Barrier Polymers and Structures 9780841217621, 9780841212794, 0-8412-1762-9

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ACS

SYMPOSIUM

SERIES

Barrier Polymers and Structures William J. Koros,

EDITOR

The University of Texas at Austin

Developed from a symposium sponsored by the Division of Polymer Chemistry, Inc. at the 197th National Meeting of the American Chemical Society, Dallas, Texas, April 9-14, 1989

American Chemical Society, Washington, DC 1990

In Barrier Polymers and Structures; Koros, W.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

423

Library of Congress Cataloging-in-Publication Data Barrier Polymers and Structures William J. Koros, editor p.

cm—(ACS Symposium Series, 0097-6156; 423).

"Developed from a symposium sponsored by The Division of Polymer Chemistry, Inc. at the 197th Meeting of the American Chemical Society, Dallas, Texas, April 9-14, 1989." Includes bibliographical references. ISBN 0-8412-1762-9 1. Food—Packaging—Congresses Congresses. I. Koros, William J., 1947. II. Polymer Chemistry, Inc. III. American Chemical Society. Meeting (197th : 1989 : Dallas, Tex.) IV. Series. TP374.B37 1990 668.4'9—dc20

90-275 CIP

The paper used in this publication meets the minimum requirements of American National Standard for Information Sciences—Permanence of Paper for Printed Library Materials, ANSI Z39.48-1984.

Copyright ©1990 American Chemical Society All Rights Reserved. The appearance of the code at the bottom of the first page 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, M A 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 the first page 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 A C S 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 U N I T E D STATES O F A M E R I C A

In Barrier Polymers and Structures; Koros, W.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

ACS Symposium Series M . Joan Comstock, Series Editor 1990 ACS Books Advisory Board Paul S. Anderson Merck Sharp & Dohme Research Laboratories V. Dean Adams Tennessee Technological University

Michael R. Ladisch Purdue University

Robert McGorrin Kraft General Foods

Alexis T. Bell University of California— Berkeley

Daniel M. Quinn University of Iowa

Malcolm H. Chisholm Indiana University

Eisa Reichmanis AT&T Bell Laboratories

Natalie Foster Lehigh University

C. M. Roland U.S. Naval Research Laboratory

G. Wayne Ivie U.S. Department of Agriculture, Agricultural Research Service

Stephen A. Szabo Conoco Inc.

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

Wendy A Warr Imperial Chemical Industries Robert A Weiss University of Connecticut

In Barrier Polymers and Structures; Koros, W.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

Foreword The ACS 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 submitted by the authors in camera-ready form. Papers are reviewed under the supervision of the Editors with the assistance of the Series Advisory Board and are selected to maintain the integrity of the symposia; however, verbatim reproductions of previously published papers are no research are acceptable, symposi y types of presentation.

In Barrier Polymers and Structures; Koros, W.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

Preface

BARRIER RESINS HAVE REVOLUTIONIZED the packaging industry and, therefore, have been the focus of intense investigation. Numerous interesting developments related to barrier applications have been reported in journals and at various technical meetings Typically however, the informatio number of outlets. The time seemed right to publish a collection of both fundamental and practical principles involved in making and using barrier polymers and structures. The project took the form of this book, which provides a comprehensive treatment of the state of science and technology in the area of barrier polymers and barrier structures. The topics covered will be of direct interest to industrial scientists making or using barrier packaging. Moreover, government regulators who work with the packaging industry should find the book useful in indicating trends in materials and processes in the industry. Academic researchers working in fundamentals of sorption and transport processes will alsofindseveral good updates in these areas.

Acknowledgments The National Science Foundation and the ACS are acknowledged for partial support of my time in the preparation of the overview chapter and the coordination of the book. It has been a sincere pleasure working with Cheryl Shanks and Beth Pratt-Dewey of the ACS on this project; their good nature and efficiency were examples for me. Thanks are extended to the session chairs and participants in the symposium on which the book is based. Finally, I express my deepest gratitude to the authors whose work is presented in this volume and the many excellent reviewers who helped perfect the papers with their thoughtful comments.

ix In Barrier Polymers and Structures; Koros, W.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

Dedication This book is dedicated to Vivian T. Stannett, whose pioneering contributions to understanding the fundamentals of barrier polymers continue to inspire and motivate scientists in this field. WILLIAM J. KOROS

The University of Texas at Austin Austin, TX 78712 January 25, 1990

x

In Barrier Polymers and Structures; Koros, W.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

Chapter 1

Barrier Polymers and Structures: Overview William J. Koros Department of Chemical Engineering, The University of Texas at Austin, Austin, TX 78712

This introductory chapter provides an overview of the papers presented at the symposium on Barrier Polymers and Barrier Structures that was sponsored by the Polymer Chemistry Division of the American Chemical Society at the Spring 1989 meeting. A total of nineteen papers from the symposium are included in this volume. Topics covered include barrier transport fundamentals, advanced composite structures, reactive surface treatments and the effects of orientation on barrier properties. Relationships between polymer molecular structure and barrier efficacy are also treated in detail. Time and history dependent phenomena associated with retorting of barrier laminates are discussed from the standpoint of theoretical modeling and experimental characterization of the barrier layers. The effects of concentration dependent diffusion, flavor scalping and nonFickian transport phenomena are also discussed. The coverage, therefore, is broad while providing sufficient depth to provide a state-of-the-art update on the major technical issues facing the barrier packaging field. The development of efficient packaging materials and containers has allowed the evolution of the modern market system based on concentrated production facilities supplying individuals residing far from the ultimate source of products. The goal of the packaging industry has been to provide increasingly more cost effective means of preserving the quality of materials with as close to their as-produced natures as possible. Modern packaging is a sophisticated technology rooted deeply in fundamental polymer science. Nevertheless, the sheer size and competitiveness of the packaging industry also makes i t an extremely practical field with an eye to applying polymer science to achieve the bottom line of cost and barrier efficiency. This book seeks to 0097-6156/90/0423-0001$06.25/0 © 1990 American Chemical Society

In Barrier Polymers and Structures; Koros, W.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

2

BARRIER POLYMERS AND STRUCTURES

r e f l e c t t h e d u a l n a t u r e o f t h i s t e c h n o l o g y by p r e s e n t i n g fundamental p r i n c i p l e s a l o n g w i t h complementing d i s c u s s i o n s o f a p p l i c a t i o n s o f t h e s e p r i n c i p l e s by l e a d e r s i n t h e f i e l d . Many e x c e l l e n t r e v i e w s o f t h e fundamental p r o c e s s e s by which s m a l l m o l e c u l e s p e n e t r a t e between t h e segments c o m p r i s i n g a polymeric f i l m a r e a v a i l a b l e (1-10). These r e v i e w s , c o u p l e d w i t h p a s t symposia p u b l i c a t i o n s r e l a t e d t o b a r r i e r s and t h e a l l i e d f i e l d o f membranes (11-18) s h o u l d a l l o w a newcomer t o t h e f i e l d t o r a p i d l y g a i n t h e needed background t o become an a c t i v e p a r t i c i p a n t i n t h i s dynamic t e c h n o l o g y . The c h a p t e r s i n t h e p r e s e n t book r e f l e c t t h e c u r r e n t r e s e a r c h and development d i r e c t i o n s o f a c t i v e u n i v e r s i t y and i n d u s t r i a l participants i n the b a r r i e r f i e l d . The o v e r v i e w p r o v i d e d by t h i s f i r s t c h a p t e r g i v e s a framework f o r a p p r e c i a t i n g t h e more d e t a i l e d subsequent d i s c u s s i o n o f t o p i c s t h a t a r e a t t h e c u t t i n g edge o f t h i s e v o l v i n g t e c h n o l o g y . T h e r e f o r e , i t i s a n t i c i p a t e d t h a t t h i s book w i l l be o f i n t e r e s t t o b o t the f i e l d . Fundamentals In a d d i t i o n t o p r o v i d i n g a c o n t a i n e r t o p r e v e n t s c a t t e r i n g and b u l k phase m i x i n g o f components, modern packages c o n t r o l t h e exchange of components between t h e package c o n t e n t s and t h e e x t e r n a l environment. F o r i n s t a n c e , t h e p r o t e c t i o n from a t t a c k by oxygen i s among t h e most common f u n c t i o n s e r v e d i n f o o d p a c k a g i n g . Even t h i s g e n e r a l r e q u i r e m e n t , however, has m a n i f o l d a s p e c t s . The d e g r e e s o f s e n s i t i v i t y of d i f f e r e n t m a t e r i a l s t o environmental e f f e c t s are c l e a r l y v e r y d i f f e r e n t and t h e oxygen b a r r i e r s r e q u i r e d t o s u c c e s s f u l l y store these products d i f f e r a c c o r d i n g l y . Similar c o n s i d e r a t i o n s a p p l y t o water p e r m e a t i o n , s i n c e f o o d s and many p h a r m a c e u t i c a l s show v a r y i n g degrees o f s t a b i l i t y i n a d r y s t a t e as compared t o t h a t i n t h e p r e s e n c e o f water. T a b l e I (19) i l l u s t r a t e s t y p i c a l ranges o f s e n s i t i v i t y t o oxygen and water vapor f o r a s p e c t r u m o f common f o o d s . The t a b l e a l s o i l l u s t r a t e s t h e m a n i f o l d r e q u i r e m e n t s needed i n terms o f o i l and f l a v o r and/or aroma component b a r r i e r s t h a t p a c k a g i n g must p r o v i d e f o r t h e d i f f e r e n t foods. As p a c k a g i n g c a p a b i l i t i e s i n c r e a s e , t h e c o m p l e x i t y o f t h e a p p l i c a t i o n s t h a t c a n be t r e a t e d a l s o i n c r e a s e s c o n s i d e r a b l y , t h e r e b y e x p l a i n i n g t h e e v e r expanding markets f o r p a c k a g i n g materials. An i n t e r e s t i n g example i n which complex r e q u i r e m e n t s must be met i n v o l v e s t h e s t o r a g e o f b l o o d p l a t e l e t s (20). Blood p l a t e l e t s a r e l i v i n g c e l l s t h a t b o t h consume oxygen t o l i v e and g e n e r a t e c a r b o n d i o x i d e as a m e t a b o l i c b y p r o d u c t . The g e n e r a t i o n o f c a r b o n d i o x i d e p r e s e n t s problems t o v i a b i l i t y , s i n c e i t tends t o cause u n d e s i r a b l e changes i n t h e n a t u r a l pH u n l e s s i t c a n e s c a p e . An added r e q u i r e m e n t e n t e r s because t h e aqueous s o l u t i o n c o n t a i n i n g the p l a t e l e t s must n o t l o s e a s i g n i f i c a n t amount o f water by permeation. T h i s c a s e , t h e r e f o r e , i l l u s t r a t e s t h e need f o r an advanced " c o n t r o l l e d atmosphere" package, o r "smart package" t h a t i s a b l e t o a l l o w r e l a t i v e l y f r e e exchange o f oxygen and c a r b o n d i o x i d e w i t h t h e e x t e r n a l environment w h i l e e s s e n t i a l l y p r e v e n t i n g outward p e r m e a t i o n l o s s e s o f water. Similar considerations apply

In Barrier Polymers and Structures; Koros, W.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

1.

KOROS

3

Overview

Table I: Permeation p r o t e c t i o n r e q u i r e d f o r various one y e a r s h e l f l i f e a t 25°C f

Food or Beverage Canned milk, meats

Estimated

Estimated

oxyge gain, ppm

wL percent

f o o d s and b e v e r a g e s f o r a

High oil

High volatile

yes

-

-3

yes

yes

-3

-

yes

lto5

+2

yes

yes

lto5

-3

lto5

Baby foods

lto5

Beer, ale, wine

ltt>5*

Instant coffee Canned vegetables, soups, spaghetti

-3

-

Canned fruits

5 to 15

-3

Nuts, snacks

5 to 15

+5

yes

Dried foods

5 to 15

+1

-

-

Fruit juices, drinks

10 to 40

-3

-

yes

yes -

Carbonated soft drinks

10 to 40*

-3

-

yes

Oils,shortenings

50to200

+10

yes

-

Salad dressings

50 to 200

+10

yes

yes

Jams, pickles, vinegars

50 to 200

-3

-

yes

Liquors

50to200

-3

-

yes

50to200

+10

yes

-

Peanut butter

* Less than 20% loss of CC>2 is also required, t (Data taken from ref. 19.)

In Barrier Polymers and Structures; Koros, W.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

4

BARRIER POLYMERS AND STRUCTURES

to

the

packaging

permeation of

of

water

Modern The

fruits

and

dioxide

vegetables

and

that

respiration

require

inward

oxygen

without

of

loss

(21).

contents

packages

and

first

dates

of

carbon

the

good

back

to

membranes

description 1831

under

describe

the

the

equivalent

=

a process

the

noted

carbon

conditions. of

contact

known

permeation (22)

of

process

coefficient

Pi

of

passage

permeation

permeability

regulate the

by

when M i t c h e l l

allowed

hydrogen

typically

environment

in

polymers

natural faster

Eq(l)

rubber

than

Mathematically, of

the

permeation.

process that

dioxide

i n terms

component

between

as

one

using

can

a

i , P-^,

(steady s t a t e f l u x of i i

(D

A /C Pi

The

permeability is defined

permeation

rate

of

normalized

partial

across

membrane

the

Accurate

description

and

pressure

partial

illustrated partial (15).

of

depending

penetrant

low

and

F i g . 2b

are

most fixed

is typical

penetrant

polypropylene permeability activity present

in

The

rubbery

a

containers are vapor

flavor fruit

like

or or

packing and

temperature

At

pressure

show

response

such

the

response

for a

more

and

upwardly

as

increases

inflecting

penetrant

for strongly

as

response

(24-27).

of

the

response

polymer

d-limonene

i n F i g . 2c general,

depends

and

the

which

(28)

due

to

under

to

their the

i s found

the

are

upon

for

magnitude the

critical

is related

in glasses 2

In

pressure

of

i n Chapter

sufficiently the

permeability

onset seen

acetone-ethyl systems.

measurements that

to

Fig.2a

the

of

glass

temperature

intersegmental hindered

heading

of

mobilities, "dual

mode"

theory.

polymers,

other

type

defects present

i s discussed

sorption

the

This

while

juices.

d e c l i n e i n permeability with gas.

layer shown

Polyethylene

i n F i g . 2b

phase

barrier

behaviors

permeability in

polymers,

plasticizing

components

i n g l a s s y polymers

transition

and the

the

each

As

penetrant

with

constant

expected

barrier composition

f u n c t i o n of

pressures,

polymer.

that

liquid aroma

p e r m e a b i l i t y vs

gases

simple

rubbery

responses

in a

interacting

The

in

a

i n contact

common.

the

permeabilities in the

be

intermediate

gases of

in a

complex

structure.

upon

the

in

the

barrier

at

f o r many

state

(23,24) .

and

penetrant

i n the

of

seen

a

dependence (a-d),

films

pressure

F i g . 2a-c

steady

i n f o r m a t i o n about

p e r m e a b i l i t y may

gases

the

F i g . 1)

the

For

soluble

barrier

requires

in Fig. 2

is

of

of

(see

considered,

in

the

course,

constituent materials

polymer

most

thickness, t

of

of

the

of

pressur

structures, of

i n terms

componen

seen

In

high

of

i n F i g . 2d. cellulose fact,

i n F i g . 2c

i n F i g . 2d

combination

plasticization

of

penetrant

the

While

system,

this

extends psia,

dual

mode

the

an

(29). at

in

i s shown

typical

of

range

for

many of

the

in permeability

response

i n F i g . 2c

high

in glassy

upturn

pressure

upturn This

response

i n F i g . 2b

the

behavior

i t i s also

i f one 900

pressures

produces

to

i s observed

response

partial

plasticization

sorbed

and

like

i s , therefore, the

concentrations.

In Barrier Polymers and Structures; Koros, W.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

1.

KOROS

Overview

5

p. = [moles of i/area] Δρ|/^ flux of i = - D i d C |

=

[flux of i]

Definition of permeability

Δ ρ | / ^

of component!

Fick's Law of diffusion

dx Sj = C j / p j

Pi =

Dj S j

Solubility coefficient

Permeability coefficient in terms of diffusion and solubility coefficients

Figure 1: Summary of solution-diffusion model relationships. Typically the diffusion coefficient, Dj, decreases as the penetrant molecular weight increases. On the other hand, the solubility coefficient, S|, tends to increase with increasing penetrant molecular weight. As a result, the permeability, Pj, may either decrease or increase with increasing penetrant molecular weight, depending upon which factor dominates.

In Barrier Polymers and Structures; Koros, W.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

6

BARRIER POLYMERS AND STRUCTURES

100

200 psia CO,

— U 2000 300 0

β C Η Ο

cm Hg

3 β

Figure 2: The pressure dependency of various penetrant-polymer systems: (a) 30°C, H /polyethylene, (b) 20°C propane/polyethylene, (c) 35°C, carbon dioxide/polycarbonate, (d) 40°C, acetone/ethyl cellulose. (Reprinted with permissionfromref. 15. Copyright 1987 John Wiley and Sons.) 2

In Barrier Polymers and Structures; Koros, W.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

12

1.

KOROS

Overview

7

Graham, who was one o f t h e f i r s t t o c o n s i d e r t h e p e r m e a b i l i t i e s o f n a t u r a l rubber f i l m s t o a wide range o f gases, found r e s p o n s e s such as t h a t seen i n F i g . 2a. The d e s c r i p t i o n he f o r m u l a t e d i n 1866 o f t h e s o - c a l l e d " s o l u t i o n - d i f f u s i o n " mechanism s t i l l p r e v a i l s t o d a y (30). He p o s t u l a t e d t h a t a p e n e t r a n t l e a v e s t h e e x t e r n a l phase i n c o n t a c t w i t h t h e membrane by d i s s o l v i n g i n the u p s t r e a m f a c e o f t h e f i l m and t h e n undergoes m o l e c u l a r d i f f u s i o n t o t h e downstream f a c e where i t e v a p o r a t e s i n t o t h e e x t e r n a l phase again. M a t h e m a t i c a l l y , one can s t a t e t h e s o l u t i o n - d i f f u s i o n model in terms o f p e r m e a b i l i t y , s o l u b i l i t y and d i f f u s i v i t y c o e f f i c i e n t s , as shown i n E q ( 2 ) . Ρ =

D S

(2)

where t h e d i f f u s i o n . c o e f f i c i e n t , D, c h a r a c t e r i z e s t h e average a b i l i t y o f t h e d i s s o l v e d p e n e t r a n t t o move among t h e polymer segments c o m p r i s i n g t h e f i l m Th solubilit coefficient S i thermodynamic i n n a t u r e an e q u i l i b r i u m s o r p t i o n isother l a r g e v a l u e o f S i m p l i e s a l a r g e tendency f o r t h e p e n e t r a n t t o d i s s o l v e , o r " s o r b " , i n t o t h e polymer. In t h e more complex c a s e s shown i n F i g . 2c-d, t h e s o l u t i o n - d i f f u s i o n mechanism s t i l l a p p l i e s ; however, t h e D and S p a r a m e t e r s can be s t r o n g f u n c t i o n s o f t h e penetrant a c t i v i t i e s or p a r t i a l pressures i n contact with the b a r r i e r a t i t s two s u r f a c e s . gas

Barrier

Materials

I f one i s u s i n g a b a r r i e r t o m i n i m i z e outward d i f f u s i o n o f a component, e.g. C O 2 the downstream c o n d i t i o n "1" i n F i g . 1 would r

c o r r e s p o n d t o t h e e x t e r n a l atmosphere. On t h e o t h e r hand, f o r m i n i m i z a t i o n o f inward d i f f u s i o n , e.g. O 2 b a r r i e r s , t h e downstream c o n d i t i o n "1" c o r r e s p o n d s t o t h e i n t e r n a l package c o n t e n t s . In e i t h e r c a s e , t h e p e n e t r a n t c o n c e n t r a t i o n i s t y p i c a l l y v e r y low a t t h e downstream f a c e . F o r such c a s e s where - 0 , one can s i m p l y r e p r e s e n t S as t h e s e c a n t s l o p e o f t h e e q u i l i b r i u m s o r p t i o n i s o t h e r m f o r t h e p a r t i c u l a r p o l y m e r - p e n e t r a n t p a i r (31). C l e a r l y , i n s e e k i n g a good b a r r i e r m a t e r i a l f o r e x c l u d i n g a g i v e n p e n e t r a n t , i t i s d e s i r a b l e t o m i n i m i z e b o t h D and S as much as p o s s i b l e w i t h o u t s a c r i f i c i n g c o s t and p r o c e s s i n g e a s e . Extensive s t u d i e s o f s y n t h e t i c r u b b e r y m a t e r i a l s were p e r f o r m e d t o i d e n t i f y p o s s i b l e replacements f o r n a t u r a l rubber(3,32-35). The development of b u t y l and n i t r i l e r u b b e r s w i t h both adequate r e s i l i e n c e and g r e a t l y r e d u c e d a i r and even h y d r o c a r b o n p e r m e a b i l i t y f o r t i r e s and g a s k e t a p p l i c a t i o n s was an e a r l y s u c c e s s o f t h e s e s t r u c t u r e permeability studies. Such m a t e r i a l s , however, a r e not a p p r o p r i a t e f o r p a c k a g i n g f i l m s , and r e g e n e r a t e d c e l l u l o s e o r c e l l o p h a n e was t h e f i r s t p r a c t i c a l g l a s s y f i l m t o f i n d l a r g e s c a l e use i n p a c k a g i n g (36). C e l l o p h a n e i s an e x c e l l e n t gas b a r r i e r i n t h e d r y s t a t e ; however, i t i s s e n s i t i v e t o m o i s t u r e and l o s e s b a r r i e r c a p a b i l i t y a t h i g h r e l a t i v e h u m i d i t i e s (4,23). Moreover, i t i s not t h e r m o p l a s t i c o r heat s e a l a b l e , so r e p l a c e m e n t s were c l e a r l y d e s i r a b l e .

In Barrier Polymers and Structures; Koros, W.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

BARRIER POLYMERS AND STRUCTURES

8

P o l y v i n y l c h l o r i d e (PVC) i s an e c o n o m i c a l t h e r m o p l a s t i c f i l m w i t h good b a r r i e r p r o p e r t i e s t o gases and water. Later developments o f even h i g h e r b a r r i e r S a r a n ® t y p e r e s i n s based on c o p o l y m e r s o f v i n y l i d e n e c h l o r i d e and v i n y l c h l o r i d e p r o d u c e d many new p a c k a g i n g o p p o r t u n i t i e s . These m a t e r i a l s a r e s t i l l v e r y p o p u l a r t o d a y (37-39) . C o n s i d e r a t i o n o f the s t r u c t u r e - p e r m e a b i l i t y r e l a t i o n s h i p s f o r t h e Saran m a t e r i a l s i n C h a p t e r 6(40) i n d i c a t e s t h a t t h e i r e v o l u t i o n i s s t i l l c o n t i n u i n g as b e t t e r u n d e r s t a n d i n g of t h e i n f l u e n c e o f sequence d i s t r i b u t i o n and even more s u b t l e i s s u e s o f c h a i n m i c r o s t r u c t u r e emerge. Concerns about h e a l t h e f f e c t s c a u s e d by low l e v e l s of r e s i d u a l monomers i n PVC and a c r y l o n i t r i l e r e s i n s promoted t h e c o n s i d e r a t i o n o f c o n d e n s a t i o n polymers such as p o l y ( e t h y l e n e t e r e p h t h a l a t e ) (PET) (41). P a c k a g i n g r e s i n s b a s e d on a c r y l o n i t r i l e a r e e x t r a o r d i n a r i l y good b a r r i e r s t h a t were c o n s i d e r e d f o r c a r b o n a t e d b e v e r a g e b o t t l e s . The f u r o r o v e r r e s i d u a l monomers i n t h e s e m a t e r i a l s s t u n t e d t h e i r growth i n s p i t e o f improve detectable levels. The s t e t e r e p h t h a l a t e ( P E T ) has i n t r i n s i c a l l the as-made polymer, and i t has become t h e m a t e r i a l o f c h o i c e f o r the c a r b o n a t e d b e v e r a g e market (42). E t h y l e n e - v i n y l a l c o h o l (EVOH) copolymers were d i s c o v e r e d t o be e x c e l l e n t oxygen b a r r i e r s and d i d not r e q u i r e d e a l i n g w i t h dangerous monomers, so t h e y became p o p u l a r soon a f t e r t h e i r i n t r o d u c t i o n i n t h e US i n t h e mid 1970's (43). These polymers combine a s t r o n g hydrogen bonded amorphous phase l i k e t h a t o f c e l l o p h a n e w i t h a p a r t i a l l y c r y s t a l l i n e nature. While t h e s e m a t e r i a l s have i n c r e d i b l y low p e r m e a b i l i t i e s t o oxygen i n the d r y s t a t e , l i k e c e l l o p h a n e , t h e y l o s e t h e i r b a r r i e r c a p a b i l i t y at h i g h r e l a t i v e h u m i d i t i e s (43). C h a p t e r s 8-11(44-47) d i s c u s s a p p r o a c h e s t o e n g i n e e r around t h i s s e n s i t i v i t y t o h u m i d i t y t h r o u g h t h e use o f a c o e x t r u d e d l a m i n a t e s t r u c t u r e . While t h i s approach m i n i m i z e s d i r e c t c o n t a c t o f the b a r r i e r l a y e r w i t h h i g h water a c t i v i t i e s , d u r i n g steam r e t o r t i n g f o r s t e r i l i z a t i o n , complex measures a r e needed t o a v o i d s e r i o u s l o n g t e r m d e g r a d a t i o n o f t h e b a r r i e r (48, 49) . As n o t e d i n t h e c a s e o f the o l d e r Saran r e s i n s , much remains t o be l e a r n e d r e g a r d i n g t h e e f f e c t s o f c h a i n m i c r o s t r u c t u r e on u l t i m a t e f i l m p r o p e r t i e s f o r the EVOH f a m i l y o f p o l y m e r s . D i r e c t i o n s i n t h e S e a r c h f o r New

Barrier Materials

The s e a r c h f o r b a s i c p r i n c i p l e s t o g u i d e the development o f more e f f e c t i v e b a r r i e r m a t e r i a l s based on condensation p o l y m e r i z a t i o n i s ongoing. Advanced p o l y e s t e r s such as p o l y ( e t h y l e n e 2,6-naphthalene d i c a r b o x y l a t e ) or PEN have been r e p o r t e d t o have as much as 5 t i m e s lower p e r m e a b i l i t y t h a n c o n v e n t i o n a l PET; however, p r o p r i e t a r y c o n s i d e r a t i o n s have p r e v e n t e d the p u b l i c a t i o n o f complete d e t a i l s c o n c e r n i n g such m a t e r i a l s . The c h a p t e r s d e v o t e d t o s t r u c t u r e - p e r m e a b i l i t y p r o p e r t i e s o f the more c o m m e r c i a l l y advanced amorphous n y l o n s (Chapter 5 (50)) and experimental polycarbonates (Chapter 7 (51)) i l l u s t r a t e many of the g e n e r a l p r i n c i p l e s needed f o r o p t i m i z i n g t h e b a r r i e r p r o p e r t i e s o f e s s e n t i a l l y any polymer f a m i l y . These c h a p t e r s c o n s i d e r the i m p o r t a n c e o f i n t e r s e g m e n t a l p a c k i n g and i n t r a s e g m e n t a l m o b i l i t y on

In Barrier Polymers and Structures; Koros, W.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

1.

KOROS

Overview

9

t h e a b i l i t y o f a s m a l l m o l e c u l e l i k e oxygen t o e x e c u t e i t s t h e r m a l l y a c t i v a t e d d i f f u s i v e motions t h r o u g h g l a s s y e n v i r o n m e n t s t y p i c a l o f high b a r r i e r polymers. The h i g h b a r r i e r amorphous n y l o n r e s i n s d i s c u s s e d i n C h a p t e r 5 (50) have t h e d e s i r a b l e p r o p e r t y o f becoming b e t t e r , r a t h e r t h a n worse oxygen b a r r i e r s as t h e h u m i d i t y i n c r e a s e s . The g l a s s y polymers such as t h e a r o m a t i c p o l y a m i d e s and p o l y c a r b o n a t e s have s i g n i f i c a n t h i n d r a n c e s t o i n t r a m o l e c u l a r mobility. The d a t a f o r t h e s e m a t e r i a l s appear t o be c o r r e l a t e d f a i r l y w e l l i n terms o f t h e " s p e c i f i c f r e e volume" d i s c u s s e d by Lee (52). S t r u c t u r a l v a r i a t i o n s t h a t s u p p r e s s t h e a b i l i t y t o pack t e n d t o reduce t h e q u a l i t y o f t h e b a r r i e r w h i l e t h o s e t h a t improve t h e a b i l i t y t o pack produce b e t t e r b a r r i e r s . The f r e e volume i n t h i s case i s d e f i n e d as t h e d i f f e r e n c e between t h e a c t u a l polymer molar volume a t t h e t e m p e r a t u r e o f t h e system and a t 0°K. T h i s l a t t e r parameter i s d e t e r m i n e d by group c o n t r i b u t i o n methods. The above approach lumps a l l volume t o g e t h e r , and f o r g l a s s y materials this oversimplificatio s c a t t e r i n such c o r r e l a t i o n s p a c k i n g a r e thought t o c o n t r i b u t apparen , b e s i d e s t h a t which a r i s e s from segmental and subsegmental o s c i l l a t i o n s and v i b r a t i o n s ( 2 5 ) . T h i s l o c a l i z e d u n r e l a x e d volume due t o p a c k i n g d e f e c t s i s n o t as l i k e l y t o c o n t r i b u t e t o d i f f u s i o n as i s t h e g e n e r a l l y d i s t r i b u t e d f r e e volume which i s c o n t r i b u t e d by segmental motions (25) . T h e r e f o r e , w h i l e lumping t h e two c o n t r i b u t i o n s t o g e t h e r s i m p l i f i e s b r o a d comparisons between d i f f e r e n t m a t e r i a l s , i t would be i n t e r e s t i n g , b u t t e d i o u s t o base comparisons on s p e c i f i c f r e e volumes t h a t have been a d j u s t e d f o r t h e c o n t r i b u t i o n o f t h e p a c k i n g d e f e c t s t o see i f s c a t t e r i n t h e c o r r e l a t i o n s i s r e d u c e d . F o r t u n a t e l y such c o r r e c t i o n s a r e o f l e s s importance f o r m a t e r i a l s such as t h e p o l y a m i d e s c o n s i d e r e d i n C h a p t e r 5 w i t h s i m i l a r g l a s s t r a n s i t i o n t e m p e r a t u r e s , so s c a t t e r i s r a t h e r s m a l l even w i t h o u t t h e i r c o n s i d e r a t i o n (50). S e v e r a l d e t a i l e d analyses of the d i f f u s i o n process i n both r u b b e r y polymers and i n h i n d e r e d g l a s s e s a r e o f f e r e d i n C h a p t e r 2 ( 2 8 ) . Approximate m o l e c u l a r i n t e r p r e t a t i o n s have been o f f e r e d f o r t h e parameters i n t h e s e models ( 2 5 ) . N e v e r t h e l e s s , more work i s needed t o v e r i f y any m o l e c u l a r s c a l e c o n n e c t i o n between such p a r a m e t e r s and t h e s t r u c t u r e s and motions o f t h e polymer backbone. S p e c t r o s c o p y and m o l e c u l a r m o d e l i n g o f t h e d i f f e r e n c e s i n segmental motions i n a s y s t e m a t i c a l l y v a r i e d f a m i l y o f polymers, e.g, t h e p o l y e s t e r s , o r p o l y a m i d e s , can o f f e r i n s i g h t i n some cases. U n f o r t u n a t e l y , t h e e x a c t segmental motions i n v o l v e d i n t h e d i f f u s i v e process are only p a r t i a l l y understood, so one must be c a u t i o u s about drawing c o n c l u s i o n s based on such s t u d i e s u n l e s s t h e y a r e s u p p o r t e d by a c t u a l complementary t r a n s p o r t d a t a . Hopefully t h e s t r u c t u r e - p r o p e r t y r e s u l t s p r e s e n t e d i n t h i s book w i l l f u r t h e r s t i m u l a t e t h i n k i n g t o improve t h e c o n n e c t i o n between s p e c t r o s c o p i c a l l y sensed motions, and d i f f u s i o n t o complement t h e c o r r e l a t i o n s b a s e d on s p e c i f i c f r e e volume i n C h a p t e r s 5 & 7 (50,51). The i m p o r t a n c e o f segmental motions i n t h e d i f f u s i o n p r o c e s s i s r e f l e c t e d by t h e A r r h e n i u s e x p r e s s i o n f o r t h e d i f f u s i o n c o e f f i c i e n t g i v e n by Eq(3) (23,24). D = D exp[ -ED/RT] 0

(3)

In Barrier Polymers and Structures; Koros, W.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

10

BARRIER POLYMERS AND STRUCTURES

The m a j o r i t y o f t h e a c t i v a t i o n energy f o r e x e c u t i o n o f a d i f f u s i o n jump i s used t o produce a t r a n s i e n t gap o f s u f f i c i e n t s i z e between s u r r o u n d i n g segments t o a l l o w movement o f t h e p e n e t r a n t o v e r t h e l e n g t h o f one d i f f u s i o n a l s t e p , λ. As shown i n F i g . (3a), a good c o r r e l a t i o n e x i s t s between t h e a c t i v a t i o n energy and t h e p r e e x p o n e n t i a l f a c t o r i n Eq(3) f o r e l a s t o m e r s . This correlation i s l a r g e l y independent o f t h e t y p e o f r u b b e r and gas t y p e ( 3 ) . The t r i a n g l e p o i n t s t h a t have been added t o t h e p l o t a r e d a t a f o r r u b b e r y s e m i c r y s t a l l i n e PET, and t h e s e p o i n t s appear t o f i t t h e c u r v e w e l l i n s p i t e o f t h e f a c t t h a t PET i s not an e l a s t o m e r l i k e t h e o t h e r m a t e r i a l s on t h e p l o t . F i g u r e (3b) compares s i m i l a r d a t a f o r p r e e x p o n e n t i a l f a c t o r s and a c t i v a t i o n e n e r g i e s f o r t h r e e g l a s s y p o l y m e r s : PET, b i s p h e n o l - A p o l y c a r b o n a t e ( P C ) and t e t r a m e t h y l b i s p h e n o l - A polycarbonate(TMPC). TMPC has a Tg o v e r 50°C h i g h e r than t h a t o f PC and almost 120°C above t h a t o f PET. Moreover, t h e sub-Tg t r a n s i t i o n o f TMPC i s o v e r 150°C h i g h e r t h a n t h a t o highly hindered. In s p i t polymers, a l l t h r e e g l a s s single correlation line. I f one c o n s i d e r e d polymers w i t h lower Tg's so t h e d i f f e r e n c e between t h e measured t e m p e r a t u r e and Tg was s m a l l , t h e s c a t t e r i n t h e g l a s s y s t a t e c o r r e l a t i o n would p r o b a b l y i n c r e a s e , r e f l e c t i n g the t r a n s i t i o n r e g i o n between t h e two s t a t e s (23) . I t appears t h a t t h e s l o p e o f t h e c o r r e l a t i o n l i n e f o r t h e g l a s s y m a t e r i a l s i s v e r y s i m i l a r t o t h a t f o r the r u b b e r y m a t e r i a l s , but t h e r e i s an o f f s e t i n t h e v a l u e o f D by r o u g h l y two o r d e r s of magnitude r e l a t i v e t o r u b b e r y m a t e r i a l s h a v i n g s i m i l a r a c t i v a t i o n energies. The f a c t t h a t r u b b e r y PET f o l l o w s t h e t o p l i n e , w h i l e g l a s s y PET f o l l o w s t h e bottom one i s i m p r e s s i v e e v i d e n c e f o r a change i n some a s p e c t o f t h e t r a n s p o r t p r o c e s s i n t h e g l a s s transition interval. The p r e e x p o n e n t i a l f a c t o r can be w r i t t e n i n terms o f t h e a c t i v a t e d s t a t e t h e o r y t o g i v e : Q

D

2

0

= κ λ kT/h exp( ASrj/R)

(4)

The Κ f a c t o r i s e s s e n t i a l l y e q u a l t o 1/6 f o r an i s o t r o p i c medium, and i t s h o u l d be t h e same f o r b o t h r u b b e r y and g l a s s y polymers. The lower v a l u e s o f D i n the g l a s s y s t a t e , t h e r e f o r e , must be due t o e i t h e r a l a r g e r e d u c t i o n i n t h e jump l e n g t h , o r t o a s i g n i f i c a n t l y lower e n t r o p y o f a c t i v i a t i o n a s s o c i a t e d w i t h f o r m a t i o n o f t h e a c t i v a t e d s t a t e i n t h e g l a s s y m a t e r i a l as compared t o t h a t i n the rubbery s t a t e . Most m o l e c u l a r v i s u a l i z a t i o n s o f t h e d i f f u s i o n p r o c e s s s u g g e s t t h a t a l t h o u g h t h e jump l e n g t h may be somewhat s m a l l e r i n g l a s s e s as compared t o r u b b e r s , t h e y a r e p r o b a b l y on the o r d e r o f one t o t h r e e c o l l i s i o n d i a m e t e r s o f t h e p e n e t r a n t i n b o t h media (53). T h e r e f o r e , t h e l a r g e d i f f e r e n c e i n t h e p r e e x p o n e n t i a l f a c t o r s i n t h e g l a s s y and r u b b e r y s t a t e s i s l i k e l y t o be due t o s i g n i f i c a n t l y lower e n t r o p i e s o f a c t i v a t i o n i n t h e g l a s s as compared t o t h e r u b b e r y s t a t e . M o l e c u l a r m o d e l i n g c a l c u l a t i o n s may one day be able to e x p l a i n systematic differences i n D for structurallyr e l a t e d m a t e r i a l s t o p e r m i t m o l e c u l a r d e s i g n of improved b a r r i e r s . A c h i e v i n g t h i s g o a l i n t h e near f u t u r e i s u n l i k e l y , so s y s t e m a t i c e x p e r i m e n t a l s t u d i e s c o u p l e d w i t h f u n d a m e n t a l l y based c o r r e l a t i o n s Q

0

In Barrier Polymers and Structures; Koros, W.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

KOROS

Overview

4.0

-3.0

I

I

10

I 20

I

I 30

I

I 40

I

I 50

EjyT (cal/mol°K)

Figure 3a: Illustration of the correlation between the preexponential factor in Eq(3) and the activation energy divided by the absolute temperature at the midpoint of the temperature range over which it was evaluated for a large number of penetrants in different elastomers. Note that all points are relatively well correlated by a single line (3). The triangle points are for He (a), ^(b), C0 (c), and CH (d) in rubbery PET (23). 2

4

In Barrier Polymers and Structures; Koros, W.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

12

BARRIER POLYMERS AND STRUCTURES

E j / T (cal/mol°K)

Figure 3b: Comparison of the correlation between the preexponential factor in Eq(3) and the activation energy divided by the absolute temperature at the midpoint of the temperature range over which it was evaluated for a large number of penetrants in different glassy and rubbery polymers. The open triangle points are the same as in Fig. 3a. The closed triangle points are for He (e), 0 (f), C0 (g), N (h) and C H (i) in glassy PET (23). The open diamonds are for H (j), Ar(k), 0 (1), C0 (m) and Kr (n) in glassy PC (23). The closed diamonds are for N (o) and 0 (p) in glassy TMPC (26). 2

2

2

2

4

2

2

2

2

In Barrier Polymers and Structures; Koros, W.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

1.

KOROS

Overview

13

l i k e t h o s e d i s c u s s e d i n C h a p t e r s 5 & 7 p r o v i d e t h e most e f f i c i e n t means f o r d i s c o v e r i n g new h i g h performance m a t e r i a l s (50,51) . Complex B a r r i e r Responses The above d i s c u s s i o n s o f t h e d i f f u s i o n c o e f f i c i e n t a r e a b i t o v e r l y s i m p l i f i e d , s i n c e b o t h t h e d i f f u s i v i t y and t h e s o l u b i l i t y c o e f f i c i e n t s i n Eq(2) may v a r y w i t h p e n e t r a n t a c t i v i t y a t a f i x e d t e m p e r a t u r e (15, 26, 54-56). Indeed, as s u g g e s t e d by t h e v a r i e t y o f p e r m e a b i l i t y r e s p o n s e s shown i n F i g . 3, D and S can be f u n c t i o n s o f t h e s t a t e o f t h e polymer and t h e a c t i v i t y o f t h e p e n e t r a n t as w e l l as t h e t e m p e r a t u r e . C h a p t e r 2(28) d i s c u s s e s the d i f f e r e n c e s between r u b b e r y and g l a s s y polymers and s u r v e y s t h e t y p e s o f t h e o r e t i c a l analyses that e x i s t f o r gas s o r p t i o n and t r a n s p o r t processes. Besides reviewing c u r r e n t l y accepted analyses of these two s t a t e s o f amorphous m a t e r i a l s , i n c l u d i n g t h e well-known d u a l mode s o r p t i o n model, t h i chapte suggest th d fo m o l e c u l a r - s c a l e modeling 6(40) where p r e l i m i n a r b a r r i e r Saran® m a t e r i a l s are c o n s i d e r e d . C h a p t e r 3(57) d i s c u s s e s t h e r e d u c e d p e r m e a b i l i t y commonly o b s e r v e d when c r y s t a l l i n i t y i s added t o e i t h e r a r u b b e r y o r g l a s s y matrix. The r e d u c t i o n i n p e r m e a b i l i t y r e s u l t s from r e d u c t i o n s i n both the s o l u b i l i t y and d i f f u s i v i t y p a r a m e t e r s (58-62) . This c h a p t e r a l s o r e v i e w s t h e i m p o r t a n t e f f e c t s o f combined o r i e n t a t i o n and c r y s t a l l i n i t y which a r e dependent on whether o r i e n t a t i o n o c c u r s d u r i n g t h e f o r m a t i o n o f t h e b a r r i e r , o r subsequent t o i t i n a c o l d drawing p r o c e s s (62-66). The e f f e c t s o f p r o c e s s - i n d u c e d o r i e n t a t i o n o f s e v e r a l commercial b a r r i e r r e s i n s i s c o n s i d e r e d i n f u r t h e r d e t a i l i n C h a p t e r 12 (67) . B e s i d e s t h e above c o n v e n t i o n a l e f f e c t s , C h a p t e r 3 summarizes d a t a s u g g e s t i n g t h e a b i l i t y o f some gases t o s o r b and d i f f u s e i n s i d e t h e a c t u a l c r y s t a l s o f p o l y ( 4 - m e t h y l - l - p e n t e n e ) (68,69). Finally, C h a p t e r 3 c o n s i d e r s l i q u i d c r y s t a l l i n e polymers, which seem t o form a new c l a s s o f m a t e r i a l s i n terms o f b a r r i e r r e s p o n s e s (57) . The h i g h b a r r i e r n a t u r e o f l i q u i d c r y s t a l polymers appears t o be l a r g e l y due t o t h e i r u n u s u a l l y low s o l u b i l i t y c o e f f i c i e n t s f o r t y p i c a l penetrants. T h i s i s q u i t e d i f f e r e n t from t h e c a s e f o r most h i g h b a r r i e r s l i k e EVOH, and p o l y a c r y l o n i t r i l e t h a t t y p i c a l l y f u n c t i o n due t o t h e u n u s u a l l y low m o b i l i t i e s o f p e n e t r a n t s i n t h e i r m a t r i c e s (70) . C h a p t e r 4 (71) f o c u s e s on t h e c h a r a c t e r i z a t i o n o f s o r p t i o n k i n e t i c s i n s e v e r a l g l a s s y polymers f o r a b r o a d s p e c t r u m o f p e n e t r a n t s r a n g i n g from t h e f i x e d gases t o organic vapors. The s o r p t i o n k i n e t i c s and e q u i l i b r i a o f t h e s e d i v e r s e p e n e t r a n t s a r e r a t i o n a l i z e d i n terms o f t h e p o l y m e r - p e n e t r a n t i n t e r a c t i o n parameter and t h e e f f e c t i v e g l a s s t r a n s i t i o n o f t h e polymer r e l a t i v e t o t h e t e m p e r a t u r e o f measurement. The k i n e t i c r e s p o n s e i s shown t o t r a n s i t i o n s y s t e m a t i c a l l y from c o n c e n t r a t i o n independent diffusion, to c o n c e n t r a t i o n dependent d i f f u s i o n , and f i n a l l y t o complex nonFickian responses. The n o n F i c k i a n b e h a v i o r i n v o l v e s s o - c a l l e d "Case I I " and o t h e r anomalous s i t u a t i o n s i n which a c o u p l i n g e x i s t s between t h e d i f f u s i o n p r o c e s s and m e c h a n i c a l p r o p e r t y r e l a x a t i o n s i n t h e polymer t h a t a r e i n d u c e d by the i n v a s i o n of t h e p e n e t r a n t (72-78) .

In Barrier Polymers and Structures; Koros, W.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

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Gomgiex B a r r i e r

Structures

As shown i n F i g . 4, one can use a v a r i e t y of b a r r i e r s t r u c t u r e s b e s i d e s t h a t o f the s i m p l e m o n o l i t h i c c o n t a i n e r o r f i l m t o c o n t r o l t h e exchange between the package and t h e e x t e r n a l environment. Cases b-d i n v o l v e t h e a d d i t i o n o f p e r m e a t i o n r e s i s t a n c e s i n s e r i e s , w h i l e c a s e s e & f i n v o l v e t h e i n t r o d u c t i o n of an e f f e c t i v e l y l o n g e r p e r m e a t i o n p a t h l e n g t h by i n t e r p o s i n g o b j e c t s with l a r g e aspect r a t i o s i n the b a r r i e r . B a r r e r (79) has p r o v i d e d a comprehensive a n a l y s i s o f b o t h t h e l a m i n a t e and f i l l e d systems, i n c l u d i n g the p o s s i b i l i t y of f i n i t e p e r m e a b i l i t y through the d i s p e r s e d phase as i n F i g . 4 f . He g i v e s e x p r e s s i o n s f o r the s t e a d y s t a t e p e r m e a b i l i t y and t r a n s i e n t time l a g a s s o c i a t e d w i t h t h e a p p r o a c h t o s t e a d y s t a t e p e r m e a t i o n f o r the case of c o n s t a n t d i f f u s i v i t i e s and s o l u b i l i t y c o e f f i c i e n t s . Examples of such b a r r i e r s t r u c t u r e s are d i s c u s s e d i n t h e f o l l o w i n g c h a p t e r s . I t was n o t e d e a r l i e oxygen b a r r i e r i n the d r transformations i n i t s b a r r i e propertie h u m i d i t y i s i n c r e a s e d (44-46). The most s i g n i f i c a n t of t h e s e changes are r e f e r r e d t o g e n e r a l l y as " r e t o r t shock" and r e f l e c t s i n t e r a c t i o n s o f water w i t h hydrogen bonds r e s p o n s i b l e f o r t h e e x c e l l e n t oxygen b a r r i e r o f t h i s r e s i n under d r y c o n d i t i o n s . Under h i g h t e m p e r a t u r e r e t o r t c o n d i t i o n s used f o r s t e r i l i z a t i o n of food, c a t a s t r o p h i c changes can be wrought i n the b a r r i e r p r o p e r t i e s of the b a r r i e r l a y e r o f l a m i n a t e f i l m s (48). T h i s phenomenon, d i s c u s s e d i n C h a p t e r s 8-11 i s a f f e c t e d by the degree of c r y s t a l l i n i t y , the e x t e n t o f o r i e n t a t i o n and t h e time and t e m p e r a t u r e o f the r e t o r t process(44-47). Severe d i s r u p t i o n s i n t h e amorphous phase hydrogen bond network and even some of t h e s m a l l , l e s s p e r f e c t c r y s t a l l i n e domains are b e l i e v e d t o o c c u r i n extreme r e t o r t shock c a s e s . U n m i t i g a t e d , t h i s e f f e c t has s e r i o u s r e s u l t s , amounting t o r o u g h l y a 300% l o s s i n oxygen b a r r i e r e f f i c a c y even a y e a r a f t e r t h e r e t o r t process. In C h a p t e r 9, m o d i f i c a t i o n of the l a m i n a t e c o n s t r u c t i o n ( F i g . 4b), i n c l u d i n g the i n c o r p o r a t i o n of d e s i c a n t s i n some c a s e s , i s shown t o p r o v i d e h o p e f u l approaches t o m i t i g a t i n g r e t o r t shock(45). The use of o t h e r m a t e r i a l s as t h e c e n t r a l l a y e r of the b a r r i e r l a m i n a t e i s , o f c o u r s e , f e a s i b l e . Obvious c a n d i d a t e s f o r t h i s a p p l i c a t i o n i n c l u d e the h i g h b a r r i e r amorphous p o l y a m i d e s (Chapter 5 (50)) and t h e l i q u i d c r y s t a l l i n e p o l y e s t e r s (Chapter 3(57)) which e i t h e r d e v e l o p s l i g h t l y improved b a r r i e r s under e l e v a t e d r e l a t i v e h u m i d i t y c o n d i t i o n s or at l e a s t do not l o s e b a r r i e r p r o p e r t i e s . No r e p o r t s are y e t a v a i l a b l e c o n c e r n i n g t h e p e r f o r m a n c e of such structures. B e s i d e s t h e c o e x t r u d e d l a m i n a t e s t r u c t u r e i n F i g . 4b, c a s e s c - f are a l s o v i a b l e s t r u c t u r e s f o r some a p p l i c a t i o n s . Chapter 11(47) d i s c u s s e s the a d d i t i o n of i n o r g a n i c f i l l e r s t o EVOH copolymer t o a c h i e v e l a r g e i n c r e a s e s i n b a r r i e r p r o p e r t i e s i n some applications. The e f f e c t s of d i f f e r e n t l o a d i n g s of mica f l a k e i n s e v e r a l polymers o t h e r t h a n EVOH was a l s o r e c e n t l y r e p o r t e d t o be e f f e c t i v e (80). R e a c t i v e m o d i f i c a t i o n s o f i n e x p e n s i v e low b a r r i e r r e s i n s as a means of o p t i m i z i n g c o s t - b e n e f i t p r o p e r t i e s of b a r r i e r s t r u c t u r e s

In Barrier Polymers and Structures; Koros, W.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

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Overview

(a) Monolithic, single polymer

(b) Coextruded laminate of two or more polymers olefin layers with tie layers for adhesion to barrier layer

Figure 4: Primary types of barrier structures.

In Barrier Polymers and Structures; Koros, W.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

16

BARRIER POLYMERS AND STRUCTURES

( F i g . 4c) can be compared t o l a m e l l a r b l e n d i n g o f s m a l l amounts of i m m i s c i b l e h i g h e r b a r r i e r polymers ( F i g . 4f) by c o n s i d e r a t i o n of C h a p t e r s 14, 15 and 13, r e s p . (81-83) The e f f i c a c i e s of t h e v a r i o u s t r e a t m e n t s f o r t h e p r e p a r a t i o n of s o l v e n t r e s i s t a n t a u t o m o t i v e g a s o l i n e t a n k s and h y d r o c a r b o n s t o r a g e b o t t l e s are t r e a t e d i n these chapters. F l a v o r S c a l p i n g by Package W a l l a Whereas most p a c k a g i n g i n v o l v e s p r e v e n t i n g t h e p a s s a g e of m a t e r i a l between t h e e x t e r n a l environment and t h e i n t e r n a l package contents, t h e p r o b l e m o f uptake o f components i n t o t h e b a r r i e r w a l l a l s o deserves a t t e n t i o n . T h i s p r o b l e m was f i r s t e n c o u n t e r e d when PET b o t t l e s were i n t r o d u c e d . I t was found t h a t a l t h o u g h a c t u a l l o s s of C0 t o t h e e x t e r n a l environment might be s m a l l , a b s o r p t i o n i n t o t h e w a l l c o u l d a c c o u n t f o r measurable d e c a r b o n a t i o n o f the c o n t a i n e d b e v e r a g e (84). More r e c e n t l y s c a l p i n g " of c r i t i c a l f l a v o i n t o the o l e f i n i n n e r l i n e r j u i c composit c o n t a i n e r s (85-87). T h i s p r o b l e m can a l s o be c o m p l i c a t e d f u r t h e r by d e s t r u c t i o n o f f l a v o r components by i n v a d i n g oxygen i n some c a s e s . 2

The complex b i o p h y s i c a l n a t u r e o f t a s t e and odor s e n s a t i o n s makes i t d i f f i c u l t t o d e f i n e t h e p r e c i s e a g e n t s t h a t a r e most c r i t i c a l i n r e t a i n i n g p l e a s i n g product p r o p e r t i e s . I t may even be p o s s i b l e t h a t major components such as d-limonene which e x i s t at h i g h l e v e l s i n f r u i t j u i c e s may not be the most c r i t i c a l compounds t h a t a r e s e n s e d t o be m i s s i n g i n o f f - f l a v o r p r o d u c t s . Nevertheless, a major component l i k e d-limonene w i t h the a b i l i t y t o i n t e r a c t s t r o n g l y w i t h t h e o l e f i n i n n e r l i n e r s may s w e l l and p l a s t i c i z e the o l e f i n s t r u c t u r e s u f f i c i e n t l y t o a l l o w t h e r a p i d p e n e t r a t i o n of t h e c r i t i c a l minor components. The c r i t i c a l component might be p r e s e n t i n such a low l e v e l t h a t i t would be u n a b l e t o s w e l l t h e sample s u f f i c i e n t l y t o a l l o w t h e d e b i l i t a t i n g l o s s i n f l a v o r t h a t can o c c u r i n t h e p r e s e n c e of t h e d-limonene. The d e t a i l e d c o n s i d e r a t i o n of the f l a v o r s c a l p i n g p r o b l e m i n r e f e r e n c e 18 and i n C h a p t e r s 16-18 (88-90) of the p r e s e n t book p r o v i d e a good i n t r o d u c t i o n t o q u a n t i f y i n g and e n g i n e e r i n g around t h i s complex problem. In a d d i t i o n t o o p t i m i z a t i o n of c r y s t a l l i n i t y and o r i e n t a t i o n f a c t o r s t o s u p p r e s s d-limonene uptake i n f l a v o r s c a l p i n g , one might e x p e c t t h a t r e a c t i v e t r e a t m e n t s such as t h o s e used f o r r e n d e r i n g gas t a n k s h i g h l y i m p e r v i o u s t o h y d r o c a r b o n uptake c o u l d h e l p w i t h such f l a v o r s c a l p i n g t h a t i s d r i v e n by d-limonene and o t h e r t e r p e n e s o r p t i o n . While t h i s i d e a appears a t t r a c t i v e , a r e c e n t a n a l y s i s has shown t h a t i t i s u n l i k e l y t o s u c c e e d i f the p r i m a r y f l a v o r l o s s o c c u r s due t o d-limonene s o r p t i o n (87). Rubhfir-Solvenf.

Sy«t-.«im

S t r o n g i n t e r a c t i o n s of v a p o r s and l i q u i d p e n e t r a n t s w i t h r u b b e r y polymers a r e d i s c u s s e d i n C h a p t e r s 19 and 20 (91-92) . These systems a r e e x t r e m e l y i m p o r t a n t i n the c o n t i n u e d improvement of g a s k e t s , hoses, and p r o t e c t i v e a p p a r e l . C h a p t e r 20(92) shows that anomalous s o r p t i o n b e h a v i o r can be o b s e r v e d i n r u b b e r s exposed to high a c t i v i t i e s of strong s w e l l i n g s o l v e n t s . Evidence i s

In Barrier Polymers and Structures; Koros, W.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

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17

presented to show that the anomalous behavior is probably due to a breakdown of nonisothermal conditions instead of inhibited segmental relaxation effects as in the case of glassy materials cited in Chapter 4(71). Similar effects have been noted for water vapor sorption due to heat transfer limitations coupled with the large heat of vaporization and rapid diffusion rates of water. The present case, however, is believed to be the first example of the problem involving an organic vapor. This study is useful reading for anyone considering the use of vapor sorption- desorption studies to estimate penetrant diffusivities in rubbery media where uptake rates are rapid and may appear to produce anomalous sorption responses. Chapter 19 considers the effects of polymer-penetrant interactions on the sorption of aromatic penetrants into a polyurethane thermoplastic elastomer(91). A direct liquid immersion approach was used, so the heat transfer problems noted above should not be important Nevertheless Fickia phenomen are s t i l l observed. Unlik elastomers achieve thei y microdomains of either crystalline or glassy hard segments. The anomalous sorption behavior presumably reflects interactions of the solvents with these microdomains (93-94). Conclusion The packaging industry is entering a period of high v i s i b i l i t y and high expectations. The technology discussed in the following chapters gives hints of the available resources in terms of materials and package structures that must be used to meet these challenges. Current estimates (85) suggest that the dollar value of polymers used to manufacture packaging alone will account for almost $16 Billion in 1990. Gaskets, hoses, protective apparel, and encapsulants or masks for microelectronics (95) add considerably to the size of the entire market that depends upon packaging related technology. Clearly, an incredible number of issues face the modern packaging engineer. Solvent attack, oxygen invasion, flavor losses, water losses or gains must be regulated using materials that hopefully will not cost an inordinate fraction of the value of the package contents. In addition to the treatment of purely technical issues treated an important social consideration involving the acceptable handling of wastes generated by the use of polymeric materials in packaging must be considered. Indeed, this issue, as much as first cost considerations of packaging approaches will become increasingly important as society awakens to both the limits of our ability to bury our wastes and also the value of polymeric wastes i f handled correctly in recycling programs. Literature Cited 1. 2.

Crank J . and Park G. S. Diffusion in Polymers; Eds.; Academic: New York, 1968. Rogers, C. E. In Physics and Chemistry of the Organic Solid State; Fox, D., Labes, M. M., and Weissberger, Α., Eds.; Wiley-Interscience: New York, 1965; Vol II, Chapt.6.

In Barrier Polymers and Structures; Koros, W.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

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26. 27. 28.

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van Amerongen, G. J . Rubber Chem. Technol., 1964, 37, 1065. Bixler, H. J . and Sweeting, O. J., In Ed., The Science and Technology of Polymer Films, Sweeting, O. J.,Ed.; John Wiley and Sons: New York, 1971; Vol. II. Hopfenberg, H. B. Permeability of Plastic Films and Coatings to Gases, Vapors and Liquids; Ed.; Plenum: New York, 1974. Crank, J . The Mathematics of Diffusion; 2nd Ed.; Clarendon Press: Oxford, UK, 1975 Felder, R. M. and Huvard, G. S. Methods of Experimental Physics, 1980, 16c, 315. Stern, S. A. and Frisch, H. L. CRC Crit. Rev. in Solid State and Mat. Sci., 1983, 11(2), 123, CRC Press: Boca Raton, F l . Hopfenberg, H. B.and Paul D. R. In Polymer Blends; Paul, D. R. and S. Newman; Eds.; Academic: New York, 1978; Chapt. 10. M. Salame In The Wiley Encyclopedia of Packaging Technology; Bakker, M., Ed.; John Wiley and Sons: New York, 1986; pp. 48-54. Koros, W. J.; Fleming G K. Jordan S M. Kim T H d Hoehn, H. H. Prog. Vieth, W. R.; Howell, , , 1976, 1, 177. Vieth, W. R. Membrane Systems,: Analysis and Design; Hanser Publishers: New York, 1988; Chapt.1-3 Stannett, V. T., Koros; W. J., Paul, D. R.; Lonsdale, H. K. and Baker, R. W. Adv. in Polym. Sci., 1979, 32, 71. Koros, W. J . and Chern. R. T. In Handbook of Separation Process Technology; Rousseau R. W., Ed.; John Wiley and Sons: New York, 1987; Chapter 20. Comyn, J . Polymer Permeability, Ed.; Elsevier Applied Science Publishers: New York, 1985. Duda, J . L. and Vrentas, J . S. In Encyclopedia of Polymer Science; Kroschwitz, J . I., Ed.; John Wiley and Sons: New York, 1986; vol. 5, p. 36. Hotchkiss, J . H. Food and Packaging Interactions, Ed.; American Chemical Society: Washington, 1988; Symposium Series No. 365. Plastics Engineering, May 1984, p. 47. PL 732™ Blood Containers, 1983 Travenol Product Guide, Travenol Labs, Inc., Deerfield IL.; p. 11. Mod. Plastics, Aug. 1985, p. 57. Mitchell, J . K. R. Inst. J., 1831, 2, 101. Stannett, V. T. Chapter 2 in Ref.1. Rogers, C. E . , Chapter 2 in Ref 16. Chern, R. T.; Koros, W. J.; Sanders, E. S.; Chen, S. H., and Hopfenberg, H. B. In Industrial Gas Separations; Whyte, T. E. Jr.; Yon, C. M. and Wagener Ε. H., Eds; American Chemical Society: Washington, 1983; Symposium Series No. 223, p. 47. Muruganandam, Ν.; Koros, W. J . and D. R. Paul J. Polym. Sci., Polym. Phys. Ed., 1987, 25, 1987. Koros, W. J . and Paul, D. R. J . Polym. Sci.. Polym, Phys. Ed., 1978, 16, 2171. Stern, S. A. and S. Trohalaki In Barrier Polymers and Barrier Structures; Koros, W. J., Ed.; American Chemical Society: Washington, D.C., 1990; Chapter 2.

In Barrier Polymers and Structures; Koros, W.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

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Overview

19

29. Jordan, S. M.; Fleming, G. K., and Koros, W. J . J . Membr. Sci., 1987, 30, 191. 30. Graham, T. Philos. Mag., 1866, 32, 401. 31. Chern, R. T.; Koros, W. J.; Hopfenberg, H. B. and Stannett, V. T. In Material Science Aspects of Synthetic Polymer Membranes; Lloyd, D. R., Ed.; American Chemical Society: Washington, 1984; Symposium Series No. 269, Chapter 2. 32. Barrer, R. M., Trans. Faraday Soc., 1939, 35, 628. 33. Barrer, R. M. and Skirrow, G., J . Polym. Sci., 1948, 3, 549. 34. Prager, S. and Long, F. Α., J . Am. Chem. Soc., 1951, 73, 4072. 35. van Amerongen, G. J., J . Polym. Sci., 1950, 5, 307. 36. Briston, J . H. In The Wiley Encyclopedia of Packaging Technology; Bakker, M., Ed.; John Wiley and Sons: New York, 1986; p. 329. 37. DeLassus, P. T., J.Vinyl Technol., 1979, 1, 14. 38. Brown, W. E. and DeLassus, P. T., Polym. Plast. Technol. Engr., 1980, 14(2), 171. 39. Brown, W. E . , In Th Technology, Bakker, , ; y , 1986; p. 692. 40. Bicerano, J., Burmester; A. F . ; Delassus, P. T. and Wessling, R. Α., in Barrier Polymers and Barrier Structures; Koros, W. J., Ed.; American Chemical Society: Washington, D.C., 1990; Chapter 6. 41 McCaul, J . P., In The Wiley Encyclopedia of Packaging Technology, Bakker, M., Ed.; John Wiley and Sons: New York, 1986; p.474. 42. Wyeth, N. C. In High Performance Polymers; Their Origin and Development; Seymour, R. B., and Kirshenbaum, G. S., Eds.; Elsevier: New York, 1986; p. 417. 43. Blackwell, A. L . , In High Performance Polymers; Their Origin and Development; Seymour, R. B., and Kirshenbaum, G. S., Eds.; Elsevier: New York, 1986; p. 425. 44. Gerlowski, L. E. In Barrier Polymers and Barrier Structures; Koros, W. J., Ed.; American Chemical Society: Washington, D.C., 1990; Chapter 8. 45. Tsai, B. C. and Wachtel, J . A. In Barrier Polymers and Barrier Structures; Koros, W. J., Ed.; American Chemical Society: Washington, D.C., 1990; Chapter 9. 46. Alger, M. M., Stanley, T. J . and Day, J . In Barrier Polymers and Barrier Structures; Koros, W. J., Ed.; American Chemical Society: Washington, D.C., 1990; Chapter 10. 47. Bissot, T. C. In Barrier Polymers and Barrier Structures; Koros, W. J., Ed.; American Chemical Society: Washington, D.C., 1990; Chapter 11. 48. Tsai, B. C., and Jenkins, B. J., J . Plastic Film & Sheeting, 1988, 4, 63. 49. U. S. Patent No. 4,407,897 issued to American Can Company, Greenwich, Conn. 50. Krizan, T. D., Coburn, J . C. and Blatz, P. S. In Barrier Polymers and Barrier Structures; Koros, W. J., Ed.; American Chemical Society: Washington, D.C., 1990; Chapter 5. 51. Schmidhauser, J . C. and Longley, K. L. In Barrier Polymers and

In Barrier Polymers and Structures; Koros, W.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

20

52. 53. 54. 55. 56. 57. 58. 59. 60. 61. 62. 63. 64. Die

65. 66. 67. 68. 69. 70. 71. 72. 73. 74. 75. 76. 77. 78. 79. 80.

BARRIER P O L Y M E R S AND STRUCTURES

Barrier Structures; Koros, W. J . , Ed.; American Chemical Society: Washington, D.C., 1990; Chapter 7. Lee, W. M., Polym. Engr. and Sci., 1980, 20(1), 65. Kumins, C. A. and Kwei, T. K.; Chapter 4 in ref. 1. Barrer, R. M., Barrie, J. A. and Slater, J. J. Polym. Sci., 1957, 23, 315. Stern, S. A. and Saxena, V. J. Membr. Sci., 1980, 7, 47. Saxena, V. and Stern, S. A. J. Membr. Sci., 1982, 12, 65. Weinkauf D. H. and Paul, D. R. In Barrier Polymers and Barrier Structures; Koros, W. J . , Ed.; American Chemical Society: Washington, D.C., 1990; Chapter 3. Doty, P. M., Aiken, W. H. and Mark, H. Ind. Engr. Chem., 1946, 38, 788. Meyers, A. W., Rogers, C. E., Stannett, V. T. and Szwarz, M., Tappi, 1958,41,716. Michaels, A. S. and Bixler, H. J. J. Polym. Sci., 1961, 50, 413. Michaels, A. S., Vieth Phys., 1963, 34, 1 Klute, C. H. J. Appl. Polym. Sci., 1959, 1, 340. Wang, L. H. and Porter, R. S. J. Polym. Sci., Polym. Phys. Ed., 1984, 22, 1645. El-Hibri, M. J. and Paul, D. R. J. Appl. Polym.Sci.,1985,30, 3649. Yasuda, H., Stannett, V. T., Frisch, H. L., and Peterlin, A. Macromol. Chem., 1964, 73, 188. Slee, J. Α., Orchard, G. Α., Bower, D. I., Ward, I. M., J. Polym. Sci., Polym. Phys. Ed., 1989, 27, 71. Shastri, R., Dollinger, S. E., Roehrs, and Brown, C. N. In Barrier Polymers and Barrier Structures; Koros, W. J . , Ed.; American Chemical Society: Washington, D.C., 1990; Chapter 12. Winslow, F. H., ACS Symposium Series No. 95, American Chemical Society: Washington, D.C., 1979, p. 11. Puleo, A. C., Paul, D. R. and Wong, K. P. Polymer, 1989, 30, 1357. Chiou, J. S. and Paul, D. R., J. Polym. Sci., Polym. Phys. Ed., 1987, 25, 1699. Berens, A. R., In Barrier Polymers and Barrier Structures; Koros, W. J . , Ed.; American Chemical Society: Washington, D.C., 1990; Chapter 4. Petropoulos, J. H. J. Membr. Sci., 1984, 17, 233. Durning, C. J., and Russel, W. B. Polymer, 1985, 26, 119. Enscore, D. J., Hopfenberg, H. B. and Stannett, V. T. Polymer, 1977, 18, 793. Hopfenberg, H. B. and Frisch, H. L. J. Polym. Sci., Part B: Polym. Phys., 1969, 7, 405. Lasky, R. C., Kramer, E. J . , and Hui, C. Y. Polymer, 1988, 29, 673. Thomas, N. L., Windle, A. H. Polymer, 1982, 23, 529. Sarti, G. Polymer, 1979, 20, 827. Barrer, R. M.; Chapter 6 in Ref. 1. Cussler, E. L., Hughes, S. E., Ward, W. J. and Aris, R., J. Membr. Sci., 1988, 38, 161.

In Barrier Polymers and Structures; Koros, W.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

1.

KOROS

21

Overview

81. Walles, W. E., In Barrier Polymers and Barrier Structures; Koros, W. J . , Ed.; American Chemical Society: Washington, D.C., 1990; Chapter 14. 82. Hobbs, J.P., Anand, M., and Campion, Β. Α., In Barrier Polymers and Barrier Washington, D.C., 1990; Chapter 15. 83. Subramanian, P. M. In Barrier Polymers and Barrier Structures; Koros, W. J . , Ed.; American Chemical Society: Washington, D.C., 1990; Chapter 13. 84. Fenelon, P. J. Polym. Engr. and Sci., 1973, 13, 440. 85. Hotchkiss, J. H.; Chapter 1 in Ref. 18. 86. Landois-Garza, J. and Hotchkiss, J. H.; Chapter 4 in Ref. 18. 87. Farrel, C. J . , Ind. Engr. Chem. Res., 1988, 27, 1946. 88. Hansen, A. P. and Arora, D. K, In Barrier Polymers and Barrier Structures; Koros, W. J . , Ed.; American Chemical Society: Washington, D.C., 1990; Chapter 16. 89. Miltz, J., Mannheim, C. H., and Harte, B. R., In Barrier Polymers and Barrie Structures Koros W J . Ed. America Chemical Society: Washington 90. Strandburg, G., DeLassus, , , , Polymers and Barrier Structures; Koros, W. J., Ed.; American Chemical Society: Washington, D.C., 1990; Chapter 18. 91. Aithal, U. S., Aminabhavi, T. M. and Cassidy, P. E., In Barrier Polymers and Barrier Structures; Koros, W. J . , Ed.; American Chemical Society: Washington, D.C., 1990; Chapter 19. 92. Waksman, L S., Schneider, N. S., and Sung, Ν., In Barrier Polymers and Barrier Structures; Koros, W. J . , Ed.; American Chemical Society: Washington, D.C., 1990; Chapter 20. 93. Koberstein, J. T. and R. S. Stein J. Polym. Sci., Polym. Phys. Ed., 1983, 21, 1439. 94. Chiang, K. T., and Sefton, M.V. J. Polym. Sci., Polym. Phys. Ed., 1977, 15, 1927. 95. Prasad, S. K., Advanced Materials & Processes; Aug.1986, p. 25. RECEIVED November 14,

1989

In Barrier Polymers and Structures; Koros, W.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

Chapter 2

Fundamentals of Gas Diffusion in Rubbery and Glassy Polymers S. A. Stern and S. Trohalaki Department of Chemical Engineering and Materials Science, Syracuse University, Syracuse, NY 13244-1190

This paper reviews some of the more important models and mechanisms of gas diffusion in rubbery and glassy poly­ mers in light of recent experimental data. Diffusion (transport) of gases in polymers is an important, and in some cases, controlling factor in a number of important applica­ tions, such as protective coatings, membrane separation processes, and packaging for foods and beverages. Therefore, a better under­ standing of the mechanisms of gas diffusion in polymers is highly desirable in order to achieve significant improvements in these applications and to develop new ones. From a formal (macroscopic) viewpoint, the diffusion process can be described in many cases of practical interest by Fick's two laws (1-5). These laws are represented by the following equations for the isothermal diffusion of a substance in or through a v-dimensional, hyperspherical polymer body of sufficiently large area [v=l for a slab or membrane (film), v=2 for a hollow cylinder, and V=3 for a spherical shell] (2) : ( V

J = -ω r v

X )

D

3

^ ^ dr

(1)

and 3c 3t

=

1 3 ( v-l 3C rV 3r~ ( D r

R , within perhaps an order of magnitude, for other penetrant/polymer systems, once values for a few representative penetrants in a given polymer are determined. Fickian kinetics are also observed for diffusion of gases and organic vapors in rubbery polymers (14), but diffusivities are much higher and much less steeply dependent upon molecular size than in glassy polymers. The same is true for glassy polymers already plasticized into the rubbery state, i.e., for experiments carried out entirely above Tg or Cg, as indicated in Figure 9. Diffusivities of several gases,vapors and liquids in plasticized PVC are compared to values in glassy PVC on a plot of log D vs molecular diameter in Figure 11 (15). The difference in diffusivity between the glassy and rubbery states increases dramatically with increasing size of the penetrant: For the small gas molecules, diffusivity is increased by about one order of magnitude upon plasticization. For common solvent molecules of 5 to 6 Â diameter, the ratio of rubbery- to glassy-state diffusivity may be 10^ to 10 , and rough extrapolation suggests this ratio might be as great as 10* for plasticizers or other additives of 8 to 10 Â diameter. These trends have interesting consequences for transport in systems involving carbon dioxide. 3

8

2

Carbon Dioxide The transport of carbon dioxide in polymers has historically been analyzed in the same manner as other simple gases (1). A number of recent studies have shown, however.

In Barrier Polymers and Structures; Koros, W.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

104

BARRIER POLYMERS AND STRUCTURES

5 h

\ He \ #

-6

\

-7 Η0 S \ 2

-8

\·ο

2

Α · \ • •CO., N

ΚΓ·\

-10

CH

4

\

\ β CH.OH \ 3

ο

-11

ο

\ G CH CI ® C H CI \ C.H.OH — G 2 5 (CH ) CO 3

-12 μ

2

3

χ

s

-13 μ

.

7

6

4

10

5

-15 μ

n-C H 0 6

-16 μ -17

2

®n-C,H OH ν C H Q O n-C H n-C.HgOH — © O n-C H 6

-14 μ

3

SF6* G C

14

\

CCI

4

.2

Figure 10.

.4

dnm

.5

.6

Effect of molecular size on diffusivity at low penetrant concentration in PVC at 30°C: log D vs. mean molecular diameter of penetrant. (Reproduced with permission from Ref. 3. Copyright 1982 Elsevier.)

In Barrier Polymers and Structures; Koros, W.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

12

BERENS

Transport of Plasticizing Penetrants

-4

-8 £

•

• •

+ + ++ +

•

•

n

•

-10 -\

-12 H + +

-14

i

°

Plasticized

+

Unplasticized

-16 H -18

-τ

1

4

1

τ

I

1ο

6 d, Α

Figure 11.

Effect of plasticization on relation of diffusivity in P V C to molecular size of penetrant. (Reproduced with permission from Ref. 15. Copyright 1989 IUPAC.)

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106

BARRIER POLYMERS AND STRUCTURES

that C 0 at high pressure resembles common organic solvents in its ability to swell and plasticize polymers (16^22). It therefore is appropriate to include studies of C 0 transport in a discussion of plasticizing penetrants in glassy polymers. We have recently applied a simple gravimetric technique to study sorption equilibria and kinetics for near-critical C 0 in a number of glassy polymers at pressures up to the saturated vapor pressure of liquid C 0 (5). Representative sorption isotherms for four glassy polymers at 25°C are shown in Figure 12, plotted against C 0 pressure. All of these isotherms may be regarded as examples of the generalized isotherm of Figure 4, with appropriate values of the interaction parameter and glass composition. For polyvinyl acetate) (PVA), a relatively low χ (strong interaction with CO^ and low Cg (since the Tg of PVA itself is only 30°) produce an isotherm of FloryHuggins, rubbery form over most of the pressure range. For PVC and polycarbonate (PC), the lower solubility of C 0 d th highe f th polymer resul i isotherms of dual-mode form at all C 0 pressures. For PMMA, the data suggest a sigmoid isotherm,with an inflection indicative of a glass transition at an intermediate C 0 pressure. 2

2

2

2

2

2

2

Further examples of the applicability of the generalized sigmoidal isotherm to C0 /glassy polymer systems have been obtained from published isotherms for several other polymers by converting the original pressure axis to an activity scale (5). Activity was taken as the fugacity ratio f/f . defining the reference state above the critical temperature by extrapolating the saturated vapor pressure from sub-critical temperatures. Figure 13, for example, shows such isotherms derived from data of Kamiya, et al. for polyvinyl benzoate) (PVBz) (23). Like the VCM/PVC system (Figure 1), the C0 /PVBz isotherms show sigmoidal form, and the dual-mode portion diminishes with increasing temperature. Superposition of the Flory-Huggins portions for different temperatures suggests a near-zero heat of mixing for C 0 with PVBz; similar results were found for PMMA and PC (5). Glass transitions of C0 /polymer systems, determined from the isotherm inflections, occur at significantly lower weight concentrations of penetrant than is the case for organic solvents. This greater plasticizing efficiency of the smaller C 0 molecule is consistent with the predictions of the Chow equation (12): Figure 14 shows T g vs weight composition calculated for C0 , VCM and toluene in PVC. The calculated T of PVC containing 8 weight % . C0 , the limiting solubility at 25° and unit activity, is about 27°C. Thus, the solubility and Cg of C 0 in PVC at room temperature are nearly equal; i.e., liquid C 0 can plasticize PVC virtually into the rubbery state at room temperature. For polymers in which C 0 is more soluble, such as PMMA or poly(vinyl benzoate), depression of the glass transition to below room temperature, as evidenced by the isotherm inflections, is quite in accord with the predicted high plasticizing efficiency of C 0 The sorption kinetics for C 0 in the glassy polymers studied (5) appear to be Fickian over the entire activity range; diffusivities have the high values anticipated for a 2

Q

2

2

2

2

2

2

g

2

2

2

2

2

In Barrier Polymers and Structures; Koros, W.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

4.

BERENS

Transport ofPlasticizing Penetrants

In Barrier Polymers and Structures; Koros, W.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

107

108

BARRIER POLYMERS AND STRUCTURES

100

Weight f r a c t i o n

Figure 14.

penetrant

Glass transition temperature vs. composition for PVC/penetrant systems, calculated from the equation of Chow (12).

ο >

° DMP + CO.

100

200

— ι —

300

400

500

Time, hrs

Figure 15.

Gravimetric sorption/desorption vs. time date for the system PVC/DMP/CO2 . (Reproduced with permission from Ref. 15. Copyright 1989 IUPAC.)

In Barrier Polymers and Structures; Koros, W.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

4. BERENS

109

Transport ofPlasticizing Penetrants

small gas molecule and increase with increasing C 0 concentration. By analogy with solvent/polymer systems, Case II sorption kinetics might be expected when the sorption of C 0 produces the glass-rubber transition, as in PMMA at high C 0 activity. The absence of Case II kinetics may be due to the relatively small change of C 0 diffusivity between the glassy and rubbery states. The observed increase of D across the glass transition is from 10 to 100-fold for C0 , far less than the change for organic solvents (cf. Figure 11). According to one recent theory of Case II transport (24), this mechanism is associated with a very sharp increase of Din the vicinity of Tg. Perhaps the change of D at Tg for C 0 is too small to produce the sharp concentration step involved in the Case II mechanism. 2

2

2

2

2

2

The high diffusivity, polymer-solubility, and plasticizing efficiency of C 0 lead to some interesting effects on the transport of other low-molecular weight penetrants in glassy polymers (25.26). Whe film simultaneousl an additive substance and to plasticizing the polymer and thereby sharply increasing the diffusivity of the additive. Upon the release of pressure, the C 0 is quickly desorbed, the degree of plasticization and additive diffusivity are sharply reduced, and the additive is effectively "trapped" in the polymer. It has been found that high-pressure C 0 remarkably accelerates the absorption of many polymer-soluble compounds whose diffusion into the polymer alone is kinetically limited. This "C0 -assisted impregnation" process is illustrated in Figure 15 with data for the model system PVC/C0 /dimethyl phthalate (DMP). In the absence of C0 , the absorption of DMP by PVC at room temperature is extremely slow; no more than 1 wt % DMP was absorbed by PVCfilmsimmersed in excess liquid DMP for 64 hours. In the presence of liquid CO2, in contrast, the DMP content of the PVC reached 40 wt % in 16 hours. After the pressure was released, over 95% of the absorbed C 0 had escaped in 24 hours, but more than 80% of the absorbed DMP remained even after over 1000 hours. In addition to its practical potential in incorporating additives into polymers, the effect of compressed CO2 on third-component diffusivities may also be relevant to the supercritical fluid extraction of low-molecular compounds from polymers, and to the attack of barrier polymers by other swelling agents during service in high-pressure C 0 environments. 2

2

2

2

2

2

2

2

Literature Cited 1. 2. 3. 4.

Crank, J.; Park, G. S. Diffusion in Polymers, Academic Press, London, 1968. Berens, A. R. Polymer 1977, 18, 697. Berens, A. R.; Hopfenberg, H. B. J. Membrane Sci. 1982, 10, 283. Berens, A. R. J. Appl. Polym. Sci. 1989, 37, 901.

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BARRIER POLYMERS AND STRUCTURES

5. Berens, A. R.; Huvard, G. S. In Supercritical Fluid Science and Technology: Johnston, K. P., and Penninger, J. M. L., Eds.; ACS Symposium Series No. 406; American Chemical Society, Washington, DC, 1989; pp. 207-223. 6. Berens, A. R. Polym. Prepr. 1974, 15, 197, 203. 7. Flory, P. J. Principles of Polymer Chemistry, Cornell University Press, Ithaca, NY, 1953; p. 514. 8. Michaels, A. S.; Veith, W. R.; Barrie, J. A. J. Appl. Phys. 1963, 34, 1. 9. Berens, A. R. Polym. Eng. Sci. 1980, 20, 95. 10. Connelly, R. W.; McCoy, N. R.; Koros, W. J.; Hopfenberg, H. B.; Stewart, M. E. J. Appl. Polym. Sci. 1987, 34, 703. 11. Stewart, M. E.; Hopfenberg, H. B.; Koros, W. J.; McCoy, N. R. J. Appl. Polym. Sci. 1987, 34, 721. 12. Chow, T. S. Macromolecules 1980, 13. 362. 13. Hopfenberg, H. B.; Frisch 14. van Amerongen, G. J. Rubbe , , 15. Berens, A. R. Makromol.Chem.,Macromol. Symp. 1989, 29, 95. 16. Fleming, G. K.; Koros, W. J. Macromolecules 1986, 19, 2285. 17. Sefcik, M. D. J. Polym. Sci, Polym. Phys. 1986, 24, 935. 18. Hirose, T.; Mizoguchi, K.; Kamiya, Y. J. Polym. Sci, Polym. Phys. 1986, 24, 2107. 19. Wissinger, R. G.; Paulitis, M.E. J. Polym. Sci., Polym. Phys. 1987, 25, 2497. 20. Wang, W. V.; Kramer, E. J.; Sachse, W. H. J. Polym. Sci, Polym. Phys. 1982, 20, 1371. 21. Chiou, J. S.; Barlow, J. W.; Paul, D. R. J. Appl. Polym Sci. 1985, 30, 2633. 22. Sefcik, M. D. J. Polym. Sci, Polym. Phys. 1986, 24, 957. 23. Kamiya, Y.; Mizoguchi, K.; Naito, Y.; Hirose, T. J. Polym. Sci, Polym. Phys., 1986, 24, 535. 24. Hui, C.-Y.; Wu, K.-C.;. Lasky, R. C.; Kramer, E. J. J. Appl. Phys. 1987, 61, 5137. 25. Berens, A. R.; Huvard, G. S.; Korsmeyer, R. W. AIChE National Meeting, Washington, DC, November 28 - December2,1988 (to be published). 26. Berens, A. R.; Huvard, G. S.; Korsmeyer, R. W., U. S. Patent 4 820 752, April 11, 1989. RECEIVED December 5, 1989

In Barrier Polymers and Structures; Koros, W.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

Chapter 5

Structure of Amorphous Polyamides Effect on Oxygen Permeation Properties Timothy D. Krizan, John C. Coburn, and Philip S. Blatz Polymer Products Department, Experimental Station, E. I. du Pont de Nemours and Company, Wilmington, DE 19880

The structure of an amorphous polyamide prepared from hexamethylenediamine and i s o p h t h a l i c / t e r e phthalic acids was modified i n order to determine the effect of chemical structure on the oxygen permeation properties. The greatest increase i n permeation was obtained by lengthening the a l i p h a t i c chain. Placement of substituents on the polymer chain also led to increased permeation. Reversal of the amide linkage d i r e c t i o n had no effect on the permeation properties. Free volume calculations and d i e l e c t r i c relaxation studies indicate that free volume i s probably the dominant factor i n determining the permeation properties of these polymers.

Barrier resins, polymers which have r e l a t i v e l y low rates of small molecule permeation, have revolutionized the packaging industry i n recent years. For food packaging applications, i t i s s p e c i f i c a l l y desirable to impede oxygen permeation. Each food type has i t s own p a r t i c u l a r packaging requirements, which leads to the use of many polymer classes at a variety of temperatures and relative humidities i n these applications. Figure 1 shows the effect of relative humidity (RH) upon the oxygen permeation values (OPV) of a few representative polymers. This data i s reported i n the units of cc-mil/(100 sq.in.-day-atm). For many polymers such as polyethylene, OPV i s e s s e n t i a l l y unaffected by changes i n RH. For polymers such as nylon 6 or poly(vinyl alcohol) which contain hydrogen bonds, OPV increases dramatically with increasing RH. The increase i n permeation i s attributed to p l a s t i c i z a t i o n of the polymer structure by the water (1), which disrupt the polymer hydrogen bonds. Selar PA, poly(hexamethylene isophthalamide/terephthalamide) or 6-I/T (the diamine components are l i s t e d f i r s t , then the d i a c i d components), i s an amorphous polyamide which i s marketed by Du Pont. As shown i n Figure 1, i t has unique properties for a b a r r i e r resin i n that the oxygen barrier properties actually 0097-6156/90/0423-0111$06.00/0 © 1990 American Chemical Society

In Barrier Polymers and Structures; Koros, W.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

BARRIER P O L Y M E R S AND STRUCTURES

OPV

20

40

60

% R E L A T I V E HUMIDITY * cc-mil/100sq. in./day/atm

Figure 1. Resins.

Effect of Relative Humidity on OPV of Selected

In Barrier Polymers and Structures; Koros, W.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

5.

KRIZAN E T A L .

Structure ofAmorphous Polyamides

improve (OPV d e c r e a s e s ) a s RH i n c r e a s e s . T h i s improvement i s o p p o s i t e from what would be e x p e c t e d f o r a polymer w h i c h c o n t a i n s a s i g n i f i c a n t amount o f hydrogen b o n d i n g . I t was o f i n t e r e s t t o examine t h e p e r m e a t i o n p r o p e r t i e s o f t h i s c l a s s o f a l i p h a t i c - a r o m a t i c p o l y a m i d e s . More s p e c i f i c a l l y , i t was d e s i r e d t o d e t e r m i n e t h e e f f e c t o f changes i n c h e m i c a l s t r u c t u r e upon OPV and upon t h e RH dependence o f OPV. I t was a l s o d e s i r e d t o d e t e r m i n e t h e f a c t o r s which l e a d t o t h e s e o b s e r v e d structural effects. Experimental F i g u r e 2 d e p i c t s t h e monomers u s e d i n t h i s s t u d y w i t h t h e a b b r e v i a t i o n s u s e d f o r each monomer. A l l p o l y a m i d e s made from a l i p h a t i c d i a m i n e s and a r o m a t i c d i a c i d c h l o r i d e s were p r e p a r e d i n t e r f a c i a l l y (2). Those made from a r o m a t i c d i a m i n e s and a l i p h a t i c d i a c i d s were p r e p a r e d b y a s o l u t i o n method u s i n g t r i p h e n y l p h o s p h i t e and pyridine i n N-methylpyrrolidinon f o r oxygen p e r m e a t i o n ha in sulfuric acid. F i l m s o f t h e s e p o l y a m i d e s were p r e p a r e d by p r e s s i n g from t h e m e l t . OPV d a t a o f t h e s e f i l m s were measured on a Modern C o n t r o l s Ox-Tran 10/50 a t 30°c D e n s i t i e s were measured i n a carbon t e t r a c h l o r i d e / t o l u e n e d e n s i t y g r a d i e n t tube. D i f f e r e n t i a l s c a n n i n g c a l o r i m e t r y d a t a (DSC) were o b t a i n e d on a Du Pont I n s t r u m e n t s DSC a t a h e a t i n g r a t e o f 20°C/minute. D i e l e c t r i c measurements were made on a Polymer Labs D i e l e c t r i c Thermal A n a l y z e r . T e s t s p e r f o r m e d on wet samples were c o n d u c t e d a f t e r immersing t h e f i l m s i n water a t 25°C f o r a minimum o f 72 hours. The samples were b l o t t e d d r y p r i o r t o t e s t i n g . R e s u l t s and D i s c u s s i o n The e f f e c t s o f t h e f o l l o w i n g s t r u c t u r a l changes on t h e OPV o f a l i p h a t i c - a r o m a t i c p o l y a m i d e s were d e t e r m i n e d : alteration of t h e a l i p h a t i c c h a i n l e n g t h ; r e v e r s a l o f t h e amide l i n k a g e ; s u b s t i t u t i o n o f groups upon e i t h e r t h e amide n i t r o g e n , t h e a l i p h a t i c c h a i n , o r a r o m a t i c r i n g ; replacement o f t h e l i n e a r a l i p h a t i c c h a i n w i t h a c y c l o a l i p h a t i c group; and u s e o f o t h e r a r o m a t i c r i n g systems. The e f f e c t o f p l a c i n g o t h e r f u n c t i o n a l groups i n t h e c h a i n was a l s o s t u d i e d , b u t t h o s e r e s u l t s w i l l n o t be d i s c u s s e d i n t h i s paper ( K r i z a n , T. D., Du Pont, u n p u b l i s h e d data). I n o r d e r t o d e t e r m i n e t h e e f f e c t s o f a g i v e n monomer on p o l y a m i d e p e r m e a t i o n p r o p e r t i e s , d a t a o b t a i n e d from copolymers where t h e monomer o f i n t e r e s t was d i l u t e d by a n o t h e r d i a m i n e o r d i a c i d were o f t e n u s e d . I t i s assumed t h a t t h e OPV d a t a f o r c o p o l y m e r s a r e w e i g h t e d a v e r a g e s o f t h e OPV d a t a f o r t h e c o n s t i t u e n t homopolymers. The u s e o f copolymers was n e c e s s i t a t e d by s e v e r a l r e a s o n s . I t was o f t e n t o o d i f f i c u l t t o form t h e homopolymer o f i n t e r e s t w i t h h i g h enough m o l e c u l a r w e i g h t t o a l l o w formation o f cohesive f i l m s . I n o t h e r c a s e s , t h e homopolymer was s e m i - c r y s t a l l i n e , which, a s w i l l be d e s c r i b e d i n t h e n e x t paragraph, i s u n d e s i r a b l e f o r t h i s study. In o r d e r t o make m e a n i n g f u l comparisons o f p e r m e a t i o n p r o p e r t i e s , i t was n e c e s s a r y t o i n s u r e t h a t no c o m p l i c a t i n g f a c t o r s were p r e s e n t i n t h e polymers under s t u d y . The major p r e c a u t i o n was t o

In Barrier Polymers and Structures; Koros, W.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

113

114

BARRIER POLYMERS AND STRUCTURES

Diamines NH ^ ^ N H 2

2

n

e

\( J]

NH (CH ) NH 2

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2

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2Me5 NH

2

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3

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2

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3

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)—CH —{

)—NH

2

2

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^C0 H

2

2

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2

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n

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2,6 Pyr Monomers Used In This Study.

In Barrier Polymers and Structures; Koros, W.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

3

5.

KRIZAN

ET AL.

Structure of Amorphous Polyamides

i n s u r e t h a t the polymers had no o b s e r v a b l e c r y s t a l l i n i t y (by DSC). I n s e m i - c r y s t a l l i n e polymers, i t i s g e n e r a l l y assumed t h a t p e r m e a t i o n o c c u r s o n l y through the amorphous r e g i o n s w h i l e the c r y s t a l l i n e r e g i o n s a r e e s s e n t i a l l y impervious (4). For t h i s study, the s i m p l e s t way t o p r e p a r e c o m p l e t e l y amorphous polymers was t o use m e t a - s u b s t i t u t e d benzenes as the s o l e a r o m a t i c component i n the a l i p h a t i c - a r o m a t i c p o l y a m i d e s . In most c a s e s , t h i s a p p r o a c h was s u f f i c i e n t t o e l i m i n a t e any o b s e r v a b l e crystallinity. E f f e c t o f C h a i n Length. The i n i t i a l p a r t o f t h i s s t u d y c o n s i s t e d o f d e t e r m i n i n g the e f f e c t o f a l i p h a t i c c h a i n l e n g t h on the p e r m e a t i o n p r o p e r t i e s o f the p o l y a m i d e s . A s e r i e s o f i s o p h t h a l amides (n-I) was p r e p a r e d where the a l i p h a t i c c h a i n l e n g t h was s y s t e m a t i c a l l y a l t e r e d from 2 t o 10 methylenes (5). Crystalline m e l t i n g p o i n t s were o b s e r v e d by DSC f o r 2-1 and 3-1, so p e r m e a t i o n d a t a was measured o n l y f o r 4-1 through 10-1. The t h e r m a l , d e n s i t y , and oxygen p e r m e a t i o i n Table I. Table

Polymer

OPV* (dry)

I.

Data f o r n-I

OPV* RH)

(80%

0.4 0.5 1.2 0.9 1.2 1.9 3.9 2.9 4.1 7.0 7.8 11.3 12.8 11.1 *cc-mi 1/(100 sq.in.-day-atm)

4-1 5-1 6-1 7-1 8-1 9-1 10-1

Polyamide S e r i e s

Density (g/mL)

1.25 1.23 1.19 1.18 1.15 1.13 1.11

Tg (°C)

141 129 123 113 114 105 97

Wet Tg (°C)

1/SFV (g/mL)

46 42 41 46 53 53

11.91 11.12 10.51 10.02 9.62 9.28 8.99

I t i s a p p a r e n t from the OPV d a t a i n T a b l e I t h a t w i t h each a d d i t i o n a l methylene group i n the polymer backbone, OPV a t b o t h 0% and 80% RH s i g n i f i c a n t l y i n c r e a s e s . T h i s t r e n d can a l s o be d i s c e r n e d i n a s e r i e s o f c o p o l y e s t e r s i n which 8-16% o f the t e r e p h t h a l i c a c i d p o r t i o n o f p o l y ( e t h y l e n e t e r e p h t h a l a t e ) (PET) i s r e p l a c e d by a l i p h a t i c d i a c i d s o f v a r i o u s l e n g t h s (6). A n o t h e r s i g n i f i c a n t f e a t u r e o f the OPV d a t a i n T a b l e I i s the v a r i a n c e i n the e f f e c t o f RH on OPV. The OPV a t 80% RH i s g r e a t e r t h a n the OPV a t 0% RH o n l y when n=4. When n=5, the d r y OPV i s s l i g h t l y g r e a t e r t h a n the OPV a t 80% RH, but i t becomes s i g n i f i c a n t l y g r e a t e r t h a n the OPV a t 80% RH as η i n c r e a s e s . The e f f e c t o f RH upon OPV o f the m a j o r i t y o f t h e s e i s o p h t h a l a m i d e s i s , t h e r e f o r e , s i m i l a r t o t h a t o b s e r v e d f o r 6-I/T. As η i n c r e a s e s , the i s o p h t h a l a m i d e s t r u c t u r e w i l l approach l i n e a r p o l y e t h y l e n e , and the e f f e c t o f RH upon OPV s h o u l d become n e g l i g i b l e . The amide d e n s i t y o f the i s o p h t h a l a m i d e s examined here i s s t i l l too h i g h , however, f o r c o n f i r m a t i o n o f t h i s p r e d i c t i o n . F a c t o r s A f f e c t i n g Polyamide OPV. I n o r d e r t o d e t e r m i n e the polymer p r o p e r t i e s which a f f e c t polyamide p e r m e a t i o n p r o p e r t i e s , the n-I s e r i e s was s t u d i e d i n more d e t a i l . F i g u r e 3 shows the

In Barrier Polymers and Structures; Koros, W.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

116

BARRIER POLYMERS AND STRUCTURES

e f f e c t o f c h a i n l e n g t h upon the g l a s s t r a n s i t i o n temperature (Tg) for t h i s s e r i e s . In the d r y s t a t e , i n c r e a s i n g the a l i p h a t i c c h a i n l e n g t h l e a d s t o lower Tg ( a l s o o b s e r v e d i n o t h e r polyamide s e r i e s (7,8)). The polymers, however, a r e a l l g l a s s y a t the p e r m e a t i o n t e s t temperature o f 30°C I t i s , therefore, impossible to a t t r i b u t e the o b s e r v e d dependence o f OPV upon η t o a t r a n s i t i o n o f the polyamide from a g l a s s t o a rubber. As shown i n T a b l e I , the wet Tg o f t h e polymer i s s t i l l above the t e s t temperature when η i s g r e a t e r t h a n o r e q u a l t o 5. T h i s means t h a t the 80% RH OPV d a t a i s o b t a i n e d from polymers which a r e s t i l l i n the g l a s s y state. I t was n o t p o s s i b l e t o observe a wet Tg f o r 4-1 i n the DSC, which may i n d i c a t e t h a t i t dropped below room temperature. I f t h i s i s the c a s e , the 80% RH OPV o f the 4-1 might be e x p e c t e d to be h i g h e r t h a n the d r y OPV due t o an i n c r e a s e i n r u b b e r y c h a r a c t e r a t h i g h RH. The f r e e volume i n a polymer i s c o n s i d e r e d t o be a v e r y i m p o r t a n t parameter a f f e c t i n g the amount o f gas p e r m e a t i o n . Unfortunately, this i s a One a p p r o a c h t h a t has bee polymers and i n f e r t h a t the denser polymer has a l e s s e r amount o f f r e e volume and t h u s lower gas p e r m e a t i o n r a t e s ( 9 , 1 Ό ) . T h i s a p p r o a c h , however, has been abused i n t h a t i t has been used t o compare the f r e e volumes o f s t r u c t u r a l l y d i s s i m i l a r polymers. S i n c e the polymers i n t h i s s e r i e s a r e homologues, t h e r e i s some j u s t i f i c a t i o n f o r u s i n g a d e n s i t y comparison t o determine r e l a t i v e amounts o f f r e e volume. F i g u r e 4 shows t h a t as η i n c r e a s e s , the p o l y i s o p h t h a l a m i d e d e n s i t y d e c r e a s e s . S i m i l a r t r e n d s were o b s e r v e d by Ridgway i n o t h e r polyamide s e r i e s (11). T h i s t r e n d i n d i c a t e s t h a t f r e e volume i s i n c r e a s i n g w i t h η and t h a t p e r m e a t i o n would be e x p e c t e d t o i n c r e a s e , which i s what i n f a c t i s observed. A more d i r e c t method t o determine the f r e e volume d i f f e r e n c e s i n t h e s e r i e s i s t o c a l c u l a t e them u s i n g the method o f Lee (12), w h i c h u s e s a group c o n t r i b u t i o n approach. T a b l e I c o n t a i n s the v a l u e s f o r 1/SFV ( s p e c i f i c f r e e volume) which were c a l c u l a t e d u s i n g the a d d i t i v e molar volumes p r o v i d e d by Van K r e v e l e n (13). F i g u r e 5 shows a p l o t o f the l o g o f the d r y OPV f o r the n-I s e r i e s a g a i n s t 1/SFV. A l i n e a r r e l a t i o n s h i p , which i s what would be e x p e c t e d i f f r e e volume i s a d e t e r m i n i n g f a c t o r i n oxygen permeation, i s obtained i n t h i s p l o t . S u b g l a s s motions a r e p o s t u l a t e d t o a i d i n the t r a n s p o r t o f gases t h r o u g h g l a s s y polymers (14-16). These t r a n s i t i o n s i n the n-I s e r i e s were examined u s i n g d i e l e c t r i c s p e c t r o s c o p y . The r e l a x a t i o n d a t a w i l l be r e p o r t e d i n g r e a t e r d e t a i l elsewhere (Coburn, J . C ; K r i z a n , T. D., Du Pont, u n p u b l i s h e d d a t a ) . A p l o t of t h e d i e l e c t r i c l o s s o f 6-I/T i s p r o v i d e d i n F i g u r e 6. The magnitude o f the l a r g e s u b g l a s s t r a n s i t i o n (beta) i s much l e s s t h a n t h a t o f the g l a s s t r a n s i t i o n due t o the hydrogen b o n d i n g w h i c h e f f e c t i v e l y reduces l o c a l segmental motion. This behavior i s n o t o b s e r v e d i n o t h e r t h e r m o p l a s t i c polymers such as PET where the magnitude o f the s u b g l a s s t r a n s i t i o n i s comparable t o t h a t o f the g l a s s t r a n s i t i o n (17). F i g u r e 7 shows a p l o t o f the temperature o f the b e t a t r a n s i t i o n a t 10 kHz a g a i n s t n. This t r a n s i t i o n occurs close to room temperature i n 4-1 and s h i f t s t o lower temperatures as the number o f methylene groups i n c r e a s e . T h i s means t h a t the amount

In Barrier Polymers and Structures; Koros, W.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

5. KRIZANETAL.

117

Structure ofAmorphous Polyamides

200 η

180

160

Tg (°C) 140 Η

120 Η

100

1

—ι—«—•—ι—· 2

•—ι—•—'—ι—•—'—ι

4

6

8

10

NUMBER OF METHYLENES Figure 3.

Effect of Chain Length Upon Isophthalamide Tg.

1.3η

DENSITY (g/ml)

1.2

1.1

—•

3

1

5

·

1

7

«

1

11>12>13>14. This order i s not observed f o r t h e p e r m e a b i l i t i e s o f t h e p o l y c a r b o n a t e s , i m p l y i n g t h a t t h i s p a r t i c u l a r m o t i o n i s n o t by i t s e l f t h e c o n t r o l l i n g f a c t o r f o r gas t r a n s p o r t r a t e s . The g l a s s y m o r p h o l o g y o f t h e 3 , 3 - d i s u b s t i t u t e d p o l y c a b o n a t e s p r e c l u d e s d i r e c t , c r y s t a l l o g r a p h i c measurement o f p o l y m e r c h a i n packing. However, t h e d e n s i t i e s o f t h e s e p o l y m e r s d e c r e a s e s as t h e substituent size increases. Thus t h i s c r u d e measure o f p a c k i n g d e n s i t y does n o t c o r r e l a t e w i t h t h e o b s e r v e d oxygen p e r m a b i l i t y d a t a . To g a i n a more a c c u r a t e v i e w o f p o l y m e r c h a i n p a c k i n g , s e g m e n t a l models o f t h e s e p o l y c a r b o n a t e s were p r e p a r e d and s t u d i e d b y s i n g l e c r y s t a l d i f f r a c t i o n methods. A summary o f t h e s e more d e t a i l e d e x a m i n a t i o n s o f e f f e c t s o f polymer segmental m o b i l i t y and p a c k i n g on t h e p e r m e a b i l i t y o f s u b s t i t u t e d p o l y c a r b o n a t e s w i l l be r e p o r t e d shortly ( B e n d l e r , J . T.; G r a b a u s k a s , M. F.; S c h m i d h a u s e r , J . C. G e n e r a l E l e c t r i c Co., u n p u b l i s h e d d a t a ) . 1

Permeability o_f P o l y m e r s Containing a Common Bisphenol. To i n v e s t i g a t e t h e importance o f monomer s t r u c t u r e v e r s u s polymer c l a s s

In Barrier Polymers and Structures; Koros, W.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

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(i.e., f u n c t i o n a l group l i n k ) i n d e t e r m i n i n g a polymer's gas transport rate, i t i s necessary t o obtain permeability data f o r s e v e r a l c l a s s e s o f p o l y m e r , t h e members o f w h i c h a r e b a s e d upon monomers w i t h a common s t r u c t u r e . To t h i s end, oxygen p e r m e a b i l i t i e s measured f o r s e v e r a l p o l y a r y l a t e s (PAr) and p o l y e t h e r i m i d e s (PEI) a r e compared i n T a b l e IV w i t h t h e p r e v i o u s l y l i s t e d t r a n s p o r t d a t a f o r p o l y c a r b o n a t e s (PC) b a s e d upon t h e same b i s p h e n o l monomers. (For comparison purposes, only p o l y a r y l a t e s c o n t a i n i n g a 1 : 1 molar r a t i o of i s o - and t e r e p h t h a l i c a c i d s a r e c o n s i d e r e d at this point.) Analysis of t h i s transport data leads to several conclusions concerning the r e l a t i o n s h i p between polymer structure and permeability. W i t h i n a p a r t i c u l a r polymer c l a s s , oxygen p e r m e a b i l i t y r a t e s v a r y w i d e l y d e p e n d i n g upon t h e d i f f e r e n t monomer s t r u c t u r e p r e s e n t . The s t r o n g dependence o f t h e gas p e r m e a b i l i t i e s o f p o l y c a r b o n a t e s on t h e s t r u c t u r e o f t h e i r r e p e a t u n i t ( r a t e s v a r y by a f a c t o r o f 63 f o r t h e s e examples) has bee of rates observed fo polyetherimides (factor ) g dependence i s g e n e r a l f o r a number o f c l a s s e s o f b i s p h e n o l derived polymers. In a d d i t i o n , i t i s found t h a t t h e same o r d e r i n g o f oxygen p e r m e a b i l i t i e s i s f o l l o w e d by members w i t h i n e a c h p o l y m e r c l a s s , so t h a t t h e s p i r o b i i n d a n e b i s p h e n o l c o n t a i n i n g m a t e r i a l s always have t h e highest transmission rates while the 1,1-bis(4-hydroxy-3-methylp h e n y l ) c y c l o h e x a n e b a s e d m a t e r i a l s always e x h i b i t t h e l o w e s t r a t e . B o t h t h e wide r a n g e and t h e s i m i l i a r o r d e r i n g o f p e r m e a b i l i t i e s w i t h i n these c l a s s e s o f polymers are evidence f o r t h e importance o f monomer s t r u c t u r e i n d e t e r m i n i n g polymer gas t r a n s p o r t p r o p e r t i e s . An a l t e r n a t i v e a n a l y s i s o f t h i s gas t r a n s p o r t d a t a c a n be made by f o c u s i n g on c l a s s e s o f p o l y m e r s w h i c h were made f r o m t h e same b i s p h e n o l monomers. T h i s c o m p a r i s o n o f t h e oxygen p e r m e a b i l i t i e s reveals the following general order: polyarylate > polycarbonate > polyetherimide. These r e s u l t s agree w i t h p r e v i o u s l y reported observations on b i s p h e n o l A based polymers (22.) . As m i g h t be expected, a bisphenol A based p o l y e s t e r c a r b o n a t e (GE L e x a n PPC, c o n t a i n i n g 20 % c a r b o n a t e l i n k s and 80 % e s t e r l i n k s ) e x h i b i t s an oxygen p e r m e a b i l i t y (1.77 B a r r e r s ) somewhere between t h a t o f t h e p u r e PC o r P A r . The p o l y a r y l a t e s have o n l y s l i g h t l y h i g h e r oxygen p e r m e a b i l i t i e s than the s t r u c t u r a l l y r e l a t e d polycarbonates, while the analogous p o l y e t h e r i m i d e s have much l o w e r p e r m é a b i l i t é s t h e n either material. O v e r a l l , t h e range o f t r a n s p o r t r a t e s e x h i b i t e d by these groups of polymers with t h e same-monomer/different-link is s m a l l e r , i . e . , n e v e r more t h a n a f a c t o r o f f i v e ( i n the case o f b i s p h e n o l A) , t h a n t h a t d i s p l a y e d by t h e j u s t d i s c u s s e d s u b s e t s o f polymers with different-monomer/same-link. Therefore, i t is concluded that f o r these sets of bisphenol containing polymers, monomer (repeat u n i t ) s t r u c t u r a l f a c t o r s , not f u n c t i o n a l group l i n k s , p l a y a dominant r o l e i n c o n t r o l l i n g polymer p e r m e a b i l i t y p r o p e r t i e s . Are P o l y e s t e r s B e t t e r Gas B a r r i e r s Than P o l y c a r b o n a t e s ? Commercial a l k y l t e r e p h t h a l a t e p o l y e s t e r s e x h i b i t lower gas p e r m e a b i l i t i e s t h a n commercial b i s p h e n o l based p o l y c a r b o n a t e s . On t h i s b a s i s , p o l y e s t e r s are generally classified as good barrier polymers while p o l y c a r b o n a t e s a r e t y p i c a l l y n o t r e g a r d e d as s u c h . Furthermore, the e s t e r f u n c t i o n a l i t y has been a s s i g n e d h i g h e r b a r r i e r v a l u e s t h a n t h e

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Table IV. Oxygen Permeabilities of Polymers Containing a Common Bisphenol

Polymer No.

Bisphenol

Polycarbonat

2

Polyeste

,

Polyetherimid

6.90 7.44 2.82

3.41 3.03

1.50 1.81 0.33 1.26 1.32

0.11 0.44 0.30

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168

BARRIER POLYMERS AND STRUCTURES

c a r b o n a t e group i n a model which attempts t o a s s i g n b a r r i e r v a l u e s t o s e g m e n t a l i n c r e m e n t s o f p o l y m e r s (23.) . However, t h e gas t r a n s p o r t d a t a l i s t e d i n T a b l e IV shows t h a t i n f o u r o f f i v e examples, t h e b i s p h e n o l c o n t a i n i n g p o l y e s t e r s e x h i b i t s t h e same o r s l i g h t l y h i g h e r oxygen p e r m e a b i l i t y t h a n t h e s t r u c t u r a l l y r e l a t e d polycarbonates. These results indicate that a polyester based upon a "polycarbonate-like" monomer (i.e., a bisphenol A derivative) e x h i b i t s a " p o l y c a r b o n a t e - l i k e " gas t r a n s p o r t r a t e . A s i m i l i a r c o m p a r i s o n can be made between p e r m e a b i l i t y d a t a o f p o l y e s t e r s and polycarbonates c o n t a i n i n g " p o l y e s t e r - l i k e " monomers. F o r example, p o l y c a r b o n a t e s 26 and 27, which c o n t a i n p r e d o m i n a n t l y bisphenols which are s t r u c t u r a l l y s i m i l i a r to the repeat unit (circled) of an alkyl terephthalate polymer (PBT), possess e s s e n t i a l l y t h e same low p e r m e a b i l i t y as t h a t p o l y e s t e r ( T a b l e V) . F u r t h e r m o r e , t h e r e has been a r e c e n t announcement (2£.) t h a t a l i p h a t i c polycarbonates, such as p o l y p r o p y l e n e and p o l y e t h y l e n e c a r b o n a t e , are b e i n g d e v e l o p e d as p o t e n t i a r e s u l t s suggest t h a t th p o l y e s t e r s and h i g h p e r m e a b i l i t i e polycarbonate not a d i r e c t consequence o f t h e e s t e r o r c a r b o n a t e l i n k s , but a r e due i n s t e a d t o t h e s t r u c t u r e o f t h e monomers t h e y a r e p r e p a r e d from, i . e . , t h e a l i p h a t i c d i o l and a r o m a t i c b i s p h e n o l , r e s p e c t i v e l y . E f f e c t o f D i a c i d S t r u c t u r e on P o l y a r y l a t e P e r m e a b i l i t y . On f i r s t i n s p e c t i o n , i t would be t e m p t i n g t o a s c r i b e t h e r e l a t i v e l y high p e r m e a b i l i t y of the p o l y a r y l a t e s to the presence of i s o p h t h a l a t e u n i t s which c o u l d l e a d t o ". . . i n t e r u p t i o n of t h e [polymer] c h a i n p a c k i n g c a u s e d by t h e random " k i n k s " . . . " (2JL) . To t e s t t h i s h y p o t h e s i s , t h e e f f e c t of d i a c i d s t r u c t u r e on t h e p e r m e a b i l i t y o f p o y l a r y l a t e s was e x p l o r e d by p r e p a r i n g s e r i e s o f p o l y m e r s c o n t a i n i n g d i f f e r e n t m o l a r r a t i o s of i s o - and t e r e p h t h a l i c d i a c i d s ( T a b l e V I ) . S u r p r i s i n g l y , i n c r e a s i n g the amount of i s o p h t h a l o y l l i n k s l o w e r e d t h e o b s e r v e d oxygen t r a n s p o r t r a t e s , w i t h an i s o p h t h a l o y l - r i c h p o l y e s t e r h a v i n g a p e r m e a b i l i t y a p p r o x i m a t e l y one h a l f t h a t o f a polymer b a s e d p r e d o m i n a n t l y on the more symmetric t e r e p h t h a l a t e g r o u p . S e v e r a l r a t i o n a l e s o t h e r t h a n monomer s t r u c t u r e were c o n s i d e r e d i n a t t e m p t s t o e x p l a i n t h e gas b a r r i e r enhancement i m p a r t e d by t h e m-phenylene u n i t s (from i s o p h t h a l i c a c i d ) . F o r example, t h e opaque a p p e a r a n c e of f i l m s p r e p a r e d f r o m i s o p h t h a l o y l - r i c h BPA 20a and DMBPC 22a p o l y a r y l a t e s suggested the p o s s i b i l i t y t h a t increased c r y s t a l l i n i t y may account f o r the lowered p e r m e a b i l i t y of these m a t e r i a l s r a t h e r t h a n any s t r u c t u r a l e f f e c t o f t h e r e p e a t u n i t s . W h i l e no d e t a i l e d s t u d y of the e f f e c t o f d i a c i d r a t i o on p o l y a r y l a t e c r y s t a l l i n i t y has been r e p o r t e d , c e r t a i n o b s e r v a t i o n s p o i n t a g a i n s t t h i s being a determining f a c t o r f o r observed trends i n the t r a n s p o r t rates. F i r s t o f a l l , an e a r l y s u r v e y o f p o l y a r y l a t e p r o p e r t i e s s t a t e s (21) t h a t b i s p h e n o l A c o n t a i n i n g p o l y e s t e r s show i n c r e a s i n g amounts o f c r y s t a l l i n i t y when t h e y c o n t a i n e i t h e r p r e d o m i n a n t l y i s o or terephthalic acid. In a d d i t i o n , v i s u a l i n s p e c t i o n and DSC a n a l y s i s of the 5,5-bis-(4-hydroxyphenyl)nonane c o n t a i n i n g polya r y l a t e s 19 a-c i n d i c a t e s t h a t a l l t h r e e o f t h e s e c o m p o s i t i o n s a r e amorphous. T h i s e v i d e n c e i n d i c a t e s t h e r e i s no g e n e r a l r e l a t i o n s h i p between e x t e n t of c r y s t a l l i n i t y c a u s e d by t h e isomeric diacid composition and oxygen p e r m e a b i l i t y o f t h e p o l y a r y l a t e s . In a s i m i l a r manner, t h e r e i s no c o r r e l a t i o n between t h e p o l y e s t e r s 1

In Barrier Polymers and Structures; Koros, W.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

7.

SCHMIDHAUSER & L O N G L E Y

Gas Transport Through Polymers

Table V. Comparison of Polyester and Polyestercarbonate Permeabilities

(Copolymer with 10 % BPA)

Table VI. Effect of Diacid Content on Polyarylate Permeability

Acid Content Polymer No.

Bisphenol

% Iso

% Tere

PrO 2

90

10

2.61

50

50

3.03

10

90

3.53

75

25

1.41

50

50

1.81

20c

25

75

2.52

22a

90

10

0.35

22b

50

50

0.44

22c

0

100

0.60

19a 19b

0H

"°-O-K>

19c

20a 20b

HO-0-|-0-OH

In Barrier Polymers and Structures; Koros, W.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

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170

BARRIER POLYMERS AND STRUCTURES

density ( T a b l e V I I ) and oxygen p e r m e a b i l i t y / u n l i k e t h e r e c e n t l y reported (2JL) f i n d i n g s o f t h e p r o p e r t i e s o f p o l y ( p h e n o l p h t h a l e i n phthalate)s. M o l e c u l a r s t r u c t u r e , r a t h e r t h a n p o l y e s t e r morphology, i s t h e r e f o r e found t o e x e r t a c o n t r o l l i n g i n f l u e n c e on t h e m a t e r i a l s gas t r a n s p o r t p r o p e r t i e s . 1

Fitting Q_f Transport Data to Structure-Permeability Models . Measurements of the oxygen p e r m e a b i l i t y p r o p e r t i e s of t h e new substituted polyarylates, polyetherimides, and p o l y c a r b o n a t e s show t h a t gas t r a n s p o r t r a t e s f o r a polymer c l a s s a r e not c e n t e r e d around a s i n g l e v a l u e , but i n s t e a d can span a l a r g e range. Given t h i s broad d i s t r i b u t i o n of p o s s i b l e p r o p e r t i e s , the a b i l i t y t o r e l a t e polymer s t r u c t u r e t o p o l y m e r t r a n s p o r t r a t e s would be most d e s i r a b l e . Of s e v e r a l p o s s i b l e s t r a t e g i e s , the approach of r e l a t i n g polymer p e r m e a b i l i t y p r o p e r t i e s to another polymer p h y s i c a l p r o p e r t y is a t t r a c t i v e due t o i t s s i m p l i c i t y . F o r example a c o r r e l a t i o n between polymer g l a s s t r a n s i t i o proposed. Unfortunately upon t h e p r o p e r t i e s of th bispheno g polymer (Tabl VII) To i l l u s t r a t e t h i s p o i n t , a p l o t o f p e r m e a b i l i t y v e r s u s Tg f o r t h e substituted polycarbonates 2-27 ( F i g u r e 2) shows no discernable trend. E v i d e n t l y a more s o p h i s t i c a t e d a p p r o a c h t o permeability m o d e l i n g i s needed. The p r o c e s s of a permeate d i f f u s i n g t h r o u g h a t r a n s i e n t network of c h a n n e l s or m i c r o v o i d s i s b a s i c t o most m o d e l s o f p o l y m e r diffusion. The v a r y i n g t i g h t n e s s of t h e p o l y m e r c h a i n p a c k i n g i s t h e r e f o r e s t r o n g l y i m p l i c a t e d as a f a c t o r i n d e t e r m i n i n g polymer permeability rates. However when t r y i n g t o p r e d i c t p o l y m e r c h a i n p a c k i n g , one i s c o n f r o n t e d by a p r o b l e m t h a t has p l a g u e d enzyme/ p r o t e i n c h e m i s t s f o r y e a r s , namely t h a t t h e r e i s c u r r e n t l y no good model f o r r e l a t i n g monomer ( p r i m a r y ) s t r u c t u r e t o p o l y m e r c h a i n folding/packing (tertiary) structure. On a v e r y s i m p l e l e v e l , t h i s i s i l l u s t r a t e d by a l a c k of c o r r e l a t i o n between monomer symmetry and gas p e r m e a b i l i t y o f t h e d e r i v e d p o l y m e r , as was observed f o r the methyl-substituted bisphenol containing polycarbonates and the iso-/terephthalic acid containing polyarylates. W i t h no direct method p r e s e n t l y a v a i l a b l e t o c a l c u l a t e t h e e x t e n t o f p o l y m e r c h a i n p a c k i n g , o t h e r a p p r o x i m a t i o n methods must be used. The f r e e volume a p p r o a c h has been an i n c r e a s i n g l y p o p u l a r method t o r e l a t e polymer s t r u c t u r e t o gas t r a n s p o r t p r o p e r t i e s . The b a s i c p r e m i s e o f t h i s t e c h n i q u e i s t h a t a p o l y m e r w i t h an open, p o o r l y p a c k e d s t r u c t u r e w i l l have a l a r g e u n o c c u p i e d f r e e volume t h r o u g h which a gas can d i f f u s e w i t h e a s e . In a t y p i c a l model, s e t f o r t h by Lee (23.) a s p e c i f i c f r e e volume, SFV, i s d e r i v e d f r o m t h e d i f f e r e n c e between t h e m o l a r volume, Vm, (determined from the experimental d e n s i t y o f a polymer) and the o c c u p i e d volume, Vo, ( c a l c u l a t e d u s i n g a group a d d i t i v e method, i n t h i s case, t h a t of Bondi) (AO.) . A p l o t o f the l o g a r i t h i m of t h e oxygen p e r m e a b i l i t y c o e f f i c i e n t s v e r s u s t h e r e c i p r o c a l of t h e SFVs, as p r o p o s e d by Lee, shows a f a i r c o r r e l a t i o n between t h e s e v a l u e s . Data f o r t h e p o l y e t h e r i m i d e s and polyarylates ( F i g u r e 3) i s meager, b u t does a l l o w some g e n e r a l c o n c l u s i o n s t o be drawn. The model does a p p a r e n t l y o v e r e s t i m a t e the p e r m e a b i l i t y f o r the b i s p h e n o l A b a s e d PEI 24. In a d d i t i o n , t h e f i t of t h e p o l y a r y l a t e d a t a w i t h the c a l c u l a t e d model i s f a v o r a b l e e x c e p t f o r an o v e r e s t i m a t i o n of the p e r m e a b i l i t y o f the bis(4-hydroxy-

In Barrier Polymers and Structures; Koros, W.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

7.

Gas Transport Through Polymers

SCHMIDHAUSER & L O N G L E Y

Table VII.

Physical Properties of Polymers Prepared from

rnier No.

G l a s s Transistion

Density

Temperature ( ° C )

(g/cc)

Bisphenols

SFV

1

150

1.199

0.138

2

137

1.159

0.141

5

179

1.208

0.129

6

228

1.147

0.131

7

155

1.222

0.124

8

145

1.212

0.119

9

155

1.203

0.114

10

150

1.238

0.109

1 1

98

1.167

0.127

12

65

1.128

0.137

13

80

1.081

0.159

14

191

1.086

0.163

15

101

1.093

0.149

16

117

1.192

0.111

1 7

131

1.184

0.096

18

278

1.127

0.175

19a

160

1.126

0.147

19b

156

1.128

0.145

19c

188

1.124

0.148

20a

186

1.208

0.135

20b

195

1.205

0.137

20c

212

1.223

0.125

21

156

1.121

0.145

22a

181

1.208

0.105

22b

193

1.177

0.127

22c

232

1.183

0.123

23

258

1.226

0.131

24

222

1.280

0.112

25

214

1.253

0.107

3 4

In Barrier Polymers and Structures; Koros, W.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

171

172

BARRIER P O L Y M E R S AND S T R U C T U R E S

8

-1.5 H 5

Figure 3.

•

1

6

Correlation between

i

1

•

1

•

7 8 Specific Free Volume (-1)

ι

—

9

polyarylate and polyetherimide oxygen

10

permeability

versus reciprocal of specific free volume.

In Barrier Polymers and Structures; Koros, W.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

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Gas Transport Through Polymers

173

3 - m e t h y l p h e n y l ) h e p t a n e b a s e d PAr 21. One s h o r t c o m i n g o f t h e g r o u p a d d i t i v e method i s t h a t t h e t a b l e s u s e d (4_L) do n o t d i s t i n g u i s h between i s o m e r i c d i s u b s t i t u t e d b e n z e n e s , w i t h t h e r e s u l t t h a t t h e same o c c u p i e d volumes a r e c a l c u l a t e d f o r p o l y a r y l a t e s c o n t a i n i n g d i f f e r e n t r a t i o s o f i s o - and t e r e p h t h a l i c a c i d . The success o f t h i s model i n a n a l y z i n g the polycarbonate p e r m e a b i l i t y d a t a i s mixed ( F i g u r e 4) . A t i t s b e s t , t h i s model c o r r e c t l y p r e d i c t s the ordering of the p e r m e a b i l i t i e s of the 3,3'-disubstituted polycarbonates 11 - 14. On t h e o t h e r hand, t h e order of p e r m e a b i l i t i e s p r e d i c t e d f o r the medium-sized ring containing bisphenol PCs 7 - 9 i s wrong. In a d d i t i o n t h e r e i s s u b s t a n t i a l s c a t t e r i n t h e r e s t o f t h e d a t a , w i t h an extreme example b e i n g SBI b a s e d PC 6 h a v i n g a much g r e a t e r p e r m e a b i l i t y t h a n i s calculated. T h e r e f o r e , a f r e e volume a p p r o a c h t o m o d e l i n g p o l y m e r p e r m e a b i l i t y , w h i l e h a v i n g m e r i t , a l s o has room f o r r e f i n e m e n t and work c o n t i n u e s towards t h a t end. Conclusions Oxygen permeabilities have been measured for polycarbonate, p o l y a r y l a t e and p o l y e t h e r i m i d e p o l y m e r s made f r o m a common s e t o f s t r u c t u r a l l y v a r i e d b i s p h e n o l monomers. W i t h i n each polymer c l a s s , the t r a n s p o r t r a t e s were f o u n d t o v a r y d r a m a t i c a l l y d e p e n d i n g upon which monomer was used, w i t h t h e same r e l a t i v e o r d e r o f r a t e s b e i n g o b s e r v e d i n each s e t o f m a t e r i a l s . By comparison, t h e d i f f e r e n c e i n p e r m e a b i l i t i e s among members o f d i f f e r e n t p o l y m e r c l a s s e s b a s e d on t h e same monomer was f o u n d t o be s m a l l e r . These r e s u l t s c a n be i n t e r p r e t e d as showing t h a t monomer s t r u c t u r e has a dominant r o l e i n determining the transport rates of these bisphenol containing polymers, while t h e type o f f u n c t i o n a l group l i n k p l a y s a secondary r o l e i n t h i s regard. A more g e n e r a l comparison o f oxygen p e r m e a b i l i t y data f o r polycarbonates and p o l y e s t e r s shows t h a t , g i v e n a s i m i l a r monomer structure, a polycarbonate e x h i b i t s t h e lower gas t r a n s p o r t r a t e . This observation i s d i f f e r e n t t h a n one would e x p e c t b a s e d upon a v a i l a b l e l i t e r a t u r e p e r m e a b i l i t y data, p o i n t i n g out t h e d i f f i c u l t y in drawing conclusions about polymer structure-permeability r e l a t i o n s h i p s u s i n g d a t a from a r e l a t i v e l y s m a l l number o f p o l y m e r s . T h r o u g h o u t t h e s e s t u d i e s , s y n t h e s i s o f new p o l y m e r s containing s p e c i f i c a l l y v a r i e d monomers p l a y e d a c r u c i a l r o l e i n d e t a i l i n g t h e complex r e l a t i o n s h i p between polymer s t r u c t u r e and gas p e r m e a b i l i t y . The f i n d i n g t h a t p o l y m e r s w i t h i n a p a r t i c u l a r c l a s s p o s s e s s a wide range o f t r a n s p o r t r a t e s s h o u l d make i t p o s s i b l e t o s y n t h e s i z e m a t e r i a l s whch e x h i b i t a b e t t e r match between p e r m e a b i l i t y and some o t h e r p r o p e r t y needed f o r a p a r t i c u l a r a p p l i c a t i o n , s u c h as t h e r m a l stability. The c h a l l e n g e comes i n a t t e m p t i n g t o d e v e l o p a model f o r p r e d i c t i n g polymer t r a n s p o r t p r o p e r t i e s b a s e d on t h e s t r u c t u r e o f t h e repeat unit. T h i s s t u d y shows t h a t t h e p r e s e n c e o f fcntJl t h e f u n c t i o n a l g r o u p l i n k and t h e s t r u c t u r e o f t h e monomer must be considered when attempting t o model polymer permeability. F u r t h e r m o r e , examples were f o u n d where t h e symmetry o f a monomer's s t r u c t u r e does n o t by i t s e l f c o r r e l a t e w i t h p o l y m e r p e r m e a b i l i t y (e.g., the iso-/terephtha1ate containing polyarylates). A d d i t i o n a l l y , no s i m p l e r e l a t i o n s h i p between polymer p e r m e a b i l i t y and o t h e r p h y s i c a l p r o p e r t i e s was found.

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7.

SCHMIDHAUSER & L O N G L E Y

Gas Transport Through Polymers

175

1

A detailed analysis of the 3, 3 -disubstituted bisphenol containing polycarbonates provided an explanation for their observed oxygen permeabilities. However such an approach is not of general utility. Using an existing free-volume approach, a qualitative relationship between monomer structure and the experimental polymer permeabilities was found to exist. Therefore, these and other results are presently being used to refine an excess-free volume type method in hopes of achieving better estimated gas transport properties based on molecular structure. Acknowledgments We thank John Bendler for performing the molecular modeling calculations and for many helpful discussions. Monty Alger and Lorraine Rogers provided some of the permeability data, while John Campbell, Dave Dardaris, Gary Faler, Ed Fewkes, Paul Howson, Rick Joyce, Jerry Lynch an described in this report Literature Cited 1. 2. 3. 4. 5. 20, 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20.

Reed, T. F . ; Thomas, G. B. Chemtech 1988, 18, 48-53. "New Generation of Membranes Developed for Industrial Separations", Chem. Eng. News, June 6, 1988, pp. 7-16. Stannett, V. T. Polym. Sci. Eng. 1978, 18, 1129-1134. Steingiser, S.; Nemphos, S. P.; Salame, M. In Kirk-Othmer Encyclopedia of Chemical Technology 3rd Ed.; Grayson, M., Ed.; Interscience: New York, 1978; Vol. 3, pp.480-502. Pye, D. G.; Hoehn, H. H.; Panar, M. J . Appl. Polym. Sci. 1976, 287-301. Sykes, G. F . ; St. Clair, A. K. J . Appl. Polym. Sci. 1986, 32, 3725-3735. Stern, S. Α.; Mi, Y.; Yamamoto, H.; St. Clair, A. K. J . Polym. Sci. Polym. Phys. Ed. 1989, 27, 1887-1909. Takada, K.; Matsuya, H.; Masuda, T . ; Hiyashimura, T. J . Appl. Polym. Sci. 1985, 30, 1605-1616. Kawakami, Y.; Kamiya, H.; Toda, H.; Yamashita, Y. J . Polym. Sci. Polym. Chem. Ed. 1987, 25, 3191-3204. Stern, S. Α.; Shah, V. M.; Hardy, B. J . J . Polym Sci. Polym. Phys. Ed. 1987, 25, 1263-1298. Cassidy, P. E.; Aminabhavi, T. M.; Thompson, C. M. Rubber Chem. Tech. 1983, 56, 594-618 . Perkins, W. Polym. Bull. 1988, 19, 397-401. Meares, P. J . Amer. Chem. Soc. 1954, 76, 3415-3422. Vieth, W. R.; Sladek, K. J . J . Colloid Sci. 1965, 20, 1014-1033. Paul, D. R.; Koros, W. J . J . Polym. Sci. Polym. Phys. Ed. 1976, 14, 675-685. Fredrickson, G. H . ; Helfand, E. Macromolecules 1985, 18, 2201-2207. Petropoulos, J . H. J . Polym. Phys. Ed. 1988, 26, 1009-1020. Koros, W. J.; Chan, A. H.; Paul, D. R. J . Membrane Sci. 1977, 2, 165-190. Koros, W. J . Ph. D. Dissertation; Univ. of Texas, Austin, 1977. Erb, A. J . M. S. Thesis, Univ. Of Texas, Austin, 1977.

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21. Schnell, H. Chemistry and Physics of Polycarbonates; Interscience: New York, 1969. 22. Pilato, L.; Litz, L. M.; Hargitay, B.; Osbourne, R. C.; Farnham, A. G.; Kawakami, J.; Fritze, P. E.; McGrath, J. E. Polym. Prepr. 1975, 16, 42-44. 23. Muruganandam, N.; Koros, W. J.; Paul, D. R. J. Polym. Sci. Polym. Phys. Ed. 1987, 25, 1999-2026. 24. Moe, M. B.; Koros, W. J.; Paul, D. R. J. Polym. Sci. Polym. Phys. Ed. 1988, 26, 1931-1945. 25. Sheu, F. R.; Chern, R. T.; Stannett, V. T.; Hopfenberg, H. B. J. Polym. Sci. Polym. Phys. Ed. 1988, 26, 883-892. 26. Barbari, Τ. Α.; Koros, W. J.; Paul, D. R. J. Polym. Sci. Polym. Phys. Ed. 1988, 26, 709-727. 27. Barbari, T. A.; Koros, W. J.; Paul, D. R. J. Polym. Sci. Polym. Phys. Ed. 1988, 26, 729-744. 28. Tsia, Η. B.; Lee, Y.-D. J. Polym. Sci. Polym. Phys. Ed. 1987, 25, 3405-3412. 29. Wirth, J. G.; Heath 30. Huglin, M. B.; Zakaria, Angew , , 1-13. 31. Mark, V.; Hedges, C. U. S. Patent 4 304 899, 1974. 32. Crank, J . ; Park, G. S. Diffusion in Polymers, Academic Press: London, 1968. 33. Yee, A. F.; Smith S. A. Macromolecules 1968, 14, 54-73. 34. Ratto, J. Α.; Inglefield, P. T.; Rutowski, R. Α.; L i , K.-L.; Jones, Α. Α.; Roy, A. K. J. Polym. Sci. Polym. Phys. Ed. 1987, 25, 1419-1430. 35. Salame, M. Polym. Eng. Sci. 1986, 26, 1543-1546. 36. Chem. Engineering November 24, 1986, p. 11. 37. Eareckson, W. M. J. Polym. Sci. 1959, 40, 399-406. 38. Sheu, F. R.; Chern, R. T. J. Polym. Sci. Polym. Phys. Ed. 1989, 27, 1121-1133. 39. Lee, W. M. Polym. Eng. Sci. 1980, 20, 65-69. 40. Bondi, A. Physical Properties of Molecular Crystals, Liquids and Gases; Wiley: New York, 1968. 41. VanKrevelen, D. W.; Hoftyzer, P. J. Properties of Polymers; Eisevier: New York 1972; pp. 410-420. RECEIVED

December 11, 1989

In Barrier Polymers and Structures; Koros, W.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

Chapter 8

Water Transport Through Polymers Requirements and Designs in Food Packaging LeonardE.Gerlowski Polymeric Materials Department, Shell Development Company, Houston, TX 77251-1380

The plastics packaging industry needs to understand the transport of water through polymers in order to properly utilize these materials in food containers. The primary goal of any food container is to protect the quality of the packaged food. To meet this goal, and account for water-polymer interactions which can affect the food, the transport of water must be properly taken into account in material selection and design of plastic containers. For example, the abundance of water in the food packaging environment for sterilization, sealing, etc., can affect the polymer properties and limit the package to only certain uses. Also, interactions of water with polymers used in food packaging can affect the organoleptic properties (taste characteristics) of food-polymer interactions. On this basis, the transport characteristics of water in polymers are reviewed from a physico-chemical basis. These transport characteristics are then included in design equations and three design calculations are presented; i . an equation to predict proper wall thickness; ii. a heuristic rule to insure the polymer is transport limiting; and iii. an example of a water mass balance to predict the effect of water sorption on transport properties common to the food packaging environment. The many benefits offered by plastics over conventional materials has led to increased use over the last decade (1). Some of these benefits include light weight, ease of use and manufacture, aesthetics, economics and many others. However, one deficiency of plastics is that a l l polymers allow small molecules to dissolve into and diffuse through this matrix at some rate. This process has been termed permeation and is of interest to protect the food from 0097-6156/90/0423-0177S06.00/0 © 1990 American Chemical Society

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external or i n t e r n a l permeants which can a l t e r the food chemistry. Permeation of water i s of p a r t i c u l a r interest i n food packaging due to i t s abundance and importance to the food ( 2 ) . Water i s used at high temperatures and pressures i n the food packaging environment for s t e r i l i z a t i o n , sealing, etc. (3). Such processes can affect the i n i t i a l package properties (2). Also, most foods require a c e r t a i n water content to provide proper taste, consistency and other properties. It is the r e s p o n s i b i l i t y of the package design to insure the package can provide such food properties over a given shelf l i f e . The objective of this work i s to b r i e f l y review water transport i n polymers and provide h e u r i s t i c rules to a i d i n proper container design. The interest of the p l a s t i c s packaging industry i n designing containers which l i m i t water transport i s two-fold. F i r s t , the quality of the packaged food must be maintained. In many cases, this requires keeping water out of dried foods or i n wet foods ( 4 ) . To design containers to transport c h a r a c t e r i s t i c properties can be highly non-linea and may require taking the water content of the food into account. With a proper understanding of these transport properties, a container can be designed which properly maintains the water content of a packaged good. Secondly, the permeability of gases other than water i n many hydrophilic polymers can be affected by water content. The second case i s of p a r t i c u l a r interest where food undergoes a high temperature, high water (steam) s t e r i l i z a t i o n process following packaging. The high water a c t i v i t y i n the b a r r i e r polymer r e s u l t i n g from s t e r i l i z a t i o n can reside with the container for long periods of time. With a proper understanding of the water transport c h a r a c t e r i s t i c s of the container, this long term effect can be properly handled i n the design. Methodologies to predict the response of p l a s t i c package designs i n such instances are also discussed i n the container design section. In order to use design methodologies, the basic data used i n such calculations (transport properties) should be well understood. Simple gases such as oxygen and carbon dioxide, under most normal food packaging conditions, follow Fickian transport mechanisms. However water, due to i t s hydrogen bonding nature and other polar interactions with the polymer backbone, tends to follow d i f f e r e n t transport mechanisms under transient conditions. These waterpolymer interactions have been generically quantified and are reviewed b r i e f l y i n the transport mechanism section. Also, examples of alterations to the polymer backbone to reduce water transport are presented i n the polymer s t r u c t u r a l c h a r a c t e r i s t i c s section. These examples are presented as guidelines to follow i n designing or a l t e r i n g polymer chemical structure to meet water transport requirements. TRANSPORT MECHANISM Water and polymer interactions have been recognized since the early developments of polymeric materials. These interactions have led to characterizing polymers as hydrophobic (e.g. p o l y o l e f i n s , etc.) and hydrophilic ( e . g . , p o l y v i n y l a l c o h o l , nylons, e t c . ) . The way i n

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Water Transport Through Polymers

which polymers interact with water can affect the transport mechanism of water through polymers. Table I contains the transport parameters of several polymers of commercial food packaging interest. The water permeabilities of these polymers vary several orders of magnitude. T y p i c a l l y , the hydrophilic polymers sorption values are also much larger than the hydrophobic polymer values. This trend indicates that water sorption plays a role i n determining the water transport c h a r a c t e r i s t i c s . The way i n which water permeates through polymers i s much the same as other vapors and has been well characterized (5,6). As i s the case with non-condensible gases, the permeability i s a function of the sorption l e v e l and the d i f f u s i v i t y of the permeating substance : Ρ

=

D χ

S

(1)

Typically, i n hydrophobi r e l a t i v e l y low, and th cases, the permeability is r e l a t i v e l y unaffected by the surrounding water (water a c t i v i t y or r e l a t i v e humidity). The transport mechanism i n these cases follows F i c k ' s Law. On the other hand, the water sorption l e v e l can be extremely high i n hydrophilic polymers. In these cases, the permeability i s greatly affected by the amount of water present. The state of water adsorbed can be considered: i.) as homogeneously distributed throughout the polymer; i i . ) to reside primarily i n l o c a l i z e d areas, or i i i . ) to exist i n clusters (5). These high levels of water content can greatly affect the transport mechanism and dominate the transport process (as opposed to d i f f u s i o n l i m i t e d ) . In these cases, the water can be thought to move as a uniform front through the polymer [often referred to as Case II sorption (7)] and the dynamics of water uptake is t y p i c a l l y l i n e a r with time. The c h a r a c t e r i s t i c water uptake equations for the three states mentioned above are reviewed b r i e f l y i n the following. Homogeneous sorption occurs i n cases where the i n t e r a c t i o n between the polymer and water is uniform. Flory (17) described this case as the polymer and water being randomly d i s t r i b u t e d . The d i s t r i b u t i o n can be described by: p

0

/ p

-

V

H20

e x

P

( V

p

+

X

V

p

2 )

(

2

)

where p^ i s the p a r t i a l pressure of water i n the environment, ρ is the t o t a l pressure, X i s the interaction parameter which can be a function of the water a c t i v i t y , and V . ' s are the volume fractions polymer and water. Other more complicated forms have been postulated to include crosslinked systems and crystalline systems (5). This interaction mechanism has been applied to cellulose, nylons, and polyvinylalcohol over limited water a c t i v i t y (8-12). Localized sorption occurs when adsorbed water i s t i g h t l y bound to the polymer molecules. In these cases, two populations of water are considered to exist i n the polymer: a bound population and a l i q u i d l i k e population. This sorption isotherm i s described by:

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BARRIER POLYMERS AND STRUCTURES

Table I.

Water Transport Properties

Ρ 100 F 90% RH„ (gm-mil/100in -d) PVDC (coatings) (films)

0.02 - 0.06 0.08 - 0.20

high density polyethylene

0.38

polypropylene

0.66

low density polyethylene

1.

PET

S* Polymer (gm/100gmpolymer) < 0.1

Reference 36,37 36,37 38

0.0071

38

1.8

0.8

38

Polyvinylchloride

2.2

1.5

36,38

PAN

2.1**

3.6**

35,18

EVAL-F -H -E

3.8 2.1 1.4

3.0

30 30 30

Nylon-6/6,11

3.8

4.0

36

Acrylonitrile/ S t y r è n e (BAREX)

6.1

PS

7.4

35

0.048

36,38

Nylon-6

10.9

36

Polycarbonate

11.4

35

Nylon-12

63.5

Cellulose acetate

76.0

Hydroxy ethyl cellulose

4.0

110

* at 25 C, 100% RH * * at 30 C

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8.

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181

Water Transport Through Polymers A Β

-

C_p_ - V C P

(1 - p ) ( l 0

0

._ )

(3)

Where A molecules are d i s t r i b u t e d over Β s i t e s and C represents the p a r t i t i o n i n g of bound and l i q u i d water. This "BET" l i k e absorption model has been modified to more s i m p l i f i e d forms for saturable sorption models. For low water a c t i v i t y , this sorption model has been applied to several hydrophilic polymers including epoxies (13) and nylons (14). Clustering accounts for the association of water molecules i n the polymer. In clusters water molecules can associate with other water molecules, but not necessarily polymer molecules. NMR evidence of these types of associations have been found i n the literature. In this case, the sorption isotherms can be described as :

G —

=

ρ

-

V

r , 1

3a

H20 H20 Where v. is the p a r t i a l molar volume, G i s the c l u s t e r i n g i n t e g r a l defined such that when G/v i s greater than -1 there i s a p r o b a b i l i t y for c l u s t e r i n g to occur. This sorption isotherm has been applied to high water adsorbing polymers [nylon (15), polyurethanes (16), and p o l y a c r y l o n i t r i l e (18)] at high water a c t i v i t i e s (5). For most applications for food packaging, sorption of water i n most hydrophilic polymers can be f i t to a saturation isotherm, such as Equation 3. A s i m p l i f i e d form would be: A

S =

p

0

(5)

where A and Β are empirically determined constants temperature. Although, this empirical approach may physico-chemical insight to polymer/water interactions, adequate information for design of p l a s t i c packages. water i n most hydrophobic polymers can be f i t to simple l i n e a r sorption: S

=

P

0

at a given not provide i t provides Sorption of Henry's law

k

(6)

where k i s the sorption c o e f f i c i e n t . The d i f f u s i v i t y can be affected by concentration of water, but i n most instances the transport of water i s sorption l i m i t e d . The temperature and surrounding r e l a t i v e humidity effects on uptake k i n e t i c s are usually more important than the water content effects. POLYMER STRUCTURAL CHARACTERISTICS As was seen from Table I for most hydrophilic polymers, the sorption of water appears to be the c o n t r o l l i n g element of the transport equation (1) . With this i n mind, there are several means to characterize this process. The p o l a r i t y of the polymer backbone i s a good i n d i c a t i o n as to the l e v e l of sorption and water transport.

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BARRIER POLYMERS AND STRUCTURES

This i n d i c a t i o n i s not as straight forward as i s the case for non-interacting transporting gases (such as oxygen and carbon dioxide). The structural permeability relationships for non-interacting gases have been established with empirical mechanisms relating permeability to a backbone interaction parameter. One method develops a "permachor" and has been established by Salame for a variety of polymers (19,20). The second method i s based on amorphous free volume (21) . Based on this approach, the more s p e c i f i c free volume i n the polymer, the higher the permeability or d i f f u s i o n c o e f f i c i e n t . Both of these methods are based on polymer-polymer interactions and predict gas transport properties of non-interacting systems w e l l . It appears that i n highly adsorbing water-polymer systems, the transport i s dependent on polymer-water interactions and these s t r u c t u r a l p r e d i c t i o n methods do not apply at high RH. With this i n mind, we consider the functional groups on a polymer backbone (Figur no free electrons for interactio water sorption and permeability are extremely low. In cases where the pendant groups are highly hydrogen bonding groups and can e a s i l y attach to water, the sorption and permeability are both high and water a c t i v i t y dependant. In cases where the pendant groups are very strong interactors - for example polyvinylidene chloride and polyvinylidene fluoride - pendant groups interact with each other more strongly than with water. In these cases, the water sorption and permeability are extremely low. Van Krevelen (22) provides a c o r r e l a t i o n of water sorption for various backbone and pendant groups at several r e l a t i v e humidities. For the hydrophobic o l e f i n backbone groups, the contribution i s 2-5 orders of magnitude below that for the hydrophilic backbone groups. In ranging water a c t i v i t y from 0.3 to 1.0, the increase i n group contribution from the o l e f i n structures i s of the order of 5. Whereas, i n the case of hydrophilic backbone groups, the increase over this water a c t i v i t y range is an order of magnitude. For pendant groups of halogen molecules or o l e f i n groups such as methane, the contribution i s similar to that from an o l e f i n backbone. However, i n the case of hydrophilic pendant groups such as alcohols, the contribution i s greater than the hydrophilic backbone groups by an order of magnitude. For cases of new polymer design, many times i t may be important to increase or decrease water transport i n a given polymer backbone for some a p p l i c a t i o n . Choices of chemical groups to a l t e r the backbone can be based on h e u r i s t i c guidelines from Van Krevelen (22). As was previously discussed, adding o l e f i n i c or halogen nature to the backbone can reduce transport and adding hydrophilic groups can increase transport. In an example of s t r u c t u r a l alterations to reduce water sorption, Barrie et a l . (23) considered halogenation of epoxy resins. They added bromine, chlorine, and trifluoramethyl groups to the benzene rings i n MY720. At 40, 50, and 60 C they found lowering of sorption levels with the largest reduction with the fluorinated system. The reduction was almost 3X based on a dual mode model with the fluorinated system. Little effect was seen on the d i f f u s i o n c o e f f i c i e n t . The non-Fickian

In Barrier Polymers and Structures; Koros, W.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

GERLOWSKI

Water Transport Through Polymers

H XI I I (-C-C-)I I H X2

XI H

H

H

CH

H

1.4

0.006

0.66

0.007

Cl

2.2

1.5

Cl

Cl

0.06

H

F

0.04

0.01

F

F

0.06

0.04

F,F

F,F

0.02

-0-

Cl, F

F,F

0.03

-0-

F,F

CF ,F

0.02

0.01

3

3

P

-

units of

S

-

gm per

gm-mil/100sqin-d

100

gm polymer

Data taken from Reference F i g u r e 1.

70% , i t s oxygen and water b a r r i e r effectiveness i s severely reduced because o f d i s r u p t i o n o f the polymer-polymer hydrogen bonds (10-12) . Also, because o f t h e h y d r o x y l g r o u p s on the polymer backbone, water tends t o be e x t r e m e l y s o l u b l e i n EvOH. F i g u r e 1 g i v e s t h e w a t e r s o l u b i l i t y i n EvOH-32^ as a f u n c t i o n o f h u m i d i t y a t 20°C and 120°C. We see t h a t the water s o l u b i l i t y i n c r e a s e s markedly as h u m i d i t y i s r a i s e d above 65-75%. Measurements made by Hopfenberg e t a l . (10) show t h a t the maximum water s o l u b i l i t y at 100% humidity i s a v e r y s t r o n g f u n c t i o n o f t h e v i n y l a l c o h o l c o n t e n t i n the polymer. F i g u r e 2 shows t h e oxygen p e r m e a b i l i t y c o e f f i c i e n t f o r EvOH-32 as a f u n c t i o n o f w a t e r c o n t e n t a t 20C. The oxygen b a r r i e r e f f e c t i v e n e s s of EvOH32 d e c r e a s e s markedly w i t h the absorption of water because of p l a s t i c i z a t i o n of t h e EvOH-32 m a t r i x . PRINCIPLES

o f a n EvOH R E T O R T A B L E FOOD

PACKAGE

Re t o r t a b l e f o o d package s t r u c t u r e s w h i c h are formed by coextrusion. The number of d i f f e r e n t layers i n a the s t r u c t u r e can v a r y ; a r e p r e s e n t a t i v e f i v e - l a y e r s t r u c t u r e i s shown i n F i g u r e 3 . O f t e n one o r more l a y e r s a r e added t o reuse s c r a p and r e g r i n d i n the package. There i s a wide range o f p o s s i b l e c o m b i n a t i o n s o f layer t h i c k n e s s e s w h i c h c a n be u s e d i n t h i s t y p e o f s t r u c t u r e . I n r e t o r t the package i s s t e r i l i z e d a t h i g h temperature, 220-270°F, f o r one t o two h o u r s . D u r i n g t h i s p e r i o d o f time the r e l a t i v e h u m i d i t y on b o t h the i n s i d e and o u t s i d e package w a l l s i s 100% . As a r e s u l t , water permeates through the o u t e r l a y e r s and i n t o the c e n t e r EvOH l a y e r . The s o l u b i l i t y of water i n EvOH a t h i g h temperatures i s r e m a r k a b l y h i g h as shown i n F i g u r e 1. S i n c e t h e EvOH l a y e r i s p r o t e c t e d from the water by the o u t e r l a y e r s , the t o t a l amount o f water absorbed i n the EvOH l a y e r d u r i n g r e t o r t i s l i m i t e d by water p e r m e a t i o n t h r o u g h these o u t e r l a y e r s . I f v e r y l o n g r e t o r t times are u s e d t h e n t h e amount o f w a t e r a b s o r b e d i n t h e EvOH l a y e r c a n be q u i t e s i g n i f i c a n t (4) . Water s o l u b i l i t y a t 120 °C i s about 35 t o 40 g/100 g EvOH (4,5). A f t e r the r e t o r t c y c l e i s complete the package i s p l a c e d i n "storage". W h i l e the i n s i d e h u m i d i t y o f the package remains at 100%, the outside humidity c a n v a r y o v e r a w i d e r a n g e d e p e n d i n g upon the e x a c t s t o r a g e c o n d i t i o n s . Changes i n temperature a r e v e r y l i k e l y t o o c c u r during storage. I f we assume that there i s no i r r e v e r s i b l e loss of EvOH b a r r i e r effectiveness f o l l o w i n g retort t h e n the p r i m a r y v a r i a b l e o f importance i s the t o t a l water absorbed by the EvOH. The w a t e r c o n t e n t i s an i n i t i a l c o n d i t i o n f o r the d r y i n g p e r i o d following retort. D u r i n g s t o r a g e t h e e x c e s s w a t e r i n t h e EvOH l a y e r d e s o r b s and the p a c k a g e a p p r o a c h e s a s t e a d y - s t a t e . D u r i n g t h i s l a t t e r phase the oxygen p e r m e a b i l i t y d e c r e a s e s as water i s desorbed from the EvOH layer i n accordance w i t h the r e l a t i o n s h i p g i v e n i n F i g u r e 2 . The drying stage can l a s t from weeks t o many months depending on the p a r t i c u l a r package l a y e r d i s t r i b u t i o n and the t o t a l w a t e r a b s o r b e d d u r i n g t h e r e t o r t p r o c e s s . A f t e r excess water has permeated out o f the EvOH l a y e r , the package reaches a steady-state c o n d i t i o n

1

EvOH- 32 r e f e r s t o e t h y l e n e - v i n y l a l c o h o l copolymer w i t h 32 mol % e t h y l e n e . A l l subsequent r e f e r e n c e s t o EvOH-32 w i l l be f o r E v a l c a E v a l - F grade m a t e r i a l .

In Barrier Polymers and Structures; Koros, W.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

206

BARRIER P O L Y M E R S AND STRUCTURES

40

+ Apicella and Hopfenberg (1982) D

this work

C\J

*

30

Ο >

120 C

LU

ο ο

20

τ­

ω

G5

20 C

£ 10

Ο)

20

40

60

80

100

Relative Humidity, % F i g u r e 1. Water S o l u b i l i t y i n EvOH-32 as a f u n c t i o n of humidity. Results a t 25°C w e r e t a k e n f r o m r e f 10, L L . and 26. R e s u l t s a t 120°C a r e from ref 5 .

I I

100

>>

•

Ikari and Motoishi (1984)

100% Humidity at 20 C

T = 20C π

1

4

1

1

8

— ι

12

1

1

16

~i

r

20

1

1

24

1 —

28

g water/100 g EvOH-32 F i g u r e 2. Oxygen P e r m e a b i l i t y i n EvOH-32 as a f u n c t i o n o f water r e g a i n a t 20°C (5.) .

In Barrier Polymers and Structures; Koros, W.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

10. ALGER ET AL.

207

Retortable Food Packages

EvOH polypropylen

Storage, 65-75% Humidity

Food Contents, 100% Humidity

tie layers F i g u r e 3. F i v e - L a y e r S t r u c t u r e w i t h P P / t i e / E v O H / t i e / P C .

In Barrier Polymers and Structures; Koros, W.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

208

BARRIER POLYMERS AND STRUCTURES

d u r i n g w h i c h the w a t e r and oxygen p e r m e a t i o n r a t e s r e a c h a c o n s t a n t v a l u e . W h i l e somewhat o f an i d e a l i z a t i o n , because storage conditions are not f i x e d i n r e a l i t y , the i m p o r t a n t a s p e c t s o f the performance of a r e t o r t a b l e package are c a p t u r e d u s i n g t h i s s i m p l i f i e d model. MODEL o f a R E T O R T A B L E

PACKAGE

To p r e d i c t t h e p e r f o r m a n c e o f a r e t o r t a b l e package b o t h w a t e r and oxygen p e r m e a t i o n must be i n c l u d e d i n t h e m o d e l . To i l l u s t r a t e a s i m p l i f i e d c a l c u l a t i o n o f t h e p e r f o r m a n c e o f a r e t o r t a b l e package we c o n s i d e r t h e i d e a l i z e d f i v e - l a y e r structure shown i n Figure 3. There are two d i s t i n c t parts of the c a l c u l a t i o n : r e t o r t and s t o r a g e . To a f i r s t approximation the r e t o r t c a l c u l a t i o n d e t e r m i n e s the t o t a l water absorbed by the EvOH l a y e r during the r e t o r t c y c l e and can be thought o f as an i n i t i a l c o n d i t i o n f o r the s t o r a g e calculation. RETORT CALCULATION D u r i n i s p r e s e n t so t h a t o n l y wate the d i f f u s i o n c o e f f i c i e n t o y hig so t h a t a p s e u d o - s t e a d y - s t a t e model, i n w h i c h l i n e a r p r o f i l e s are assumed a c r o s s a l l s t r u c t u r a l l a y e r s , can be used. The EvOH layer i s assumed to be at a u n i f o r m w a t e r a c t i v i t y a t a g i v e n time which i s good a s s u m p t i o n based on w a t e r p e r m e a b i l i t y m e a s u r e m e n t s i n EvOH a t h u m i d i t i e s found i n package applications (13). W r i t i n g a mass b a l a n c e f o r water i n the s t r u c t u r e g i v e s the f o l l o w i n g e q u a t i o n f o r r e g a i n , Φ [g water/100 g EvOH]: d - " d t

P^T) - - - - P

W (*V

5

EvOH EvOH

*Η

Ε ν 0 Η

1

) - /> (RH w

Ev0H

- RH )>

(1)

2

2

We use the term " r e g a i n " t o r e f e r t o an amount o f water i n the EvOH l a y e r a t a g i v e n c o n d i t i o n and " s o l u b i l i t y " a t a g i v e n water a c t i v i t y to r e f e r to the a m o u n t o f w a t e r a b s o r b e d w h e n t h e EvOH i s i n e q u i l i b r i u m w i t h t h e surrounding water a c t i v i t y . I n the water mass b a l a n c e , e f f e c t i v e water t r a n s m i s s i o n rates, /3 , are d e f i n e d f o r t h e l a y e r s b e t w e e n t h e f o o d a n d EvOH l a y e r W

k _ 1

β„

-

( Σ

. 1=1

1

and

δ c--))"

Ί

f o r the layers

< Σ

. , . i=k+l

1

(2)

w. ι

between N

"2

Ρ

ambient

a n d t h e EvOH

s. ( Ρ w.

—

(

3

)

ι

For the g e n e r a l c a s e , the k ^ l a y e r i s EvOH and i n t h i s example k=3 and N=5. δι i s the t h i c k n e s s and Ρ i s the water permeability of l a y e r i , r e s p e c t i v e l y .

In Barrier Polymers and Structures; Koros, W.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

10.

Retortable FoodPackages

ALGER ET AL.

209

The temperature p r o f i l e d u r i n g r e t o r t can be s p e c i f i e d as any general f u n c t i o n of t i m e ; however, we have used o n l y c o n s t a n t r i s e and f a l l r a t e s and f i x e d r e t o r t t e m p e r a t u r e . By so d o i n g we assume t h a t h e a t t r a n s f e r through the package and r a t e o f h e a t i n g the c o n t e n t s i s v e r y r a p i d . T h i s assumption i s r e a s o n a b l e ; t o r e l a x i t w o u l d s e r v e o n l y t o c o m p l i c a t e the c a l c u l a t i o n . An i m p o r t a n t p a r t o f the r e t o r t s i m u l a t i o n f o r LEP and PEP structures i s t h e t e m p e r a t u r e dependence o f the p o l y p r o p y l e n e and p o l y c a r b o n a t e water p e r m e a b i l i t i e s . Measurements o f p o l y p r o p y l e n e and p o l y c a r b o n a t e water p e r m e a b i l i t i e s were made u s i n g a MOCON Permatran-W™ and i t was found t h a t p o l y c a r b o n a t e has a water p e r m e a b i l i t y which decreases s l i g h t l y with temperature whereas p o l y p r o p y l e n e i n c r e a s e s w i t h temperature. Near r e t o r t temperature the water p e r m e a b i l i t i e s of both materials are s i m i l a r ; at storage p o l y c a r b o n a t e has a water p e r m e a b i l i t y about t e n times g r e a t e r than p o l y p r o p y l e n e . Measurements shown i n F i g u r e 4 are i n good agreement w i t h previously reported r e s u l t s f o r polycarbonate (2,14-18) and polypropylene (5,1921) . The r e t o r t s i m u l a t i o u s i n g the i s o t h e r m f o r EvOH-wate f o r p o l y p r o p y l e n e and p o l y c a r b o n a t e shown i n F i g u r e 4. S i n c e v o l u m e t r i c measurements are n o t a v a i l a b l e , the EVOH-32 was assumed t o have a constant d e n s i t y o f 1.19 g/cm^ i n t h e c a l c u l a t i o n . STORAGE CALCULATION D u r i n g s t o r a g e the temperature and o u t s i d e package h u m i d i t y are b o t h lowered. Storage temperatures i n the range o f 10 to 40°C and h u m i d i t i e s from 50 t o 85% RH are common. A g a i n , w h i l e i t i s p o s s i b l e to use a g e n e r a l i z e d time-dependent s t o r a g e h u m i d i t y and temperature, most o f the i n t e r e s t i n g performance c h a r a c t e r i s t i c s are r e a l i z e d w i t h constant values. The i n s i d e package h u m i d i t y was s e t t o 100% f o r a l l c o n d i t i o n s we considered f o r wet f o o d a p p l i c a t i o n s . Modelling of v a r i a b l e package humidity coupled w i t h o x y g e n p e r m e a t i o n h a s b e e n d i s c u s s e d b y Hows mon a n d P e p p a s (2_2) . The water r e g a i n i n the EvOH l a y e r i s c a l c u l a t e d as a f u n c t i o n o f time u s i n g eqn 1. The i n i t i a l l o a d i n g o f water i s t a k e n d i r e c t l y from the r e t o r t c a l c u l a t i o n ; o u t s i d e h u m i d i t y i s imposed o n l y on the outside of the package. The f o o d c o n t e n t s a r e a s s u m e d t o r e m a i n a t 100% h u m i d i t y a t a l l t i m e s . With the water c o n t e n t o f the EvOH a v a i l a b l e from i n t e g r a t i o n of eqn 1, t h e o x y g e n t r a n s m i s s i o n r a t e o f t h e package, OxTr, i s c a l c u l a t e d from:

OxTr -

( I

i=l

(— i - ) f

ρ 0

1

(4)

2

i s

β

where P Q ^ i - ^ oxygen p e r m e a b i l i t y o f l a y e r i . Normally a l l of the oxygen b a r r i e r i s s u p p l i e d by the EvOH l a y e r . The OxTr can then be i n t e g r a t e d w i t h time t o c a l c u l a t e accumulated oxygen: 2

.t 3 cm ( s t p ) = 0.21 A

OxTr d t 0

In Barrier Polymers and Structures; Koros, W.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

(5)

210

BARRIER POLYMERS AND STRUCTURES

20 C

0.0025

0.0029

0.0033

temperature, 1/Kelvin F i g u r e 4. Water P e r m e a b i l i t y for Polypropylene and Polycarbonate. P o l y p r o p y l e n e measurements were made on 9.5 m i l extruded sheet ; polycarbonate measurements were made on 10 and 20 mil extruded sheet. Note that permeability has water pressure i n the denominator which is d i s t i n c t l y d i f f e r e n t from a WVTR (water vapor transmission rate) .

In Barrier Polymers and Structures; Koros, W.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

10. ALGER ET AL.

Retortable Food Packages

211

where A is the surface area of the package and the factor of 0.21 is added for ambient oxygen pressure. Oxygen concentration i s calculated from t o t a l cm^(stp) found from eqn (5) and the volume of the package. An expression for the EvOH oxygen permeability as a function of humidity and temperature is r e q u i r e d to i n t e g r a t e eqn ( 5 ) . Using any standard numerical routine, the above set of coupled equations, eqns 1- 5 , can be solved to simulate both the retort and storage of a multilayer package. RESULTS o f SIMULATED PACKAGE PERFORMANCE F i g u r e 5 shows the c a l c u l a t e d ppm' s of oxygen vs time for symmetrical LEP and PEP structures s t o r e d both at 65 and 75% RH. Figure 6 gives the water desorption i n the EvOH l a y e r as a f u n c t i o n of time corresponding to the ppm' s of oxygen i n F i g u r e 5. From s o l u t i o n of the retort model equations we find that the use of a p o l y c a r b o n a t e outs ide l a y e r decreases the amount of oxygen which w i l l permeate into a package fo following. At r e t o r t condition p o l y p r o p y l e n e are comparable, as shown i n Figure 4 , so that the amount of water allowed into the EvOH during r e t o r t is s i m i l a r i n the two structures. However , under t y p i c a l storage c o n d i t i o n s , polycarbonate has a water p e r m e a b i l i t y w h i c h i s a p p r o x i m a t e l y 10 times g r e a t e r than that of p o l y p r o p y l e n e ; thus, the EvOH dried out faster i n the LEP structure and, t h e r e f o r e , l e s s oxygen can diffuse through the LEP during dry out. Some early studies suggested that the ratio of polycarbonate to polypropylene water permeabilities was the same under retort conditions as under storage conditions w h i c h w o u l d r e s u l t i n a " f l o o d " of water p a s s i n g through the outer polycarbonate skin into the EvOH layer during r e t o r t . As seen i n Figure 4 , t h i s a s s u m p t i o n i s a p o o r one. While the above model c e r t a i n l y captures the performance of a retortable package, package tests have shown that complications arise from the retort process which lead to an i r r e v e r s i b l e change i n EvOH barrier properties ( 4 ) . Since the exact nature of t h i s change i s very p o o r l y understood, i t is necessary to develop a test methodology to provide insight into mechanisms o p e r a t i n g i n the p a c k a g e to make a t r u l y q u a n t i t a t i v e m o d e l . OXYGEN INGRESSION EXPERIMENTS

on FOOD PACKAGES

An oxygen head-space analysis was used to measure oxygen ingression into packages as a function of time following r e t o r t . Packages were placed in an oxygen-free glove box, f i l l e d with 1-2 cc of oxygen-free water, and then sealed. The packages were retorted under predetermined conditions and then placed i n storage at a fixed humidity. At various time intervals individual packages were s e l e c t e d and destructively tested using a M0C0N/T0RAY LC7OOF™ headspace analyzer to measure oxygen concentration i n the package. The procedure can be modified by f i r s t r e t o r t i n g the package, flushing with nitrogen, and placing i t in storage (8). While usually not a problem, this l a t t e r approach w i l l 'miss ' any oxygen which permeates into the package during the r e t o r t process . We have found that dissolved oxygen i n the package material can diffuse i n during r e t o r t . This is illustrated by Figure 7 which shows ppm oxygen vs time following r e t o r t for a LEP package. In Figure 7 a 2 ppm oxygen offset was observed immediately following r e t o r t . The offset was presumed to be a r e s u l t of the very thick polycarbonate outer wall in which there was dissolved oxygen available for d i f f u s i o n into the package during r e t o r t .

In Barrier Polymers and Structures; Koros, W.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

212

BARRIER POLYMERS AND STRUCTURES

18.0 "PEP"

16.0 Storage Humidity = 75%

14.0 c Φ

12.0

CD

._ _

Ε

8.0

ίΓ 100 ο Ω­ α.

6.0

= 75%

"LEP"

4.0 2.0

65%

0.0

1

— ι

1

0

1

r

1

— ι

40

1

1

1

1

1

80 120 time, days

1

1

1

1

r~

160

200

F i g u r e 5 . C a l c u l a t e d ppm' s oxygen vs time f o r s y m m e t r i c a l LEP and PEP s t r u c t u r e s ( 10 m i l PP/2 EvOH/10 PP o r PC ; R e t o r t 90 m i n u t e s , 120°C) .

12

C\J CO I X

10

Ο >

LU Ο) Ο ο CO

Ο

a

Φ

•Η

Ο

υ •rl «Η «Η

1β-14+

Φ

Ο

d α 0 ^

ο

3 «Η «Η •Η Ρ

le-15-

4-

5

4-

β

7

4-

8

9

10

Number of Carbons i n E s t e r F i g u r e 6. D i f f u s i o n C o e f f i c i e n t a t 85'C o f E s t e r s i n a V i n y l i d e n e C h l o r i d e Copolymer F i l m

Ο.ΙΟΟτ

8 α Φ

•Η

υ •Η *Η «Η

^

Φ οβ 0 Û4

ο2

0.010+

i

3 0

ο ο

CO

0.0015

4-

4-

6

7

4-

8

θ

10

Number of Carbons i n E s t e r Figure

7. S o l u b i l i t y C o e f f i c i e n t a t 85'C o f E s t e r s i n a V i n y l i d e n e C h l o r i d e Copolymer F i l m

In Barrier Polymers and Structures; Koros, W.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

18.

STRANDBURG ET A L

Diffusion and Sorption of Linear Esters

O.lOOr

43 α Φ

•Η

U

•Η «Η «Η

0

s i

°

ο

g 0.010+

* "Sa

Ο Ο

rH •Η ιΗ

Ο Ο

Ο CO

J

0.001

100

120

140

160

180

200

220

B o i l i n g Point Figure

8. S o l u b i l i t y C o e f f i c i e n t s o f E s t e r s a t 85*C i n a V i n y l i d e n e C h l o r i d e Copolymer F i l m

32

T

30f u

φ4 α

28+ Φ

§^

26+

•rl rH 4J (β

•Η

43

«3

Ο

O

24+

>X

Ο

Ο

^

22+ 20+

10 Number of Carbons i n E s t e r Figure

9. A c t i v a t i o n Energy f o r D i f f u s i o n o f E s t e r s i n a V i n y l i d e n e C h l o r i d e Copolymer F i l m

In Barrier Polymers and Structures; Koros, W.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

343

344

BARRIER P O L Y M E R S AND STRUCTURES

-4-r

-5-

-7--

Ο

o " - ^

°

-10»

ο

-11-

Ο

-12-13"

-14-1

1

1

1-

10 Number of Carbons

F i g u r e 10. Heats o f S o l u t i o n f o r E s t e r s i n a V i n y l i d e n e C h l o r i d e Copolymer F i l m

ΙΟΟΟτ

43 •rl rH

ri CSJ

0 ^

J

100

1

1

1

1

h

5

6

7

8

9

Number of Carbons Figure

11.

Permeability Coefficient of E s t e r s Through EVOH

a t 85'C

In Barrier Polymers and Structures; Koros, W.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

18.

Diffusion and Sorption of Linear Esters

STRANDBURGETAL.

α •Η

υ

10 •14 +

•Η «Η «Η

Ο ϋ

^
M

•H

^

43

30+

3

25-

+-

5

6

7

8

9

Number of Carbons F i g u r e 14. A c t i v a t i o n Energy o f D i f f u s i o n f o r E s t e r s Through EVOH

-1+ -2+

δ

el

-3+

% -4+

-5+ -6+ -7·

4-

1

6

7

f—

8

Number of Carbons F i g u r e 15. Heats o f S o l u t i o n f o r E s t e r s i n an EVOH Copolymer F i l m

In Barrier Polymers and Structures; Koros, W.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

18. STRANDBURG ET AL.

347

Diffusion and Sorption ofLinear Esters

F i g u r e s 16 t h r o u g h 18 show the

results

f o r the

LDPE f i l m .

Discussion. In each f i l m the p e r m e a b i l i t i e s i n c r e a s e as t h e s i z e o f the e s t e r i n c r e a s e s . The d a t a show t h a t t h i s r e s u l t i s d r i v e n by the s o l u b i l i t y c o e f f i c i e n t . The d i f f u s i o n c o e f f i c i e n t s d e c r e a s e as the s i z e o f the e s t e r i n c r e a s e s ; however, t h e s o l u b i l i t y c o e f f i c i e n t s i n c r e a s e g r e a t l y as t h e s i z e o f t h e e s t e r i n c r e a s e s . The q u a l i t a t i v e e f f e c t o f permeant s i z e on the d i f f u s i o n c o e f f i c i e n t was e x p e c t e d . A r e c e n t s i m p l e a n a l y s i s q u a n t i f i e d the r e l a t i o n s h i p between D and permeant s i z e f o r s p h e r i c a l permeants up t o Cg i n s e v e r a l polymers ( 10) . That a n a l y s i s i s n o t d i r e c t l y a p p l i c a b l e h e r e s i n c e t h e s e permeants a r e l i n e a r and l a r g e r . A n o t h e r s t u d y q u a n t i f i e s t h e r e l a t i o n s h i p between D and permeant s i z e f o r l i n e a r a l k a n e s i n p o l y e t h y l e n e (11). The d i f f u s i o n c o e f f i c i e n t a t 25"C was o b s e r v e d t o d e c r e a s e from 3.9 χ 1 0 " m^/s f o r n-hexane t o 2.2 χ 10 m^/s f o r n-nonane. The d i f f u s i o n c o e f f i c i e n t seem t o change comparabl d i f f u s i o n c o e f f i c i e n t s f o r the e s t e r s d e c r e a s e b y about 3x as t h e number o f atoms ( e x c l u d i n g hydrogen) i n the backbone i n c r e a s e s from 6 t o 9 (5 carbons p l u s one oxygen t o 8 carbons p l u s one oxygen). The d i f f e r e n c e would be g r e a t e r a t 25'C because t h e a c t i v a t i o n e n e r g i e s f o r d i f f u s i o n are l a r g e r f o r the l a r g e r e s t e r s . This i s c o n s i s t a n t w i t h the r e s u l t s o f L a n d o i s - G a r z a and H o t c h k i s s f o r an e s t e r s e r i e s i n p o l y v i n y l a l c o h o l (12). At 30"C i n LDPE, t h e d i f f u s i o n c o e f f i c i e n t s d e c r e a s e b y about 30% as t h e number o f atoms i n the backbone o f t h e e s t e r i n c r e a s e s f r o m 6 t o 9. S t u d i e s o f a l k a n e s i n low d e n s i t y p o l y e t h y l e n e show s i m i l a r t r e n d s (13). 11

_

1

1

8

io T

o ο ο

100 +

ο

5

6

7

8

9

10

Number o f Carbons F i g u r e 16.

P e r m e a b i l i t y a t 30 C f o r e

Library 115S 16tft St., N.W. In Barrier Polymers and Structures; Koros, W.; Washington, O.C. 20038 ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

348

BARRIER POLYMERS A N D STRUCTURES

43

10r l 2 4 0 1«

•H

CQ «H «H •H

Q

4

5

-+6

7

Number

8

9

10

o f Carbons

F i g u r e 17. D i f f u s i o n C o e f f i c i e n t o f E s t e r s i n a LDPE F i l m

a t 30'C

l.OOOr

0.100+

«Η «Η

ο +> μ •H rH •Η

w

0.010+

rH

0 CO

0.001

—I 100

1 120

1 140

Boiling

1 160 Point

F— 180

200

°C

F i g u r e 18. S o l u b i l i t y C o e f f i c i e n t a t 30*C v s . B o i l i n g P o i n t o f E s t e r i n a LDPE F i l m

In Barrier Polymers and Structures; Koros, W.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

18. STRANDBURG ET AL»

349

Diffusion and Sorption of Linear Esters

The a c t i v a t i o n e n e r g i e s f o r d i f f u s i o n i n t h e co-VDC and EVOH f i l m s i n c r e a s e as t h e s i z e o f t h e e s t e r i n c r e a s e s . T h i s i s e x p e c t e d s i n c e c o o p e r a t i v e m o t i o n o f l a r g e r zones o f t h e polymer m a t r i x a r e necessary t o c r e a t e l a r g e r holes f o r passage. The a c t i v a t i o n e n e r g i e s f o r d i f f u s i o n i n LDPE were n o t d e t e r m i n e d . The s o l u b i l i t y c o e f f i c i e n t s make i n t e r e s t i n g c o n t r i b u t i o n s t o the p e r m e a b i l i t i e s . The s o l u t i o n p r o c e s s o f t h e s e gas phase permeants c a n be s e p a r a t e d f o r a n a l y s i s i n t o two components c o n d e n s a t i o n and m i x i n g . Table I I I contains the heats of condensation f o r the experimental e s t e r s . Table I I I also contains t h e h e a t s o f s o l u t i o n i n t h e polymer f i l m s from l i n e a r f i t s o f t h e d a t a i n F i g u r e s 10 and 15. F i n a l l y , T a b l e I I I c o n t a i n s t h e h e a t s o f mixing f o r the e s t e r s i n the f i l m s . The h e a t s o f m i x i n g were c a l c u l a t e d u s i n g e q u a t i o n 9.

Table III Heats of Condensation and Solution for Experimental Esters Number o f

AH

ΔΗ

Q

Carbons

a)

1

3

ΔΗ

3

AH

m i x

AH

m i x

co-VDC

EVOH

co-VDC

EVOH

-8..4

-1..6

0..4

7.3

5

-8..88

6

-9..47

7

-10. .48

1

-9..7

-2..8

0..8

7.7

8

-11. ,20

2

-10, .3

-3,.5

0..9

7.7

9

-11. .92

-10, .9

-4,.1

1..0

7.9

10

-12, .87

-9..1

2

-11, .5

—— ——

0..4

1..4

kcal/mole

1) average

o f two v a l u e s

2) e x t r a p o l a t e d v a l u e s

The h e a t s o f c o n d e n s a t i o n a r e r e l i a b l e d a t a : On t h e o t h e r hand, there i s c o n s i d e r a b l e experimental u n c e r t a i n t y i n the heats of solution. The t r e n d s i n t h e heat o f s o l u t i o n a r e more c e r t a i n ; however, t h e a c t u a l v a l u e s a r e n o t so c e r t a i n . This uncertainty a r i s e s from two s o u r c e s . F i r s t , the s o l u b i l i t y c o e f f i c i e n t s are d e r i v e d from t h e p e r m e a b i l i t i e s and t h e d i f f u s i o n c o e f f i c i e n t s . Hence, t h e r e l a t i v e u n c e r t a i n t i e s o f b o t h Ρ and D t r a n s f e r t o S. The h e a t s o f s o l u t i o n a r e d e r i v e d by f i t t i n g e q u a t i o n 7 t o o n l y 3 o r 4 data. This i s not i d e a l . F u t u r e work w i l l expand t h e d a t a base to test t h i s preliminary analysis. Nevertheless, the trends f o r the heats of mixing a r e probably c o r r e c t as shown i n T a b l e I I . As t h e e s t e r s g e t l a r g e r , t h e m o l e c u l e becomes more l i k e an a l k a n e , and t h e p o l a r e s t e r group i s diluted. The t r e n d i n t h e h e a t s o f m i x i n g f o r t h e co-VDC f i l m i s interesting. As t h e e s t e r s become l a r g e r t h e h e a t s o f m i x i n g become l e s s f a v o r a b l e i n t h e co-VDC f i l m . This i s a r e f l e c t i o n of the

In Barrier Polymers and Structures; Koros, W.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

350

BARRIER POLYMERS AND STRUCTURES

d i l u t i o n of the ester function. Esters are known to interact well with chlorine containing polymers. The heats of mixing f o r the esters i n the EVOH f i l m are unfavorable. They are more unfavorable than the heats of mixing i n the co-VDC f i l m . The heats of mixing i n the EVOH f i l m became more unfavorable as the size of the ester increases. This behavior may be predicted by s o l u b i l i t y parameters. Future work w i l l include a variety of functional groups to study the s p e c i f i c interactions of polymer and permeant. SUMMARY 1. 2. 3. 4. 5. 6. 7.

The d i f f u s i o n c o e f f i c i e n t decreases modestly as the ester size increases f o r low density polyethylene, EVOH and co-VDC f i l m s . The s o l u b i l i t y c o e f f i c i e n t undergoes a much greater change with increasing ester s i z e . Thus the permeability increases with increased ester s i z e . P l a s t i c i z i n g the polyme i n the d i f f u s i o n c o e f f i c i e n t Important flavor an compound package by sorption. The amount of flavor loss to the package i s dependent on both d i f f u s i o n and s o l u b i l i t y c o e f f i c i e n t s . The heats of mixing are dependent on s p e c i f i c interactions between polymer and permeant.

LITERATURE CITED 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13.

DeLassus, P. T.; Hilker, B. L . , Proc. Future-Pak '85 (Ryder), 1985, p. 231. Tou, J. C.; Rulf, D. C.; DeLassus, P. T.; accepted for publiciation in Analytical Chemistry. DeLassus, P. T.; Tou, J. C.; Babinec, M. A.; Rulf, D. C.; Karp, Β. K.; Howell, B. A.; ACS Symposium Series 365, Food and Packaging Interactions (J. H. Hotchkiss, ed.) 1988, p. 11. Zobel, M. G. R., Polymer Testing 1989, 5, 153. Lange's Handbook of Chemistry 13th Edition, McGraw-Hill, N.Y., 1985, pp 10-28. DeLassus, P. T.; Strandburg, G., Howell, Β. Α., TAPPI Journal, 1988, 71(11), 177. Ziegel, K. D.; Frensdorff, Η. K.; Blair, D. E . , J. Polym. Sci., Part A-2, 1969, 7, 809. Pasternak, R. Α.; Schimscheimer, J. R.; Heller, J., J. Polym. Sci., Part A-2, 1970, 8, 467. Zobel, M. G. R., Polymer Testing 1982, 3, 133. Berens, A. R.; Hopfenberg, H. B., J. Membrane Sci., 1982, 10, 283. Peters, H.; Vanderstracten, P.; Verhoeye, L.; J. Chem. Tech. Biotech., 1979, 29, 581. Landois-Garza, J.; Hotchkiss, J. H., Food and Packaging Interactions, ACS Symposium Series 365, 1987, 53. Asfour, A. F. Α.; Saleem, M.; DeKee, D., Journal of Applied Polymer Science, 38, 1989, 1503.

RECEIVED January 31, 1990

In Barrier Polymers and Structures; Koros, W.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

Chapter 19

Sorption and Diffusion of Monocyclic Aromatic Compounds Through Polyurethane Membranes 1

1

U. Shanthamurthy Aithal , Tejraj M.Aminabhavi ,and PatrickE.Cassidy

2

1

Department of Chemistry, Karnatak University, Dharwad, 580003, India Department of Chemistry, Southwest Texas State University, San Marcos, TX 78666 2

Sorption and transpor aromatics through a polyurethane membrane have been investigated at 25, 44 and 60°C based on an immersion/weight gain method. Activation parameters for the process of diffusion, permeation and sorption are found to follow the conventional wisdom that larger molecules exhibit lower diffusivities and higher activation energies. From a temperature dependence of the sorption constant, the standard enthalpy and entropy have been determined. The molar mass between c r o s s l i n k s has been determined by using the Flory-Rehner theory. Furthermore, results have been discussed in terms of the thermodynamic interactions between the polymer and penetrants. P o l y u r e t h a n e e l a s t o m e r s a r e known t o e x h i b i t unique mechanical p r o p e r t i e s , p r i m a r i l y as a r e s u l t of two p h a s e m o r p h o l o g y (_1 ). These m a t e r i a l s a r e alternating b l o c k copolymers made of h a r d segments of a r o m a t i c groups from t h e d i i s o c y a n a t e / c h a i n e x t e n d e r and soft segments of a l i p h a t i c c h a i n s from t h e d i o l ( e t h e r o r e s t e r ) . The h a r d and soft segments a r e c h e m i c a l l y i n c o m p a t i b l e and m i c r o p h a s e s e p a r a t i o n of t h e h a r d segments into domains d i s p e r s e d i n a m a t r i x of soft segments c a n occur i n v a r y i n g d e g r e e s . In v i e w of the i m p o r t a n c e of p o l y u r e t h a n e as a b a r r i e r m a t e r i a l i n s e v e r a l e n g i n e e r i n g areas (_2»_3), i t i s i m p o r t a n t t o know i t s t r a n s p o r t characteristics w i t h r e s p e c t to common o r g a n i c s o l v e n t s . Thus, k n o w l e d g e of t h e t r a n s p o r t mechanisms as m a n i f e s t e d b y s o r p t i o n , diffusion and permeation of o r g a n i c liquids (penetrants) i n a polyurethane matrix i s helpful f o r establishing the relationships between structures and p r o p e r t i e s under severe application c o n d i t i o n s . A l t h o u g h some p r e v i o u s s t u d i e s (4 -10) have been made on s o l v e n t t r a n s p o r t t h r o u g h p o l y u r e t h a n e , more e x p e r i m e n t a l data a r e s t i l l needed f o r a b e t t e r u n d e r s t a n d i n g of t h e t h e r m o d y n a m i c interactions between t h e polymer and solvents. The p r i n c i p a l o b j e c t i v e of t h i s paper i s to f o l l o w t h e t r a n s p o r t p r o p e r t i e s of

0097-6156/90/0423-0351$07.50/0 © 1990 American Chemical Society

In Barrier Polymers and Structures; Koros, W.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

352

BARRIER POLYMERS AND STRUCTURES

monocyclic aromatic liquids through a commercially available p o l y u r e t h a n e membrane. I t i s e x p e c t e d t h a t a s y s t e m a t i c change i n s o l v e n t power would l e a d to r e s u l t s w h i c h c o u l d be i n t e r p r e t e d by c o n s i d e r i n g t h e p o s s i b l e i n t e r a c t i o n s w i t h soft and h a r d segments of the polymer. Transport properties v i z . , diffusivity, D, permeability, P, and s o r p t i o n , S, have been s t u d i e d o v e r an i n t e r v a l of t e m p e r a t u r e from 25 to 60°C, to p r e d i c t t h e A r r h e n i u s p a r a m e t e r s f o r each of t h e t r a n s p o r t p r o c e s s e s i n v o l v e d . F u r t h e r ­ more, t h e r e s u l t s have been d i s c u s s e d i n terms of t h e r m o d y n a m i c i n t e r a c t i o n s between p o l y u r e t h a n e and t h e l i q u i d p e n e t r a n t s . Molar mass, M^, between c r o s s l i n k s have been e s t i m a t e d b y u s i n g t h e F l o r y - R e h n e r model (11,12). EXPERIMENTAL REAGENTS AND MATERIALS. P o l y u r e t h a n e ( P U ) used was o b t a i n e d from PSI, A u s t i n , T e x a s i n s h e e t s of 0.250 cm t h i c k n e s s T h e base polymer i s a Vibrathan the r e a c t i o n of p o l y p r o p y l e n (TDI). T h e base p o l y m e r was c u r e d w i t h 4, 4 ' - m e t h y l e n e - b i s ( o - c h l o r o a n i l i n e ) i . e . , MOCA, to g i v e t h e p o l y u r e t h a n e . T h u s , t h e t w o - p h a s e m o r p h o l o g y of PU c o n s i s t e d of p o l y e t h e r d i o l as t h e soft segment and t h e a r o m a t i c d i i s o c y n a t e a c t i n g as t h e h a r d segment. The d r i v i n g f o r c e f o r t h e phase s e p a r a t i o n i s t h e i n c o m p a t i b i l i t y of t h e h a r d and soft segments. A s c h e m a t i c a l d e s c r i p t i o n of t h e m o l e c u l a r s t r u c t u r e of t h e p o l y u r e t h a n e used i s g i v e n below : V^A/W-

AC — A B A B A B A B A B A — ι

J—ι

soft where,

1

CA — i

hard

_

soft

A : +-CM- C H 4 j 4 2

i

BABABAB-vvwvw, ι

ι

hard Β : —f- O — f C H 4 y 4 2

Some r e p r e s e n t a t i v e e n g i n e e r i n g p r o p e r t i e s of PU a r e t e n s i l e s t r e n g t h , 387 k g / s q . cm ( 5500 p s i ) ; maximum p e r c e n t elongat­ i o n , 430; modulus f o r 300% e l o n g a t i o n , 155 k g / s q . cm ( 2200 p s i ) ; tear s t r e n g t h , 5 kg/sq.cm(70 p s i ) (ASTM D- 470) and s p e c i f i c g r a v i t y 1.101. T h e Τ of t h e p o l y m e r was found to be -43.27°C w i t h a heat f l o w of - 1.420 Watts/g as d e t e r m i n e d by differential scanning c a l o r i m e t r y , duPont model 951. The s o l v e n t s g i v e n i n T a b l e I a r e of reagent grade and d o u b l e d i s t i l l e d b e f o r e u s e . DIFFUSION E X P E R I M E N T S : P o l y u r e t h a n e e l a s t o m e r s were cut into u n i f o r m s i z e c i r c u l a r p i e c e s ( d i a m e t e r = 1.9 cm) u s i n g a s p e c i a l l y designed, sharp-edged, s t e e l d i e and d r i e d o v e r n i g h t i n a d e s i ­ c c a t o r b e f o r e use. T h e t h i c k n e s s of t h e sample was measured at s e v e r a l points using a micrometer ( p r e c i s i o n ± 0.001 cm) and t h e mean v a l u e was taken as 0.250 cm. D r y w e i g h t s of t h e cut s a m p l e s were taken b e f o r e i m m e r s i o n into t h e a i r t i g h t , m e t a l - c a p p e d test b o t t l e s c o n t a i n i n g t h e l i q u i d . A f t e r i m m e r s i o n into t h e r e s p e c t i v e l i q u i d s , t h e b o t t l e s were p l a c e d i n a t h e r m o s t a t i c a l l y c o n t r o l l e d oven (± 0.5°C).

In Barrier Polymers and Structures; Koros, W.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

19.

AITHAL ET AL.

Sorption and Diffusion Through Polyurethane Membranes 353

At p e r i o d i c i n t e r v a l s , the samples were r e m o v e d from the b o t t l e s , the wet s u r f a c e s were d r i e d between f i l t e r paper w r a p s and w e i g h e d i m m e d i a t e l y to the nearest 0.05 mg by p l a c i n g i t on a c o v e r e d w a t c h g l a s s w i t h i n the chamber of the balance. The samples w e r e p l a c e d back into test b o t t l e s and were t r a n s f e r r e d to the oven. The e x p e r i m e n t s were p e r f o r m e d at 25, 44 and 60°C. A p o s s i b l e source of e r r o r i n t h i s method i s t h a t the sample has to be removed from the l i q u i d to a l l o w w e i g h i n g ; i f t h i s i s done q u i c k l y (say w i t h i n 30-50 sees) c o m p a r e d to the time a sample spent i n the l i q u i d i n between c o n s e c u t i v e w e i g h i n g s , the sample r e m o v a l e x e r t s a n e g l i g i b l e e f f e c t . F o r those l i q u i d s whose d e n s i t y i s g r e a t e r than the p o l y m e r i t s e l f , the sample was k e p t submerged i n the l i q u i d by a g l a s s - p l u n g e r a t t a c h e d to the s c r e w cap. RESULTS AND SORPTION

DISCUSSION

KINETICS.

Roo 1

as p e r c e n t penetrant u p t a k e , Q ( t ) , v e r s u s s q u a r e root of t i m e , t are d i s p l a y e d i n F i g u r e s 1 and 2. A p e r u s a l of the s o r p t i o n c u r v e s g i v e n i n F i g u r e 1 suggests a s y s t e m a t i c t r e n d i n the s o r p t i o n behavior of methyl substituted benzenes. For instance, the s o r p t i o n of benzene i s h i g h e r than o t h e r homologues; a l s o , benzene reaches sorption equilibrium more quicker than toluene, px y l e n e and m e s i t y l e n e . T h i s may be due to the presence of more b u l k i e r -CH^ groups r e n d e r i n g a s l u g g i s h movement of the s o l v e n t w i t h i n the p o l y m e r m a t r i x . The s o r p t i o n c u r v e s of o t h e r a r o m a t i c s , namely, bromobenzene, o-dichlorobenzene, nitrobenzene, chlorobenzene and a n i s o l e are p r e s e n t e d i n F i g u r e 2. Here, bromobenzene e x h i b i t s a maximum Q(t) of about 147% (the maximum of a l l the p e n e t r a n t s ) w h e r e a s a n i s o l e has o n l y about 80%. T h u s , i t i s c l e a r that c h l o r o , bromo, n i t r o and methoxy s u b s t i t u t i o n s on the benzene moiety tend to i n c r e a s e the extent of s o r p t i o n w i t h the p o l y u r e t h a n e membrane. The maximum s o r p t i o n of a l l the penetrants follow the sequence : Bromobenzene > o - d i c h l o r o b e n z e n e > c h l o r o b e n z e n e κ n i t r o b e n z e n e > a n i s o l e > benzene > toluene > p - x y l e n e > m e s i t y l e n e . S o r p t i o n data a l s o s e r v e as a guide to s t u d y the e f f e c t of t e m p e r a t u r e on the o b s e r v e d transport behavior. Temperature v a r i a t i o n of s o r p t i o n c u r v e s f o r some r e p r e s e n t a t i v e penetrants namely, benzene, m e s i t y l e n e , a n i s o l e , bromobenzene and o - d i c h l o r o ­ benzene a r e i n c l u d e d i n F i g u r e s 3-7. In almost a l l c a s e s , the s h a p e s of s o r p t i o n c u r v e s at 25°C a r e s i m i l a r to those at the two h i g h e r t e m p e r a t u r e s , a l t h o u g h the change i n s l o p e i s more pronoun­ ced between 25 and 44°C than between 44 and 60°C. F o r a l l the penetrants except benzene and bromobenzene, the maximum s o r p t i o n v a l u e s at 44 and 60°C seem to be more or l e s s i d e n t i c a l . 2

F o r a F i c k i a n b e h a v i o r , the p l o t s of Q(t) v e r s u s t should increase linearly up to about 50% sorption. Deviations from the F i c k i a n s o r p t i o n a r e a s s o c i a t e d w i t h the time t a k e n by the p o l y m e r segments to r e s p o n d to a s w e l l i n g s t r e s s and r e a r r a n g e

In Barrier Polymers and Structures; Koros, W.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

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BARRIER P O L Y M E R S AND STRUCTURES

]

Figure 1 Percentage mass uptake Q(t) of the polymer versus square root of time t for polyurethane (PU) + solvent pairs at 25 C. β

In Barrier Polymers and Structures; Koros, W.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

AITHAL ET AL.

0

Sorption and Diffusion Through Polyurethane Membranes 355

15

30

45 v 1 (min) 7

60

75

90

•

Figure 2 Percentage mass uptake Q(t) of the polymer versus square root of time t for polyurethane (PU) + solvent pairs at 25 C. β

In Barrier Polymers and Structures; Koros, W.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

1/2

356

BARRIER POLYMERS AND STRUCTURES

0

15

30

45

60

J t (min) Figure 3 Temperature dependence of percentage mass uptake Q(t) of the polymer versus t for polyurethane + Benzene system. 1/2

In Barrier Polymers and Structures; Koros, W.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

AITHAL E T

Sorption and Diffusion Through Polyurethane Membranes 357

Figure 4 Temperature dependence of percentage mass uptake Q(t) versus t polyurethane + Mesitylene system.

In Barrier Polymers and Structures; Koros, W.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

1/2

for

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BARRIER POLYMERS AND STRUCTURES

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Sorption and Diffusion Through Polyurethane Membranes 359

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BARRIER POLYMERS AND STRUCTURES

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Sorption andDiffusion Through Polyurethane Membranes 361

themselves to accommodate the solvent molecules. This usually r e s u l t s i n t h e s i g m o i d a l s h a p e s f o r t h e s o r p t i o n c u r v e s : T h u s , nonFickian diffusion involves the tension between swollen (soft segments) and the u n s w o l l e n ( h a r d segments) p a r t s of polyurethane as the l a t t e r tend to r e s i s t f u r t h e r s w e l l i n g . However, during early stages of s o r p t i o n , the s a m p l e s may not r e a c h the t r u e e q u i l i b r i u m c o n c e n t r a t i o n of the penetrant and t h u s , t h e r a t e of s o r p t i o n b u i l d s up s l o w l y to p r o d u c e s l i g h t c u r v a t u r e s as shown i n F i g u r e s 1-7. T h i s i s i n d i c a t i v e of the d e p a r t u r e from the F i c k i a n mode and i s further confirmed from an a n a l y s i s of s o r p t i o n data by u s i n g the f o l l o w i n g e q u a t i o n (JJ^.J^) : log

(M

/M ) œ

= log k + η log t

(1)

Here, the constant k d e p e n d s on the structural characteristics of the p o l y m e r i n a d d i t i o n to i t s i n t e r a c t i o n w i t h the s o l v e n t ; and are r e s p e c t i v e l y b r i u m , t . The magnitud η transpor ; i n s t a n c e , a v a l u e of η = 0.5 suggests the F i c k i a n mode and f o r η = 1, a n o n - F i c k i a n d i f f u s i o n mode i s p r e d i c t e d . H o w e v e r , the intermediate values ranging from η = 0.5 to u n i t y suggest the p r e s e n c e of anomalous t r a n s p o r t mechanism. F r o m a l e a s t - s q u a r e s a n a l y s i s , t h e v a l u e s of η and k have been e s t i m a t e d and these are included in Table I, and F i g u r e 8 r e p r e s e n t s a t y p i c a l p l o t f o r benzene and toluene. The a v e r a g e u n c e r t a i n t y i n the e s t i m a t i o n of η i s around ±0.007. The v a l u e of η do not i n d i c a t e any s y s t e m a t i c v a r i a t i o n w i t h t e m p e r a t u r e H o w e v e r , a general v a r i a t i o n of η from a minimum v a l u e of 0.53 to a maximum of 0.74 i n d i c a t e s t h a t the anomalous t y p e t r a n s p o r t mechanism is operative and the diffusion is slightly deviated from the F i c k i a n t r e n d . T h i s f a c t can be further substantiated 3

from the c u r v a t u r e d e p e n d e n c i e s of Q(t) v s . t p l o t s shown i n Figures 1-7. Such o b s e r v a t i o n s a r e a l s o e v i d e n t from the work of N i c o l a i s et a l . (_15) f o r n-hexane t r a n s p o r t i n g l a s s y p o l y s t y r e n e A t e m p e r a t u r e dependence of k suggests t h a t i t i n c r e a s e s w i t h a r i s e i n t e m p e r a t u r e f o r a l l the p e n e t r a n t s e x c e p t j D - x y l e n e and bromobenzene. F u r t h e r m o r e , k a p p e a r s to d e p e n d on s t r u c t u r a l characteristics of the penetrant molecules i.e., it decreases successively from benzene to mesitylene; this decrease in k parallels the decrease i n the values of sorption equilibrium. S i m i l a r l y , f o r c h l o r o b e n z e n e to n i t r o b e n z e n e via o-dichlorobenzene k d e c r e a s e s s u c c e s s i v e l y . T h u s , i t a p p e a r s t h a t k not o n l y depends on the structural characteristics of the p o l y m e r and penetrant molecules, but a l s o on s o l v e n t i n t e r a c t i o n s w i t h the polyurethane c h a i n s . At any r a t e , the g r e a t e r tendency f o r n o n - F i c k i a n c o e f f i ­ cients in Equation (1) seen f o r 60°C, may r e f l e c t some a s p e c t of s w e l l i n g of i n t e r p h a s e r e g i o n s between the h a r d and soft domains.

—

TRANSPORT

COEFFICIENTS:

portion

the

of

sorption

From curves

the

slope

i . e . , Q(t)

j

initial

t ,

the

θ , of the vs.

In Barrier Polymers and Structures; Koros, W.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

linear

diffusion

362

BARRIER POLYMERS AND STRUCTURES

Table

I

Analysis o f Penetrant Transport at Various

Molecular Penetrant

Volume χ 1 0

Temperatures

Exponent 2 3

(g/g-hr)102 Q(t) (Eq 1) (X o f d r y sample wt)

Temp

η

(cm3/molecule)

(°C)

(Eq 1)

Benzene

14.85

25 44 60

0.554 0.593 0.599

2.809 3.295 3.975

71.01 72.15 74.14

Toluene

17.75

44 60

0.600 0.599

3.207 3.874

59.09 60.08

£-xylene

20.48

25 44 60

0.561 0.557 0.595

2.295 3.334 3.021

49.68 49.95 49.31

Mesitylene

23.19

25 44 60

0.532 0.571 0.602

1.261 2.203 2.444

40.15 41.52 41.99

Anisole

18.16

25 44 60

0.566 0.585 0.581

2.166 2.765 3.227

80.25 83.89 82.80

Nitrobenzene

17.14

25 44 60

0.602 0.584 0.618

1.134 1.797 1.839

106.32 107.17 114.81

Chlorobenzene

16.98

25 44 60

0.588 0.569 0.584

2.371 3.597 3.739

105.52 109.08 108.84

o-Dichlorobenzene

18.78

25 44 60

0.583 0.565 0.589

1.658 2.441 2.625

131.36 131.17 131.52

Bromobenzene

17.53

25 44 60

0.575 0.586 0.736

2.300 2.790 1.600

147.50 150.06 151.58

m a x

In Barrier Polymers and Structures; Koros, W.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

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Sorption and Diffusion Through Polyurethane Membranes 363

τ

J 1.2

I

I

L

1.5

1.8

2.1

log t Figure 8 Log

Γ

1

•

versus log t for polyurethane + Benzene and polyurethane +

Toluene systems.

In Barrier Polymers and Structures; Koros, W.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

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BARRIER POLYMERS AND STRUCTURES

c o e f f i c i e n t s D, h a v e been c a l c u l a t e d

b y using (16,17)

D = π(ηθ/4Μ

)

2

(2)

00 Here, M has t h e same meaning as b e f o r e and h i s t h e i n i t i a l sample t h i c k n e s s ; t h e s l o p e Q , i s u s u a l l y o b t a i n e d b e f o r e 50% completion of s o r p t i o n . The values of D d e t e r m i n e d in this manner can be r e g a r d e d as independent of c o n c e n t r a t i o n , and a r e t h u s , a p p l i c a b l e f o r t h e F i c k i a n mode of t r a n s p o r t . A t r i p l i c a t e evaluation of D from sorption curves gave us D v a l u e s with an e r r o r of ±0.003 u n i t s at 25°C and ±0.005 u n i t s at 60°C f o r a l l polymer-penetrant systems. These uncertainty estimates regarding diffusion coefficients suggest that the half times were very r e p r o d u c i b l e ( t o w i t h i n a f e w tens of s e c o n d s ) . T h e v a l u e s of D are compiled i n Tabl I n c l u d e d i n t h e same t a b l S, as computed from t h e p l a t e a u r e g i o n s of t h e s o r p t i o n c u r v e s and permeability coefficient, Ρ as c a l c u l a t e d from the simple relation U 8 ) : Ρ = D.S (3) œ

In a l l c a s e s , both p e r m e a b i l i t y and d i f f u s i v i t y of m e t h y l - s u b s t i t u t e d benzenes v a r y i n an i n v e r s e manner w i t h t h e i r m o l e c u l a r volumes (as c a l c u l a t e d b y d i v i d i n g t h e m o l e c u l a r w e i g h t by d e n s i t y and A v o g a d r o number to y i e l d t h e volume p e r m o l e c u l e ) . For o t h e r p e n e t r a n t s , namely, a n i s o l e , n i t r o b e n z e n e , c h l o r o b e n z e n e , o-dichlorobenzene and bromobenzene, t h e volume p e r molecule v a r i e s i n t h e range 17-19. However, t h e i r d i f f u s i o n t r e n d s a r e q u i t e d i f f e r e n t . F o r i n s t a n c e , though n i t r o b e n z e n e and c h l o r o b e n z e n e 2 3

3

h a v i n g almost t h e same m o l e c u l a r volume ( ~ 1 7 x l 0 ~ cm /molecule), y i e l d w i d e l y d i f f e r e n t d i f f u s i v i t y and p e r m e a b i l i t y ; h o w e v e r , t h e maximum S v a l u e s of both t h e penetrants a r e almost identical. Similarly, f o r bromobenzene, the diffusive trends are higher whereas, jo-dichlorobenzene exhibits somewhat intermediatory transport behavior between c h l o r o - and bromobenzene. However, a n i s o l e e x h i b i t s d i f f u s i v e t r e n d s that a r e i n between m e s i t y l e n e and _p-xylene. When our r e s u l t s a r e compared with the literature data, a good agreement c o u l d be seen. F o r e x a m p l e , i n a s t u d y by S c h n e i d e r and c o w o r k e r s (10) f o r t h e t r a n s p o r t of o - d i c h l o r o ­ benzene in a segmented polyurethane elastomer at 25°C, a -7 2 v a l u e of D = 0.95 χ 10 cm /s agrees w i t h our data at 25°C -7 2 ( i . e . , 2.01 χ 10 cm fs). S i m i l a r l y , f o r benzene, toluene, and chlorobenzene at 30°C i n a p o l y u r e t h a n e , Hung (_5) found t h e -7 -7 -7 2 v a l u e s of D as 1.29 χ 10 , 1.43 χ 10 and 1.37 χ 10 cm /s respectively, which agree somewhat w i t h our v a l u e s , 2.90, 2.60 -7 2 and 3.42 χ 10 cm /s r e s p e c t i v e l y , at 25°C.

In Barrier Polymers and Structures; Koros, W.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

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Sorption and Diffusion Through Polyurethane Membranes 365

Table

II.

S o r p t i o n and T r a n s p o r t Data of P o l y u r e t h a n e - P e n e t r a n t Systems 7

Temp Penetrant

/

χ ( C) β

Λ

S , , * (g/g)

*

7

DxlO

v

PxlO

s

,

/ ( mmol/g)

2. . (cm /s) E q . (2)

,

2, . (cm /s) E q . (3)

25 44 60

0. 741

9. 49

8. 09

6. 00

Toluene

25 44 60

0. 602 0. 591 0. 601

6. 53 6. 41 6. 52

2. 60 5. 71 7. 52

1. 56 3. 37 4. 52

p-Xylene

25 44 60

0. 497 0. 500 0. 493

4.,68 4.,71 4.,64

2. 34 4. 33 7. 02

1. 16 2.,16 3.,46

Mesitylene

25 44 60

0. 402 0. 415 0. 420

3.,34 3.,45 3.,49

0. 86 2. 24 3. 76

0.,34 0.,93 1.,58

Anisole

25 44 60

0. 803 0. 839 0. 828

7.,42 7,.76 7,.66

1. 66 3. 57 4.82

1.,33 2,,99 3..99

Nitrobenzene

25 44 60

1. 063 1. 071 1. 148

8,.64 8,.71 9..23

0. 87 1. 57 2.,47

0,.92 1..68 2,.83

Chlorobenzene

25 44 60

1. 055 1. 091 1. 088

9,.38 9,.69 9,.67

3.,42 5.,41 6.,90

3,.61 5,.90 7,.50

o-Dichlorobenzene

25 44 60

1.,314 1.,312 1.,315

8,.94 8,.92 8,.95

2.,01 2.,87 4.,39

2,.64 3,.77 5,.77

Bromobenzene

25 44 60

1,,475 1.,501 1.,516

9 .39 9 .56 9 .65

2.,81 4..56 7,.19

4,.14 6 .85 10 .90

Benzene

In Barrier Polymers and Structures; Koros, W.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

366

BARRIER POLYMERS AND STRUCTURES

The d i f f u s i o n c o e f f i c i e n t s as c a l c u l a t e d from E q u a t i o n 2 have been used i n E q u a t i o n 4 to generate t h e t h e o r e t i c a l s o r p t i o n c u r v e s (19-22) :

oo

= 1 - (Α) π

M /M

η

Σ "

— 5 - exp [ - D ( 2 n + l ) (2η+1)

υ

2

2 ï ï

2

t/h l

(4)

Equation 4 d e s c r i b e s t h e F i c k i a n d i f f u s i o n mode. The s i m u l a t e d sorption curves are compared i n Figures 9 and 10 w i t h t h e experimental profiles f o r some r e p r e s e n t a t i v e penetrants. The o v e r a l l agreement i s o n l y f a i r . During e a r l y stages of s o r p t i o n the agreement i s not so good and t h e e x p e r i m e n t a l curves show s l i g h t c u r v a t u r e s ; t h i s suggests that t h e t r a n s p o r t i s not s t r i c t l y of F i c k i a n t y p e . TEMPERATURE E

D

and E

p

EFFECTS

The A r r h e n i u s

activation

parameters v i z .

f o r the processe

computed from a c o n s i d e r a t i o n of t h e t e m p e r a t u r e D and Ρ r e s p e c t i v e l y , b y u s i n g t h e r e l a t i o n : log

X = log X

- ( E / 2 . 3 0 3 RT)

X

refers

to D

or Ρ

of

(5)

v

O

where

variation

A

and X

q

i s a constant

representing

D

Q

and P ; E ^ denotes t h e a c t i v a t i o n energy f o r t h e p r o c e s s under c o n s i d e r a t i o n and RT has t h e c o n v e n t i o n a l meaning. The e s t i m a t e d parameters Ε and E a r e g i v e n i n T a b l e I I I . The A r r h e n i u s p l o t s q

β

p

are g i v e n i n F i g u r e 11. The E and E D

for

t h e penetrants

under

p

values

vary

study.

As

from regards

about 16 to 35 k J / m o l the effect

s u b s t i t u t i o n on t h e benzene molecule ( i . e . , on going from to m e s i t y l e n e ) t h e r e i s a s y s t e m a t i c i n c r e a s e i n E ^ and E

of CH^-

p

benzene values.

These r e s u l t s c o u l d be e x p l a i n e d on t h e b a s i s of E y r i n g ' s h o l e t h e o r y (_23), a c c o r d i n g to w h i c h , t h e energy r e q u i r e d to "open a h o l e " i n t h e p o l y m e r m a t r i x to accommodate a d i f f u s i n g molecule bears a d i r e c t r e l a t i o n s h i p w i t h E^. Thus, t h e l a r g e r molecules in

a

related

series

will

have

larger

E^

and

smaller

diffusion

coefficients. This i s i n conformity with the experimental obser­ vations r e p o r t e d here. A t t e m p t s have a l s o been made to c a l c u l a t e t h e e q u i l i ­ b r i u m s o r p t i o n c o n s t a n t s , K^, from c o n s i d e r a t i o n s on t h e e q u i l i b r i u m process occurring i n the l i q u i d pressure (5). Thus,

phase

at constant

number of moles of penetrant

temperature

sorbed

unit mass of t h e p o l y m e r

In Barrier Polymers and Structures; Koros, W.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

and

19.

AITHALETAK

Sorption and Diffusion Through Polyurethane Membranes 367

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368

BARRIER POLYMERS AND STRUCTURES

0

10

20

30

40

50

60

I

1

1

1

1

1

Γ

7 t (min )

•

Figure 10 Comparison between experimental and simulated sorption curves for polyurethane + solvent systems.

In Barrier Polymers and Structures; Koros, W.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

19.

Table III.

369

Sorption and Diffusion Through

AITHALETAL.

A c t i v a t i o n P a r a m e t e r s and T h e r m o d y n a m i c F u n c t i o n s f o r P o l y u r e t h a n e - P e n e t r a n t Systems E

D

E

p

AS

0

ΔΗ°

Penetrant

Benzene Toluene p-Xylene Mesitylene Bromobenzene Anisole Chlorobenzene o-Dichlorobenzene Nitrobenzene

kJ/mol

kJ/mol

J/mol.K

24. 17 25.43 25.83 34.95 22.04 25.42 16.64 18.20 24. 54

25.,13 25..40 25..72 35.,90 22..67 26.,24 17.,40 18..23 26,.21

21.6 15.3 12.3 13.7 20.8 19.4 21.2 18.3 23.7

In Barrier Polymers and Structures; Koros, W.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

kJ/mol 0.99 - 0.08 -0.17 1.08 0.64 0.79 0.76 0.02 1.74

370

BARRIER POLYMERS AND STRUCTURES

π

1

1 Ο

Benzene

Δ

Toluene

Ο

Mesitylene

•

Bromobenzene

•

o-Dichlorobenzene

φ

Nitrobenzene

3

Anisole

•

Chlorobenzene

•

p-Xylene

3.0

3.1

3.2 3

3.3

3.4

1

±χ10 (Κ" ) τ Figure 11 Arrhenius plots for diffusivity.

In Barrier Polymers and Structures; Koros, W.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

19.

Sorption and Diffusion Through Polyurethane Membranes 371

AITHALETAK

_

m moles of penetrant g membrane

S Figure

12

weights

shows

at

25,

the

44

dependence

and

60°C;

of

a

Kg

on

systematic

penetrant decrease

molecular

in

Kg with

molecular weight can be seen for benzene to mesitylene an inverse dependence of Kg on molecular weight of the

suggesting penetrant.

This may be more logical because larger molecules tend to occupy more free volume in the amorphous regions of PU chains than smaller molecules. On the other hand, anisole, nitrobenzene, bromobenzene, chloro- and o-dichlorobenzene show positive devia­ tions ( i . e . , higher solubility) from linearity. This could be a t t r i ­ buted to the structural and polarity similarities of the solvents. Another factor might be the affinity of these solvents towards polyurethane. Following the generalization "like absorbs like" this explanation is consisten the data of Kg we hav ( i . e . , heat of sorption) Δ Η ° , and standard entropy of by using van't Hoff relationship (24) :

log Κ

=

ς

s

The

plots

within the of

of

log

Kg versus

AS°

estimated error in about ±1 J / m o l / K . For vely

decreases

(1)

2 . 303R

temperature

ΔΗ° and

-J*l 2. 303 R

1/T as

interval of

are

also

Δ Η ° is

benzene,

shown

AS° is

in

±4 J/mol about

(7)

13,

are linear

The estimated

Table

III.

whereas

The

for

AS° is

about

values average

A S ° , it

22 J/mol. Κ and it

for which

AS°,

Τ

in Figure

25 to 6 0 ° C .

included

about

up to p-xylene

sorption

is

progressi­ 12

J/mol.K;

A S ° for mesitylene is slightly higher than j?-xylene. However, for the remaining penetrants we could not observe any systematic trend in A S ° values because cal sizes and interact This further confirms not involve polymeric essentially from the movement within the

these penetrants possess more or less identi­ differently with the polyurethane segments. that the diffusive portion of absorption does cooperation to a greater extent, but results positioning of penetrant molecules during pre-existing available sites of the polymer

matrix (21). The ΔΗ° values are small and positive excepting toluene and p-xylene for which negative values are observed. THERMODYNAMIC

ANALYSIS.

For

a

comprehensive

understanding

of the structure-property relationships of the elastomeric materials in the presence of a solvent, it is necessary to know the magnitude of polymer-solvent interaction parameter χ , and hence the molar mass between crosslinks, M^. The criterion for swelling equilibrium

In Barrier Polymers and Structures; Koros, W.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

372

BARRIER POLYMERS AND STRUCTURES

C H Br6

5

cx

9 H

C

X6 6

H

N0

6 5 2 ©

C H Cl 6

C

Ό

\ 6 5 3 H

6

4

2

C,H OCH, c

CH

Κ

6

(0)-25*C

Ν. C H (CH ) ^\ 6

4

3

2

(Δ) - 60"C C H (CH )3^ 6

3

.. . 1

3

1 110

90

Mol.Wt.

1 130

1

150

>

Figure 12 Dependence of sorption constant (K ) on molecular weight of the solvents at (O) 25 * C; (•) 44 C; and ( Δ ) 60 C. β

β

In Barrier Polymers and Structures; Koros, W.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

19.

Sorption and Diffusion Through Polyurethane Membranes 373

AITHAL ET A K

was f i r s t r e c o g n i z e d by F r e n k e l (25) and was l a t e r d e v e l o p e d by F l o r y and Rehner (11,12) i n t o a general t h e o r y . F o r a s u c c e s s f u l c a l c u l a t i o n of M by t h i s t h e o r y we must have i n hand r e l i a b l e data c

of χ f o r t h e s o l v e n t - p o l y m e r p a i r . A number of methods of d e t e r ­ mining χ h a v e been suggested i n t h e l i t e r a t u r e and these h a v e been r e c e n t l y r e v i e w e d by T a k a h a s h i ( 2 6 ) . A l l t h e s e methods a r e e m p i ­ r i c a l and r e q u i r e t h e use of s o l u b i l i t y parameter of t h e s o l v e n t . I n s t e a d , we suggest an a l t e r n a t i v e phenomenological treatment f o r the c a l c u l a t i o n of χ. T h i s a p p r o a c h i s based on e x p r e s s i n g t h e F l o r y - R e h n e r e q u a t i o n into a d e r i v a t i v e of volume f r a c t i o n of the polymer, φ , i n the c o m p l e t e l y s w o l l e n state w i t h r e s p e c t to temperature. Thus,

(

di, d

χφ

=

T

(-ί Ν = — (

χ =

W/dT)

- -

) (9)

1 / 3

-

Φ

fi)

[{ φ / ( 1 - φ ) } + Ν In (1-φ)

[2φ

The

( 8 )

2ΧΦ -

where

so t h a t

2 / τ

(άφ/άΊ)

2

-

φ Ν

-

+ Νφ

]

( 1 Q )

2

φ /Τ]

volume f r a c t i o n of t h e s w o l l e n p o l y m e r i s c a l c u l a t e d as :

φ = [1

Here,

and

are

after

p

M

=

(

(11)

i ) - —^- ]

respectively,

the

mass

of

polymer

before

i s s o l v e n t d e n s i t y and ρ i s d e n s i t y of b ρ polyurethane. Computation of the c o e f f i c i e n t of volume fraction (ά(\>/άΊ) can be done from a l e a s t - s q u a r e s f i t of t h e φ data v e r s u s t e m p e r a t u r e , T. The molar mass between c r o s s l i n k s can then be o b t a i n e d from a m o d i f i c a t i o n of F l o r y - R e h n e r r e l a t i o n as :

and

swelling,

+ — Β

C

P v p

(φ)

1 / 3

(12) [In (1-φ)

+ φ+

Χ

2

φ ]

w h e r e V i s molar volume of s o l v e n t and t h e parameter χ t o be used here has been o b t a i n e d from Equation 10. The estimated q u a n t i t i e s a r e c o m p i l e d i n T a b l e IV. Wide d i s p a r i t y i n χ and

In Barrier Polymers and Structures; Koros, W.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

374

BARRIER POLYMERS AND STRUCTURES

1

1-0

1

1

I

C^Cl

Γ P C

H

6 5

N 0

=

I

& C H Br 6

5

= ·— *-

2*

o—

C H OCH 6

5

—A

C

H

6 6

3

—m C

•

H

6 5

C H

3

C

W

H

}

3 2

0-6 C H (CH ) 6

0

3

3

3

—Λ

1

1

3-0

3.1

Ο 1

3.2 3

1

1X10 (K' )

1

1

3.3

3.4

•

τ

Figure 13 van't HofFs plots for polyurethane + solvent systems. Symbols:

(I)

Chloro- and Bromobenzene;

(0)

Benzene; (•)

(Δ)

^-xylene; and (o) Mesitylene;

Table IV.

Anisole; (•)

Toluene; (A)

Nitrobenzene.

Results of Flory-Rehner Theory

Penetrant Benzene Toluene p-Xylene Mesitylene Bromobenzene Anisole Chlorobenzene o-Dichlorobenzene Nitrobenzene

X 0.47 0.29 0.23 0.37 0.36 0.41 0.39 0.24 0.58

M c 860 551 469 517 1018 886 1017 848 1793

In Barrier Polymers and Structures; Koros, W.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

19. AITHAL ET AL. Sorption and Diffusion Through Polyurethane Membranes 375 values are observed w h i c h depend on t h e nature of t h e s o l v e n t s u s e d . F o r toluene, p - x y l e n e and o - d i c h l o r o b e n z e n e , both χ and respectively,

vary

from

:χ =

0.23 to 0.29 and M

c

= 470 to 850.

T h i s suggests that t h e r e i s a c o n s i d e r a b l e s o l v e n t i n t e r a c t i o n w i t h the p o l y u r e t h a n e segments. F o r benzene χ = 0.47 and M = 860, c

whereas

nitrobenzene

e x h i b i t s χ = 0.58 and M

c

= 1793, t h e h i g h e s t

among t h e l i q u i d s c o n s i d e r e d . On t h e o t h e r hand, m e s i t y l e n e , a n i s o l e , c h l o r o - and bromobenzene e x h i b i t almost i d e n t i c a l v a l u e s of χ i . e . , 0.36 to 0.41; among t h e s e , the latter two e x h i b i t higher (

~

1020) than

respectively,

either

mesitylene

517 and 886. Such

or

anisole

variations

in

for which data

are

indicate the

s e r i o u s l i m i t a t i o n s of t h e F l o r y - R e h n e r t h e o r y to s t u d y polymer swelling. However, the complexity of s o l v e n t interactions may p r o b a b l y affect i n variou of PU. T h i s will alte contributing to t h e v a r i a b i l i t y of M^ r e s u l t s . T h i s i s further i n d i c a t i v e of t h e s u b t l e the s o l v e n t - p o l y u r e t h a n e

n o n - F i c k i a n e f f e c t s as o b s e r v e d pairs.

f o r most of

ACKNOWLEDGMENTS The a u t h o r s a p p r e c i a t e t h e f i n a n c i a l s u p p o r t from t h e Robert A. Welch Foundation (Grant AI-0524); TMA and USA thank t h e U n i v e r s i t y Grants C o m m i s s i o n , New D e l h i , I n d i a f o r t h e a w a r d of a t e a c h e r f e l l o w s h i p to Mr. A i t h a l to s t u d y at Karnatak U n i v e r s i t y .

LITERATURE CITED 1. Cooper, S. L . ; Tobolsky, Α. V. J. Appl. Polym. Sci. 1966, 10, 1837. 2. Gibson, P. E. Properties in Polyurethane Block Copolymers in Developments in Block Copolymers; Goodman, I., Ed.; Elsevier : London, 1982. 3. Trapps, G. In Advances in Polyurethane Technology; Buist, J. M . ; Gudgeon, H . , Eds.; Interscience : New York, 1968 ; p 63. 4. Hung, G. W. C.; Autian, J. J. Pharm. Sci. 1972, 61, 1094. 5. Hung, G. W. C. Microchem. J. 1974, 19, 130. 6. Hopfenberg, Η. B . ; Schneider, N. S.; Votta, F. J. Macromol. Sci.-Phys. 1969, B3(4), 751. 7. Nierzwicki, W.; Majewska, Z. J. Appl. Polym. Sci. 1979, 24, 1089. 8. Sefton, M. V . ; Mann, J. L. J. Appl. Polym. Sci. 1980, 25, 829.

In Barrier Polymers and Structures; Koros, W.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

376 BARRIER POLYMERS AND STRUCTURES

9.

Yokoyama, T.; Furukawa, M. In International Progress in Urethanes; Ashida, K.; Frisch, K. C., Eds.; Technomic Publishing Co., Inc., 1985; Vol. 4, Chapter 1. 10 Schnieder, N. S.; Illinger, J. L . ; Cleaves, M. A. Polym. Eng. Sci. 1986, 26, 1547. 11. Flory, P. J . ; Rehner, Jr, J. J. Chem. Phys. 1943, 11, 521. 12. Flory, P. J. J. Chem. Phys. 1950, 18, 108. 13. Lucht, L. M.; Peppas, N. A. J. Appl. Polym. Sci. 1987, 33, 1557. 14. Chiou, J. S.; Paul, D. R. Polym. Eng. Sci. 1986, 26, 1218. 15. Nicolais, L . ; Drioli, E . ; Hopfenberg, Η. B.; Apicella, A. Polymer 1979, 20, 459. 16. Britton, L. N.; Ashman P. E. J. Chem. Educ 17. Aminabhavi, T. M.; Cassidy, P. E. Polym. Commun. 1986, 27, 254. 18. Cassidy, P. E . ; Aminabhavi, T. M.; Thompson, C. M. Rubb. Chem. Technol., Rubb. Revs. 1983, 56, 594. 19.

24.

Crank, J. The Mathematics of Diffusion; 2nd Ed; Clarendon Press : Oxford, 1975. Gent, A. N.; Tobias, R. H. J. Polym. Sci. Polym. Phys. Ed. 1982, 20, 2317. Enscore, D. J . ; Hopfenberg, H. B.; Stannett, V. T. Polym. Eng. Sci. 1980, 20, 102. Garrido, L . ; Mark, J. E . ; Clarson, S. J . ; Semlyen, J. A. Polym. Commun. 1984, 25, 218. Zwolinski, B. J . ; Eyring, H.; Reese, C. E. J. Phys. Colloid Chem. 1949, 53, 1426. Golden, D. M. J. Chem. Educ. 1971, 48, 235.

25. 26.

Frenkel, J. Rubb. Chem. Technol. 1940, 13, 264. Takahashi, S. J. Appl. Polym. Sci. 1983, 28, 2847.

20. 21. 22. 23.

RECEIVED

December 5, 1989

In Barrier Polymers and Structures; Koros, W.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

Chapter 20

Toluene Diffusion in Natural Rubber 1,3

1,4

2

Lawrence S. Waksman , Nathaniel S. Schneider , and Nak-Ho Sung 1

Polymer Research Branch, SLCMT-EMP, U.S. Army Materials Technology Laboratory, Watertown, MA 02172 Department of Chemical Engineering, Tufts University, Medford, MA 02152

2

Immersion swellin experiments were lightly crosslinked natural rubber sample with varying amounts of carbon black. Only small differences in equilibrium swelling were found. The sorption isotherms were superimposable for samples at all carbon black levels up to an activity of 0.9 and could be fitted with the Flory-Rehner relation. Sorption and desorption curves, above 25% toluene, showed slight "S" shaped curvature. Diffusion constants, D, obtained by the half-time method or the Joshi-Astarita analysis of coupled diffusion and relaxation, showed a similar maximum in D with concentration. When converted to solvent mobilities, D , the values leveled out rather than extrapolating to the self-diffusion coefficient of toluene, D . Application of the Armstrong-Stannett treatment of heating effects during sorption lead to significant corrections in D and to better agreement with an empirical extrapolation to D*1. 1

1

Studies o f the d i f f u s i o n of benzene i n natural rubber represent some of the e a r l i e s t d e t a i l e d examinations of the i n t e r a c t i o n of an organic solvent with a polymer. Hayes and Park c a r r i e d out measurements a t low concentrations by the vapor sorption method (1), and a t higher concentrations by determining the concentration d i s t r i b u t i o n using an interferometric method (2). Complementary measurements by vapor transmission to determine the d i f f u s i o n c o e f f i c i e n t from time-lag data were c a r r i e d out a t low concent r a t i o n s by Barrer and Fergusson (3). The main results of these studies have been summarized i n F u j i t a ' s review (4) of organic vapor d i f f u s i o n i n polymers above the glass t r a n s i t i o n temperature. However, the problems with these measurements were not referenced. In the work o f Hayes and Park, the calculated solvent m o b i l i t i e s extrapolated to a value, a t unit solvent volume f r a c t i o n , which 3

Current address: United Technologies, Hamilton Standard, 1 Hamilton Standard Road, Windsor Locks, C T 06096

4

Address correspondence to this author. 0097-6156/90A)423-0377$06.00/0 © 1990 American Chemical Society

In Barrier Polymers and Structures; Koros, W.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

378

BARRIER POLYMERS AND STRUCTURES

exceeded the s e l f - d i f f u s i o n c o e f f i c i e n t of pure benzene by two orders of magnitude. This lead the authors to question the thermodynamic correction factors used i n computing the solvent m o b i l i t i e s . In Barrer and Fergusson's study, the values of the d i f f u s i o n c o e f f i c i e n t from steady state were higher than from the time-lag, leading to the conclusion that the behavior might be complicated by relaxation e f f e c t s . Thus, i t appears that even i n t h i s c l a s s i c a l system there are problems which deserve consideration. The goals of the present study were to reexamine vapor sorption i n a l i g h t l y crosslinked rubber, both as an u n f i l l e d sample and i n samples containing two types of carbon black i n varying amounts. Complications i n the vapor sorption-rate curves motivated a more d e t a i l e d study of the d i f f u s i o n problems as the main area of concern. EXPERIMENTAL SAMPLE PREPARATION. Samples of natural rubber were prepared by mixing a l l ingredients i n a Haake-Buchler system 40 internal mixer, using an accelerated sulfur cure and excluding any o i l extender or p l a s t i c i z e r which could leach out i n the immersion experiments. Two types of carbon black were used; N110, a fine p a r t i c l e , high structure black and N774, a large p a r t i c l e , low structure black. A two-stage mixing procedure was used to minimize scorch. The carbon black was incorporated i n the f i r s t stage, followed by l a t e r addition of the curatives, since the addition and mixing of carbon black tends to elevate the batch temperature to unacceptable l e v e l s The formulations and the outline of the procedure are summarized i n Table 1. The compounded rubber was m i l l e d to a thickness of 60 mil and cured a t 121 C f o r 84 minutes i n a hydraulic press using a picture frame shim with a thickness of 20 m i l . The low cure temperature was chosen i n order to maximize the scorch time so that the uncured rubber could flow and f i l l the frame.

Table 1. Natural Rubber Formulations and Processing

Stage 1 Natural Rubber Stearic A c i d Zinc Oxide Agerite Resin D Carbon Black Volume Fraction

PHR

TOD 2 4 1

0-50 0-20

Stage 2 Sulfur Santocure

PHR

0.8

Stage 1 mix: 80°C, 77 RPM m i l l :; T