Investigations into the Structure of Liquid ... - ACS Publications

Chapter 30

Investigations into the Structure of Liquid-Crystalline Polymer—Liquid-Crystalline Polymer Blends Downloaded by VIRGINIA TECH on February 27, 2015 | http://pubs.acs.org Publication Date: August 24, 1990 | doi: 10.1021/bk-1990-0435.ch030

1

2

Mark T. DeMeuse and Michael Jaffe 1

Enimont America, Inc., 2000 Princeton Park Corporate Center, Monmouth Junction, NJ 08852 Hoechst Celanese Corporation, 86 Morris Avenue, Summit, NJ 07901 2

Blends in which both of the components are capable of forming l i q u i d c r y s t a l l i n e phases in the melt have been investigated. Rheological data as well as s o l i d state data i s presented which suggests that not a l l of the blends behave the same. This observation i s in contrast to ideas of small molecule l i q u i d c r y s t a l s . The r e s u l t s are interpreted in terms of present theories regarding blends. Much work h a s a p p e a r e d r e c e n t l y i n t h e p o l y m e r l i t e r a t u r e c o n c e r n i n g b l e n d i n g o f p o l y m e r s a s a mechanism f o r t a i l o r i n g p h y s i c a l p r o p e r t i e s to o b t a i n a c e r t a i n performance l e v e l . Many o f t h e s e e f f o r t s have f o c u s e d on d e t e r m i n i n g m i s c i b i l i t y c r i t e r i a f o r the b l e n d s . V a r i a b l e s such as m o l e c u l a r weight, temperature and c o m p o s i t i o n o f t h e two component m a t e r i a l s h a v e been shown t o b e i m p o r t a n t i n u n d e r s t a n d i n g t h e p h a s e b e h a v i o r present i n these systems (!). T h e m a j o r i t y o f t h e work done t h u s f a r h a s d e a l t w i t h m i x i n g o f two random c o i l p o l y m e r s . T h e F l o r y - H u g g i n s formalism i s u s u a l l y used to d e s c r i b e t h e e x p e c t e d f r e e energy o f m i x i n g . As such, m i s c i b i l i t y i s n o r m a l l y d o m i n a t e d by t h e e n t h a l p i c p a r t o f t h e f r e e energy. The o n e m a j o r e x c e p t i o n t o t h i s o b s e r v a t i o n h a s involved s t u d i e s of molecular composites (2.3). In t h o s e s y s t e m s , t h e two p o l y m e r s b e i n g m i x e d h a v e v a s t l y d i f f e r e n t molecular conformations. A s p r e d i c t e d by F l o r y ( 2 . 3 ) . phase s e p a r a t i o n i s e a s i l y induced i n such systems. I t s h o u l d be r e c o g n i z e d t h a t t h i s s e p a r a t i o n i s b a s e d s o l e l y on e n t r o p i c e f f e c t s .

0097-6156/90/0435-0439$06.00/0 © 1990 American Chemical Society In Liquid-Crystalline Polymers; Weiss, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

Downloaded by VIRGINIA TECH on February 27, 2015 | http://pubs.acs.org Publication Date: August 24, 1990 | doi: 10.1021/bk-1990-0435.ch030

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LIQUID-CRYSTALLINE POLYMERS

Two e x t r e m e c a s e s h a v e , t h e n , been e s t a b l i s h e d i n the l i t e r a t u r e . Random c o i l / r a n d o m c o i l m i x i n g i s d o m i n a t e d by e n t h a l p i c e f f e c t s w h i l e random c o i l / r i g i d r o d m i x i n g i s d o m i n a t e d by e n t r o p i c e f f e c t s . The p r e s e n t i n v e s t i g a t i o n w i l l be p a r t o f an o n g o i n g s t u d y to b r i d g e t h e gap between t h e s e two e x t r e m e s by e x a m i n i n g m i x t u r e s of two l i q u i d c r y s t a l p o l y m e r s . The c o n f o r m a t i o n o f these polymers i s best d e s c r i b e d as being s e m i - f l e x i b l e . By a n a l o g y w i t h s m a l l m o l e c u l e l i q u i d c r y s t a l s , w h e r e t h e t y p e of l i q u i d c r y s t a l f o r m e d i s u s e d a s a t e s t f o r m i s c i b i l i t y , i t i s expected that a l l polymer m o l e c u l e s t h a t f o r m t h e same t y p e of l i q u i d c r y s t a l l i n e p h a s e w i l l be m i s c i b l e ( 4 ) . T h i s i s i n c o n t r a s t to more t r a d i t i o n a l polymers where m i s c i b i l i t y i s the e x c e p t i o n r a t h e r than t h e r u l e . The p r e s e n t work w i l l s u g g e s t w h i c h of t h e s e c o n c e p t s i s a p p l i c a b l e to l i q u i d c r y s t a l polymer b l e n d systems. A r e l a t e d s t u d y h a s r e c e n t l y been r e p o r t e d by C i f e r r i , e t a l ( 5 ) u s i n g t h e s y s t e m c e l l u l o s e a c e t a t e and (hydroxypropyl) c e l l u l o s e dissolved in N , N - D i m e t h y l a c e t a m i d e (DMAC). The two p o l y m e r s e x h i b i t s i m i l a r c o n f o r m a t i o n s and b o t h e x i s t i n a n i s o t r o p i c p h a s e s above a c e r t a i n c r i t i c a l c o n c e n t r a t i o n . I t was f o u n d t h a t when t h e t e r n a r y s y s t e m s w e r e e x a m i n e d , two a n i s o t r o p i c p h a s e s e x i s t a b o v e t h e c r i t i c a l volume fraction. F u r t h e r , each of the s t a b l e a n i s o t r o p i c phases c o n t a i n pure polymer. The a u t h o r s c o n c l u d e t h a t m i s c i b i l i t y between p o l y m e r s f o r m i n g t h e same t y p e o f mesophase i s n ' t n e c e s s a r i l y o b s e r v e d , i n c o n t r a s t to l o w molecular weight l i q u i d c r y s t a l s . The p u r p o s e of t h e p r e s e n t work i s to e x t e n d t h e above s t u d i e s to b l e n d s y s t e m s c o n s i s t i n g of two c o m p o n e n t s , e a c h of w h i c h i s c a p a b l e of f o r m i n g a l i q u i d c r y s t a l l i n e phase i n the m e l t . S u c h b l e n d s become o f i n c r e a s i n g importance w i t h the r e c e n t l y r e p o r t e d f i n d i n g (6) t h a t i t i s p o s s i b l e to o b s e r v e s y n e r g i s m s i n mechanical p r o p e r t i e s in these systems. Our p r e v i o u s s t u d i e s i n t h i s a r e a h a v e s u g g e s t e d t h a t t h e t h e o r i e s of t r a d i t i o n a l p o l y m e r b l e n d s y s t e m s ( 7 , 8 ) a r e a p p l i c a b l e to these blends. T h i s p a p e r i s a f u r t h e r s t u d y of t h e a p p l i c a b i l i t y of t h e s e c o n c e p t s . Experimental

Procedure

The s a m p l e s u s e d i n t h e p r e s e n t s t u d y a r e ( 1 ) a c o p o l y m e r c o n s i s t i n g o f p - h y d r o x y b e n z o i c a c i d (HBA) and 6 - h y d r o x y - 2 - n a p h t h o i c a c i d (HNA) i n t h e m o l e f r a c t i o n r a t i o s 7 3 : 2 7 and ( 2 ) a p o l y m e r w h i c h c o n t a i n s HBA, HNA, t e r e p h t h a l i c a c i d (TA) and h y d r o q u i n o n e (HQ) i n t h e ratios 57:41:1:1. I t w i l l be shown l a t e r t h a t t h e i n c o r p o r a t i o n of t h e s m a l l amount of TA and HQ i n t o t h e l a t t e r s t r u c t u r e makes t h e m a t e r i a l l e s s s h e a r - t h i n n i n g t h a n t h e c o p o l y m e r c o n t a i n i n g o n l y HBA and HNA, when examined r h e o l o g i c a l l y . It i s generally believed that a random s e q u e n c e d i s t r i b u t i o n of monomers o c c u r s i n b o t h

In Liquid-Crystalline Polymers; Weiss, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.


30. DEMEUSE & JAFFE

Investigations into the Structure ofLCP-LCP Blends 441

of these polymers. T h i s c o n c l u s i o n h a s been r e a c h e d b a s e d on X - r a y d i f f r a c t i o n work done by b o t h S t a m a t o f f (9) and B l a c k w e l l , e t a l ( 1 0 . 1 1 ) w h i c h h a s shown t h a t t h e m e r i d o n a l s c a t t e r i n g p a t t e r n of these copolymers i s c o n s i s t e n t w i t h a random c o p o l y m e r c o n c e p t . The b l e n d s w e r e p r o d u c e d v i a a m e l t b l e n d i n g procedure. The e q u i p m e n t u s e d f o r t h i s p r o c e d u r e was a H a a k e - B u c h l e r R h e o c o r d . The s a m p l e s i z e was 7 0 . 0 grams w h i c h was d e t e r m i n e d by t h e d e n s i t y of t h e s a m p l e . Since t h e s a m p l e s u s e d i n t h i s work a r e c a p a b l e o f u n d e r g o i n g h y d r o l y t i c d e g r a d a t i o n ( 1 2 ) , d r y i n g b e f o r e any m e l t i n g p r o c e s s i s r e q u i r e d . The s a m p l e s i n t h e p r e s e n t s t u d y w e r e d r i e d o v e r n i g h t a t 120 C p r i o r to m e l t i n g . S t a n d a r d m i x i n g c o n d i t i o n s were to m e l t b l e n d the s a m p l e s a t 300 C f o r f i v e m i n u t e s a t a r o t o r s p e e d of 40 RPM. The e f f e c t o f m i x i n g h i s t o r y on t h e s t r u c t u r e o f t h e LCP/LCP b l e n d s h a s n o t been i n v e s t i g a t e d i n t h e present work. S u c h a s t u d y h a s been r e p o r t e d by M e h t a and B a i r d ( 1 3 ) . T h e s a m p l e s w e r e , t h e n , g i v e n two m i n u t e s a t 5 RPM f o l l o w e d by p r o g r a m m i n g t h e r o t o r s p e e d to 100 RPM i n a two m i n u t e p e r i o d . The s p e e d i s , t h e n , i n s t a n t a n e o u s l y l o w e r e d back down to 5 RPM i n two minutes. T h i s i s f o l l o w e d by two m i n u t e s a t 5 RPM to d e t e r m i n e w h e t h e r t h e ramp t e s t h a s a f f e c t e d t h e s a m p l e i n any way. The X - r a y d i f f r a c t i o n work w h i c h i s r e p o r t e d was p e r f o r m e d on s t r a n d s o f b o t h t h e n e a t m a t e r i a l s and a 50/50 c o m p o s i t i o n b l e n d . M e r i d o n a l s c a n s w e r e p e r f o r m e d u s i n g a f o c u s e d beam on a h i g h r e s o l u t i o n Huber d i f f r a c t o m e t e r u t i l i z i n g CuK^ r a d i a t i o n on a h i g h i n t e n s i t y r o t a t i n g anode. The beam, f o c u s e d a t t h e s a m p l e p o s i t i o n , was a b o u t 1 X 1 mm i n s i z e . A &-2& c o u p l i n g was p e r f o r m e d u s i n g t h e t o t a l e x t e r n a l r e f l e c t i o n of a g l a s s s l i d e . A s t e p s c a n was p e r f o r m e d i n t r a n s m i s s i o n mode f r o m 5 ° t o 60° (2-0$ w i t h 0 . 1 s t e p s i z e f o r a p e r i o d o f 60 s e c o n d s and f o r e x p a n d e d s c a n s f r o m 8 ° to 1 6 ° ( 2 < e ) , t h e s t e p p i n g t i m e was 120 s e c o n d s p e r step. DSC e x p e r i m e n t s w e r e p e r f o r m e d by h e a t i n g powdered s a m p l e s o f b o t h t h e b l e n d s and component m a t e r i a l s i n a n i t r o g e n a t m o s p h e r e a t 99 C/min to 300 C, l e t t i n g t h e s a m p l e s a m b i e n t l y c o o l and r e s c a n n i n g u s i n g a h e a t i n g r a t e of 20 C/min. B o t h c r y s t a l l i z a t i o n t r a c e s upon c o o l i n g and m e l t i n g t r a c e s upon r e h e a t i n g w e r e r e c o r d e d . Dynamic m e c h a n i c a l e x p e r i m e n t s w e r e p e r f o r m e d on c o m p r e s s i o n m o l d e d b a r s w h i c h had been m o l d e d f r o m powders. Compression m o l d i n g of the samples produced s p e c i m e n s a d e q u a t e f o r s o l i d s t a t e e v a l u a t i o n but n o t s u f f i c i e n t for physical property e v a l u a t i o n . Samples w e r e m o l d e d u s i n g a s t a n d a r d Wabash p r e s s and a m o l d i n g t e m p e r a t u r e of 300 C. E v a l u a t i o n of t h e m o l d e d s a m p l e s was p e r f o r m e d u s i n g a P o l y m e r L a b o r a t o r i e s DMTA o p e r a t i n g i n t h e b e n d i n g mode. The s a m p l e s w e r e e v a l u a t e d i n t h e d u a l c a n t i l e v e r


442


mode under c o n d i t i o n s o f c o n s t a n t s t r a i n and f r e q u e n c y . T y p i c a l h e a t i n g r a t e s w e r e e i t h e r 2 or 5 C/min. Rheology A s d e m o n s t r a t e d p r e v i o u s l y ( 7 ) , t h e r a m p i n g of r o t o r s p e e d s can be u s e d to o b t a i n s i m i l a r r h e o l o g i c a l i n f o r m a t i o n a s i s o b t a i n e d v i a more t r a d i t i o n a l c a p i l l a r y rheometer methods. S p e c i f i c a l l y , i t was shown t h a t t h e f o l l o w i n g r e l a t i o n i s a p p l i c a b l e to t o r q u e and r o t o r speed d a t a


M = K •C(n)-S

n

(1)

In t h i s r e l a t i o n , M i s t h e t o r q u e , K i s t h e f a m i l i a r c o n s i s t e n c y index from v i s c o s i t y v e r s u s shear r a t e r e l a t i o n s h i p s , S i s t h e r o t o r s p e e d , and n i s t h e power-law index, a l s o f a m i l i a r from v i s c o s i t y versus shear r a t e p l o t s . I t can be s e e n f r o m E q u a t i o n 1 t h a t a l o g - l o g p l o t of torque v e r s u s r o t o r speed s h o u l d y i e l d a s t r a i g h t l i n e w i t h i n t e r c e p t g i v e n by l o g (K* C ( n ) ) and s l o p e g i v e n by n , t h e p o w e r - l a w i n d e x . The p o w e r - l a w v a l u e s w h i c h a r e o b t a i n e d f o r t h e p r e s e n t b l e n d s a r e d i s p l a y e d i n F i g u r e 1. The v a l u e s w h i c h a r e r e p o r t e d t h e r e a r e a v e r a g e s of v a l u e s o b t a i n e d on two s e p a r a t e r u n s on t h e same c o m p o s i t i o n b l e n d s . A l s o , on t h e same r u n , v a l u e s f o r t h e up p a r t o f t h e r a m p i n g p r o c e d u r e a s w e l l a s t h e down p a r t h a v e been averaged. In e s s e n c e , t h e n , t h e v a l u e s r e p o r t e d c o r r e s p o n d to a v e r a g e s o f f o u r d a t a p o i n t s . In g e n e r a l , t h e p o w e r - l a w i n d e x v a l u e s d i f f e r e d by no more t h a n 0 . 0 3 on t h e d u p l i c a t e e x p e r i m e n t s . S e v e r a l i n t e r e s t i n g p o i n t s can be o b s e r v e d f r o m an a n a l y s i s of t h e d a t a . F i r s t , i n d a t a not p r e s e n t e d h e r e , i t c a n be shown t h a t t h e v a l u e o f L o g ( K » C ( n ) ) p a s s e s t h r o u g h a minimum a t t h e 50/50 c o m p o s i t i o n b l e n d . R e c a l l i n g t h a t the v a l u e of K « C(n) r e a l l y c o r r e s p o n d s to t h e t o r q u e g e n e r a t e d by t h e s a m p l e s a t a r o t o r s p e e d of 1 RPM, t h i s r e s u l t s u g g e s t s t h a t t h e t o r q u e g e n e r a t e d a t v e r y l o w r o t o r s p e e d s p a s s e s t h r o u g h a minimum a t t h e 50/50 c o m p o s i t i o n b l e n d . In more f a m i l i a r r h e o l o g y t e r m s , t h e v i s c o s i t y s h o u l d p a s s t h r o u g h a minimum a t l o w s h e a r r a t e s a t t h e 50/50 c o m p o s i t i o n b l e n d . Another i n t e r e s t i n g p o i n t which i s noted from the p l o t of the power-law index v e r s u s weight percent d i s p l a y e d i n F i g u r e 1 i s that the power-law index v a l u e s r e a c h a maximum a t t h e 50/50 c o m p o s i t i o n b l e n d . The p o w e r - l a w v a l u e of a b o u t 0 . 7 o b t a i n e d f o r t h i s b l e n d i s h i g h e r t h a n f o r e i t h e r o f t h e component m a t e r i a l s . This r e s u l t i n d i c a t e s t h a t t h e b l e n d h a s a f l o w c u r v e more c l o s e l y a p p r o x i m a t i n g Newtonian i n shape than e i t h e r of t h e component m a t e r i a l s . I t i s p o s s i b l e to g e t f u r t h e r i n s i g h t i n t o how b l e n d i n g a f f e c t s t h e r h e o l o g i c a l r e s p o n s e of t h e two



30. DEMEUSE & JAFFE


10

0

25 50 75 100 WEIGHT PERCENT POLVMER (D

F i g u r e 1. Power-law values obtained f o r v a r i o u s composition blends.



444


m a t e r i a l s by c o n s i d e r i n g t h e a c t u a l m e a s u r e d t o r q u e v a l u e s w h i c h a r e g e n e r a t e d a t a r o t o r s p e e d o f 5 RPM. What i s o b s e r v e d i s t h a t t h e t o r q u e v a l u e s a g a i n p a s s t h r o u g h a minimum a t t h e 50/50 b l e n d c o m p o s i t i o n , v e r y s i m i l a r to t h e b e h a v i o r o b s e r v e d f o r t h e c a l c u l a t e d t o r q u e v a l u e s a t 1 RPM. Han and Kim ( 1 4 ) have o b s e r v e d a s i m i l a r phenomenon in a c o m p l e t e l y d i f f e r e n t system c o n s i s t i n g of polypropylene blended w i t h p o l y s t y r e n e . The c o n c l u s i o n of t h a t work was t h a t t h e p o l y s t y r e n e and p o l y p r o p y l e n e have no i n t e r a c t i o n a t t h e i n t e r f a c e and do n o t f o r m an interphase. Han and c o w o r k e r s ( 1 4 ) have s t u d i e d o t h e r systems which d i s p l a y minima i n the v i s c o s i t y v e r s u s c o m p o s i t i o n g r a p h s and h a v e c o n c l u d e d t h a t t h e r e i s l i t t l e c h e m i c a l i n t e r a c t i o n between t h e two p h a s e s . The s u g g e s t i o n t h a t two l i q u i d c r y s t a l p o l y m e r s a r e i n c o m p a t i b l e w i t h e a c h o t h e r i s c o n t r a r y to i d e a s w h i c h are w e l l e s t a b l i s h e d for small molecule l i q u i d c r y s t a l s (4). In f a c t , m i s c i b i l i t y w i t h o t h e r l i q u i d c r y s t a l s i s one o f t h e c r i t e r i a s o m e t i m e s u s e d to e s t a b l i s h t h e t y p e of l i q u i d c r y s t a l b e i n g d e a l t w i t h . On t h e o t h e r h a n d , i f the r h e o l o g i c a l c r i t e r i a e s t a b l i s h e d f o r other polymer b l e n d systems a r e v a l i d f o r l i q u i d c r y s t a l polymer b l e n d s y s t e m s a s w e l l , t h e two m a t e r i a l s b e i n g d i s c u s s e d i n t h e p r e s e n t work must be i n c o m p a t i b l e . The n e x t s e c t i o n s o f t h i s p a p e r c o n t a i n t h e r e s u l t s of s o l i d s t a t e s t r u c t u r e i n v e s t i g a t i o n s of t h e s e b l e n d samples. The s u g g e s t i o n w h i c h w i l l be a d v a n c e d i s t h a t i d e a s c o n c e r n i n g t r a d i t i o n a l polymer b l e n d systems a r e more a p p l i c a b l e f o r p r o v i d i n g an u n d e r s t a n d i n g of t h e s e r e s u l t s than a r e i d e a s c o n c e r n i n g s m a l l m o l e c u l e l i q u i d crystals. X-rav

Diffraction

X - r a y p a t t e r n s w e r e o b t a i n e d on s t r a n d s w h i c h w e r e manually p u l l e d from the samples a f t e r the m i x i n g o p e r a t i o n was c o m p l e t e d . A s shown by G u t i e r r e z , e t a l (2)i s i g n i f i c a n t c h a n g e s o n l y o c c u r i n t h e f i r s t and second meridonal peaks f o r copolymers which c o n t a i n only HBA and HNA. S i n c e component (2) c o n t a i n s o n l y one m o l e p e r c e n t TA and HQ, i t i s e x p e c t e d t h a t s i m i l a r o b s e r v a t i o n s h o l d t r u e f o r components ( 1 ) and (2) i n t h e present blends. Due to t h i s f a c t , t h e p r e s e n t e x a m i n a t i o n w i l l f o c u s on t h e c h a n g e s i n t h e f i r s t two p e a k s w h i c h o c c u r upon b l e n d i n g . F i g u r e 2 c o n t a i n s t h e m e r i d o n a l s c a n s of component ( 1 ) , (2), and t h e 50/50 b l e n d . F i g u r e 3 c o n t a i n s an expanded p l o t of t h e 6-7 & r e g i o n o f t h e d i f f r a c t i o n patterns. I t i s seen t h e r e t h a t the p o s i t i o n of the s e c o n d peak i n t h e component m a t e r i a l s i s too c l o s e to make any d e f i n i t e s t a t e m e n t a b o u t w h a t ' s h a p p e n i n g i n t h e blend. The p o s i t i o n of t h e f i r s t peak i s s e p a r a t e d by 0 . 4 4 ft i n t h e component m a t e r i a l s . To s e e i f t h i s i s a



F i g u r e 2 . X - r a y m e r i d o n a l s c a n s of component ( 2 ) , and a 50/50 b l e n d o f components.

component t h e two

(1),


in

I

5.'

Ι

d

Ρ


Figure 3. region·

Expanded p l o t o f F i g u r e 2 i n t h e 6 - 7 &


i


30.

DEMEUSE & JAFFE

Investigations into the Structure of LCP-LCP Blends 447

l a r g e enough d i f f e r e n c e to be observed, a p h y s i c a l mixture of the two component m a t e r i a l s was made by p h y s i c a l l y p l a c i n g f i b e r s next to each other i n the d i f f r a c t o m e t e r . In the p h y s i c a l mixture, i t i s d i f f i c u l t to d i s t i n g u i s h the two o v e r l a p p i n g peaks based on peak p o s i t i o n alone. However, the width of the peak showed an increase from 0.76 to 0.95 . T h i s suggests that the p h y s i c a l mixture indeed c o n s i s t s of two o v e r l a p p i n g peaks. Since the peak width i n the 50/50 blend i s 0.76 rather than 0.95 , as p r e d i c t e d from the p h y s i c a l mixture r e s u l t s , the 50/50 blend i s suggested to have a s i n g l e peak i n t h i s region of the d i f f r a c t i o n p a t t e r n . One suggestion f o r what may occur when blends of two l i q u i d c r y s t a l polymers are produced i s that an e s t e r exchange r e a c t i o n w i l l occur and a s i n g l e copolymer with intermediate composition w i l l r e s u l t . Such an i s s u e has been addressed in the work of Mehta and B a i r d (13).The present r e s u l t s suggest that such i s not the case f o r t h i s system. If a s i n g l e random copolymer of intermediate composition was being produced in the present system, a peak centered at 12.56 & i s expected. Instead, a peak centered at 12.85 & i s observed. The d i f f e r e n c e between the expected r e s u l t and the observed r e s u l t i s o u t s i d e of experimental e r r o r . It can be speculated that the e s t e r exchange r e a c t i o n has already occurred but complete randomization of the monomers has not yet occurred. In that case, a blocky s t r u c t u r e would be expected. However, as w i l l be demonstrated l a t e r , the observed DSC t r a c e s are not c o n s i s t e n t with the occurrence of a blocky s t r u c t u r e . Thus, the c o n c l u s i o n i s reached that the present data suggests that t r a n s e s t e r i f i c a t i o n i s not a dominant mechanism i n determining the behavior of the present blends. DSC

Results

F i g u r e s 4 and 5 c o n t a i n the DSC t r a c e s which are obtained upon second heating f o r each of the component m a t e r i a l s . The small endotherm observed i s t y p i c a l of these m a t e r i a l s . T h i s i s a t t r i b u t e d to the small entropy change which occurs at the c r y s t a l - n e m a t i c t r a n s i t i o n temperature (15)• It should also be noted that the two polymers d i s p l a y t r a n s i t i o n temperatures which d i f f e r by about f o r t y degrees. Thus, i n the blend samples, i f two endotherms are present, there should be no problem i n d i s c e r n i n g them. A t y p i c a l c o o l i n g curve f o r a blend sample i s shown in F i g u r e 6. There i s t y p i c a l l y an undercooling of about f o r t y degrees which i s observed f o r these samples. In a l l cases, only a s i n g l e c r y s t a l l i z a t i o n exotherm i s noted, suggesting that c o c r y s t a l l i z a t i o n i s o c c u r r i n g . The subsequent reheat of samples that had been ambiently cooled from the melt y i e l d e d only a s i n g l e endotherm. The p o s i t i o n of thjls.endo.therm as a f u n c t i o n

American Chemical Society Library 1155 15th St., N.W.

In Liquid-Crystalline Polymers; Weiss, R., et al.; Washington, D.C. Society: 20036Washington, DC, 1990. ACS Symposium Series; American Chemical


Figure 4. heating.

DSC t r a c e of

component ( 1 ) on

second


ι

si

ί !

Ε ο


Figure 5. heating.

DSC t r a c e of

component ( 2 ) on

second


I

r

Η

3

G

ρ


F i g u r e 6.

Cooling curve for

blend

sample.


η

ι

Β ο

©

in

30.

DEMEUSE& JAFFE


o f b l e n d c o m p o s i t i o n i s t a b u l a t e d i n T a b l e I and d i s p l a y e d i n F i g u r e 7. A l s o d i s p l a y e d i n F i g u r e 7 i s the l i n e w h i c h c o r r e s p o n d s to what i s e x p e c t e d f r o m a w e i g h t e d a v e r a g e o f t h e amount o f component m a t e r i a l values. I t i s o b s e r v e d f r o m t h i s p l o t t h a t a s t h e amount of copolymer i s i n c r e a s e d i n the b l e n d , the p o s i t i o n of t h e DSC e n d o t h e r m t e n d s t o w a r d what i s e x p e c t e d f r o m a weighted average c a l c u l a t i o n . However, f o r a l l c o m p o s i t i o n s , only a s i n g l e endotherm i s o b s e r v e d .


T a b l e I. P o s i t i o n o f DSC e n d o t h e r m a s a f u n c t i o n of b l e n d c o m p o s i t i o n .

Weight % Polvmer (1) 0 25 50 75 85 90 95 100

DSC Endotherm

CO

241 248 256 268 272 275 276 278

The p r e s e n t DSC r e s u l t s a l s o a r g u e a g a i n s t t h e p o s s i b i l i t y of c h e m i c a l r e a c t i o n b e i n g the dominant mechanism i n t h e b l e n d s . I f t h e two component m a t e r i a l s had c o m p l e t e l y r e a c t e d t o f o r m a s i n g l e random c o p o l y m e r , a DSC e n d o t h e r m f o r t h e 50/50 b l e n d a t a b o u t 270 C i s expected. I n s t e a d , f o r t h i s b l e n d , an e n d o t h e r m a t 256 C i s observed. The o t h e r p o s s i b i l i t y i s t h a t t h e two m a t e r i a l s have r e a c t e d b u t t h e r e a c t i o n h a s n o t p r o g r e s s e d t o t h e l e v e l o f a c o m p l e t e l y random c o p o l y m e r . T h i s a l s o d o e s not seem to be t h e c a s e b e c a u s e s u c h a b l o c k c o p o l y m e r w o u l d be e x p e c t e d t o d i s p l a y two DSC endotherms, something w h i c h i s not o b s e r v e d . V e r y l i t t l e work h a s a p p e a r e d i n t h e l i t e r a t u r e w h i c h d e a l s w i t h b l e n d s i n w h i c h t h e component m a t e r i a l s can c o c r y s t a l l i z e . It i s generally b e l i e v e d (16.17) that a r e q u i r e m e n t f o r c o c r y s t a l l i z a t i o n i s t h a t t h e r e must be a c l o s e m a t c h i n g o f t h e p o l y m e r c h a i n c o n f o r m a t i o n s and of c r y s t a l l i n e d i m e n s i o n s . A l s o , some l e v e l o f m i s c i b i l i t y s h o u l d e x i s t between t h e two p o l y m e r s and t h e c r s t a l l i z a t i o n k i n e t i c s c a n n o t be v e r y d i f f e r e n t . C e r t a i n l y , i n the c a s e of l i q u i d c r y s t a l l i n e p o l y m e r s , i n g e n e r a l , t h e s e r e q u i r e m e n t s w o u l d be e x p e c t e d t o be m e t . Some of our r e c e n t work ( 8 ) h a s s u g g e s t e d , h o w e v e r , t h a t n o t a l l l i q u i d c r y s t a l p o l y m e r s do c o c r y s t a l l i z e . The p r e s e n t work s u g g e s t s t h a t i n c e r t a i n c a s e s i t may be p o s s i b l e to a c h i e v e t h i s e f f e c t .


452



Dynamic M e c h a n i c a l R e s u l t s The d y n a m i c m e c h a n i c a l r e s p o n s e o f l i q u i d c r y s t a l l i n e p o l y m e r s has r e c e i v e d a g r e a t d e a l of a t t e n t i o n i n the recent l i t e r a t u r e . Yoon and J a f f e ( 1 8 ) e x a m i n e d t h e r e s p o n s e i n t e n s i o n of a n n e a l e d , h i g h l y o r i e n t e d s t r a n d s of v a r i o u s c o m p o s i t i o n l i q u i d c r y s t a l l i n e p o l y m e r s . More r e c e n t l y , Ward, et a l ( 1 9 . 2 0 ) s t u d i e d the dynamic m e c h a n i c a l r e s p o n s e i n b o t h t e n s i o n and t o r s i o n o f t h e s e same p o l y m e r s . The f o l l o w i n g b r i e f summary t a k e s i n t o account the r e s u l t s of b o t h of those w o r k s . The l i q u i d c r y s t a l p o l y m e r s o f i n t e r e s t t o t h e present i n v e s t i g a t i o n d i s p l a y three t r a n s i t i o n s in the t e m p e r a t u r e r a n g e - 1 0 0 to 200 C. There i s a low t e m p e r a t u r e t r a n s i t i o n w h i c h o c c u r s a t a b o u t - 7 0 C and i s v e r y weak i n i n t e n s i t y . T h i s h a s been a t t r i b u t e d to motion of the p - p h e n y l e n e u n i t s . A t r a n s i t i o n which i s h i g h l y dependent on c o m p o s i t i o n o c c u r s i n t h e r e g i o n 20 80 C. T h i s t r a n s i t i o n h a s been a t t r i b u t e d to r e o r i e n t a t i o n of t h e n a p h t h a l e n e g r o u p s . F i n a l l y , there i s a t r a n s i t i o n w i t h a l a r g e a c t i v a t i o n energy i n the r a n g e 100 - 150 C. T h i s t r a n s i t i o n i s a n a l o g o u s to t h e g l a s s t r a n s i t i o n o f t r a d i t i o n a l p o l y m e r s and h a s been a t t r i b u t e d to d e l o c a l i z e d o r i e n t a t i o n . Most of the samples f o r t h e s e s t u d i e s were compression molded b a r s . In t h e c a s e of t h e b l e n d w h i c h c o n t a i n s 25 p e r c e n t o f component ( 2 ) , a l a r g e enough s a m p l e f o r i n j e c t i o n m o l d i n g e v a l u a t i o n was p r e p a r e d . A s p e c i a l n o t e w i l l be made o f t h e s e s a m p l e s when they a r e discussed. The m a i n d i f f e r e n c e between t h e i n j e c t i o n m o l d e d and c o m p r e s s i o n m o l d e d s a m p l e s i s t h e a b s o l u t e m a g n i t u d e of t h e m o d u l u s v a l u e s w h i c h a r e o b t a i n e d . The main f o c u s o f t h e p r e s e n t work w i l l be on t h e « - t r a n s i t i o n , w h i c h h a s c h a r a c t e r i s t i c s s i m i l a r to a glass transition. The c r i t e r i a w h i c h w i l l be u s e d f o r m i s c i b i l i t y i s the appearance of a s i n g l e ^ . - t r a n s i t i o n w h i c h i s i n t e r m e d i a t e i n t e m p e r a t u r e to t h e ^ - t r a n s i t i o n of t h e component m a t e r i a l s . The e f f e c t of b l e n d i n g on t h e s e c o n d a r y t r a n s i t i o n s i s o u t s i d e t h e s c o p e of t h e present i n v e s t i g a t i o n . F i g u r e 8 d e p i c t s the dynamic m e c h a n i c a l r e s p o n s e of t h e two component m a t e r i a l s a s w e l l a s a b l e n d w h i c h c o n t a i n s 75 w e i g h t p e r c e n t o f t h e c o p o l y m e r component. I t can be s e e n f r o m t h a t f i g u r e t h a t t h e c c - t r a n s i t i o n s of t h e two component m a t e r i a l s a r e s e p a r a t e d by about 20 degrees. I f t h e b l e n d i s an i m m i s c i b l e m i x t u r e , i t i s e x p e c t e d t h a t t h e r e w o u l d be two t r a n s i t i o n s o b s e r v e d w h i c h a r e 20 d e g r e e s a p a r t , o r , a t l e a s t , i t i s e x p e c t e d t h a t a s i n g l e s i g n i f i c a n t l y b r o a d e n e d t r a n s i t i o n w o u l d be observed. I t can be s e e n f r o m F i g u r e 8 t h a t s u c h i s not t h e c a s e i n t h e 75/25 b l e n d . I n s t e a d , what i s o b s e r v e d i s a s i n g l e t r a n s i t i o n w h i c h i s a s s h a r p a s i n t h e two component m a t e r i a l s . This observation holds true for a l l of t h e b l e n d c o m p o s i t i o n s s t u d i e d . The p o s i t i o n of t h e



30. DEMEUSE & JAFFE


TEMPERATURE (C) F i g u r e 7. P o s i t i o n of m e l t i n g endotherm a s a f u n c t i o n of b l e n d c o m p o s i t i o n .

POLYMER (1)

75/25 BlfivID

POLYMER (2) *

i

50

•

'—'—•—i——>— —•—i— —

Investigations into the Structure of Liquid ... - ACS Publications

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