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Chapter 5
Structure of Amorphous Polyamides
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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.
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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.
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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
In Barrier Polymers and Structures; Koros, W.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.
113
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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
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e
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NH (CH ) NH 2
NH(CH2) NH
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η Ν Η? ^
DMe6
.NH? NH \
NH CH CH(CH ) NH 2
2
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c h
CIMPD
2Me5 NH
2
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3
2
NH /
3
Pip
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)—NH
2
2
PACM
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2
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n
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H0 C 2
2
Ν
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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
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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
In Barrier Polymers and Structures; Koros, W.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.
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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
In Barrier Polymers and Structures; Koros, W.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.
5. KRIZANETAL.
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Structure ofAmorphous Polyamides
200 η
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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
<
9
r
-
11
NUMBER OF METHYLENES Figure 4.
Effect of Chain Length Upon Isophthalamide Density.
In Barrier Polymers and Structures; Koros, W.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.
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BARRIER POLYMERS AND STRUCTURES
100
10
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OPV'
11
10
12
1/SFV (g/ml) * cc-mil/100 sq. in./day/atm Figure 5 Volume.
Correlation
F i g u r e 6.
of
Isophthalamide
Dielectric
OPV to S p e c i f i c
Loss o f 6 - I / T
Polyamide.
In Barrier Polymers and Structures; Koros, W.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.
Free
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KRIZAN
ET A L .
Structure ofAmorphous Polyamides
20 π
10 τ (°C)
ο -
-10 Η
1 4.0
ι
1 5.0
1
1 6.0
1
1 7.0
1
1 8.0
NUMBER O F METHYLENES
Figure 7. Effect of Chain Length Upon Beta Transit Temperature at 10 kHz.
In Barrier Polymers and Structures; Koros, W.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.
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o f segmental m o t i o n o c c u r r i n g a t room temperature i n c r e a s e s w i t h i n c r e a s i n g methylene c o n t e n t . The gamma t r a n s i t i o n , w h i c h o c c u r s near -100°C, i s o b s e r v e d when t h e r e a r e s i x o r more methylenes i n t h e a l i p h a t i c c h a i n . The magnitude o f t h i s t r a n s i t i o n i n c r e a s e s w i t h i n c r e a s i n g methylene c o n t e n t . I t i s a t t r i b u t e d t o motion i n v o l v i n g the methylene groups and o c c u r s i n t h e same temperature range a s t h e gamma p r o c e s s i n a l i p h a t i c polyamides (18) and p o l y e t h y l e n e (19). A minimum o f f o u r c o n s e c u t i v e methylene groups i s u s u a l l y r e q u i r e d t o observe t h i s t r a n s i t i o n ( 2 0 ) . The t r e n d s o b s e r v e d i n b o t h the b e t a t r a n s i t i o n ( i n c r e a s e d m o t i o n a t room temperature w i t h i n c r e a s i n g methylene c o n t e n t ) and t h e gamma t r a n s i t i o n ( i n c r e a s e d magnitude o f t h e t r a n s i t i o n w i t h i n c r e a s i n g methylene c o n t e n t ) a r e c o n s i s t e n t w i t h t h e o b s e r v e d e f f e c t s o f η on OPV. I t i s l i k e l y , however, t h a t t h e amount o f f r e e volume i n t h e polymer i s t h e dominant f a c t o r i n d e t e r m i n i n g t h e OPV f o r t h e n-I s e r i e s ( a l t h o u g h f r e e volume and s u b g l a s s m o t i o n a r e n o t c o m p l e t e l y independent p r o p e r t i e s ) . This h y p o t h e s i s i s based on t h e l i n e a r i t y o f t h e l o g OPV a g a i n s t 1/SFV l i n e p l o t i n F i g u r e 5, and the magnitude o f t h e s u b g l a s s t r a n s i t i o n s r e l a t i v e t o the g l a s s t r a n s i t i o n s . A strong dependence o f p o l y e s t e r OPV on t h e amount o f s u b g l a s s motion h a s been r e p o r t e d ( 1 5 ) , b u t i n t h a t c l a s s o f polymers, t h e magnitude o f t h e b e t a t r a n s i t i o n i s comparable t o t h a t o f t h e g l a s s transition. I t i s t h e r e f o r e reasonable t o expect t h a t subglass m o t i o n w i l l have g r e a t e r importance i n d e t e r m i n i n g 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 a t s e r i e s t h a n f o r t h e amorphous polyamide s e r i e s . O t h e r M o d i f i c a t i o n s o f Polymer S t r u c t u r e . I n o r d e r t o more f u l l y d e t e r m i n e t h e e f f e c t s o f s t r u c t u r a l change upon polyamide OPV, several other m o d i f i c a t i o n s o f the b a s i c a l i p h a t i c - a r o m a t i c backbone were performed. One simple m o d i f i c a t i o n i s t o r e v e r s e the d i r e c t i o n o f t h e amide l i n k a g e . The d a t a i n T a b l e I I i n d i c a t e t h a t f o r a t l e a s t t h e s h o r t e r c h a i n p o l y i s o p h t h a l a m i d e s , amide r e v e r s a l h a s no measurable e f f e c t upon the OPV a t e i t h e r 0% o r 80% RH. I t i s s u r p r i s i n g t o note t h a t t h e amide d i r e c t i o n does n o t seem t o a f f e c t e i t h e r d e n s i t y o r Tg i n a d d i t i o n t o OPV. F u r t h e r more, Morgan and Kwolek r e p o r t e d t h a t t h e amide d i r e c t i o n had l i t t l e , i f any, e f f e c t upon the m e l t i n g p o i n t s on a l i p h a t i c t e r e p h t h a l a m i d e s and t h e i r a n a l o g s ( 2 1 ) . I t i s due t o t h e i n d i f f e r e n c e o f polyamide p r o p e r t i e s t o amide d i r e c t i o n t h a t d a t a f o r MPD-14 i s i n c l u d e d i n F i g u r e s 3 and 5.
Table I I .
E f f e c t o f Amide R e v e r s a l on Polyamide P r o p e r t i e s
Polymer
MPD-6
OPV* (dry) TT~4 0.4
6-1 1.9 MPD-8 2.1 * c c - m i l / ( 1 0 0 sq.in.-day-atm)
OPV* (80% RH)
Density (g/mL)
Ô7B" 0.5
T^5 1.26
1.2 1.2
1.19 1.19
Tg (°C) Τ4Γ
144 123 129
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5.
KRIZAN
ET A L .
Structure of Amorphous Polyamides
Incorporation of Ν,Ν'-dialkyldiamines into the polymer chain would disrupt the normal hydrogen bonding since the repeat units would have no available amide hydrogens. Comparison of the OPV data i n Table III for DMe6/6-I (25/75) to that for unsubstituted 6-1 indicates that the dry OPV i s increased much more dramatically than the 80% RH OPV. The reason for t h i s observation may be due to the two d i f f e r e n t e f f e c t s of the N-methyl groups. Not only do the methyl groups o b l i t e r a t e 25% of the hydrogen bonds r e l a t i v e to 6-1, but they also increase the free volume by p h y s i c a l l y increasing the interchain distance. At 0% RH, both e f f e c t s are operative and the combination leads to a large increase i n permeation. At 80% RH, the hydrogen bonding disruption imparted by the methyl groups i s inconsequential as there i s more than enough water present to provide the same disruption. The only observed difference i n OPV at 80% RH i s due to the s t e r i c e f f e c t s of the methyl groups, which i s s l i g h t compared to the e f f e c t s of hydrogen bond disruption. The chain a l k y l a t i o n data described below confirm t h i s conclusion.
Table I I I .
Oxygen Permeation Data for Modified Polyamides
Polymer
DMe6/6-I (25/75) 2Me5-I 6-I/T (from nylon salt) MPD/5C1MPD-8 (50/50) Pip/6-I (20/80) PACM/6-I/T (50/50-70/30) 6-2,6Pyr/I (50/50) *cc-mil/(100 sq.in./day/atm)
OPV* (dry)
OPV* (80% RH)
3~4 1.4 3.5 5.2 2.8 5.6 3J
Γ75 0.9 1.8 2.3 1.8 3.7 2.6
Table III contains data for both 5-1 and the chain alkylated 2Me5-I. In t h i s case, the methyl group leads to a s l i g h t increase i n OPV at 0% RH and a n e g l i g i b l e difference at 80% RH. Additional chain a l k y l groups lead to even greater increases i n permeation. For example, Trogamid T, an amorphous polyamide made by Dynamit Nobel from two trimethylhexamethylenediamine isomers and terephthalic acid, has an OPV of 5.1 cc-mil/(100 sq.in.-day-atm) at 80% RH. This OPV i s nearly three times greater than the OPV of unsubstituted 6-I/T (Table III) made i n the same manner. (The difference i n OPV between 6-1 and 6-I/T reported here i s not due to differences i n the I/T r a t i o as i n other polymer classes (see, for example: Schmidhauser, J . C ; Longley, K. L., t h i s volume). I t i s instead related to differences i n synthetic method (more branching i n the polymer prepared from the nylon salt) or processing (more unrelaxed free volume i n films which are cast through an extruder die and onto a quench r o l l ) ) . The e f f e c t of placing substituents on the aromatic ring was studied. Several polymers containing substituted meta-phenylenediamines and isophthalic acids were prepared. Great d i f f i c u l t y was generally encountered i n preparing polymers i n high molecular weight from these monomers, so 6-1 or MPD-8 copolymers containing 10-50 mol% of the monomer of interest were prepared. Although
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rings s u b s t i t u t e d i n the 5-position with sulfonate, a l k y l , n i t r o , and carboxamide groups were examined, t h e MPD/5C1MPD-8 example i n T a b l e I I I s e r v e s a s a r e p r e s e n t a t i v e due t o t h e r e l a t i v e l y h i g h l e v e l o f monomer i n c o r p o r a t i o n . A s i n t h e p r e v i o u s examples o f chain s u b s t i t u t i o n , s u b s t i t u t i o n o f the rings, a t l e a s t i n the 5 - p o s i t i o n , l e a d s t o i n c r e a s e d OPV. T h i s i s p r o b a b l y due t o i n c r e a s e d i n t e r c h a i n d i s t a n c e which l e a d s t o i n c r e a s e d f r e e volume. I t i s d i f f i c u l t t o evaluate the e f f e c t o f incorporating c y c l o a l i p h a t i c groups due t o t h e p r e s e n c e o f o t h e r c o m p l i c a t i n g f a c t o r s i n commonly a v a i l a b l e monomers o f t h i s c l a s s . The P i p / 6 - I polymer l i s t e d i n T a b l e I I I w i l l serve a s an example. In t h i s c a s e , t h e p i p e r a z i n e c o n t a i n s o n l y two carbons between amide n i t r o g e n s . A s demonstrated e a r l i e r , t h i s would tend t o lower OPV. On t h e o t h e r hand, t h e l a c k o f hydrogen bonding imparted by t h e p i p e r a z i n e m o i e t i e s would be e x p e c t e d t o i n c r e a s e OPV. I n f a c t , the p r e s e n c e o f t h e p i p e r a z i n e i n c r e a s e s p e r m e a t i o n . A n o t h e r example o f c y c l o a l i p h a t i c group i n c o r p o r a t i o n i n T a b l e I I I i s t h e d a t a f o r PACM/6-I/T (made from the n y l o n s a l t ) ( V a s s a l l o , D. Α., DuPont, u n p u b l i s h e d r e s u l t s ) . I n t h i s case, hydrogen b o n d i n g i s p o s s i b l e , b u t t h e d i s t a n c e between amide n i t r o g e n s has i n c r e a s e d . I t i s d i f f i c u l t from these examples t o d e l i n e a t e t h e e f f e c t o f t h e a l i p h a t i c r i n g s on polyamide OPV, although i t i s l i k e l y that t h e i r presence i n c r e a s e s i n t e r c h a i n d i s t a n c e much a s a c h a i n s u b s t i t u e n t would. A f i n a l m o d i f i c a t i o n o f t h e b a s i c polymer s t r u c t u r e examined was t h e s u b s t i t u t i o n o f a h e t e r o c y c l e f o r t h e benzene r i n g . Table I I I c o n t a i n s d a t a f o r 6 - 2 , 6 P y r / I . The p y r i d i n e r i n g i n c r e a s e s p e r m e a t i o n r e l a t i v e t o i s o p h t h a l i c a c i d , which may be due t o i n c r e a s e d oxygen s o l u b i l i t y imparted by t h e p y r i d i n e n i t r o g e n . E f f e c t o f RH on OPV. I t was a l s o o f i n t e r e s t 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 a d e c r e a s e i n OPV w i t h i n c r e a s i n g RH i n amorphous p o l y a m i d e s . As n o t e d above, t h i s b e h a v i o r i s unique f o r commercial oxygen b a r r i e r m a t e r i a l s . T h i s phenomena, however, a p p e a r s t o be g e n e r a l f o r amorphous p o l y a m i d e s , so t h e d i s c u s s i o n which f o l l o w s w i l l assume t h a t t h e OPV d e c r e a s e i s caused by t h e same e f f e c t i n a l l c a s e s . A l t h o u g h t h e OPV d e c r e a s e w i t h i n c r e a s i n g RH i s u n i q u e f o r b a r r i e r m a t e r i a l s , d e c r e a s e d gas t r a n s m i s s i o n r a t e s i n membrane m a t e r i a l s i n t h e p r e s e n c e o f m o i s t u r e have been p r e v i o u s l y n o t e d . For example, workers a t DuPont found s i g n i f i c a n t r e d u c t i o n s i n t h e p e r m e a b i l i t y o f hydrogen and methane t h r o u g h p o l y i m i d e f i l m s i n the p r e s e n c e o f water vapor ( 2 2 ) . Koros and coworkers a l s o o b s e r v e d r e d u c t i o n s i n t h e p e r m e a b i l i t y o f carbon d i o x i d e t h r o u g h Kapton p o l y i m i d e f i l m s i n t h e p r e s e n c e o f m o i s t u r e ( 2 3 ) . Both groups p r o p o s e d t h a t t h e gas p e r m e a b i l i t y d e c r e a s e was due t o c o m p e t i t i o n between t h e gases and the water vapor f o r t h e e x c e s s f r e e volume i n t h e polymer m a t r i x . Because o f t h i s c o m p e t i t i o n , the pathways a v a i l a b l e f o r d i f f u s i o n a r e e f f e c t i v e l y reduced. The d a t a o b t a i n e d so f a r f o r t h e amorphous polyamide s e r i e s i n d i c a t e s t h a t t h e same e f f e c t i s o p e r a t i v e . For t h e above e x p l a n a t i o n t o be v a l i d , t h e r e must be u n r e l a x e d f r e e volume i n t h e polymer m a t r i x a f t e r exposure t o moisture. T h i s means t h a t t h e temperature a t which t h e p e r m e a t i o n i s t e s t e d must be below t h e polymer Tg a t h i g h RH. As shown i n
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5.
KRIZAN
ET
AL.
Structure ofAmorphous Polyamides
Table I, the wet Tg for every polyamide tested with more than f i v e carbons i n the a l i p h a t i c chain meets t h i s requirement (OPV measured at 30°C). The Tgs of wholly a l i p h a t i c polyamides such as nylon 66 drop below this temperature at high RH (18), so i t i s not surprising that these polyamides exhibit increases i n permeation with increasing RH. Figure 8 shows a detailed dependence of the OPV of 6-I/T on RH. The majority of the permeation decrease occurs at low RH. This drop i s c l e a r l y not due to the hydrogen bond disruption which accounts for the drop i n polyamide Tg with increasing RH (24). Starkweather found that the Tg drop of the s t r u c t u r a l l y similar 6-1 nylon i s much more linear with increasing RH (Starkweather, H. W., DuPont, unpublished data). The d i s s i m i l a r i t y i n behavior between Tg drop and OPV drop with increasing RH i s indicative that the hydrogen bond disruption induced by water does not play a dominant role i n the OPV drop observed. As discussed e a r l i e r , the fact that DMe6/6-I has a higher dry OPV than 6-1 also indicates that hydrogen bond disruption i n and of i t s e l f leads to an increase, and not a decrease, i n OPV. A f i n a l piece of evidence deals with the e f f e c t of moisture upon polyamide density. Sorption of water i n excess free volume should lead to a increase i n density while sorption with concurrent swelling should result i n the a d d i t i v i t y of volumes (25,26). In the case of 6-I/T, the density of a dry f i l m sample i s 17Ί78 g/mL while the density of a sample after immersion i n water i s 1.189 g/mL. A l i k e l y explanation for the observed increase i s f i l l i n g of the excess free volume of 6-I/T by water, which must dominate the effects of the concurrent p l a s t i c i z a t i o n by the water. Conclusions Through systematic modification of the polymer backbone, the e f f e c t s of chemical structure upon the oxygen permeation properties of aliphatic-aromatic amorphous polyamides were determined. In t h i s class of polymers, the greatest effects were obtained by a l t e r a t i o n of the chain length and disruption of the amide hydrogen bonding by N-alkylation. I t i s remarkable that reversal of the amide linkage has no effect whatsoever on the permeation properties of the examples studied. In an attempt to determine the factors which determine the barrier properties of t h i s polyamide series, i t was found that the permeation results were consistent with both the r e l a t i v e l e v e l s of subglass motion as measured by d i e l e c t r i c spectroscopy and the r e l a t i v e levels of free volume as calculated using a group contribution approach. I t appears that free volume i s the dominant e f f e c t i n determining the OPV due to the r e l a t i v e l y small magnitude of the subglass transitions as compared to the glass transition. The substantial decline i n OPV as relative humidity increases, which i s unique for an oxygen barrier resin, was studied. It was concluded that this decline i s due to water occupying the excess free volume through which the oxygen would otherwise t r a v e l .
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KRIZAN ET AL.
Structure of Amorphous Polyamides
Acknowledgments We would like to thank DuPont Polymer Products Department for supporting this work and allowing its publication. We thank Gerald Horack and Robert Tomczak for performing the OPV tests and Michael Panco for obtaining the dielectric relaxation data. D. A. Vassallo is acknowledged for many helpful discussions in the early days of this program.
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RECEIVED October 17, 1989
In Barrier Polymers and Structures; Koros, W.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.
