Measurement of Polarization in Thermoplastic Elastomers with

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Measurement of Polarization in Thermoplastic Elastomers with Application to Morphology Studies M. A. VALLANCE and S. L. COOPER University of Wisconsin—Madison, Department of Chemical Engineering, Madison, WI 53706 The measurement of electrical polarization is proposed for studying morphological heterogeneities down to the nanometer size range in segmented copolymers such as segmented polyurethanes. The technique relies on measurement of frequency-dependent, electric polariza tions related to frequency-dependent, electric-field redistribution in microdomains of unequal permittivity and conductivity. Due to the low overall conductivi ties of typical polymers, the polarizations usually become significant at subaudio frequencies. The con struction of two processor-aided dielectric spectrom eters designed for use in the frequency range 10 to 10 Hz are described. Spectra for segmented poly ether-polyurethanes and a polyether-polyester are compared to an adjustable model of the polarization process. From these comparisons and DSC results, a correlation between intradomain ordering and lamellar morphology is seen in such polymers. There appears to be a relation between the asymmetry of the polariza tions' frequency dependence and the character of the diffuse transition zones between dissimilar microdomains. -4

+2

Segmented p o l y u r e t h a n e s and r e l a t e d t h e r m o p l a s t i c e l a s t o m e r s a r e b l o c k c o p o l y m e r s w i t h ( A B ) c h e m i c a l a r c h i t e c t u r e w h e r e A and B a r e segments o f d i s s i m i l a r homopolymers. These c o p o l y m e r s a r e s y n t h e s i z e d i n one o r two s t e p s by c o n d e n s a t i o n . As an example, a t y p i c a l segmented p o l y u r e t h a n e m i g h t be f o r m e d b y r e a c t i n g e x c e s s d i i s o c y a n a t e w i t h a p o l y o l " s o f t " segment w h i c h i n t u r n c o u l d be r e a c t e d w i t h a d i o l c h a i n e x t e n d e r t o f o r m t h e " h a r d " s e g m e n t s . The r e s u l t i n g p o l y m e r i s composed o f many segments ( n a v e r a g e s 10 t o 1 0 0 ) , w h i l e t h e i n d i v i d u a l segment l e n g t h s a r e b r o a d l y d i s t r i b u t e d about a r e l a t i v e l y s h o r t average l e n g t h (number a v e r a g e segment m o l e c u l a r w e i g h t s t y p i c a l l y r a n g e 500 n

0097-6156/82/0197-0277$06.00/0 © 1982 American Chemical Society Provder; Computer Applications in Applied Polymer Science ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

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COMPUTER APPLICATIONS IN APPLIED POLYMER SCIENCE

to 5,000). I n almost a l l c a s e s thermodynamic f o r c e s d r i v e the s y s t e m t o w a r d s s e g r e g a t i o n o f A and B s e g m e n t s . However, t h e c h e m i c a l a r c h i t e c t u r e c o n s t r a i n s t h e m i c r o d o m a i n s so f o r m e d t o have d i m e n s i o n s on t h e o r d e r o f 10 nm. The r e s u l t i n g " m i c r o composite" morphology e x h i b i t s e l a s t o m e r i c p r o p e r t i e s i f the use temperature i s s e l e c t e d i n t e r m e d i a t e to the s o l i d - m e l t or 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 o f t h e two p h a s e s . In t h i s case the r i g i d , h a r d - s e g m e n t r i c h d o m a i n s a c t a s v i r t u a l c r o s s l i n k s and f i l l e r p a r t i c l e s f o r the v i s c o u s , soft-segment r i c h phase. Thermodynamic t r e a t m e n t s o f b l o c k c o p o l y m e r m o r p h o l o g y , s u c h a s t h o s e o f H e l f a n d (1,2,3) and M e i e r ( 4 ) , w h i c h a r e based on a n i o n i c a l l y p o l y m e r i z e d , m o n o d i s p e r s e segment AB and ABA t y p e block copolymers, are not a p p l i c a b l e i n the present case. The l a r g e number o f s h o r t segments i n e a c h m o l e c u l e , t h e p o s s i b i l i t y of c r y s t a l l i n i t y i n one o r b o t h p h a s e s , and t h e l i k e l i h o o d o f m e t a s t a b l e m o r p h o l o g i e s work t o g e t h e r t o p r e c l u d e the a n a l y t i c a l approaches p r e s e n t l y a v a i l a b l e . The most d i r e c t e x p e r i m e n t a l t e c h n i q u e f o r o b s e r v i n g t h e morphology i s t r a n s m i s s i o n e l e c t r o n microscopy. Although several m i c r o g r a p h s o f segmented p o l y u r e t h a n e m o r p h o l o g i e s a r e r e c o r d e d i n the l i t e r a t u r e showing n o d u l a r t e x t u r e s of r e a s o n a b l e p e r i o d i c i t y ( 5 , 6 , 7 ) , a r e c e n t m a t h e m a t i c a l a n a l y s i s has shown t h a t i n a l l p r o b a b i l i t y these t e x t u r e s are defocus a r t i f a c t s ( 8 ) . A recent s t u d y o f an OsO^ s t a i n e d , s o l u t i o n - c a s t , s e m i c r y s t a l l i n e , s e g mented p o l y u r e t h a n e has p r o d u c e d m i c r o g r a p h s w h i c h show more d e t a i l ( 9 ) . N o n e t h e l e s s , l a c k o f c o n t r a s t and t h e d i f f i c u l t y o f producing s u f f i c i e n t l y t h i n f o i l s w i t h r e p r e s e n t a t i v e morphologies r e n d e r TEM u n s u i t a b l e f o r g e n e r a l u s e . Measurement o f s m a l l - a n g l e x - r a y s c a t t e r i n g (SAXS) has r e c e n t l y gained i n s t a t u r e f o r use i n m o r p h o l o g i c a l i n v e s t i g a t i o n s . T h i s t e c h n i q u e p r o v i d e s i n f o r m a t i o n r e g a r d i n g t h e p e r i o d i c i t y and magnitude of e l e c t r o n d e n s i t y f l u c t u a t i o n s . The t e c h n i q u e has been u s e d on segmented p o l y u r e t h a n e s and r e l a t e d p o l y m e r s t o s t u d y domain s p a c i n g , p h a s e p u r i t y , and t h e w i d t h o f t h e t r a n s i t i o n zone between d i s s i m i l a r domains ( 1 0 , 1 1 , 1 2 ) . I n t h e p r e s e n t s t u d y we have u s e d a r e l a t i v e l y new t e c h n i q u e to study the morphology of these systems. A p p l i c a t i o n o f an a l t e r n a t i n g e l e c t r i c f i e l d to a m u l t i p h a s e morphology r e s u l t s i n a frequency-dependent e l e c t r i c - f i e l d d i s t r i b u t i o n due t o t h e d i f f e r e n t p e r m i t t i v i t i e s and c o n d u c t i v i t i e s o f d i s s i m i l a r p h a s e s . M e a s u r e d p e r m i t t i v i t y i n c r e a s e s w i t h d e c r e a s i n g f r e q u e n c y V = 0)/27T, approaching a low f r e q u e n c y l i m i t e as the f i e l d d i s t r i b u t i o n t r a n s i t s from p e r m i t t i v i t y dominated t o c o n d u c t i v i t y dominated. F o r most s i m p l e t w o - p h a s e g e o m e t r i e s t h e f r e q u e n c y d e p e n d e n c e o f t h e p o l a r i z a t i o n a p p r o a c h e s a s i m p l e overdamped r e s o n a n c e : Q

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17.

VALLANCE AND COOPER

Polarization

in Thermoplastic

Elastomers

279

r e a l and i m a g i n a r y p a r t s . The p a r a m e t e r s e and 0 ) a r e t h e h i g h f r e q u e n c y l i m i t o f £ and t h e "damped r e s o n a n t f r e q u e n c y " . The i m a g i n a r y component e" v a n i s h e s i n t h e h i g h and l o w f r e q u e n c y l i m i t s w h i l e p e a k i n g a t 03 = co . I f e q u a t i o n ( 1 ) i s s o l v e d f o r e" i n t e r m s e , w h e r e b y 0) d e p e n d e n c e c a n be e l i m i n a t e d , i t i s s e e n that t h e equation has t h e form o f a s e m i c i r c l e . A s w i l l be shown 9 oo> d ^ a r e i n p a r t d e t e r m i n e d by m o r p h o l o g y so t h a t experimental determination of these q u a n t i t i e s w i l l provide certain morphological insights. A complication i s the divergence of most r e a l s p e c t r a f r o m t h e d e p e n d e n c e o f e q u a t i o n ( 1 ) . A d i s c u s s i o n of a p o s s i b l e source of the divergence i s included i n the d i s c u s s i o n o f measured s p e c t r a . I n t e r f a c i a l p o l a r i z a t i o n s have been r e c o r d e d f o r many m u l t i phase s o l i d - s t a t e polymer systems: s e m i c r y s t a l l i n e polyamides and p o l y u r e t h a n e s ( 1 3 ) , segmented p o l y u r e t h a n e s ( 1 4 ) , segmented p o l y e s t e r s (15,16), polyethylene o x i d e - p o l y s t y r e n e d i b l o c k c o p o l y m e r s ( 1 7 ) , and s t y r e n e - b u t a d i e n e - s t y r e n e t r i b l o c k s y s t e m s (18). In g e n e r a l , due t o t h e l o w c o n d u c t i v i t i e s o f p o l y m e r i c s o l i d s , t h e p o l a r i z a t i o n s a r e f o u n d a t s u b a u d i o f r e q u e n c i e s , where s t a n d a r d b r i d g e e x p e r i m e n t s do n o t o p e r a t e . I n order to study t h e s e p o l a r i z a t i o n s two p r o c e s s o r - c o n t r o l l e d d i e l e c t r i c s p e c t r o m e t e r s have been d e s i g n e d t o o p e r a t e i n t h i s f r e q u e n c y range. One u s e s l o w - f r e q u e n c y , A.C. e x c i t a t i o n i n c o n j u n c t i o n w i t h c r o s s - c o r r e l a t i o n a n a l y s i s w h i l e t h e other uses step e x c i t a tion with Laplace-transform analysis. The r e m a i n d e r o f t h i s paper i n c l u d e s d e s c r i p t i o n s o f t h e a p p a r a t u s , a t h e o r e t i c a l c o n t e x t i n w h i c h t o v i e w t h e m e a s u r e d s p e c t r a , and t h e p r e s e n t a t i o n and a n a l y s i s o f some e x p e r i m e n t a l d a t a . OT

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D i e l e c t r i c s p e c t r o s c o p y c a n be c a r r i e d o u t by o b s e r v i n g a m a t e r i a l ' s s t e a d y - s t a t e r e s p o n s e t o an o s c i l l a t i n g e l e c t r i c f i e l d o r by o b s e r v i n g t h e t r a n s i e n t r e s p o n s e t o a s i n g l e e v e n t s u c h a s a s t e p change i n f i e l d . The s i m p l e s t sample g e o m e t r y i n e i t h e r case i s t h a t o f t h e d i e l e c t r i c i n a p a r a l l e l - p l a t e c a p a c i t o r . Edge e f f e c t s a r e m i n i m i z e d b y t h e u s e o f a t h r e e - t e r m i n a l , g u a r d r i n g c o n f i g u r a t i o n . T y p i c a l d i s k - s h a p e d s a m p l e s measure 1.5 cm i n d i a m e t e r and 0.01 cm i n t h i c k n e s s . The e x p e r i m e n t i s s e t up so t h a t t h e sample r e s i d e s i n a vacuum o r a d e s i c c a t e d n i t r o g e n atmosphere, thus m i n i m i z i n g water r e g a i n i n h y d r o s c o p i c polymer s y s t e m s . Good p r a c t i c e d i c t a t e s t h e a p p l i c a t i o n o f i n t e g r a l m e t a l l i c , u s u a l l y gold, coatings to thef l a t surfaces of the sample. T h i s p r o v i d e s f o r low-impedance e l e c t r i c a l c o n t a c t . The a p p a r a t u s u s e d , w i t h s l i g h t m o d i f i c a t i o n s , i s t h e same for both experiments. The p r o c e s s o r i s a DEC L S I 11/3 w i t h 64K b y t e s o f RAM. C o m m u n i c a t i o n w i t h t h e p r o c e s s o r i s done u s i n g a CRT t e r m i n a l . A p a i r o f d o u b l e - d e n s i t y , f l e x i b l e - d i s k d r i v e r s a r e u s e d f o r s t o r a g e o f s o f t w a r e and d a t a . Other system components i n c l u d e a 1 MHz p r o g r a m m a b l e t i m e r , 1 2 - b i t A/D a n d



280

D/A c o n v e r t e r s , and a d i g i t a l p l o t t e r . A Nicolet Explorer I I I d i g i t a l o s c i l l o s c o p e capable of high-speed data a c q u i s i t i o n i s i n t e r f a c e d t o t h e p r o c e s s o r v i a 45 TTL l i n e s w h i c h p r o v i d e h i g h speed d a t a t r a n s f e r i n b o t h d i r e c t i o n s . The o s c i l l o s c o p e i s used p r i n c i p a l l y f o r h i g h frequency ( 1 0 " t o 1 0 Hz) t e s t i n g and s e r v e d o n l y a s b a c k - u p i n t h e p r e s e n t c a s e s . For transducing c u r r e n t s i n t h e n a n o - t o p i c o a m p e r e r a n g e a K e i t h l y 427 c u r r e n t a m p l i f i e r i s u s e d w i t h a d j u s t a b l e r i s e t i m e down t o 10 y s . 2

+ 3

Steady-State Experiment. A simple harmonic s i g n a l i n t h e f r e q u e n c y r a n g e 20 Hz and down i s s y n t h e s i z e d by s t e p p i n g t h e o u t p u t a t a D/A c o n v e r t e r a t e q u a l l y s p a c e d i n t e r v a l s up t o 5,000 t i m e s p e r c y c l e . The a m p l i t u d e i s s e l e c t e d up t o 10 v o l t s . The r e s u l t i n g s i g n a l , w h i c h i s somewhat n o t c h y , i s smoothed by low pass f i l t e r i n g ( f o u r t h - o r d e r B u t t e r w o r t h ) . Figure 1 i s a schematic of the experiment. The f i l t e r e d s i g n a l i s a p p l i e d t o the sample c e l l w h i l e t h e sample's c u r r e n t r e s p o n s e i s m o n i t o r e d a t t h e g u a r d e d measurement e l e c t r o d e . B e c a u s e o f t h e l o w - l e v e l c u r r e n t s i n v o l v e d , s h i e l d e d c a b l e and s h o c k m o u n t i n g a r e u s e d throughout t h e system. R e m a i n i n g n o i s e c a n be e l i m i n a t e d by s e l e c t i o n of a s u i t a b l y h i g h r i s e time on t h e c u r r e n t a m p l i f i e r . P r o p o r t i o n a l v o l t a g e o u t p u t o f t h e c u r r e n t a m p l i f i e r and t h e f i l t e r e d d r i v i n g s i g n a l a r e s i m u l t a n e o u s l y s a m p l e d o v e r one c o m p l e t e c y c l e a t up t o 5,000 e q u a l l y s p a c e d i n t e r v a l s o n c e s t e a d y s t a t e r e s p o n s e i s a t t a i n e d . The v o l u m e - a v e r a g e e l e c t r i c f i e l d f(cot) and t h e v o l u m e - a v e r a g e c u r r e n t d e n s i t y t h r o u g h t h e s a m p l e i(cot) c a n be r e p r e s e n t e d a s f ( o ) t ) = F cos(o)t + a )

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17.

VALLANCE AND COOPER

Polarization

in Thermoplastic

Elastomers

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VALLANCE AND COOPER

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in Thermoplastic

291

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t


292


was t e s t e d i n t h e a n n e a l e d and u n a n n e a l e d s t a t e s . Samples were d e s i c c a t e d i n h i g h vacuum w h e r e g o l d e l e c t r o d e s were subsequently applied. A l l samples were s u b j e c t e d t o a 150-hour, 24°C, s t a b i l i z a t i o n p e r i o d p r i o r t o t e s t i n g a t 24°C. F i g u r e 8 shows t h e t h r e e d e c a y f u n c t i o n s w h i c h a r e a c t u a l l y t h e n o r m a l i z e d d e p o l a r i z a t i o n c u r r e n t s i / F . The d e p o l a r i z a t i o n s f o r t h e UET48-1 s y s t e m s o c c u r o v e r a l o n g e r p e r i o d (100 t o 700 s) t h a n t h a t f o r HET49-1 (~30 s ) , and t h e UET48-1 d e p o l a r i z a t i o n c u r r e n t s are h i g h e r i n magnitude. F i g u r e 9 shows t h e t r a n s f o r m e d e s p e c t r a ; t h e e" s p e c t r a a r e n o t shown. A G e n e r a l R a d i o 1620-A b r i d g e a s s e m b l y was u s e d t o e v a l u a t e k* and k" as w e l l as t o p r o v i d e t h e h i g h f r e q u e n c y ( 1 0 - 10 * Hz) d a t a . The two UET48-1 s a m p l e s show l a r g e p o l a r i z a t i o n s o f s i m i l a r s t r e n g t h and shape a l t h o u g h t h e p o l a r i z a t i o n f o r t h e a n n e a l e d p o l y m e r i s s h i f t e d about 0.7 l o g a r i t h m i c d e c a d e s h i g h e r i n f r e q u e n c y . HET49-1 shows o n l y s l i g h t p o l a r i z a t i o n i n t h i s f r e q u e n c y r e g i m e , a s s e e n i n F i g u r e s 8 and 9. The HET49-1 s p e c t r u m f l a t t e n s o u t a t a b o u t 0.1 Hz. The f r e q u e n c y s h i f t c a u s e d by a n n e a l i n g UET48-1 i s r e l a t e d t o h i g h e r s o f t p h a s e c o n d u c t i v i t y , w h i c h i n t u r n r e s u l t s f r o m d e p r e s s i o n o f t h e g l a s s t r a n s i t i o n due t o annealing. S u c h a change w o u l d n e c e s s a r i l y c a u s e a f r e q u e n c y s h i f t , t h o u g h i t need n o t c a u s e a change i n p o l a r i z a t i o n strength. The l a c k o f a l a r g e d i s t i n c t p o l a r i z a t i o n i n HET49-1 i s s u g g e s t i v e of a n o n - l a m e l l a r morphology. C r y s t a l l i z a t i o n of HET49-1 i s hampered by t h e p r e s e n c e o f t h r e e d i s t i n c t c o n f i g u r a t i o n a l isomers i n the d i i s o c y a n a t e used to form t h i s polymer. In f a c t a 20°/min DSC s c a n r e v e a l s a t w o - p h a s e m o r p h o l o g y w i t h e n d o t h e r m i c d i s r u p t i o n o f t h e h a r d p h a s e o c c u r r i n g a t 78°C, i n d i c a t i n g a l a c k of s t r o n g o r d e r i n g i n those domains. Thus t h e r e a p p e a r s t o be a c o r r e l a t i o n between t h e f o r m a t i o n o f h a r d segment c r y s t a l l i n i t y and l a m e l l a r m o r p h o l o g y i n t h e s e s h o r t - b l o c k segmented c o p o l y m e r s . T h i s i s not to say t h a t l a m e l l a r morphologies occur only f o r s e m i c r y s t a l l i n e polymers. In t o t a l l y amorphous AB and ABA b l o c k c o p o l y m e r s , s u c h a s p o l y s t y r e n e - p o l y b u t a d i e n e d i b l o c k s , l a m e l l a r m o r p h o l o g i e s can a l s o form, d e p e n d i n g on r e l a t i v e f r a c t i o n s o f t h e two b l o c k s and c a s t i n g parameters. f

2

1

Summary L a r g e p o l a r i z a t i o n s a r e seen t o o c c u r i n m u l t i p h a s e polymer s y s t e m s , p r i n c i p a l l y due t o t h e d i f f e r e n t c o n d u c t i v i t i e s i n t h e d i s s i m i l a r microdomains. Due t o t h e l o w o v e r a l l c o n d u c t i v i t i e s of t h e s e polymers, the p o l a r i z a t i o n s o c c u r a t low, o f t e n subaudio, frequencies. I n o r d e r t o s t u d y t h e s e p r o c e s s e s two c o m p u t e r a i d e d d i e l e c t r i c s p e c t r o m e t e r s have b e e n d e v e l o p e d . They d i f f e r i n the type of e x c i t a t i o n used: A.C. v e r s u s s t e p . E a c h s p e c t r o m e t e r has c e r t a i n a d v a n t a g e s i n t e r m s o f a c c u r a c y , r e s o l u t i o n and c o n v e n i e n c e . A n a l y t i c a l m o d e l l i n g i s used to compare m e a s u r e d s p e c t r a t o p r o p o s e d m o r p h o l o g i e s . Although


.

Polarization

VALLANCE AND COOPER

i

1E+3

CO

CD +

+

UJ

\

Elastomers

1E+2

1E+1

1E+1

\

.JJET4) M(80° : anneal £•*•»..

o o

HET

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\ *9-f \

CO

m

•

•

o

e

CD

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293

+

1E+2 8.

1E+0

Q

in Thermoplastic

1E+0

1E-1 UET48

• * •

fl

>

•

1E-2

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o

1E-3

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Time (s) Figure 8. Measured polarization decay junctions at 24° C jor three segmented polyether-polymethanes. HET49-1 is tested after annealing at 80 °C followed by stabilization at 24°C. VET48-1 is tested with and without annealing. The decay function is the depolarization current density normalized by the electric field value used for polarizing.


294


Figure 9. Transformed e' spectra at 24°C for three segmented poly ether-poly methanes. Key: O, UET48-1 (80°C anneal); •, UET48-1; and A , HET49-1 (80°C anneal).


17.

VALLANCE AND COOPER

Polarization

in Thermoplastic

Elastomers

295

p r e c i s e f i t s of models t o e x p e r i m e n t a l s p e c t r a i s d i f f i c u l t f o r complex m o r p h o l o g i e s , t h e experiment c a n l e a d t o u s e f u l morphological i n s i g h t s , p a r t i c u l a r l y i n comparative studies. A c k n o w l e d g ement The a u t h o r s w i s h t o a c k n o w l e d g e p a r t i a l s u p p o r t o f t h i s r e s e a r c h by G r a n t # DMR 81-06888 o f t h e P o l y m e r s S e c t i o n o f NSF's D i v i s i o n o f M a t e r i a l s R e s e a r c h . The a u t h o r s a l s o w i s h t o thank I n t e r n a t i o n a l Harvester f o r p r o v i d i n g a f e l l o w s h i p t o M i c h a e l A. V a l l a n c e f o r work o n t h i s p r o j e c t .

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van Beek, L. K. H. Progress in Dielectrics 1967, 7, 69. Brown, Jr., W. F. J. Chem. Phys. 1955, 23, 1514. Sillars, R. W. J. Inst. Elect. Engrs. 1937, 80, 378. Fricke, H. J. Phys. Chem. 1953, 57, 934. Saito, S.; Sasabe, H.; Nakajima, T.; Yada, K. J. Polym. Sci., A2 1968, 6, 1297. Harrel, Jr., L. L. Macromolecules 1969, 2, 607. Flory, P. J. "Polymer Chemistry", Cornell, Ithaca, 1953, 568. Lilaonitkul, A.; Cooper, S. L. Rubber Chem. Technol. 1977, 50, 1. Davidson, D. W.; Cole, R. H. J. Chem. Phys. 1951, 19, 1484. Fong, F. K.; Smyth, C. P. J. Phys. Chem. 1963, 67, 226. Maxwell, J. C. "Electricity and Magnetism", Vol. 1, Clarendon, Oxford, 1892, 452.

RECEIVED May 4, 1982.


Measurement of Polarization in Thermoplastic Elastomers with

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