Solvent-Induced Changes in the Glass Transition Temperature of

Chapter 31

Solvent-Induced Changes in the Glass Transition Temperature of Ethylene—Vinyl Alcohol Copolymer Studied Using Fourier Transform IR and Dynamic Mechanical Spectroscopy Downloaded by PURDUE UNIV on June 28, 2016 | http://pubs.acs.org Publication Date: May 5, 1995 | doi: 10.1021/bk-1995-0598.ch031

Marsha A. Samus and Giuseppe Rossi Ford Research Laboratory, Ford Motor Company, P.O. Box 2053, M a i l Drop 3198, Dearborn, M I 48121-2053

Results of dynamic mechanical spectroscopy and Fourier-transform infrared spectroscopy are presented for a glassy polymer (ethylene vinyl alcohol copolymer, EVOH) exposed to a p l a s t i c i zing solvent (methanol), a non-plasticizing fluid (toluene), and mixtures of the two. Quantitative FTIR measurements on thin films of EVOH are used to analyze the sorption and d i f f u sion characteristics of the systems. The presence of even minute quantities of methanol i n toluene allows toluene to penetrate EVOH to a significant extent. We interpret the observed large increase in toluene uptake in the presence of small amounts of methanol as due to the fact that the methanol present in the solvent mixture i s preferentially absorbed by the EVOH and plasticizes it, thereby lowering i t s glass transition temperature and eliminating the kinetic constraints preventing penetration by toluene. P o l y m e r f i l m s a r e o f t e n u s e d as b a r r i e r l a y e r s t o p r e v e n t o r r e d u c e t h e t r a n s p o r t o f g a s e s o r l i q u i d s f r o m one s y s t e m to another (1) . F o r example, p o l y ( v i n y l c h l o r i d e ) and p o l y e t h y l e n e a r e u s e d as b a r r i e r s t o w a t e r m i g r a t i o n o r evaporation, while nylon is an e f f e c t i v e hydrocarbon barrier (2) . B a r r i e r p r o p e r t i e s a r e o f t e n due t o the g l a s s y nature of the polymer m a t e r i a l . In these i n s t a n c e s t h e y may be c o m p r o m i s e d i f t h e f l u i d b e i n g c o n f i n e d is e i t h e r contaminated o r i n t e n t i o n a l l y m o d i f i e d t o i n c l u d e an i n g r e d i e n t which p l a s t i c i z e s the b a r r i e r . A g l a s s y p o l y m e r i n c o n t a c t w i t h a low m o l e c u l a r w e i g h t f l u i d c a n be p l a s t i c i z e d b y t h e f l u i d i f t h e m o l e c u l a r i n t e r a c t i o n s a r e s u f f i c i e n t l y f a v o r a b l e (3) . In the o t h e r 0097-6156/95/0598-0535S12.00/0 © 1995 American Chemical Society Urban and Provder; Multidimensional Spectroscopy of Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1995.

536

MULTIDIMENSIONAL SPECTROSCOPY OF POLYMERS

extreme, the i n t e r a c t i o n s between the p o l y m e r and t h e f l u i d may be so u n f a v o r a b l e t h a t l i q u i d m o l e c u l e s w i l l e n t e r t h e p o l y m e r o n l y i n v e r y m i n u t e amounts n o t u s u a l l y d e t e c t a b l e by weight g a i n measurements. In these i n s t a n c e s , the p o l y m e r w i l l r e m a i n g l a s s y (4-9). I n t h e p r e s e n t work we c o n s i d e r a n e t h y l e n e - v i n y l a l c o h o l c o p o l y m e r (EVOH) (which i s g l a s s y a t room t e m p e r a t u r e ) i n c o n t a c t w i t h two l i q u i d s , methanol and t o l u e n e , which are examples, r e s p e c t i v e l y , o f the f i r s t and second type o f b e h a v i o r . R e c e n t l y , we u s e d d y n a m i c m e c h a n i c a l measurements a n d s t a n d a r d s o r p t i o n and d e s o r p t i o n e x p e r i m e n t s t o s t u d y t h e d i f f u s i o n b e h a v i o r i n the methanol/EVOH b i n a r y system at t e m p e r a t u r e s f r o m a b o u t 10 t o 50 ° C b e l o w t h e 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 (T ) o f t h e EVOH. (Samus, M . A . a n d R o s s i , G . , s u b m i t t e d t o Polymer, 1994.) We f o u n d t h a t , a l t h o u g h s o r p t i o n curves taken i n t h i s temperature range e x h i b i t s i g m o i d a l b e h a v i o r (5), the main f e a t u r e s o f the s o r p t i o n and d e s o r p t i o n p r o c e s s c a n be a c c o u n t e d f o r b y F i c k i a n d i f f u s i o n w i t h a d i f f u s i o n c o e f f i c i e n t whose d e p e n d e n c e o n c o n c e n t r a t i o n e x h i b i t s a d r a s t i c i n c r e a s e at the methanol c o n c e n t r a t i o n at which the polymer i s p l a s t i c i z e d . I n t h e p r e s e n t work we f i r s t b r i e f l y summarize t h e r e s u l t s o f o u r dynamic m e c h a n i c a l a n a l y s i s on the e f f e c t o f s o l v a t i o n on T f o r the EVOH/methanol system: these data are c e n t r a l to our i n t e r p r e t a t i o n of the b e h a v i o r of t o l u e n e - m e t h a n o l m i x t u r e s i n c o n t a c t w i t h EVOH. We t h e n u s e F o u r i e r - t r a n s f o r m i n f r a r e d (FTIR) s p e c t r o s c o p y t o show t h a t t h e s o r p t i o n (and d i f f u s i o n ) behavior of the nonp l a s t i c i z i n g l i q u i d (toluene) i s i t s e l f c r i t i c a l l y a f f e c t e d by the polymer 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 . F i n a l l y , we e x a m i n e i n some d e t a i l t h e t o l u e n e - m e t h a n o l - E V O H t e r n a r y system. We e x p l o i t the fact that quantitative FTIR s p e c t r o s c o p y on t h i n EVOH f i l m s a l l o w s d e t e c t i o n o f s m a l l amounts of penetrant fluid a n d makes it possible to d i f f e r e n t i a t e between d i s s i m i l a r f l u i d s p e c i e s . In t h i s way we a r e a b l e t o o b t a i n s e p a r a t e s o r p t i o n c u r v e s for methanol and t o l u e n e . We f i n d t h a t r a i s i n g t h e t e m p e r a t u r e above T and s o l v a t i n g the p o l y m e r w i t h the p l a s t i c i z i n g l i q u i d (methanol) have s i m i l a r e f f e c t s o n t h e s o l u b i l i t y (and d i f f u s i v i t y ) o f t h e n o n - p l a s t i c i z i n g l i q u i d .

Downloaded by PURDUE UNIV on June 28, 2016 | http://pubs.acs.org Publication Date: May 5, 1995 | doi: 10.1021/bk-1995-0598.ch031

g

g

g

Experimental The d e p r e s s i o n i n t h e T w i t h i n c r e a s i n g m e t h a n o l c o n c e n t r a t i o n i s measured by dynamic m e c h a n i c a l spectroscopy. The d y n a m i c s h e a r s t o r a g e modulus (G') a n d l o s s tangent ( t a n δ) were o b t a i n e d f o r "neat" a n d s o l v e n t - s o a k e d s p e c i mens o n a R h e o m e t r i c s RMS 800 m e c h a n i c a l s p e c t r o m e t e r i n t o r s i o n r e c t a n g u l a r geometry. I s o c h r o n a l s p e c t r a were o b t a i n e d f r o m -70 o r -50 ° C t o +100 ° C f o r d e t e r m i n a t i o n o f T . The s p e c t r a were o b t a i n e d a t .2% s t r a i n ( v e r i f i e d t o be w i t h i n t h e linear viscoelastic region), 240 grams t e n s i o n a n d 1 Hz f r e q u e n c y . g

g

Urban and Provder; Multidimensional Spectroscopy of Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1995.

31.

SAMUS AND ROSSI

537

Glass Transition Temperature ofEVOH

T h i n (.7 mm) m o l d e d s l a b s o f EVAL F101 EVOH (a random copolymer of ethylene a n d v i n y l a l c o h o l c o n t a i n i n g 32% e t h y l e n e , T = 68 ° C ) , G110 EVOH (44% e t h y l e n e , T = 58 ° C ) and L101 EVOH (27% e t h y l e n e , T = 73 ° C ) were e q u i l i b r a t e d i n s e a l e d v i a l s f o r a time s u f f i c i e n t l y l o n g t o assure t h a t a l l c o n c e n t r a t i o n g r a d i e n t s had d i s a p p e a r e d . The e q u i l i b r a t i o n t i m e was c h o s e n o n t h e b a s i s o f a d i f f u s i o n c o e f f i cient d e t e r m i n e d from s t a n d a r d s o r p t i o n measurements. However, t h e d y n a m i c m e c h a n i c a l measurements themselves p r o v i d e a check f o r whether e q u i l i b r i u m has been r e a c h e d . I n d e e d , i n s a m p l e s where c o n c e n t r a t i o n g r a d i e n t s (solvated s h e l l , u n s o l v a t e d core) are p r e s e n t , the o b s e r v e d dynamic m e c h a n i c a l s p e c t r a e x h i b i t two p e a k s i n t h e l o s s t a n g e n t measurement ( G " / G ' ) / as shown i n F i g u r e 1. The p r e s e n c e o f two w e l l s e p a r a t e d p e a k s i n n o n - e q u i l i b r a t e d s p e c i m e n s indicates a s h a r p d i f f u s i o n f r o n t t h r o u g h t h e EVOH: a u n i f o r m change i n s o l v e n t c o n c e n t r a t i o n a s a f u n c t i o n o f p e n e t r a t i o n d i s t a n c e would r e s u l t i n a v e r y b r o a d , flat t e n e d t a n δ. I n f r a r e d s p e c t r a o f .012 mm F101 e x t r u d e d p o l y m e r f i l m s h e l d b e t w e e n two K B r p l a t e s were r e c o r d e d w i t h a n i t r o g e n p u r g e d M a t t s o n G a l a x y S e r i e s 5000 F T I R s p e c t r o m e t e r w i t h a room t e m p e r a t u r e DTGS d e t e c t o r . A b s o r b a n c e s p e c t r a were p r o d u c e d b y r a t i o i n g t h e s i n g l e beam s p e c t r a o f t h e s a m p l e s t o t h e s i n g l e beam s p e c t r a o f t h e b a c k g r o u n d . E a c h r a t i o e d absorbance spectrum c o n s i s t e d o f 16 b a c k g r o u n d a n d 16 sample scans r e c o r d e d u s i n g t r i a n g u l a r a p o d i z a t i o n w i t h a p p r o x i m a t e l y 1 wavenumber r e s o l u t i o n . Baseline correc t i o n s were made as n e c e s s a r y . The p e a k a b s o r b a n c e o f t h e i n t e r n a l n o n - r e a c t i n g ( s e c o n d a r y ) C - O H s t r e t c h i n g mode a t 1091 c m a s s o c i a t e d w i t h t h e EVOH f i l m was u s e d t o c o r r e c t f o r p o t e n t i a l c h a n g e s i n sample t h i c k n e s s due t o s w e l l i n g . g

g


g

-1

Results

and D i s c u s s i o n

E x a m p l e s o f o u r d y n a m i c m e c h a n i c a l r e s u l t s a r e shown i n F i g u r e 2, w h i c h g i v e s s t o r a g e m o d u l i a n d l o s s t a n g e n t s ( t a n δ) a s f u n c t i o n s o f t e m p e r a t u r e f o r a s e r i e s o f EVOH (F101) specimens e q u i l i b r a t e d at d i f f e r e n t l e v e l s of methanol uptake. The t r a n s i t i o n s o f t h e amorphous p h a s e f r o m t h e g l a s s y low t e m p e r a t u r e r e g i m e t o t h e r u b b e r y h i g h t e m p e r a t u r e b e h a v i o r a r e smooth a n d r e l a t i v e l y n a r r o w (10). As t h e amount o f m e t h a n o l w i t h i n t h e s a m p l e i n c r e a s e s , the t r a n s i t i o n r e g i o n ( e . g . , T ) i s s h i f t e d towards lower and lower temperatures. F i g u r e 3 summarizes the relation between methanol c o n c e n t r a t i o n w i t h i n the polymer and c h a n g e i n T f o r t h e t h r e e c o p o l y m e r s t h a t we e x a m i n e d . The v a l u e s o f T r e p o r t e d i n t h e F i g u r e c o r r e s p o n d t o t h e t e m p e r a t u r e a t w h i c h t h e maximum i n t h e loss tangent occurs. Although T is different for different (ethyle n e / v i n y l a l c o h o l ) r a t i o s , the s h i f t s i n T f a l l a p p r o x i m a t e l y o n t h e same c u r v e . A r e l a t i v e l y s m a l l amount o f methanol is sufficient to d r a s t i c a l l y lower T : for e x a m p l e a 3% u p t a k e i n d u c e s a d r o p i n T o f a b o u t 40 ° C . g

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Temperature, °C Figure 1. Equilibrated and non-equilibrated dynam ic-mechanical specimen response. The storage modulus (G') and the loss tangent (tan δ) are given as a function of temperature.


SAMUS AND ROSSI

Glass Transition Temperature of EVOH


τ—ι—ι—ι—ι—ι—ι—ι—ι—ι—I

1 I I \ ι ι—ι—ι—ι—ι—ι—ι—ι—ι—ι—ι—ι—Γ

Temperature (°C)

Figure 2. Effect of methanol concentration on the equilibrated dynamic-mechanical specimen response of EVOH F101 polymer. The storage modulus (G') and the loss tangent (tan δ) are given as a function of temperature.


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31.

SAMUS AND ROSSI

541


T h e s e r e s u l t s show t h a t m e t h a n o l b e h a v e s a s a p l a s t i c i z e r f o r EVOH: they are s i m i l a r to r e s u l t s r e p o r t e d i n the l i t e r a t u r e f o r p o l y v i n y l a l c o h o l i n the presence o f water o r g l y c e r o l (a s t a n d a r d p l a s t i c i z e r f o r PVA) ( 1 2 ) . F i g u r e 4 shows FTIR s p e c t r a o b t a i n e d f r o m a p u r e EVOH f i l m ( c u r v e a ) , f r o m m e t h a n o l ( c u r v e b) a n d f r o m t o l u e n e ( c u r v e c) i n t h e r e g i o n o f 1200 t o 675 cm" . In o r d e r to d e t e r m i n e t h e amount o f m e t h a n o l p r e s e n t i n s o a k e d EVOH s a m p l e s we c h o s e t h e p e a k a t 1031 cm" i n t h e m e t h a n o l s p e c t r u m as the c h a r a c t e r i s t i c (signature) absorbance: t h i s peak i s a t t r i b u t e d t o the C-OH s t r e t c h o f the p r i m a r y alcohol. To i d e n t i f y t o l u e n e , t h e a b s o r b a n c e a t 696 cm" , which i s c h a r a c t e r i s t i c of the C-H o u t - o f - p l a n e deformation o f a m o n o s u b s t i t u t e d b e n z e n e , was s e l e c t e d . This choice a l l o w s us t o d e t e c t t o l u e n e i n EVOH f i l m b y F T I R m e a s u r e ments t o c o n c e n t r a t i o n s as low as .3% b y w e i g h t . U s i n g a c o m b i n a t i o n o f FTIR s p e c t r o s c o p y a n d l o n g t e r m w e i g h t g a i n e x p e r i m e n t s we t r i e d t o a s c e r t a i n t h e d e p e n dence o f the s o l u b i l i t y and d i f f u s i o n c o e f f i c i e n t on t h e s t a t e of aggregation (glassy or rubbery) of the polymer i n the toluene-EVOH b i n a r y system. We f o u n d t h a t , unlike methanol, toluene does not s o l v a t e EVOH u n d e r a m b i e n t c o n d i t i o n s t o any d e t e c t a b l e e x t e n t . Evidence supporting t h i s r e s u l t i s shown i n c u r v e (a) o f F i g u r e 5: i t gives t h e s p e c t r u m o f a 12 μπι t h i c k EVOH (F101) f i l m s o a k e d i n t o l u e n e a t room t e m p e r a t u r e f o r 24 h o u r s a n d d o e s n o t exhibit any s i g n o f the s i g n a t u r e t o l u e n e absorbance. I n d e e d a s e r i e s o f l o n g t e r m room t e m p e r a t u r e s o r p t i o n e x p e r i m e n t s o n t h i c k e r f i l m s p r o d u c e d no m e a s u r a b l e w e i g h t gain. On t h e o t h e r h a n d , t h e s i g n a t u r e t o l u e n e a b s o r b a n c e i s c l e a r l y e v i d e n t i n t h e FTIR s p e c t r u m o f a f i l m h e l d a t 9 8 ° C i n the presence of toluene (shown i n c u r v e (b) o f Figure 5). From w e i g h t u p t a k e measurements we d e t e r m i n e d t h a t the c o r r e c t e d i n t e n s i t y of the toluene absorbance i n t h i s i n s t a n c e c o r r e s p o n d s t o 0.8 wt% t o l u e n e i n t h e EVOH film. I n o t h e r words i n c r e a s i n g t h e t e m p e r a t u r e f r o m 23 t o 98 ° C i n c r e a s e s t h e e q u i l i b r i u m volume f r a c t i o n o f t o l u e n e i n EVOH f r o m n e a r z e r o t o a b o u t 1%. In o r d e r to c o r r e l a t e p r e c i s e l y t h i s increase i n s o l u b i l i t y with the e f f e c t s of t h e g l a s s t r a n s i t i o n r e g i o n , we o b t a i n e d F T I R s p e c t r a f r o m a series of EVOH f i l m s held i n toluene at different temperatures f o r several hours. T h i s a l l o w e d us t o n a r r o w t h e t e m p e r a t u r e window where t h e i n c r e a s e i n s o l u b i l i t y takes place: t h e r e i s no e v i d e n c e o f t o l u e n e a f t e r a 22 h o u r s o a k a t 50 ° C (about 18 ° C b e l o w T ) , w h i l e the t o l u e n e s i g n a t u r e i s c l e a r l y p r e s e n t a t 78 ° C ( o n l y 10 ° C above T ) . 1

1


1

g

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We a l s o f o u n d t h a t f o r EVOH f i l m s w h i c h were first e x p o s e d t o t o l u e n e a t t e m p e r a t u r e s w e l l above T a n d t h e n q u e n c h e d t o room t e m p e r a t u r e , t h e a b s o r b e d t o l u e n e r e m a i n s "trapped" i n the f i l m . S p e c i f i c a l l y , the c h a r a c t e r i s t i c t o l u e n e a b s o r b a n c e i s e s s e n t i a l l y t h e same i n s p e c t r a t a k e n i m m e d i a t e l y a f t e r quenching and s e v e r a l days l a t e r . This suggests that, even f o r t h e s e v e r y t h i n f i l m s , little g




542

Figure 4. Fourier-trans form infrared spectra showing characteristic absorbances used for identification of components: (a) pure EVOH F101 film, (b) methanol, and (c) toluene.


31. SAMUS AND ROSSI

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Solvent-Induced Changes in the Glass Transition Temperature of

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