Emulsion Polymers and Emulsion Polymerization - ACS Publications

3 Well-Characterized Monodisperse Polystrene Latexes as Model Colloids J. W. VANDERHOFF

Downloaded by COLUMBIA UNIV on April 18, 2013 | http://pubs.acs.org Publication Date: October 7, 1981 | doi: 10.1021/bk-1981-0165.ch003

Emulsion Polymers Institute and Department of Chemistry, Lehigh University, Bethlehem, PA 18015 An ideal model colloid should comprise monodisperse spherical particles stabilized with a known number of chemically bound surface groups. Monodisperse polystyrene latexes (1) are a good starting point for the preparation of such an ideal model colloid. Their particle size can be varied systematically over a wide range, and their bound surface groups are the endgroups of the polymer molecules, so that variation of polymer molecular weight and latex particle size can give different surface charge densities. Moreover, the preparation of these latexes using persulfate initiator gives sulfate endgroups, which can be hydrolyzed to hydroxyl groups, which, in turn, can be oxidized to carboxyl groups, to give particles stabilized with the same number of three different surface groups. The purpose of this paper is to describe the preparation and characterization of such model colloids, starting with the emulsion polymerization of styrene using persulfate initiator. The preparation of a latex by emulsion polymerization comprises two stages: (i) particle nucleation; (ii) particle growth. For the latex to be monodisperse, the particle nucleation stage must be short relative to the particle growth stage. Despite many investigations, there is disagreement as to the locus of particle nucleation: (i) monomer-swollen emulsifier micelles; (ii) adsorbed emulsifier layer; (iii) aqueous phase; (iv) monomer droplets. Whatever the locus of particle nucleation, the decomposition of persulfate is considered to produce sulfate ion-radicals in the aqueous phase. These ion-radicals are repelled by the negatively charged surface of a monomer-swollen emulsifier micelle, polymer particle or monomer droplet and therefore remain in the aqueous phase and add hydrophobic monomer units to form an oligomeric radical. When this oligomeric radical attains a critical chain length, it becomes surface-active and adsorbs on the surface of a micelle, polymer particle, or monomer droplet. If it cannot adsorb on a suitable surface, it continues to grow until it exceeds its solubility in water and therefore precipitates to nucleate a latex particle. Initiation in micelles is significant 0097-6156/81 /0165-0061 $05.75/0 © 1981 American Chemical Society

In Emulsion Polymers and Emulsion Polymerization; Bassett, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.


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EMULSION POLYMERS AND EMULSION POLYMERIZATION

o n l y i n t h e b e g i n n i n g o f t h e p o l y m e r i z a t i o n , and t h e monomer d r o p l e t s usually are not a s i g n i f i c a n t locus of p a r t i c l e nucleation. T h e r e f o r e , once t h e p a r t i c l e n u c l e a t i o n stage i s completed, t h e monomer-swollen polymer p a r t i c l e s a r e t h e p r i n c i p a l l o c u s o f p o l y merization. The a d s o r p t i o n o f an o l i g o m e r i c s u l f a t e r a d i c a l t o i n i t i a t e a p o l y m e r i c r a d i c a l and t h e t e r m i n a t i o n o f t h i s p o l y m e r i c r a d i c a l by a d s o r p t i o n o f a n o t h e r o l i g o m e r i c s u l f a t e r a d i c a l s h o u l d g i v e two s u r f a c e s u l f a t e g r o u p s f o r e a c h p o l y m e r m o l e c u l e f o r m e d a s s u m i n g t h a t no t r a n s f e r r e a c t i o n s o c c u r t o i n t r o d u c e a d i f f e r e n t e n d g r o u p , t h a t t e r m i n a t i o n i n s i d e t h e p a r t i c l e o c c u r s b y comb i n a t i o n r a t h e r t h a n by d i s p r o p o r t i o n a t i o n , a n d t h a t none o f t h e s u l f a t e e n d g r o u p s become b u r i e d i n s i d e t h e p a r t i c l e . P o l y s t y r e n e l a t e x e s have been p r e p a r e d u s i n g p e r s u l f a t e i n i t i a t o r f o r many y e a r s , b u t o n l y r e c e n t l y h a v e methods b e e n d e v e l o p e d t o d e t e r m i n e t h e number a n d l o c i o f t h e s u l f a t e s u r f a c e g r o u p s . To d e t e r m i n e t h e s e s u r f a c e g r o u p s , t h e l a t e x i s c l e a n e d to remove t h e a d s o r b e d e m u l s i f i e r and s o l u t e e l e c t r o l y t e , t h e n the s u r f a c e s u l f a t e groups i n the H form a r e t i t r a t e d conductom e t r i c a l l y w i t h b a s e . The l a t e x e s c a n be c l e a n e d e f f e c t i v e l y by i o n e x c h a n g e ( 2 - 5 ) o r serum r e p l a c e m e n t ( 6 ) ; d i a l y s i s i s n o t e f f e c t i v e i n r e m o v i n g t h e a d s o r b e d e m u l s i f i e r and s o l u t e e l e c t r o l y t e (3,5,6). I n i o n e x c h a n g e , t h e aqueous p h a s e i o n s a r e r e p l a c e d w i t h H and 0H~ i o n s . I f t h e a q u e o u s p h a s e i o n s a r e i n e q u i l i b r i u m w i t h t h e a d s o r b e d i o n s , t h e i r r e m o v a l f r o m t h e aqueous p h a s e c a u s e s d e s o r p t i o n o f the adsorbed ions to m a i n t a i n the e q u i l i b r i u m u n t i l a l l o f t h e a d s o r b e d i o n s h a v e b e e n removed. I n p r a c t i c e , t h i s removal i s q u a n t i t a t i v e (2-5). I o n exchange i s r a p i d and e a s i l y c a r r i e d o u t ; h o w e v e r , c o m m e r c i a l i o n exchange r e s i n s c o n t a i n l e a c h a b l e p o l y e l e c t r o l y t e s which adsorb on l a t e x p a r t i c l e s u r f a c e s ; t h e s e p o l y e l e c t r o l y t e s c a n be removed o n l y by a n a r d u o u s p u r i f i c a t i o n process (2-5). I n serum r e p l a c e m e n t ( 6 ) , t h e l a t e x i s c o n f i n e d i n a c e l l w i t h a s e m i - p e r m e a b l e membrane, e . g . , N u c l e p o r e f i l t r a t i o n memb r a n e , and w a t e r i s pumped t h r o u g h t h e l a t e x t o l i t e r a l l y r e p l a c e t h e serum. The r e m o v a l o f a d s o r b e d i o n s i s q u a n t i t a t i v e p r o v i d e d the a d s o r p t i o n - d e s o r p t i o n e q u i l i b r i u m i s maintained. The N a and K i o n s a r e r e p l a c e d by i o n s by pumping d i l u t e h y d r o c h l o r i c a c i d t h r o u g h t h e l a t e x f o l l o w e d by w a t e r t o remove t h e e x c e s s acid. Serum r e p l a c e m e n t t a k e s l o n g e r t h a n i o n e x c h a n g e , b u t a v o i d s t h e a r d u o u s r e s i n p u r i f i c a t i o n s t e p ; m o r e o v e r , t h e serum i s recovered q u a n t i t a t i v e l y i n a form s u i t a b l e f o r a n a l y s i s . I n d i a l y s i s , the l a t e x i s c o n f i n e d w i t h a semi-permeable membrane ( e . g . , i n a d i a l y s i s b a g ) , a n d t h e aqueous phase i o n s a r e removed by d i f f u s i o n a c r o s s t h i s s e m i - p e r m e a b l e membrane. The r a t e o f t h i s d i f f u s i o n c a n be i n c r e a s e d by c h a n g i n g t h e w a t e r o u t s i d e t h e b a g more o f t e n o r by u s i n g d i a l y s i s membranes o f greater surface area (e.g., hollow f i b e r d i a l y s i s ) . In principle, t h e a d s o r b e d i o n s s h o u l d be removed q u a n t i t a t i v e l y p r o v i d e d t h e adsorption-desorption e q u i l i b r i u m i s maintained. In practice, +

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however, the removal o f adsorbed i o n s a s w e l l as the replacement o f N a and K~*~ i o n s b y Y& i o n s i s i n c o m p l e t e ( 2 , 3 , 6 ) . The d i a l y s i s membrane must b e c o n s i d e r e d a t h i r d p h a s e i n t h i s p r o c e s s ; m o r e o v e r , t h e r a t e o f d i a l y s i s d e c r e a s e s d r a s t i c a l l y w i t h dec r e a s i n g c o n c e n t r a t i o n g r a d i e n t a c r o s s t h e membrane ( 7 ) . The c l e a n e d l a t e x i n t h e H f o r m i s t i t r a t e d c o n d u c t o m e t r i c a l l y o r p o t e n t i o m e t r i c a l l y w i t h base t o determine the s u r f a c e charge (2,5,8). C o n d u c t o m e t r i c t i t r a t i o n i s t h e more s e n s i t i v e method when t h e o v e r a l l c o n d u c t a n c e i s v e r y l o w a s i n a s a m p l e c l e a n e d b y i o n exchange o r serum r e p l a c e m e n t ; p o t e n t i o m e t r i c t i t r a t i o n i s t h e more s e n s i t i v e when t h e c o n d u c t a n c e i s r e l a t i v e l y h i g h . F i g u r e 1 shows a t y p i c a l c o n d u c t o m e t r i c t i t r a t i o n c u r v e (8). The s o l i d l i n e shows t h e e x p e r i m e n t a l c u r v e and t h e d a s h e d l i n e the t h e o r e t i c a l curve. The t h e o r e t i c a l c u r v e l i e s s l i g h t l y above t h e e x p e r i m e n t a l c u r v e ; moreover, the s l o p e o f i t s decendi n g l e g i s much g r e a t e r . The d i f f e r e n c e i n t h e d e s c e n d i n g s l o p e s r e s u l t s from the d i s t r i b u t i o n of the H c o u n t e r i o n s i n the double l a y e r ; some a r e h e l d c l o s e t o t h e p a r t i c l e s u r f a c e and t h e r e f o r e do n o t c o n t r i b u t e much t o t h e o v e r a l l c o n d u c t a n c e . The l o w e r t h e e l e c t r o l y t e c o n c e n t r a t i o n and t h u s t h e g r e a t e r t h e e x p a n s i o n o f the e l e c t r i c double l a y e r , the s m a l l e r the s l o p e o f the descendi n g l e g . The h i g h e r p o s i t i o n o f t h e t h e o r e t i c a l c u r v e r e s u l t s because the c o n d u c t o m e t r i c t i t r a t i o n i n v o l v e s i o n exchange a s w e l l a s n e u t r a l i z a t i o n , which decreases the o v e r a l l conductance. +


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C h a r a c t e r i z a t i o n b y I o n E x c h a n g e and

Conductometric

Titration

E a r l i e r w o r k (2,3,5,9) d e s c r i b e d t h e c h a r a c t e r i z a t i o n o f m o n o d i s p e r s e p o l y s t y r e n e l a t e x e s o f 25-447nm p a r t i c l e d i a m e t e r prepared u s i n g p e r s u l f a t e i n i t i a t o r , b i c a r b o n a t e b u f f e r , and four d i f f e r e n t e m u l s i f i e r s . The l a t e x e s w e r e i o n e x c h a n g e d u s i n g r i g o r o u s l y p u r i f i e d Dowex 50W(H )-Dowex 1 (0H~) m i x e d r e s i n s and t i t r a t e d c o n d u c t o m e t r i c a l l y ; t h i s p r o c e s s was r e p e a t e d u n t i l t h e s u r f a c e c h a r g e was c o n s t a n t . T h e s e s u r f a c e c h a r g e s shown i n T a b l e I (3,5,9) c o r r e s p o n d t o 1.0-1.3 s u l f a t e e n d g r o u p s p e r p o l y s t y r e n e m o l e c u l e ( m o l e c u l a r w e i g h t d e t e r m i n e d b y o s m o m e t r y ) . To d e t e r m i n e i f any s u l f a t e g r o u p s w e r e b u r i e d i n s i d e t h e p a r t i c l e , t h e p o l y m e r was r e c o v e r e d f r o m t h e i o n - e x c h a n g e d l a t e x , d i s s o l v e d i n d i o x a n e - w a t e r m i x t u r e , i o n e x c h a n g e d a g a i n , and t i t r a t e d c o n ductometrically. The t o t a l number o f s u l f a t e e n d g r o u p s was i n t h e r a n g e 1.0-1.6 p e r p o l y s t y r e n e m o l e c u l e ( e x c e p t f o r one l a t e x w h i c h had a v a l u e o f 2 . 0 7 ) , s u g g e s t i n g t h a t t h e r e a r e o t h e r endgroups b e s i d e s s u l f a t e s . One p o s s i b i l i t y i s h y d r o x y l e n d g r o u p s , w h i c h may b e f o r m e d by a s i d e r e a c t i o n o f s u l f a t e i o n - r a d i c a l s t o f o r m h y d r o x y l r a d i c a l s (9) o r h y d r o l y s i s o f t h e s u r f a c e s u l f a t e g r o u p s . To d e t e r mine i f h y d r o x y l groups were p r e s e n t , the ion-exchanged l a t e x e s w e r e o x i d i z e d b y h e a t i n g w i t h p e r s u l f a t e and 1 0 " ^ s i l v e r i o n a t 90°, t h e n i o n e x c h a n g e d and t i t r a t e d c o n d u c t o m e t r i c a l l y t o determine the c a r b o x y l groups. T a b l e I I (9) shows t h a t some s u l +



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Figure 1. Conductometric titration of ion-exchanged 234-nm-diameter monodisperse polystyrene latex: (1) theoretical curve calculated assuming 100% dissociation; (2) experimental curve (S).

0

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6

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0. 0203N NaOH, c. c. Marcel Dekker



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Monodisperse

Polystyrene

65

Latexes

f a t e groups were hydrolyzed and o x i d i z e d to c a r b o x y l groups duri n g t h i s process, but the c a r b o x y l t i t r a t i o n values c o r r e c t e d f o r t h i s h y d r o l y s i s accounted s a t i s f a c t o r i l y f o r the r e q u i s i t e two endgroups per p o l y s t y r e n e molecule. The extent of the s i d e r e a c t i o n o f s u l f a t e i o n - r a d i c a l s with water to produce hydroxyl r a d i c a l s was p o s t u l a t e d to i n c r e a s e w i t h decreasing pH. Therefore, p o l y m e r i z a t i o n s were c a r r i e d out u s i n g p e r s u l f a t e i n i t i a t o r but with the pH of the p o l y m e r i z a t i o n adjusted to values i n the range of pH range 2-8 ( 9 ) . Table I I I (9) shows t h a t , a t the lowest pH, the endgroups were about 90% hydroxyls and 10% s u l f a t e s ; at pH 7-8, they were a l l s u l f a t e s . These r e s u l t s showed that monodisperse p o l y s t y r e n e p a r t i c l e s prepared u s i n g p e r s u l f a t e i n i t i a t o r and bicarbonate b u f f e r cont a i n both s u l f a t e and hydroxyl s u r f a c e groups. Some s u l f a t e groups are on the p a r t i c l e s u r f a c e , while others are b u r i e d i n s i d e the p a r t i c l e . A l l of the hydroxyl groups are on the part i c l e s u r f a c e . These r e s u l t s a l s o showed that c a r e f u l c o n t r o l o f the pH d u r i n g p o l y m e r i z a t i o n can produce l a t e x p a r t i c l e s s t a b i l i z e d w i t h only s u r f a c e s u l f a t e groups. C h a r a c t e r i z a t i o n by Serum Replacement and Conductometric

Titration

For serum replacement (6), the l a t e x i s confined i n a c e l l w i t h a uniform-pore-size Nuclepore f i l t r a t i o n membrane. D i s t i l l e d , d e i o n i z e d water i s pumped through the l a t e x u n t i l the conductance of the e f f l u e n t stream i s about the same as that o f the d i s t i l l e d , d e i o n i z e d water. T h i s serum replacement removes the adsorbed e m u l s i f i e r and s o l u t e e l e c t r o l y t e q u a n t i t a t i v e l y and allows recovery o f the serum i n a form s u i t a b l e f o r f u r t h e r ana l y s i s ; however, i t does n o t r e p l a c e the N a and counterions of the s u r f a c e groups w i t h H i o n s . To do t h i s , d i l u t e hydroc h l o r i c a c i d (ca. 10"^N) i s pumped through the l a t e x , followed by d i s t i l l e d , d e i o n i z e d water to remove the excess a c i d . The l a t e x i s then t i t r a t e d c o n d u c t o m e t r i c a l l y to determine the s u r f a c e charge. The pore s i z e of the Nuclepore f i l t r a t i o n membranes used i s 0.50-0.75 times the p a r t i c l e diameter of the l a t e x to be cleaned. The l a t e x i n the c e l l i s a g i t a t e d c l o s e to the membrane t o p r e vent c l o g g i n g by deposited p a r t i c l e s . Even so, c l o g g i n g i s observed a t higher pumping pressures and l a t e x s o l i d s contents, e.g., f o r 10-15% s o l i d s l a t e x at 5 p s i pressure, while 5-10% s o l i d s l a t e x a t 2 p s i gives no c l o g g i n g . S u c t i o n f i l t r a t i o n can a l s o be used to i n c r e a s e the r a t e o f serum replacement. Seven p o l y s t y r e n e l a t e x e s prepared with p e r s u l f a t e i n i t i a t o r and bicarbonate b u f f e r were c h a r a c t e r i z e d to demonstrate the e f f i c a c y of t h i s method (6). Three were monodisperse l a t e x e s p r e pared using c o n v e n t i o n a l e m u l s i f i e r s ; four were prepared u s i n g sodium styrene s u l f o n a t e o r sodium v i n y l t o l u e n e s u l f o n a t e as ccmanomeric e m u l s i f i e r s . Each l a t e x was subjected to serum replacement with +

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Table I Monodisperse Polystyrene Latexes S u r f a c e C h a r g e and Number o f S u l f a t e E n d g r o u p s / P o l y m e r M o l e c u l e (3,5,9) Endgroups/Molecule Particle Surface ^ Sulfate Total Latex Surface D i a m e t e r , nm C h a r g e , uC/cm A-l 13.0* 12.4* 25 0.5 A-2 1.57 88 0.98 3.3 A-3 1.59 0.87 234 2.0 B-l 1.21 1.26 158 4.2 B-2 1.23 1.02 248 5.7 C-l 0.94 254 0.95 5.4 D-l 0.95 109 1.6 D-2 0.90 187 1.8 D-3 1.63 285 1.03 4.9 D-4 2.07 0.93 447 8.1 A = A e r o s o l MA ( b i s - 1 , 3 - d i m e t h y l b u t y l s o d i u m s u l f o s u c c i n a t e ) B = S i p o n WD ( s o d i u m l a u r y l s u l f a t e ) C = A e r o s o l OT ( d i - 2 - e t h y l h e x y l s o d i u m s u l f o s u c c i n a t e ) D = potassium o l e a t e ueq/gm p o l y m e r Table I I M o n o d i s p e r s e P o l y s t y r e n e L a t e x A-2 O x i d a t i o n o f S u l f a t e and H y d r o x y l E n d g r o u p s IQ-^N s i l v e r n i t r a t e ; 6 h o u r s a t 90°) Number o f E n d g r o u p s , ue

Emulsion Polymers and Emulsion Polymerization - ACS Publications

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