Barrier Polymers and Structures - ACS Publications - American

Chapter 5

Structure of Amorphous Polyamides

Downloaded by UNIV OF CALIFORNIA IRVINE on October 17, 2014 | http://pubs.acs.org Publication Date: May 9, 1990 | doi: 10.1021/bk-1990-0423.ch005

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


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 p y r i d i n e i n N - m e t h y l p y r r o l i d i n o n e (3). A l l polymer samples t e s t e d f o r oxygen p e r m e a t i o n had a minimum i n h e r e n t v i s c o s i t y o f 0.6 d L / g 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


113

114

BARRIER POLYMERS AND STRUCTURES

Diamines NH ^ ^ N H Downloaded by UNIV OF CALIFORNIA IRVINE on October 17, 2014 | http://pubs.acs.org Publication Date: May 9, 1990 | doi: 10.1021/bk-1990-0423.ch005

2

2

n

e

\( J]

NH (CH ) NH 2

NH(CH2) NH

2

2

CH/

CH

MPD

η Ν Η? ^

DMe6

.NH? NH \

NH CH CH(CH ) NH 2

2

2

CI

c h

CIMPD

2Me5 NH

2

(

3

2

NH /

3

Pip

)—CH —{

)—NH

2

2

PACM

H0 C^

^C0 H

2

2

H0 C—((

j)—C0 H

2

H0 C(CH ) . C0 H 2

2

η Figure 2.

n

2

2

JUL

H0 C 2

2

Ν

^C0 H 2

2,6 Pyr Monomers Used In This Study.


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 n d a t a f o r t h i s s e r i e s a r e c o n t a i n e d 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



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 . U n f o r t u n a t e l y , t h i s i s a v e r y d i f f i c u l t parameter t o q u a n t i f y . One a p p r o a c h t h a t has been u s e d i s t o compare the d e n s i t i e s o f two 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


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

Barrier Polymers and Structures - ACS Publications - American

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