Chapter 5
Chemical Grafting of Poly(ethylene oxide) onto Polystyrene Latex Synthesis, Characterization, and Rheology 1
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H. J. Ploehn and J . W. Goodwin Department of Physical Chemistry, University of Bristol, Bristol, England
A novel method u t i l i z i n g a water-soluble carbodiimide was used to chemically graft 100,000 and 615,000 g/mol PEO onto PS latex particles with mean diameters of 375 and 347 nm. Electrophoretic mobility measurements show that the PEO is truly grafted, not just adsorbed. Capillary viscometry yields a layer thickness of 23 nm for grafted 615,000 g/mol PEO. Steady shear and oscillatory rheological measurements indicate a t r a n s i t i o n from primarily l i q u i d - l i k e behavior to s o l i d - l i k e behavior at an effective volume f r a c t i o n between 0.528 and 0.550. The rheological behavior is interpreted in terms of long-range repulsive p a r t i c l e interactions due to the grafted polymer layers. These measurements thus provide a useful probe of the relationship between polymeric interactions, suspension microstructure, and macroscopic material properties. Economic and environmental considerations have led to renewed interest in polymer latices which employ water as the suspending medium. It is well known that interparticle electrostatic forces play an important role in controlling the s t a b i l i t y and rheological properties of aqueous suspensions. Although the particle electrostatics can be adjusted so that the latex has desirable properties and behavior, in many situations the electrostatics impose serious l i m i t a t i o n s on the 1
Current address: Department of Chemical Engineering, Texas A&M University, College Station, TX 77843-3122
0097-6156/91/0462-0088$06.00/0 © 1991 American Chemical Society
In Polymers as Rheology Modifiers; Schulz, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1991.
Downloaded by NORTH CAROLINA STATE UNIV on August 6, 2012 | http://pubs.acs.org Publication Date: May 13, 1991 | doi: 10.1021/bk-1991-0462.ch005
5.
P L O E H N & GOODWIN
Chemical Grafting of PEO onto PS Latex
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formulation o f t h e s u s p e n s i o n and i t s performance i n practical situations. Dissolved polymer also modifies particle i n t e r a c t i o n s t h r o u g h any o f s e v e r a l mechanisms ( 1 ) . For i n s t a n c e , p o l y m e r i n s o l u t i o n can be e x c l u d e d f r o m t h e i n t e r s t i t i a l r e g i o n between two p a r t i c l e s ; t h e r e s u l t a n t o s m o t i c a t t r a c t i o n b e t w e e n t h e p a r t i c l e s may lead to depletion flocculation. Physical adsorption of homopolymers, b l o c k and random c o p o l y m e r s , p o l y e l e c t r o lytes, and proteins onto p a r t i c l e s occurs i n many systems. A t low a d s o r b e d amounts, p o l y m e r m o l e c u l e s may a d s o r b on two o r more p a r t i c l e s s i m u l t a n e o u s l y , producing a t t r a c t i o n between t h e p a r t i c l e s . The p o l y m e r - m e d i a t e d i n t e r a c t i o n a l s o has a r e p u l s i v e component t h a t i n c r e a s e s w i t h a d s o r b e d amount due t o t h e e x c l u s i o n o f segments f r o m s a t u r a t e d s u r f a c e s and u n f a v o r a b l e p o l y m e r - p o l y m e r i n t e r a c t i o n s i n good s o l v e n t s . Strongly repulsive interactions which impart " s t e r i c " o r p o l y m e r i c s t a b i l i z a t i o n (2) can be achieved by chemically g r a f t i n g polymer to the particles. R e l a t i v e l y low m o l e c u l a r weight s t a b i l i z e r s (
