Polymers as Rheology Modifiers 9780841220096, 9780841213180, 0-8412-2009-3

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Polymers as Rheology Modifiers

In Polymers as Rheology Modifiers; Schulz, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1991.

In Polymers as Rheology Modifiers; Schulz, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1991.

Downloaded by 89.163.34.136 on August 6, 2012 | http://pubs.acs.org Publication Date: May 13, 1991 | doi: 10.1021/bk-1991-0462.fw001

ACS

SYMPOSIUM

SERIES

462

Polymers as Rheology Modifiers Downloaded by 89.163.34.136 on August 6, 2012 | http://pubs.acs.org Publication Date: May 13, 1991 | doi: 10.1021/bk-1991-0462.fw001

Donald N. Schulz,

EDITOR

Exxon Research and Engineering Company

J. Edward Glass,

EDITOR

North Dakota State University

Developed from a symposium sponsored by the Division of Polymeric Materials: Science and Engineering at the 198th National Meeting of the American Chemical Society, Miami Beach, Florida, September 10-15, 1989

American Chemical Society, Washington, DC 1991 In Polymers as Rheology Modifiers; Schulz, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1991.

Library of Congress Cataloging-in-Publication Data Polymers as rheology modifiers / editors p.

Donald N. Schulz, J. Edward Glass,

cm—(ACS symposium series; 462)

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Developed from a symposium sponsored by the Division of Polymeric Materials: Science and Engineering at the 198th national meeting of the American Chemical Society, Miami Beach, Florida, September 10-15, 1989. Includes bibliographical references and index. ISBN 0-8412-2009-3 1. Fluid dynamic measurements. 2. Rheology. 3. Polymer solutions. I. Schulz, Donald N., 1943. II. Glass, J. E. (J. Edward), 1937- . III. American Chemical Society. Division of Polymeric Materials: Science and engineering. IV. Series. TA357.5.M43P65 1991 620.1'06—dc20

91-11687 CIP

The paper used in this publication meets the minimum requirements of American National Standard for Information Sciences—Permanence of Paper for Printed LibraryMaterials,ANSI Z39.48-1984. Copyright © 1991 American Chemical Society All Rights Reserved. The appearance of the code at the bottom of thefirstpage of each chapter in this volume indicates the copyright owner's consent that reprographic copies of the chapter may be made for personal or internal use or for the personal or internal use of specific clients. This consent is given on the condition, however, that the copier pay the stated per-copy fee through the Copyright Clearance Center, Inc., 27 Congress Street, Salem, MA 01970, for copying beyond that permitted by Sections 107 or 108 of the U.S. Copyright Law. This consent does not extend to copying or transmission by any means—graphic or electronic—for any other purpose, such as for general distribution, for advertising or promotional purposes, for creating a new collective work, for resale, or for information storage and retrieval systems. The copying fee for each chapter is indicated in the code at the bottom of thefirstpage of the chapter. The citation of trade names and/or names of manufacturers in this publication is not to be construed as an endorsement or as approval by ACS of the commercial products or services referenced herein; nor should the mere reference herein to any drawing, specification, chemical process, or other data be regarded as a license or as a conveyance of any right or permission to the holder, reader, or any other person or corporation, to manufacture, reproduce, use, or sell any patented invention or copyrighted work that may in any way be related thereto. Registered names, trademarks, etc., used in this publication, even without specific indication thereof, are not to be considered unprotected by law. PRINTED IN THE UNITED STATES OF AMERICA

In Polymers as Rheology Modifiers; Schulz, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1991.

ACS Symposium Series M. Joan Comstock, Series Editor

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1991 ACS Books Advisory Board V . Dean Adams Tennessee Technological University Paul S. Anderson Merck Sharp & Dohme Research Laboratories Alexis T. Bell University of California—Berkeley Malcolm H . Chisholm Indiana University

Bonnie Lawlor Institute for Scientific Information John L . Massingill Dow Chemical Company Robert McGorrin Kraft General Foods Julius J. Menn Plant Sciences Institute, U.S. Department of Agriculture

Natalie Foster Lehigh University

Marshall Phillips Office of Agricultural Biotechnology, U.S. Department of Agriculture

Dennis W. Hess University of California—Berkeley

Daniel M . Quinn University of Iowa

Mary A . Kaiser Ε. I. du Pont de Nemours and Company

A . Truman Schwartz Macalaster College

Gretchen S. Kohl Dow-Corning Corporation Michael R. Ladisch Purdue University

Stephen A . Szabo Conoco Inc. Robert A . Weiss University of Connecticut

In Polymers as Rheology Modifiers; Schulz, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1991.

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Foreword A C S SYMPOSIUM SERIES was founded in 1974 to provide a medium for publishing symposia quickly in book form. The format of the Series parallels that of the continuing ADVANCES IN CHEMISTRY SERIES except that, in order to save time, the papers are not typeset, but are reproduced as they are submit­ ted by the authors in camera-ready form. Papers are reviewed under the supervision of the editors with the assistance of the Advisory Board and are selected to maintain the integrity of the symposia. Both reviews and reports of research are acceptable, because symposia may embrace both types of presentation. However, verbatim reproductions of previously published papers are not accepted.

In Polymers as Rheology Modifiers; Schulz, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1991.

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Preface

A D V A N C E D MATERIALS BRING TO MIND materials with highperformance solid-state properties such as mechanical strength, electrical conductivity, and nonlinear optical properties. However, advanced materi­ als can also be fluids, and polymers can be the agents that convert them from simple fluids to high-performance fluids. Small amounts of polymer can have a profound effect on fluid rheological properties such as viscosification, drag reduction, and antimisting. Such complex fluids have found usefulness in applications as diverse as paints, coatings, fuels, lubri­ cants, cosmetics, personal care products, and foods. Numerous books cover rheology, and even more volumes cover poly­ mers. However, few books are devoted primarily to polymers as materials for modifying or controlling fluid rheology. This volume aims to fill this void. The first three chapters present basic rheological concepts. Chapter 1 contains a section on electrorheological fluids, that is, polymer suspen­ sions whose viscosities change with electric fields. Chapters 4-7 emphasize gels and latices and describe synthesis, characterization, and fluid properties. The next seven chapters feature the special properties, such as shear thickening rheology, of associating polymer systems. These chapters give examples of ionic and Η-bonding association groups on hydrocarbon backbones and hydrophobic associating groups on watersoluble backbones. Chapter 14 is the first report of the use of surfactants in combination with associative thickeners. The next four chapters describe rheology control based on polymer-polymer and polymer-solvent interactions. Chapters 19 and 20 examine deformationrelated orientations in bulk and gelled polymer systems. Acknowledgments We wish to thank all the participants and contributors to this book and to the symposium on which it is based. We also acknowledge the support and encouragement of our families and our secretaries, Arlene Ozbun and xi

In Polymers as Rheology Modifiers; Schulz, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1991.

Penny Clancy. Finally, we thank A . Maureen Rouhi, the acquisitions edi­ tor at the A C S Books Department who kept us on task in a firm but patient manner. DONALD N . SCHULZ Exxon Research and Engineering Company Annandale, NJ 08801 J. E D W A R D GLASS

North Dakota State University Fargo, N D 58105

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February 13, 1991

xii

In Polymers as Rheology Modifiers; Schulz, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1991.

Chapter 1

Polymers as Rheology Modifiers A n Overview 1

2

J. Edward Glass , Donald N. Schulz , and C. F. Zukoski

3

1

Polymers and Coatings Department, North Dakota State University, Fargo, ND 58105 Corporate Research Laboratories, Exxon Research and Engineering Company, Annandale, NJ 08801 Department of Chemical Engineering, University of Illinois, Champaign, IL 61820

2

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3

The chapter summarizes the basis of the use of polymers to con­ trol or influence fluid rheology and outlines typical applications. Electrorheologicalfluids,a new class of polymeric rheology modif­ iers, are discussed. Polymers are best known for their use as bulk materials (e.g., elastomers, plastics, and fibers). There is another world, however, where polymers are used to control solution and dispersion rheology. This world includes fields as diverse as fuels, lubricants, oil field chemicals, water treatment chemicals, coatings, and food applications. In these fields polymers affect the shear and elongational flow behavior (defined in Chapters 2 and 3) and thereby the performance of the fluid during and after application. Polymers modify rheology by virtue of their high molecular weights, chain entanglements, and polymer—solvent interactions. Additional pro­ perty control can be achieved by use of phase changes and associations. In special cases, polymer fluids also can be made to respond to external electrical fields. In the sections to follow, general flow behaviors are dis­ cussed, followed by the rheology requirements as they are desired or required in various applications. In the last section of this chapter, a relatively new class of polymeric rheology modifiers, electrorheological fluids, is discussed. These materials have not yet realized their full com­ mercial acceptance but show promise for doing so. Basic Concepts The power of polymers to influence fluid rheology arises from the greater volume of a macromolecule in solution compared to the total of the molecular dimensions of the repeating units. The solution volume swept out by the polymer coil is known as the hydrodynamic volume (HDV), 0097-6156/91/0462-0002$06.00/0 © 1991 American Chemical Society

In Polymers as Rheology Modifiers; Schulz, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1991.

1.

GLASS E T

AJL

3

Polymers as Rheology Modifiers: An Overview

which is determined by polymer structural parameters (e.g., chain length and chain stiffness) and polymer—solvent interactions, as well as polymer associations or repulsions. H D V also has a temperature, concentration, molecular weight, and deformation rate dependence. For random-coil polymers, the effective H D V of a macromolecule is proportional to the cube of the root-mean-square end-to-end distance, ^ , and is generally equated to the product [n]M, defined by the key equation in Flory's theory (i): 2

1

2

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[η] =

Σ

(1) M where M is molecular weight, [n] is the intrinsic viscosity of the polymer, and φ is a universal constant for all polymer types, φ is influenced by the interaction between the polymer and solvent. This interaction is generally defined by an expansion coefficient, a, which is determined by comparing ^r2 l/2 j j under solvent conditions where strong interaction forces are developed (e.g., a polar polymer in a polar solvent) to < / > ^ or [n] values under poor or theta solvent conditions. In the latter, the interaction is minimized, and the subscript is in reference to unperturbed dimensions.


Q r

n

r2

1

0

Q

Molecular Weight Effects Polymer solutions exhibit a linear increase in viscosity with increasing molecular weight. At a certain molecular weight (M ), the dependence of viscosity on molecular weight increases, generally by an exponential power of 3.4. For a polymer with a given molecular weight, the solution viscos­ ity also increases monotonically with concentration, until a critical concen­ tration is reached (Figure 1). Both observations reflect the onset of inter­ chain associations. Data related to this behavior can be superimposed (2) by relating the solution viscosity dependence to a dimensionless parame­ ter, [n]c, reflecting both the size, through [n], the intrinsic viscosity, and its concentration. c

Shear Thinning Behavior Related phenomena also can be observed when polymer solutions are stu­ died with increasing shear rate. The viscosity of a polymer solution will decrease with shear rate, a consequence, in part, of the disruption of overlapping chains faster than their ability to associate at higher deforma­ tion rates. The shear rate at which non-Newtonian behavior occurs increases with decreasing molecular weight (5), reflecting a shorter relaxa­ tion time of lower molecular weight polymers. This phenomenon also is apparent with increasing concentration for a polymer of constant molecu­ lar weight (Figure 2).

In Polymers as Rheology Modifiers; Schulz, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1991.

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4

POLYMERS AS R H E O L O G Y MODIFIERS

c c*

(1c) Figure 1. The relationship between low-shear specific viscosity and concentration for HP-guar. (la) Plot of log shear specific viscosity vs concentration (g/dL); (lb) log shear specific viscosity vs dimensionless concentration ε[η]. The break points in these curves occur where c = c* (lc). (Reproduced from reference 2. Copyright 1986 American Chemical Society.)

In Polymers as Rheology Modifiers; Schulz, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1991.

1.

GLASS E T A L

Polymers as Rheology Modifiers: An Overview

5

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This behavior will be discussed in this volume, with a variety of uniquely different polymers, in both organic and aqueous media. This will occur at concentrations and molecular weights well below that noted with traditional structures; the phenomenon arises from units that effect atypi­ cal associations: mesogenic groups (liquid crystals), ionic interactions (ionomers) in organic media, and hydrophobic interactions in aqueous media. These interactions effect higher viscosities at low shear rates but without the high elastic response in high application deformation rate processes noted with truly high-molecular-weight polymers. This is a key factor in the acceptance of some of the novel rheology modifiers exam­ ined in this volume. In many applications, disperse phases will be present. In the absence of polymers, they also exhibit shear thinning. Viscosities of such disper­ sions at low shear rates are greater than the viscosity of the individual disperse components; the phenomenon arises from multiplet rotations in a shear field and, with true agglomerates, entrapment of the continuous phase. As the shear deformation is increased, these multiple "associations" are disrupted, with an attendant drop in the viscosity. This phenomenon has been reported for a variety of disperse phase compositions (e.g., bentonite, coal particles, and titanium dioxide), but the mechanism for disperse systems has been defined (4) in greatest detail for monodisperse latices (Figure 3). Among disperse phases, at equal formulation volume fractions, smaller particles will impart greater viscosities because of greater surface area and the impact of hydration on the relative "effective volume fractions" of these dispersions. Shear Thickening Behavior Structure formation in polymer solutions under flow results in a number of experimental observations (5); one of these is shear thickening. Shear thickening occurs in both polymer solutions and in dispersions. In poly­ mer solutions the phenomenon has been attributed to either intramolecu­ lar hydrodynamic interactions due to nonuniform changes of molecular distances during coil deformation of large chains, particularly in viscous solvents (6), or the transition of intra- to greater intermolecular associa­ tions of entanglement junctions. Shear thickening in polymers was observed in ionomers (7 and Chapter 9, this volume) and interpreted in terms of an increase in tem­ porary associations among chains made possible by elongation under shear flow of the charged polymers. Established statistical properties of polymers have been used (8) to support such a mechanism. This theoretical work supports the original observations that the phenomenon is highly depen­ dent on chemical composition and is maximized with only a few associat­ ing groups per chain. The shear thickening phenomenon also has been observed in aqueous systems utilizing hydrophobic bonding (Chapter 11, this volume), in ionic complexes (9a,b and Chapter 10, this volume), and

In Polymers as Rheology Modifiers; Schulz, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1991.

6

POLYMERS AS R H E O L O G Y MODIFIERS

-

5

-

4

-

3

-

2

-

1

0

1

2

3

Log γ (sec )

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1

Figure 2. Viscosity data for solutions of high-molecular-weight poly(isobutylene) in decalin at 25 ° C obtained on a cone and plate viscometer. (Reproduced with permission from reference 3. Copyright 1955 Academic)

aΡ eP cP (c)

(b)

(a)

(3a) 26 ο · "

24 -cv

22

Nl

20

TV

ΒΖΟΗ "-CRES0L H0 2

V

18 16 14 12 10 0.0

0.03

0.10

0.30 1.0 Or (3b)

3.0

10.0

30.0

Figure 3. Shear thinning behavior for dispersions. (3a) Schematic drawing showing that proximity doublets, formed by Brownian diffusion, are destroyed by shear: (a) fluid motion if particles were to rotate independently, (b) rigid dumbbell rotation, and (c) a more realistic flow pattern, intermediate between (a) and (b). (3b) Relationship between relative viscosity and reduced shear stress for monodispersions of polystyrene spheres of various sizes in different media. (Reproduced with permission from reference 4. Copyright 1972 Elsevier.)

In Polymers as Rheology Modifiers; Schulz, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1991.

1.

GLASS E T A L .

7

Polymers as Rheology Modifiers: An Overview

in surfactant-modified, water-soluble Chapters 13 and 14, this volume).

polymers

(Figure 4;

10a,b and

One of the most complete experimental study of shear thickening has been in aqueous solutions with certain sodium borate/poly(vinyl alcohol) combinations (11), where the rheological behavior was correlated with B NMR chemical shifts. The results are in agreement with the original mechanism proposed for ionomers. Many of the crosslinks in the system at rest were intramolecular. Shearing at sufficiently high rates was sug­ gested to elongate the aggregates, and the equilibria of spontaneously breaking and reforming crosslinks shifted to more inter- rather than intramolecular associations. This creates a flow unit with a more effec­ tive hydrodynamic volume and higher viscosities. The mechanism is most significant at the point where the time scale of deformation (inverse shear rate) becomes shorter than the molecular relaxation time. At still higher shear rates the shear stresses become large enough to prevent interchain junctions reforming at a rate equal to breakage, and shear thinning is observed.

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n

In high-molecular-weight polymer solutions at low concentrations, dilational flow occurs when individual high-molecular-weight chains are stretched in extensional flow with severe constrictions (e.g., in irregular channels related to subterranean media; 12), leading to increased resis­ tance factors (Figure 5) (pressure differential measurements taken during core testing that can be related to relative viscosities). The phenomenon has been studied as a function of polymer structure, molecular weight, solvent quality, and ionic environment (13—15). Shear thickening also is observed in concentrated dispersions (16-22). Most of the work has been on monodispersed latices (Figure 6), but the phenomenon also occurs in multimodal latices and pigments. Light scattering and light diffraction techniques, complementing rheological studies, have suggested that the dilatant flow behavior arises from an order—disorder transition (21). Topical Applications As Viscosifiers. Since polymers have an intrinsically large hydrodynamic volume (HDV), only low concentrations of polymer are needed to sub­ stantially increase the viscosity of the fluid. Viscosity index (VI) impr­ overs, for automotive lubricant oils, are examples of a nonaqueous rheol­ ogy modifier. Viscosity index is an empirical number that indicates the resistance of a lubricant viscosity to changes in temperature. High VI values indicate greater resistance to thinning at high temperatures (23, 24). These polymers exist as compact coils in cold oil ("poor" solvent) and expand with increased temperature because of increased solvation. This inverse temperature response of polymer solutions enables the formulation of multigrade lubricating oils with flatter viscosity—temperature curves.

In Polymers as Rheology Modifiers; Schulz, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1991.

8

POLYMERS AS R H E O L O G Y MODIFIERS

100 :

-

H RAM ΑΑ

m % C8

:

1.25

• AAAA

A

Αλ

A

AAAAA* A

: 1.0 ο ο ο

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!ΗΡΑΜ

• •

A

ο

OOOOOOOOOOOOOOOOQ

• •••••••••••••••

I 111 Hill I L-IJJJul 0.01

0.1

ι ι ι ι inn

1

A

•

1—ι ι ι in

ι ι ι ι inn

10

100

1000

Shear Rate, 1/sec

Figure 4. Shear thickening behavior for hydrophobically associating acrylamide polymers as a function of hydrophobe level. (Repro­ duced from reference 10a. Copyright 1989 American Chemical Society.)

I 50 -

8

Φ

oc

Η Ρ Α Μ , 340ppm 20g/1 NaCI ph=8,0=3O°C

/

_

2 0

/

υ CO

>

1

I

10

5

i

Glass-bead pack(400-500 μιτι) 1

/

-

1

^

Jf

5H Ele!m ts,i -£1>4C Cen K

L = ° H ·•-•- Shear Viscosity . . . Apparent Viscosity 1_ ni ι ι ι ι 11 ill I I I I I I III I I I I I I 'III ι ι 1L 10 10 10 10 1/r

#

1

H

1

2

3

4

Wall Shear Rate, s'

1

Figure 5. Comparison of rheological behavior in a glass-bead pack and in a model including successive constrictions; HPAM is hydrolyzed polyacrylamide. (Reproduced with permission from reference 53. Copyright 1981 Society of Petroleum Engineers.)

In Polymers as Rheology Modifiers; Schulz, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1991.

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GLASS E T A L .

Polymers as Rheology Modifiers: An Overview

1

1

Vol Fraction PVÇ

3

°CL in 10

• 057 • 0J55 A Q51 ο 0.49 • 0.47

A

A

(0 Ο ο

g

10

2

•.^o-o-o'o'o-o-o-o10 1t)

(12)

where r

cos^ = η'-y max

max

, and r max

s i n ^ s «"γ

.

max

In Polymers as Rheology Modifiers; Schulz, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1991.

(13)

22

POLYMERS AS R H E O L O G Y MODIFIERS 11

This defines the two dynamic v i s c o s i t y c o e f f i c i e n t s η' and η . At low frequencies η' approaches the zero-shear-rate v i s c o s i t y measured i n steady shear. Alternately, c o e f f i c i e n t s can be defined i n terms of the maximum s t r a i n instead of the s t r a i n rate. r

- -G'7 yx

cos(û>t) - G"7

sin(ot)

max

(14)

max

where r

cos^ = G ' 7

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max

, r max

s i n ^ = G"7

max

.

(15)

max

This defines the two functions G' and G" which are the storage and loss moduli, respectively. G', proportional to the stress in-phase with s t r a i n , provides information about the e l a s t i c i t y of a material. For example, an i d e a l e l a s t i c rubber band would have a l l of i t s stress in-phase with s t r a i n or displacement. G", the loss modulus, i s proportional to stress out-of-phase with displacement and, therefore, in-phase with rate-of-displacement or shear-rate. For a purely viscous l i q u i d , a l l of the stress would be out-of-phase with displacement. I t should be kept i n mind that l i n e a r v i s c o e l a s t i c i t y assumes that the stress i s l i n e a r l y proportional to s t r a i n and that the stress response involves only the f i r s t harmonic and not higher harmonics i n frequency ( i . e . , the stress i s a s i n u s o i d a l ) . Experimentally,both of these conditions should be verified. These l i n e a r v i s c o e l a s t i c dynamic moduli are functions of frequency. They have proven to be sensitive probes of the structure of polymer solutions and gels. Figure 2 shows the dynamic moduli for a polymer solution during gelation (7). The material begins as a solution i n F i g . 2a and ends as a s o l i d gel i n F i g . 2d. For a polymer solution at low frequency, e l a s t i c stresses relax and viscous stresses dominate with the r e s u l t that the loss modulus, G", i s higher than the storage modulus, G'. Both decrease with decreasing frequency, but G' decreases more quickly. For a gel the stress cannot relax and, therefore, i s independent of frequency. Also, because the gel i s highly e l a s t i c the storage modulus, G' i s higher than the loss modulus, G". Linear v i s c o e l a s t i c measurements can also be used i n conjunction with c l a s s i c a l polymer k i n e t i c theory to relate the storage modulus of a gel to the number density of c r o s s l i n k s . By following the storage modulus with time, the chemical k i n e t i c s of gel formation can be measured (8,9). Polymer k i n e t i c theory (6) shows that the frequency independent, low frequency l i m i t of the storage modulus for a gel i s given by G' - G° - ι/kT + G

en

(16)

where G° i s the equilibrium shear modulus, ν i s the number density of network strands, k i s Boltzmann's constant, Τ i s the absolute temperature, and G i s a contribution a r i s i n g from entanglements that are not covalent crosslinks. en

In Polymers as Rheology Modifiers; Schulz, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1991.

2. P R U D ' H O M M E

10

10

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3

:

10

10' ο

or/ ο

23

Rheological Measurements

/ ζ *

UJ

10

Ë /

G

'

(A) -12°C

10 1 0.1

0.1 1 10 10 FREQUENCY, RAD/S

2

(C) -18°C 1 10 10 FREQUENCY, RAD/S

2

ο ο

0.1

1 10 10* FREQUENCY, RAD/S

10

2

1 10 10 FREQUENCY, RAD/S

2

Figure 2. Dynamic moduli versus frequency during the process of gelation. The material is polystyrene in carbon disulfide that gels upon cooling. (Reproduced from reference 7. Copyright 1983 American Chemical Society.)

In Polymers as Rheology Modifiers; Schulz, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1991.

POLYMERS AS R H E O L O G Y MODIFIERS

24 Step Shear Rate

In this experiment, the shear rate i s changed instantaneously from one value to another and the shear stress i s monitored with time. Most commonly, the i n i t i a l shear rate i s zero and the r e s u l t i n g material function i s the "stress growth function":

r

xy

(t) - 5 ( 7

t)7

xyi

(17)

xy

which defines the growth from an i n i t i a l shear rate 7 to the f i n a l shear rate 7 . There are corresponding functions f o r the transient normal stresses: x

y

i

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x y

r

xx

- r

yy

- Ν (t) • 1

r yy - r z z - Η2 (t)

t

1

(7

- *2 «

xyi 4

, t)

-y

(18)

2

xy

2

xyi

. t) -γxy

(19)

Step Strain. In step s t r a i n experiments, an instantaneous s t r a i n i s applied to the material and the decay i n the stress i s monitored with time. This defines the shear modulus, G(t), f o r an applied shear s t r a i n of magnitude 7 : 0

r

x y

(t) - G(t) 7

0

defines shear modulus

(20)

and i t defines the Young's modulus, E ( t ) , i f an elongational s t r a i n of magnitude c i s applied. 0

r„(t)

- r ( t ) - E(t) c y y

0

defines Young's modulus

(21)

Constant Stress Creep Experiments. The creep test i s the inverse of the step shear rate experiment; a constant stress r i s applied to the material, and the s t r a i n i s monitored with time. This defines the compliance J : x y

7

xy

(t) - J ( t ) r°

xy

defines the compliance J(t)

(22)

Thixotropic Loop. A deformation h i s t o r y that provides q u a l i t a t i v e information about time-dependent f l u i d rheology i s the thixotropic loop where the shear rate i s continuously ramped from zero to a higher value over a prescribed time period. The r e s u l t i n g shear stress i s measured. This test i s sensitive to the k i n e t i c s of structure evolution which can be important i n aggregated c o l l o i d a l dispersions. I f the structures i n the material are broken apart by shear and cannot reform during the time of the shear rate ramp, then the stresses during the decreasing shear rate ramp w i l l be lower than the stresses during the increasing l e g . The region between the stress and shear rate curves i n the increasing and decreasing ramps i s known as the "thixotropic loop". I t i s d i f f i c u l t to quantify the results observed i n a thixotropic loop experiment because the response i s a complex convolution of the k i n e t i c s of shear induced structure breakdown and the k i n e t i c s of

In Polymers as Rheology Modifiers; Schulz, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1991.

2.

PRUD'HOMME

Rheological Measurements

25

aggregation. But the thixotropic loop often provides a useful "finger p r i n t " of the material. Experimental Geometries and Simple Flows. Although the material functions are defined for the flows s p e c i f i e d i n the previous section, i t i s often most convenient to measure the material functions using alternate geometries or experiments that approximate the i d e a l flow geometry. Table I gives several examples of geometries from which material functions can be determined for low v i s c o s i t y f l u i d s (10).

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Examples of Material Behavior Since rheology i s frequently used as a probe of molecular structure and interactions, i t i s h e l p f u l to have a general idea of what the material functions look l i k e for commonly encountered fluids. Steady Shear V i s c o s i t y and Normal Stresses. Low molecular weight f l u i d s and resins generally are Newtonian f l u i d s . which means that they have a constant v i s c o s i t y independent of shear rate. They also display no e l a s t i c normal stresses. For polymer melts, f i l l e d polymers, polymer solutions and dispersions, the v i s c o s i t y i s not constant, but decreases at higher shear rates as shown i n F i g . 3. Whereas many systems display shear thinning v i s c o s i t i e s , only long chain polymeric molecules exhibit high values of e l a s t i c normal stresses. These e l a s t i c stresses are responsible for phenomena such as die swell, where a polymer extruded through an o r i f i c e swells to a diameter greater than the o r i f i c e diameter. S o l i d f i l l e r s reduce the l e v e l of normal stresses as shown i n F i g . 4, which shows the v i s c o s i t y and normal stress of a polypropylene melt f i l l e d with 50% wt calcium carbonate f i l l e r (11,12,13). Two points should be noted, the v i s c o s i t y increases with f i l l e r concentration, and the primary normal stress difference decreases. For dispersions of s o l i d s i n non-polymeric media, the v i s c o s i t y may also show shear thinning (14), exactly l i k e the polymeric analog; but there w i l l be v i r t u a l l y no normal stresses. For both Newtonian and polymeric continuous phases, v i s c o s i t y increases with increasing volume f r a c t i o n up to a c r i t i c a l volume f r a c t i o n above which the v i s c o s i t y diverges to i n f i n i t y and the material w i l l not flow. This c r i t i c a l volume f r a c t i o n i s about 63% for non-interacting, monodisperse, spheres and decreases markedly when the p a r t i c l e s have long aspect r a t i o s (e.g., chopped glass f i b e r s ) or when the p a r t i c l e s are strongly i n t e r a c t i n g as i s the case when p a r t i c l e sizes are below 1 μπι and surface forces become dominant. I t can be increased to about 80% for a broad d i s t r i b u t i o n of sphere sizes. The e f f e c t of p a r t i c l e aspect r a t i o i s seen i n F i g . 5, which i s the v i s c o s i t y versus shear rate for polyamide 6 melts f i l l e d with glass f i b e r s of d i f f e r e n t aspect r a t i o s , a l l at 30% by weight loading (15). The higher aspect r a t i o f i l l e r s produce higher v i s c o s i t i e s . The e f f e c t of p a r t i c l e surface interactions i s seen for the polystyrene calcium carbonate system i n Fig. 6, where decreasing p a r t i c l e size from 17 μπι to 0.07 μπι at the same volume f r a c t i o n of p a r t i c l e s (φ - 30% by volume) r e s u l t s i n a ten-fold increase i n v i s c o s i t y (16). The e f f e c t of surface

In Polymers as Rheology Modifiers; Schulz, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1991.

26

POLYMERS AS R H E O L O G Y MODIFIERS

Table I. Experimental Geometries for Measuring Fluid Rheology Measured Quantities

Experimental Geometry Row in a tub* (capillary viscometer) - Shearing surface

'Jτ*

shear and Line of path line particle

Q Δ/ R L 5

Volume rate of flow Pressure drop through tube Tube radius Tube length

-y = Shear rate at tube wall R

T

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= = = =

= Shear stress at tube wall

r

Torsional flow between a cone and disk

R = Radius of circular plate θ = Angle between cone and plate (usually less than 100 mm) W = Angular velocity of cone Τ = Torque on plate F = Force required to keep tip of cone in contact with circular plate 0

0

Shearing surface

" Une of shear and particle path line Torsional flow between parallel plates

R = Radius of disks H = Separation of disks W = Angular velocity of upper disk Τ - Torque required to rotate upper disk F = Force required to keep separation of two disks constant 0

- Line of shear and particle path line Torsional flow between concentric cylinders (Couette geometry)

- Shearing surface

- Line of shear and particle path line

R,,R =

°f inner and outer cylinders H = Height of cylinders WVW2 = Angular velocities of inner and outer cylinders Τ = Torque on inner cylinder R

a

d

i

i

2

In Polymers as Rheology Modifiers; Schulz, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1991.

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2.

27

Rheological Measurements

PRUD'HOMME

Table I. Continued Material Function Determination Flow in a tube (capillary viscometer) /· ι

Ύ ρ

=

T

R

Π»

d In (Ο/π/ΡΠ

- (TgûAr/? ) TRCITR 3

Torsional flow between a cone and disk η(Ύ) '

Torsional flow between parallel plates

2π/? γ 3

L

d l n

Torsional flow between concentric cylinders

η(Ύ) =

2ir/??/y(IV - IV, j 2

γ = WQ/ΘΟ

In Polymers as Rheology Modifiers; Schulz, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1991.

^J

POLYMERS AS R H E O L O G Y MODIFIERS

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28

1 U

1(F

ïir

5

ûr*

ûr

1

ΐ

ίο

ίο

ici

5

5

îô

4

Figure 3. Viscosity versus shear rate for a low-density polyethylene melt at several temperatures. Data at shear rates below 5 χ 10~~ s"" were taken on a rotational viscometer, and viscosities at higher shear rates were taken on a capillary viscometer. (Reproduced with permission from reference 23. Copyright 1971 Hanser.) 2

1

Figure 4. Viscosity, η (open symbols), and first normal stress difference, N . (closed circles), as a function of shear stress for polypropylene melts filled with C a C 0 (50% wt) with and without a titanate coupling agent: (•, • ) pure propylene, (O, · ) with titanate treatment, and ( Δ , A) without titanate treatment. (Reproduced with permission from reference 11. Copyright 1981 Society of Plastics Engineers.) 3

In Polymers as Rheology Modifiers; Schulz, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1991.

2.

PRUD'HOMME

29

Rheological Measurements

10*

FILLER

d [μχη]

•

GLASS FIBERS

10

25

1

Δ

GLASS FIBERS

13.5

25

1

Ο

GLASS FIBERS

10

10

1

ο

GLASS FIBERS

13.5

7.3

1

—•

GLASS BEADS

15-40

1

1

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10'

10

10 - 2

°r

NONE

10

10°

10'

SHEAR RATE γ

10'

1

10°

(S" ) 1

Figure 5. Viscosity versus shear rate for polyamide 6 melts filled with glass fibers of different aspect ratios (a ) and diameters (d) at a constant mass fraction of 30% fibers. (Reproduced with permission from reference 15. Copyright 1984 Steinkopff.) f

1

1

'T

τ " '

«

ι

«

PS/CQC03

φ 10

«0.3

\

6

0.07/im

^^^^v

0.5ftm

17/im

^ ^ ^ • Ν Λ •

1

Y

.

PS

.

(s" ) 1

Figure 6. Viscosity versus shear rate for polystyrene melts with C a C 0 fillers of various particle sizes shown on the figure. The filler loading is constant at 30% vol. (Reproduced with permission from reference 16. Copyright 1983 Wiley.) 3

In Polymers as Rheology Modifiers; Schulz, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1991.

POLYMERS AS R H E O L O G Y MODIFIERS

30

treatments i s also seen i n F i g . 4, f o r the two f i l l e d polypropylene melts at the same solids loading with two d i f f e r e n t surfaces: one i s treated with a titanate surface treatment that minimizes p a r t i c l e surface interactions and the other i s untreated. The treated f i l l e r has a lower v i s c o s i t y since p a r t i c l e aggregation i s reduced. Additional information on f i l l e d melt rheology and p a r t i c l e orientation f o r non-spherical p a r t i c l e s i s found i n a recent review (17). Steady shear v i s c o s i t y measurements are also used f o r polymer molecular weight characterization i n two ways. The measurement of the v i s c o s i t y η, of a d i l u t e polymer solution at a succession of concentrations, C , can be used to determine the i n t r i n s i c v i s c o s i t y , [η]:

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p

[η] =

il» c -o p

V-P*C f; p

(23)

s

where η i s the solvent v i s c o s i t y . The i n t r i n s i c v i s c o s i t y i s r e l a t e d to molecular weight, M^, through the Mark-Houwink expression: Λ

[if] - Κ lC (24) where Κ and a are constants tabulated f o r each polymer i n standard references (18). Also, the zero shear v i s c o s i t y , η , of a polymer solution or melt can be used to determine i t s molecular weight. Figure 7 (6) shows s i m i l a r v i s c o s i t y molecular weight behavior f o r several amorphous, l i n e a r polymers. The data can be represented by 0

η

0

- M^)

for if < M w

3

η

0

- Κ (Μ^) · 2

4

c

for J* < M w

c

(25) (26)

The constants Kj and K are tabulated (18) and the t r a n s i t i o n from the f i r s t order to 3.4 order dependence on molecular weight comes from the t r a n s i t i o n from unentangled behavior at low molecular weight [corresponding to material II i n the following section] to entangled behavior at high molecular weight [corresponding to material I I I i n the following section]. 2

Uniaxial Extension/Compression. Polymeric melts and v i s c o e l a s t i c f l u i d s display elongational v i s c o s i t i e s as a function of elongational s t r a i n rate as shown i n F i g . 8. The data are shown f o r a polystyrene melt at 170°C at several constant elongation rates (24). The elongational v i s c o s i t y increases with time i n i t i a l l y , may reach a constant asymptotic value that i s equal to three times the zero shear v i s c o s i t y , and then may increase u n t i l the f i b e r breaks. The onset of the r i s e i n v i s c o s i t y i s related to the rate of elongation, and the onset occurs e a r l i e r f o r higher elongation rates. This f i n a l " s t r a i n hardening" i s one of the mechanisms s t a b i l i z i n g the stretching of polymer f i b e r s and i s therefore very desirable. However, i t makes r e l i a b l e "steady elongational v i s c o s i t y " data very d i f f i c u l t to obtain. Most data on f i b e r

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2.

PRUD'HOMME

Rheological Measurements

31

Ο Polydimethylsiloxane

i CO C Ο

Polyisobutylene

U Polyethylene Polybutadiene Polytetra-methyl-p-silphenylene, siloxane

Polymethylmethacrylate

. Polyethylene glycol Polyvinyl acetate Polystyrene

Constant + log (cM) Figure 7. Log of the zero-shear-rate viscosity versus log of concentration times molecular weight. The data are shifted along the vertical and horizontal axes; the two shift factors are tabulated for many polymer systems. The molecular weight at the break point between the region of slope 1 and the region of slope 3.4 at higher molecular weight defines the critical molecular weight for entanglement, Mc. (Reproduced with permission from reference 24. Copyright 1968 Springer.)

In Polymers as Rheology Modifiers; Schulz, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1991.

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32

POLYMERS AS R H E O L O G Y MODIFIERS

10

_

7

3*

10

0

3

Ν

Ethylene ( m o l e %)

(CH2) (CH3)

(NMR)

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(FTIR)

EP-1

60

4.2

0.35

2.0

EP-2

58

3.9

0.30

2.1

EP-3

70

4.8

0.56

5.1

EP-4

80

6.5

0.62

7.8

EP-5

80

6.1

0.65

7.5

3.50-

3.00-

Tetralin

È 82.50-)

•Q Toluene

to

>

Isooctane

>

• Hexane

Ε 2.00-j

d

-X- Methyl Cyclohexane 1.50H

1.00—τ .050

1 .100

,

,

,

,

,

,

1

.150 .200 .250 .300 .350 .400 .450 CONCENTRATION OF EP-5 (gm/100 ml)

1

,

.500

.550

Figure 1. Relative viscosities of EP-5 in different solvents at 20 ° C.

In Polymers as Rheology Modifiers; Schulz, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1991.

277

POLYMERS AS R H E O L O G Y MODIFIERS

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278

Figure 2. Specific viscosities in methyl cyclohexane at -10 ° C.

1.20-,

In Polymers as Rheology Modifiers; Schulz, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1991.

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16.

RUBIN & SEN

Solution Rheology of Ethylene-Propylene Copolymers

Huggins equation (7), and the d i m e n s i o n l e s s parameter K l is the Huggins constant. I n t r i n s i c v i s c o s i t i e s were o b t a i n e d from t h i s equation by e x t r a p o l a t i o n of p l o t s of η,ρ/c vs. c to i n f i n i t e d i l u t i o n while Huggins constants were c a l c u l a t e d from the s l o p e s o f the plots. Figure 4 shows the temperature dependence of intrinsic viscosities in methyl cyclohexane. For a l l cases, intrinsic viscosities decreased modestly as the temperature r o s e ; t h e b i g g e s t d e c r e a s e i n [ η ] , a b o u t 20 %, was obtained for EP-5. This indicates some deterioration in polymer-solvent interaction and solubility with increasing temperature. Analogous curves for toluene are plotted in Figure 5. The i n t r i n s i c v i s c o s i t i e s f o r the amorphous samples changed only modestly with temperature, f i r s t i n c r e a s i n g as the temperature rose from -10*C t o 20*C, and then remaining reasonably constant or d e c r e a s i n g s l i g h t l y as i t rose f u r t h e r t o 5 0 * C . T h i s was n o t t h e c a s e f o r t h e p a r t i a l l y crystalline samples EP-4 and E P - 5 . For these samples, [n] i n c r e a s e d b y o v e r 500 % b e t w e e n -10*C and 10*C, after which i t i n c r e a s e d much more m o d e r a t e l y when t h e s o l u t i o n s were warmed f u r t h e r t o 50* C . Such a rapid rise in [n] with t e m p e r a t u r e has been a s c r i b e d t o an endothermal heat of m i x i n g and i n c r e a s i n g s o l v e n t power with r i s i n g temperature (6,8,9). Plots of the r a t i o of i n t r i n s i c v i s c o s i t i e s for the copolymers i n methyl cyclohexane and toluene at the two extreme temperatures, -10*C and 50*C, a g a i n s t mole % e t h y l e n e a r e shown i n F i g u r e 6. T h i s r a t i o e x c e e d e d one for a l l copolymers i n methyl cyclohexane, indicating a d e c r e a s e i n [n] w i t h temperature but was essentially independent of molecular weight and composition. It was less than one in toluene, showing the improved polymer-solvent interaction as the temperature was r a i s e d and decreased enormously f o r the two copolymers with 80 mole % ethylene and small amounts of crystallinity. I n t r i n s i c v i s c o s i t i e s and Huggins c o n s t a n t s between -10* C and 50* C f o r EP-1, E P - 2 , and EP-5 i n hexane, i s o o c t a n e and tetralin are summarized i n T a b l e s I I - I V . In t e t r a l i n , i n t r i n s i c v i s c o s i t i e s f o r EP-1 and EP-2 d i d not change s i g n i f i c a n t l y w i t h temperature. However, the viscosity for E P - 5 i n c r e a s e d b y 200% b e t w e e n - 1 0 * C a n d 20*C and r e m a i n e d e s s e n t i a l l y c o n s t a n t between 20*C and 50*C. T e t r a l i n t h u s e x h i b i t e d t h e same g e n e r a l features as t o l u e n e , b e h a v i n g as a poor solvent for partially crystalline E P c o p o l y m e r s b e l o w 20" C a n d a satisfactory one for the amorphous samples. The viscositytemperature relationships for EP-1 and EP-2 i n hexane and isooctane were essentially similar to those d i s c u s s e d above f o r t o l u e n e and t e t r a l i n . For instance, f o r E P - 1 a n d E P - 2 , [n] v a l u e s i n h e x a n e decreased from 1.05 and 1.70 d l / g m a t -10 C t o 0.74 and 1.25 dl/gm at 50*C. T h e c o r r e s p o n d i n g [n] d e c r e a s e s i n i s o o c t a n e w e r e

In Polymers as Rheology Modifiers; Schulz, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1991.

279

280

POLYMERS AS R H E O L O G Y MODIFIERS

4.003.50-

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&3.00H

X EP--1

S ^2.50H "K

•O EP--2

12.00-1

•

- * EP--3

ο ο

ο "S

EP--4

+ EP--5

|I.5OH

x-

χ

χ

40

50

i.ocH 0.50—ι -20 -10

1

0

1 1 r10 20 30 Temperature (%)

ι 60

Figure 4. Intrinsic viscosities in methyl cyclohexane.

2.50-,

2.00E Χ ΕΡ--1 1.50H

•Ο ΕΡ--2

m

- * ΕΡ--3

δ

•

1.00-

ΕΡ--4

+ ΕΡ--5

c s c 0.50H

0.00—ι -20 -10

1

0

1

1

1

10 20 30 Temperature (°C)

1

40

1

50

1

60

Figure 5. Intrinsic viscosities in toluene.

In Polymers as Rheology Modifiers; Schulz, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1991.

16.

Solution Rheology of Ethylene-Propylene Copolymers

RUBIN & SEN

1.40-1

1.20-

σ

1.00-

8 ·*· Methyl Cyclohexane Downloaded by PENNSYLVANIA STATE UNIV on August 6, 2012 | http://pubs.acs.org Publication Date: May 13, 1991 | doi: 10.1021/bk-1991-0462.ch016

~0.80-

• Toluene

g0.60JL §0.400.2050

V 1

1

1

1

1

1

1

1

55

60

65

70

75

80

85

90

Ethylene Content (mole X)

0.00-

Figure 6. Ratio of intrinsic viscosities at two extreme temperatures, -10 and 50 °C.

TABLE I I .

I n t r i n s i c v i s c o s i t i e s , [η], and Huggins constants, K l , o f E P - 1 , EP-2 and EP-5 i n hexane

(°C)

[n] (dl/gm)

EP-5

EP-2

EP-1 Kl

[n] (dl/gm)

Kl

[n] (dl/gm)

Kl

•10

1.05

0.52

1.70

1.54

1.20

5.12

0

1.05

0.53

1.65

1.52

2.20

4.85

10

0.90

0.56

1.55

1.54

2.30

4.53

20

0.93

0.63

1.50

1.55

2.20

4.32

30

0.75

0.69

1.35

1.60

2.10

4.26

40

0.73

0.65

1.30

1.67

2.05

4.30

50

0.74

0.62

1.25

1.66

2.00

4.28

In Polymers as Rheology Modifiers; Schulz, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1991.

281

282

POLYMERS AS R H E O L O G Y MODIFIERS

TABLE I I I *

I n t r i n s i c v i s c o s i t i e s , [η], and Huggins constants, K l , o f EP-1, EP-2, and EP-5 i n isooctane

EP-1

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Temperature (°C)

[n] (dl/gm)

EP-2 Kl

[n] (dl/gm)

EP-5 Kl

[n] (dl/gm)

Kl

-10

1.10

0.62

1.70

0.83

0.45

3.06

0

0.98

0.64

1.60

0.82

1.10

3.10

10

0.95

0.67

1.50

1.33

1.85

2.67

20

0.95

0.67

1.50

1.37

1.90

2.64

30

0.90

1.02

1.40

1.56

1.80

2.47

40

0.85

1.04

1.35

1.86

1.80

2.82

50

0.80

1.02

1.30

1.82

1.75

2.84

TABLE IV. I n t r i n s i c v i s c o s i t i e s , [η], and Huggins constants, K l , o f EP-1, EP-2, and EP-5 in tetralin

EP-1 Temperature (°C)

[n] (dl/gm)

EP-2 K l

[n] (dl/gm)

EP-5 K l

[n] (dl/gm)

K l

-10

0.90

0.35

1.10

1.33

0.70

2.55

0

0.95

0.32

1.10

1.17

1.15

2.67

10

1.00

0.38

1.15

1.25

1.90

1.50

20

0.95

0.38

1.15

1.33

2.10

1.58

30

1.00

0.42

1.15

1.38

2.10

1.60

40

1.00

0.50

1.15

1.35

2.05

1.63

50

0.95

0.46

1.10

1.34

2.05

1.62

In Polymers as Rheology Modifiers; Schulz, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1991.

16.

Solution Rheology of Ethylene—Propylene Copolymers

RUBIN & SEN

e

from 1.10 and 1.70 d l / g m a t - 1 0 C t o 0.80 and 1.30 dl/gm at 50 C. Intrinsic viscosity changes of EP-5 were likewise similar to those i n toluene and t e t r a l i n . In h e x a n e , t h e v a l u e i n c r e a s e d r a p i d l y between -10 and 10*C from 1.20 dl/gm to 2.30 dl/gm and then decreased s l o w l y t o 2.00 d l / g m as the temperature r o s e to 50*C. In isooctane, i t i n c r e a s e d from 0.45 d l / g m a t -10*C t o 1.90 d l / g m a t 20*C and t h e n g r a d u a l l y f e l l o f f to 1.75 dl/gm at 50 C. Hexane and i s o o c t a n e thus a l s o behaved as poor s o l v e n t s f o r EP-5 below 10-20*C. e

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e

HUGGINS

CONSTANTS

Huggins constants, K l , for a l l copolymers i n methyl cyclohexane and toluene are p l o t t e d as a f u n c t i o n o f t e m p e r a t u r e i n F i g u r e s 7 and 8. In methyl cyclohexane, a l l K l values increased with temperature. In toluene, a much p o o r e r s o l v e n t , they passed through a minimum and then rose. A s e x p e c t e d f r o m t h e [n] v a l u e s , results for E P - 5 showed t h e g r e a t e s t c h a n g e ; K l d e c r e a s e d from 2.94 at - 1 0 * C t o 2.35 a t 20*C and then i n c r e a s e d t o 2.58 at 50*C. The K l v a l u e s i n the other solvents, shown in Tables II-IV, exhibited similar trends with respect to their intrinsic viscosities. In general, the trend for the K l values was i n t h e o p p o s i t e d i r e c t i o n f r o m t h a t observed for i n t r i n s i c v i s c o s i t i e s , as has been r e p o r t e d in the l i t e r a t u r e (9,10).

INTERPRETATION OF RESULTS In considering the results discussed above, it is necessary to take into account both the solvents and copolymers. I t i s c l e a r t h a t m e t h y l c y c l o h e x a n e was the best solvent as it gave the highest relative and intrinsic viscosities for a l l copolymers. Intrinsic viscosities i n methyl cyclohexane decreased with rising temperature, indicating that heats of mixing were negative over the entire temperature range. Since [n] decreased, copolymer-solvent i n t e r a c t i o n d i m i n i s h e d as temperature rose and, therefore, the EPs contributed less to solution viscosity at elevated temperatures, just the opposite of what is required for good VI improvers. Intrinsic viscosity-temperature data i n the other s o l v e n t s showed t h a t t h e y were good solvents for the amorphous copolymers, EP-1, EP-2 and E P - 3 , though not q u i t e as good as methyl c y c l o h e x a n e . F i g u r e 5, w h i c h is typical for these solvents, showed that [n] either increased slightly or did not change much with temperature. However, below 10-20*C t h e s e s o l v e n t s had v a s t l y r e d u c e d s o l v a t i n g power f o r the two copolymers

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POLYMERS AS R H E O L O G Y MODIFIERS

•X EP--1

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-O EP--2 - * EP--3 •

•O-

-•

-20

χ

X

-*

*

*

X

X·

"

X· "

"

1

1

1

1

-10

1

0

10 20 30 Temperature (°C)

*

"*

χ

x

1

40

1

50

EP--4

-+-EP--5

1

60

Figure 7. Huggins constants in methyl cyclohexane.

•XEP--1 •O EP--2 - * EP--3 •

EP--4

+ EP--5

-20

1

-10

1

0

1

1

1

10 20 30 Temperature (°C)

1

40

1

50

1

60

Figure 8. Huggins constants in toluene.

In Polymers as Rheology Modifiers; Schulz, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1991.

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16.

RUBIN & SEN

Solution Rheology of Ethylene-Propylene Copolymers

with high ethylene content and small amount of crystallinity. Our data showed that the naphthenic structure favored solubility and solvent-polymer i n t e r a c t i o n i n comparison with p a r a f f i n i e solvents and that the copolymers were least soluble in aromatics. T h e d i s t i n c t i o n b e t w e e n g o o d a n d p o o r s o l v e n t s was most pronounced for the p a r t i a l l y c r y s t a l l i n e copolymers EP-4 and E P - 5 a t low t e m p e r a t u r e . The e f f e c t o f the EP s t r u c t u r e on s o l u b i l i t y can be seen most c l e a r l y by comparing the behavior of the copolymers with 58-70 mole % ethylene to those w i t h 80%, particularly a t low temperatures. In the poorer solvents, the i n t r i n s i c viscosities of the two partially crystalline copolymers decreased rapidly as the temperature was lowered below about 10* C . No s u c h decrease was o b s e r v e d for the three amorphous copolymers. T h e s m a l l [n] v a l u e s f o r E P - 4 a n d EP-5 at low temperatures in a l l solvents except methyl cyclohexane can be understood by examining the differences in structure between the two sets of copolymers. A s shown i n T a b l e I , E P - 4 a n d EP-5 had a larger fraction of longer ethylene sequences than the t h r e e t o t a l l y amorphous c o p o l y m e r s , and i t i s presumably these longer ethylene sequences which formed the crystallites i n the bulk copolymer. We b e l i e v e t h a t at low temperature in reasonably poor solvents these ethylene sequences can organize into partially ordered domains o r aggregates which are h e l d i n s o l u t i o n by the more soluble mixed ethylene-propylene sequences and shorter ethylene and propylene segments i n c a p a b l e of crystallizing. T h i s p i c t u r e i s s i m i l a r t o one proposed some time ago by F i l i a t r a u l t and Delmas (11). These p a r t i a l l y o r d e r e d domains would l e a d to contraction of the copolymer in solution and some reduction in viscosity. In a d d i t i o n , they could also give solutions with fewer c h a i n entanglements and i n c r e a s e d copolymer mobility, leading to further viscosity decrease (12). It is thus c l e a r from t h i s study t h a t the presence of a high amount of ethylene ( 8 0 m o l e %) and l o n g e r ethylene sequences i n EP copolymers can significantly lower the solution viscosity of EP copolymers in relatively poor solvents at temperatures below 20"C. Dilute solution viscosities o f amorphous EP copolymers show relatively l i t t l e variation with temperature under s i m i l a r conditions.

REFERENCES 1. 2. 3.

Kapuscinski, M. M.; Sen, Α.; Rubin, I. D. SAE Pub. No. 892152, 1989. Randall, J . C. Polymer Sequence Determination. C-13 NMR Method, Academic Press, New York, 1977. Johnston, J . E . ; Bloch, R.; Ver Strate, G. W.; Song, W. R. US Patent 4,507,515, 1985.

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4.

Ham, G. E. High Polymers, Interscience, New York, 1964. 5. Fox, T. G., J r . ; Fox, J. C . ; Flory, P. J. J. Amer. Chem. Society. 73, 1901, 1951. 6. Bohdanecky, M.; Kovar, J. Viscosity of Polymer Solutions, Elsevier Publishing, New York, 1982. 7. Huggins, M. L. J. Am. Chem. Soc. 64, 2716, 1942. 8. Maderek, E . ; Wolf, B. A. Angew. Makromol. Chemie, 161, 157, 1988. 9. Schott, N.; Will, B.; Wolf, B. A. Makromol. Chem., 189, 2067, 1988. 10. Schmidt, J. R.; Wolf, B. A. Macromolecules, 15, 1192, 1982. 11. Filiatrault, D.; Delmas, G. Macromolecules, 12, 65, 69, 1979. 12. Rubin, I. D.; Stipanovic , A. J.; Sen, Α., presented at 1990 meeting of Society of Tribologists and Lubrication Engineers (STLE), Denver, May 1990. Received July 18, 1990

In Polymers as Rheology Modifiers; Schulz, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1991.

Chapter 17

Rheological Properties of High-Molecular­ -Weight Poly[(methyl methacrylate)-co­ -ethylacrylate-co-butylacrylate] Solutions Influence of Polymer—Solvent Interactions

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Wendel J. Shuely and Brian S. Ince U.S. Army, SMCCR-RSC-P, Aberdeen Proving Ground, MD 21010-5423

The effects of polymer-solvent interactions on rheological viscoelastic properties i s being investigated. Relatively small volume fractions, 0.02-0.06, of an u l t r a high molecular weight rheological processing aid, terpolymer poly(methylmethacrylate­ -co-ethylacrylate-co-butylacrylate), poly (MMA/EA/BA), form polymer solutions in the semidilute regime. Over 30 solutions were formulated to define several interaction categories. Polymer-solvent interactions were characterized by several methods: polymer cohesion phase diagram coordinates, limiting viscosity number, proton donating strength, and s o l u b i l i t y with control homopolymers. Rheological measurements included steady shear f i r s t normal stress difference, apparent viscosity, hysteresis, transient, dynamic viscosity and storage and loss moduli. Relationships between degree or type of interaction and rheological properties have been formulated. The fluid dynamics of liquids can be modified and controlled by the a d d i t i o n o f polymer additives (1-3). U l t r a h i g h , megadalton ( m i l l i o n gm/mole) m o l e c u l a r w e i g h t (MW) p o l y m e r s a r e e f f e c t i v e a t low c o n c e n t r a t i o n s . Most specific i n d u s t r i a l process applications involving the control of f l u i d dynamics are p r o p r i e t a r y ; examples of applications that involve free surface flow are r o l l splatter, bulk liquid spraying, aero-stripping, antimisting of fuels, and spray droplet distribution control

This chapter not subject to U.S. copyright Published 1991 American Chemical Society

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in crop pest management. The process phenomena of interest i n a p p l i c a t i o n s o f t e n are c h a r a c t e r i z e d by l a r g e nonlinear deformations, simultaneous shear and extensional flows, complex geometries and free surface flows. The ideal geometries and flows imposed by rheological evaluation are a useful framework for u n d e r s t a n d i n g the e f f e c t s of m a t e r i a l p r o p e r t i e s on t h e s e complex processes. The large deformations present in the process phenomena o f i n t e r e s t suggest correlations with steady shear measurements instead of the small strains from dynamic mode measurements. However, oscillatory measurements were a l s o r e c o r d e d f o r p o t e n t i a l c o r r e l a t i o n to steady shear rheological properties at low s h e a r rates. Given this selection of steady shear measurements, two r h e o l o g i c a l p r o p e r t i e s were c o n s i d e r e d : first normal s t r e s s d i f f e r e n c e and apparent viscosity, i n c l u d i n g t h e power law coefficient for the shear rate dependence. The most r e l i a b l e q u a n t i t a t i v e measure of solution elasticity under simple steady shear is first normal stress difference. Correlations of both of these rheological properties with solvent effects were investigated, although f i r s t normal s t r e s s d i f f e r e n c e had shown more p r o m i s i n g p r e d i c t i o n o f p r o c e s s f l u i d d y n a m i c s and had allowed d i r e c t comparisons of polymer solutions over a wider v a r i e t y of concentration, MW a n d s o l v e n t regimes. Polymer-Solvent I n t e r a c t i o n s : T h e r h e o l o g i c a l and viscoelastic properties of polymer solutions are i n f l u e n c e d by the polymer MW a n d MW d i s t r i b u t i o n ( M W D ) , the chemical s t r u c t u r a l f e a t u r e s and c o n f i g u r a t i o n , the c o n c e n t r a t i o n , and p o l y m e r - s o l v e n t i n t e r a c t i o n s (4). The quantitative range of influence of polymer-solvent interactions is relatively limited when compared w i t h these other variables. Furthermore, concentration, MW and s t r u c t u r a l p r o p e r t i e s p r o v i d e a c o n t i n u u m o f solution properties, although the values of these v a r i a b l e s might produce solutions in different c o i l density regimes. On the other hand, solvent effects are f i n i t e i n that they are bounded between nominally theta solvents and maximally interacting solvents. Most s t u d i e s o n l y employ theta and 'good' solvents to bracket the extent of solvent effects. One p u r p o s e o f t h i s i n v e s t i g a t i o n was to determine the influence of the full spectrum of solvents between near theta conditions and highly expanded p o l y m e r c o i l s on r h e o l o g i c a l properties and, i n addition, determine the influence of several other qualitatively unique solvent sets on rheological behavior. Among these solvent sets are: (1) solvents that are insoluble with the major comonomer components and o n l y ' d i s s o l v e d ' by p r e f e r e n t i a l i n t e r a c t i o n w i t h t h e m i n o r i t y comonomer c o m p o n e n t s , (2) s o l v e n t s w i t h specific hydrogen b o n d i n t e r a c t i o n as p r o t o n d o n o r , weak a c c e p t o r solvents, a n d (3) solvents that precipitate during shear.

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17.

SHUELY & INCE

S e m i d i l u t e Regime:

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289

Poly (MMA—EA—BA) Solutions Variables

held

constant

were

polymer composition, MW, MWD, concentration and temperature. The concentration of 4.7 + 0.1 g/dL resulted in coil density based on a concentration χ limiting viscosity number (LVN) p r o d u c t o f ca. 5-20, spanning the semidilute regime. Characterization: S e v e r a l physical, chemical and rheological c h a r a c t e r i z a t i o n methods a r e b e i n g applied. Polymer Cohesion Phase Diagrams (PCPD) o f t h e terpolymer (5) and homopolymer c o n t r o l s were employed to identify solvents with specific solution interactions. The phase diagrams consist of a bounded area on a plot of s o l u b i l i t y or i n s o l u b i l i t y i n terms of a s e l f - c o n s i s t e n t solvent parameter set (Hansen p a r a m e t e r s , ASTM D3132 solvent parameters (6), etc.). The boundary c o o r d i n a t e s approximate t h e t a c o n d i t i o n s or ' p o o r ' s o l v e n t s and the coordinates toward the center provide a selection of 'good' solvents. This semi-quantitative selection of s o l v e n t s was t h e n f u r t h e r quantified by measurement of LVN. LVN o r i n t r i n s i c v i s c o s i t y measurement p r o v i d e d an e s t i m a t e of c o i l expansion and degree of i n t e r a c t i o n (7). Linear S o l v a t i o n E n e r g y R e l a t i o n s h i p (LSER) s c a l e s were used to i d e n t i f y and quantify p r o t o n d o n a t i n g , weak o r nonacceptor solvents (8). The LSER s y s t e m of scaling solvent parameters is more rigorous than the various cohesion parameter systems and i n c l u d e s s p e c i f i c terms for proton donating or acceptor strength. Unique solvents with relatively strong proton donating capability relative to proton acceptor strength (e.g. c h l o r o f o r m , methylene c h l o r i d e ) were i d e n t i f i e d employing LSER d a t a (8). Straightforward solubility determinations (6) were employed to classify solvent/nonsolvent interactions with homopolymer controls f o r each of the terpolymer repeat units. Rheological measurements provided a characterization of v i s c o u s and viscoelastic p r o p e r t i e s : steady shear hysteresis, f i r s t normal stress difference (FNSD), apparent v i s c o s i t y and c r i t i c a l shear r a t e f o r o n s e t o f FNSD a r e u n d e r e v a l u a t i o n . Dynamic and t r a n s i e n t d a t a have a l s o been r e c o r d e d f o r evaluation.

Experimentation Materials,

The s p e c i f i c p o l y m e r i c additive investigated is poly(methylmethacrylate-co-ethylacrylate-co-butylacrylate), (poly(MMA/EA/BA)), Rohm and Haas Acryloid (now Paraloid) K125 (9). The Mw is 1.5 megadalton (million gm/mole) by l i g h t s c a t t e r i n g ( L S ) a n d Mw/Mn = 1.8 (10). Size exclusion chromatography (SEC) a n d L i g h t S c a t t e r i n g (LS) show t h e t a i l e x t e n d i n g f r o m 1 t o c a . 15 megadalton. The structure is that o f a l i n e a r random copolymer produced by aqueous emulsion polymerization ( r e c o v e r e d by s p r a y drying). A lot (3-6326) o f several hundred Kg has been set aside for detailed c h a r a c t e r i z a t i o n and r h e o l o g i c a l p r o c e s s correlations.

In Polymers as Rheology Modifiers; Schulz, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1991.

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The structure and molar monomeric ratios were d e t e r m i n e d b y NMR w i t h Mass Spectral (MS) c o n f i r m a t i o n and a r e (MMA:82, E A : 1 2 , BA:6) ( 1 1 ) . A l l monomeric r e p e a t units are dipolar, proton acceptors. The homopolymer control samples were employed only for solubility determinations and were as follows: polymethyl­ methacrylate) (PMMA), DuPont Elvacite 2041, lot no. 2 2 0 0 , Mw c a . 0.4 m e g a d a l t o n ; poly(ethylacrylate) (PEA), Rohm and Haas, l o t no. P S - 1 , Hw c a . 2 megadalton; and poly(butylacrylate) (PBA), Scientific Polymer Products, lot no. 3, Mw c a . 0.06 megadalton. The commerical q u a l i t y s o l v e n t s were used as received. Procedures. Solubility Determinations: The poly(MMA/ EA/BA) s o l u t i o n s were p r e p a r e d a t a c o n c e n t r a t i o n o f 4.7 +0.1 g/dL. T h e P E A was r e c e i v e d as an aqueous emulsion and p r e c i p i t a t e d u s i n g m e t h a n o l . T h e PBA was r e c e i v e d as a 26% s o l u t i o n i n t o l u e n e and dried to constant weight. A1J p o l y m e r s were d r i e d o v e r n i g h t i n a vacuum oven a t c a . 40 C b e f o r e s o l u t i o n preparation. A l l h o m o p o l y m e r PMMA, PEA, PBA s o l u t i o n concentrations ranged from ca. 5-10 g/dL and a l l s o l u t i o n s were p r e p a r e d a t room t e m p e r a t u r e (ca. 25 C ) . A l l s o l u t i o n s were p l a c e d i n a shaker ca. 1-5 w e e k s b e f o r e f i n a l visual solubility determinations were made. LVN: T h e L V N was e m p l o y e d a s a m e a s u r e o f t h e coil expansion of the polymer coil in the solvent. A poor solvent, approaching a theta solvent, has a low LVN relative to a good solvent, which has a high LVN. Procedures for o b t a i n i n g the LVN from dilute solution e x t r a p o l a t i o n s have been p u b l i s h e d (7). LVN measurements were p e r f o r m e d by S p r i n g b o r n L a b o r a t o r i e s , E n f i e l d , C T . Rheology: R h e o l o g i c a l measurements were performed using t h e R h e o m e t r i c s F l u i d s R h e o m e t e r (RFR) M o d e l 7800 w i t h cone and p l a t e geometry at a temperature of 25.0 + 1.0 C . The i n i t i a l r h e o l o g i c a l data from the steady r a t e sweep e x p e r i m e n t were further analyzed; viscosity data were reduced to o b t a i n power law coefficients. FNSD versus shear rate squared was reduced to o b t a i n FNSD coefficients and z e r o s h e a r FNSD. F o r many s o l v e n t s , the onset of measurable FNSD occurred in or above the t r a n s i t i o n to the n o n l i n e a r r e g i o n which has complicated data analysis. T h e l i n e a r r e g i o n a n a l y z e d was d e f i n e d as occurring above 1% f u l l s c a l e n o r m a l t r a n s d u c e r output and i n c l u d i n g the linear data until the correlation coefficient dropped below 0.98. Results

and

Discussion

Solvent Sets. Preferential polymer-solvent interactions can be viewed i n c o m p l e x i t y as a r a n g e from a m a t r i x o f a homopolymer with a single solvent through a copolymer with cosolvents. The system investigated consists of a terpolymer with single solvents. The i n v e s t i g a t i o n was

In Polymers as Rheology Modifiers; Schulz, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1991.

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17.

SHUELY & INCE

Poly(MMA-EA-BA) Solutions

291

structured to q u a l i t a t i v e l y determine solvent-nonsolvent classes for each solvent with the three comonomeric components by independent determination of solubility with homopolymer analogs of the comonomers. Each of the t h r e e h o m o p o l y m e r s , PMMA, P E A a n d P B A , was i n d i v i d u a l l y t e s t e d f o r s o l u b i l i t y i n each s o l v e n t . For a terpolymer, this generates eight hypothetical classes. Based on the qualitative r h e o l o g i c a l r e s u l t s and t h e s m a l l number of solvents in several classes, t h e s e were c o l l a p s e d into two c l a s s e s as shown by t h e d i v i s i o n i n Table I. Note that i n the nomenclature used h e r e i n , the " / " denotes the usual copolymerized polymer, poly(MMA/EA/BA). The " - " i s used for homopolymer solubility classes based on combinations of independent homopolymer solubility experiments; f o r example, PEA-PBA represents a group of solvents i n w h i c h b o t h PEA and PBA a r e s o l u b l e , but not PMMA. Polymer-solvent interactions were further evaluated according to a matrix of nonpolar, p o l a r , and hydrogen-bond interactions. The poly(MMA/EA/BA) is a completely aprotic dipolar structure. The s o l v e n t sets are defined below.

Table

1. 2. 3. 4. 5. 6. 7. 8.

I.

Formation of S o l u b i l i t y C l a s s e s by P e r m u t a t i o n of S o l u b i l i t y (S) a n d I n s o l u b i l i t y (I) of Component Homopolymers o f t h e Terpolymer

Terpolymer Component Homopolymer S o l u b i l i t y PBA Solubility Classes PEA PMMA S PMMA-PEA-PBA Soluble S S PMMA-PEA S o l u b l e S I S D PMMA-PBA Soluble S S I I PMMA S o l u b l e S I PEA-PBA Soluble S S I PEA S o l u b l e S I I 2) S PBA Soluble I I PMMA-PEA-PBA Insoluble I I I (Not a p p l i c a b l e ) 1) S o l u b l e w i t h 82-100% o f comonomer c o n t e n t , e.g. MMA, M M A - B A , M M A - Ε A o r M M A - E A - B A . 2) S o l u b l e w i t h 6-18% o f c o m o n o m e r c o n t e n t , e.g. BA, EA o r E A - B A , l a b e l l e d " N o n s o l v e n t W/0.82+".

Aprotic Solvent Set: The m a j o r i t y of s o l v e n t s were aprotic dipolar solvents, further classified by a nonsolvent/solvent determination for each terpolymer component, as d e s c r i b e d above. Aprotic is used i n the usual sense here as indicating absence of a proton capable of hydrogen bonding. Theta Solvent Set: Solvents were identified that were theta o r n e a r - t h e t a s o l v e n t s and a l s o belonged to the class demonstrating solubility with a l l three

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P O L Y M E R S AS R H E O L O G Y MODIFIERS

terpolymer components, that is, controls were insoluble with solvent.

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Nonsolvent

W/0.82+

Solvent

none o f t h e homopolymer the terpolymer theta

Set: T h e t e r m " N o n -

solvent W/0.82+" defines an operational subset of solvents i n which the solvent dissolves the terpolymer b u t was a n o n s o l v e n t for at least t h e m a j o r i t y 0 . 8 2 MMA fraction based on a s o l u b i l i t y d e t e r m i n a t i o n w i t h PMMA homopolymer. A comparison of the r h e o l o g i c a l properties of "Nonsolvent W/0.82+" set with theta s o l v e n t s was of interest since the low viscometric coil size estimates were similar but based on different physiochemical phenomena. The t h e t a solvents appear to systematically extend t h e r e l a t i o n s h i p o f i n c r e a s i n g FNSD a n d apparent viscosity with decreasing LVN. The s o l v e n t s t h a t are i n the "Nonsolvent W/0.82+" set display extremely high viscoelasticity as evidenced by FNSD, a p p a r e n t viscosity and dynamic properties. In general, hysteresis e x p e r i m e n t s do not demonstrate structure formation in these s o l v e n t s a l t h o u g h o v e r 82% (PMMA) o f t h e c h a i n is nominally insoluble i n the solvent. Proton Donor Solvent Set: T h e r e a r e a s e l e c t n u m b e r of solvents with low to moderate proton donating strength, but even weaker a c c e p t o r strength (in most media); examples are chloroform, methylene chloride, pentachloroethane, trichloroethylene and pentachlorocyclopropane. These solvents should specifically i n t e r a c t w i t h p r o t o n a c c e p t o r c a r b o n y l and e s t e r m o i e t i e s of the polymer solute. A l l such solvents investigated showed enhanced c o i l expansion evidenced by LVN values clustered higher than any other 'good' solvents and c o r r e s p o n d i n g l y low v a l u e s f o r r h e o l o g i c a l properties. Shear Induced P r e c i p i t a t i o n Solvent S e t : P o l y m e r s o l u t i o n s made f r o m four solvents were discovered to undergo shear induced p r e c i p i t a t i o n during rheological measurements: the solvents were dimethyl methylphosphonate, dimethyl formamide, 1-methy1-2-pyrrolidone and t r i m e t h y l phosphate. Since only 17 instances of nonaqueous shear induced p r e c i p i t a t i o n have been r e p o r t e d (12), the p o s s i b i l i t y of additions to t h i s unique class o f p o l y m e r - s o l v e n t s y s t e m s was f u r t h e r i n v e s t i g a t e d . The s t e a d y s h e a r FNSD and apparent v i s c o s i t y were r e c o r d e d d u r i n g the shear induced p r e c i p i t a t i o n p r o c e s s as l o n g as homogeneous s o l u t i o n was p r e s e n t b u t n e i t h e r t h e FNSD n o r the apparent v i s c o s i t y from the p r e c i p i t a t i o n experiments were used in the regression analyses. Clear viscous solutions had been i n t r o d u c e d i n t o the cone and plate fixture and low viscosity solvent with solid white polymer p i e c e s were r e c o v e r e d . The p r e c i p i t a t i o n s were r e p e a t a b l e and c o u l d n o t be prevented by u t i l i z a t i o n of various sample p r e p a r a t i o n and measurement procedures. The water c o n t e n t o f the s o l v e n t s was a n a l y z e d a n d f o u n d t o be s e v e r a l t i m e s h i g h e r i n t h e p r e c i p i t a t e d solutions.

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The p o l y m e r p a r t i c l e s were found to r e d i s s o l v e i n f r e s h , low water content solvent. The phase diagram boundaries of the p o l y m e r - s o l v e n t system w i t h r e s p e c t to s o r p t i o n of water as a c o s o l v e n t were e v a l u a t e d . The precipitating s o l v e n t s have c o o r d i n a t e s on o r n e a r t h e b o u n d a r y between solubility/insolubility. Thermodynamically, an addition of a s m a l l volume f r a c t i o n of sorbed water as a cosolvent would produce a nonsolvent mixture. The shear then may only i n i t i a t e p r e c i p i t a t i o n or accelerate the k i n e t i c s of precipitation. Previously published polymer-solvent pairs showing shear-induced precipitation will be evaluated f o r s i m i l a r water cosolvent effects.

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Correlations,

Solvent

Interaction Effect

range of solvent i n f l u e n c e o n FNSD c a n inspection of Figure 1. FNSD vs shear graphed f o r examples of each s o l v e n t set. o f FNSD d a t a i s p r e s e n t e d including high well into the nonlinear region. Descrip a b b r e v i a t i o n s and s o l u b i l i t y c l a s s e s are II.

Table

No. 1 2 3 4 5 6 7 8

II.

on FNSD: T h e

be viewed by rate squared is The f u l l range shear rate data tions of solvent l i s t e d i n Table

Codes and S o l u b i l i t y C l a s s e s for Representative Solvents ( F i g u r e 1)

Codes CHCL^ ACP DEM 2HP 3HP 4HP DPGMME

Solvent Solubility Class Chloroform P r o t o n d o n a t i n g PMMA-PEA-PBA* Acetophenone Good PMMA-PEA-PBA Moderate PMMA-PEA-PBA D i e t h y l malonate Near t h e t a PMMA-PEA-PBA 2-heptanone N e a r t h e t a PMMA-PEA 3-heptanone 4-heptanone Preferential PEA-PBA Dipropyleneglycol Intramolecular monomethylether PMMA-PEA-PBA TPPO Tripropyl phosphate Preferential PEA-PBA See T a b l e I f o r T e r p o l y m e r S o l u b i l i t y c l a s s codes.

Figure 2 contains a plot of First Normal Stress Difference (FNSD) v s L i m i t i n g V i s c o s i t y Number (LVN) f o r a moderately high shear rate of 400/sec. The LVN v a l u e s provide a quantitative indication of degree of solvent interaction; t h e L V N v a l u e s r a n g e f r o m a b o u t 1.2 t o 4.1 r e p r e s e n t i n g a low t o h i g h d e g r e e o f c o i l e x p a n s i o n . One can note t h a t c e r t a i n s o l v e n t s e t s f a l l i n t o l i m i t e d LVN ranges. Only the proton donating solvents ('+','•') have LVN values i n the h i g h range between 3.3 t o 4 . 1 . Even the lowest values for a proton donating solvent were h i g h e r t h a n the b e s t 'good' s o l v e n t based on n o n - s p e c i f i c interactions. The LVN range from about 1.4 to 3.3 on the regression line in Figure 2 contains three solvent sets.

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100009000 H

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8000 1: 2: 3: 4: 5: 6: 7: 8:

7000υ σ

6000 — 50004000

CHCL flCP DEM 2HP 3HP 4HP DPGMME TPPO 3

3000—L 20001000-

"Ί 0.0

0.5

I

1

1

1.0

1.5

2.0

1

1

1

1

2.5 3.0 3.5 4.0 X10 SHEAR RATE SQUARED. 1/SEC. SCL

1— 4.5

5

Figure 1. F i r s t Normal S t r e s s D i f f e r e n c e (FNSD) vs Shear Rate Squared f o r Examples of A l l Solvent Sets with Poly(MMA/EA/BA), f o r S o l u t i o n s a t 4.7 + 0.1 g/dL a n d 25.0 + 1.0 C: Proton Donating PMMA-PEA-PBA(1), P M M A - P E A - P B A (2,3,4), P M M A - P E A (5), P E A - P B A ( 6 , 8 ) and Intramolecular P M M A - P E A - P B A (7). See T a b l e s I and II for abbreviations.

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25000

1.0

1.5

2.0

2.5

3.0

3-5

4.0

4.5

LVN, DL/G Figure 2. F i r s t Normal S t r e s s D i f f e r e n c e (FNSD) vs L i m i t i n g V i s c o s i t y Number (LVN) f o r A l l S o l v e n t Sets with Poly(MMA/EA/BA) f o r S o l u t i o n s a t 4.7 + 0.1 g/dL and 2 5 . 0 + 1.0 C : R e g r e s s i o n L i n e = PMMA-PEA-PBA (*) and P r o t o n D o n a t i n g PMMA-PEA-PBA (+ ) . Not i n c l u d e d i n the Regression f i t : I n t r a m o l e c u l a r PMMA-PEA-PBA ( ο ) , PMMA-PEA (Ο), Proton D o n a t i n g PMMA-PBA ( £ ) / P E A - P B A (Δ) and P r o t o n Donating PEA-PBA (•). See T a b l e I for abbreviations.

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The m a j o r i t y o f the solvents are contained i n the set d e f i n e d as s o l u b l e w i t h a l l t e r p o l y m e r components (Code = PMMA-PEA-PBA; * ) . The solvent set consisting of solvents soluble w i t h b o t h PMMA a n d P E A a r e contained w i t h i n t h i s r a n g e (Code = PMMA-PEA; ' 0 ' ) · Qualitatively, the PMMA-PEA set, s o l u b l e w i t h 94% o f the terpolymer, a p p e a r s t o be following the general trend of the 100% soluble PMMA-PEA-PBA set, however, due t o the limited data set, t h e s e have not been included in the regression i n F i g u r e 2. The t h i r d s o l v e n t s e t i n t h e 1.4 t o 3 . 3 range has been d e f i n e d as i n t r a m o l e c u l a r hydrogen bonded solvents. These solvents form both c y c l i c (usually 5-7 member rings) intramolecular hydrogen bonds and acyclic i n t e r m o l e c u l a r hydrogen bonds. A l t h o u g h t h e i r LVN v a l u e s fall within the usual range of 1.4 to 3.3, their viscoelasticity, a s m e a s u r e d b y FNSD v a l u e s , are quite scattered, do n o t f a l l on the regression l i n e and, i n some cases, are not w i t h i n the Figure 2 a x i s . In fact, o n l y one i n t r a m o l e c u l a r hydrogen bond s o l v e n t f a l l s into the plot axis (Code = PMMA-PEA-PBA; Ό ' ) , methyl salicylate, w i t h an ortho hydroxy p r o t o n bonded to the adjacent carbonyl moiety. The other intramolecular hydrogen bonded solvent, dipropyleneglycolmonomethylether, with a six-member i n t r a m o l e c u l a r r i n g c o n t a i n i n g a hydroxy proton hydrogen bonded to the e t h e r moiety, is off-scale i n F i g u r e 2, a t an LVN v a l u e o f 1.64 a n d FNSD value of 28,000. From t h e l i m i t e d d a t a i n t h i s s e t , one cannot yet determine if this set will form a unique regression equation or will be generally anomalous. T r a c e i m p u r i t i e s , such as water o r o t h e r p r o t i c solvents, can i n f l u e n c e the e q u i l i b r i u m between i n t r a m o l e c u l a r and i n t e r m o l e c u l a r hydrogen bonding s o l v e n t conformations and thereby c o n t r i b u t e to the appearance of anomalous solvent effects. The LVN range between 1.5 to 2.0 appears quite complex, at first. T h e FNSD v a l u e s s e e m t o be rather randomly d i s t r i b u t e d between 3,000 and 25,000 d y n e s / s q . cm., a l t h o u g h some s o l v e n t s l i e u p o n t h e r e g r e s s i o n line. Actually, FNSD values range off-scale up to 900,000 dynes/sq. cm. (not p l o t t e d ) w i t h i n t h i s 1.5 t o 2.0 LVN range. One c a n b e g i n to unravel the apparent random nature of the v i s c o e l a s t i c FNSD m e a s u r e m e n t s by noting the solubility classifications and solvent data sets associated with the data points. F o r t h e 1.5 t o 2.0 LVN r a n g e , no i n s t a n c e o f a n o m a l o u s l y h i g h ( d e f i n e d a s : above the regression line 95% confidence interval) FNSD measurements were r e c o r d e d for solvents soluble with the 82% t o 94% m a j o r i t y c o m o n o m e r c o n t e n t . A l l solvents that were s o l u b l e w i t h the 18% comonomer fraction (Code = PEA-PBA; 'Δ') h a v e FNSD v a l u e s a t o r a b o v e t h e regression 95% c o n f i d e n c e interval; this includes several PEA-PBA solvents that are off-scale a t v e r y h i g h FNSD v a l u e s .

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/

/

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FNSD coefficient can be independent of solvent effects for solutions of high MW-concentration products within the concentrated, network entangled regime (4). Within the semidilute regime f o r C χ LVN = 5-20, and C χ Mw = 7.1 g/dL megadalton, the FNSD increases with d e c r e a s i n g L V N as shown i n Figure 2. T h e FNSD d a t a h a s been a n a l y z e d as a f u n c t i o n of s p e c i f i c shear rates of 100, 400, 1000/sec and as a f u n c t i o n o f FNSD coefficient at the onset of s i g n i f i c a n t (>1% n o r m a l f o r c e transducer f u l l scale) normal force response. The r e l a t i o n s h i p s at v a r i o u s shear r a t e s are s i m i l a r and data o b t a i n e d to date can be summarized by t h e f o l l o w i n g equation.

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FNSD

(@ 4 0 0 / s e c )

=

8900

-

(2000

X LVN)

The e q u a t i o n r e p r e s e n t s a l l o f t h e 17 P M M A - P E A - P B A soluble solvents i n c l u d i n g the proton donors ('*' and '+'). E x c l u d e d a r e t h e 11 o f 28 s o l v e n t s c o m p r i s i n g t h e intramolecular PMMA-PEA-PBA ( Ο ) , PMMA—PEA s o l u b l e (O), p r o t o n d o n a t i n g P M M A - P B A fà) , P E A - P B A s o l u b l e (Δ)/ proton donating E B (•) and s h e a r - i n d u c e d p r e c i p i t a t i o n solvent set (not shown on p l o t ) . Solvent Influence on Apparent V i s c o s i t y : T h e s o l v e n t influence as estimated by LVN i s summarized i n the equation.

Apparent

Viscosity

(@ 1 0 / s e c )

= 8 . 1 x e

*

^

ν Γ ,

>

The r e s u l t s i n c l u d e o v e r 16 s o l v e n t s s o l u b l e i n at l e a s t 82 m o l e p e r c e n t o f t h e copolymer content and cover all solubility classes. The slope shows a nominal decrease of apparent v i s c o s i t y w i t h i n c r e a s i n g LVN but is not statistically different from a zero slope at a polymer concentration of 4.7 g/dL. Limited experiments at lower c o n c e n t r a t i o n s o f 2-3 g/dL found a slightly increasing apparent viscosity with increasing LVN; at higher c o n c e n t r a t i o n s o f 6.0 g / d L a s l i g h t l y decreasing apparent viscosity with increasing L V N was found. Therefore, the i n c i p i e n t s l o p e change from positive to n e g a t i v e o c c u r s b e t w e e n C χ Mw v a l u e s o f 4 . 5 a n d 7 . 1 g / d L megadalton. Overall, ten additional solvents were studied to better define the FNSD vs LVN r e l a t i o n s h i p ; additional apparent viscosity vs LVN d a t a did not s i g n i f i c a n t l y influence the slope.

Conclusions Range o f Solvent

Effects. It would be d e s i r a b l e t o be a b l e to summarize the magnitude of the solvent e f f e c t on rheological viscoelastic properties i n the same manner t h a t MW a n d concentration have been presented i n the past. T h e e f f e c t o f MW a n d c o n c e n t r a t i o n o n r h e o l o g i c a l p r o p e r t i e s has been a p p r o p r i a t e l y summarized i n terms of

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the slope of the property-variable relationship, for e x a m p l e , c h a n g e i n F N S D p e r d e c a d e c h a n g e i n Mw ( 4 ) . S o l v e n t e f f e c t s a r e more complex i n that the slope and even the d i r e c t i o n of the r e l a t i o n s h i p depend on the coil density regime; in addition, there is increased complexity due to the various s p e c i a l solvent effects considered here (preferential, proton donating). One approach to summarizing the solvent effect data takes advantage of the finite extent of solvent interaction between t h e two e x t r e m e s o f i n s o l u b l e o r t h e t a conditions and maximal coil expansion. One can then r a t i o the rheological properties a t o r n e a r t h e s e two limits in solvent interaction. One can also view the range or r a t i o of r h e o l o g i c a l properties with respect to the range i n LVN or i n t r i n s i c v i s c o s i t y over the s o l v e n t set. These LVN, apparent viscosity a n d FNSD v a l u e s , and the r a t i o s for the l i m i t s of the solvent set are listed in Table III. Note t h a t these r a t i o s apply to a specific semi-dilute concentration-MW value and that solvent e f f e c t s d i s a p p e a r a t h i g h v a l u e s o f FNSD a t t h e h i g h c o i l densities within the entangled regime. The ratio of high/low rheological values are listed in Table III for the example of a moderate coil density within the semidilute regime of C χ Mw = 4.7 χ 1 . 5 = 7 . 1 g/dL megadalton.

Table

III.

S o l v e n t E f f e c t Ranges and R a t i o s f o r Rheological P r o p e r t i e s of Polymer Solutions at a Moderate C o i l Density (Concentration χ Mw = 7 . 1 g / d L m e g a d a l t o n ) (See T a b l e I f o r Abbreviations) Solvent Set Within Within Within Between PMMA-PEA-PBA PEA-PBA P r o t o n Donor Soluble P r o t o n Donor Soluble and PEA-PBA (3.03/1.27) (4.08/3.36) (2.12/1.27) (4.08/1.27) = 2.4 = 1.2 = 1.7 = 3.2

Property LVN (dL/g) Apparent V i s c o s i t y (11.7/4.05) (poise) = 2.9 @ 10/sec F i r s t Normal Stress (6800/1420) Difference =4.8 (dynes/sq. cm.) é 400/sec

(16.1/2.52) = 6.4

(29.2/5.18) = 5.6

(29.2/2.52) = 11.6

(6660/100.) =67.

(900K/9400) =96.

(900K/100.) = 9000.

Two types of ranges are l i s t e d ; 'within' a solvent class refers to the ratio between the highest/lowest values for a single class. 'Between' solvent classes

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refers to the ratio between the highest value of one c l a s s and the lowest of the other c l a s s . The properties l i s t e d i n the f i r s t column are LVN, apparent v i s c o s i t y at low s h e a r ( 1 0 / s e c ) , and f i r s t normal s t r e s s d i f f e r e n c e at a moderate shear rate (400/sec). The solvent sets d e f i n e d i n a d j a c e n t columns i n c l u d e : w i t h i n PMMA—PEA-PBA s o l u b l e , w i t h i n p r o t o n donor, w i t h i n PEA-PBA s o l u b l e , and between p r o t o n donor and PEA-PBA soluble. The PMMA-PEA-PBA s o l u b l e s o l v e n t s e t p r o v i d e d a LVN r a t i o of 2.4, r a n g i n g from near t h e t a s o l v e n t s to maximal values from non-specific interactions. The range of apparent viscosity and first normal stress values measured from this range is about 3X and 5X, respectively. The PMMA-PEA-PBA soluble solvent set of nonspecific interactions is bracketed at higher LVN v a l u e s by t h e p r o t o n donor s e t and a t lower LVN v a l u e s by the preferential PEA-PBA soluble set. Both of these solvent sets have LVN ranges of about half of the 'regular' solution range, although the preferentially soluble solvent set and near theta s o l v e n t LVN v a l u e s overlap. Both of these solvent s e t s have wider ranges and l a r g e r r a t i o s of apparent viscosity ( a b o u t 6X) a n d first normal stress difference (67X-96X) than the PMMA-PEA-PBA s o l u b l e set. The l a s t column r e c o r d s the measurement ranges and their ratios between the extremely high LVN and low rheological property values of the proton donor set and the low LVN and extremely high rheological property values of the preferentially PEA-PBA s o l u b l e s e t . The LVN range has been extended to 3.2X, the apparent viscosity ratio to 11.6X, and first normal stress difference to 9000X. Therefore, although the normal range of solvent influence on rheological properties between 'good' and theta solvents is limited, the s e l e c t i o n of polymer-solvent p a i r s can extend the viscous properties by about one decade and viscoelastic p r o p e r t i e s by over t h r e e decades. Molecular Interactions. At the molecular level, the interactions can be interpreted in the u s u a l manner, whereby the s p e c i f i c proton donor i n t e r a c t i o n s further minimize polymer-polymer interactions and thereby m i n i m i z e the f r i c t i o n f a c t o r i n f l u e n c i n g v i s c o u s flow and t r a n s i e n t entanglements influencing viscoelasticity. The s e t o f s o l v e n t s s o l u b l e w i t h o n l y 6-18% o f t h e comonomer can be interpreted in a general way as containing substantial s e q u e n c e l e n g t h s o f t h e i n s o l u b l e PMMA; b o t h i n t r a c o i l and i n t e r c o i l polymer-polymer contacts would occur and high frictional interaction and transient entanglement d e n s i t i e s would be present. Rheo-optical studies might detect any non-statistical, ordered structure in these solutions.

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12.

Rangel-Nafaile, C . ; Metzner, A. B.; Wissbrun, K. F. Macromolecules, 1984, 17, 1187. Received November 26, 1990

In Polymers as Rheology Modifiers; Schulz, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1991.

Chapter 18

Thermal Characteristics of Waxy, High-PourPoint Crudes That Respond to Rheology Modifiers 1

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D. S. Schuster and J . H. Magill

2

1

Department of Chemical Engineering, Bucknell University, Lewisburg, PA 17837 Department of Chemical and Petroleum Engineering and Department of Materials Science and Engineering, University of Pittsburgh, Pittsburgh, PA 15261

2

Characteristics of waxy, high pour point crude oils that respond to chemical rheology modifying pour point depressants were determined using Differential Scanning Calorimetry. Waxy oils that show a reduced pour point after treatment crystallize over a broad temperature range, have a broader∆H than do their saturate components, and show a reduced∆H after treatment. After treatment these oils also show a phase transition as do their saturate components. c

c

Waxy, high pour point crudes are low v i s c o s i t y , Newtonian f l u i d s at high temperatures but exhibit non-Newtonian behavior owing to the p r e c i p i t a t i o n of waxes as the crude i s cooled. At the pour point temperature a s o l i d mass i s formed. Transporting these crudes presents technical and economic problems, the magnitude of which depends on the pour point of the o i l , transportation method, and ambient temperature (1-5). For example, a 32.2°C (90°F) pour point crude w i l l present congealing problems i n most ambient conditions. I t may congeal on the walls of tankers r e s u l t i n g i n a stock loss, and i f i t congeals i n a pipeline, the required r e s t a r t pressure may exceed the burst pressure of the l i n e . Whereas, a 10°C (50°F) pour point o i l presents few congealing d i f f i c u l t i e s , but wax may p r e c i p i t a t e out of solution and s t i c k to pipe walls or form a sludge at the bottom of a storage tank even i n warm climates. This wax deposition can block flow l i n e s , reduce throughput, clog pumps, and i n h i b i t the performance of metering devices that measure transferred crude o i l (4). The goal of successful treatment of high pour crudes with a polymeric pour point depressant chemical i s to reduce the temperature of congealing, i n h i b i t waxes from p r e c i p i t a t i n g out of solution, reduce the y i e l d strength of congealed crude, and decrease the crude's v i s c o s i t y . Unfortunately, pour point depressants are not used to t h e i r f u l l e s t potential because either the treatment cost i s p r o h i b i t i v e , or they are only marginally e f f e c t i v e on c e r t a i n crudes regardless of the treatment l e v e l (5). 0097-6156/91A)462-0301$06.00/0 © 1991 American Chemical Society

In Polymers as Rheology Modifiers; Schulz, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1991.

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In order to improve t h e i r effectiveness and economics, research has been conducted to improve pour point depressant formulations and to study the mechanism of pour point depression (4,6-10). Irani suggests that the interactions of the various p r e c i p i t a t i n g high molecular weight species has resulted i n s p e c i f i c additives being e f f e c t i v e only with very s p e c i f i c types of crude. A U.S. patent claims that polars interfere with the propagation of the wax, and thus improve the pour point of the o i l (8). Additionally, the presence of asphaltenes i n the waxes appears to correlate with the maximum extent of pour point depression that can be obtained with a crude o i l (7). In conjunction with these studies, c r y s t a l l i z a t i o n of the waxes have been explored. Morphology of the waxes has been related to pour point depression by several researchers (9) and another has suggested that the response of the wax to a pour point depressant i s a function of i t s c r y s t a l structure (10). A number of researchers have suggested i t i s the c r y s t a l l i z a t i o n k i n e t i c s that may be the dominating factor. Holder (9) established that mixing d i f f e r e n t carbon number alkanes increased the extent of pour point depression and suggested that this occurred since the mixtures tended to c r y s t a l l i z e more slowly. Reddy, (10) also suggested that additives have more time to interact with slow growing c r y s t a l s and i n h i b i t t h e i r growth. D i f f e r e n t i a l Scanning Calorimetry (DSC) i s used i n this study to determine i f c r y s t a l l i z a t i o n differences e x i s t between high pour point o i l s that do and do not respond to rheology modifiers. Kawamura (11) and others have used the DSC to study waxes. Weslowski (12), using the DSC, noted that some lube o i l waxes undergo phase t r a n s i t i o n s . After our work was completed (13), Redelius (14) used the DSC technique to characterize parafin wax i n mineral o i l products. Experimental Three waxy, high pour point crude o i l s were analyzed i n this study. The three o i l s were selected since they respond quite d i f f e r e n t l y to pour point depressants as seen i n Table I. Crude A, a West A f r i c a n crude, responds extremely well to rheology modifiers, while crude C, a crude from the United States, shows no response to pour point depressants. These o i l s are c h a r a c t e r i s t i c of waxy o i l s that do and do not respond to rheology modifiers based on previous chemical analysis (6,7). Sample Preparation. Each of the three crudes were separated into t h e i r saturate, aromatic, polar, and asphaltene fractions, using High Performance Liquid Chromatography (HPLC). Gel Permeation Chromatography and Gas Chromatography were used to v e r i f y the p u r i t y of the saturate f r a c t i o n s . Results of the GPC analyses for the crudes used i n t h i s study and several others were presented previously (6,7). The HPLC separation was performed with a s i l i c a gel column equipped with a r e f r a c t i v e index detector. The HPLC apparatus has been discussed elsewhere (15). The saturates acquired from the separation are a l l species which contain only carbon and hydrogen. The aromatic hydrocarbons contain at least one aromatic r i n g .

In Polymers as Rheology Modifiers; Schulz, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1991.

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Waxy, High-Pour-Point Crudes

303

Nitrogen containing compounds, phenolic and carboxylic acids comprise the polar compounds. Asphaltenes, by d e f i n i t i o n , are a s o l u b i l i t y c l a s s i f i c a t i o n , and for this analysis include a l l hexane insoluble material. Samples treated with pour point depressants were heated along with the pour point depressant to 82°C i n a pressurized c e l l to prevent vaporization of the l i g h t ends. Crudes A, Β and C were treated with 2000 ppmw of Paradyne-85 (Exxon Chemicals) for subsequent DSC and pour point analysis. A centrifuge was used to separate the portion of the crude that responds to pour point depression from the portion that does not respond to pour point depressants. Crude Β was treated with the pour point depressant, and was transferred to a centrifuge tube at 10°C above the treated crude's pour point. The sample was centrifuged at t h i s temperature, and the top l i q u i d phase was decanted from the bottom wax phase. This l i q u i d phase contained the waxes that were not treated by the rheology modifier, and was subjected to further analysis. Pour Point Test. The pour point test i s a standardized ASTM procedure (ASTM D97) for measuring the congealing temperature of a hydrocarbon f r a c t i o n . This procedure recommends heating the sample to 46°C and then cooling i t i n 2.8°C (5°F) decrements to a temperature at which the sample shows no f l u i d i t y over a period of 5 seconds when subjected to a pressure head of 2.54 cm. The test i s performed i n standardized pour point tubes which automatically provide the required head. The pour point i s reported as the next highest 2.8°C (5°F) increment above the recorded congealed value. A pour point i s only accurate to +/- 2.8°C (5°F). Since a temperature of 46°C was believed to be inadequate for the complete d i s s o l u t i o n of a l l waxes, an alternate congealing temperature procedure was developed. In this modified procedure, the sample was heated to 82°C i n a sealed rotating pressured c e l l to assure that a l l of the dissolved waxes were i n solution. The container was then slowly cooled at a rate of 10°C/hr to 43°C and transferred to a standard pour point tube and s t e a d i l y cooled at the same cooling rate u n t i l congealing occurred. A programmable, controlled temperature water bath was used for cooling. D i f f e r e n t i a l Scanning Calorimetry Analysis. Thermal measurements were made with a Perkin-Elmer DSC-2B calorimeter with a scanning auto-zero accessory. A l l studies were made on samples 2 to 4.5 mg. i n weight. The samples were tested i n hermetically sealed aluminum pans to prevent vaporization losses. Helium was used to purge the DSC head, and nitrogen was used i n the dry box. A l l samples were loaded at ambient temperature. High instrument s e n s i t i v i t y was used for a l l scans. Temperature c a l i b r a t i o n was performed using 99.9%(+) pure samples of benzene, indium, and mercury. Heats of c r y s t a l l i z a t i o n were estimated with respect to an Indium standard. Typical scans required that each sample be heated to 87°C to assure that a l l wax p a r t i c l e s were i n solution. Cooling curves were acquired at 40°C/min. using l i q u i d nitrogen as the coolant. Similar results were also acquired with slower cooling rates (5°C/min), where

In Polymers as Rheology Modifiers; Schulz, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1991.

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the onset of n u c l e a t i o n / c r y s t a l l i z a t i o n was only suppressed on the temperature scale (13). Since the DSC-2B measures the rate of energy absorption or evolution by the sample, heats of transitions were determined. A Hewlett Packard 9816 computer equipped with a d i g i t i z e r was used to integrate the areas under the cooling curves for each DSC scan.

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Results and Discussion The crudes are i d e n t i f i e d i n Table I as being responsive or nonresponsive to polymeric pour point depressant chemicals. Data i s presented f o r Paradyne-85 as i t i s representative of several of the more e f f e c t i v e pour point depressant chemicals. I t should be noted that Crude C and the nonresponsive portion of crude Β (denoted as b*) have never been affected by any of 49 pour point depressant chemicals that were available at the time of this study (5). D i f f e r e n t i a l Scanning Calorimetry curves are presented i n Figure 1 f o r the crudes, i n Figure 2 for the crudes treated with 2000 ppmw of Paradyne-85, and i n Figure 3 for the saturates portion of the crude o i l s . Nucleation temperatures and heats of c r y s t a l l i z a t i o n are also noted on the figures. Non-isothermal cooling was studied because i t i s representative of the s i t u a t i o n that a high pour point o i l experiences during treatment with a pour point depressant. Besides, i t has been documented that isothermal k i n e t i c s can d i f f e r from nonisothermal results (11). Additionally, i n i t i a l microscopic studies indicated that isothermal c r y s t a l l i z a t i o n proceeded at extremely slow rates, and the s e n s i t i v i t y of the DSC instrument could not measure quantitatively the energy t r a n s i t i o n s . When analyzing the differences i n responsive crudes (A and B) and non-responsive (C and b*) i n Figure 1, the most s t r i k i n g difference i s seen i n the temperature span of c r y s t a l l i z a t i o n . The responsive crudes c r y s t a l l i z e over a much broader range of temperatures. Differences i n nucleation temperature and heats of c r y s t a l l i z a t i o n are apparent for each of the o i l s , but the data does not support a systematic trend between the treatable and nontreatable o i l s . When treated with a pour point depressant chemical, the scans i n Figure 2 show an immediate difference between the responsive and nonresponsive o i l s . The responsive o i l s (A and B) show a phase t r a n s i t i o n at approximately -90°C. Again, nucleation temperatures and heats of c r y s t a l l i z a t i o n of the treated crudes do not show a systematic trend between the successfully treated A and B, and the non-responsive b* and C. However, when compared with the untreated case i n Figure 1, Crudes A and Β show a decrease i n the heats of c r y s t a l l i z a t i o n when treated, whereas the non-responsive crudes do not. In Figure 3, the saturates isolated from the responsive crudes A and Β show a phase t r a n s i t i o n similar to that exhibited by the crudes a f t e r treatment. I t appears that the additive unmasks the phase t r a n s i t i o n undergone by the saturates and that the other o i l components suppress i t . Comparison of the crudes i n Figure 1 to the saturates i n Figure 3 show that the saturates i n the non-responsive crudes b* and C have higher heats of c r y s t a l l i z a t i o n than the entire crude o i l exhibits. This indicates that not a l l of the saturates

In Polymers as Rheology Modifiers; Schulz, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1991.

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Table I Response of Crude O i l s to Pour Point Depressant Chemicals

Treated Pour Point °C (°F)

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Crude

A Β b* C

21,.1 (70) 35,.0 (95) 35,.0 (95) 43,.3 (110)

Pour Point °C (°F)

Treatment

-17..8 18..3 35..0 43..3

2000 ppmw Paradyne-85 II II II

(0) (65) (95) (110)

b* i s the non-treatable portion of crude Β i s o l a t e d by the technique presented i n reference 7 and described below,

exothermic ^

Temperature Scale (°C)

ι

1

1

1

1

-140

1

1

1

1

1

1

1

1 1

0

0

0

34

A AH

c

-

27

cal/g

Figure 1. DSC scans of waxy, high pour point crude o i l s at 40 C/min. cooling rate.

In Polymers as Rheology Modifiers; Schulz, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1991.

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exothermic 1 I

Temperature Scale (°C) I

I

I

1

1

1

1

1

1

1

1

1

Figure 2. DSC scans of waxy, high pour point crude oils after treatment with 2000 ppmw of Paradyne-85 at 40 ° C/min cooling rate.

In Polymers as Rheology Modifiers; Schulz, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1991.

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Figure 3. DSC scans of saturates isolated from waxy, high pour point crude oils at 40 ° C/min cooling rate.

In Polymers as Rheology Modifiers; Schulz, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1991.

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c r y s t a l l i z e i n the non-responsive crudes. In contrast, the responsive crudes A and Β have higher heats of c r y s t a l l i z a t i o n than do the saturates from these crudes, indicating that components other than the saturates also c r y s t a l l i z e i n these o i l s . The r e s u l t s depict the complexity that exists i n these systems and no simple picture describes i t . Various cooling rates are needed where isothermal data may p i n down one extreme end of the scale of interactions between the additive and the systems. Of course, one can invent an "exothermicity value" defined simply i n cal./g./degree of c r y s t a l l i z a t i o n undercooling to delineate responsive and nonresponsive systems. A l t e r n a t i v e l y the value of pour point of the crudes (°K) divided by t h e i r respective c r y s t a l l i z a t i o n range may provide another p r a c t i c a l way of doing i t . Higher numbers would function as a guideline indicating non-responsive systems i n which non-interacting additives f a i l to interact or suppress the onset of n u c l e a t i o n / c r y s t a l l i z a t i o n and do not prolong the f l u i d i t y span of the system responsible for sustaining p o u r a b i l i t y . Conclusions Based upon the analysis of 3 crude o i l s i t appears that c r y s t a l l i z a t i o n differences exist between waxy crude o i l s that do and do not respond to a pour point depressant chemical. Although inference to a l l o i l s and pour point depressant chemicals can not be made based on only the 3 o i l s studied, the following c h a r a c t e r i s t i c s are offered f o r consideration of responsive and non-responsive crude oils. Responsive: • • • •

Crude: C r y s t a l l i z e s over a broad temperature range Crude AHQ > Saturate AHç Crude ΔΗ^ > Saturate AHQ > Treated Crude ΔΗ