SynopsisA heterogeneous model for the monomer-polymer particle in styrene emulsion polymerization is presented. I n this model, the growing particle consists of an expanding polymer-rich core surrounded by a monomer-rich shell which serves as the major locus of polymerization, This core-shell model was suggested by kinetic studies with continuously uniform latices which showed that the systems of interest were of the Smith-Ewart case I1 type but that the dynamic-as opposed to equilibrium swelling-particle monomer concentrations were continuously variable. Supporting evidence for the suggested morphology was obtained by electron microscope observation of ultrathin sections of latex particles.

synopsisContinuously uniform latices were applied in an experimental study of molecular weight development in constant-rate styrene emulsion polymerization. The formulation around which this study centered exhibited Smith-Ewart, case 11 kinetics from zero to about 60% conversion with a constant conversion rate of 13 f 277/hr and a final particle diameter of 2300 A. By utilizing an inhibitor perturbation technique, we directly confirmed that free radicals are generated from K2SeOs by a first-order process with 100% efficiency. We further confirmed that, in contrast-to currtnt theories for constant rate polymerization, both the instantaneous values of M , and Me may increase 6-to 9-fold. Little or no chain branching is evidenced. We interpret these findings to mean that radicals are not utilized with 100% efficiency in emulsion polymerization.

The effect of rubber particle size on the tensile properties of rolled and unrolled acrylonitrile-butadiene-styrene has been studied by considering model systems consisting of mixtures of a small particle (0.1 micron diam) rubber, S, and a large particle (0.56 micron diam) rubber, L, in an SAN matrix. Before rolling, tensile toughness is characterized by crazing. While both rubbers induce matrix crazing, ABS systems containing only the S rubber exhibits early failure due to crack formation, before crazing is propagated very far along the tensile axis. The inefficiency of the small particle rubber is interpreted in terms of high composite yield stress and insufficient distance between particles to allow craze branching. The efficiency of the small particle rubber is improved via the addition of a small amount of large particle, L, rubber to the composite or by a slight degree of cold rolling, both of which enhance craze propagation in the tensile direction. With further rolling, the tensile deformation mode changes from one of localized crazing, which is propagated, to one of uniform deformation, which occurs without crazing.