Polymer Alloys, Blends, and Ionomers - ACS Symposium Series (ACS

Chapter 1

Polymer Alloys, Blends, and Ionomers An

1

Overview

2

3

L. A. Utracki , D. J. Walsh , and R. A. Weiss

Downloaded by UNIV OF SOUTH AUSTRALIA on October 25, 2012 | http://pubs.acs.org Publication Date: July 21, 1989 | doi: 10.1021/bk-1989-0395.ch001

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Industrial Materials Research Institute, National Research Council of Canada, Boucherville, Québec J4B 6Y4, Canada E. I. du Pont de Nemours and Company, Experimental Station, Wilmington, DE 19898 Polymer Science Program and Department of Chemical Engineering, University of Connecticut, Storrs, CT 06269-3136 2

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This chapter provides a broad overview of the subjects of polymer blends and ionomers. Specific topics concerning polymer blends include the thermodynamics of mixing of polymer-polymer p a i r s , polymer interfaces, rheology, and mechanical properties. For ionomers, the chemistry, s t r u c t u r e , rheology and s o l u t i o n properties are discussed.

M u l t i p h a s e polymer systems a r e becoming an i n c r e a s i n g l y i m p o r t a n t t e c h n i c a l a r e a o f polymer s c i e n c e . By d e f i n i t i o n , a m u l t i p h a s e polymer i s one t h a t has two o r more d i s t i n c t phases. The phases may d i f f e r i n c h e m i c a l c o m p o s i t i o n and/or t e x t u r e . Thus, i n i t s b r o a d e s t sense, the term i n c l u d e s not o n l y multi-component systems, such a s i m m i s c i b l e polymer b l e n d s and f i l l e d - p o l y m e r s , b u t a l s o s e m i - c r y s t a l l i n e p o l y m e r s , b l o c k copolymers, segmented p o l y m e r s , and ionomers. The l a t t e r f o u r systems a r e c h a r a c t e r i z e d b y a m i c r o p h a s e - s e p a r a t e d morphology w h e r e i n a s i n g l e polymer chain participates i n more than one phase. I n addition, even homopolymers t h a t have e x p e r i e n c e d complex t h e r m a l and m e c h a n i c a l h i s t o r i e s , such a s encountered i n most common polymer p r o c e s s i n g o p e r a t i o n s , may p o s s e s s m o r p h o l o g i e s c o n t a i n i n g more than one c r y s t a l l i n e t e x t u r e . These may a l s o be c o n s i d e r e d m u l t i p h a s e materials. Because o f the great d i v e r s i t y o f multiphase polymers, coverage o f the e n t i r e f i e l d i n a s i n g l e volume i s n e i t h e r p o s s i b l e nor p r a c t i c a l . I n s t e a d , t h i s book c o n c e n t r a t e s on two s p e c i f i c subjects: polymer b l e n d s , i n c l u d i n g interpenetrating polymer networks, and ionomers. Even with this specialization, a comprehensive t r e a t i s e on b o t h s u b j e c t s i s n o t p o s s i b l e , and t h i s book f o c u s s e s on s e l e c t e d contemporary t o p i c s from t h e two f i e l d s . The purpose o f t h i s o v e r v i e w c h a p t e r i s t o p r o v i d e a c u r s o r y 0097-6156/89/0395-O001$09.75A) o 1989 American Chemical Society

In Multiphase Polymers: Blends and Ionomers; Utracki, L., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989.

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MULTIPHASE POLYMERS: BLENDS AND IONOMERS

i n t r o d u c t i o n t o these s u b j e c t s and t o o u t l i n e t h e o r g a n i z a t i o n o f the book. Those r e q u i r i n g a more d e t a i l e d r e v i e w o f polymer b l e n d s and ionomers a r e d i r e c t e d t o o t h e r monographs and r e v i e w a r t i c l e s (1-22). POLYMER BLENDS


There i s some c o n f u s i o n i n t h e l i t e r a t u r e r e g a r d i n g polymer b l e n d nomenclature. Here t h e f o l l o w i n g d e f i n i t i o n s a r e a s s i g n e d t o t h e commonly used terms: POLYMER BLEND (PB) - t h e a l l - e n c o m p a s s i n g term f o r any m i x t u r e of homopolymers o r copolymers; HOMOLOGOUS POLYMER BLENDS - a s u b - c l a s s o f PB l i m i t e d t o m i x t u r e s of c h e m i c a l l y i d e n t i c a l polymers d i f f e r i n g i n m o l a r mass; POLYMER ALLOYS - a s u b - c l a s s o f PB r e s e r v e d f o r p o l y m e r i c m i x tures w i t h s t a b i l i z e d morphologies; MISCIBLE POLYMER BLENDS - a s u b - c l a s s o f PB encompassing t h o s e b l e n d s which e x h i b i t s i n g l e phase b e h a v i o r ; IMMISCIBLE POLYMER BLENDS - A s u b - c l a s s o f PB r e f e r r i n g t o those blends that exhibit two o r more phases a t a l l c o m p o s i t i o n s and t e m p e r a t u r e s ; PARTIALLY MISCIBLE POLYMER BLENDS - a s u b - c l a s s o f PB i n c l u d i n g those b l e n d s t h a t e x h i b i t a "window" o f m i s c i b i l i t y , i.e., are miscible only a t some c o n c e n t r a t i o n s and temperatures; COMPATIBLE POLYMER BLENDS - a u t i l i t a r i a n term, i n d i c a t i n g c o m m e r c i a l l y u s e f u l m a t e r i a l s , a m i x t u r e o f polymers w i t h o u t s t r o n g r e p u l s i v e f o r c e s t h a t i s homogeneous t o t h e eye; INTERPENETRATING POLYMER NETWORK (IPN) - a s u b - c l a s s o f PB r e s e r v e d f o r m i x t u r e s o f two polymers where b o t h components form c o n t i n u o u s phases and a t l e a s t one i s s y n t h e s i z e d o r c r o s s l i n k e d i n the presence of the other. From the standpoint of commercial applications and developments, polymer b l e n d i n g r e p r e s e n t s one o f t h e f a s t e s t growing segments o f polymer t e c h n o l o g y . Both t h e open and t h e p a t e n t l i t e r a t u r e have become voluminous. In p r i n c i p l e , blending two m a t e r i a l s t o g e t h e r i n o r d e r t o a c h i e v e a b a l a n c e o f p r o p e r t i e s not o b t a i n a b l e w i t h a s i n g l e one i s an o b v i o u s and w e l l - f o u n d e d p r a c t i c e , one t h a t has been s u c c e s s f u l l y e x p l o i t e d i n m e t a l l u r g i c a l science. W i t h polymers, however, t h e thermodynamics o f m i x i n g do not u s u a l l y f a v o r mutual s o l u b i l i t y and most b i n a r y polymer m i x t u r e s form two d i s t i n c t phases. T h i s i s a d i r e c t consequence o f t h e i r h i g h m o l e c u l a r mass. S t i l l , many i m m i s c i b l e systems form u s e f u l p r o d u c t s and a r e commercial. Key examples i n c l u d e r u b b e r -



1.

UTRACKI ETAL.

Polymer Alloys, Blends, and Ionomers

3

toughened p l a s t i c s such a s h i g h impact p o l y s t y r e n e (HIPS) and ABS r e s i n s and b l e n d s o f s y n t h e t i c rubber w i t h n a t u r a l rubber. The problems and c h a l l e n g e s i n h e r e n t t o d e v e l o p i n g u s e f u l materials with o p t i m a l m o r p h o l o g i e s and p r o p e r t i e s from an i m m i s c i b l e o r p a r t i a l l y m i s c i b l e polymer b l e n d a r e n o t t r i v i a l and have spawned c o n s i d e r a b l e i n d u s t r i a l and academic r e s e a r c h . Work on polymer m i s c i b i l i t y , c o m p a t i b i l i z i n g a g e n t s , r e a c t i v e systems, and t h e i n f l u e n c e o f f l o w on t h e s t r u c t u r e and p r o p e r t i e s o f b l e n d s i s described i n l a t e r chapters. The major t e c h n o l o g i c a l problem i n t h e u s e o f polymer b l e n d s concerns d e t e r m i n i n g c o r r e l a t i o n s between c o m p o s i t i o n , p r o c e s s i n g , structure and p r o p e r t i e s . Each variable has inherent c h a r a c t e r i z a t i o n problems, e.g., o f t h e p r e p a r a t i o n p r o c e s s , o f t h e c h e m i s t r y and morphology, and o f what a r e m e a n i n g f u l p r o p e r t i e s . None o f these c o r r e l a t i o n s o r c h a r a c t e r i z a t i o n s a r e easy t o make o r p a r t i c u l a r l y w e l l understood. Because polymer s c i e n c e i s by n a t u r e i n t e r d i s c i p l i n a r y , t h e s o l u t i o n o f t h e above p r o b l e m i n v o l v e s c o n t r i b u t i o n s from many fields, i n c l u d i n g chemistry, p h y s i c s , and e n g i n e e r i n g . The d i s c u s s i o n t h a t f o l l o w s w i l l h i g h l i g h t a number o f areas where p r o g r e s s has r e c e n t l y been made i n u n d e r s t a n d i n g the subject. C o n s i d e r a b l y more d e t a i l w i l l be found i n t h e subsequent c h a p t e r s of t h i s book. M e c h a n i c a l M i x i n g o f Polymer

Blends

Most commercial polymer alloys and b l e n d s a r e p r e p a r e d by m e c h a n i c a l m i x i n g , l a r g e l y because o f i t s s i m p l i c i t y and low c o s t . The p r e f e r r e d i n d u s t r i a l method o f m e c h a n i c a l m i x i n g i s t o use a screw compounder o r e x t r u d e r t h a t c a n be r u n c o n t i n u o u s l y and generate a product i n a c o n v e n i e n t form f o r f u r t h e r p r o c e s s i n g . Not s u r p r i s i n g l y , much e f f o r t has gone i n t o t r y i n g t o u n d e r s t a n d the f l o w o f polymer b l e n d s . M i x i n g from Ternary

Systems and by R e a c t i o n

Other methods f o r forming b l e n d s such a s by e v a p o r a t i o n o f a s o l v e n t o r by p o l y m e r i z a t i o n o f a monomer i n t h e p r e s e n c e o f a polymer i n v o l v e a t l e a s t t h r e e components i n t h e p r e p a r a t i o n process. M i x i n g i n a common s o l v e n t f o l l o w e d b y i t s removal i s a convenient way o f making b l e n d s on a l a b o r a t o r y s c a l e , b u t h a s obvious commercial d i s a d v a n t a g e s due t o t h e c o s t and d i f f i c u l t y o f s o l v e n t r e c o v e r y as w e l l a s t h e p o t e n t i a l e n v i r o n m e n t a l h a z a r d s a s s o c i a t e d w i t h h a n d l i n g l a r g e volumes o f o f t e n t o x i c c h e m i c a l s . In s p e c i f i c a p p l i c a t i o n s , however, such as membrane f o r m a t i o n o r p a i n t s and c o a t i n g s where t h i n f i l m s a r e r e q u i r e d , t h e u s e o f solvents i s unavoidable. The t h i r d component o f such a b l e n d , i . e . , t h e s o l v e n t , and the k i n e t i c s o f i t s removal c a n i n f l u e n c e t h e r e s u l t i n g morphology. For example, i f two m i s c i b l e polymers a r e c a s t from a common s o l v e n t , one does n o t n e c e s s a r i l y o b t a i n a homogeneous m i x t u r e . A two-phase r e g i o n can e x i s t i n t h e t e r n a r y phase diagram a s shown i n F i g . l a , and as t h e s o l v e n t evaporates t h e c o m p o s i t i o n may e n t e r the two- phase r e g i o n as shown by p r o g r e s s i n g f r o m p o i n t A t o p o i n t


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MULTIPHASE POLYMERS: BLENDS AND

IONOMERS

B. As the e v a p o r a t i o n of s o l v e n t c o n t i n u e s , the c o m p o s i t i o n may l e a v e the two-phase r e g i o n , but at t h a t p o i n t t h e v i s c o s i t y may be too h i g h and the phase s i z e s may be too l a r g e f o r h o m o g e n i z a t i o n t o occur. The more common s i t u a t i o n , i l l u s t r a t e d i n F i g . l b , i s where the two polymers are i m m i s c i b l e but form a homogenous s o l u t i o n i n a common s o l v e n t . In t h i s case, f i l m c a s t i n g along the l i n e C to D g e n e r a t e s a v a r i e t y of s t r u c t u r e s depending on the s e l e c t e d s o l v e n t (and i t s i n t e r a c t i o n p a r a m e t e r s X-jo * X-j^)» the c h e m i c a l n a t u r e of the two polymers (X23) w e l l as on t h e k i n e t i c s o f t h e process. Three p h a s e - s e p a r a t e d t y p e s o f m o r p h o l o g i e s can r e s u l t : c o - c o n t i n u o u s , d i s p e r s e d , and l a y e r e d . The c o - c o n t i n u o u s morphology w i t h the polymers forming i n t e r p e n e t r a t i n g networks i s the most i n t e r e s t i n g . This s t r u c t u r e , w h i c h i s known t o e x i s t even a t c o n c e n t r a t i o n s as low as 10 t o 15 vol%, can be created by j u d i c i o u s l y s e l e c t i n g the casting c o n d i t i o n s t o a s s u r e dominance of the s p i n o d a l d e c o m p o s i t i o n (SD) mechanism of phase s e p a r a t i o n . The c o r r e l a t i o n l e n g t h s of t h e g e n e r a t e d s t r u c t u r e s v a r y w i t h time from a few nanometers t o about a m i c r o n (23, 24). T h i s morphology a l l o w s f o r c o e x i s t e n c e of the b e s t c h a r a c t e r i s t i c s of each polymer i n the b l e n d (25). For example, the combination of good m e c h a n i c a l p r o p e r t i e s with p e r m e a b i l i t y , accomplished w i t h a blend composition above the p e r c o l a t i o n t h r e s h o l d , has y i e l d e d a h i g h l y s u c c e s s f u l membrane technology (26). The p h a s e - s e p a r a t e d d r o p l e t / m a t r i x morphology i s an outcome of the n u c l e a t i o n and growth mechanism (NG) of phase s e p a r a t i o n . The phase d i m e n s i o n s a r e s i m i l a r t o t h o s e o b s e r v e d f o r SD, but i n t h i s case the p r o p e r t i e s are dominated by the m a t r i x polymer w i t h t h e d i s p e r s e d phase p l a y i n g the r o l e of a c o m p a t i b i l i z e d f i l l e r . A s i m i l a r d i s p e r s e d morphology, but w i t h l a r g e d r o p s , can be o b t a i n e d by a l l o w i n g the SD o r NG system t o r i p e n . The c o a r s e n i n g u s u a l l y l e a d s t o a n o n - u n i f o r m i t y of p r o p e r t i e s . The l a y e r e d s t r u c t u r e of a c a s t f i l m i s c o n t r o l l e d by t h e surface properties during evaporation. S i g n i f i c a n t compositional g r a d i e n t s can be g e n e r a t e d by making use of t h e n a t u r a l t e n d e n c i e s of one polymer t o m i g r a t e toward the a i r - p o l y m e r i n t e r f a c e and t h e other toward the substrate. H y d r o p h o b i c i t y / h y d r o p h i l i c i t y of macromolecules i s o f t e n c i t e d as the d r i v i n g f o r c e (27, 28). R e a c t i v e m i x i n g f i n d s a p p l i c a t i o n i n many commercial b l e n d s such as HIPS and r u b b e r m o d i f i e d t h e r m o s e t s . Many IPN's can a l s o be i n c l u d e d h e r e . I n the case of t h e p o l y m e r i z a t i o n of monomer i n the p r e s e n c e of a p o l y m e r , the monomer-l/polymer-1/polymer-2 t e r n a r y phase diagram a l s o p l a y s a r o l e i n d e t e r m i n i n g the f i n a l morphology. Where the two polymers are i m m i s c i b l e , such as p o l y s t y r e n e and p o l y b u t a d i e n e , a two-phase m i x t u r e w i l l r e s u l t . However, i n cases where the polymers a r e m i s c i b l e , s i n g l e phase morphologies are not always a c h i e v e d . For example, i n the p o l y m e r i z a t i o n of v i n y l c h l o r i d e i n the p r e s e n c e of p o l y ( b u t y l a c r y l a t e ) a two-phase r e g i o n i s p r e s e n t i n the phase diagram, F i g . 2. P o l y m e r i z a t i o n pathways t h a t pass through t h i s r e g i o n , such as l i n e A-B i n F i g . 2, may y i e l d a two-phase system f o r t h e same r e a s o n s as d e s c r i b e d above f o r s o l v e n t e v a p o r a t i o n from a b l e n d . a n

(4)

K

where A i s t h e v i s c o s i t y r a t i o o f d i s p e r s e d and m a t r i x liquids, A = /n . Note t h a t i n t h e f u l l range o f 0 < A 3.8 o n l y f o r m a t i o n i n t o an e l l i p s o i d is possible. Also f o r X < 0.05 the o v e r a l l e f f i c i e n c y of d i s p e r s i n g decreases. However, on the one hand t h e d e c r e a s e i s l e s s d r a m a t i c than t h a t f o r h i g h v a l u e s of X and on the o t h e r hand t h e r e a r e two mechanisms p r e s e n t : the s a t e l l i t e b r e a k i n g and so c a l l e d t i p s p i n n i n g , s c h e m a t i c a l l y i l l u s t r a t e d i n F i g s . 6a and 6b, respectively. D r o p l e t breakup i n u n i a x i a l e x t e n s i o n a l f l o w i s more e f f i c i e n t ( 1 0 ) . The t h e o r e t i c a l c a l c u l a t i o n s e s t i m a t e t h a t ( 2 1 ) : 9

d /d £

a

= 3.3

[1 + ( 1 . 9 X K ) ] 2

1 / 2

/ [ 1 + 1.5K]

(7)

where s u b s c r i p t s e and a i d e n t i f y the e q u i l i b r i u m drop d i a m e t e r i n e x t e n s i o n and i n shear, respectively. Moreover, i n u n i a x i a l e x t e n s i o n , the p r o c e s s i s i n s e n s i t i v e t o e l o n g a t i o n a l v i s c o s i t y ratio, X = n * f i n e d i s p e r s i o n i s always o b t a i n e d . Only a s l i g h t l o s s ' of c t f s p e r s i n g e f f i c i e n c y i s n o t e d a t the l i m i t i n g v a l u e s of X ^ . The most e f f i c i e n t mechanism o f drop breakup i n v o l v e s i t s d e f o r m a t i o n i n t o a f i b e r f o l l o w e d by the t h r e a d d i s i n t e g r a t i o n under the i n f l u e n c e of c a p i l l a r y f o r c e s . F i b r i l l a t i o n o c c u r s i n b o t h steady s t a t e shear and u n i a x i a l e x t e n s i o n . I n shear (= r o t a t i o n + e x t e n s i o n ) the p r o c e s s i s l e s s e f f i c i e n t and l i m i t e d t o l o w - X r e g i o n , e.g. X < 2. In i r r o t a t i o n a l u n i a x i a l extension ( i n absence of the i n t e r p h a s e s l i p ) the phases codeform i n t o t h r e a d like structures. The method of morpholgy control should involve the f i b r i l l a t i o n s t e p . When f i n e d r o p l e t d i s p e r s i o n i s d e s i r a b l e , t h e r e s i n s s h o u l d be s e l e c t e d t o a s s u r e low X , t h e u n i a x i a l e x t e n s i o n s h o u l d be maximized and i m p o s i t i o n of a f i b e r d i s t u r b i n g dynamic f i e l d s h o u l d be used a t t h e end of t h e p r o c e s s . On t h e o t h e r hand, i f p r e s e r v a t i o n of extended morphology i s d e s i r e d , ( i . e . , the an(

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12 Downloaded by UNIV OF SOUTH AUSTRALIA on October 25, 2012 | http://pubs.acs.org Publication Date: July 21, 1989 | doi: 10.1021/bk-1989-0395.ch001

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\oqX F i g u r e 5. Reduced d r o p l e t d i a m e t e r v s . v i s c o s i t y r a t i o , X, i n shear and e x t e n s i o n a l f l o w s . The type o f shear drop d e f o r m a t i o n w i t h i n each of the f o u r zones o f X i s i n d i c a t e d .

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(b) F i g u r e 6. Schematic r e p r e s e n t a t i o n o f drop breakup i n shear f i e l d a t two d i f f e r e n t v a l u e s o f the v i s c o s i t y r a t i o X: (a) 0.1 < X < 1, (b) X < 0 . 0 1 .



12


IONOMERS

f i b r i l l a s i n u n i a x i a l extension or lamellae i n b i a x i a l elongation) then a system w i t h l a r g e X and l a r g e i n i t i a l drop d i a m e t e r s s h o u l d be used. D i s i n t e g r a t i o n of the extended structures w i l l be p r e v e n t e d i f the m a t e r i a l forming the d i s p e r s e d phase shows the apparent y i e l d s t r e s s b e h a v i o r . I t was r e c e n t l y r e p o r t e d t h a t extended s t r u c t u r e s a r e a l s o c r e a t e d i n low a m p l i t u d e o s c i l l a t o r y shear f i e l d ( 3 8 ) . However, j u d g i n g by the geometry of the g e n e r a t e d t h r e a d s , c o a l e s c e n c e r a t h e r than drop e l o n g a t i o n was the r e s p o n s i b l e mechanism. The c o a l e s c e n c e a l s o t a k e s p l a c e i n convergent ( e l o n g a t i o n a l ) as w e l l as i n the shear f l o w f i e l d s . Elmendorp s t r e s s e d the need f o r taking the coalescence into account while evaluating m i c r o r h e o l o g i c a l models ( 3 7 ) . Thus, the d i s p e r s i n g p r o c e s s must be seen as a dynamic e v e n t , t e n d i n g t o d i f f e r e n t " e q u i l i b r i u m " s t a t e s under d i f f e r e n t s e t s of imposed c o n d i t i o n s . I n summary, m i c r o r h e o l o g y , combining t h e thermodynamic and r h e o l o g i c a l p r i n c i p l e s , provides a powerful t o o l f o r understanding and c o n t r o l l i n g the m i x i n g p r o c e s s . However, w h i l e m i c r o r h e o l o g y l o o k s a t the i n f l u e n c e of each f a c t o r i n i s o l a t i o n , i n commercial compounding a l l of these t a k e p l a c e a t once; t h e r e i s an i n t e r p l a y between v a r i o u s d e f o r m a t i o n f i e l d s , t h e i r i n t e n s i t i e s , and d i v e r s e process v a r i a b l e s . Blend Rheology The most f r e q u e n t l y used r h e o l o g i c a l f u n c t i o n i s the shear v i s c o s i t y , n. An o l d argument s t a t e s t h a t i n m u l t i p h a s e systems the shear s t r e s s , o^~> a continuous f u n c t i o n at the i n t e r f a c e whereas the shear r a t e , y i s not ( 1 0 ) . Of c o u r s e , the argument i g n o r e s the e x i s t e n c e o f systems with interlayer slip where both and y a r e d i s c o n t i n u o u s , but i n d e e d t h e s e a r e l e s s frequent. Thus, c o n d i t i o n a l l y a c c e p t i n g t h e argument, t h e b l e n d r h e o l o g i c a l f u n c t i o n s s h o u l d be examined a t c o n s t a n t s t r e s s ; i . e . , n = n (a ) not n = n (y) and G = G (G") not G' = G (u>). C o n s e q u e n t l y , l e t us s e t P = n = nfa-^) i n Eq ( 8 ) . Note t h a t a t low enough s t r e s s n e q u a l s t h e z e r o - s h e a r v i s c o s i t y , n = n ( a ^ ^ O ) • I t i s c o n v e n i e n t t o d e f i n e the l o g - a d d i t i v i t y r u l e as: i s

1

T

f

Q

ZnP

= Z w lriP ±

±

(8)

±

where P i s a r h e o l o g i c a l f u n c t i o n (e.g., v i s c o s i t y , n, dynamic s t o r a g e and l o s s m o d u l i , G and G", s t r e s s , $12* *) * * the weight f r a c t i o n of i n g r e d i e n t " i " i n the m i x t u r e . E q u a t i o n (8) can now be used t o s e p a r a t e a l l b l e n d s i n t o f o u r categories: 1. a d d i t i v e b l e n d s where n f o l l o w s the r e l a t i o n , i.e., t h e r e i s agreement between the e x p e r i m e n t a l v a l u e of v i s c o s i t y , n, measured f o r the b l e n d and t h a t c a l c u l a t e d from v i s c o s i t i e s of neat r e s i n s by means o f Eq. ( 8 ) : n = n , ; 2. p o s i t i v e l y d e v i a t i n g b l e n d s , PDB, w i t h n > n ^ ; 3. n e g a t i v e l y d e v i a t i n g b l e n d s , NDB, w i t h n < n ; and 4. mSxecf, p o s i t i v e l y and n e g a t i v e l y d e v i a t i n g b l e n d s , pfiSA? Hopefully, different flow mechanisms can be a s s i g n e d t o each of t h e s e c a t e g o r i e s . A g e n e r a l p i c t u r e w h i c h emerges from a n a l y s i s o f many a l l o y s and b l e n d s i s t h a t a l l f o u r c a t e g o r i e s a r e found i n m i s c i b l e and i m m i s c i b l e 1

etc

an(


w

s


1.

UTRACKI ETAL.

13


systems. D i f f e r e n t mechanisms a r e r e s p o n s i b l e f o r t h e d e v i a t i o n i n m i s c i b l e systems than i n i m m i s c i b l e ones and t h e i n t e n s i t y o f d e v i a t i o n from the l o g - a d d i t i v i t y r u l e i n m i s c i b l e b l e n d s i s n e v e r v e r y l a r g e . U n f o r t u n a t e l y , however, one cannot say t h a t a l l b l e n d s of a g i v e n phase b e h a v i o r b e l o n g t o a s i n g l e r h e o l o g i c a l c a t e g o r y (21). I n m i s c i b l e systems t h e c a t e g o r y o f In n v s . w. r e l a t i o n depends on t h e f r e e volume v a r i a t i o n w i t h c o m p o s i t i o n , f = f (w). For n o n - i n t e r a c t i n g , homologous l i q u i d s ( f o r example: f r a c t i o n s o f a g i v e n p o l y m e r ) , t h i s r e l a t i o n can be computed from s t a t i s t i c a l thermodynamics and v a r i a t i o n s o f v i s c o s i t y w i t h c o m p o s i t i o n have been p r e d i c t e d w i t h good p r e c i s i o n ( 3 9 ) . I n t h a t case o n l y a s m a l l PDB was observed. For most m i s c i b l e b l e n d s the m i s c i b i l i t y stems from s p e c i f i c i n t e r a c t i o n s w h i c h u s u a l l y l e a d t o a r e d u c t i o n o f volume w h i c h i n t u r n causes an i n c r e a s e i n n; thus PDB i n m i s c i b l e b l e n d s predominates. On t h e o t h e r hand, i n systems where m i s c i b i l i t y o r i g i n a t e s from r e d u c t i o n o f u n f a v o r a b l e i n t e r a c t i o n s between groups o f t h e same polymer an e x p a n s i o n o f volume and NDB a r e expected (21). I n i m m i s c i b l e b l e n d s t h e changes i n r h e o l o g i c a l p r o p e r t i e s may o r i g i n a t e e i t h e r from the volume change i n the i n t e r p h a s e r e g i o n o r from the p r e s e n c e o f the second phase. The e f f e c t o f t h e d i s p e r s e d phase can be modeled by e i t h e r comparing the b l e n d t o an e m u l s i o n (X>1); i n b o t h c a s e s n r a p i d l y i n c r e a s e s w i t h c o m p o s i t i o n l e a d i n g t o PDB. About 60% o f b l e n d s b e l o n g t o t h i s type. NDB i m m i s c i b l e systems a r e about h a l f a s numerous. The o n l y mechanism w h i c h e x p l a i n s t h i s b e h a v i o r i s i n t e r l a y e r s l i p , and i n d i r e c t e v i d e n c e o f t h i s h a s been shown w i t h m i c r o g r a p h s o f extruded n o n - c o m p a t i b i l i z e d blends. A more d i r e c t a f f i r m a t i o n was r e c e n t l y p r o v i d e d by Lyngaae-Jorgensen ( 4 0 ) , who p r e p a r e d a m u l t i l a y e r e d sandwich c o n s i s t i n g o f hundreds o f l a y e r s o f s e l e c t e d i m m i s c i b l e polymers p e r m i l l i m e t e r o f specimen t h i c k n e s s . The v i s c o s i t y showed s t r o n g n e g a t i v e d e v i a t i o n and was a n a l y s e d by c o n s i d e r i n g t h e b l e n d a s a t h r e e component system, i n c l u d i n g an i n t e r p h a s e regime. The v i s c o s i t y o f t h i s r e g i o n was found t o be about o n e - t e n t h o f t h e v i s c o s i t y o f t h e l e s s v i s c o u s component. I n t e r l a y e r s l i p i s a s s o c i a t e d w i t h t h e r e p u l s i v e f o r c e s between i m m i s c i b l e polymers w h i c h l e a d t o a r e d u c t i o n o f d e n s i t y a t t h e interface. I n most c a s e s , the v i s c o s i t y f o l l o w s t h e t h e o r e t i c a l r e l a t i o n (41): 1/n

- [1 + ( 3 / o ) 1 2

(W W ) 1

2

1 / 2

]-[W /T 1

1 ] L

+ w /n ] 2

2

(9)

where 3 i s a c h a r a c t e r i s t i c i n t e r l a y e r s l i p f a c t o r f o r the system. Eq (9) p r e d i c t s a r e d u c t i o n o f t h e i m p o r t a n c e o f s l i p with i n c r e a s i n g a, « The r e l a t i o n i s s y m m e t r i c a l f o r e c a s t i n g a maximum s l i p e f f e c t f o r a 50:50 c o m p o s i t i o n , though due t o asymmetry o f t h e thermodynamic i n t e r a c t i o n s , t h i s i s not a l w a y s o b s e r v e d . At low c o n c e n t r a t i o n o f the d i s p e r s e d phase d i s c r e t e d r o p l e t s are expected. As t h e c o n c e n t r a t i o n i n c r e a s e s t h e shear field e x t e n d s t h e d r o p s , c o a l e s c e s them and f i n a l l y a t t h e c r i t i c a l 2


14


IONOMERS

volume f r a c t i o n , , forms a co-continuous s t r u c t u r e and i n v e r t s . P a u l and Barlow proposed the s i m p l e f o r m u l a f o r t h e phase i n v e r s i o n (42): • /(l-* ) = X ±

(10)

±

More e l a b o r a t e formulas a r e a l s o a v a i l a b l e i n t h e l i t e r a t u r e ( 2 1 ) . The c o n c e n t r a t i o n at phase i n v e r s i o n , .^, r e p r e s e n t s the m i d - v a l u e of two s determined f o r b o t h polymers as t h e m a t r i x , though there are no t h e o r i e s w h i c h can p r e d i c t . E x p e r i m e n t a l l y t h e b r e a d t h of the c o - c o n t i n u o u s r e g i o n , ^ - 2» i n c r e a s e s w i t h l o w e r i n g the component v i s c o s i t i e s , n ( 4 J ) . ° From the m o r p h o l o g i c a l p o i n t of v i e w , the e x i s t e n c e of these c r i t i c a l c o n c e n t r a t i o n s , has a more p r o f o u n d i n f l u e n c e on some r h e o l o g i c a l f u n c t i o n s than on o t h e r s , i e . , s t r a i n r e c o v e r y , S^. Thornton et a l . (44) r e p o r t e d s t e p - i n c r e a s e s i n S v s . w^ f u n c t i o n at c o n c e n t r a t i o n s and (J^. The s t e p s were not v i s i b i l e i n n v s . w_£ p l o t s . I n e m u l s i o n s and s u s p e n s i o n s the i n t e r a c t i n g p a r t i c l e s may form a t h r e e - d i m e n s i o n a l network t h a t results in a yield stress, a . In b l e n d s , a has been r e p o r t e d f o r b o t h d i s p e r s e d and c o - c o n t i n u o u s m o r p h o l o g i e s , i . e . , below and above (J> . Presence of a i n c r e a s e s PDB b e h a v i o r i n Zn n v s . w. p l o t s . ^PNDB o c c u r s f o r b l e n d s t h a t are m i s c i i l e o n l y w i t h i n a s m a l l range of concentration. To understand these systems, the i n t e r a c t i o n between the f l o w and the phase s e p a r a t i o n must be considered. T h i s has become an a r e a of c o n s i d e r a b l e s c i e n t i f i c activity. One needs t o c o n s i d e r b o t h how t h e phase s e p a r a t i o n i n f l u e n c e s the r h e o l o g i c a l b e h a v i o r and how the f l o w (or s t r e s s ) a f f e c t s the thermodynamics of phase s e p a r a t i o n . T h e o r e t i c a l c a l c u l a t i o n s f o r l i q u i d / l i q u i d systems p r e d i c t t h a t the v i s c o s i t y goes through a maximum a t the s p i n o d a l . Depending on the type of system and i t s r e g u l a r i t y , the i n c r e a s e may be q u i t e l a r g e ; f o r example, L a r s o n and F r e d e r i c k s o n (45) p r e d i c t e d t h a t f o r b l o c k copolymers. These a u t h o r s concluded t h a t i n the s p i n o d a l r e g i o n a t h r e e - d i m e n s i o n a l network i s formed and that the system exhibits non-linear viscoelastic behavior. E x p e r i m e n t a l l y , sharp i n c r e a s e s of n n e a r t h e phase s e p a r a t i o n have been r e p o r t e d f o r low m o l a r mass s o l u t i o n s as w e l l as f o r o l i g o m e r i c and p o l y m e r i c m i x t u r e s (21). M e l t f l o w , however, a l s o a f f e c t s the phase s e p a r a t i o n , u s u a l l y enhancing the m i s c i b i l i t y f o r p a r t i a l l y m i s c i b l e b l e n d s t h a t show LCST b e h a v i o r . From Lyngaae-Jorgensen work (46) one may d e r i v e the f o l l o w i n g r e l a t i o n between the shear s t r e s s and the change i n t h e s p i n o d a l temperature, T , (21), f


c

R

c

g

a

2 1 2

- a(T-T )T g

(11)

where a i s a c h a r a c t e r i s t i c m a t e r i a l parameter. S e v e r a l groups have observed r e d u c t i o n of o p a c i t y of sheared i m m i s c i b l e polymer b l e n d s by measuring l i g h t s c a t t e r i n g d u r i n g the f l o w w i t h i n the two-phase r e g i o n (47-49). An important q u e s t i o n t h a t a r i s e s from these r e s u l t s i s whether they r e p r e s e n t t r u e phase m i x i n g or are due t o a r e d u c t i o n of the phase s i z e below the l i m i t needed t o



1.


UTRACKI ET AL.

15

scatter light. W i n t e r (50) h a s r e c e n t l y shown by f l u o r e s c e n c e measurements t h a t i n the case o f p o l y s t y r e n e / p o l y v i n y l m e t h y l e t h e r b l e n d s t h a t h i g h s t r a i n (e = 44) f l o w s l e a d t o m o l e c u l a r m i x i n g . The s t r u c t u r e o f i m m i s c i b l e b l e n d s i s seldom a t e q u i l i b r i u m . In p r i n c i p l e , the c o a r s e r the d i s p e r s i o n t h e l e s s s t a b l e i t i s . There a r e two a s p e c t s o f s t a b i l i t y i n v o l v e d : the c o a l e s c e n c e i n a s t a t i c system and d e f o r m a b i l i t y due t o f l o w . As d i s c u s s e d above the c r i t i c a l parameter f o r b l e n d d e f o r m a b i l i t y i s the t o t a l s t r a i n i n shear y = t y , o r i n e x t e n s i o n , e = t e . Provided t i s large enough i n steady s t a t e the s t r a i n s and d e f o r m a t i o n s can be q u i t e s u b s t a n t i a l ; one s t a r t s a t e s t w i t h one m a t e r i a l and ends w i t h another. T h i s means t h a t n e i t h e r the s t e a d y s t a t e s h e a r i n g n o r e l o n g a t i o n a l f l o w can be used f o r c h a r a c t e r i z a t i o n o f m a t e r i a l s w i t h deformable s t r u c t u r e . F o r these systems the o n l y s u i t a b l e method i s a low s t r a i n dynamic o s c i l l a t o r y t e s t . The t e s t i s s i m p l e and r a p i d , and a method o f d a t a e v a l u a t i o n l e a d i n g t o unambiguous d e t e r m i n a t i o n o f the s t a t e of m i s c i b i l i t y i s d i s c u s s e d i n a l a t e r chapter. There a r e f o u r p o p u l a r measures o f l i q u i d e l a s t i c i t y : t h e f i r s t normal s t r e s s d i f f e r e n c e , the e x t r u d a t e s w e l l , B, the Bagley e n t r a n c e - e x i t p r e s s u r e drop c o r r e c t i o n , P^, and t h e s t o r a g e shear modulus, G . For m u l t i p h a s e systems t h e r e i s no s i m p l e c o r r e l a t i o n between N^ and G , a l t h o u g h the S p r i g g s t h e o r e t i c a l r e l a t i o n ( 5 1 ) : 1

T

f

f

o\ Y = G"(o))/C; N (y) = 2G (o))/C 12 1 9

2

Where C = a>/y

(12)

has been found t o work when 1 < C < 2. Q u a l i t a t i v e l y C i n c r e a s e d w i t h h e t e r o g e n e i t y o f the m e l t ( 2 1 ) . Both N^ and G ( a t c o n s t a n t s t r e s s ) show a d d i t i v i t y , PDB, NDB o r PNDB b e h a v i o r p a r a l l e l i n g the n v s . w^ p l o t . The extrudate swell, B, dependence on c o m p o s i t i o n i s determined by X. F o r deformable drops i n systems where X < 4, s t r o n g PDB b e h a v i o r i s o b s e r v e d , independent o f t h e n v s . w relation. T h i s b e h a v i o r o r i g i n a t e s i n s h r i n k i n g of f i b r i l s c r e a t e d i n t h e convergent f l o w r e g i o n a t t h e c a p i l l a r y e n t r a n c e . The tendency f o r the d r o p l e t t o r e g a i n s p h e r i c i t y causes the e x t r u d a t e to s w e l l c o n s i d e r a b l y . T h i s h a s l i t t l e t o do w i t h t h e mechanism r e s p o n s i b l e f o r e x t r u d a t e s w e l l i n homogeneous m e l t s . On the o t h e r hand, i n b l e n d s where X > 4 t h e s w e l l i s s m a l l , f r e q u e n t l y NDB, and independent o f the n v s . w^ b e h a v i o r . The h i g h l y v i s c o u s drop can be equated w i t h a s o l i d p a r t i c l e ; T h e o l o g i c a l l y these blends resemble s u s p e n s i o n s . The f o u r t h measure o f l i q u i d e l a s t i c i t y , P , r e f l e c t s t h e e x t e n s i o n a l p r o p e r t i e s r a t h e r than e l a s t i c i t y . For polymer b l e n d s i t i s d i f f i c u l t t o determine P w i t h a s u f f i c i e n t degree o f accuracy. The d a t a i n d i c a t e t h a t t h i s parameter i s most s e n s i t i v e to m o r p h o l o g i c a l changes. F o r example, t h e degree o f d r o p l e t c o a l e s c e n c e i n t h e i n s t r u m e n t r e s e r v o i r d r a s t i c a l l y changes t h e v a l u e s o f P^. Polymer p r o c e s s e s such a s the f i l m b l o w i n g , blow m o l d i n g o r w i r e c o a t i n g i n v o l v e e x t e n s i o n a l f l o w s and a r e , t h e r e f o r e , most a p p r o p r i a t e l y s t u d i e d u s i n g e x t e n s i o n a l rheometry. Extensional f l o w d a t a p r o v i d e unique i n f o r m a t i o n on s t r e s s h a r d e n i n g , SH, w h i c h a r e i m p o r t a n t f o r good b u b b l e s t a b i l i t y d u r i n g f i l m b l o w i n g . Some f

£


16


IONOMERS

p o l y m e r s , such as low d e n s i t y p o l y e t h y l e n e , LDPE, have s t r o n g SH, whereas some l i k e p o l y p r o p y l e n e , PP, do not show i t a t a l l . B l e n d i n g o f f e r s an easy way t o i n t r o d u c e SH i n t o m a t e r i a l s l a c k i n g this ability. SH can be i n c o r p o r a t e d i n t o b o t h m i s c i b l e and i m m i s c i b l e b l e n d s , a l t h o u g h i n the l a t t e r case the phenomenon seems t o be r e s t r i c t e d t o systems w i t h c o - c o n t i n u o u s s t r u c t u r e s .


Compounding and P r o c e s s i n g Polymer B l e n d s Commercial blends are designed either to optimize their morphological s t a b i l i t y or t h e i r a b i l i t y to a t t a i n a s p e c i f i c morphology. In the f i r s t case the m a n u f a c t u r e r ' s aim i s g e n e r a t i o n of m a t e r i a l s t h a t are r e l a t i v e l y i n s e n s i t i v e t o the method of processing. Most of these b l e n d s c o n t a i n w _< 30% of w e l l d i s p e r s e d and c o m p a t i b i l i z e d minor phase i n g r e d i e n t . Stabilization of morphology a t h i g h e r c o n c e n t r a t i o n i s more d i f f i c u l t , but p o s s i b l e , as evidenced by new, c o m m e r c i a l l y s u c c e s s f u l PA/ABS = 50/50 b l e n d s . The second c a t e g o r y i n c l u d e s b l e n d s i n w h i c h the minor phase must deform i n t o l a m e l l a e o r f i b e r s d u r i n g the f i n a l processing step. There i s no s i n g l e r e c i p e f o r p r e p a r a t i o n of b l e n d s . Before the i n g r e d i e n t s are s e l e c t e d and the compounding/processing equipment i s chosen, one s h o u l d have a c l e a r i d e a of the use of the b l e n d . A program of b l e n d development was r e c e n t l y proposed ( 2 1 ) . P a s t e x p e r i e n c e t e a c h e s t h a t synergism i s r a r e and s e l e c t i o n of b l e n d components i s u s u a l l y based on compensation of n a t u r a l properties. For example, the r a t i o n a l e f o r blending poly (phenylene e t h e r ) , PPE, w i t h h i g h impact p o l y s t y r e n e , HIPS, o r w i t h p o l y a m i d e , PA, i s shown i n T a b l e I . The compensation of p r o p e r t i e s i n PPE/HIPS and PA/PPE b l e n d s i s o b v i o u s . What i s l e s s c e r t a i n i s the method of c o m p a t i b i l i z a t i o n . Since P P E / P o l y s t y r e n e b l e n d s a r e m i s c i b l e t h e r e i s no need f o r a c o m p a t i b i l i z e r i n PPE/HIPS b l e n d s . However PA and PPE are a n t a g o n i s t i c a l l y immiscible and c o m p a t i b i l i z a t i o n i s r e q u i r e d ; low o r h i g h m o l a r mass a d d i t i v e s w i t h a c i d ( o r a n h y d r i d e ) groups are o f t e n used i n a r e a c t i v e compounding o p e r a t i o n . The advantage of b l e n d s w i t h f i n e d r o p l e t d i s p e r s i o n i s t h e i r homopolymer-like b e h a v i o r . These m a t e r i a l s a r e easy t o p r o c e s s i n a r e p r o d u c i b l e manner, w e l d l i n e s do not cause s e r i o u s problems and s c r a p r e c y c l i n g i s p o s s i b l e . On the o t h e r hand, the ability of a n i s o t r o p i c r e i n f o r c e m e n t s by oriented inclusions i s lost. F i b r i l l a r o r l a m e l l a r m o r p h o l o g i e s a r e d e s i r a b l e i n some p r o d u c t s e i t h e r as r e i n f o r c e m e n t s o r b a r r i e r s a g a i n s t gas or l i q u i d permeation. Here the compounding step must g e n e r a t e r e l a t i v e l y l a r g e domains of a d i s p e r s e d , c o m p a t i b l i z e d component, and the d e s i r e d morphology i s d e v e l o p e d i n the subsequent p r o c e s s i n g s t e p . P r o c e s s i n g of b l e n d s u s u a l l y i n v o l v e s two s t e p s : compounding, and forming (52). In about 80% of c a s e s t h e s e two s t e p s a r e carried out at different l o c a t i o n s , e.g. the material i s precompounded e i t h e r by the r e s i n m a n u f a c t u r e r o r a compounder t h e n transformed into the final article by a processor. Most compounding i s c u r r e n t l y done i n a t w i n - s c r e w e x t r u d e r , t w i n - s h a f t continuous i n t e n s i v e m i x e r o r o t h e r type of e x t r u d i n g machine ( s i n g l e - s c r e w , d i s k , s p e c i a l i t y coupounder, e t c . ) . The t w i n - s c r e w i



1.

UTRACKI ETAL.


17

e x t r u d e r s are e x p e n s i v e but they p r o v i d e f l e x i b i l i t y f o r changes, u n i f o r m i t y o f m i x i n g , s h o r t r e s i d e n c e t i m e s and narrow r e s i d e n c e time d i s t r i b u t i o n . W i t h s i n g l e screw e x t r u d e r s , c o n t r o l i s not a s good. During t h e compounding step the ingredients are often c o m p a t i b i l i z e d by a d d i t i o n of a t h i r d component (e.g., copolymer o r c o - s o l v e n t ) o r by one o f s e v e r a l methods o f r e a c t i v e processing. Sometimes the g e n e r a t e d morphology must be f u r t h e r s t a b i l i z e d by enhanced c r y s t a l l i z a t i o n o r p a r t i a l c r o s s l i n k i n g o f one phase. As e v i d e n c e d by the e x t e n s i v e p a t e n t l i t e r a t u r e , r e a c t i v e compounding dominates t h e f i e l d . These p r o c e s s e s r e q u i r e f i n e c o n t r o l o f p r o c e s s v a r i a b l e s , narrow d i s t r i b u t i o n o f r e s i d e n c e times and u n i f o r m s t r e s s f i e l d s , t h a t c a n be p r o v i d e d o n l y b y t h e more s o p h i s t i c a t e d , and t h e r e f o r e , e x p e n s i v e machines. Compounding can add from 50 t o 500 US$ per m e t r i c t o n t o the c o s t of a b l e n d . The forming o p e r a t i o n s f o r polymer a l l o y s and b l e n d s use the same methods and equipment a s f o r homopolymers, b u t the p r o c e s s v a r i a b l e s u s u a l l y need be more p r e c i s e l y c o n t r o l l e d . The key t o a s u c c e s s f u l p r o d u c t i s o p t i m i z a t i o n of the morphology; e x c e e d i n g the recommended temperature o r shear s t r e s s can r e a d i l y a n n i h i l a t e the f r a g i l e b a l a n c e of f o r c e s r e s p o n s i b l e f o r the b l e n d s t r u c t u r e . F o r example, i n i n j e c t i o n m o l d i n g , l a y e r i n g and d e l a m i n a t i o n i s always possible. I n s p i t e of c o m p a t i b i l i z a t i o n there i s i n v a r i a b l y a s k i n / c o r e morphology w h i c h may l e a d t o h i g h n o t c h - s e n s i t i v i t y . With the growing p o p u l a r i t y o f b l e n d s compounded by the r e a c t i v e processing method t h e r e i s a l s o a new danger. F r e q u e n t l y two b a t c h e s o f the same b l e n d , r e a c t e d t o a d i f f e r e n t degree, behave l i k e two i m m i s c i b l e p o l y m e r s . Adding too much r e c y c l e o r m i x i n g d i f f e r e n t b a t c h e s may r e s u l t i n sudden d r a m a t i c weakening o f w e l d l i n e s o r , i n extreme c o n d i t i o n s , d e l a m i n a t i o n o f the p r o c e s s e d article. Mechanical

Properties

P r e d i c t i n g the modulus o f a two phase s t r u c t u r e i s not a t r i v i a l problem, but i t can be measured r e a s o n a b l y w e l l . F a i l u r e p r o p e r t i e s are more d i f f i c u l t t o measure i n a r e p r o d u c i b l e and m e a n i n g f u l way t o g i v e a measure o f m a t e r i a l p r o p e r t i e s and not j u s t a f u n c t i o n of the t e s t method and sample geometry. The b e s t s o l u t i o n l i e s i n f r a c t u r e mechanics (53). The b a s i s o f f r a c t u r e mechanics i s the a n a l y s i s o f the s t r e s s around a p r e - e x i s t i n g c r a c k o f known s i z e as shown i n F i g . 7. The toughness, g i v e n b y K the stress concentration factor i s : K

c

= Yaa. 1

1 / 2

(13)

where a i s the s t r e s s at b r e a k , a. i s the c r a c k l e n g t h and Y i s a f a c t o r w h i c h depends on the s p e c i f i c geometry. The problem i n the case o f b l e n d s i s t h a t the p l a s t i c zone must be s m a l l e r than the o t h e r dimensions a s shown i n F i g . 7. I n p a r t i c u l a r , the t h i c k n e s s , B, must be l a r g e enough t o m a i n t a i n a p l a n e s t r a i n c o n d i t i o n , o t h e r w i s e excess y i e l d i n g a l o n g the w i d t h of the c r a c k , i . e . , changing B, o c c u r s . For tough p l a s t i c s , w h i c h


18

MULTIPHASE POLYMERS: BLENDS AND IONOMERS


T a b l e I . Advantages and D i s a d v a n t a g e s of S e l e c t e d Polymers

Polymer

Advantage

Disadvantage

1

PPE

High Heat D i s t o r t i o n T, R i g i d i t y , Flammability, Moisture Absorption

Processability, Impact S t r e n g t h , Solvent Resistance

2

HIPS

Processability, Impact S t r e n g t h

Heat D i s t o r t i o n Rigidity

3

PA

Processability, Impact S t r e n g t h , Solvent Resistance

Heat D i s t o r t i o n T, Rigidity, Moisture Absorption

No.

F i g u r e 7. Compact t e n s i o n specimen showing around the c r a c k t i p .

the p l a s t i c


T,

zone


1.

UTRACKI ETAL.


19

i n c l u d e many p l a s t i c / r u b b e r b l e n d s , B needs t o be s e v e r a l c e n t i m e t e r s , w h i c h i s not v e r y p r a c t i c a l . V a r i o u s ways have been t r i e d t o overcome t h i s problem, an example of w h i c h i s the J method. A t h r e e p o i n t b e n d i n g method i s used and the energy per u n i t a r e a f o r i n i t i a t i o n o f c r a c k growth measured as the a r e a under a l o a d - d e f l e c t i o n c u r v e up t o the p o i n t of i n i t i a t i o n . I n o r d e r t o determine the p o i n t o f i n i t i a t i o n , s e v e r a l samples are loaded t o d i f f e r e n t p o s t - i n i t i a t i o n s t a g e s , the amount o f c r a c k growth measured a f t e r f r e e z i n g and b r e a k i n g , and t h i s i s e x t r a p o l a t e d t o Aa.=0 w i t h a c o r r e c t i o n b e i n g made f o r crack b l u n t i n g . Measurements o f J have been r e p o r t e d f o r a v a r i e t y o f polymers and comparison w i t h e q u i v a l e n t K v a l u e s found t o be s a t i s f a c t o r y (54). I n p r a c t i c e , much of the l i t e r a t u r e s t i l l r e p o r t s n o t c h e d I z o d numbers, but these can be d e c e p t i v e , e s p e c i a l l y f o r m a t e r i a l s c l o s e to a d u c t i l e - b r i t t l e transition. Better indicators of r e a l toughness a r e o b t a i n e d when samples o f d i f f e r e n t t h i c k n e s s a r e considered. As one adds more rubber t o a p l a s t i c , a c r i t i c a l concentration i s often found where t h e toughness increases dramatically. I n some c a s e s , e.g., polymers t h a t are c a p a b l e o f shear y i e l d i n g , t h i s c o r r e l a t e s b e t t e r w i t h t h e i n t e r p a r t i c l e d i s t a n c e o r "ligament t h i c k n e s s " than w i t h any o t h e r v a r i a b l e . Fatigue i s probably t h e commonest cause o f f a i l u r e i n p l a s t i c s , but i t i s l e s s s t u d i e d than o t h e r forms o f f a i l u r e . I t has been reviewed by B u c k n a l l ( 6 ) . Data i s o f t e n shown i n terms o f a p l o t o f t h e s t r e s s a m p l i t u d e a g a i n s t t h e l o g o f the number o f cycles t o failure. I t i s n o t a b l e t h a t f o r unnotched f a t i g u e t e s t s , h i g h impact p o l y s t y r e n e a c t u a l l y p r e f o r m s more p o o r l y t h a n pure p o l y s t y r e n e when s t u d i e d t h i s way. I t i s p o s s i b l e t h a t the ease o f i n i t i a t i o n of m u l t i p l e c r a z e s , w h i c h g i v e s r i s e t o improved i m p a c t , a c t u a l l y l e a d s t o f a i l u r e i n a f a t i g u e t e s t . W i t h notched samples s t u d i e d by the methods o f f r a c t u r e mechanics one p l o t s a c r a c k growth r a t e a g a i n s t the s t r e s s i n t e n s i t y f a c t o r . IONOMERS The i n t r o d u c t i o n o f a s m a l l amount o f bonded s a l t groups i n t o a r e l a t i v e l y n o n p o l a r polymer has a p r o f o u n d e f f e c t on i t s s t r u c t u r e and p r o p e r t i e s . These m a t e r i a l s , termed ionomers, have been used i n a v a r i e t y o f a p p l i c a t i o n s , i n c l u d i n g p e r m s e l e c t i v e membranes, thermoplastic elastomers, p a c k a g i n g and f i l m s , v i s c o s i f i e r s and compatibilizing agents f o r polymer b l e n d s . F o r a l l these applications, t h e i n t e r a c t i o n s o f t h e i o n i c groups and t h e m o r p h o l o g i e s t h a t r e s u l t are c r i t i c a l f o r e s t a b l i s h i n g the u n i q u e properties. D e s p i t e t h i s , however, the s p a t i a l arrangement o f t h e i o n s i n ionomers remains an open q u e s t i o n , and c o n s i d e r a b l e r e s e a r c h has been and c o n t i n u e s t o be devoted towards u n d e r s t a n d i n g the m i c r o s t r u c t u r e o f t h e s e m a t e r i a l s . T h i s , however, does n o t appear t o have i n h i b i t e d p r o g r e s s i n the t e c h n o l o g i c a l e x p l o i t a t i o n of ionomers as n o v e l m a t e r i a l s . Comprehensive r e v i e w s and d i s c u s s i o n s o f ionomers can be found i n a number of monographs and r e v i e w a r t i c l e s (55-64). Four general topics are discussed here: (1) new m a t e r i a l s , (2) s t r u c t u r e , (3) s o l u t i o n s , and (4) membranes.


20


IONOMERS


S y n t h e s i s of Ionomers Ionomers have been p r e p a r e d by two g e n e r a l r o u t e s : (1) copolymerization of a low level of f u n c t i o n a l i z e d monomer w i t h an o l e f i n i c a l l y u n s a t u r a t e d comonomer o r (2) d i r e c t f u n c t i o n a l i z a t i o n of a preformed polymer. Almost a l l ionomers of p r a c t i c a l i n t e r e s t have c o n t a i n e d e i t h e r c a r b o x y l a t e o r s u l f o n a t e groups as t h e i o n i c species. Other salts, such as phosphonates, sulfates, t h i o g l y c o l a t e s , ammonium, and p y r i d i n i u m s a l t s have been s t u d i e d , but nowhere t o the e x t e n t of the c a r b o x y l a t e and sulfonate anionomers. (An aniomer i s d e f i n e d as an ionomer i n w h i c h the a n i o n i s bonded t o the polymer. C o n v e r s e l y , ionomers t h a t have the c a t i o n bonded t o the polymer are termed c a t i o n o m e r s ) . Relatively l i t t l e i n f o r m a t i o n i s a v a i l a b l e on the s t r u c t u r e and p r o p e r t i e s of these t y p e s of ionomers. T y p i c a l l y , c a r b o x y l a t e ionomers are p r e p a r e d by d i r e c t cop o l y m e r i z a t i o n of a c r y l i c o r m e t h a c r y l i c a c i d w i t h ethylene, s t y r e n e o r s i m i l a r comonomers by f r e e r a d i c a l copolymerization (65). More r e c e n t l y , a number of c o p o l y m e r i z a t i o n s i n v o l v i n g s u l f o n a t e d monomers have been d e s c r i b e d . For example, Weiss e t a l . (66-69) p r e p a r e d ionomers by a f r e e - r a d i c a l , e m u l s i o n c o p o l y m e r i z a t i o n of sodium s u l f o n a t e d s t y r e n e w i t h b u t a d i e n e or styrene. Similarly, A l l e n et a l . (70) c o p o l y m e r i z e d n - b u t y l a c r y l a t e w i t h s a l t s of s u l f o n a t e d s t y r e n e . The ionomers p r e p a r e d by t h i s r o u t e , however, were r e p o r t e d t o be " b l o c k y " w i t h r e g a r d t o the i n c o r p o r a t i o n of the s u l f o n a t e d s t y r e n e monomer. Salamone e t a l . (71-76) prepared ionomers based on the c o p o l y m e r i z a t i o n of a n e u t r a l monomer, such as s t y r e n e , m e t h y l m e t h a c r y l a t e , or n - b u t y l a c r y l a t e , with a c a t i o n i c - a n i o n i c monomer p a i r , 3-methacrylamidopropyltrimethylammonium 2-acrylamide-2-methylpropane s u l f o n a t e . The second method used to prepare ionomers involves f u n c t i o n a l i z i n g a preformed polymer. T h i s has been t h e more common s t r a t e g y f o r o b t a i n i n g s u l f o n a t e - i o n o m e r s . Makowski e t a l . (77, 78) prepared lightly sulfonated polystyrene (SPS) and ethylenepropylene-diene terpolymers (SEPDM) by r e a c t i o n of the polymers w i t h a c e t y l s u l f a t e i n homogeneous s o l u t i o n . T h i s c h e m i s t r y y i e l d s a c o n t r o l l e d c o n c e n t r a t i o n and a random d i s t r i b u t i o n of s u l f o n a t e groups. Bishop et a l . 079 , 80) d e s c r i b e d the s u l f o n a t i o n of p o l y ( e t h e r e t h e r ketone) w i t h c h l o r o s u l f o n i c a c i d o r s u l f u r i c a c i d . Zhou and E i s e n b e r g (81) p r e p a r e d s u l f o n a t e d p o l y - c i s - 1 , 4 - i s o p r e n e u s i n g a c e t y l s u l f a t e , but t h i s r e a c t i o n a l s o y i e l d e d a c o n s i d e r a b l e amount of c y c l i z e d polymer. R a h r i g e t a l . (82) s u l f o n a t e d p o l y p e n tenamer u s i n g a 1:1 complex of SO^ w i t h t r i e t h y l p h o s p h a t e . Polyamp h o l y t e s were prepared by P e i f f e r et a l . (83) who s u l f o n a t e d a copolymer of s t y r e n e and 4 - v i n y l p y r i d i n e w i t h a c e t y l s u l f a t e . Huang e t a l . (84) made z w i t t e r i o n o m e r s by r e a c t i n g a p o l y u r e t h a n e segmented copolymer w i t h g-propane s u l t o n e . T h i s was c o n v e r t e d t o an anionomer (85) by r e a c t i o n w i t h a m e t a l a c e t a t e . A s p e c i a l c l a s s of ionomer i n w h i c h the s a l t groups a r e o n l y a t the c h a i n ends, i . e . , t e l e c h e l i c ionomers, have r e c e n t l y r e c e i v e d c o n s i d e r a b l e a t t e n t i o n as model ionomer systems. Kennedy and coworkers (86-88) p r e p a r e d l i n e a r and t r i - a r m s t a r t e l e c h e l i c s u l f o n a t e d p o l y i s o b u t y l e n e by heterogeneous s u l f o n a t i o n of an olefin-terminated polyisobutylene with acetyl sulfate. Omeis e t


1.

UTRACKI ETAL.


21

al. (89) s y n t h e s i z e d telechelic sulfonated p o l y s t y r e n e and polybutadiene by t e r m i n a t i n g an a n i o n i c p o l y m e r i z a t i o n with 1,3-propane s u l t o n e . I n a l a t e r chapter, S t o r e y and George d e s c r i b e the s y n t h e s i s o f s t a r - b r a n c h e d t e l e c h e l i c b l o c k copolymer ionomers.


Blends As d i s c u s s e d e a r l i e r i n t h i s c h a p t e r , p h y s i c a l b l e n d i n g o f two polymers g e n e r a l l y r e s u l t s i n a two-phase m a t e r i a l a s a consequence of a p o s i t i v e e n t h a l p y o f m i x i n g . V a r i o u s approaches have been taken to introduce specific interacting f u n c t i o n a l i t i e s on polymer p a i r s i n o r d e r t o improve t h e i r m i s c i b i l i t y . The most common method, though by no means the o n l y method, i s t o employ hydrogen bonding (90). Recent work has shown t h a t i t i s p o s s i b l e t o improve m i s c i b i l i t y o f two i m m i s c i b l e polymers by u s i n g i n t e r a c t i o n s i n v o l v i n g one o r more i o n i z e d s p e c i e s . E i s e n b e r g and coworkers have employed a c i d - b a s e i n t e r a c t i o n s t o improve the m i s c i b i l i t y o f a number o f polymer-polymer p a i r s . M i s c i b l e b l e n d s were p r e p a r e d u s i n g a c i d - b a s e i n t e r a c t i o n s , e.g., with SPS ( a c i d derivative) and p o l y (ethylacrylate-co-4v i n y l p y r i d i n e ) ( 9 1 ) , s u l f o n a t e d p o l y i s o p r e n e and p o l y ( s t y r e n e - c o 4 - v i n y l p y r i d i n e ) ( 9 2 ) , and u s i n g i o n - d i p o l e i n t e r a c t i o n s , e . g . , p o l y ( s t y r e n e - c o - l i t h i u m m e t h a c r y l a t e ) and p o l y ( e t h y l e n e o x i d e ) (93). S i m i l a r l y , Weiss e t a l . (94) prepared m i s c i b l e b l e n d s o f S P S ( a c i d ) and a m i n o - t e r m i n a t e d p o l y ( a l k y l e n e o x i d e ) . I n a d d i t i o n to miscibility improvements, t h e i n t e r a c t i o n s between two f u n c t i o n a l i z e d polymers o f f e r s the p o s s i b i l i t y f o r a c h i e v i n g u n i q u e molecular a r c h i t e c t u r e w i t h a polymer b l e n d . Sen and Weiss d e s c r i b e the p r e p a r a t i o n o f g r a f t - c o p o l y m e r s by t r a n s i t i o n m e t a l c o m p l e x a t i o n of two f u n c t i o n a l i z e d polymers i n a n o t h e r c h a p t e r . STRUCTURE There i s a c o n s i d e r a b l e body o f e x p e r i m e n t a l and t h e o r e t i c a l e v i d e n c e f o r two types o f i o n i c aggregates termed m u l t i p l e t s and c l u s t e r s ( 9 5 ) . The m u l t i p l e t s are c o n s i d e r e d t o c o n s i s t o f s m a l l numbers o f a s s o c i a t e d c o n t a c t i o n - p a i r s t h a t a r e d i s p e r s e d i n the m a t r i x o f low d i e l e c t r i c c o n s t a n t , but do not t h e m s e l v e s c o n s t i t u t e a second phase. The number o f i o n - p a i r s i n a m u l t i p l e t i s s t e r i c a l l y l i m i t e d by the f a c t t h a t the s a l t groups are bound t o the polymer c h a i n . On the o t h e r hand, c l u s t e r s a r e c o n s i d e r e d t o be small microphase separated regions (

Polymer Alloys, Blends, and Ionomers - ACS Symposium Series (ACS

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