Multiphase Polymers: Blends and Ionomers - American Chemical

Chapter 17

X-Ray Analysis of Ionomers 1

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Richard A. Register , Y. Samuel Ding , Marianne Foucart , R. Jérôme , Stevan R. Hubbard , Keith O. Hodgson , and Stuart L. Cooper 3,5

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Department of Chemical Engineering, University of Wisconsin, Madison, WI 53706 Laboratory of Macromolecular Chemistry and Organic Catalysis, University of Liège, Sart Tilman B6, B-4000 Liège, Belgium Department of Chemistry and Department of Applied Physics, Stanford University, Stanford, CA 94305

Multiphase Polymers: Blends and Ionomers Downloaded from pubs.acs.org by UNIV LAVAL on 04/24/16. For personal use only.

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X-ray analysis techniques, such as small-angle x-ray scattering (SAXS), have been profitably applied in probing ionomer morphology for many years. We discuss below the application of two newer techniques, ex tended x-ray absorption fine structure (EXAFS) spec troscopy and anomalous SAXS (ASAXS), in two recent studies of ionomer morphology and structure-property relationships. The combination of SAXS and EXAFS was used to explain the modulus enhancement, due to inter locking loops of polymer chain, and high-strain behavior, due to aggregate cohesion, of five carboxy-telechelic polyisoprenes neutralized with divalent cations. ASAXS was used to show that the up turn near zero angle commonly observed in the SAXS patterns of ionomers is due to scattering from ca tions, possibly reflecting an inhomogeneous distribu tion of dissolved ionic groups. The i n c o r p o r a t i o n o f a s m a l l amount o f bound i o n i c f u n c t i o n a l i t y , t y p i c a l l y l e s s t h a n t e n mole p e r c e n t , has a p r o f o u n d e f f e c t on t h e p r o p e r t i e s o f a polymer. These m a t e r i a l s , termed "ionomers", c a n e x h i b i t marked i n c r e a s e s i n such i m p o r t a n t m a t e r i a l p r o p e r t i e s as modulus, a d h e s i v e s t r e n g t h , t e a r and a b r a s i o n r e s i s t a n c e , melt v i s c o s i t y , and impact s t r e n g t h [ 1 - 3 ] . I t i s now g e n e r a l l y accepted t h a t these e f f e c t s r e s u l t from aggregation of t h e ions into m i c r o d o m a i n s [ 4 , 5 ] , which a c t as p h y s i c a l c r o s s l i n k s i n t h e m a t e r i a l . The i o n i c groups may be spaced randomly a l o n g t h e polymer c h a i n , l o c a t e d a t r e g u l a r i n t e r v a l s a l o n g t h e c h a i n (such as i n t h e p o l y urethane ionomers[6]), o r l o c a t e d o n l y a t t h e c h a i n ends ( t h e 4

5

Current address: Baxter Healthcare Corporation, Round Lake, IL 60073 Current address: Department of Biochemistry and Molecular Biophysics, College of Physicians and Surgeons of Columbia University, New York, N Y 10032 0097-6156/89A)395-0420$06.00/0 ο 1989 American Chemical Society


17.

REGISTER ET AL.

X-Ray Analysis of Ionomers

421

" t e l e c h e l i c " ionomers[7,8]). W h i l e t h e placement of i o n i c groups i s expected t o m o d i f y t h e a g g r e g a t i o n p r o c e s s , a l l of t h e s e m a t e r i a l s e x h i b i t m i c r o p h a s e s e p a r a t i o n under c e r t a i n c o n d i t i o n s . Small-angle x-ray s c a t t e r i n g (SAXS) has been used f o r t w e n t y y e a r s t o c h a r a c t e r i z e ionomer morphology, s i n c e t h e p i o n e e r i n g observation by W i l s o n e t a l . [ 4 ] t h a t e t h y l e n e / m e t h a c r y l i c a c i d ionomers, upon n e u t r a l i z a t i o n , developed an i n t e n s e peak a t 20 ~ 4 . T h i s peak was i m m e d i a t e l y t a k e n as e v i d e n c e f o r i o n i c a g g r e g a t i o n , and has s i n c e been found t o be a n e a r l y u n i v e r s a l f e a t u r e of ionomers. A l s o , many r e s e a r c h e r s have noted a s t r o n g u p t u r n i n s c a t t e r e d i n t e n s i t y as z e r o a n g l e i s approached[6,8,9]. The origin of t h i s f e a t u r e has remained unknown u n t i l r e c e n t l y and w i l l be d i s c u s s e d below. The prominence of SAXS i n t h e a n a l y s i s of ionomers a r i s e s f r o m t h e s t r o n g e l e c t r o n d e n s i t y c o n t r a s t between t h e i o n i c aggregates and t h e polymer m a t r i x , g i v i n g a s t r o n g s i g n a l , as well as t h e d i f f i c u l t y i n o b t a i n i n g u s e f u l i n f o r m a t i o n by e l e c t r o n microscopy[10]. The use o f m o r p h o l o g i c a l models, through which a n a l y t i c a l e x p r e s s i o n s may be o b t a i n e d f o r t h e SAXS i n t e n s i t y , a l l o w i n v e s t i g a t o r s t o e s t i m a t e parameters such as t h e a g g r e g a t e s i z e and interaggregate spacing. With t h e r e c e n t a v a i l a b i l i t y of h i g h - i n t e n s i t y , e n e r g y - t u n a b l e x - r a y s from s y n c h r o t r o n s o u r c e s , t h e number of p r a c t i c a l x-ray a n a l y s i s t e c h n i q u e s has increased dramatically. I n t h e a r e a of s c a t t e r i n g , t h e t e c h n i q u e of anomalous s m a l l - a n g l e x - r a y scattering (ASAXS) has emerged r e c e n t l y as a p o w e r f u l t e c h n i q u e f o r i s o l a t i n g t h e s c a t t e r i n g due t o a p a r t i c u l a r element i n a m a t e r i a l . By t u n i n g t h e x - r a y energy near an a b s o r p t i o n edge of t h a t e l e m e n t , i t s s c a t t e r i n g power can be s t r o n g l y a l t e r e d , w h i l e l e a v i n g t h a t of the other elements unchanged. ASAXS i s p a r t i c u l a r l y u s e f u l f o r ionomers, s i n c e t h e elements of g r e a t e s t i n t e r e s t a r e t h e c a t i o n s , many of whose a b s o r p t i o n edges l i e i n an energy range a v a i l a b l e a t current synchrotron sources. Another v a l u a b l e t o o l i s extended x - r a y a b s o r p t i o n f i n e s t r u c t u r e (EXAFS) s p e c t r o s c o p y . T h i s a b s o r p t i o n t e c h n i q u e makes use of t h e m o d u l a t i o n i n a b s o r p t i o n c o e f f i c i e n t above an element a b s o r p t i o n edge, due t o b a c k s c a t t e r i n g of t h e e j e c t e d p h o t o e l e c t r o n by n e i g h b o r i n g atoms. By s u i t a b l e d a t a t r e a t m e n t , i t i s p o s s i b l e t o determine t h e e l e m e n t a l t y p e , number, and d i s t a n c e of t h e atoms coordinated t o the absorbing c a t i o n . Thus, EXAFS probes the a t o m i c - s c a l e s t r u c t u r e i n t h e m a t e r i a l , w h i l e SAXS and ASAXS probe the microdomain-scale s t r u c t u r e . By u s i n g b o t h s c a t t e r i n g and abs o r p t i o n t e c h n i q u e s , a f u l l p i c t u r e of ionomer morphology may be developed. To demonstrate t h e u t i l i t y of t h e x - r a y a n a l y s i s t e c h n i q u e s i n t h e s t u d y of ionomers, two r e c e n t r e s e a r c h p r o j e c t s w i l l be discussed. The f i r s t [ 1 1 ] employs SAXS and EXAFS t o e x p l a i n unexpected features in the mechanical behavior of a series of c a r b o x y - t e l e c h e l i c p o l y i s o p r e n e ionomers n e u t r a l i z e d w i t h d i v a l e n t c a t i o n s . The second p r o j e c t [ 1 2 ] uses ASAXS t o i d e n t i f y t h e origin of t h e z e r o - a n g l e u p t u r n i n s c a t t e r e d i n t e n s i t y commonly o b s e r v e d f o r ionomers.

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MULTIPHASE POLYMERS: BLENDS AND IONOMERS

Structure-Property Relationships in Telechelic Polvisoprenes S y n t h e s i s and C h a r a c t e r i z a t i o n . The s y n t h e s i s o f c a r b o x y - t e l e c h e l i c polyisoprenes was c a r r i e d o u t a s d e s c r i b e d p r e v i o u s l y [ 7 ] , i n t e t r a h y d r o f u r a n a t -78 C u s i n g sodium n a p h t h a l i d e a s i n i t i a t o r . A p p r o x i m a t e l y 2-3 u n i t s o f α-methylstyrene per l i v i n g end were added a f t e r c o m p l e t i o n o f t h e i s o p r e n e p o l y m e r i z a t i o n t o reduce t h e r e a c t i v i t y o f t h e c h a i n ends, which were t h e n t e r m i n a t e d b y t h e a d d i t i o n of gaseous C 0 The f u n c t i o n a l i t y o f t h e s e polymers i s 1.90, as d e t e r m i n e d b y p o t e n t i o m e t r i c t i t r a t i o n , and t h e m o l e c u l a r w e i g h t was 8000 a s d e t e r m i n e d by g e l p e r m e a t i o n chromatography. The m a t e r i a l s were n e u t r a l i z e d w i t h 95% o f t h e s t o i c h i o m e t r i c m e t a l methoxide ( C a , S r ) o r a c e t a t e ( N i , Zn, Cd) i n t o l u e n e s o l u t i o n , and t h e methanol o r a c e t i c a c i d b y p r o d u c t was removed b y a z e o t r o p i c distillation. F i n a l l y , a p p r o x i m a t e l y 1% b y w e i g h t o f t h e a n t i o x i d a n t I r g a n o x 1010 was added t o p r e v e n t chemical c r o j s l i n k i n g of the polyisoprene u n i t s . The m a t e r i a l s were f o u n d b y H FTNMR t o c o n t a i n t h e p o s s i b l e 3,4/1,2/1,4 r e p e a t u n i t s i n t h e r a t i o 59/41/0, and g l a s s transition t e m p e r a t u r e s measured b y d i f f e r e n t i a l s c a n n i n g c a l o r i m e t r y were found t o l i e i n t h e range 15-19°C r e g a r d l e s s o f c a t i o n . Specimens f o r t e s t i n g were compression-molded f o r f i v e m i n u t e s a t 70 C i n t o s h e e t s a p p r o x i m a t e l y 0.5 mm t h i c k , o r d i s k s a p p r o x i m a t e l y 2 mm thick. A l l samples were s t o r e d i n a d e s i c c a t o r over CaSO, u n t i l 4 use. Instrumental Conditions. Dogbone samples f o r t e n s i l e t e s t i n g were stamped o u t w i t h a s t a n d a r d ASTM D1708 d i e . Measurements were p e r formed i n a i r , a t 30 C, on an I n s t r o n TM a t a c r o s s h e a d speed o f 0.5 i n . / m i n . A l l r e p o r t e d d a t a a r e t h e average o f t h r e e t e s t s . The small-angle x - r a y s c a t t e r i n g (SAXS) e x p e r i m e n t s were p e r formed w i t h an E l l i o t GX-21 r o t a t i n g anode x - r a y g e n e r a t o r operated w i t h a copper t a r g e t a t 40 kV a c c e l e r a t i n g p o t e n t i a l and 15 mA emis s i o n c u r r e n t . Cu Κα x - r a y s were s e l e c t e d b y f i l t e r i n g w i t h nickel foil and b y p u l s e - h e i g h t a n a l y s i s a t t h e d e t e c t o r . An Anton-Paar compact K r a t k y camera was used t o c o l l i m a t e t h e x - r a y s i n t o a l i n e m e a s u r i n g 0.75 cm b y 100 /im. The s c a t t e r e d x - r a y s were d e t e c t e d w i t h a TEC 211 l i n e a r p o s i t i o n s e n s i t i v e d e t e c t o r , p o s i t i o n e d a t a sample-to-dejector d i s t a n c e o f 60 cm f o r a q range ^=4π8ΐη0/λ) o f 0.15-5.4 nm The d a t a were c o r r e c t e d f o r d e t e c t o r sensitivity, empty beam s c a t t e r i n g , and sample a b s o r p t i o n . A moderate amount o f c u b i c s p l i n e smoothing was a p p l i e d t o t h e d a t a , which were t h e n desmeared by the iterative method o f L a k e [ 1 9 ] u s i n g an e x p e r i m e n t a l l y - d e t e r m i n e d w e i g h t i n g f u n c t i o n . The d a t a was p l a c e d on an a b s o l u t e i n t e n s i t y s c a l e b y comparison w i t h a c a l i b r a t e d L u polen polyethylene standard[13]. To e l i m i n a t e s c a t t e r i n g from t h e r m a l f l u c t u a t i o n s , t h e d a t a i n t h e range 3.0-5.0 nm were f i t t o Porod's Law[20] p l u s a c o n s t a n t background term. T h i s background was t h e n s u b t r a c t e d from t h e c u r v e s . The t r a n s m i s s i o n extended x - r a y absorption f i n e structure (EXAFS) s p e c t r a were c o l l e c t e d on t h e A-2, C - l , and C-2 s t a t i o n s o f t h e C o r n e l l H i g h Energy S y n c h r o t r o n Source (CHESS). Data r e d u c t i o n f o l l o w e d a s t a n d a r d p r o c e d u r e o f pre-edge and post-edge background r e m o v a l , e x t r a c t i o n o f t h e EXAFS o s c i l l a t i o n s x ( k ) , t a k i n g t h e F o u r i e r - t r a n s f o r m o f x ( k ) , and f i n a l l y a p p l y i n g an i n v e r s e t r a n s f o r m


2 >

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to i s o l a t e the space[14-16].

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X-Ray Analysis of Ionomers EXAFS

c o n t r i b u t i o n from a s e l e c t e d r e g i o n i n r e a l

T e n s i l e T e s t i n g R e s u l t s . S t r e s s - s t r a i n c u r v e s a r e shown i n F i g u r e 1 f o r a l l f i v e m a t e r i a l s , where d i f f e r e n c e s between t h e m a t e r i a l s neu t r a l i z e d w i t h d i f f e r e n t c a t i o n s can be c l e a r l y seen. Note t h a t a l l t h e c a t i o n s used h e r e a r e d i v a l e n t , so t h a t t h e c a t i o n charge i s not a variable. F i g u r e l a shows t h e m a t e r i a l s n e u t r a l i z e d w i t h t h e a l k a l i n e e a r t h c a t i o n s Ca and S r , w h i l e t h o s e n e u t r a l i z e d w i t h the t r a n s i t i o n m e t a l c a t i o n s N i , Zn, and Cd a r e shown i n F i g u r e l b . Note t h a t t h e Cd ionomer a c t u a l l y extended t o a p p r o x i m a t e l y 1600%, and e x h i b i t e d e s s e n t i a l l y no r e c o v e r y upon b r e a k i n g . The Ca and N i m a t e r i a l s , by c o n t r a s t , snapped back t o w i t h i n 40% of t h e i r un s t r e s s e d l e n g t h a f t e r b r e a k i n g . The Zn and Sr t e l e c h e l i c s e x h i b i t e d intermediate behavior. The z e r o - s t r a i n Young's m o d u l i a r e l i s t e d i n Table I. Table I. Cation

Ε

Ca Sr Ni Zn Cd

T e l e c h e l i c T e n s i l e T e s t i n g and SAXS M o d e l l i n g R e s u l t s (MPa)

4.4 5.6 3.7 3.2 3.2

(nm)

1.06 1.04 0.95 0.92 0.90

R

2

(nm)

2.01 1.95 2.02 2.10 2.06

ν

(nm

262 250 205 194 197

)

PI"PQ

(

163 139 208 169 185

NM

)

N

34 33 27 25 26

The two a l k a l i n e e a r t h m a t e r i a l s have an average modulus n e a r l y 50% h i g h e r t h a n t h e average of t h e t h r e e t r a n s i t i o n m e t a l m a t e r i a l s , d e s p i t e h a v i n g i d e n t i c a l i o n c o n t e n t s and m o l e c u l a r w e i g h t s . Moreo v e r , t h e s e modulus v a l u e s a r e h i g h e r t h a n t h e v a l u e s p r e d i c t e d by rubber e l a s t i c i t y t h e o r y , i f we c o n s i d e r t h e e f f e c t i v e network e l e ments t o be t h o s e p o l y i s o p r e n e c h a i n s t e r m i n a t e d a t b o t h ends w i t h m e t a l c a r b o x y l a t e groups. In t h i s case, the z e r o - s t r a i n t e n s i l e modulus Ε i s g i v e n b y [ 1 7 ] : Ε = 3[(g-2)/g]i/RT

(1)

where g i s t h e c r o s s l i n k f u n c t i o n a l i t y , u i s t h e d e n s i t y o f e l a s t i c a l l y e f f e c t i v e c h a i n s , R i s t h e gas c o n s t a n t , and Τ i s the a b s o l u t e t e m p e r a t u r e . The b r a c k e t e d q u a n t i t y r e f l e c t s t h e m o b i l i t y of t h e c r o s s l i n k s w i t h i n t h e m a t r i x . Assuming t h a t a l l c h a i n s with m e t a l c a r b o x y l a t e groups a t b o t h ends a r e e l a s t i c a l l y e f f e c t i v e , and t h a t t h e 5% of a l l c h a i n ends which a r e not c a r b o x y l a t e groups are randomly d i s t r i b u t e d among a l l t h e c h a i n |nds, t h e u v a l u e f o r t h e s e t e l e c h e l i c s i s a p p r o x i m a t e l y 107 /imole/cm . I f we f u r t h e r assume t h a t t h e f u n c t i o n a l i t y of an i o n i c c r o s s l i n k i s l a r g e , such t h a t t h e b r a c k e t e d q u a n t i t y i n E q u a t i o n 1 approaches u n i t y , we c a l c u l a t e an upper bound on Ε e q u a l t o 0.81 MPa. The o b s e r v e d v a l u e s l i e i n t h e range 3.2-5.6 MPa, so some f a c t o r must be a c t i n g t o enhance t h e mod ulus. Because of t h e r e l a t i v e l y low c o n c e n t r a t i o n o f i o n i c g r o u p s , t h e i o n i c a g g r e g a t e s do not occupy a s i g n i f i c a n t f r a c t i o n of the materials volumes, so t h e f i l l e r e f f e c t i s s m a l l , as w i l l be shown 1


424


i n t h e f o l l o w i n g s e c t i o n . Moreover, t h e volume f r a c t i o n o f i o n i c a g g r e g a t e s s h o u l d be n e a r l y t h e same f o r a l l f i v e m a t e r i a l s . S t r a i n - h a r d e n i n g b e h a v i o r can a l s o be observed i n F i g u r e 1 f o r t h e Ca t e l e c h e l i c b e g i n n i n g near 50% e l o n g a t i o n , and t o a l e s s e r e x t e n t f o r t h e N i ionomer near 100% e l o n g a t i o n . T h i s f e a t u r e i s absent i n t h e other t h r e e m a t e r i a l s . As t h e s e t e l e c h e l i c s a r e copolymers o f t h e two t y p e s o f v i n y l a d d i t i o n , s t r e s s - i n d u c e d c r y s t a l l i z a t i o n cannot be t h e s o u r c e o f t h e s t r a i n - h a r d e n i n g , nor i s i t due t o the f i n i t e e x t e n s i b i l i t y of the primary c h a i n s , of molecular weight 8000, which s h o u l d o c c u r above 300% e x t e n s i o n . Trapped entanglements o f t e n c o n t r i b u t e a p p r e c i a b l y t o t h e modu l i o f networks formed by c h e m i c a l l y c r o s s l i n k i n g i n b u l k [ 1 8 ] . The s i t u a t i o n f o r t h e t e l e c h e l i c ionomers i s somewhat d i f f e r e n t , s i n c e t h e p r i m a r y c h a i n s a r e o f low m o l e c u l a r weight and would n o t be e x p e c t e d t o be h i g h l y e n t a n g l e d . However, entanglements c a n s t i l l form when t h e i o n i c groups a g g r e g a t e , and a l a r g e f r a c t i o n o f t h e s e a r e l i k e l y t o be i n t e r l o c k i n g " l o o p s " . I n a d d i t i o n , t r a p p e d e n tanglements between two l i n e a r c h a i n s which c r o s s a r e p o s s i b l e (as i n c h e m i c a l l y - c r o s s l i n k e d networks) o r , between a s i n g l e l o o p and a l i n e a r c h a i n , nonw o f which would be p r e s e n t i n t h e n o n i o n i c p o l y isoprene precursor. I t has l o n g been r e c o g n i z e d [ 5 ] t h a t a l a r g e f r a c t i o n o f t h e i o n i c s i t e s would, upon i o n i c a g g r e g a t i o n , c o a l e s c e i n t o t h e same a g g r e g a t e as t h e i r t o p o l o g i c a l n e i g h b o r . F o r a l i n e a r t e l e c h e l i c ionomer, t h i s c o r r e s p o n d s t o b o t h ends r e s i d i n g i n t h e same a g g r e g a t e , c r e a t i n g a " l o o p " o f polymer c h a i n . I n b u l k , t h i s l o o p w i l l be e n t a n g l e d w i t h o t h e r l o o p s , and as l o n g as t h e i o n i c a g g r e g a t e s a r e n o t d i s r u p t e d under s t r e s s , t h e s e i n t e r l o c k i n g l o o p s a r e e l a s t i c a l l y - e f f e c t i v e entanglements. These l o o p s will p u l l t a u t a t a much lower e x t e n s i o n t h a n would be r e q u i r e d t o f u l l y e x t e n d t h e e n t i r e polymer c h a i n . When t h e s e i n t e r l o c k i n g l o o p s p u l l t a u t , one o f two e f f e c t s may result. I f t h e i o n i c a g g r e g a t e s a r e h i g h l y c o h e s i v e and do n o t r u p t u r e , t h e n t h e polymer c h a i n s must b r e a k , and t h e m a t e r i a l w i l l snap back t o a p p r o x i m a t e l y i t s u n s t r e s s e d l e n g t h . I f t h e a g g r e g a t e s a r e weakly cohesive, the stressed entanglements can r e l a x by " i o n - h o p p i n g " , o r p u l l i n g t h e i o n i c groups o u t o f t h e a g g r e g a t e s . Two q u e s t i o n s r e m a i n , however: why i s t h e modulus enhancement, a t t r i b u t e d p r i m a r i l y t o t h e s e i n t e r l o c k i n g l o o p s , g r e a t e r f o r t h e Ca and S r ionomers t h a n f o r t h e N i , Zn, and Cd t e l e c h e l i c s , and why i s t h e s t r e s s - h a r d e n i n g b e h a v i o r e x h i b i t e d o n l y by t h e t e l e c h e l i c s neut r a l i z e d w i t h Ca and N i ? 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) and e x tended x - r a y a b s o r p t i o n f i n e s t r u c t u r e (EXAFS) s p e c t r o s c o p y were employed t o a d d r e s s t h e s e q u e s t i o n s . SAXS R e s u l t s . The SAXS p a t t e r n s f o r t h e t e l e c h e l i c s a r e shown i n F i g u r e 2; t h e c u r v e s have each been o f f s e t by 1200 i n t e n s i t y units for c l a r i t y . Due t o t h e low c o n c e n t r a t i o n o f i o n s i n t h e s e m a t e r i a l s , t h e y a r e poor s c a t t e r e r s compared w i t h many ionomers, b u t t h e s e c u r y e s do show two t y p i c a l f e a t u r e s : a peak, seen h e r e from 1.2-1.5 nm , and a s t e e p u p t u r n a t v e r y low a n g l e . To q u a n t i f y t h e morp h o l o g i c a l d i f f e r e n c e s between t h e m a t e r i a l s , t h e SAXS p a t t e r n s were f i t t o t h e Y a r u s s o l i q u i d - l i k e m o d e l [ 9 ] , which a t t r i b u t e s t h e peak to i n t e r p a r t i c l e scattering. A schematic diagram o f t h e Y a r u s s o model i s shown i n F i g u r e 3. The i o n i c aggregates have a c o r e r a d i u s R , b u t a r e c o a t e d w i t h an i m p e n e t r a b l e sheath o f polymer due t o t h e


425


REGISTER ET A L

Figure 2. Desmeared, background-subtracted SAXS patterns for carboxy-telechelic polyisoprenes. Curves are offset 1200 intensity units for clarity.

426


c h a i n u n i t s near t h e i o n i c groups. As a r e s u l t , t h e r a d i u s c l o s e s t a p p r o a c h , R , i s g r e a t e r t h a n R . The i o n i c a g g r e g a t e s assumed t o have a u n i f o r m e l e c t r o n d e n s i t y p^, w h i l e t h e m a t r i x an e l e c t r o n d e n s i t y p . The e q u a t i o n f o r t h e s c a t t e r e d i n t e n s i t y g i v e n by: Q

2

I/I V = (l/v )(4^/3) (p -p )^(qR )S(q,R ,v ) e

1

p

0

1

2

p

of are has is

(2)

where ν i s t h e t o t a l volume o f m a t e r i a l per i o n i c a g g r e g a t e ( r e c i p r o c a l ^ a g g r e g a t e number d e n s i t y ) , Φ(χ) = 3 ( s i n x - x c o s x ) / x , and S ( q , R , v ) i s t h e i n t e r f e r e n c e f u n c t i o n . The P e r c u s - Y e v i c k [ 2 1 ] t o t a l c o r r e l a t i o n f u n c t i o n was employed f o r S, as suggested by K i n n i n g and Thomas[22]. The Y a r u s s o model produces good f i t s of t h e ionomer peak when a p p l i e d t o s u l f o n a t e d p o l y s t y r e n e [ 9 ] and p o l y u r e t h a n e [ 6 ] ionomers, w i t h p h y s i c a l l y s a t i s f y i n g parameters. However, i t does n o t p r e d i c t t h e o b s e r v e d i n t e n s i t y u p t u r n a t v e r y low a n g l e , so o n l y d a t a above 0.65 nm was c o n s i d e r e d i n t h e f i t s . The b e s t - f i t parameters of t h e Y a r u s s o model t o t h e SAXS d a t a a r e g i v e n i n T a b l e I. Note t h a t t h e s e v a l u e s a r e s l i g h t l y d i f f e r e n t from t h o s e r e p o r t e d p r e v i o u s l y [ 1 1 ] ; t h i s i s due t o c o r r e c t i o n s t o our desmearing p r o c e d u r e and a b s o l u t e i n t e n s i t y c a l i b r a t i o n . However, s i n c e t h e d i s c u s s i o n was o r i g i n a l l y based on t r e n d s r a t h e r t h a n a b s o l u t e v a l u e s , t h e d i s c u s s i o n h e r e w i l l p a r a l l e l t h e oçiginal[ll]. The volume f r a c t i o n o f i o n i c a g g r e g a t e s , e q u a l t o (4TTR /3v ), i s l e s s t h a n 2.0% i n a l l cases. Therefore, the f i l l e r e f f e c t f l ? ] i n these m a t e r i a l s l e a d s t o l e s s t h a n 5% modulus enhancement, and t h u s cannot be t h e s o u r c e o f t h e 300-600% enhancement observed h e r e . The R^ v a l u e s f o r t h e Ca and Sr ionomers a r e l a r g e r t h a n t h o s e f o r t h e N i , Zn, and Cd ionomers, s u g g e s t i n g t h a t t h e a g g r e g a t e s i n t h e former two m a t e r i a l s c o n t a i n more i o n i c groups. However, t h e s i z e o f t h e i o n i c a g g r e g a t e s i s a l s o i n f l u e n c e d by t h e s i z e o f t h e n e u t r a l i z i n g c a t i o n , any hydrocarbon i n c o r p o r a t e d i n t o t h e a g g r e g a t e s , and any water absorbed from t h e atmosphere. These t e l e c h e l i c ionomers s h o u l d be h i g h l y p h a s e - s e p a r a t e d [ 8 ] , due t o t h e r e g u l a r i t y of t h e c h a i n a r c h i t e c t u r e . Assuming t h a t a l l t h e i o n s r e s i d e i n agg r e g a t e s , a more r e l i a b l e i n d i c a t o r of t h e number o f c a t i o n s per i o n i c aggregate i s ν . Based on t h e m o l e c u l a r w e i g h t , f u n c t i o n a l i t y , and n e u t r a l i z a t i o n l e v e l o f t h e s e t e l e c h e l i c s , t h e r e i s an average o f 0.13 ions/nm . M u l t i p l y i n g t h i s f a c t o r by t h e ν v a l u e y i e l d s t h e number of i o n s per a g g r e g a t e , n, l i s t e d i n T a b l e I ? There i s a c l e a r d i v i s i o n i n η between t h e Ca and Sr m a t e r i a l s (n=33-34) and t h e N i , Zn, and Cd m a t e r i a l s (n=25-27). L a r g e r v a l u e s of ν and η i n d i c a t e t h a t t h e a g g r e g a t i o n p r o c e s s has proceeded 1


2

furtRer

for

This w i l l , concluded t h e Ca and l o o p s as a

the

Ca

and

Sr

t e l e c h e l i c s , forming larger

aggregates.

i n t u r n , l e a d t o more t r a p p e d entanglements. I t can be t h a t t h e h i g h e r s m a l l - s t r a i n t e n s i l e m o d u l i observed f o r Sr m a t e r i a l s i s due t o a g r e a t e r d e n s i t y of interlocking r e s u l t o f t h e l a r g e r aggregates i n t h e s e m a t e r i a l s .

EXAFS R e s u l t s . I n o r d e r t o e s t i m a t e t h e c o h e s i v e n e s s of t h e i o n i c a g g r e g a t e s , we a p p l i e d EXAFS s p e c t r o s c o p y t o examine t h e c o o r d i n a t i o n s t r u c t u r e about t h e c a t i o n . The EXAFS s i g n a l i s a m o d u l a t i o n of t h e x - r a y a b s o r p t i o n c o e f f i c i e n t on t h e h i g h - e n e r g y s i d e o f an e l e m e n t a l a b s o r p t i o n edge. The p h o t o e l e c t r o n s t h a t a r e e j e c t e d by t h e absorbed x - r a y s can be b a c k s c a t t e r e d by atoms c o o r d i n a t e d t o t h e


17.

REGISTER ET AL.

427


absorbing atom; s u p e r p o s i t i o n of t h e o u t g o i n g and b a c k s c a t t e r e d e l e c t r o n waves g i v e s r i s e t o an interference pattern. The EXAFS s i g n a l x(k), where k i s t h e p h o t o e l e c t r o n w a v e v e c t o r , c o n t a i n s i n f o r m a t i o n on t h e number Ν. and t y p e of atoms i n c o o r d i n a t i o n shell j, t h e d i s t a n c e R. t o ^ t h i s s h e l l , and t h e s t a t i c and v i b r a t i o n a l d i s o r d e r of t h e shelî, measured as t h e Debye-Waller f a c t o r

Multiphase Polymers: Blends and Ionomers - American Chemical

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