Polymer Science Overview - American Chemical Society

12 The Interfacial Macromolecule F. R. EIRICH

Downloaded by CORNELL UNIV on July 20, 2016 | http://pubs.acs.org Publication Date: December 10, 1981 | doi: 10.1021/bk-1981-0175.ch012

Polytechnic Institute of New York, Brooklyn, NY 11201

The now so popular r e t e l l i n g of developments of ideas which led to a discovery or new understanding, may not serve a wider purpose than that of the h i s t o r i a n , or our general interest in the drama of the growth of an idea and the dynamics of i t s formation. Still, i t is quite possible that the account of the many false s t a r t s , the many b l i n d alleys or a l t e r n a t i v e routes to a s o l u t i o n , in the unpredictable ways of i n t e l l e c t u a l connections, might stimulate some readers to take-off on a t r i p of t h e i r own. The following, then, is the story of a development s t a r t i n g from an afternoon conversation with H. Mark and still winding i t s way today through the minds of many investigators. During the f i r s t years of the 1950's at the Polymer Institute of the Polytechnic I n s t i t u t e , Brookl y n , N . Y . , and following e a r l i e r work on the v i s c o s i t y of c o l l o i d s and macromolecular s o l u t i o n s , one of my interests centered on the magnitude of the volume affected by the macromolecular c o i l s in solution as expressed by t h e i r v i s c o s i t y .

W. Kuhn

(1) a l m o s t

t w e n t y y e a r s e a r l i e r h a d shown t h a t t h e " f l o w - t h r o u g h " r e s i s t a n c e o f t h e s o l v e n t i s so h i g h t h a t t h e f l u i d w i l l move a r o u n d r a t h e r t h a n t h r o u g h a c o i l . L a t e r we h a d shown w i t h m o d e l s how t h e v i s c o s i t y o f a s u s p e n s i o n c o n t a i n i n g r i g i d r o d - l i k e p a r t i c l e s i s l o w e r e d when t h e p a r t i c l e s become f l e x i b l e ( 2 ) . T h e " f l o w - t h r o u g h " r e s i s t a n c e had j u s t been c a l c u l a t e d f o r a s p e c i a l model by D e b y e a n d B u e c h e ( 3 ) , w h i l e S c h e r a g a a n d C e r f (4) h a d

©

0097-6156/81/0175-0143$5.25/0 1981 American Chemical Society

Stahl; Polymer Science Overview ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

POLYMER SCIENCE OVERVIEW

144


p o i n t e d o u t t h a t p a r t o f t h e v i s c o s i t y and f l o w b i r e fringence of macromolecular s o l u t i o n s was n o t o n l y due t o o r i e n t a t i o n a l , b u t e v e n more t o d é f o r m â t i o n a l c o n t r i b u t i o n s f r o m t h e c o i l , a p o i n t l a t e r e l a b o r a t e d by Peterlin(5). When we c o m p a r e d t h e v i s c o s i t i e s o f s o l u t i o n s o f n a t u r a l r u b b e r and o f g u t t a p e r c h a and o f o t h e r e l a s t o mers and l a t e r o f p o l y e t h y l e n e v s . ( p o l y ) c i s - b u t a d i e n e , w i t h s u c h b u l k p r o p e r t i e s as m o d u l i , d e n s i t i e s , X-ray s t r u c t u r e s , and a d h e s i v e n e s s , we w e r e g r e a t l y h e l p e d i n u n d e r s t a n d i n g t h e s e b e h a v i o r a l d i f f e r e n c e s by t h e s t u d i e s o f Wood (6) on t h e t e m p e r a t u r e a n d s t r e s s d e p e n d e n t ,melt i n g and f r e e z i n g , h y s t e r e s i s o f n a t u r a l r u b b e r , and by t h e w o r k o f T r e l o a r (7) and o f F l o r y (8) on t h e e l a s t i c i t y a n d c r y s t a l l i n i t y o f e l a s t o m e r s on stretching. M o l e c u l a r s y m m e t r y a n d s t i f f n e s s among c l o s e l y s i m i l a r c h e m i c a l s t r u c t u r e s , as t h e y a f f e c t t h e e n t h a l p y , t h e e n t r o p y , and p h a s e t r a n s i t i o n s ( p e r h a p s b e s t e x p r e s s e d by AH d by Clapeyron s T

m

T

A

?

a

n

S

m~ e q u a t i o n ) , w e r e o b v i o u s l y as p o w e r f u l l y b e h a v i o r d e t e r m i n i n g f a c t o r s i n m a c r o - as i n s m a l l e r m o l e c u l e s , f o r which such s t e r i c v a r i a n t s as s u c c i n i c , a n d m a l e i c and f u m a r i c , o r s t e a r i c v e r s u s o l e i c and e l a i d i c a c i d s have been s c h o o l book examples f o r a l o n g t i m e . m

Working w i t h l e s s d i l u t e s o l u t i o n s o f elastomers one c a n n o t f a i l t o n o t i c e t h e i n f l u e n c e ( t h e s t i f f e r t h e g r e a t e r t h e e f f e c t ) o f m o l e c u l a r s t r u c t u r e on t h e o n s e t a n d c o u r s e o f n o n - N e w t o n i o n f l o w , on g e l a t i o n a n d s w e l l i n g , a n d t h e i n f l u e n c e o f t h e s o l v e n t as exp r e s s i n g i t s e l f by v i r i a l c o e f f i c i e n t s , m o l e c u l a r d i m e n s i o n s i n s o l u t i o n , s p i n n a b i l i t y , and f i l m f o r m i n g . The s e n s i t i v i t y w i t h w h i c h t h e t a c k o f a d h e s i v e s , demonstrated by p r e s s u r e s e n s i t i v e t a p e s w h i c h a t t h a t t i m e r e a c h e d t h e m a r k e t , d e p e n d s on t h e s t r u c t u r e a n d c o m p o s i t i o n o f t h e e l a s t o m e r was s i m i l a r l y striking and r a i s e d t h e q u e s t i o n , w h i c h m o l e c u l a r s t r u c t u r e o r s t a t e was b e s t s u i t e d t o e x h i b i t t a c k y a d h e s i o n , o r adhesion per se. F r o m my w o r k i n s u r f a c e a n d c o l l o i d c h e m i s t r y , I was a w a r e o f t h e e n e r g e t i c s a n d m e c h a n i c s o f w e t t i n g and s p r e a d i n g . I t f o l l o w e d that cis-compounds w i t h t h e i r greater entropy of fusion and f l e x i b i l i t y m u s t a l w a y s s p r e a d b e t t e r and s t a y t a c k i e r on t h e s u r f a c e under a g i v e n set of c o n d i t i o n s . I discussed this with D r . M a r k d u r i n g an a f t e r n o o n s t r o l l i n 1951. He agreed and a l s o p o i n t e d o u t t h a t c i s - c h a i n s w i t h t h e i r more



12.

EIRICH

Interfacial

Macromolecule

145

convoluted c o i l s would form thicker surface layers than the s t i f f rods of the trans forms and would not contain their ordered shear planes. At that time also data just published by Jenkel and Rumbach (9), and some e a r l i e r work of other authors (10), showed that polymers were adsorbed from dilute solutions i n quantities amounting to 10 to 20 monolayers of monomeric units which indicated either a dense, semicrystalline packing or, more l i k e l y , long segments or loops dangling from the adsorption sites. "When these macromolecular c o i l s collapse on removal of the solvent, or are being applied i n bulk (melt) to the surface, the wetting should then occur by p a r t i a l l y adsorbed molecules wliich extend into the adjacent phase. Surface layers thus structured should assist adhesion. In view of the great importance of adsorption and adhesion for a wide range of natural phenomena, including biological, pharmaceutical and industrial processes, I undertook a study of adsorption that was to continue for over 20 years (II). Exploring the adsorption of macromolecules and i t s underlying mechanism has since, because of i t s implications (see Table I ) , attracted many other workers and become a major f i e l d of theoretical and experimental study. I f i r s t approached my theoretition friend and co-WDrker of many years, R. Simha, for statistical-mechanical assistance, and we obtained further the cooperation of H. Prisch, then just completing his Ph.D. at the Polytechnic Institute. The model we evolved was that of a macromolecule i n solution c o l l i d i n g f i r s t with one of i t s segments with a solvent-solid interface, becoming adsorbed when a complicated set of energetics becomes negative. This set included the free energies of desolvation of the adsorption sites and adsorbed segment, the free energy of the segmental adsorption step, and the loss of conformational entropy when the rest of the c o i l becomes restrained i n i t s number of available conformations. Once one segment i s held, further segments w i l l at f i r s t be more easily adsorbed, followed later by a greater r e s i s tance against adsorption when the restraints on the chain conformations become severe.


146



The e x t e n t o f e q u i l i b r i u m a d s o r p t i o n o f a g i v e n c h a i n , t h u s , w o u l d depend on t h e c o u n t e r a c t i o n between t h e t h r e e e n t h a l p i c and e n t r o p i e components, p l u s t h e negative entropie contribution of the chain r e s t r a i n t . The r a t e o f a d s o r p t i o n s h o u l d be f a i r l y f a s t , i n f a c t , a c c e l e r a t e d a f t e r t h e f i r s t s e g m e n t was h e l d , w h i l e t h e r a t e o f d e s o r p t i o n r e q u i r i n g s i m u l t a n e o u s m u l t i p l e des o r p t i o n s t e p s s h o u l d be v e r y s l o w , s o s l o w i n d e e d a s t o be b a r e l y m e a s u r a b l e e x c e p t when a s s i s t e d b y a d i s placing species. B a s e d on t h i s , t h e t h e o r y d e v e l o p e d by F r i s c h and S i m h a ( 1 2 ) a c c o u n t e d f o r t h e a l r e a d y known t e m p e r a t u r e and s o l v e n t d e p e n d e n c i e s a n d a l s o t h e number a n d l e n g t h of the attachments. I t gave f u r t h e r t h e n o t y e t obs e r v e d , and n o t r e a d i l y e v i d e n t , r e s u l t t h a t t h e l e n g t h of the unattached l o o p s w o u l d be p r o p o r t i o n a l t o t h e m a c r o m o l e c u l a r w e i g h t (MW) t o t h e 1/2 p o w e r ; t h e amount a d s o r b e d s h o u l d show o n l y a weak d e p e n d e n c e o n t h e macromolecular weight. For weakly adsorbed macromolecules, that i s u s u a l l y f o r p h y s i c a l a d s o r p t i o n i n the absence o f s t r o n g s u r f a c e - s e g m e n t i n t e r a c t i o n s u c h as a c i d b a s e r e a c t i o n s , s a l t - o r complex-formation, or o f porous surf a c e s w i t h much e n t r a p m e n t , o r o f s u r f a c e condensation reactions etc., the extension of p a r t i a l l y held c o i l s i n t o t h e s o l v e n t may be a l a r g e f r a c t i o n o f t h e d i m e n sion of the free c o i l s i n s o l u t i o n . Over t h e y e a r s , t h i s t h e o r y h a s b e e n much d e b a t e d , h a s b e e n i m p r o v e d , a l t e r n a t i v e t h e o r i e s d e v e l o p e d b y many o t h e r authors ( 1 3 ) , s p e c i a l f e a t u r e s and l i m i t i n g c a s e s analyzed; much e x p e r i m e n t a l w o r k was s t i m u l a t e d ( 1 4 ) w h i c h a d d e d a g r e a t d e a l o f i n f o r m a t i o n b u t a l s o i n t r o d u c e d new complications f o r our understanding. B e f o r e o u r t h e o r y was f u l l y d e v e l o p e d , e x t e n s i v e w o r k b y J . K o r a l i n c o o p e r a t i o n w i t h R. U l l m a n ( 1 5 ) confirmed i n d e t a i l and w i t h c o n s i d e r a b l e a c c u r a c y a l l p r e v i o u s l y known f e a t u r e s . They a s c e r t a i n e d , i n a d d i t i o n , the p a r t i c u l a r s o f the adsorption isotherms fora number o f p o l y m e r s a n d d i s p e r s e d a d s o r b a t e s and e s t a b l i s h e d t h e r e m a r k a b l e d e g r e e t o w h i c h most isotherms c o u l d be a p p r o x i m a t e d b y 2 - p a r a m e t e r e q u a t i o n s , like Langmuir's isotherm f o r monolayers o f small molecules. T h e y f o u n d t h e d e p e n d e n c e o f t h e a d s o r p t i o n on MW t o be weak a n d d e t e r m i n e d t h e a r e a p e r a d s o r b e d m o l e c u l e . The c o n c e p t o f i n t e r m o l e c u l a r r e p u l s i o n a n d c o m p r e s s i o n w i t h i n t h e a d s o r b e d m o n o l a y e r and between t h e s u r f a c e l a y e r s o f 2 d i s p e r s e d p a r t i c l e s was a l s o invoked.



12.

EIRICH

Interfacial

Macromolecule

147

However, t h e most e s s e n t i a l i n f o r m a t i o n f o r a p h y s i c a l p i c t u r e of the adsorbed l a y e r s ( F i g . 1), t h a t of t h e i r dimensions p e r p e n d i c u l a r to the a d s o r b i n g face, e . g . , t h e l e n g t h o f t h e l o o p s , was l a c k i n g up t o t h a t time. Experiments y i e l d i n g such i n f o r m a t i o n are diffic u l t to design. E v e n t u a l l y , I d e c i d e d on h y d r o d y n a m i c d e t e r m i n a t i o n s which, w h i l e not y i e l d i n g the a c t u a l , but o n l y the e f f e c t i v e , t h i c k n e s s e s of the adsorbed l a y e r s , a r e s e n s i t i v e t o t h e MW and t h e s o l v e n t . The work o f O e h r n ( 1 7 ) , and o f T i j n m a n and Hermans (17) s h o w e d t h a t t h e r e was p r o m i s e i n o b t a i n i n g t h e n e e d e d d a t a f r o m f l o w m e a s u r e m e n t s b e f o r e a n d a f t e r macromol e c u l a r a d s o r p t i o n onto the c a p i l l a r y w a l l s . The drawb a c k was that adsorption isotherms in individual capill a r i e s , needed to o b t a i n the n e c e s s a r y i n f o r m a t i o n , are most d i f f i c u l t t o e s t a b l i s h . T h u s , we b e g a n t o w o r k with suspensions of n e a r l y i s o d i a m e t r i c pigment part i c l e s ( T 1 O 2 , CaC03, S i 0 2 , c a r b o n b l a c k ) f o r w h i c h t h e r e l a t i o n b e t w e e n t h e i r o r i g i n a l d i m e n s i o n s and a v e r a g e v o l u m e s and t h e i r s e d i m e n t a t i o n and d i f f u s i o n r a t e s , a n d i n t r i n s i c v i s c o s i t i e s w e r e w e l l known. The adsorpt i o n isotherms f o r p o l y m e r f r a c t i o n s on d i s p e r s i o n s o f the p r e v i o u s l y d e f i n e d p a r t i c l e s were t h e n e s t a b l i s h e d . U n f o r t u n a t e l y , the s e t t l i n g r a t e s o f p u r e and coated p a r t i c l e s c o u l d n o t be u s e d b e c a u s e b o u n d a r i e s o f s e t t l i n g o r r i s i n g swarms a r e b e s e t by c o n v e c t i o n i n stabilities. T h u s we d e t e r m i n e d t h e v i s c o s i t y o f p i g ment d i s p e r s i o n s , o f p r e d e t e r m i n a t e d Einstein factors a n d o f known s u r f a c e a r e a s , s t a b i l i z e d f i r s t w i t h the a i d o f s m a l l soap m o l e c u l e s . T h e same p i g m e n t d i s p e r s i o n s were m e a s u r e d a f t e r b e i n g s t a b l y d i s p e r s e d by adsorbed polymers (18). From t h e i n c r e a s e i n v i s c o s i t y w i t h i n c r e a s i n g MW o f t h e p o l y m e r , t h e t h i c k n e s s e s o f t h e a d s o r b e d l a y e r s w e r e c a l c u l a t e d a n d f o u n d t o be approximately equal to the c o i l diameters i n s o l u t i o n . By k n o w i n g t h e s u r f a c e a r e a s , t h e l a y e r d e n s i t i e s , o r t h e d e g r e e o f s u r f a c e c o v e r a g e r e s p e c t i v e l y , c o u l d be d e t e r m i n e d and, i n p a r t i c u l a r , t h e d e p e n d e n c e on MW w h i c h i n d e e d t u r n e d o u t t o be c l o s e t o MW^/^; e v e n more i n t e r e s t i n g l y , t h e l a y e r t h i c k n e s s e s w e r e e x a c t l y p r o p o r t i o n a l to the i n t r i n s i c v i s c o s i t i e s of f r a c t i o n s of the adsorbed polymer. T h i s proved not o n l y t h a t the c o i l s w e r e h e l d by r e l a t i v e l y few s e g m e n t s , b u t a l s o that the conformations d a n g l i n g or l o o p i n g i n t o the s o l v e n t w e r e a f f e c t e d b y MW, s o l v e n t , or temperature w i t h r e s p e c t t o t h e i r d i m e n s i o n s i n t h e same way, as i f t h e y were f r e e i n s o l u t i o n . I t s h o u l d be n o t e d t h a t , a l t h o u g h t h e t h e o r y o f a d s o r p t i o n has been d e v e l o p e d

American Chemical Society Library 1155 ieth St. N. w. Stahl; PolymerD.Science Washington, C. Overview 20036

ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

148


TABLE I


Adsorption

o f (or onto) polymers p l a y s a key r o l e i n :

Adhesion Coatings Paints Lamination Reinforcements Emulsions Suspensions Detergent a c t i o n Flotation D r i l l i n g and c u t t i n g Solid lubricants Crystallization Precipitation Agglomeration

Corrosion Aging o f composites Crack r e s i s t a n c e Drag r e d u c t i o n Textile finishing Flocculants Chromatography S o i l structure Films and membranes Biological agglutination Immune r e a c t i o n s C e l l recognition Drug d i r e c t i o n Genetic reproduction

Figure 1. Hypothetical conformations of chain molecules (16) adsorbed on solidliquid interfaces: (a) lying on surface; (b) standing on end, (c) looping, (a) coiled, (e) flat multilayer. a,c,d single chains adsorbed with decreasing affinity; b,e, condensed surface layers.


12.

EIRICH

Interfacial

Macromolecule

149

f o r f l a t s u r f a c e s , the c u r v a t u r e o f s m a l l e r p a r t i c l e s o f f e r s no d i f f i c u l t i e s i n p r i n c i p l e . P a r t i c l e s smaller than the c o i l s become e n g u l f e d w i t h i n the l a t t e r . Only when the p a r t i c l e s s i z e s become comparable to t h o s e o f the adsorbed t r a i n s , w i l l s t e r i c h i n d e r a n c e s a f f e c t the l a t t e r so t h a t fewer segments and m o l e c u l e s adsorb p e r a r e a ; perhaps, i t would now be more c o r r e c t to say t h a t the p a r t i c l e s become adsorbed on the c h a i n s . We have l i t t l e i n f o r m a t i o n on the way low m o l e c u l a r weight m o l e c u l e s and o l i g o m e r s adsorb (19). A p p a r e n t l y below DP s o f about 100 they l i e f l a t on the s u r f a c e f o r c o n c e n t r a t i o n s up to a monolayer o f segments, then seem to form t h i c k e r i s l a n d s o f s m e c t i c or nematic s t r u c t u r e . Ordered condensed mono, - d i , -or m u l t i - l a y e r s are p r i m a r i l y the arrangements o f s m a l l e r , e s p e c i a l l y amphipat i c m o l e c u l e s on l i q u i d - l i q u i d i n t e r f a c e s . Polymers are too l a r g e to adsorb, i n the o r d i n a r y sense, on m i c e l l e s but segments o f l i n e a r polymers may a c t as n u c l e a t i o n c e n t e r s f o r m i c e l l e s o f s m a l l m o l e c u l e s which p r o b a b l y i s one o f the mechanisms f o r the l i p i d - , or d e t e r g e n t - , polymer i n t e r a c t i o n .


1

These p r i n c i p a l r e s u l t s were c o n f i r m e d by f u r t h e r work o f Rowland (19), Wadman (20), Chough (21), Chao (22) and K u d i s h (23) employing a c a p i l l a r y ( f r i t t e d g l a s s or s i n t e r e d s t e e l wool f i l t e r ) f l o w method. Their r e s u l t s showed a g a i n t h a t f o r MW's s u f f i c i e n t l y h i g h to f o l l o w F l o r y ^ v i s c o s i t y r e l a t i o n (24), the l o o p s r e s ponded to changes i n temperature, s o l v e n t , and a d d i t i v e s i n p r a c t i c a l l y the same way as i f they were f r e e . This r e s u l t means f u r t h e r t h a t f o r s o - c a l l e d p h y s i c a l adsorpt i o n ( n o n - s p e c i f i c i n t e r a c t i o n ) the d a n g l i n g c h a i n port i o n s are l o n g enough and the r e s t r a i n t s on them weak enough, to a l l o w them to behave l i k e random c o i l s . For s t r o n g e r i n t e r a c t i o n s , e.g., between charged s u r f a c e s and o p p o s i t e l y charged p o l y e l e c t r o l y t e s , the s i t u a t i o n may be v e r y d i f f e r e n t as i n d i c a t e d f i r s t by the work on such i s o t h e r m s o f L o p a t i n (25) and L a u r i a (26), to be d i s c u s s e d f u r t h e r below. The f i n a l p r o o f o f the p h y s i c a l r e a l i t y o f our model o f macromolecular a d s o r p t i o n was p r o v i d e d by s i m u l t a n e o u s independent work, by an e n t i r e l y d i f f e r e n t method, at the N a t i o n a l Bureau o f Standards (27). Stromberg e t a l . , a l l o w e d p o l y s t y r e n e f r a c t i o n s , b e s i d e s o t h e r polymers, to become adsorbed on f e r r o c h r o m e p l a t e s and determined the t h i c k n e s s e s o f the adsorbed l a y e r s


150

POLYMER SCIENCE

OVERVIEW

by e l l i p s o m e t r y w i t h t h e h e l p o f assumptions on t h e segment d e n s i t y d i s t r i b u t i o n . They found t h e t h i c k nesses a g a i n t o be p r o p o r t i o n a l t o MW^/2, except f o r the v e r y h i g h e s t MW s , i . e . , above 106 (see F i g . 2 ) . They a l s o o b s e r v e d f a s t a d s o r p t i o n , v e r y slow d e s o r p t i o n and proved the important f a c t , by working w i t h tagged polymers, t h a t a d s o r b a t e exchange o c c u r r e d r e g u l a r l y a t the i n t e r f a c e , i . e . t h a t t h i s type o f a d s o r p t i o n was truly reversible.


T

The s u b s t a n t i a l t h i c k n e s s o f adsorbed polymer has important consequences f o r many systems t h a t c o n t a i n macromolecular components. When adsorbed, e.g., on the p a r t i c l e s o f f l u i d d i s -

layers

Figure 2. Root-mean-square thickness in the plateau region of polystyrene, adsorbed on chrome ferrotype as a function of the square root of the molecular weight, in cyclohexane, 36.4°C.


12.

EIRICH

Interfacial

Macromolecule

151

p e r s i o n s so as t o cover most o f t h e p a r t i c l e s u r f a c e s macromolecules w i l l r e s i s t c l o s e i n t e r - p a r t i c l e approach and c o n t a c t s because t h i s would i n c r e a s e segmental den s i t y . Resistance to r e d u c t i o n of conformation surface g i v e s r i s e t o d i s p e r s i o n s t a b i l i t y . In o t h e r words, the s t a y i n g " c h a r a c t e r o f t h e d a n g l i n g p o r t i o n s i s the b a s i s f o r d i s p e r s i o n and emulsion s t a b i l i z a t i o n , and o f macromolecular d e t e r g e n t a c t i o n . I f t h e polymers were p o l y e l e c t r o l y t e s , s t a b i l i z a t i o n would f u r t h e r be due t o the r e p u l s i o n between t h e i r e l e c t r i c a l double l a y e r s . On t h e o t h e r hand, i f t h e d a n g l i n g c h a i n s would c a r r y 2 types o f , l o c a l l y s e p a r a t e d , groups which a r e c a p a b l e of a s s o c i a t i n g with t h e i r counterparts, t h i s could lead to mutual p a r t i c l e ( c e l l ) r e c o g n i t i o n and a s s o c i a t i o n , or t o t a r g e t e d a d s o r p t i o n o f t h e macromolecules on s p e c i f i c c e l l s , or c e l l s i t e s . B r i d g i n g , i . e . , simul taneous a d s o r p t i o n o f one long c h a i n on 2 or more par t i c l e s , l e a d s t o e a s i e r p a r t i c l e f i l t e r a b i l i t y or f l o c c u l a t i o n ; i f h y d r o p h o b i c s i d e groups, o r b l o c k s o f h y d r o p h o b i c segments, a r e p a r t o f t h e macromolecules, they w i l l adsorb p r e f e r e n t i a l l y , by h y d r o p h o b i c bonding on l e s s h y d r o p h i l i c s u r f a c e s o r s u r f a c e s i t e s , e x a c t l y analogous t o d e t e r g e n t a c t i o n : t h e polymer w i l l func t i o n now as a long c h a i n , i o n i c or n o n - i o n i c , soap. I f the h y d r o p h o b i c segment a d s o r p t i o n o c c u r s on w a t e r - a i r i n t e r f a c e s , such as o f a i r b u b b l e s , i t w i l l a l l o w f l o t a t i o n , whereas j o i n t a d s o r p t i o n on p a r t i c l e s and l a r g e l i q u i d - s o l i d i n t e r f a c e s i s t h e b a s i s f o r pigment c o a t ing. B l o c k p o l y m e r s , made up o f 2 s e c t i o n s , one o f a c o h e s i v e d e n s i t y c l o s e t o t h a t o f t h e d i s p e r s e d phase and l y i n g m o s t l y on t h e i n t e r f a c e , t h e o t h e r e x t e n d i n g i n t o the c o n t i n u o u s s o l v e n t phase, w i l l be e x c e e d i n g l y good p o l y m e r i c soaps o r c o m p a t i b i l i z e r s ( 2 8 ) .


f,

The p a r a l l e l i s m between 6 and [η] does not h o l d when [η] grows e i t h e r much f a s t e r , o r slower, than α MW^*^. The former happens f o r r a t h e r extended mole c u l e s o f low a f f i n i t y , o r f o r p o l y e l e c t r o l y t e s o f low charge a t t r a c t i o n , t h e l a t t e r f o r s t r o n g a f f i n i t i e s , e.g., due t o m o l e c u l a r group r e a c t i o n s between adsorbate and adsorbant s u r f a c e , such as p o l y e t h e r s on p o l y c a t i o n i c p a r t i c l e s , p o l y a c r y l a m i d e on s t r o n g l y hydrogen bonding, o r PVP on n e g a t i v e , s u r f a c e s and, i n p a r t i c u l a r , f o r t h e a d s o r p t i o n o f p o l y e l e c t r o l y t e s on s u r f a c e s o f o p p o s i t e n e t charge. Examples f o r the l a t t e r cases o f " b i n d i n g " by i o n p a i r i n g a r e : P o l y ( s u l f o n a t e s ) and p o l y ( a c r y l a t e s ) on c a l c i u m c a r b o n a t e (29), o r c o l l a g e n on hydroxy a p a t i t e ( 3 0 ) .


152

POLYMER

SCIENCE OVERVIEW

I n t e r e s t i n g o b s e r v a t i o n s are made, i f a p o l y m e r i c a d s o r b a t e i s c a p a b l e o f more than one mechanism o f adsorption. I f p o l y ( s t y r e n e s u l f o n a t e ) (PSS) i s adsorbed on carbon b l a c k which c o n t a i n s m u l t i v a l e n t c a t i o n s on i t s s u r f a c e , the a d s o r p t i o n depends s t r o n g l y on the n a t u r e o f the c a t i o n s and on pH, w i t h a maximum near pH 7. At v e r y a l k a l i n e p H s , the OH" i o n s compete w i t h the s u l f a t e s f o r the c a t i o n s and the PSS a d s o r p t i o n drops to the low v a l u e s o f h y d r o p h o b i c bonding between the carbon i t s e l f and the PSS backbone. The l a t t e r low v a l u e s are a l s o o b t a i n e d , i f PSS i s adsorbed on the same carbon a f t e r p u r i f i c a t i o n , i . e . when oxygen and/or the c a t i o n s were removed. T h i s a d s o r p t i o n o f PSS i s now pH independent (29).


T

The f a c t t h a t the c o n f o r m a t i o n s o f the f r e e , s o l vent immersed, segments remains the same i f , i n s t e a d o f a few adsorbed segment t r a i n s , one or a few segments are s t r o n g l y , e.g. c o v a l e n t l y , bonded to the i n t e r f a c e , i s the b a s i s f o r the u n i n h i b i t e d f u n c t i o n o f enzymes, and o t h e r p o l y m e r i c r e a g e n t s , when bound to s o l i d supports. T h i s has been important f o r many forms of s o l i d support syntheses and enzyme e n g i n e e r i n g . On the o t h e r hand, polymers t h a t grow to v e r y h i g h MW, or s t a y adsorbed on the s o l i d , or s u p p o r t e d , c a t a l y s t a f t e r they have grown and become detached from a Z i e g l e r - N a t t a s i t e , i n h i b i t monomer d i f f u s i o n to s t i l l a c t i v e s i t e s or o t h e r growing c h a i n s , and thus reduce c a t a l y s t e f f i c i e n c y . As the c o n c e n t r a t i o n o f the macromolecules i n e q u i l i b r i u m w i t h the i n t e r f a c e i n c r e a s e s , so t h a t the c o i l p o p u l a t i o n becomes crowded, a sudden r e a c h i n g o f a v e r y g e n t l y upward s l o p i n g p l a t e a u i s o b s e r v e d , but h a r d l y ever any m u l t i l a y e r f o r m a t i o n , s i n c e no p r o p e r o u t e r boundary o f the adsorbed l a y e r o f c o i l s develops (see below). I n s t e a d , as more c o i l s compete f o r the same i n t e r f a c i a l a r e a , a d s o r p t i o n of f u r t h e r m o l e c u l e s o c c u r s as interpénétration i s s t i l l r e s i s t e d , at the expense of the s i t e areas h e l d per m o l e c u l e , i . e . , the adsorbed t r a i n s become s h o r t e r , the loops l o n g e r , and the a r e a under the c o i l s s m a l l e r : the c o i l s become sideways and upwards compressed, (15) (16), F i g . 3. Experimentally,



EIRICH

Interfacial

Macromolecule

153

C Figure 3. (a) Conformation and coil interpénétration of polymer chains at the interface as a function of polymer solution concentration, or from melt (basic conformation corresponds to d of Figure 1). (b) Corresponding adsorption isotherms.


154

POLYMER

SCIENCE OVERVIEW


t h i s i s seen as a slow t h i c k e n i n g o f the adsorbed l a y e r in conjunction with gradually r i s i n g adsorption isotherms, u n t i l the number o f segments h e l d amounts to up to 10 to 12 monolayers. Such l a y e r s o f adsorbed v e r y h i g h MW polymers can r e a c h t h i c k n e s s e s o f s e v e r a l thousand nonameters and thus impede the f l o w through c a p i l l a r i e s (22) and porous media. This f a c t i s of greatest commercial importance d u r i n g t e r t i a r y o i l r e c o v e r , when the simultaneous p a r a l l e l f l o w through water b e a r i n g rock beds i s r e t a r d e d by water s o l u b l e polymers, but not the f l o w o f o i l (23). At a s t a g e a t which the d e n s i t y o f the segments per c o i l volume reaches the d e n s i t y o f a t h e t a s t a t e , c o i l interpénétration and entanglement become e x t e n s i v e ; from t h e r e on, the amounts adsorbed may r i s e s t e e p l y , as the s o l v e n t i s r e p l a c e d by polymer and an i n d e t e r minate b u i l d - u p o f l a y e r t h i c k n e s s e s and compaction p r o ceeds. Depending on the thermodynamics o f the c a s e , t h e r e a r e two c o u r s e s . I f the adsorbed polymer and the s o l v e n t are f u l l y m i s c i b l e , t h e r e f o l l o w s a c o n t i n u o u s d e n s i f i c a t i o n or s o l i d i f i c a t i o n . But i f the polymer has an e f f e c t i v e s o l u b i l i t y l i m i t , i t w i l l p r e c i p i t a t e or p l a t e - o u t on the i n t e r f a c e s , forming d i s c r e t e p a t c h e s of the a d s o r b i n g s p e c i e s . When d u r i n g e i t h e r p r o c e s s two s u r f a c e s which c a r r y such c o n c e n t r a t e d adsorbed and/or b u i l t - u p polymer l a y e r s are brought i n t o c o n t a c t , these l a y e r s w i l l merge and the s u r f a c e s adhere by autoh e s i o n . ( 3 1 ) . T h a t i s , two a d h e r i n g l a y e r s w i l l become one c o h e r e n t i n t e r l a y e r whose v i s c o s e r e s i s t a n c e (32), i f those two s u r f a c e s were to move a p a r t a g a i n , a g a i n s t f l o w i n g or s p l i t t i n g , m a k e s i t s e l f f e l t as a d h e s i o n . In a d i s p e r s e d system, c o l l i d i n g p a r t i c l e s at t h i s adsorbent l a y e r d e n s i t y w i l l aggregate i n s t e a d o f b e i n g r e p u l s e d and the p a r t i c l e s become e v e n t u a l l y imbedded i n the polymer as a c o n t i n u o u s phase: a f i l l e d reinforced composite system has been formed. On complete removal o f the s o l v e n t from the polymer, the r e m a i n i n g i n t e r l a y e r whether rubbergy, g l a s s y , or p a r t l y c r y s t a l l i n e c o n s t i t u t e s a t r u l y a d h e s i v e bond, whose v i s c o e l a s t i c c h a r a c t e r a l l o w s f o r i n t e r n a l s t r e s s r e l a x a t i o n and energy d i s s i p a t i o n , c r a z i n g , a n d c r a c k f o r m a t i o n or healing. These l a t t e r mechanism are w e l l u n d e r s t o o d f o r polymers i n b u l k and may now be seen on the m o l e c u l a r l e v e l by analogy to the n a t u r e o f t a c k (33) r e f e r r e d to above. As the p l a t e s j o i n e d by an a d h e r i n g macrom o l e c u l a r i n t e r l a y e r s e p a r a t e i n the d i r e c t i o n normal to t h e i r p l a n e s , they e x p e r i e n c e the opposing t e n s i l e



12.

EIRICH

Interfacial

Macromolecule

155

f o r c e a g a i n s t the s e p a r a t i o n due the v i s c o u s r e s i s tance a g a i n s t the inward flow of the f l u i d . I f the motion becomes so f a s t t h a t the t e n s i l e v i s c o u s s t r e s s e s r e a c h the c a v i t a t i o n s t r e s s i n the l i q u i d , v o i d s w i l l form. For narrow s e p a r a t i o n s and h i g h v i s c o s i t i e s t h i s may happen even at low s e p a r a t i o n r a t e s . I f the l i q u i d i s p u r e l y v i s c o u s , e s p e c i a l l y when i t i s shear t h i n n i n g , the v o i d s w i l l r a p i d l y merge and the r e s i s t a n c e break down at a low l e v e l o f s t r e s s . I f the f l u i d i s shear t h i c k e n i n g , e.g., s o l u t i o n s above the macromolecular en tanglement p o i n t , or t u r n i n g more g l a s s y or c r y s t a l l i n e under shear, and becomes s t r o n g r e l a t i v e to the s u r f a c e t e n s i o n , such f l u i d s are s p i n n a b l e and, i n s t e a d o f , F i g . 4, b r e a k i n g e a r l y , w i l l form s t r o n g t h r e a d s which remain a t t a c h e d to the s u r f a c e s and are being spun out f u r t h e r as the p l a t e s s e p a r a t e (38). Such " t a c k " can be s u b s t a n t i a l . Adhesion p r o p e r w i l l be f e l t , e i t h e r i f the v i s c o s i t y and the c a v i t a t i o n s t r e s s e s are h i g h enough to p r e v e n t n o t i c e a b l e f l o w , o r stop the f l u i d from break i n g under the a p p l i e d f o r c e s f o r the a p p l i e d time,or i f the f l u i d has s o l i d i f i e d and the r u p t u r e and d e w e t t i n g ( d e s o r p t i o n ) s t r e s s e s are h i g h e r than the ones a p p l i e d . Even minor debonding, i . e . , a p a r t i a l d e s o r p t i o n due to volume c o n t r a c t i o n , by c r e a t i n g f l a w s , f r o z e n s t r a i n s and s t r e s s c o n c e n t r a t i o n s along the i n t e r f a c i a l c o n t a c t w i l l weaken the m a c r o s c o p i c a d h e s i o n i n a major way. S o l i d b o d i e s do not wet. Thus, the essence of good a d h e s i o n r e s i d e s i n the c l o s e adherence of the m o l e c u l a r boundary l a y e r to the a d j o i n i n g s u r f a c e s on account of m u l t i p l e m o l e c u l a r a d s o r p t i o n i . e . , w e t t i n g which r e s i s t s r a p i d d e w e t t i n g w h i l e o t h e r s e c t i o n s o f the same m o l e c u l e s extend i n t o and are e n t a n g l e d w i t h m o l e c u l e s of the o v e r l y i n g , a l s o macromolecular, ad hesive. The a d h e s i v e must be s p i n n a b l e , or be c r o s s l i n k e d to form a f l u i d network, or be p a r t i a l l y a s t r o n g l y c o h e s i v e v i s c o e l a s t i c s o l i d f a c i n g the i n t e r f a c e s w i t h d u c t i l e domains. A c t u a l l y , the s t r e n g t h of a l l tough m a t e r i a l s , i n c l u d i n g composites, d e r i v e s from an analogous b e h a v i o r , the a b i l i t y to reduce imposed s t r e s s e s by s t r a i n r e l a x a t i o n , p a r t i c u l a r l y i n f r o n t o f c r a c k s . There, the l e v e r a g e a c t i o n by the moment of : ( c r a c k depth χ a p p l i e d f o r c e ) , t h a t i s the s t r e s s c o n c e n t r a t i o n , i s the most f r e q u e n t cause of m a t e r i a l f a i l u r e . As the c r a c k ad vances by c o n v e r t i n g excess f r e e energy of d e f o r m a t i o n i n t o s u r f a c e f r e e energy, the mode of forming of new s u r f a c e s i s analogous to the s e p a r a t i o n of two a d h e s i v e l y connected s u r f a c e s . I f the m a t e r i a l i s a macromo-



156

POLYMER

SCIENCE OVERVIEW

l e c u l a r l y e n t a n g l e d v i s c o s e l a s t i c polymer (34) , molec u l e s must become s t r u n g out f o r the c r a c k to advance. M e c h a n i c a l energy w i l l be consumed i n t h i s m o l e c u l a r movement, w h i l e any p o t e n t i a l alignment and c r y s t a l l i z a t i o n tend t o b l u n t the c r a c k and to r a i s e the modulus ahead of i t , as the m a t e r i a l changes m o r p h o l o g i c a l l y from i s o t r o p y to a n i s o t r o p y o r i e n t e d p e r p e n d i c u l a r to the c r a c k p l a n e . Cracks advance t h e r e f o r e d u r i n g b u l k c o h e s i v e f a i l u r e by e x a c t l y the same mechanism as i n a d h e s i v e f a i l u r e without debonding. Even f o r t h i s , F i g . 5., l a t t e r case W i l l i a m s has shown (35) t h a t the separat i o n of two phases along the i n t e r f a c e can be t r e a t e d by r e g u l a r f r a c t u r e mechanics. The f a c t , then, t h a t a l l good a d h e s i v e s are p o l y m e r i c d e r i v e s from t h e i r v i s c o e l a s t i c n a t u r e as w e l l as from the n a t u r e o f t h e i r adsorption. The p r o c e s s o f macromolecular a d s o r p t i o n improves the bonding, w h i l e where bonding f a i l s , v i s c o e l a s t i c i t y w i l l tend to change the f a i l u r e mode from an i n t e r f a c i a l to the b u l k t y p e . One can expand on t h i s by s a y i n g t h a t the s t a t e ments: "an a d h e s i v e bond i s as s t r o n g as the a d h e s i v e " , or " . . . . i s o n l y as s t r o n g as the i n t e r f a c e " , are t r i v i a l o n l y as l o n g as one remains i g n o r a n t o f the mechanism of how i n t e r f a c i a l bonds are s e t up. I f one s u b s c r i b e s to the g e n e r a l l y a c c e p t e d n o t i o n t h a t p h y s i c a l adsorpt i o n (absence of s p e c i a l bonds), l i k e m i x i n g , o c c u r s (36) o n l y when the sum: a^+b^-ab, becomes n e g a t i v e , where a and be are the e n e r g i e s of c o h e s i o n w i t h i n the m a t e r i a l s a and b i n b u l k , and ab r e p r e s e n t s the i n t e r a c t i o n energy, then the i n t e r f a c i a l l a y e r , ab, s h o u l d always be s t r o n g e r than the weaker component, and the break always occur w i t h i n the weaker m a t e r i a l , a or b. H o w e v e r , i n t e r f a c i a l c o n t a c t , t h a t i s w e t t i n g , i s never i d e a l or p e r f e c t . Any f l a w w i t h i n the zone of a-b c o n t a c t s a c t s as a s t r e s s r a i s e r from which c r a c k s w i l l s t a r t and e i t h e r l e a p - f r o g through n e i g h b o r i n g f l a w s c a u s i n g bonding f a i l u r e , or propagate i n t o the weaker medium ( c o h e s i v e f a i l u r e ) whichever i s e n e r g e t i c a l l y more f a v o r a b l e . Macromolecular a d s o r p t i o n o f f e r s many c o m p o s i t i o n and d e s i g n o p t i o n s by a l l o w i n g to v a r y i n t e r f a c i a l v s . i n t e r l a y e r s t r u c t u r e and s t r e n g t h . Other advantages o f p o l y m e r i c a d h e s i v e n e s s over t h a t by s m a l l m o l e c u l e s , at the same degree of w e t t i n g , are the s u b s t a n t i a l f r a c t i o n of c o v a l e n t bonds which l i e i n the immediate f i r s t p l a n e a d j a c e n t to the i n t e r f a c e , a l l o w i n g to b r i d g e s m a l l flaws by c h a i n s , and encouraging s t r e s s d i s s i p a t i o n and t r a n s m i s s i o n through t h e i r loops away from the i n t e r f a c e .


EIRICH

Interfacial

Macromolecule

157


12.

Figure 5.

Representation of stress at advancing crack tip, and below, the state of viscoelastic strain indicated by Voigt models (35).



158

POLYMER SCIENCE

OVERVIEW

A s p e c i a l l y i n t e r e s t i n g case o f i n t e r p a r t i c l e adh e s i o n i s p r e s e n t e d by b l o c k p o l y m e r s i n b u l k . They a r e s o l i d d i s p e r s i o n s i n which the component o f h i g h e r melting o r g l a s s t r a n s i o n temperature formed the f i r s t d i s c o n t i n u o u s phase, r a t h e r u n i f o r m l y d i s t r i b u t e d i n t h e sea o f the i n t e r p e n e t r a t i n g d a n g l i n g s e c t i o n s o f t h e second component. Depending on t h e volume r a t i o s o f the components, the major one w i l l e v e n t u a l l y be t h e c o n t i n u o u s phase, w h i l e t h e segments o f t h e minor component, depending on t h e amount, may arrange themselves to become s p h e r i c a l , c y l i n d r i c a l o r l a m e l l a r i s l a n d s . Such b l o c k p o l y m e r systems amount t o d i s p e r s i o n s w i t h s e c t i o n s d a n g l i n g from a s i n g l e c o v a l e n t l y bonded s i t e . Compound systems p r e c i p i t a t e d from p a r t i c l e s on which another polymer was d e n s e l y adsorbed, a r e s t r u c t u r a l l y s i m i l a r composite m a t e r i a l s . One might r a i s e t h e q u e s t i o n why we d i d not cons i d e r , o r o b s e r v e , s h e e t s o f adsorbed monolayers o f u n i f o r m l y f o l d e d m o l e c u l e s as t h e s t r u c t u r e o f t h e s e v e r e l y compressed o r h i g h l y c o n c e n t r a t e d l a y e r s t o wards t h e end o f monolayer a d s o r p t i o n , t h a t i s a k i n d of e p i t a c t i c , s m e c t i c , c o n d e n s a t i o n . We found f o r r o d l i k e m o l e c u l e s , e.g. p r o l i n I i n water ( 2 2 ) , i n d i c a t i o n s of a nematic a r r a y o f t h e adsorbed m o l e c u l e s , but never any e v i d e n c e f o r p l a n e s o f f o l d s when the polymer molec u l e s were i n t h e i r random c o i l form. Out o f hundreds of cases o b s e r v e d we encountered o n l y t h r e e i n s t a n c e s of a step-up i n the i s o t h e r m s above the f i r s t p l a t e a u to be e x p e c t e d i f an even m o d e r a t e l y o r d e r e d l a y e r o f c h a i n s e x i s t e d ; a l l t h e s e t h r e e cases were H-bonding polymers ( a c e t a t e s , a c r y l i c s , p e p t i d e s ) . When t h e second l a y e r formed, t h e s t e p i n the isotherms was i n v a r i a b l y l e s s than t h a t f o r the f i r s t monolayer l a i d down. A l s o , smooth monolayer p l a n e s , even w i t h t i e m o l e c u l e s between them, would o f f e r n a t u r a l shear p l a n e s and thus n o t n e a r l y e x h i b i t t h e observed shear s t r e n g t h of a d h e s i v e bonds which i s found t o be, o r t o exceed, the shear s t r e n g t h o f t h e a d h e s i v e i t s e l f . Finally, though e x c e l l e n t as v i s c o s i t y s t a b i l i z i n g a d d i t i v e s , polymers a r e not good l u b r i c a n t s i n themselves (except as B e i l b y l a y e r s i n s e l f l u b r i c a t i o n ) , a g a i n a s i g n t h a t the f i r s t adsorbed monolayer does not o f f e r a smooth p l a n e t o the o u t s i d e , as good adsorbed l u b r i c a n t s would do. I have r e p o r t e d so f a r o n l y on one n o n - s p e c i f i c adsorption, t h a t o f random c o i l s . Thus, o r d e r e d monolayers o f polymers were r a r e and randomly packed f i r s t l a y e r s the r u l e . There a r e o f c o u r s e many o t h e r types o f macromolecules and t h e i r a d s o r p t i o n , and an


12.

EIRICH

Interfacial

Macromolecule

159

i n f i n i t e v a r i e t y when i t comes to the a d s o r p t i o n of more than one s p e c i e s onto nonhomogeneous, or p a t t e r n e d , surfaces. I want to r e f e r here o n l y to one system, t h a t of p o l y e l e c t r o l y t e s and ampholytes on charged and un charged s u r f a c e s , n o t i n g our main o b s e r v a t i o n s : Poly e l e c t r o l y t e s adsorb o n l y i f t h e i r net charge i s o p p o s i t e to the net s u r f a c e charge; and compensation a f f e c t s o n l y the net c h a r g e s , so t h a t o p p o s i t e l y charged i n t e r f a c e s and p o l y e l e c t r o l y t e s i n t e r a c t l i k e any two p o l y e l e c t r o l y t e s , at f i r s t by an exact compensation, so p r e c i s e t h a t net s u r f a c e charges can be t i t r a t e d to n o r m a l i t i e s o f 1 0 " , or l e s s , by o p p o s i t e l y charged p o l y e l e c t r o l y t e s , u s i n g d i s p e r s i o n s t a b i l i t y , c o n d u c t i v i t y , or p o t e n t i o m e t r i c minima as i n d i c a t o r s . C o u n t e r - c h a r g e d p o l y e l e c t r o l y t e s l i e f l a t at the i n t e r f a c e , but may form l o o p s , i f they c a r r y excess c h a r g e s . For v e r y weakly charged i n t e r f a c e s and p o l y t e l e c t r o l y t e s of the same s i g n a van der Waals or h y d r o p h o b i c , bonding f o r the c o r r e s p o n d i n g s e c t i o n s o f s u r f a c e and p o l y e l e c t r o l y t e i s p o s s i b l e , w h i l e any charged s e c t i o n s of the polymer s t a y i n loops o f f the c a r r i e r s u r f a c e . Unbonded segments of polyamp h o l y t e s form e l e c t r o s t a t i c a l l y determined loops whose e x t e n s i o n s from the s u r f a c e respond c l o s e l y to the pH and s a l t c o n t e n t o f the s o l u t i o n . A g a i n , these loops p a r a l l e l the dimensions of the f r e e a m p h o l y t i c c o i l s i n s o l u t i o n , showing f o r i n s t a n c e , a minimum f o r the i s o e l e c t r i c p o i n t (23).


5

The perhaps most important case o f the e f f e c t o f the c o n f o r m a i t o n a l s t a t e , or i t s changes, on the ac t i v i t i e s of adsorbed l a y e r s i s t h a t of the g l o b u l a r p r o t e i n s , i n v i v o and i n v i t r o . The n a t i v e conforma t i o n s of t h e s e p r o t e i n s are p a r t l y h e l i c a l , α-helical, (3-sheeted or randomly c o i l e d , f i x e d i n t e r n a l l y by S-S, Η-bonding, and h y d r o p h o b i c a t t r a c t i o n . On becoming adsorbed they may or may not s t a y c o n f o r m a t i o n a l l y constant. The former i s the case f o r serum albumin whose l i n i n g o f b l o o d v e s s e l s a s s i s t s the e n d o t h e l i a l p r o s t a c y c l i n i n keeping the o t h e r b l o o d c o n s t i t u e n t s from a d h e r i n g to the w a l l s . Adsorbed on i m p l a n t e d p a r t s , serum albumins reduce b l o o d c l o t t i n g . I f they, or o t h e r plasma p r o t e i n s , become d e n a t u r e d as a r e s u l t of some damage to the organism, so t h a t t h e i r n a t i v e c o n f o r m a t i o n opens up t h e y , l i k e s y n t h e t i c l i n e a r p o l y e l e c t r o l y t e s become more e x t e n s i v e l y adsorbed on the w a l l s o f the b l o o d v e s s e l s , o r o t h e r formed elements and t h u s , become a d s o r p t i v e f o r , and t r a p , b l o o d p l a t e l e t s which then open up and t r i g g e r plaque f o r m a t i o n . I t r e q u i r e s the c o - a d s o r p t i o n o f s t r o n g l y n e g a t i v e ions l i k e c i t r a t e , or of polymers l i k e h e p a r i n , to o f f s e t the


160


a d s o r p t i v e a f f i n i t y o f denatured p r o t e i n s by o v e r l a y i n g them w i t h a r e p u l s i v e , s t a b i l i z i n g n e g a t i v e charge.


Among o t h e r methods to r e n d e r s u r f a c e s o f i m p l a n t e d materials bio-compatible, i n p a r t i c u l a r to p r e v e n t them from i n i t i a t i n g b l o o d c l o t t i n g , one may a c t i v a t e the s u r f a c e s by r a d i a t i o n and then g r a f t water s o l u b l e , or n e g a t i v e l y charged, polymers onto the a c t i v e s i t e s . These g r a f t e d polymers, though anchored c o v a l e n t l y a t one end, a c t l i k e l o o s e l y adsorbed polymers o f the same n a t u r e , except t h a t t h e i r number o f c o n t a c t p o i n t s per m o l e c u l e w i l l be s m a l l and the c h a i n s extend f u r t h e r i n to the f l u i d . I have t r i e d to r e c o u n t as a matter o f p e r s o n a l r e c o l l e c t i o n the course over two decades o f a c o n c e p t , g o d f a t h e r e d by Dr. Mark and i n c l u d i n g the i n p u t from many co-workers and c o l l e a g u e s ; a l s o to d e s c r i b e the b a s i c i d e a s and development and r a m i f i c a t i o n s o f the results. What s t a r t e d as a v e r y l i m i t e d i n q u i r y i n t o the r o l e o f macromolecules f o r a d h e s i o n , became one into, t h e i r s t a t e at i n t e r f a c e s . Since i t developed i n d e p e n d e n t l y t h a t macromolecules and i n t e r f a c e s are at the c o r e o f much o f m a t e r i a l s c i e n c e , e n g i n e e r i n g and b i o l o g y , the answers a c q u i r e d a broad s i g n i f i c a n c e . Macromolecules, p a r t l y f i x e d and p a r t l y r e a c h i n g out from an i n t e r f a c e act l i k e t e n t a c l e s , e i t h e r c o l l e c t i n g , h o l d i n g , or keeping a p a r t , the domains o f d i s p e r sed m u l t i - p h a s e d or c e l l u l a r systems. Remaining r e s p o n s i v e to the environment, they may become the mediat o r s o f i n t e r d o m a i n mechanics i n the c h e m i s t r y o f s o l i d s , o f complex r e a c t i o n sequences, and o f s t a b i l i z ing s t r u c t u r e s i n b i o l o g y . I have been p a r t i a l i n emphasizing where we b e l i e v e to have u s e f u l answers r a t h e r than where we miss them. I n e v i t a b l y , every r e s e a r c h opens more q u e s t i o n s than i t answers. Thus, we have no good way o f f i n d i n g e x p e r i m e n t a l l y the d e n s i t y d i s t r i b u t i o n o f the m o l e c u l a r segments away from the s u r f a c e i n a g i v e n c a s e , whether they extend i n t o a m o b i l e or i n t o a mixed s o l i d b u l k phase. We do not know why, d u r i n g the i n c r e a s e o f the s u r f a c e p o p u l a t i o n , t h e r e seem to be no c r i t i c a l p o i n t s at which d e n s i t y or phase changes o c c u r d i s c o n t i n u o u s l y , as one f i n d s f o r the s u r f a c e isotherms o f s m a l l molecules. Macromolecules at i n t e r f a c e s are i n a sense c r o s s - l i n k e d by way o f t h e i r a d h e r i n g segments, but t h e s e l i n k s w i l l t r a v e l d y n a m i c a l l y a l o n g the c h a i n s . W i l l t h i s be a dominant f e a t u r e o f "good adhesiveness, as w e l l as o f the m e c h a n i c a l advantage of multi-phase 11



12.

EIRICH

Interfacial

161

Macromolecule

systems w i t h macromolecular i n t e r p h a s e s over the p e r formance o f homogeneous systems? May t h e r e be m e r i t i n s t u d y i n g o t h e r systems which are a l s o c r o s s - l i n k e d i n a g l i d i n g way, e.g., i s t h i s i n essence j u s t another case of e n t a n g l e d m o l e c u l e s ? B e s i d e s the case d e s c r i b e d h e r e , t o which e x t e n t i s the s t r u c t u r e o f the f i r s t monolayer, ab, a f f e c t e d by s p e c i f i c m o l e c u l a r i n t e r a c t i o n s o f types a and b e x p l o r e d by so many s t u d i e s (36) (37)? How do polymers adsorb d u r i n g c r y s t a l l i z a t i o n on seed s u r f a c e s , as random c o i l s which o r d e r t h e m s e l v e s , or do they f o l d on a r r i v a l ? When macromol e c u l a r c o i l s , i n s t e a d on i n t e r f a c e s , a t t a c h themselves to each o t h e r , do they i n t e r p e n e t r a t e or f o l d , or o r d e r i n some f a s h i o n ? Would the l a t t e r p r o c e s s o f macromol e c u l a r a d s o r p t i o n a k i n to t h a t o f macromolecular agg r e g a t i o n s , i n c l u d i n g the r e a s s e m b l i n g o f f u s e d h e l i c a l l y i n t e r t w i n i n g macromolecules? How do s u r f a c e f e a t u r e s i n f l u e n c e the c o n f o r m a t i o n s o f the l o o p s and ,thereby , i s an amount o f " i n f o r m a t i o n " a l r e a d y n o t i c a b l e some d i s tance away from the i n t e r f a c e ? How i s the c h e m i s t r y o f r e a c t i v e groups, a l r e a d y g r e a t l y m o d i f i e d and u s u a l l y more s p e c i f i c and c a t a l y s t i c a l l y more a c t i v e when t i e d to polymers than when they are f r e e , f u r t h e r changed by adherence o f the c h a i n to an i n t e r f a c e ? Could we des i g n a c a t a l y t i c system such t h a t the a c t i v e s i t e (or enzyme) i s adsorbed on, or f i x e d t o , an i n t e r f a c e which a l s o adsorbs and c o n c e n t r a t e s the c a t a l y s t s s u b s t r a t e ? I f adsorbed c o i l s b i l l o w above a porous l a y e r , how can they snake t h e i r way t h r o u g h c h a n n e l s o n l y wide enough f o r one segment? I f a c h a i n p a s s e s t h r o u g h a number of low m o l e c u l a r m i c e l l e s , i s i t s t a b i l i z i n g or d e s t a b i l i z i n g the l a t t e r , or may i t a c t as a c o r e o f a c y l i n drical micelle? T

When p r o t e i n s are absorbed i n t o , or t o g e t h e r w i t h , l i p i d s and p h o s p h o - l i p i d s to form c e l l membranes, are the dimensions o f the amphoteric domains s e n s i t i v e enough to pH and i o n c o n c e n t r a t i o n , so t h a t by s w e l l i n g or s h r i n k i n g w i t h changing g r a d i e n t s , they may a c t as gates or r e g u l a t o r s , o f i o n i c f l o w s ? How i s c o l l a g e n adsorbed on hydroxy a p a t i t e , or v i c e v e r s a ? How does the s t a t e o f a d s o r p t i o n between the domains o f a composite m a t e r i a l a f f e c t i t s s t r e n g t h and o t h e r m e c h a n i c a l - c h e m i c a l properties? I t w i l l be seen t h a t s t a r t i n g even from our l i m i t e d i n s i g h t s i n t o the s t a t e o f adsorbed macromolecules, a l a r g e number o f i n q u i r i e s may be u n d e r t a k e n towards a b e t t e r u n d e r s t a n d i n g o f an e x t r e m e l y wide range of n a t u r a l and t e c h n o l o g i c a l l y posed phenomena. Who would


162

POLYMER

SCIENCE OVERVIEW

have expected, when we f i r s t asked why f l e x i b l e mater i a l s a r e t h e b e t t e r a d h e s i v e s , t h a t we would answer some so v e r y d i f f e r e n t q u e s t i o n s and r a i s e so many others?

References 1. 2.


3. 4. 5. 6. 7. 8. 9. 10.

11. 12.

13.

14.

15.

Kuhn, W., Z. physik. Chem., A161. 1 (1932) Sverak, J . and Eirich F., Trans. Faraday Soc.,43, 57 (1946) Debye, P. and Bueche, A . M . , J. Chem. Phys. 16, 573 (1948) Cerf, R. and Scheraga, Α . , Chem. Rev. 51, 185(1952) Peterlin, Α . , Adv. Macromol. Chem., 1, 225 (1968) Wood,L.Α., Adv. Colloid S c i . , 2, (1936); Interscience, N.Y. Treloar, L . R . G . , "The Physics of Rubber Elasticity" Clarendon Press, Oxford (1958) Flory, P.J., J . Chem. Phys., 15, 397 (1947) Jenkel, E., and Rumbach, Β . , Z. Elektro. chem., 55 612 (1951) Claesson, I . , and Claesson, S., Arkiv Kemi Mineral. Geol. 19A, No. 5 (1945) Heller, W., and Tanaka, W., Phys. Rev., 82, 302 (1951) Eirich, F . R . , J. Coll. and Surface S c i . , 58, 423 (1977) Simha, R., Frisch, H . L . , and Eirich, F . R . , J. Phys. Chem., 57, 584 (1953) Frisch, H . L . , and Simha, R., in "Rheology", Vol. I (F.R.Eirich, Ed.) p. 525, Acad. Pr. (1956) Frisch, H . L . , and Simha, R., J. Chem. Phys. 27,702 (1957) Silberberg, Α . , J. Phys. Chem., 66, 1872 (1962); Silberberg, Α . , J. Chem. Phys., 46, 1105 (1967) ibid. 48, 2835 (1968); Silberberg, Α . , Faraday Disc. No. 59, p.203 (1975) Hoeve, C . , J. Chem. Phys., 44, 1505 (1966) Hoeve, C . , DiMarcio, E., and Peyser, P . , J . Chem. Phys. 42, 2558 (1965) Roe, R., J. Chem. Phys., 43, 1951 (1965) DiMarcio, E., and Rubin, R., Amer. Chem. Soc.Polym. Preprints,11, 1239 (1970) Stromberg, R.R., in "Treatise on Adhesion and Ad hesives," I, (R.Patrick, Ed.) Dekker, N.Y. 1967 Lipatov, Yu. S., and Sergeeva, L . M . , "Adsorption of Polymers", Wiley, N.Y. 1974 Koral, L., Ullman, R., and E i r i c h , F . R . , J . Phys. Chem. 62, 5 41 (1958)


12. EIRICH Interfacial Macromolecule

16. 17. 18. 19.


20. 21. 22. 23.

24. 25. 26. 27. 28. 29. 30.

31. 32. 33. 34.

163

Eirich, F.R., in "Interface Conversion for Polymer Coatings" (Phil. Weiss, Ed.), Amer. Elsevier Co. 1969 Oehrn, O.E., J . Polym. Sci., 17, 137 (1955) Tuijnman, C.A.F., and Hermans, J.J., J . Polym. Sci. 25, 385 Rowland, F . , Bulas, R., Rothstein, E . , and Eirich, F.R., Ind. Engin. Chem., 57, 46 (1965) Rowland, F . , and Eirich, F.R., J . Polym. Sci., A l , 4, 2033, 2401 (1966) Wadman, J.W., Dissertation, Polytech, Inst., Bklyn, 1966 Chough, E . , Thesis, Polytechnic Inst., Bklyn, 1968 Chao, H.Y., Dissertation,Polytech. Inst., Bklyn, 1974 Kudish, Α., Dissertation,Polytech. Inst., Bklyn, 1972 Chao, H.Y., Chough, Ε., Earl, Ch., Lopatin, G., Wadman, J., and Eirich, F.R., preprints, 48th Natl. Colloid Symp., Austin, Tex., 1974, p. 165 Flory, P . J . , "Principles of Polymer Chemistry", Cornell Univ. Press, 1953, p. 602 ff Lopatin, G., and Eirich, F.R., Proceed. 2nd. Inter. Congr. Surface Activity", Β, p 97, 1961 Lauria, R., Dissertation, Polytechnic Inst.,Bklyn, 1962 Stromberg, R.R., Tutas, D . J . , and Passaglia, E . , J. Phys. Chem. 69, 3955 (1965) Molau, G.E., "Block Polymers" (S.L. Aggarwal, Ed.), Plenum Pr., N.Y. 1970 p. 79 Earl, Ch., Dissertation, Polytechnic Inst., Bklyn, 1970, see also Ref. 18 Chen, T.M., Report to NIH on Train. Grant DE00099, 1973 Eirich, F.R., in "Dental Adhesive Materials", (Moskowitz, Ward, Woolridge, Eds.), Brookdale Den tal Ctr. Symp., New York, 1973, p. 53 Kaelble, D., "Physical Chemistry of Adhesion", Wiley-Interscience, 1971; p.397 Eirich, F.R., and Tabor, D.,Proceed. Cambr. Philosoph. Soc., 44, 566 (1948) Beatty, J.R., Rubber Chem., Technol., 42, 1040 (1969) Eirich, F.R., Trans. Intern. Conf. Mechan. Behavior Materials, Vol. III, 405, Kyoto (1971) Eirich, F.R., Pure and Appl. Chem., 46, 115 (1976)


164

35.

36. 37.


38.


Williams, M.L., Proc. 5th U.S. Natl. Congr. Appl. Mech., ASME, (1966); Williams, M.L., in "Fracture of Solids" (D.Drucker and J. Gilman, Eds.), p.157, Wiley-Interscience, N.Y. 1963 Kaelble, D., Ref. 31, p. 489 Mark, H.F., Adhesive Age, 22,, 35, 45 (1979) Mark, H.F., Dechema Monograph, 51, 59 (1964) Adamson, A.W., "Physical Chem. of Surfaces", 2nd Ed., Interscience-Wiley, 1967 Van der Hoff, B.M.E. and Buckler, E . J . , J . Macro. Sci., 1, 747 (1967)

RECEIVED

March 16, 1981.


Polymer Science Overview - American Chemical Society

Recommend Documents