of the silver adhesive containing hexahydrophthalic anhydride was higher than that of containing methylcyclohexane-1,2-dicarboxylic anhydride, indicating that adhesive containing hexahydrophthalic anhydride had higher crosslink density.10 The higher crosslink density of the silver adhesive caused a shrinkage increase resulting in a dense structure which formed more contact points and increased the electrical conductivity.10 The electrical resistivity of the cured silver adhesives decreased when the quantity of the curing agent increased from 25 to 85 wt%.10 The adhesive containing 85 wt% of the hexahydrophthalic anhydride curing agent had the largest enthalpy (-974.16 J/g) and produced the lowest electrical resistivity of 2.76x1024 Ω-cm.10 The excessive amount of curing agent leaves the unreacted curing agent molecules, lowering enthalpy and increasing electrical resistivity.10 The distance between carbon black particles has an exponential relation with resistivity; therefore the resistivity is sharply increased as long as the distance is even slightly increased.11

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3.4.6 FLAME RETARDANCY The phosphorus-containing dicyclopentadiene novolac curing agent in cured epoxy resin produced a LOI value of 31.6% and helped to achieve the UL 94 V-0 rating.12 The peak heat release rate and total heat release were significantly decreased.12 A halogen-free flame retardant (9,10-dihydro-9-oxa-10-phosphaphenanthrene-10oxide-containing H-benzimidazole) was synthesized and subsequently used as co-curing agent of 4,4’-diamino-diphenylmethane for diglycidyl ether of bisphenol-A.13 Resin cured with 7.45 wt% of co-curative achieved UL 94 V-0 rate and had limiting oxygen index of 35.6%.13 With very low phosphorus content in the system (0.18 wt%), the cured epoxy achieved UL-94 V-0 rating with the limiting oxygen index of 29.3%.14 The methylhexahydrophthalic anhydride used as the co-curing agent of diglycidyl ether of bisphenol A caused that the cured epoxy resin passed V-0 rating of UL 94 test with the limiting oxygen index of 32.7 vol% when the phosphorus content was only 1.5 wt%.15 Phosphorus-containing compound diphenyl-(1, 2-dicarboxylethyl)-phosphine oxide was synthesized and used as a flame retardant curing agent for epoxy resins together with phthalic anhydride used as co-curing agent.16 Thermosets successfully passed UL-94 V-0 flammability rating and the LOI value of 33.2%.16 The intumescent flame-retardant curing agent, poly-(meta-xylylenediamine spirocyclic pentaerythritol bisphosphonate) used with 3.01% phosphorus content for curing epoxy resin, exhibited the best combination of properties, including a high glass transition temperature (147oC), excellent thermal stability, an initial weight loss temperature of 269oC, and a LOI of 31.2%.17 The vertical burning test reached the UL94 V-0 level.17 Curative was prepared from hexachloro-cyclo-triphosphazene by nucleophilic substitution with isophorone diamine, and its curing capability was compared with the original isophorone diamine.18 The development curative lowered total heat release, total smoke release, and total smoke production as compared with the epoxy resin cured using isophorone diamine.18 The hydrophilic curing agent was prepared from hexamethylene diisocyanate trimer, polyethylene glycol monomethyl ether, and 2-((2-aminoethyl)amino)-ethanesulfonic acid monosodium salt and used in two-component waterborne polyurethanes and their coatings.19 The best LOI and UL-94 of modified flame-retardant two-component waterborne polyurethane were 29.2% and V-0, respectively.19 3.4.7 FLEXIBILITY The thiodiphenyl epoxy resin was modified with a dimeric fatty acid at an epoxy resin:fatty acid molar ratio of 4:1.20 The tensile and impact strengths of the resin indicated improved flexibility and toughness.20 3.4.8 FLEXURAL STRENGTH Biobased epoxy resins were synthesized from diglycidyl ether of bisphenol A and epoxidized castor oil using triethylenetetramine as a curing agent.21 The blend prepared using 20 wt% of epoxidized castor oil showed optimum impact and flexural strength.21 The glycidyl polyhedral oligomeric silsesquioxane (4 phr)/bisphenol-A cyanate ester system had the maximum flexural strength (125.85 MPa) and flexural modulus (2.47 GPa), which were increased by 27.78% and 4.22% compared with that of pure bisphenolA cyanate ester, respectively.22 This can be attributed to three reasons.22 First, the epoxy

3.4.9 Fracture

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groups in the glycidyl polyhedral oligomeric silsesquioxane molecule can react with NCO groups in bisphenol-A cyanate ester, leading to improved interfacial bonding strength between the inorganic silica core and the organic bisphenol-A cyanate ester matrix.22 The second cause was the flexibility of Si–O–Si bonds in glycidyl polyhedral oligomeric silsesquioxane, as well as the tough oxazolidinone rings and the polyether structures resulting from the reactions between the triazine rings and epoxy groups.22 Third, the introduction of glycidyl polyhedral oligomeric silsesquioxane with the three-dimensional cage porous structure reduced the packing density of the polymer chain and increased the free volume,22 followed by increase of mechanical properties of the glycidyl polyhedral oligomeric silsesquioxane/bisphenol-A cyanate ester system when the content of glycidyl polyhedral oligomeric silsesquioxane increased from 0 to 7 phr.22 Amino-functionalized carbon nanotubes used as curing and reinforcing agent in epoxy resin improved flexural strength of glass fiber composite.23 Bismaleimide enhanced the thermal capability at the cost of toughness and flexural strength of phenolic resins.24 A compromise in properties is generally observed for an allyl:bismaleimide ratio of 1:2.24 3.4.9 FRACTURE Fracture toughness of glassy polymers depends on the plastic flow that dissipates energy near the crack tip region.25 The fracture energy of epoxy resins strongly depends on the functionality of epoxy resin and the ratio of the curing agent (amine) and epoxide.25 The fracture energy exhibits a maximum value within the range of conversion degrees from 65 to 95% which can be attributed to the combined effects of structural rigidity and deformability.25 The fracture toughness of a typical epoxy system can be enhanced by increasing the initial curing agent ratio beyond stoichiometric ratio.25 3.4.10 GEL FRACTION AND TIME Determination of gel fraction in cured polymers is a direct method to evaluate the progress of curing reaction.26 The gel fraction is affected by the concentration of reactive groups, the curing process, the catalyst type and content, the moving ability of curing agent, etc.26 The gel time is the lapse of time during curing at which resin becomes elastic.22 The gel time of pure bisphenol-A cyanate ester was remarkably higher than the gel time of glycidyl polyhedral oligomeric silsesquioxane/bisphenol-A cyanate ester system composite at the same curing temperature.22 When the curing temperature increased, the gel time of the system was reduced.22 3.4.11 GLASS TRANSITION TEMPERATURE There is always competition between two different factors, such as a high glass transition temperature and high conductivity.27 Demand for polymers of high mechanical stability, secured by the high glass transition temperature limits the ion mobility and furthermore conductivity.27 In the glassy region, the increase in the degree of functionality of epoxy resin cured with hexahydrophthalic anhydride leads to a decrease in the flexibility of systems and an increase in crosslink density.28 The intumescent flame-retardant curing agent, poly-(meta-xylylenediamine spirocyclic pentaerythritol bisphosphonate) used for curing epoxy resin increased its glass transition temperature from 129 to 147oC as compared with resin cured with mxylylenediamine.17

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The functionalized graphene oxide co-curing agent (diamine-functionalized graphene oxide) was used to cure the epoxy resin.29 Due to the higher crosslink density, the glass transition temperature of nanocomposite increased from 160.7 to 183.4°C.29 The glass transition temperature decreased (from 117 to 114.3oC) with the increase of diphenyl-(1, 2-dicarboxylethyl)-phosphine oxide content used together phthalic anhydride.16 The effect can be attributed to a larger space structure of diphenyl-(1, 2-dicarboxylethyl)-phosphine oxide that hinders the curing reaction, lowering the glass transition temperature.16 A halogen-free flame retardant of 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10oxide-containing H-benzimidazole was synthesized and subsequently used as co-curing agent of 4,4’-diamino-diphenylmethane for diglycidyl ether of bisphenol-A.13 Compared with the cured pristine resin, the glass transition temperature was increased by 6.9oC.13 Pairs of sterically hindered and unhindered linear aliphatic and aromatic diamines were synthesized and used as curatives for the diglycidyl ether of bisphenol A.30 The steric hindrance was caused by methyl group substitution of a hydrogen atom adjacent to the amine.30 For each pair, the hindered diamine cure had a higher glass transition.30 Another pair of diamines was synthesized for which the methyl group was replaced by ethyl and butyl side chains; for these resins, the glass transitions decreased.30 3.4.12 HEALING The reversible Diels-Alder reaction system was formed using furfuryl-terminated polybutadiene as the main resin, N,N’-1,3-phenylenedimaleimide as the curing agent and trifurfuryl propane as the chain extender.31 The repair efficiency of the crack was 88%.31 A material capable of self-healing microcracks was developed from diglycidyl ether of bisphenol A with amine curing agents (triethylenetetramine and an aminated polydimethylsiloxane), and poly(urea-formaldehyde) as microcapsule wall.32 Heating was necessary to initiate a cure and the self-healing.32 Microcapsules containing multi-functional reactive isocyanate-terminated polyurethane prepolymer as a healing agent were very stable (after 10 months, and they just lost less than 7 wt% of their loaded isocyanate molecules).33 This healing agent was prepared from isophorone diisocyanate with 2-ethyl-2-hydroxymethyl-1,3-propanediol and encapsulated with a polyurethane shell.33 Polyurethane-isophorone microcapsules were used for self-healing of an automotive clearcoat.34 The microcapsules were dispersed in an acrylic-melamine clearcoat to improve scratch resistance.34 The microcapsules increased the scratch resistance of coatings.34 Self-repair in aeronautical structures requires solvation of complex issues, such as self-healing activity at low working temperatures (can reach values as low as -50°C).35 The metathesis polymerization of 5-ethylidene-2-norbornene/dicyclopentadiene blend activated by Hoveyda-Grubbs’1st generation catalyst, dispersed at the molecular level in the matrix had a self-healing efficiency of about 72%.35 3.4.13 IMPACT STRENGTH Impact strength of glycidyl polyhedral oligomeric silsesquioxane (co-curing agent) and bisphenol-A cyanate ester cured with triethylamine was increased by 158% to 23.76 kJ/ m2.22

3.4.14 Morphology

207

The amine-terminated polydimethylsiloxane, glycidyl-terminated polydimethylsiloxane, and glycidyl-terminated polyhedral oligomeric silsesquioxane were used as toughening agent for the biobased epoxy resin.36 The amine-terminated and glycidyl-terminated polydimethylsiloxane did not affect tensile and flexural properties but led to a deterioration of the impact strength.36 The glycidyl-terminated polyhedral oligomeric silsesquioxane enhanced the impact strength and elongation at break of the bioresins.36 3.4.14 MORPHOLOGY Various sizes and shapes of liquid crystalline epoxy domains can be formed in the matrix resin, depending on the cure schedule and the liquid crystalline epoxy curative composition.37 Preferred molecular orientation along the graphite fibers can be achieved if the liquid crystalline epoxy resin is cured so as to promote extensive linear chain extension, thus giving a high concentration of mesogenic segments.37 3.4.15 OPTICAL PROPERTIES Cellulose nanocrystals mechanically reinforce polymers and have impressive optical properties that these rod-like particles exhibit at certain concentrations in aqueous solutions.38 Cellulose nanocrystals suspensions are able to self-organize into liquid crystalline arrangement.38 This arrangement can be preserved when the suspensions are dried carefully.38 3.4.16 REACTION ORDER AND RATE Isothermal kinetic parameters including reaction orders (m, n), rate constants (k1, k2), activation energy, and pre-exponential factor were estimated using Kamal autocatalytic model.39 The model gives a good description of curing kinetics at various temperatures prior to the onset of vitrification.39 The reactions between glycidyl azido polymer and isocyanate curatives follow second-order kinetics.7 The reaction orders of different NCO/OH ratios were slightly lower than that for the polyurethane formation of the isocyanate-hydroxyl reaction.7 The diffusion control, which caused a deviation from the second-order kinetics, cannot be neglected.7 A liquid crystalline epoxy network with exchangeable disulfide bonds was synthesized by polymerizing a biphenyl-based epoxy monomer with an aliphatic dicarboxylic acid curing agent containing a disulfide bond.40 The presence of the disulfide bonds resulted in an increase of the reaction rate, leading to a reduction in liquid crystallinity.40 The addition of 4,4’-dithiodibutyric acid resulted in an increase in the reaction rate, which was attributed to the reversible cleavage and reformation of the disulfide bonds at high temperatures that increased the collision probability between epoxy and acid groups.40 The reaction rate of polyetheramines increased with decreasing molecular weight of curing agents.41 3.4.17 SHAPE MEMORY The aliphatic epoxy curing agents containing ring structures were synthesized from rosin acid and isosorbide.42 The diglycidyl ether bisphenol A cured with these curatives had similar shape memory that the petroleum-based curative (phthalic acid derivative), especially curative based on isosorbide.42 The isosobide-cured resin had good shape fixity, shape recovery, and thermal stability.43

208

Curatives

The shape memory polyhydroxyurethanes are highly branched, semicrystalline polyamidoamine curing agents tailored by amidation of a polyamine-terminated hyperbranched polyethylenimine with semicrystalline long chain behenic acid.44 The incorporation of hydrophobic, crystalline n-alkyl side chains significantly lowers hydrophilicity.44 The behenic amide side chains account for nanophase separation producing nanocrystalline polyhydroxyurethanes with programmable triple-shape memory materials memorizing two different shapes in addition to their original shape within a single shape memory cycle.44 Shape memory materials with high thermal and chemical stability are produced from several polymers, such as polyphenylenesulfide cured with sulfur, silica, a quinone, a peroxy compound, a metal oxide, a metal peroxide, or their combination.45 Shape memory polymer was useful as material of construction of elements for a variety of downhole applications, particularly those which require the sealing of a portion of a borehole, or constricting the spacing around an element.45 3.4.18 STORAGE STABILITY The thermal latent curing agent, a 2-(2-hydroxyphenyl)imidazole derivative, having an intramolecular hydrogen bond between the phenolic hydroxyl group and the nitrogen atom of the imidazole ring was developed to improve the storage stability of a liquid epoxy resin containing imidazoles.46 The system is stable at room temperature and requires 150oC to cure.46 The methacrylic acid and dodecyl methacrylate copolymer was used to encapsulate an imidazole curing agent to improve its stability in epoxy resin applications.47 The shelflife of up to 18 days was obtained.47 The development of efficient latent catalysis is important for enhancement of both storage stability and handling of thermosetting resins because they can simplify the curing operation to achieve one-pot synthesis systems.24 The N-benzylpyrazinium hexafluoroantimonate had good latent thermal initiator properties.24 The cure reaction of the blend system using N-benzylpyrazinium hexafluoroantimonate as a curing agent depended on the cure temperature and proceeded through an autocatalytic kinetic mechanism that was accelerated by a hydroxyl group.24 3.4.19 STRESS RELAXATION Physical stress relaxation in thermosets was limited by crosslinks, which impeded segmental motion and restricted relaxation to network defects.48 The curing shrinkage associated with polymerization of thermoset leads to the development of internal residual stress that cannot be effectively relaxed.48 Chemical relaxation processes are exploited to relax stress by incorporation of temporary crosslinks into the polymer network.48 Physical relaxation is enhanced by the incorporation of organometallic sandwich moieties into the backbone of the polymer network in which a standard epoxy resin is cured with a diamine derivative of ferrocene.48 The ferrocene-based thermoset had reduced cure stress when cure temperature was increased and stress relaxation occurred above its glass transition in the fully cured state.48

3.4.20 Tensile strength

209

3.4.20 TENSILE STRENGTH Biobased epoxy blends were synthesized from diglycidyl ether of bisphenol A and epoxidized castor oil and triethylenetetramine.21 The tensile strength of the blend was reduced with the increasing epoxidized castor oil content which reduced crosslink density.21 The tensile strength and modulus were increased when the amount of co-curing agent, glycidyl polyhedral oligomeric silsesquioxane, has been increased up to 7 phr.22 The addition of larger amount of silsesquioxane caused its agglomeration and reduced tensile properties.22 Functionalized graphene oxide used as co-curing agent increased tensile strength of composite by 15% when only a small amount was added (0.1%).29 Further increase in concentration did not change tensile strength.29 The mechanical properties of an epoxy composite containing silica supported on ionic liquid were improved because of improved interfacial interaction, reinforcement, and participation in curing the epoxy resin.49 The tensile strength increased by more than 60% with the addition of 30 phr silica supported on an ionic liquid as compared to the addition of silica alone.49 3.4.21 THERMAL CONDUCTIVITY The silver adhesive containing an 100 wt% of epoxy resin mixed with 85 wt% of hexahydrophthalic anhydride, 1.0 wt% (weight of epoxy resin) of 2-ethyl-4-methyl-1H-imidazole-1-propanenitrile, and 80 wt% (weight of epoxy resin) of hybrid silver particles (40 wt% 15 μm and 40 wt% 1.25 μm) had the best thermal conductivity of 3.2 W/mK.10 3.4.22 THERMAL STABILITY The thermal stability in both nitrogen and air atmospheres of phthalonitrile resin was improved by use of ZnCl2/4,4’-diaminodiphenylsulfone as a curing agent.50 Quinoline was found to be an effective curing accelerator of two-step synthesis of polyimide.51 It provided a material with good thermal stability because of its moderate base strength, low steric crowding effect, and moderate boiling point.51 Functionalized poly(phthalazinone ether sulfone ketone) was synthesized by successive chloromethylation and azidation, followed by curing reaction with the propargyl endgroups of various molecular weight crosslinking agents in the presence of Cu(I) catalyst via the azide-alkyne click reaction.26 The resultant polymers had substantially higher thermal stability, thermal decomposition temperatures, and higher char-yielding properties.26 A siliconized epoxy system had higher thermal stability and a heterogeneous morphology compared to a regular epoxy system.36 Crown ethers 4-aminobenzo-15-crown-5, 1,4,10,13-tetraoxa-7,16-diazacyclooctadecane, tetraazacyclododecane-1,4,7,10-tetraacetic acid, and tetraazacyclododecane-1,4,7,10-tetraacetamide used as curing agents for bisphenol A diglycidyl ether increased thermal stability of cured resins by 30-60oC.52 3.4.23 TOUGHNESS The self-emulsifiable waterborne amine-terminated curing agent for epoxy resin based on glycidyl tertiary carboxylic ester improved the toughness of cured resin.53 Epoxidized soybean oil can be used to enhance the toughness of epoxy resin, and its addition to epoxy also helps to decrease the cost of the final product.36 The fracture toughness parameters, critical stress intensity, and the critical strain of blends increased with 20 wt% epoxidized

210

Curatives

castor oil as compared with the virgin matrix.21 The incorporation of silica supported on ionic liquid simultaneously enhanced the tensile strength and toughness of epoxy resin.49 3.4.24 TRANSPARENCY The transparency of methacrylate-based poly(siloxane-silsesquioxane) for optoelectronic application was improved with the use of trisilanolphenyl-polyhedral oligosilsesquioxane.54 The refractive indices ranged from 1.5550 to 1.5625 with increasing POSS contents as compared to 1.5500 without POSS.54 3.4.25 WETTABILITY The resin and curing agent droplets readily wetted the polydimethylsiloxane surface, with their static contact angles being relatively small, about 10° and 5°, respectively, which was essential for the performance as self-healing agent.55 References 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40

Xu, F; Li, D; J. Polym. Environ., 25, 1304-12, 2017. Dai, F; Chen, F; Wang, T; Feng, S; Hu, C; Wang, X; Zheng, Z, J. Mater. Sci., 51, 4320-7, 2016. Min, BS; Kim, CK; Ryoo, MS; Kim, SY, Fuel, 136, 165-71, 2014. Smith, CT; Burczak, I, WO2013192283A1, Zeon Chemicals L.P., Dec. 27, 2013. Jucker, B; Oertli, M; Bütikofer, P-A; Burckhardt, U, EP1981925B1, Sika Technology AG, Aug. 2, 2017. Lyu, J; Liu, T; Xi, Z; Zhao, L, J. Cellular Plast., 53, 2, 181-97, 2017. Tao, J; Jin, B; Peng, R; Chu, S; Polym. Testing, 71, 231-7, 2018. McCoy, JD; Ancipink, WB; Clarkson, CM; Kropka, JM; Fredj, N, Polymer, 105, 243-54, 2016. Dean, K; Cook, WD; Burchill, P; Zipper, M, Polymer, 42, 8, 3589-3601, 2001. Chiang, ZH; Lin, Y-C; Chen, Y-F; Chen, E-Y, J. Appl. Polym. Sci., 133, 43587, 2016. Luo, Y; Wang, C; Li, Z; Synthetic Metals, 157, 8-9, 390-400, 2007. Yang, J-W; Wang, Z-Z; Liu, L, J. Appl. Polym. Sci., 134, 44599, 2017. Wang, Y; Yuan, Y; Zhao, Y; Liu, S; Zhao, J, High Performance Polym., 29, 1, 94-103, 2017. Dong, C; Wirasaputra, A; Luo, Q; Yuan, Y; Zhao, J; Fu, Y, Materials, 9, 10008, 2016. Wirasaputra, A; Yao, X; Zhu, Y; Liu, S; Yuan, Y; Zhao, J; Fu, Y, Macromol. Mater. Eng., 301, 982-91, 2016. Zhang, H; Xu, M; Li, B, Polym. Adv. Technol., 27, 860-71, 2016. Liang, B; Wang, G; Hong, X; Long, J; Tsubaki, N, High Performance Polym., 28, 1, 110-8, 2016. Zarybnicka, L; Bacovska, R; Vecera, M; Snuparek, J; Alberti, M; Rychly, J; Kalenda, P, J. Appl. Polym. Sci., 133, 42917, 2016. Yin, X; Dong, C; Luo, Y, Colloid Polym. Sci., 295, 2423-31, 2017. Kim, D; Kim, J-H; Kwon, SH; Lee, SO; Seo, B; Lim, C-S, Polym. Bull., 74, 4595-4605, 2017. Sudha, GS; Kalita, H; Mohanty, S; Nayak, SK, Macromol. Res., 25, 5, 420-30, 2017. Jiao, J; Zhao, L-z; Xia, Y; Wang, L, High Performance Polym., 29, 4, 458-66, 2017. Garg, M; Sharma, S; Mehta, R, Composite Interfaces, 24, 2, 233-53, 2017. Nair, CPR, Prog. Polym. Sci., 29, 5, 401-98, 2004. Meng, Z; Bessa, MA; Xia, W; Liu, WK; Keten, S, Macromolecules, 49, 9474-83, 2016. He, Q-z; Wang, J-y; Song, L; Jian, X-g, Chinese J. Polym. Sci., 34, 10, 1208-19, 2016. Maksym, P; Tarnacka, M; Dzienia, A; Matuszek, K; Chrobok, A; Kaminski, K; Paluch, M, Macromolecules, 50, 3262-72, 2017. Yang, T; Zhang, C; Hou, X; Cheng, J; Zhang, J, High Performance Polym., 28, 7, 854-60, 2016. Yu, JW; Jung, J; Choi, Y-M; Choi, JH; Yu, J; Lee, JK; You, N-H; Goh, M, Polym. Chem., 7, 36-43, 2016. Balizer, E; Duffy, JV, Polymer, 33, 10, 2114-22, 1992. Liang, C; Li, J; Li, G; Luo, Y, Polymers, 9, 200, 2017. Cervi, G; Pezzin, SH; Meier, MM, Revista Mater., 22, 2, 2017. Haghayegh, M; Mirabedini, SM; Yeganeh, H; J. Mater. Sci., 51, 3056-68, 2016. Kardar, P, Pigment Resin Technol., 45, 2, 73-8, 2016. Raimondo, M; Longo, P; Mariconda, A; Guadagno, L, Adv. Compos. Mater., 24, 6, 519-29, 2015. Abdelwahab, MA; Misra, M; Mohanty, AK, J. Appl. Polym. Sci., 132, 42451, 2015. Sue, H-J; Earls, JD; Hefnerr, RE; Villarreal, MI; Plummer, CJG, Polymer, 39, 20, 4707-14, 1998. Khelifa, F; Habibi, Y; Bonnaud, L; Dubois, P, ACS Appl. Mater. Interfaces, 8, 10535-44, 2016. Barghamadi, M, High Performance Polym., 19, 7, 827-35, 2017. Li, Y; Zhang, Y; Rios, O; Keum, JK; Kessler, MR, Soft Mater., 13, 5021-7, 2017.

3.4.25 Wettability

41 42 43 44 45 46 47 48 49 50 51 52 53 54 55

211

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Index

213

Index Numerics 1,1,1-trimethylolpropane triacrylate 103 1,2-dimethylimidazole 31-32, 170 1,3,5-triaminophenoxybenzene 139 1,3-bis(3-aminophenoxy)benzene 191 1,3-bis(t-butyl peroxyisopropyl) benzene 11, 38 1,4-benzenedimethanol 87 1,4-butanediol 89 1,6-divinylperfluorohexane 38 1,6-hexamethylene diisocyanate 22 1,6-hexanediol diglycidyl ether 57 1.3-butadienediepoxide 21 1-butyl-3-methylimidazolium 104 1-ethyl-3-3-dimethyl aminopropyl carbodiimide 125 1 H chemical shift 52 NMR spectroscopy 52 2,2,6,6-tetramethylpiperidinyloxyl nitroxide 108 2,2′-azobis(2-methylpropionitrile) 45 2,4,5,6-tetraaminopyrimidine 140 2,6-di-tert-butylphenol 112 2-ethyl-4-methylimidazole 121, 130, 172 2-ethylimidazole 172 2-furanmethanethiol 33 2-methyl tetrahydrofuran 59 2-methylimidazole 170, 172 2-octyl cyanoacrylate 29 2-phenyl-2-imidazoline 170

2-tert-butylperoxydopropyl) benzene 82 3,3'-diaminodiphenylsulfone 156 3,4-dihydroxyphenylalanine 115 3-aminopropyltriethoxysilane 33, 122 3D printing 67 matrix 97 printed product 112 4,4 -diaminodiphenylmethane 162 4,4'-(l,3-phenyleneoxy)aniline 191 4,4'-oxydianiline 161 4,4‘-diaminediphenylsulfone 171, 191, 209 4,4’-dithiodibutyric acid 207 4,4′-diaminodiphenyl methane 35, 160 sulfone 160, 188, 198 4,4′-diglycidyloxybiphenyl epoxy 198 4,4-diamino-diphenylmethane 166 4,4-diphenyl diisocyanate 139 4,4-diphenylmethane diisocyanate 89 4-hydroxybenzophenone 66 4-methylcoumarin 44 4-nitroaniline 165 A α,α'-dimethoxydeoxybenzoin 50 α-chymostrypsin 69 α-eleostearic acid 33 α-helix 97 abrasion resistance 93

214

abrasive wear 18, 126 absorbed dose 109 absorption 142 capacity 141 accelerated crosslinking 37 accelerator 141, 164 acetylene black 111 acid rain 202 resistance 23, 116 acidic environment 184 acrylamide 5 gel 5 homopolymerization 132 monomer 5 acrylate 112, 200 acrylic acid 10, 141 content 141 end-capped polyurethane dispersion 91 latex coating 111 monomer 9 paint 9 polymer 8, 9 pressure-sensitive adhesive 8-9, 115, 149 resin 9, 111, 149, 201 -melamine clearcoat 195, 206 acrylonitrile-butadiene rubber 11, 121, 136, 141 activated ester monomer 47 activation 31, 182 energy 64, 108-109, 139, 154, 156-157, 162, 166, 170, 175, 189, 195, 198-199, 201, 207 temperature 102 active hydrogen compound 100

Index

actuation 50, 134 magnitude 134-135 properties 109 stress 50 actuator 134 addition reaction 172 additive 1 adhesion 14, 31, 102, 115, 131, 149, 155, 157, 169, 182-183 at interface 116 promoter 116 promoting curative 202 strength 115, 202 adhesive 7, 115, 167, 182, 203 hydrogel 39 interaction 67 photo-responsive 115 strength 39, 163 wear 126 adhesiveness 169 adipic acid 94, 141 dihydrazide 16, 52, 125 adipose-derived stem cell 45 adipoyl dichloride 57 adsorption capacity 83 aerogel 75 aeronautical structure 206 aerospace 160, 191 composite 188 agar 13, 139, 142 fiber 13 agglomeration 209 aging process 43 time 188 aircraft 180 aldehyde donor 54 group 97

Index

alginate strand 151 aliphatic 155 amine 162 chain length 49, 50 dicarboxylic acid 157 ketimine 162 polycarbonate 111 polyether diamine 156 alkali treatment 24 alkaline anion exchange membrane 142 environment 24 fuel cell 83 application 133 alkanol amine ligand 196 alkoxysilane 183 crosslinker 100 alkoxysilicone sealant 116 alkyd binder 14 film 14 resin 14, 149 -acrylic dispersion 149 alkyl chain 94 radical intermediate 81 alkylene diacrylate 29 alkylphenol-formaldehyde resin 20 allophanate 89, 195 allylic hydrogen 141 alumina counterface 111 aluminum acetylacetonate 149 phosphate 63 -air batteries 6 amidation 190 amide bond 130 amidoamine 155, 157

215

amine 1, 155 activator 120 group 163 value 179 -epoxy reaction 163 -functionalized carbon nanotube 158 -modified carbon nanotube 158 aminoisobutyric acid 95 aminopropyltriethoxysilane 98-99, 140, 149 aminosilane 116 aminosilicone 13 aminotrimethoxysilane 162 ammonia 155 ammonium peroxydisulfate 74 persulfate 5 amorphous behavior 76 elastic material 90 phase 122 region 73, 124 structure 139 thermoplastic polymer 87 amphigel 56 amphoteric behavior 48 anhydride 158, 170 curing agent 159, 176 group 168 anion exchange membrane 83, 87, 141 anisotropic hydrogel 74 annealing 73 antenna 80 anthocyanin 73 antibacterial activity 23, 117 coating 86

216

properties 116 antibiotic 116 anticancer treatment 125 anticorrosion protection 194 anticorrosive coating 159 performance 168, 177, 179 anti-fogging film 173 antifouling 86, 121 antimicrobial agent 94 antioxidant activity 23, 116 enzyme 125 antistatic properties 117 anti-water-treeing performance 73 apparent crosslink density 52 appliance 119 aromatic 155 amine 160, 162, 167, 170, 188 diamine 191 ester amine 161 hydrocarbon 70 polyamide 57, 120 ring 65, 159-160 Arrhenius law 168 artificial muscle 49, 134 artwork 195 atmospheric humidity 174 oxygen 21 atom transfer radical polymerization 74 atomic force microscopy 133 autocatalytic 154 curing model 199 kinetic mechanism 208 model 160, 172 reaction model 198 autocatalyzed rate constant 198

Index

automobile 168 automotive 87 application 37 clearcoat 206 autonomous shape-memory actuator 49 autoxidation 14 average cell size 119, 202 Avrami exponent 122 axle polymer 8 azidation 209 azide group 87, 89 azidization 79 azido polymer 189 -poly(N-isopropylacrylamide 79 aziridine 23, 116, 127 azo crosslinker 49, 108, 134 azobenzene 35 azobisisobutyronitrile 77, 137, 142 B β-carotene 94 β-sheet 97 bacteria 55 bacterial colonization 86 bactericidal 86 polymer 86 baking mold 102 barium chloride 13 titanate 99, 122 barrier effect 11 material 97 properties 72 protection 165 base strength 193, 209 basicity 160

Index

battery 106 cycling 72 electrode separator membrane 106 hot area 106 behenic acid 190, 208 amide side chain 208 bending modulus 175 strength 175, 179 benzamide 184 benzene ring 189 content 199 benzimidazole 166, 175, 206 benzoguanamine 21 benzoxazine 31, 87, 127, 199 benzoyl peroxide 87, 112, 120, 137 bifunctional coating 86 bimodal 132 bimolecular crystal morphology 51 nucleophilic substitution reaction 198 bioactive compound 116 molecule 40 bioactivity 23 bioadhesive 29 bioavailability 125 biobased epoxy resin 175, 182, 207 biocompatibility 44, 117, 130, 134 biocompatible medical product 56 polymer 55 biocomposite 136 biodegradability 96, 130 biodegradable chitosan 17 synthetic material 90, 117

217

biodegradation 23, 62, 125 biofilm formation 117 biofouling 182, 194 biofuel 182 biohydrogel 128 bio-inspired adhesive 115 biological regenerative method 40 biomaterial 97, 125 biomedical 39 application 23, 39, 120, 174 biomimetic precipitation 17 bionanocomposite ionogel 50 bioplastic 13, 94, 139 biopolymer 17-18, 125 carrier 17 coating 17 bioprosthetic heart valve 117 biosorbent 13 bis(2,4,6-trimethylbenzoyl) phenyl phosphine oxide 125 bis(2-diphenylphosphinophenyl) ether 152 bis(t-butyl peroxy-isopropyl) benzene 33, 108 bis-chloro-ethoxy-ethane 94 bismaleimide 161, 205 crosslinking 108 side reaction 161 bismuth carboxylate 196 neodecanoate 196 biuret 195 blend 135 homogeneity 31 block copolymer host 63 blocked polyisocyanate 92 crosslinker 93 blowing agent 27, 100, 118, 121

218

body temperature 134 tissue 25 weight 26 boiling point 209 bond strength 197 -exchange reaction 167 bone cement 174 regeneration 17 substitute 117 -like fibrous structure 17 borate crosslinked gel 42 borehole 170, 208 boric acid 94 boronate linkage 8 boronated biopolymer crosslinking 18 boundary between crystal domains 139 bovine serum albumin 23, 41, 55 branching 123 brittle failure 137 bromelain 69 brominated butyl rubber 33-34, 119 bromobutyl rubber 19, 121 bromotetradecane 94 bubble formation 100 nucleation 100 building block 162 industry 119 bulk process 108 bundles-promoted crystallization 123 burst release 112 butyl rubber 20, 152

Index

C cable connector 27 insulation 73 caged octa(aminophenyl) silsesquioxane 139 calcification 117 calcium alginate 151 cation 151 ion concentration 202 can coating 174 cancer cells 125 capillary chromatography 50 car window 35 carbodiimide 39, 89, 117, 127 carbon black 118, 198, 203 aggregate 198 dioxide 119, 190, 202 fiber 140, 158, 188 membrane 72 nanotube 118, 158, 205 carboxyl group 141 carboxylate group 9 carboxylic acid 102 group 149 carboxymethyl cellulose 5, 94, 120, 124, 137, 141 chitosan 17 carboxymethylated guar gum 41, 131 carboxy-reactive material 63 cardanol 159, 179 cardanol-based flexibilizer 188 carp 39 carrier density 198 casting materials 182

Index

castor oil 15, 22, 210 catalyst 37, 68, 154, 158, 175, 190, 194 type 205 catalytic activity 14, 156 efficiency 101 catechol 125 cationic photoinitiator 9 cell 55, 111 adhesion 125 breaking 203 culture 40 delivery 120 density 27, 118-119, 121, 126, 203 encapsulation 16 morphology 118, 202 opening 126 operation 105 proliferation 13 size 27, 118-119, 121, 126 distribution 118 viability 125 cellular cytotoxicity 125 interactions 117 uptake 125 cellulose 50 acetate butyrate 21 propionate 22 fiber 135 nanocrystal 30, 117, 128, 162 nanofiber 47, 134, 136 polarity 22 cementing casing 170 cephalosporin 117 cerium ammonium nitrate 50

219

chain 99, 103 entanglement density 52 extender 1, 90, 118 extension 195 growth 100 length 99, 180 between crosslinking sites 180 mobility 81, 115, 133 movement 198 packing 73 scission 20, 35, 43, 65, 81, 103, 111, 129, 130, 180 segment 165 transfer agent 9, 132 polymerization 79, 132 -extender 119 chains stacked in lamellas 123 channel adhesion 116 char residue 166 yield 167 charge potential 6 Charlesby-Pinner equation 20 char-yielding properties 209 C−H bond insertion 60 chelator 17 chemical composition 1, 200 conversion 172 crosslink 195 crosslinking 13, 39, 47-48, 72-73, 94, 109, 124, 139, 195 reaction 139 foaming extrusion 119 linkage 135 mechanism 172 modification 128 performance 159

220

reaction 108 relaxation process 208 resistance 160, 178 shift difference 52 stability 15, 17, 208 structure 157 chemotherapy agent 125 chitosan 17, 23, 25, 106-107, 116, 125, 128, 136, 142 droplet 23 fiber 23, 116 film 24 gel 24 hybrid scaffold 23 hydrogel 23-24 layer 106 membrane 24, 130 sorbent 25 chlorinated polyethylene 27, 111, 118 foam 121 chlorine resistance 57 chloroalkylsilane 184 chloromethylation 209 chloroprene 28 chlorosulfonated polyethylene 31 cholesterol 26 chondrocytes 125 chronotherapeutics 74 cis-1,4-polyisoprene 141 citric acid 24, 39, 97, 108-109, 120 click chemistry 79, 83, 117 reaction 209 closed cell foam 100 clustered 132 coagulation 13 bath 75 coalescence 100

Index

coating 14 applications 180 composition 17 cobalt bomb 20 co-condensation reaction 149 co-curing agent 165, 170-171 coefficient of friction 126 cohesion 115 strength 202 cohesive energy density 109 failure 115 strength 9, 29, 91 collagen 116 scaffold 117 collision probability 207 colloidal emulsions 24 silica surface 157 colon-specific delivery 74 color change 182 stability 160 transition 181-182 combustion 202 parameters 167 commercial coating 198 compact char 35 compatibility 34, 133 compatibilization 128 compatibilizing agent 33 complex viscosity 37, 122 complexation 202 component ratio 198, 199 composite 35, 94 hydrogel 47-48, 134 insulator 102 material 153 membrane 45, 106, 133

Index

restoration 201 compression 87, 94 molding 112 set 11, 19-20, 118-119, 122, 137 strength 94 compressive creep 120 strength 120, 164 concentration 108, 125 concrete 157, 164 condensation 47, 69, 179 cure reaction 68 mechanism 100 polymerization 139 reaction 188 conducting amine 172 conductive adhesive 158 conjugate electron 189 conjugated double bond 9 consolidation composition 35 construction 180 contact angle 68, 106, 120, 175, 210 hysteresis 120 points 203 contraction 50 controllable drug release 24 controlled degradation 184 release 23 conversion 91, 108, 133, 198 degree 53, 108-109, 198-199 rate 113, 156 coordination metal 153 structure 8 copolymerization 51, 128, 152 copper 17 chelator 17

221

core-shell morphology 9 corn protein 96 corrosion protection 161, 165, 183 resistance 157, 165, 169, 183 cosmetic composition 77 cosmetics 24 cotton fabric 116 Coulombic interactions 157 coumarin 111-112 group 65 coupling agent 33, 103 covalent agent 25 bond 1, 9, 42, 115, 129, 136, 172 interactions 157 crosslinking 25, 39, 83, 87 covalently crosslinked polymer 56 smart hydrogel 94 covulcanizing agent 136 crack bank 194 growth 130 propagation 31, 194 repair 31 tip region 205 craze 65 creep 50 modulus 73 strain 73 critical strain 209 stress intensity 209 crosslink 81, 91, 104 degree 60, 73 crosslink density 6-8, 11, 14, 16, 18-20, 27, 33-35, 38, 47, 50, 52-53, 81, 85, 89, 94, 102-103, 108,

222

110-112, 115-116, 118-119, 121-122, 124, 126, 128, 130-132, 134, 139-140, 149, 160-162, 165, 171, 175, 180, 191, 203, 205-206, 209 distribution 137 formation 47 inhomogeneity 132 junction 42 network 34 point 124 structure 103, 121 crosslinkable group 35 silicone rubber 102 crosslinked agar 139 blend 80 chitosan 17 copolyester 62 epoxy system 110 fibrous membrane 72 gel electrolyte 63 hydrogel 94 membrane 83, 138 network 49, 50 polybenzimidazole 60 polydimethylsiloxane 68 polyethylene 73 polymer film 21 resin network 66 separator 106 structure 8 surface layer 9 topography 72 crosslinker 1, 3, 6, 11, 24, 26, 29, 38, 68, 70, 74, 93, 98, 100, 102, 120, 125-127, 133, 135, 137 concentration 94, 108-109, 142

Index

diffusion 109 distribution 127 dosage 79 reactivity 100 crosslinking 2, 11, 14, 17, 19, 23, 31, 37-38, 41, 47, 65, 76, 103, 106, 111, 119 agent 21, 28, 31, 50, 57, 63, 69, 71, 76, 80, 116, 139 concentration 115 behavior 81 bundle 123 coagent 109 degree 67, 73, 83-84, 87, 91, 108, 122-123 density 91, 112 dynamics 81 efficiency 33, 81- 82, 97, 122 enthalpy 122 enzyme 40 extender 97 kinetics 122, 127 mechanism 13, 52, 65, 98-99 network 89 parameters 108 polymerization 5 procedure 72 process 9, 11, 108, 111 progress 121 reaction 11, 35, 46, 60, 73, 97-98, 108, 111, 113, 130, 150, 173 site 99 structure 70 system 31 temperature 80, 111 treatment 57 weight 67 yield 67 crown ether 162

Index

cryogenic tensile strength 166 crystal fraction 64 growth 73, 124 lamella 123 size 110 spherulite size 123 structure 139 velocity constant 64 crystalline colloidal array self-assembly 48 region 124 structure 124 crystallinity 64, 73, 90, 117, 122-125, 130, 139 degree 62, 124-125 crystallite size 139 crystallization 76, 90, 110, 117, 123-124, 139, 158, 160, 180 behavior 190 enthalpy 123 rate 62, 64, 123, 125 temperature 62, 123, 125 cumene hydroperoxide 168 curative 1, 147, 155, 169, 171, 176, 202 diffusion 203 curcumin 109 cure behavior 38 control additive 69 exotherm 158 kinetics 164 mechanism 160 process 116 rate 19, 155, 160 reaction 208 reversion 81 schedule 207

223

stress 208 temperature 169 time 141, 181 cured network 156 resin matrix 156 curing accelerator 170, 209 agent 49-50, 111, 149, 152, 157, 176-178, 200, 203 droplet 210 moving ability 205 behavior 203 degree 198 exotherm 175 kinetic analysis 156 kinetics 168, 199, 207 mechanism 1, 177 process 191, 198 profile 149 reaction 1, 2, 150, 155, 198, 200, 205 reactivity 175 shrinkage 182, 208 temperature 133, 153, 164, 191, 200, 205 time 129, 201 current density 106 cyanate 153 concentration 154 conversion 154 ester 153, 205 resin 153 cyanoacrylate 29 cyclic carbonate 190 ladder-like poly(aminophenyl) silsesquioxanes 139

224

cycloaliphatic 155, 162 amine 162-165, 181 diepoxide 111 epoxy 165 group 63 outdoor epoxy resin 176 ring 175 cyclobutane ring 112 cyclohexane 77 cyclotrimerization 153 cyclotriphosphazene 169 cylindrical part 196 cystine disulfide bond 117 cytotoxicity 24, 96, 125 D damping factor 122 dangling chain 99 data receiver/transmitter 80 deacetylation 24 debonding failure 91, 115 Debye equation 139 decay time constant 180 decellularized meniscus 125 decomposition temperature 111, 123 defect formation 153 deformability 199, 205 degradation product 159 resistance 128 temperature 140 degree of phonon scattering 139 dehydrated castor oil 15 dehydrocondensation reaction 102 dehydrocoupling reaction 68 dehydrodimer 67 dehydrothermal treatment 39

Index

delayed crosslinking 93 release 74 system 74 dendrite 107 dense structure 203 density 31, 118 gradient 85, 136 dental cement 120 dentin 29 dessert 40 deterioration 188 deuterium lamp 67 devolatilization process 84 di(ethylene glycol) dimethacrylate 74 diabetic wound 44 model 45 diacetone acrylamide functional group 52 diacrylate crosslinker 67 diagnostic radiology 5 dialdehyde carboxymethyl cellulose 17 dialkoxyxanthogendisulfide 28 diamine 70, 72, 93 crosslinked polymer 43 crosslinker 47, 75, 79 -terminated polyamide 180 diaminodiphenyl sulfone 165 diaroyl peroxide 67 diazabicycloamine 202 dibromoxylene 60 dibutyl phosphate 93 tin oxide 37 dibutyltin diacetate 149 dilaurate 68, 93, 194

Index

dicarboxylic acid 94 dichloromethane 161 dicumyl peroxide 19, 27, 34, 37-38, 52-53, 73, 98, 108, 111, 113, 115, 118-119, 122-124, 128, 135, 137, 140 dicyandiamide 87, 167, 169 dicyandiamides 170 dicyclopentadiene novolac 177-178, 204 dielectric constant 21, 63 loss coefficient 63 Diels-Alder coupling 136 reaction 15, 33, 64, 76, 135, 161, 168 diene mesogen 50, 134 diethanolamine 118, 126, 168, 203 diethylenetriamine 162, 177, 179 diethyltoluenediamine 161 diethyltriamine 183 differential scanning calorimeter 122 calorimetry 156 diffusion 112, 159 coefficient 94 control 207 controlling stage 108 process 116 rate 98, 203 -controlled 198 reaction 108 digestive enzyme 26 diglycidyl ether of bisphenol A 87, 156, 162, 170, 175, 177, 194, 198-199, 203, 206, 209 dihydrazide 93 diisocyanate 13

225

diketone 120 diluent 164 dimensional change 165 stability 22, 60, 142, 192 dimer acid 158 dimeric fatty acid 158, 204 dimethyl itaconate 136 -methyl hydrogen-siloxane 194 dimethylol propionic acid 89 dimethylsulfoxide 108 diol sequence length 93 dipolarophile 202 directional freezing 74 disorder 76 state 139 dispersibility 167 dispersion polymerization 47 display devices 48 dissolution properties 104 disulfide bond 157, 207 formation 11 crosslink 11 exchange 135 reaction 157 group 30, 44 rearrangement 31 di-t-butyl peroxide 38, 67, 111, 122, 124, 127, 131 di-tert butyl cumyl peroxide 73 dithiodibutyric acid 30 dithiol 132 diurethane 119 divinyl benzene 77 crosslinking 79 divinylbenzene 83, 126-127

226

divinylsulfone 141 DNA crosslinking agent 125 lesion 125 synthesis 125 dodecanedioic acid 31 dodecyl methacrylate copolymer 208 dodecyldimethylamine 156 domain boundary potential 198 dopamine 202 dose 87, 102, 109-111, 141 double bond 11 conversion 112 -quantum NMR spectroscopy 34 downhole application 80, 208 draw ratio 81 drilling operation 18 drinking water treatment membrane 117 dripping 63 drug 17 carrier 17 controlled release 48 delivery system 174 loading 109 release 5, 17, 41, 94 rate 126 dry composition 42 drying speed 14 time 14, 16, 200 dual crosslinking network 38 network 128 ductile failure 137 ductility 130, 171 dyeing 116

Index

dynamic bonds 135 covalent bond 135 curing 199 loading 90 polymer network 135 vulcanization 53, 111 E e-beam 131 irradiation 48, 109-110 radiation 123 dose 110 ε-caprolactam 93 effective activation energy 109 foaming 27, 121 Einstein equation 99 elastic adhesive 202 modulus 67, 89, 113, 120, 129, 136, 165 elasticity 90, 117, 129 elastomer 38 phase 84 elastomeric toughener 168 electric power transmission 73 electrical 87 conductivity 203 conductor 27 equipment 159 insulation 73 application 176 properties 160 resistivity 203 electrochemical battery cell 63 potential 177 electrode separator membrane 87

Index

electrolyte membrane 61, 70 system 106 electron accelerator 9 acceptor 97 beam 103, 124, 140 crosslinking system 127 irradiation 31, 70, 73, 87, 131 radiation 63 irradiation 102 lone pair 163 withdrawing effect 172 electronic 87 components 170 devices 161, 188 sensor 30 electronics packaging 191 electro-optic activity 46 electrophilic crosslinking agent 56 esterification 189 group 56 electropositivity 189 electrospinning 55, 72 electrospun collagen 17 gelatin nanofiber 17 electrostatic attraction 9 interaction 135 elemental sulfur 70 elevated temperature 158 elongation 39, 94, 108, 118-119, 121-122, 180, 207 embrittlement 195 emulsion polymerization 9 enamine-imine tautomerism 174 encapsulant 35, 125, 149

227

encapsulated glycerol 195 imidazole 170 encapsulation 37, 161 efficiency 109 end -capped polyurethane 133 -capping agent 100 -crosslinking 79, 132 energetic binder 189 energy dissipation 137 entanglement 47 critical length 99 density 99 enthalpy 203 of melting 122 environmental adaptability 135 elements 188 remediation 13 stress cracking 66 resistance 84 enzymatic crosslinking 40, 128-129, 139 oxidation 41 treatment 97 EPDM 33, 108, 110, 122, 128, 137 epichlorohydrin 173, 182, 188, 202 epoxidation 49 ratio 31 epoxide ring 198 epoxidized castor oil 204, 209 natural rubber 30-32, 127, 131, 137-138, 141 soybean oil 175, 179, 209 epoxy 53, 63 acrylic 173 composite 172, 209

228

decomposition 160 foam 121, 202 functionality 110 group 30, 35 hardener 188 homopolymerization 175 matrix 130, 194, 198 monomer 157 mortar 163 nanocomposite 115 phenolic novolac resin 53 resin 35, 50, 60, 110, 155, 157-158, 162-163, 165, 169, 176, 184, 198, 204, 206, 208, 210 adhesive 168 film 167 ignition-resistant 35 system 156 ring 163 value 53 -anhydride network 159 -based mesogen 49, 134 -novolac coating 188 -terminated poly(ethylene oxide) 132 equilibrium conditions 135 swelling 68 erosion rate 165 essential parameters 198 ester foam 195 linkage 158 type diamine 161 esterification 29, 41 crosslinking 133 estrogenic activity 175 etherification reaction 163 ethyl cellulose film 172

Index

ethylene glycol diglycidyl ether 41 propylene diene monomer 119 ethylene sequence 124 vinyl acetate 108, 121-122, 125 copolymer 37 crosslinking 108 -octene copolymer 123 -vinyl alcohol copolymer 123 ethylenediamine 38, 47, 157, 177 ethyleneglycol diglycidyl methacrylate 76 eugenol 175 eutectic point 200 exchange reaction 135 exchangeable disulfide bond 157 exothermic energy 156 heat release 156 interval 172 reaction 167, 200 expectation guidelines 202 explosion 202 explosives 189 exposure time 112 extracellular matrix 94, 174 protein 108 stability 117 extraction 161 extrusion 135 F facial mask 94 fast drying 180 fatigue crack propagation 111 improvement 31 resistance 196

Index

fatty acid 157, 168 dimer acid 180 FDA approval 67 ferric ions 202 ferrocene-based thermoset 208 fiber length 39 mat 39 optic cable 27 fibroblast 55 fibrous protein 96 scaffold 96 Fick's second law 188 filament winding 181 filled elastomer 102 filler 91, 102 content 111 function 91 -rubber interaction 103 filling degree 99 film formation 52, 133, 173 hardness 129 thickness 111, 200 -formation 14 -forming 9 temperature 9 filter effect 112 filtration 72, 86 final conversion 198 product 2 fire risk 165 first-order kinetics 172 fish gelatin 17 flame resistance 169

229

retardancy 35, 167, 178, 188, 204 retardant 30, 34-35, 63, 69, 166, 188, 204 properties 35 flammability rating 204 flash corrosion 149 flexibility 8, 14, 37, 66, 73, 182-183, 195, 198, 204-205 flexibilizer 181, 183, 188 flexible display 8 gel electrolyte 50 flexural modulus 204 properties 207 strength 164, 204 Flory-Rehner equation 20 flower-like nanogel 130 fluorescence intensity 156 fluorescent contrast 9 fluorinated group 120 polyol 93 fluoroelastomer 38 fluororubber 38 Flynn-Wall-Ozawa model 122 foam 27, 100, 111, 118 density 37, 100 morphology 126 foamability 171 foaming 111, 118 agent 37 food 24 industry 102 formaldehyde 23, 26, 53, 197 /phenol ratio 53 formic acid 93 forward osmosis 106 membrane 130

230

fouling trend 106 foundry binder system 168 Fourier transform infrared spectroscopy 182 fracture energy 199, 205 process 130 surface 137 toughness 111, 165, 199, 205 parameters 209 fracturing 18 fluid 42 fragmentation chain transfer 45 polymerization 44 free radical 63, 73 copolymerization 13 crosslinking mechanism 98 initiator 123 polymerization 86, 94 volume 35, 73, 79, 103, 137, 205 freezing 180 friction 17, 126 coefficient 31, 126-127 composite 31 Friedman method 199 front speed 14 -forming drier 14 fuel cell 61, 104, 106, 142 durability 142 performance 60 fumed silica 113 functional graphene oxide 9 group 1, 46, 115 functionalization 158 functionalized graphene oxide 165

Index

reduced graphene oxide 23-24 furan 136 furfuryl 161 amine 161 -terminated polybutadiene 206 G γ-aminopropyltriethoxysilane 194 γ-ray 124, 135 irradiated polycarbonate 66 irradiation 57, 141 radiation 20, 64 resistance 34 galactomannan 41-42 galactose oxidase 41 gallic acid 39 gamma radiation 63, 73, 110 radiosterilization 66 gas conductivity 119 permeability 20, 61 permeable membrane 111 permeation performance 132 separation 61, 75 membrane 95 gastrointestinal stability 17 tract 26 gel bead 151 content 81, 122-123, 127, 135, 140 electrolyte 6, 63 formation 57 fraction 70, 141, 205 layer 41 network 129, 151 point 57

Index

properties 129 rigidity 113 gelatin 39, 125, 128 fiber 39 hydrogel 39, 111, 117 gelation 39, 79, 151 behavior 132 point 109, 127 process 141 progression 129 rate 122 solvent 47 time 47, 108 generation time 159 genetic disorder 17 genipin 23, 86, 109 concentration 125 gentamycin 24 glass fiber 63 composite 205 transition 161 disappearance 159 temperature 16, 33, 35, 46, 50, 57, 73, 84, 89, 91, 93, 103-104, 108-110, 121-123, 131, 158, 161-162, 165-167, 172, 175, 178, 180, 184, 188, 194, 198-200, 203-206 glassy region 205 state 35, 203 gloss 93, 183 glue 182 glutaraldehyde 18, 26, 39, 52, 74, 96, 109, 117, 124-125, 131, 133, 137, 139 concentration 112

231

glycerol 89 dimethacrylate 47 glycerolysate 129 glycidyl azide 202 polymer 89, 189, 207 methacrylate 149 glyoxal 26, 94, 96 grafting 19, 70, 128-129, 166 density 120 process inhibition 100 ratio 129 graphene 19, 108 functionalization 166 nanoplatelet 166 nanoribbon 38 oxide 9, 48, 57-58, 94, 141, 165, 179, 206, 209 aggregate 58 colloidal solution 48 surface 19 graphite 31, 111 fiber 207 green composite 96 ground tire rubber 53 growth factor 40 guar 41, 42 gum 17, 41-42 powder 42 gum karaya 41 H hair keratin 97 halloysite 50, 89 nanotube 89 halogenophosphazene 169 hard domain 91, 118 hardener 35, 54, 164, 180 functionality 163

232

type 163 hardness 1, 14, 31, 89, 129, 169 healing 135, 157 agent 195, 206 heat aging 52 capacity 35 measurement 91 deformation temperature 170 distortion temperature 140, 163 generation 106, 160, 179 molding 38 release rate 165, 167 resistance 11, 37, 160, 192 stability 87 treatment 73 -shrinkable tubing 27 -treatment temperature 94 heating rate 175 heavy ion 117 hectorite 5 helix denaturation 17 hemiacetal bond 41 hemicellulose 197 hemispherical polymethylmethacrylate bead 76 hemocompatible 13 hen egg white lysozyme 41 herbicidal control 53 heterogeneous nucleating agent 118 nucleation effect 118 process 159 hexachloro-cyclo-triphosphazene 204 hexahydrophthalic anhydride 159, 203, 205, 209 hexamethoxymethylmelamine 22

Index

hexamethylene diisocyanate 89, 108, 130 ester 57 trimer 204 hexamethylenediamine 165 hexamethylenetetramine 53-54 high solids coating 157 paints 174 temperature 200 vinyl polybutadiene rubber 141 voltage 73 -density polyethylene 112, 139-140, 142 hindered alkyl urea moiety 135 diamine 206 hindrance mechanism 171 homogeneous condition 113 kinetics 108 homolysis 44 homopolymerization 161 horseradish peroxidase 128 hose 119 hot spot 107 Hoveyda-Grubbs’ 1st generation catalyst 206 Huisgen cycloaddition 46 human pathogen 117 tissue 90 -motion monitoring 30 humid conditions 157 humidity 100 hyaluronic acid 39, 44-45, 116, 120, 151

Index

hybrid 183 alkyd/acrylic resin 149 chemistry 149 cure 119 epoxy 183 material 14, 153 resin 149 hydratable salt 102 hydraulic fracturing 41 hydrazide 20, 120 hydrazine 169 hydrocarbon reservoir 41 hydrodynamic diameter 47 hydrogel 5-6, 16-17, 25, 41, 56, 74, 112, 117, 120, 131 dressing 142 film 108 formation 47 hydrogen abstraction 60, 67, 82, 122 bond 30, 135 crosslink density 140 bonding 131, 195 peroxide 97 sulfide 182 hydrogenated nitrile rubber 43 hydrolysis 15, 159, 174 hydrolytic condensation 68, 183 enzyme 69 stability 29 hydrophilic coating layer 130 crosslinking 47 curing agent 195, 204 group 94 nature 130 skin 106 surface 102

233

modification 121 hydrophilicity 106, 117, 120, 130, 137, 159, 208 hydrophobic interaction 41 modification 128 polymer matrix 133 structure 167 hydrophobicity 93, 106 hydrophylic polymer 95 hydrosilylation 20, 50, 134 catalyst 102 reaction 120, 132 hydroxyapatite 17, 94, 117 hydroxyethylcellulose 141 hydroxyl 1 group 11, 41, 69, 97, 162, 208 hydroxymethylated resorcinol 197 hydroxypropyl guar gum 41 hydroxysilane 116 hydroxyurethane 190 hygienic textile 116 hygroscopic performance 94 hygrothermal performance 167 hyperbranched alkyd 15 polyamide 179 polyamidoamine 157-158 polycarbosilane 132 polyethylenimine 190, 208 polyimide 46 polymer 44-46 I icephobic coating 120 ignition 188 illumination conditions 49

234

imidazole 156, 167, 170, 172, 175, 200, 208 compound 172 curing agent 208 derivative 170, 208 moiety 172 ring 171, 200, 208 -based compound 172 imidazoline reaction 180 ring 158 imidazolium ionic liquid 141 imidization temperature 193 imidocarbonate 153 imine hydrolysis 174 immiscible polymer pair 131 impact energy 163 properties 174 resistance 11, 84 strength 31, 110, 130, 166, 178, 204, 206-207 impedance module 183 impermeability 19 impermeable skin 14 implant 73, 94, 135 induction period 81 inert atmosphere 191 gas 38 inflammatory response 117 inhomogeneous heating 127 initial film thickness 200 tack 109 initiator 5, 38, 137 injectable drug carrier 94 hydrogel 44

Index

hyperbranched polymer 45 inner liner 19 interaction 39 potential 67 interchain 86 packing 161 inter-crosslinking 57-58 interface adhesion strength 91, 115 failure 91 strengthening 158 interfacial bonding strength 205 failure 116 interaction 39, 172, 209 properties 158 interlamellar contact 81 mechanical properties 158 shear strength 153 interlocked architecture 8 intermolecular crosslinking 113 internal plasticization 93 residual stress 208 stress 153 interpenetrating network 99, 203 polymer network 131 inter-strand crosslink 125 intrachain 86 intra-crosslinking 58 intragroup 86 intramolecular crosslinking 113 hydrogen 171, 208 bond 170 bonding 200

Index

intumescent 178 flame-retardant 204 iodododecane 94 ion aggregation 50 beam irradiation 106 channel 105 conductivity 83, 104, 141 conductor 104 exchange capacity 106 fluence 65 mobility 205 track 68 transport 133, 141 ionic bond 1, 124 bonding 104 cluster 50 conducting promoter 50 conductivity 6, 71, 172 crosslink 24 crosslinking 25, 86 density 25 interaction 107 liquid 50, 172-173, 200, 209-210 polycation 104 photoacid generator 35 polymer 130 ionically crosslinked structure 86 ionizing radiation 20, 55 ionomer 139 iridium catalyst 102 iron oxide magnetic nanoparticle 109 -based complex 102 irradiated composite 31 ion 68 silicone rubber 129

235

irradiation 44, 47, 102, 129 dose 31, 34, 103, 109 source 112 stability 34 temperature 87 isobutylene/isoprene copolymer 20 isocyanate 1, 115-116, 178, 189-190, 195, 202 curative 207 group 168 -free curing agent 189 -terminated polyurethane prepolymer 206 isocyanurate 202 isolated crosslinked nanodomain 99 isophorone diamine 35, 156, 164, 169, 181, 204 diisocyanate 89, 93, 109, 195, 206 isoprene 152 isopropyl trioleic titanate 33 isosorbide 173, 207 epoxy 174 isotactic 82 isothermal conversion 156 curing 108, 198 differential scanning calorimetry 182 isotropic state 50 isotropization temperature 50 transition 50, 134 K Kamal autocatalytic model 198, 207 keratin 97 fiber 117

236

ketal peroxide 27 ketimine 174 nucleophilicity 174 keto-hydrazide crosslinking system 9 reaction 52 kinetic constant 198 data 172 parameter 167 Kissinger method 108 knee arthroplasty 73 Knudsen effect 119 L lac dye 116 laccase 17, 97 lactic acid 142 lamellae surface 81 thickness 124 lamellar arrangement 124 stacking 73 structure 50 thickness 109 laminates 35, 167 lamination 125 of PV modules 127 lap shear stress 8 laser diode 112 fluence 129 scanning microscopy 68 latency 172 latex particle 9 layer-by-layer assembly 55 crosslinking 17

Index

self-assembly 104 lead 10 leather material 190 length 134 change 49 Lennard-Jones interactions 157 lifetime 11 ligand 196 light stimulus 6 -activated hydrosilation catalyst 102 -emitting diode 170 -switchable self-healing 6 lignin 162, 174, 197 -epoxy system 175 limestone 202 limiting oxygen index 166, 169, 204 linear aliphatic segment 35 chain extension 207 low-density polyethylene 131 resin 66 linking site 67 linseed oil 14 lipid droplet 17 intake 26 lipogel 56 lipophilic model drug 94 liquid crystal display cell 170 device 22 crystalline arrangement 207 elastomer 49-50, 134 epoxy

Index

curative 207 domain 207 network 157 resin 207 mesogenic unit 50 monomer 50 network 157 phase 50 properties 157 crystallinity 50, 207 lithium battery separator 72 lithium ion battery 63 liver 17 local heterogeneities 162 locust bean gum 42 long chain crosslinking 90, 117 loss factor 33, 108, 122 low Earth orbit 165 working temperature 206 -temperature curing agent 181 lubricating grease 22 M macrocrosslinker 5 macrocycle 162 macromolecular radical mobility 108 macromonomer 81 macroradical 57 trapping 81 macroscopic liquid crystalline orientation 157 magic angle spinning 53 magnesium oxide 5 peroxide 87 magnetic fluid 9 resonance 180

237

magnetite nanoparticle 9 -coated silica 15 maleic anhydride 21, 37, 128, 141, 159, 168, 179 imide 136, 161 group, 172 maleopimaric acid 175 marine application 191 mass ratio 200 transfer resistance 203 material heterogeneity 112 mathematical model 94 maturation 89 maximum degradation temperature 140 gas permeability 111 mean free path 119 measurement standard deviation 127 mechanical performance 158 properties 1, 7, 53, 112 stability 205 stiffness 89 strength 9, 23, 67, 80, 104, 141 medical application 34 device 16, 66 membrane 117 part 87 melamine 21, 63 resin 54 melt blending 131 index 37 mixing 69

238

melting point 84, 108, 110, 123-124, 169, 198, 200 temperature 122, 131, 179 depression 110 melt -spinning 53 -state crosslinking 81 membrane 20, 25, 45, 57, 67, 94, 136 applications 180 crosslinking 38 filters 48 hydrophilicity 57 surface 57, 120 mercaptan group 181 mercaptosilane 116 mesogenic segment 207 meso-scale level 33 mesoscopically periodic material 48 metal catalyst 154 chelate 149 paint 14 -ligand complex 9 crosslink 9 -metal bonding 168 metallocene polyethylene 137 metathesis polymerization 206 methanol permeability 104 methoxy group 68 methoxylation 125 methyl group substitution 206 hexahydrophthalic anhydride 175-176 maleopimarate 175

Index

side group 67 methylcellulose 131 methylene diphenyl diisocyanate 109, 121, 136, 141 group 120 methylhexahydrophthalic 158 methylnadic anhydride 158 methyltetrahydrophthalic 158 methyltrimethoxysilane 136 mica 33, 122 Michael addition 157 microcapsule 53, 195, 206 wall 156 microcellular chlorinated polyethylene rubber foam 129 epoxy foam 121, 171 foam 126, 130 structure 121, 171 microcrack 206 microcrystalline cellulose 17 microdevices 134 microdomain formation 39 microelectronic application 163 microencapsulation 23 micro-fibrillation 65 microfluidization 17 microgel 9, 10, 24, 136 particle 47 -reinforced hydrogel 5 microheterogeneity 131 microhole 68 micro-melting 72 crosslinking 72 micropillar 68 microporous substrate 106 microshear bond strength 29 microwave irradiation 53

Index

migration stability 60 mild steel substrate 165 minimally-invasive vascular application 134 miscibility 131 miscible polymer pair 131 mobile bearing 73 diol unit 93 mobility 158 modulus of elasticity 164, 198 moisture 189 curable system 119 diffusion 174 ingress 188 resistance 120, 167 sensitivity 158 sensor 21 uptake 188 -curing epoxy resin 174 -resistant adhesion 161 molar mass between crosslinks 52, 99 distribution between crosslinks 52 ratio 157 mold release 38 molecular chain 118 design 8 level 34 mobility 91, 109, 201 orientation 207 separation performance 60 stiffness 62 structure 164 weight 1, 9, 68-69, 79, 91, 117, 132-134, 155, 160, 179-180, 200

239

between crosslinks 119, 132, 137 molecularly imprinted polymer nanoparticles 50 molten state 87 monomer 1 monosulfidic bridge 119 crosslink 141 montmorillonite 202 morphology 129, 203 mortar 120 motor vehicle 180 movable crosslinker 8 multifunctional crosslinker 47 curing agent 30 phosphine 152 multi -step processes 109 -walled carbon nanotube 38 museum collections 195 N N,N'-methylenebisacrylamide 40, 48, 85, 136 crosslinking 85 N-(acryloyloxy)succinimide 47 nanoadditive 116 nanoclay 124 nanocomposite 19 film 104 nanocrystalline cellulose 133 polyhydroxyurethane 208 nanodomain surface 99 nanofiber 55 nanofibrillated cellulose 41 nanofiller 29

240

nanogel 5, 9, 125 nanographite 118 content 118 nanoindentation 94 nanoparticles 202 nanophase separation 208 nanoscopic domain 46 nanosphere 15 nanostructure 183 nanostructured conductive layer 30 nanotherapeutics 85 nanotribological properties 74 natural rubber 31, 52, 112, 136-138, 141 carbon black-reinforced 52 latex 133 particle 52 nervous system 17 network connectivity 135 crosslink density 50 defect 34, 208 formation 161 growth 201 heterogeneity 33, 108 mobility 35, 159 structure 198 nitrene addition reaction 87 nitrile group 11 rubber 119 additives 11 microgel 11 seal 11 nitrobenzyloxycarbonyl 115 nitrogen 118, 121 atom 200 nitrosamine 52 nitroxyl functionality 81

Index

nonwoven fabric 116 membrane 72 nonylphenol 154 norbornene polymer 126 novolac epoxy resin 63 oligomer 188 resin 53, 178, 188 structure 53 nuclear facility 11 magnetic resonance 53 nucleating agent 57, 62, 123, 125 effect 123 nucleation energy barrier 118 nucleophilic attack 163 substitution 169, 204 reaction 198 nucleophilicity 172 number of collisions 119 O octene content 123 odor 157 oil and gas recovery 53 -in-water emulsion 195 oleogel 22 oleophobicity 93 oligomer 1 oligomerization 81 oligonucleotides 16 oligosilsesquioxane 210 one-part adhesives 169 onset crystallization temperature 123

Index

decomposition temperature 180 open cell content 119 time 182 operating temperature 73 operational temperature 49 ophthalmic eye drops 125 optical compensation sheet 22 filters 48 limiters 48 properties 149, 207 switches 48 optically anisotropic layer 51 optimal conversion 201 morphology 51 optoelectronic application 194, 210 oral dosage form 67 organic acid 93 peroxide 11, 43 solvent nanofiltration 60 titanium compound 95 organocyclosiloxane 120 organosilane 120 organotin catalyst 98 orientation 124, 139 level 50 oriented material 50 structure 81 oscillational rheology 182 osmotic force 86 pressure 142 outdoor electrical applications 176 outer layer 43 overcure reversion stability 122

241

overheating 107 overload protection 73 oxazolidine 54 oxazolidinone ring 205 oxidation 11, 180 diffusion control 112 reaction 11 test 81 oxidative chemical stability 60 cure 9, 16 drying pattern 14 oxidized carboxymethylcellulose 39, 117 graphite 111 sucrose 96 sugar 97 oxirane group 9, 188 oxygen 159, 169 consumption 14 diffusion 11, 14 gradient 14 inhibition 119 oxygenated hydrocarbon 75 ozone action 20 P packaging 97 packing 39 density 205 paclitaxel 125 paints 167 paper 120 paraformaldehyde 53-54, 75, 199 parameter 108 partial discharge 159 particle interior 85 morphology 84

242

size 99 surface 85 -like reinforcing agent 46 particulate contaminant 107 peak heat release rate 169, 204 smoke production rate 166 pectin nanofiber 125 mat 125 peel strength 8, 109, 115 pencil hardness 111 pendant alkyl chain 134 amine 67 epoxy group 79 furan functional group 161 group 86 pendent 15 penetration depth 49 pentaerythritol 68, 190 propoxylate triacrylate 47 triacrylate 47 pentaethylenehexamine 177 pentasodium tripolyphosphate 24-25, 130 pepsin 69 peptide 97 permanent deformation 47, 134 permeability 121, 132, 201 permittivity 182 peroxidase 97 peroxide 31, 82, 122, 124, 135 concentration 82, 122 crosslinking 121, 122 cure 120 homolysis 67 level 52 radical 82 system 37

Index

type 82 vulcanization 33 -crosslinking process 73 peroxiredoxin 125 personal care composition 42 disposable hygiene product 130 pervaporation 75, 106 performance 106 separation 45 pH 136, 202 indicator 182 variation 182 -responsive 55 pharmaceutical application 152 pharmaceuticals 24 pharmacological activity 116 phase change material 89 separation 39, 126, 190, 195 transition temperature 134 phenalkamide 162 phenalkamine 177-178 phenathrene 34 phenol 154 novolac resin 188 -terminated polysulfone 87 phenolic compound 116 fiber 53 novolac 54 resin 20, 53, 72, 178, 188 phenylacetylene 35 phenylbenzimidazole 175 phenylenedimaleimide 206 phenylethynyl group 192 pendant 140 phenylmaleimide 27

Index

phenyltriethoxysilane 9 phloretic acid 128 phonon scattering 139, 161 phosgene 190 phosphaphenanthrene group 171 phosphazene 169, 191 phosphinate salt 63 phosphine 19 oxide 167, 206 phosphoric acid 138-139 doping 104 phosphorus 178 content 204 photoactuating liquid crystalline elastomer 108 polymer 134 photoactuation 49, 134 mechanism 49 photocatalytic effect 178 photo-crosslinking kinetics 124 photocycloaddition reaction 112 photo-Fries rearrangement 65 photoinitiator 112, 125 concentration 91 system 50 photopolymerization 112 photoradical initiator 44 photo-response 65 adhesive 86 photoresponsive change 134 photovoltaic module 121, 125, 149 lamination 37 phthalic acid 207 anhydride 14, 167, 204, 206 phthalonitrile resin 191, 200, 209 physical associations 134 crosslink 63, 91

243

crosslinker 50 crosslinking 122, 128, 195 physiological conditions 26 phytic acid 30, 165 Pickering emulsification 24 emulsion 24 Piers-Rubinsztajn reaction 100 pillars 194 pine oil 182 piperazine 163, 165 piperidine 163 plasma discharge 112 treatment 39 plasticity 29 plasticization 188 effect 35, 127, 188, 198 plasticizer 33 platinum 100, 102 platinum catalyst 100-102 plugging 170 pneumatic tire 95 polar bond 130 group 120 hydrogenated phenanthrene ring 140 polarization energy 198 polarizing optical device 21 plate 22 pollutant 178, 202 poly(2-ethyl-2-oxazoline) 55 poly(2-hydroxyethyl methacrylate) 74 poly(2-methyl-2-oxazoline) 55 poly(2-oxazoline) 55 poly(acrylic acid) 9, 55

244

poly(amic acid) 192 oligomer 139 poly(amidoamine) 157 poly(aryl ether sulfone) 139 poly(butyl acrylate) 131 poly(butylene succinate) 126, 139 poly(butylene succinate-cobutylene fumarate) 62, 122-123 poly(butylene terephthalate) 63, 130 poly(ether imide) 112 poly(ethylene acrylate) 21 poly(ethylene glycol) 29, 45, 89, 106, 130 chain 120 diacrylate 121 macromer 44 poly(ethylene oxide) 55 poly(ethylene terephthalate) 136 poly(ethylene-co-vinyl acetate) 124 poly(lactic acid) foam 119 poly(methyl acrylate) 131 poly(methylmethacrylate-cohydroxyethyl acrylate) 78 poly(methylvinyldimethyl) siloxane 98 poly(N-isopropylacrylamide) 47, 48, 79, 132 poly(N-vinyl pyrrolidone) 142 poly(phenylene sulfide) 80 poly(propylene glycol) bis(2-aminopropyl ether) 178 poly(siloxane-silsesquioxane) 194 poly(styrene-b-butadiene) diblock copolymer 118 poly(sulfobetaine methacrylate) 86, 115 poly(urea-formaldehyde) 156

Index

poly(vinylidene fluoride) 117, 124, 142 polyacrylamide 8 crosslink 132 polyacrylonitrile 124 polyaddition kinetics 90 polyaldehyde 96 polyallylic compound 63 polyamide 57, 179 11 140 6 57, 109-110 610 57 layer 58 oligomer 179-180 polyamidoamine 180, 190, 208 polyamine 35, 72, 75, 140, 179, 181-182, 194, 199 adduct 162 polyanion 104 polyarylene 80 polyaspartic ester 196 polybenzimidazole blend 60 membrane 59, 61 polybutadiene 127-128, 201 polycaprolactone 64, 90 chain 64, 90 triol 90, 117 polycarbonate 65 branching 66 crosslinking 65 film 117 photothermal aging 65 polycarboxylic acid 32, 120 polycondensation 16, 192 polydiene chain 111 polydimethylsiloxane 50, 67-69, 98, 108-109, 111, 116, 131-132, 134, 156, 182, 194, 206-207, 210

Index

chain 67, 99, 100 modifier 194 network 67 polydopamine 166 polyelectrolyte behavior 48 brush 74 polyester 120 chain 90 resin 66 polyetheramine 110, 132, 156, 166, 180-181, 207 polyetheretherketone 70, 140 polyetherimide 72 fibrous membrane 72 polyetherketoneketone 71 polyethersulfone 58, 87, 121, 130, 188 polyethylene 73, 112, 115, 124, 127, 131, 135 ultrahigh density 73, 110, 112, 126 polyethyleneamine 180 polyethylenepolyamine 158 polyethyleneimine 95 polyfunctional monomer 57 polyhedral oligomeric silsesquioxane 104 polyhydroxybutyrate 123 polyhydroxyurethane 190, 208 polyimide 75, 111, 209 aerogel 139-140 deposition 130 film 192 membrane 72, 75 polyisocyanate 96, 127, 149 polylactide 110, 123, 128 polymer aggregation 47

245

backbone 139 chain 1, 139 crosslinking rate 112 mobility 158 order 192 scission 112 chemistry 1 concrete 164 electrolyte 71, 87 gel dosimeter 5 electrolyte 6 matrix 172 mobility 89 morphology 133 network 208 radical 122 segmental dynamics 115 viscosity 113 -silica interface 102 polymercaptan 181-182, 194 polymeric foam 118 loop 130 polymerization 161 initiator 16 process 100 polymethylhydrosiloxane 120 polymethylmethacrylate 76-77, 120, 129, 131 polymethylmethoxysiloxane 68 polymethylvinylsiloxane 118 polynorbornene 120 polyol 89, 91, 97, 133 poly(meth)acrylate 63 storage 196 polyolefin 69, 73 polyorganosiloxane 102 polyoxazoline 56

246

polyoxyethylenediamine 156 polyoxypropylene 180 polyoxypropylenediamine 156 polyphenol oxidase 40 polyphenolic anthraquinone 23 compound 116 polyphenylenesulfide 208 polyphenylsulfone 80 polyphosphazene 169 polyphosphoric acid 84 polypropylene 34, 81, 111, 116, 119, 122, 128, 131, 137 isotactic 122 polyrotaxane 8 crosslinker 8 polysaccharide 42 crosslinking enzyme 17 gum 151 polysiloxane 120 polysilsesquioxane 75, 140 polystyrene 17, 83-84, 111, 118, 137 anion exchange membrane 133 backbone 133 beads 83 waste 83 -co-poly(N-isopropylacrylamide) microgel 85 polysulfide resin 181-182 polysulfidic crosslink 141 polysulfone 87, 130 polytetrafluoroethylene 129 polytetramethylene glycol 89 polyurea spray coating 109 polyurethane 89-91, 93, 104, 115, 117, 129, 133, 190, 199, 201, 204 acrylic 173 chemistry 1 composition 93

Index

crosslinking 195 curing 90 dispersion 93 foam 118-119, 126, 195 ink 115 molecular chain 91 prepolymer 196 scaffold 90 shell 195, 206 spray-foam 196 topcoat 183 polyvinylalcohol 77, 94-95, 112, 120, 125, 131, 136-137, 140-141 solution 94 polyvinylchloride 11, 31, 110, 131 porcine pericardium tissue 117 pore size 17, 130 distribution 106 porosity 133 porous scaffold 90, 125 structure 74 positron annihilation lifetime spectroscopy 167 post -crosslinking 97, 121 -curing 191 -lamination crosslinking 37, 121 -polymerization crosslinking 47 -synthesis crosslinking 75 pot life 93, 102, 149, 155, 157-158, 163-164, 167, 170, 172, 178-181 powder coating 93 paints 169 density 6 power generation 191 pre-curing degree 203 precursor tip 65

Index

prednisolone 67 premature crosslinking 111 prepolymer 1, 89, 134 prepreg 167, 188 pressure-sensitive adhesive 35 primary 155 amination 174 amine 155 group 190 diamine 198 drier 14 primer 7 printed circuit board 184, 188 wiring board 35 process kinetics 109 processing time 200 proliferation 125 propagation 182 propanediamine 156 propargylhydroquinone 189 propellant 189 property variations 89 proppant 53 pack conductivity 41 propylene carbonate 123 prosthesis lifetime 110 protective cover layer 22 silica layer 165 protein 16, 17, 41, 55, 97, 108 adsorption 55 chain 39 denaturation 117 desorption 117 proton beam irradiation 57, 109 conducting membrane 60 conductivity 70, 87, 104-106, 142

247

exchange membrane 104, 133, 139, 142 fuel cells 104 migration 105 transport ability 133 transportation tunnel 133 proxyphylline 74, 112 pseudo -equilibrium 159 -second-order reaction 108 pull-off adhesion 111 pyrolysis 68 Q quantum dot nanocrystal 109 quaternized polyether sulfone 87 quinoline 193 R radial distance 47 radiant energy 201 radiation crosslinking 63, 110, 130, 142 dose 64, 73, 108, 110-111, 142 dosimetry 5 resistance 87 responsiveness 6 sterilization 55 -induced crosslinking 70 polymerization 74 radical attack 47 concentration 82 copolymerization 47 crosslinking 108, 113 reaction 98, 111 diffusion 98 polymerization 44, 127 -crosslinking 117

248

thiol-ene crosslinking 55 -mediated mechanism 135 radiolytic species 128 radiopharmaceutics 55 radiotherapy 5, 55 radius size 13 random coil 97 rate constant 122, 207 conversion 199 -controlled reaction 108 reactant diffusion 203 reaction accelerator 19 activity 137 kinetics 37 mechanism 81, 158, 198 order 207 rate 207 scheme 92, 106 step 109 temperature 200, 203 reactive adhesive 197 coalescing agent 9 composition 102 diluent 173, 180 extrusion 76 group 116 polyamide 179 site 58 reactivity 11, 158 recombination 60, 81 recovered stress 134 recovery magnitude 135 rate 90 recyclability 157 recycling 135

Index

redox initiator 112 reduced graphene oxide 87 refractive index 182, 210 regenerative capabilities 44 medicine 39, 94 reinforced composite 158 reinforcement 94, 128, 209 relative humidity 162 permittivity 184 release kinetics 94 remedial operation 170 renewable bis-epoxide 175 repair efficiency 206 replication 125 reprocessability 157 reprocessing 135 residual peroxide 81 stress 161 thiol group 55 resin 2 resistive heating 135 resistivity 159 resole 53 resorcinol 197 -formaldehyde aerogel 130 respiratory sensitizer 177 retardant imidazole curing agent 171 retarder 93 retro-Diels-Alder reaction 161 reverse order 68 osmosis membrane 57, 120 reaction 65 reversible absorption of moisture 102

Index

actuation 135 addition 44-45 addition-fragmentation chain transfer 78 bond exchange 135 cleavage 207 crosslink 76, 135 Diels-Alder reaction 206 extension 50, 134 liquid crystalline phase transition 157 rheological behavior 99 properties 41 rheology 6 rhodium 102 catalyst 102 rigid polyester chain 117 -rod-like structure 161 rigidity 99, 140 ring opening metathesis polymerization 120, 126, 135 polymerization 68, 175 reaction 32, 50 rocket flight 202 rod-like nanoparticles 162 room temperature vulcanizate 99 rosin 98-99, 140 acid 207 -crosslinked silicone rubber 98 rotationally casting 196 rotomolded compound 11 rotor blades 180 roughness 93, 133 RTV silicone rubber 99 rubber chain 11

249

composite 138 matrix 31 phase 31, 84 -modified epoxy mixture 113 rubbery material 149 run-off solution 202 ruthenium 102 S sacrificial bond 9 salmon 39 salt spray 178 sample thickness 111-112 sand 53 control 54 scaffold 23, 117 scanning electron microscopy 68 Schiff base 70 scorch 69 safety 19 scratch closure 64 resistance 93, 162, 206 resistant coating 195 article 35 sealant 116, 202 sealing material 38, 170, 182 seaweed extract 151 secondary 155 amine 160, 198 drier 14 second-order kinetics 207 rate law 154 segmental mobility 33, 108, 122 motion 109, 208 segmented structure 195

250

selectivity 106 self -adhesive layer 9 properties 9 -crosslinking 9, 168 -healable gel 132 -healing 78, 206 ability 156 activity 206 coating 195 hydrogel 5 semi -crosslink point 124 -interpenetrating polymer network 142 semicrystalline polyhydroxyurethane network 190 sensitivity 5 sepiolite 38 sequential interpenetrating network 131 shape change 190 hysteresis 134 memory 49, 173, 190, 207-208 application 157 effect 50 polymer 134 recovery 64, 134, 207 shear adhesion strength 109 strain 115 strength 9 thinning 99 sheet 27 shelf life 167, 170, 174 shell layer 9 shock resistance 179, 182

Index

Shore A hardness 121, 129 short circuit 73 -segmented block copolyester 62 shrinkage 87, 118, 158, 203 shrinking velocity 47 side chain 87, 161, 175 oligomerization 81 reaction 89, 122, 195 silane 27, 63, 100-101, 202 bridge 115 crosslinking 69 surface treatment 115 silanol group 30 silica 121, 128, 137, 209 core 205 hybrid 30, 137 surface 115 silicatein 69 silicone 120 chain 101 mobility 98 elastomer 98-99, 102, 127 feel 77 foam 100 molecular weight 100 oil 67, 77, 127 rubber 98, 100, 102, 129, 137, 140 degradation mechanism 101 foam 118 product 102 shield 102 surface 86, 127 terpolymer 38 siloxane 183 chain 50 –polyurethane 89

Index

silsesquioxane 14, 29, 75, 104, 153, 184, 204,-207, 209 structure 184 silver adhesive 170, 209 silyl compound 174 ether amine 184 silylalkyl radical 67 silylated polymer 119 single phase morphology 131 step process 109 site-directed coupling 39 skeletal structure 203 skin 155 formation 14 -protection layer 94 slide ring 8 sliding coefficient of friction 127 small-bore incision 134 smaller cell size 118 smoke 169 generation 169 smoking article filter 26 sodium alginate 151 benzoate 124 metabisulfite 74 periodate 96 tetraborate 95 soft actuator 134 block 195 lithography 68 magnetic nanocomposite 10 microgel 10 tissue 94 softening temperature 93 solar cell 35, 37

251

solid composite propellants 202 electrode 106 insulation medium 159 solubility 108 parameter 109 solvation 86 solvent polarity 47 resistance 111, 131, 196 resistant nanofiltration 75 retention 200 sorbic acid 33, 110 sorption 137 effectiveness 25 sound velocity 112 soybean oil 17, 126, 175 fatty acids 175 protein 96 spacer 1 spatial distribution of the crosslinks 34 specific capacitance 50 wear rate 127 spectral hyperfine structure 82 spherulite 62, 139 structure 139 spherulitic diameter 139 spirocyclic pentaerythritol bisphosphonate 178, 204 splice 27 spontaneous emulsification 149 sport articles 180 spraying process 195 spun filament 53 stable nitroxide radical 20 steel substrate 183

252

stem cell 44 delivery 44 -based therapy 44 stereolithography 112 steric crowding effect 193, 209 effect 100 hindrance 82, 122, 135, 160, 206 stiffness 109, 136 Stille reaction 51 stimuli-responsive material 85 stoichiometric mixture 156 ratio 184, 199, 205 stone 178 surface 202 storage condition 174 modulus 89, 123, 131, 136, 162, 174, 180 stability 168, 171-172, 208 time 97 strain actuation 134 crystallization 138 -induced crystallization 138 stress 200 fracture 197 level 134 relaxation 135 strontium 104 structural adhesive 168 defect 139 degeneration 117 integrity 17 rearrangement 76 rigidity 199, 205 strength 141

Index

reinforcement 195 styrene -butadiene rubber 103, 111, 135-136 -ethylene-butadiene-styrene block copolymer 111 styrenic block copolymer 111 subsequent crosslinking 51 subterranean formation 42 succinic acid 141 sucrose 96, 175 soyate 175 sugar absorption 26 beet pectin 17 sulfonated polyetheretherketone 104 polysulfone 106-107 sulfonating solution 70 sulfonation 106 degree 104 sulfone 191 group 104 sulfonic acid group 50 group 104 sulfonyl azide 60 sulfur 19, 31, 82, 136, 141, 152 content 103 crosslinking 120 cure 122 monochloride 20 vulcanization 141 -peroxide 37 -vulcanized 31 sulfuric acid 106 superabsorbent 9 hydrogel 9, 130, 141 material 20

Index

supercapacitor 50 supercooled water droplet 120 supercritical carbon dioxide 126 superwettability 117 surface 39 adsorption 55 area 133 attachment 55 conversion value 200 crosslinked layer 112 crosslinking 72, 74 deformation 67 drying rate 162 free energy 93 layer 74, 112 modification 9, 47, 106 morphology 133 quality 167 roughness 133 tension 68 suspended cell 40 sustained drug release 17 swelling 41, 74, 79, 87, 97, 121, 130, 136, 141, 161 behavior 41 degree 47, 56, 84, 141 index 127 kinetics 94 ratio 85, 89, 124, 136-137, 142 resistance 94 switch-like shape recovery 134 swollen form 47 syndiotactic polypropylene 82, 122 synergic association 31 synergism 100 synergistic effect 122, 161 synthesis route 91 synthetic elastomer 152

253

pathway 83 route 44, 99 T tack 8-9 free time 182 tacky surface 119 tall oil 14 tara gum 42 tautomeric equilibrium 174 tear path 137 resistance 196 specimen 68 strength 118-119, 121, 137-138 tearing strength 68 telechelic poly(N-isopropylacrylamide) 79 temperature 48, 108, 137, 141, 149, 160, 174 gradient 200 resistance 120 temporary crosslink 208 tendon fibroblast 125 tensile elongation 134 load 50, 134 strain 132 strength 11, 31, 37, 39, 53, 60, 72, 89, 94, 104, 110, 118, 121-122, 129, 131, 136, 138-139, 142, 158, 162, 165, 180, 198, 209-210 stress 65, 121 tension 94 terephthalic acid 21 terminal ester group 157 galactosyl unit 41 silane group 63

254

termination 182 termination reaction 91 tertiary amine 112, 155, 168, 198 amino group 156 butyl cumyl peroxide 38 carbon atom 159 tetracycline 23-24 tetraethoxysilane 98, 183 tetraethylenepentamine 157 tetraethylorthosilicate 68, 100 tetrafunctional epoxy resin 199 tetramethyl substitution 65 thiuram disulfide 82 monosulfide 82 tetramethylguanidine 181 tetrapropoxysilane 37 tetra-thiol-based crosslinker 55 tetravinyl molecule 50 tetravinylsilane 134 textile effluents 13 textiles 23 therapeutic composition 40 thermal conductivity 139, 159, 198, 209 crosslinker 33 crosslinking 45, 72, 101, 121 curing 192 cycling 50 decomposition 160, 191 temperature 209 degradation 139, 161, 167 discoloration resistance 194 expansion coefficient 110 initiator 208 insulation 119, 126 latency 172

Index

latent curing agent 208 oxidation 35, 124 resistance 169 stability 19-20, 72-73, 83, 87, 93-94, 99-100, 103, 119, 122, 131, 135, 140, 158, 161-163, 166-167, 169, 175, 178, 180, 183, 188, 204, 207, 209 transition 161 treatment 64 thermally crosslinked polyester 63 thermomechanical programming 190 stability 73 thermooxidation stability 140, 191 thermoplastic 66 elastomer 91 thermo-reversible crosslinker 161 rearrangement 135 thermoset 110, 135, 208 thickness 112, 200 of a part 108 thin wall product 66 thiofuran 33 thiol 117 functionality 72 disulfide exchange 135 -ene click reaction 117 coupling 132 reaction 45, 56 -epoxy polymer network 182 thiolate 135 thiophene 51 thiophosphoryl disulfide 28 thiourethane 116 thiourethane group 121, 141

Index

three-dimensional cage 205 crosslinking 163 threshold stress 134 through -cure 112 -dry time 16 tie molecular chain 73, 124 tie-chain 81 tilapia skin 39 time 108 tin 100 tire carcass adhesion 19 inner liner 19 tread 95 tissue engineering 23, 39, 48 scaffold 90 regeneration 40, 120 titanium compound 42 toluene 136 tolylenediisocyanate 21 topcoat 183 tosyl group 174 total crosslink density 122 heat release 167, 169, 175, 204 smoke production 169, 204 release 169, 204 toughening 184, 188 agent 188 toughness 136, 160, 169, 204, 209-210 toxic 155 damage 17 T-peel strength 91 trans-1,4-polyisoprene 141

255

transamination 67 transcription 125 transesterification 76, 184 transglutaminase 17, 40, 128, 139 transient stage 115 transparency 167, 194, 210 transportation 191 cyanurate 21, 57, 63, 109-110, 130 isocyanate 33 isocyanurate 21, 29, 58, 108-109, 122, 140, 202 trimesate 123 triazine 21, 153 derivative 184 hardener 184 ring 153 triazole 202 tridecafluorooctyl methacrylate 120 triethylamine 153, 206 triethylenetetramine 156-158, 163, 177, 204, 209 trifunctional amine 109 isocyanate 135 trifurfuryl propane 206 tri-glycidyl ether 87 trimesoyl chloride 59 trimethylene chloride 58 trimethylol butane triacrylate 76 melamine 94, 120, 137 methane tetracrylate 76 triacrylate 76 propane 91, 115 triacrylate 21, 29, 110, 131, 135 trimethacrylate 124, 126-127, 135 trimethylacrylate 110, 121

256

triphenylphosphine 137 triple-shape memory material 208 tripolyphosphate 23 tris(2-carboxyethyl) phosphine hydrochloride 117 trismaleimide 64 trithiocarbonate 132 trypsin 69 tube model theory 99 tubing 58 tumorous cytotoxicity 125 tunable elastic properties 67 microstructure 90 properties 44 tung oil 15, 33 fatty acid 168 tuning potential 74 turbidity 47 twin-screw extruder 142 two-way shape memory 50, 134 polymer 135 tyramine 39 tyrosinase 39, 97 U ultimate strain 164 ultrasonic online control 112 ultraviolet irradiation 6, 67 crosslinking 51 irradiation 78, 86, 111 time 132-133 spectrum 67 stability 149, 158 weathering 176 UVA irradiation 44 UVC dose 44 unimodal 132

Index

unsaturated double bond 141 polyester 120 resin 136 unsaturation 33, 62 urea 121, 141 uretdione 89 urethane 202 crosslink 67 crosslinker 67 curing system 202 ionic group content 126 ionomer 126 linkage 90, 117 V vanillin 175 vapor permeation 38 vegetable oil 82 vertical burning test 169, 204 vinyl acetate content 37 group 47, 98 radicalization 67 monomer 44, 66 vinyltriethoxysilane 120 vinyltrimethoxysilane 115, 119, 122, 128, 131, 142 grafting 122 homopolymer 33 viscosified treatment 18 viscosity 1, 69, 98-100, 108, 122-123, 158, 163, 174, 180, 198, 201-203 build-up 126 measurement 113 visible light 6 vitrification 207 temperature 200

Index

time 200 vitrimer 67, 135, 167 network 67 void formation 200 fraction 119 volatility 191 voltage profile 6 volume phase transition 48 resistivity 33, 122 vulcanizate 131 vulcanization 112 rate 141 reaction 141 system 52, 137 time 119 vulcanizing agent 53 W waste polystyrene 83 tire rubber 135 water absorption 188 contact angle 182, 194 diffusion 89 behavior 165 flux 106, 121 impact 90 penetration 41 permeation flux 106 resistance 111, 139, 182 sensitivity 9 sorption 24, 29 swelling 83 treatment membrane 106 uptake 87, 104, 133, 141-142, 159, 162

257

ratio 174 -in-oil emulsion 9 waterborne polyurethane 89, 91, 131, 204 wear 73 behavior 17, 126 rate 31 resistance 110 resistant 112 wearable electronics 30 weather resistance 38, 80 weatherability 183 weight loss 23, 116, 204 wet spinning 13 process 13 wettability 106, 130, 196 whey protein 97 Wilson's disease 17 wind turbines 180 windmill blade 156 wood 197 flour 96, 142 particle 115 plastic 131 pulp 182 wool 117 fabric 139 wound dressing 16, 86, 94, 116, 151 healing 44, 117 X xenogenetic tissue 117 X-ray fluence 76 radiation 73 sensitivity 5 XRD pattern 76 xylenediamine 164

258

xylylenediamine 178, 198, 202, 205 Y yield stress 119 Young’s modulus 112, 122, 157, 180 Z zein 39 zinc acetate dihydrate 168 borate 63 derivative 152 dibenzyldithiocarbamate 120 oxide 19, 116, 120-121, 140 zirconium acetylacetonate 9, 149 crosslinker 41 zwitterionic poly(sulfobetaine methacrylate) 86 polymer 86

Index