Polymer Adsorption and Dispersion Stability - American Chemical

15 Interactions Between Polymer-Bearing Surfaces JACOB KLEIN and YAACOV ALMOG Polymer Department, Weizmann Institute of Science, Rehovot 76100, Israel

Downloaded by EMORY UNIV on February 4, 2016 | http://pubs.acs.org Publication Date: February 10, 1984 | doi: 10.1021/bk-1984-0240.ch015

PAUL LUCKHAM Cavendish Laboratory, Madingley Road, Cambridge CB3 0HE, England

The forces acting between atomically smooth mica sur faces immersed i n both organic and aqueous l i q u i d media have been determined i n the range of surface separations 0 - 300 nm, both in the absence and the presence of ad sorbed polymer layers. In this way the interactions between the adsorbed macromolecular layers themselves were determined. We present results for the following cases: i ) Poor solvent, ii) Θ - solvent, iii) good solvent, iv) polyelectrolytes i n aqueous media. Our results show that interactions between the adsorbed layers may be either attractive or repulsive, depending on the nature of the solvent and the extent of adsorp t i o n . For the polyelectrolyte case the interactions are a combination of field-type (electrostatic) and s t e r i c forces. The f o r c e s a c t i n g b e t w e e n c o l l o i d p a r t i c l e s i n d i s p e r s e d s y s t e m s may be s t r o n g l y m o d i f i e d b y a d s o r p t i o n o f p o l y m e r i c o r m a c r o m o l e c u l a r l a y e r s o n t o t h e p a r t i c l e s u r f a c e s (1). T h i s e f f e c t has b e e n u t i l i z e d e m p i r i c a l l y s i n c e h i s t o r i c a l t i m e s ( a s f o r example by t h e a n c i e n t E g y p t i a n s , who s t a b i l i z e d aqueous c a r b o n b l a c k d i s p e r s i o n s b y a d s o r b e d l a y e r s o f gum a r a b l e , a w a t e r - s o l u b l e , n a t u r a l l y o c c u r r i n g p o l y s a c c h a r i d e , t o form s t a b l e i n k s ) : nowadays s u c h a d s o r b e d l a y e r s a r e commonly u s e d b o t h f o r s t a b i l i z a t i o n a n d d e s t a b i l i z a t i o n i n a w i d e r a n g e o f s y n t h e t i c (1) and n a t u r a l l y o c c u r r i n g ( 2 ) c o l l o i d a l s y s t e m s . The c r i t e r i a f o r c o l l o i d a l s t a b i l i t y depend o n s e v e r a l f a c t o r s , s u c h a s h y d r o dynamic i n t e r a c t i o n s , k i n e t i c s o f Brownian c o l l i s i o n s and t h e e x t e n t t o w h i c h t h e s u r f a c e r e g i o n s a r e i n a s t a t e o f thermo dynamic e q u i l i b r i u m , i n a d d i t i o n t o the b a s i c n a t u r e o f t h e surface-surface forces. Nonetheless, i t i sthe l a t t e r which i s i n g e n e r a l t h e dominant f a c t o r i n d e t e r m i n i n g the b e h a v i o u r 0097 6156/ 84/ 0240 0227506.00/0 © 1984 American Chemical Society

In Polymer Adsorption and Dispersion Stability; Goddard, E. D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1984.

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of systems w i t h l a r g e surface-to-volume r a t i o s , and w h i c h must be u n d e r s t o o d i n o r d e r t o h a v e f u n d a m e n t a l i n s i g h t i n t o t h e q u e s t i o n of s t a b i l i t y i n such systems. S t e r i c s t a b i l i z a t i o n and t h e r e l a t e d phenomenon o f p o l y m e r a d s o r p t i o n have been i n t e n s i v e l y s t u d i e d o v e r the p a s t t h i r t y y e a r s ( 1 ) . However, i t i s o n l y i n a v e r y few r e l a t i v e l y r e c e n t c a s e s t h a t a t t e m p t s w e r e made t o measure d i r e c t l y t h e f o r c e s between s u r f a c e s b e a r i n g adsorbed m a c r o m o l e c u l a r l a y e r s . Sur f a c e - b a l a n c e (3) and c o m p r e s s i o n - c e l l (4) t e c h n i q u e s h a v e b e e n u s e d t o measure t h e p r e s s u r e b e t w e e n c o l l o i d a l p a r t i c l e s ( b e a r i n g a d s o r b e d p o l y m e r s ) i n two- and t h r e e - d i m e n s i o n a l a r r a y s . More r e c e n t l y the d i s j o i n i n g p r e s s u r e between l a y e r s of polymer ad s o r b e d a t t h e two l i q u i d - a i r i n t e r f a c e s o f a f i l m o f p o l y m e r s o l u t i o n was m e a s u r e d as a f u n c t i o n o f f i l m t h i c k n e s s ( 5 ) ; and t h e f o r c e b e t w e e n l a y e r s o f p o l y m e r a d s o r b e d o n t o smooth r u b b e r s p h e r e s was d e t e r m i n e d as a f u n c t i o n o f t h e d i s t a n c e b e t w e e n t h e s p h e r i c a l s u r f a c e s 0 6 ) . A l l t h e s e methods w e r e l i m i t e d by n o t b e i n g a b l e t o measure a t t r a c t i o n , i f any, b e t w e e n t h e a d s o r b e d l a y e r s . I s r a e l a c h v i l i and c o - w o r k e r s ( 7 ) , m o d i f y i n g an a p p r o a c h p i o n e e r e d by T a b o r and c o - w o r k e r s 08,9) m e a s u r e d t h e i n t e r a c t i o n f o r c e (F(D) as a f u n c t i o n o f d i s t a n c e D b e t w e e n two smooth m i c a s h e e t s immersed i n an aqueous s o l u t i o n o f a commer c i a l - g r a d e r e s i n c o n s i s t i n g of h i g h l y polydisperse polyethylene o x i d e (PEO). T h e i r r e s u l t s showed s t r o n g h y s t e r e t i c e f f e c t s , and a c o n t i n u o u s b u i l d up w i t h t i m e o f a d s o r b e d l a y e r t h i c k n e s s , and c o u l d n o t be d e s c r i b e d i n t e r m s o f an e q u i l i b r i u m f o r c e l a w . Over t h e p a s t few y e a r s , b o t h a t t h e Weizmann I n s t i t u t e and i n t h e C a v e n d i s h L a b o r a t o r y , we h a v e u s e d a m o d i f i c a t i o n o f t h e 'mica a p p r o a c h t o measure t h e i n t e r a c t i o n f o r c e s F(D) b e t w e e n two m i c a s u r f a c e s , a d i s t a n c e D a p a r t , i m m e r s e d i n o r g a n i c and aqueous l i q u i d m e d i a , b o t h i n t h e a b s e n c e and i n t h e p r e s e n c e of polymer l a y e r s absorbed onto the m i c a from the l i q u i d . We have s t u d i e d a number o f m o d e l s y s t e m s c o v e r i n g a w i d e r a n g e o f c o n d i t i o n s , using monodispersed, w e l l c h a r a c t e r i z e d polymers. I n t h i s p a p e r we b r i e f l y d e s c r i b e t h e a p p a r a t u s and experimental method, then c o n s i d e r the i n t e r a c t i o n s between i ) l a y e r s of p o l y s t y r e n e i n c y c l o h e x a n e u n d e r p o o r - s o l v e n t and i i ) Θ - s o l vent c o n d i t i o n s , i i i ) t h e i n t e r a c t i o n s b e t w e e n a d s o r b e d PEO l a y e r s i n a good ( a q u e o u s ) s o l v e n t and i v ) t h e s u r f a c e f o r c e s between l a y e r s of adsorbed p o l y - L - l y s i n e , a c a t i o n i c p o l y e l e c t r o l y t e , i n aqueous s a l t s o l u t i o n s . We c o n s i d e r b r i e f l y t h e i m p l i c a t i o n s of our r e s u l t s f o r the c u r r e n t t h e o r e t i c a l u n d e r s t a n d i n g . 1

A p p a r a t u s and

Method

M u s c o v i t e m i c a may be c l e a v e d t o p r o v i d e t h i n ( -2 ;im) s h e e t s t h a t a r e m o l e c u l a r l y smooth on b o t h s i d e s and w h i c h can be u s e d as s u b s t r a t e s i n s t u d y i n g s u r f a c e f o r c e s (8). F i g u r e 1 shows s c h e m a t i c a l l y t h e e s s e n t i a l f e a t u r e s o f t h e a p p a r a t u s (10) (used a t W . I . ) . Two smooth c u r v e d m i c a s h e e t s a r e g l u e d on


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c y l i n d r i c a l lenses and mounted opposite each other i n a crossed c y l i n d e r c o n f i g u r a t i o n (to avoid problems of alignment a s s o c i a t e d with two p a r a l l e l s u r f a c e s ) : the top sheet i s r i g i d l y mounted ( v i a arm B, f i g u r e 1) on a p i e z o c r y s t a l while the bottom surface i s mounted onto the r i g i d , movable arm A v i a a f l e x i b l e l e a f s p r i n g of s p r i n g constant K. The c l o s e s t d i s t a n c e D between the surfaces i s measured by an o p t i c a l method i n v o l v i n g m u l t i p l e beam w h i t e - l i g h t i n t e r f e r o m e t r y (10) between the mica sheets (which are h a l f - s i l v e r e d on t h e i r glued side) and may be determined to w i t h i n ± 0.3 nm. D i s c o n t r o l l e d v i a a three-stage mechanism, of which the p i e z o c r y s t a l i s the f i n e c o n t r o l stage (to ± 0.3 nm) . The f o r c e F(D) between the surfaces i s measured by applying a known r e l a t i v e displacement AD between the two r i g i d supports A and B, (by applying a known voltage to the p i e z o c r y s t a l , say) and at the same time observing - using the o p t i c a l i n t e r f e r o m e t r y technique - the a c t u a l motion AD of the surfaces r e l a t i v e to each other. I f there i s no f o r c e between the surfaces then we expect AD = AD , and i f they a t t r a c t , Δ0 > AD ; i f the surfaces r e p e l each other, t h e i r r e l a t i v e displacement AD w i l l be l e s s than ADo, and i f they a t t r a c t , AD > AD (the d i f f e r e n c e i n both cases being taken up by the l e a f s p r i n g ) . In general,


Q

Q

0

C

F ( D + AD) + K ( A D

0

- AD)

(1)

(K being the l e a f - s p r i n g constant) and by s t a r t i n g measurements with the surfaces f a r apart, where F ( D ) * 0, f o r c e p r o f i l e s may be determined. The method a l s o allows the measurement of the mean r e f r a c t i v e index n(D) of the medium between the surfaces ( 9 ) , and from t h i s the amount Γ of adsorbed species per u n i t area of mica may be evaluated. F i n a l l y the mean radius of curvature R of the c y l i n d r i c a l l y curved mica surfaces near the contact area may a l s o be c a l c u l a t e d from the shape of the i n t e r f e r e n c e f r i n g e s . Procedure. P r i o r to an experiment a l l parts of the apparatus com ing i n contact with s o l u t i o n are thoroughly cleaned and d r i e d (glass parts by standing i n sulphachromic a c i d overnight, metal and D e l r i n p a r t s by s o n i c a t i o n i n degreasing agents and d i l u t e a c i d , and a l l p a r t s f i n a l l y r i n s e d i n f i l t e r e d ethanol and d r i e d i n a laminar flow c a b i n e t ) . The apparatus i s then assembled, closed and mounted i n a v i b r a t i o n - f r e e , thermally i n s u l a t e d box. The mica surfaces are brought to contact i n a i r , and the o p t i c a l parameters of the system are noted: pure solvent i s added t o the g l a s s c e l l ( f i g u r e 1) so as to immerse the s u r f a c e s , and F(D) i s measured ( i n the absence of polymer). A t t h i s stage the presence of dust or other contaminant on the surface may be noted, and only experiments f r e e of such a r t e f a c t s are taken to the next stage. Polymer i s then added t o the r e q u i r e d c o n c e n t r a t i o n , and the surfaces allowed to incubate i n the s o l u t i o n f o r ( g e n e r a l l y ) s e v e r a l hours, to permit adsorption to take p l a c e . F(D) i s then again measured, and i f necessary the polymer s o l u t i o n may be r e -



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p l a c e d by s o l v e n t ( t o l e a v e o n l y t h e s u r f a c e a d s o r b e d p o l y m e r i n i n t h e s y s t e m ) , and F(D) m e a s u r e d once more. V a r i a t i o n s on t h i s p r o c e d u r e f o r t h e d i f f e r e n t s y s t e m s s t u d i e d w i l l be d e s c r i b e d i n t h e R e s u l t s and D i s c u s s i o n ' S e c t i o n . Materials. U n l e s s o t h e r w i s e s t a t e d , a l l c h e m i c a l s and s o l v e n t s w e r e a n a l y t i c a l g r a d e m a t e r i a l s ( F l u k a and B.D.H.), and w e r e u s e d as r e c e i v e d . The w a t e r u s e d f o r w a s h i n g and p r e p a r a t i o n o f a q ueous e l e c t r o l y t e s o l u t i o n s was d e i o n i z e d and f r e s h l y d o u b l e - d i s tilled in a fused-silica s t i l l . A l l l i q u i d s a r e f i l t e r e d (0.22 pn M i l l i p o r e or Fluoropore f i l t e r s ) p r i o r to i n t r o d u c t i o n i n t o the glass c e l l . The m i c a u s e d t h r o u g h o u t was B e s t Q u a l i t y FS/GS g r a d e 2 M u s c o v i t e Ruby m i c a , m i n e d i n K e n y a ( M i c a and M i c a n i t e L t d . , U.K.). Downloaded by EMORY UNIV on February 4, 2016 | http://pubs.acs.org Publication Date: February 10, 1984 | doi: 10.1021/bk-1984-0240.ch015

1

R e s u l t s and

Discussion

The f o r c e - d i s t a n c e p r o f i l e s p r e s e n t e d i n t h e f o l l o w i n g s e c t i o n s a r e g e n e r a l l y p l o t t e d as F ( D ) / R v . D , i . e . t h e f o r c e a x i s i s 'nor m a l i z e d ' by d i v i d i n g F(D) by t h e mean r a d i u s R o f t h e m i c a s h e e t s . In the Derjaguin approximation (11), F ( D ) / R = 2πΕ(ϋ)

(2)

where E(D) i s t h e i n t e r a c t i o n e n e r g y p e r u n i t s u r f a c e a r e a b e t w e e n two f l a t , p a r a l l e l p l a t e s a d i s t a n c e D a p a r t , o b e y i n g t h e same f o r c e law. I n t h i s way t h e e f f e c t o f d i f f e r e n t c u r v a t u r e o f t h e m i c a s u r f a c e s i n t h e d i f f e r e n t e x p e r i m e n t s i s e l i m i n a t e d . Where t h i s i s not done, the v a l u e of R i s g i v e n e x p l i c i t y i n the f i g u r e caption. P o l y s t y r e n e i n Cyclohexane at Τ < θ The f o r c e p r o f i l e b e t w e e n b a r e m i c a s u r f a c e s immersed i n p u r e c y c l o h e x a n e a t 24°C i s shown i n f i g u r e 2. No f o r c e ( w i t h i n e r r o r ) i s d e t e c t e d as t h e s u r f a c e s a p p r o a c h f r o m l a r g e D down t o D « 12 nm. when a t t r a c t i o n s e t s i n . A t t h e p o i n t J ( f i g u r e 2) t h e s u r f a c e s jump i n t o t h e i r a i r c o n t a c t p o s i t i o n ( w i t h i n e r r o r ) . Such jumps a r e due t o a m e c h a n i c a l i n s t a b i l i t y w h e n e v e r 3F(D)/8D ^ K, t h e s p r i n g c o n s t a n t o f t h e l o w e r l e a f - s p r i n g ( f i g u r e 1 ) . The b r o k e n l i n e i s t h e t h e o r e t i c a l van d e r W a a l s a t t r a c t i o n F(D) = -AR/6D , e x p e c t e d b e t w e e n c r o s s e d c y l i n d e r s o f r a d i u s R (R = 0.66 cm) w h e r e A i s t h e a p p r o p r i a t e Hamaker c o n s t a n t ( e s t i m a t e d as 1 χ 10""^0 j f o r m i c a i n c y c l o h e x a n e ( 1 2 ) ) . The d a t a o f f i g u r e 2 c o u l d be c o n s t r u e d as i n d i c a t i n g a v a n - d e r - W a a l s - l i k e i n t e r a c t i o n b e t w e e n t h e m i c a s u r f a c e s , t h o u g h we n o t e t h a t H o r n e t a l (12) have r e c e n t l y p r e s e n t e d d a t a i n d i c a t i n g o s c i l l a t i n g f o r c e s between m i c a s u r f a c e s i n d r i e d c y c l o h e x a n e f r o m w h i c h w a t e r had b e e n t h o r o u g h l y removed. These o s c i l l a t i o n s d i s a p p e a r e d , h o w e v e r , when t h e s o l v e n t was n o t e s p e c i a l l y d r i e d , i n accord w i t h the present o b s e r v a t i o n s . Poly s t y r e n e was t h e n i n t r o d u c e d i n t o t h e c e l l t o a c o n c e n t r a t i o n o f


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Surfaces

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Transmitted light

2 cm Collimated white light

F i g u r e 1. S e c t i o n o f a p p a r a t u s u s e d t o measure s u r f a c e - s u r face forcesbetween mica sheets. (Reproduced w i t h p e r m i s s i o n from r e f e r e n c e 1 0 , C o p y r i g h t 1983, R o y a l S o c i e t y o f C h e m i s t r y ) .

20

40

60

80

D(nm) F i g u r e 2. F o r c e b e t w e e n c u r v e d , b a r e m i c a s u r f a c e s ( r a d i u s R) a d i s t a n c e D a p a r t i n C y c l o h e x a n e . • » R = 0.35 cm.; 0, R = 0.66 cm. B r o k e n l i n e i s t h e o r e t i c a l v a n d e r Waals a t t r a c t i o n ( s e e text). (Reproduced w i t h p e r m i s s i o n from r e f e r e n c e 10, C o p y r i g h t 1983, R o y a l S o c i e t y o f C h e m i s t r y ) .


232


7 ± 2 yg m l and t h e s u r f a c e s l e f t t o i n c u b a t e i n t h e p o l y m e r s o l u t i o n f o r some 10 h o u r s , a l o n g way a p a r t (D - 3mm). Two a n i o n i c a l l y p o l y m e r i z e d p o l y s t y r e n e samples ( P r e s s u r e Chemicals) w e r e u s e d , P S I (M^ = 6.10$) and PS2 (M^ = 1.0 χ 10$. Μ^/Μ £ΐ.1 for both polymers). F o l l o w i n g i n c u b a t i o n F ( D ) was m e a s u r e ( a t 24° ± 2°C), and t h e s o l u t i o n was t h e n r e p l a c e d by p u r e c y c l o h e x a n e and F ( D ) meas ured again. The r e s u l t s a r e shown i n f i g u r e 3, f o r ΡSI. Once a g a i n t h e r e i s l i t t l e i n t e r a c t i o n b e t w e e n t h e s u r f a c e s as t h e y a p p r o a c h f r o m a l o n g way a p a r t , b u t a t a r o u n d D ^ 60 nm an a t t r a c t i o n i s o b s e r v e d , w h i c h i n c r e a s e s u n t i l t h e s u r f a c e s jump i n f r o m t h e p o i n t A t o a new e q u i l i b r i u m p o s i t i o n a t B . F u r t h e r ap p r o a c h r e s u l t s i n i n c r e a s i n g r e p u l s i o n as shown ( a l s o i n s e t t o figure 3). On s e p a r a t i o n , t h e f o r c e d e c r e a s e s , b e c o m i n g a t t r a c t i v e again (F(D) < 0 ) , u n t i l t h e s u r f a c e s jump a p a r t f r o m t h e p o i n t C o u t t o a new e q u i l i b r i u m p o s i t i o n a t E; f u r t h e r s e p a r a t i o n shows l i t t l e i n t e r a c t i o n b e y o n d E. Further compression-de c o m p r e s s i o n c y c l e s f u l l y r e p r o d u c e d t h e above F v. D b e h a v i o u r . The jumps a t A and C, as n o t e d e a r l i e r , a r e due t o i n s t a b i l i t i e s , and o c c u r b e c a u s e a t t h e s e p o i n t s 3F(D)/3D ^ K , t h e l e a f - s p r i n g constant. The r e f r a c t i v e i n d e x n ( D ) o f t h e medium s e p a r a t i n g t h e m i c a s u r f a c e s , b o t h b e f o r e and a f t e r a d s o r p t i o n o f p o l y m e r , was a l s o d e t e r m i n e d , and t h e r e s u l t s a r e shown i n f i g u r e 4. Within e r r o r n ( D ) does n o t change on r e p l a c i n g t h e p o l y m e r s o l u t i o n by pure s o l v e n t , nor f o l l o w i n g s e v e r a l compression/decompression cycles. This strongly i n d i c a t e s a q u a s i - i r r e v e r s i b l e adsorption of the polymer onto the m i c a s u r f a c e s , i n a c c o r d w i t h independent micro-balance experiments of p o l y s t y r e n e a d s o r p t i o n onto mica (13). The v a l u e o f t h e a d s o r b a n c e Γ ( f o r PS1) d e d u c e d f r o m n ( D ) i s 6 ± 1 mg m~2 o f m i c a s u r f a c e , a g a i n i n a c c o r d w i t h i n d e p e n d e n t a d s o r b a n c e measurements ( 1 3 ) . The F ( D ) v. D p r o f i l e s f o l l o w i n g i n c u b a t i o n o f PS2 (M = 1 0 ) a r e n o t shown, b u t f o l l o w e d t h e same q u a l i t a t i v e t r e n d : no i n t e r a c t i o n down t o D - 25 nm, when a t t r a c t i o n s e t i n , and f i n a l l y , on c l o s e r a p p r o a c h , a s t r o n g r e p u l s i v e w a l l a t D ^ 6 nm. The m a i n f e a t u r e s o f t h e f o r c e - d i s t a n c e p r o f i l e s a r e i ) an e f f e c t i v e e x t e n s i o n β f r o m t h e s u r f a c e o f some 1.5 R f o r each ad sorbed l a y e r , where R i s the u n p e r t u r b e d r a d i u s of g y r a t i o n f o r the r e s p e c t i v e p o l y m e r s . These v a l u e s o f δ a r e comparable w i t h e l l i p s o m e t r i c (14) and v i s c o m e t r i c (15) s t u d i e s o f a b s o r b e d l a y e r t h i c k n e s s e s i n v a r i o u s s y s t e m s u n d e r Θ-conditions; i i ) an i n i t i a l a t t r a c t i o n f o l l o w e d u l t i m a t e l y by a s t r o n g r e p u l s i o n . T h i s may be u n d e r s t o o d by c o n s i d e r i n g t h e i n t e r a c t i o n s b e t w e e n o p p o s i n g segments as t h e y come i n t o o v e r l a p ( 1 0 ) : i n t h e p o o r s o l v e n t c o n l i t i o n s (T = 24°C < Θ = 35°C) o f t h e p r e s e n t i n v e s t i g a t i o n t h e o s m o t i c i n t e r a c t i o n s a r e a t t r a c t i v e s o l o n g as t h e c o n c e n t r a t i o n o f o v e r l a p p i n g segments i s w i t h i n t h e r a n g e o f c o n c e n t r a t i o n s e x p e c ted f o r the b i p h a s i c r e g i o n , i . e . the range of c o n c e n t r a t i o n s f o r w h i c h p o l y s t y r e n e i n c y c l o h e x a n e w o u l d f l o c c u l a t e . F o r PS1 a t 24°C t h i s c o r r e s p o n d s t o c o n c e n t r a t i o n s o f up t o 25% p o l y m e r ( 1 0 ) :


η

5


15.

KLEIN ET AL,


Surfaces

233

1001-

z

• • 0

1


Q ST

Κ)

•ο Oh - ^

20

D/nm

Êfé # « H * ^ - o ^ - o * - » I estimated error

-5

50

100 D/nm

Figure 3 F o r c e v . D i s t a n c e D between curved m i c a s u r f a c e s (R = 0.66 cm) f o l l o w i n g 10 h r s . i n c u b a t i o n i n P S 1 s o l u t i o n a t 24°C. ,0 - i n s o l u t i o n , · - f o l l o w i n g r e p l a c e m e n t o f s o l u t i o n b y pure cyclohexane. (Reproduced w i t h p e r m i s s i o n from r e f e r e n c e 10, Copyright 1983, R o y a l S o c i e t y o f C h e m i s t r y ) .

1.60 h 1.56h & 152h

i

1 48 !44h 1 40

60

80

D/nm F i g u r e 4. V a r i a t i o n o f r e f r a c t i v e i n d e x n ( D ) o f medium s e p a r a t i n g m i c a s u r f a c e s D a p a r t . 0 - i n p u r e c y c l o h e x a n e ; Δ-following i n c u b a t i o n i n PS1 s o l u t i o n ; à - a f t e r r e p l a c i n g s o l u t i o n by pure cyclohexane. ( a ) - η(bulk p o l y s t y r e n e ) ; ( b ) - η(pure c y c l o hexane) . (Reproduced w i t h p e r m i s s i o n from r e f e r e n c e 10, C o p y r i g h t 1983, R o y a l S o c i e t y o f C h e m i s t r y ) .



234


f o r concentrations higher than t h i s the osmotic i n t e r a c t i o n s are once again r e p u l s i v e . Since the adsorption of polymer onto the mica i s e s s e n t i a l l y i r r e v e r s i b l e , compression of the surfaces (ie. reducing D) e v e n t u a l l y increases the polymer concentration i n the gap beyond t h i s l i m i t , and the i n t e r a c t i o n changes from a t t r a c t i o n to r e p u l s i o n . Reference to the r e f r a c t i v e index p r o f i l e n(D) shows that the mean polymer volume f r a c t i o n i n the gap f o r D - 20 nm, i . e . , at the p o i n t where the changeover begins from a t t r a c t i o n to r e p u l s i o n ( f i g u r e 3), i s i n f a c t about 25%, i n sup port of the q u a l i t a t i v e explanation above. A more q u a n t i t a t i v e model f o r the form of the f o r c e - d i s t a n c e p r o f i l e between adsorbed polymer l a y e r s i n poor solvent c o n d i t i o n s , based on the above ideas, has r e c e n t l y been presented by Pincus and one of us (16). This i s an approach ( f i r s t described by De Gennes (17) f o r the case of adsorbed polymers i n good- and Θs o l v e n t s ) whereby the excess f r e e energy E(D) of adsorbed polymer i n the gap between two surfaces a d i s t a n c e D apart i s c a l c u l a t e d with respect to the polymer segmental d i s t r i b u t i o n across the gap. By minimizing E(D) i t i s p o s s i b l e to deduce the e q u i l i b r i u m segmental d i s t r i b u t i o n and the value of E(D) f o r any D, and hence the d i s j o i n i n g pressure ird = - 8E(D)/8D. The p r e d i c t i o n s of these c a l c u l a t i o n s (16) are i n good q u a l i t a t i v e and q u a n t i t a t i v e accord with the present r e s u l t s f o r Τ < Θ. Polystyrene i n Cyclohexane at Τ £ Θ By mounting a small heating element w i t h i n the g l a s s c e l l ( f i g u r e 1) i t becomes p o s s i b l e to heat the polymer s o l u t i o n , monitoring the temperature v i a a small thermocouple c l o s e to the mica s u r f a c e s . Because of the extreme s e n s i t i v i t y of the present approach to changes i n D, i t i s not p r a c t i c a b l e to thermostat the system, but r a t h e r one allows a steady-state temperature to be reached, where the power provided by the h e a t i n g c o i l equals the heat l o s s e s from the system. In t h i s way s u f f i c i e n t l y steady temperatures may be a t t a i n e d over the time of the f o r c e measurements. F i g u r e 5 shows the f o r c e - d i s t a n c e p r o f i l e between l a y e r s of PS1 adsorbed on mica: f o l l o w i n g overnight adsorption at room temperature F(D) was measured at 23°C (curve ( a ) , f i g u r e 5); the r e s u l t i n g p r o f i l e i s very s i m i l a r to that obtained p r e v i o u s l y ( f i g u r e 3) under the same c o n d i t i o n s . The c e l l was then heated to 37.2°C and F(D) was again measured (curve ( b ) , F i g . 5 ) . The r e s u l t i n g f o r c e - d i s tance p r o f i l e at the higher temperature s t i l l shows a p e r s i s t e n t (though considerably weaker) a t t r a c t i v e r e g i o n , of s i m i l a r range to the p r o f i l e at room temperature (curve ( a ) ) . However, the distance of c l o s e s t approach on s t r o n g l y compressing the surfaces i s now about h a l f (6.5 nm) of i t s previous value (~ 12 nm) f o r an equivalent compression. On r e - c o o l i n g the system to room temper ature, the behaviour i n d i c a t e d i n curve (a) was recovered, and on r e - h e a t i n g , curve (b) was again obtained.


15.

KLEIN ET AL.



f 4000

235

Surfaces

N/m at 65A

1000 | f t 3000/xN/m (3 120 A PS(6-10 )/CH, D

500

23°C

P S ( 6 - 1 0 ) / C H , 37°C - ·

1

5

Ί Jt tr

ο

U-500|

-1000

.a

500

1000

D(A)

Figure 5. F(D)/R v . d i s t a n c e D between m i c a s u r f a c e s b e a r i n g a d s o r b e d P S 1 l a y e r s , a t 23°C and 37°C. ( A d a p t e d f r o m r e f e r e n c e 18).


P O L Y M E R A D S O R P T I O N AND DISPERSION STABILITY

236

The p e r s i s t e n t a t t r a c t i o n between the adsorbed PS1 l a y e r s at 37.2°C (higher than the Θ-temperature f o r the system, 34.5°C) i s at f i r s t s i g h t p u z z l i n g , i n view of our previous i n t e r p r e t a t i o n of the a t t r a c t i o n at room temperature being due to osmotic e f f e c t s r e s u l t i n g from the Τ < Θ c o n d i t i o n s . There are two p o s s i b l e ex planations for t h i s : the concept of a Θ-temperature d e r i v e s from a c o n d i t i o n i n f r e e s o l u t i o n , when the net van der Waals a t r a c t i o n s between the segments of a polymer e x a c t l y compensate the r e p u l s i v e 'hard-core excluded volume i n t e r a c t i o n s . I t may not be j u s t i f i e d to assume that the e f f e c t i v e 'Θ-temperature' i n the c l o s e v i c i n i t y of a w a l l i s i d e n t i c a l to the bulk value (35°C), and i n that case one may need to increase the temperature beyond the present value of 37°C i n order to e n t i r e l y e l i m i n a t e osmotic a t t r a c t i o n e f f e c t s (18). The other p o s s i b i l i t y i s suggested by the f a c t that the strong r e p u l s i v e 'wall' at 37°C, at D ~ 6.5 nm, ( f i g u r e 5 curve (b)) i s much c l o s e r i n than at 23°C ( f i g u r e 5 curve ( a ) ) . This i m p l i e s some desorption of polymer has occurred due to r a i s i n g the temperature : q u a l i t a t i v e l y one may imagine that such desorption has r e l e a s e d surface b i n d i n g s i t e s on both mica s u r f a c e s , and thus f a c i l i t a t e d a t t r a c t i v e ' b r i d g i n g ' e f f e c t s by adsorbed polymer across the i n t e r - s u r f a c e gap. In f a c t some c a l c u l a t i o n s by Scheuj tens and F l e e r (19) seem to p r e d i c t such a t t r a c t i o n at Τ = Θ, but s i n c e t h e i r model assumes thermodynamic e q u i l i b r i u m c o n d i t i o n s ( i . e . the surface-adsorbed polymer may ex change with a f r e e polymer s o l u t i o n , so that complete desorption at D = 0 i s p o s s i b l e ) , and i n f a c t p r e d i c t s a t t r a c t i o n even i n good solvent c o n d i t i o n s , i t i s probably not appropriate i n the present case of i r r e v e r s i b l y adsorbed polymer. A more promising approach i s v i a the g e n e r a l i z e d van der Waals approach used by De Gennes ( 1 7 ) , and Pincus and K l e i n (_16), where the c o n d i t i o n of i r r e v e r s i b l e adsorption may be e x p l i c i t l y i n c l u d e d . Experimental support f o r the suggestion that depleted s u r f a c e l a y e r s r e s u l t i n a t t r a c t i v e f o r c e s (at Τ ^ Θ) has come from recent experiments (J.K. and Y . A . , submitted) where mica surfaces par t i a l l y covered by polystyrene i n cyclopentane above the Θ-temper ature show a c l e a r mutual a t t r a c t i o n , which disappears when f u l l surface coverage by the polymer i s a t t a i n e d . 1

1


1

Polyethylene Oxide (PEO)

i n a Good Aqueous Solvent

The i n t e r a c t i o n between bare mica surfaces i n 0.1 M KNO3 (pH 5.5) aqueous e l e c t r o l y t e i s shown i n f i g u r e 6a. The s t r a i g h t l i n e (curve (a)) i n d i c a t e d on the l o g a r i t h m i c - l i n e a r p l o t i s i n accord with the exponential r e l a t i o n F/R « Θ -

κ Β

(3)

expected due to overlap of the e l e c t r o s t a t i c double-layers asso c i a t e d w i t h the mica surfaces as they approach each other, where κ i s the Debye-Huckel parameter ( 9 ) ; the corresponding surface


15.

KLEIN ET AL.


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30


ε ζ

t Q

«

Polymer Adsorption and Dispersion Stability - American Chemical

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