16 Techniques for Measuring Particle Swelling of Carboxylic Emulsion Polymers 1
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Downloaded by UNIV OF ROCHESTER on June 13, 2013 | http://pubs.acs.org Publication Date: October 7, 1981 | doi: 10.1021/bk-1981-0165.ch016
D. R. BASSETT, E. J. DERDERIAN, J. E. JOHNSTON, and T. B. MacRURY Technical Center, Union Carbide Corporation, South Charleston, WV 25303
Carboxylic monomers, such as acrylic or methacrylic acid, are included in emulsion polymerization recipes for a variety of reasons: to increase the stability of the latex particles, to improve the adhesion of the resultant films to various substrates, to provide functional crosslinking sites for interparticle thermosetting reactions and to control the viscosity of latex formulations via particle swelling upon neutralization. This swelling or partial solubilization, has been qualitatively investigated by titration (1) and turbidity measurements (2) as well as by viscometry and microscopy (3). More recently Nishida, et al. (4), characterized the swelling behavior of methyl methacrylate/methacrylic acid emulsion polymers using viscometry and light scattering. It is clear that particle swelling is influenced by the type and concentration of carboxylic monomer incorporated into the particle, the relative hydrophilicity of the comonomers employed in the polymerization, the stiffness (T ) of the polymer chains, and the molecular weight of the polymer. While most studies of particle swelling have dealt with emulsion polymers containing high acid levels, we have restricted our attention to latexes containing relatively low incorporated acid (ca.2-3%) to avoid substantial solubilization of the particles. Questions under current consideration are related to the location of incorporated carboxyl groups within the latex particles, the morphology of expanded particles, and the detailed mechanism of expansion. Information of this type is valuable not only from a fundamental standpoint but is essential in the investigation of very practical problems such as the viscosity stability of latex formulations. Central to any investigation of particle swelling is the use of a reliable method of measuring the size of the particles undergoing expansion. A sedimentation method was described previously (5) which can be used to explore the expansion characteristics of carboxylic emulsion polymers. In the present report, we present a comparison of sedimentation results with those obtained with two g
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Current address: Current address:
Exxon Chemicals, Linden, New Jersey IMC Corporation, Terre Haute, Indiana
0097-6156/81/0165-0263$05.00/0 © 1981 American Chemical Society
In Emulsion Polymers and Emulsion Polymerization; Bassett, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.
EMULSION POLYMERS AND EMULSION POLYMERIZATION
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other q u a n t i t a t i v e methods, p r e c i s i o n viscometry and photon c o r r e l a t i o n spectroscopy. Of p a r t i c u l a r concern i n t h i s comparison i s the e f f e c t o f i n t e r p a r t i c l e i n t e r a c t i o n s . The s p e c i a l problems a s s o c i a t e d with the use o f l i g h t s c a t t e r i n g techniques to probe the i n t e r n a l s t r u c t u r e of expanded p a r t i c l e s i s described elsewhere i n t h i s Monograph ( 6 ) .
Downloaded by UNIV OF ROCHESTER on June 13, 2013 | http://pubs.acs.org Publication Date: October 7, 1981 | doi: 10.1021/bk-1981-0165.ch016
EXPERIMENTAL Latex P r e p a r a t i o n . Model l a t e x e s were prepared by a semi-batch technique i n which a monomer mix was c o n t i n u o u s l y fed into a s t i r r e d r e a c t o r at 80°C i n the presence of ammonium p e r s u l f a t e and a s i n g l e a n i o n i c s u r f a c t a n t , Aerosol OT (American Cyanamid). The i n i t i a l s u r f a c t a n t c o n c e n t r a t i o n i n the r e a c t o r was adjusted to produce a p a r t i c l e s i z e of approximately 0 . 1 μ \ η . The standard a c r y l i c polymer composition was 40 parts methyl m e t h a c r y l a t e , 54 parts ethyl a c r y l a t e and 6 parts butyl a c r y l a t e . The p o l y m e r i z a t i o n was a r b i t r a r i l y d i v i d e d i n t o two s t a g e s , Stage I being the f i r s t h a l f of the monomer feed and Stage II being the l a s t h a l f of the feed. Since previous expansion s t u d i e s (5j revealed that p a r t i c l e s w e l l i n g i s accentuated by l a t e a d d i t i o n o f the c a r b o x y l i c monomer, the Stage II a d d i t i o n was chosen f o r the present comparison. A c r y l i c a c i d , amounting to 2 percent of the t o t a l monomer mix, was added to the l a s t h a l f of the monomer feed used to prepare the model c a r b o x y l i c l a t e x . A second a c r y l i c l a t e x was prepared under the same c o n d i t i o n s without the monomeric a c i d . Several much harder l a t e x e s were a l s o prepared using methyl methacrylate with 2 or 3 percent a c r y l i c a c i d added in Stage I I . Sedimentation. The sedimentation method, described p r e v i o u s l y ( 5 ) , i s based on the r e l a t i v e sedimentation r a t e s of swollen and unswollen latex p a r t i c l e s . S t a r t i n g with the Stokes expression f o r c e n t r i f u g a l sedimentation, an equation can be developed f o r the r a t i o o f the sedimentation c o e f f i c i e n t o f a p a r t i c l e , S , to the sedimentation c o e f f i c i e n t , S, of the same p a r t i c l e having a surface l a y e r :
V S
2
r (
