SWELLING OF POLY(STYRENE-CO-METHACRYLIC ACID) LATEX BY ISOPRENE

Carboxylated polystyrene latex was used as seed and isoprene as the se cond-stage monomer in an inhibited, seeded emulsion polymerization rec ipe for studies of monomer swelling kinetics at 80 degrees C during in terval III of an emulsion polymerization. The isoprene was added to th e reactor in small portions using a syringe, and changes in the reacto r pressure were continuously measured. Isoprene was added until a free liquid monomer phase was formed; that was, interval II was reached, a s indicated by no further pressure increase upon the addition of more monomer. When the observed pressure increment, Op(i), per unit isopren e added was plotted as a function of the volume fraction of polymer in the latex particles, v(p), the graph could be divided into 3 domains. The break points in the Op(i) curve could, in an analogous emulsion p olymerization, be identified as the glass transition temperature for t he polymer, the so-called gel point in interval III and the onset of i nterval III. In the second domain, where the v(p) was between the glas s transition temperature, T-g, for the seed polymer and the gel point, the value of Op(i) decreased significantly with increasing monomer co ncentration in the latex particles. This was due to the entropy of mix ing and the monomer acting as a plasticizer in the seed polymer. The r ate of sorption of monomer to the latex particles was low at high valu es of v(p). It then increased rapidly with increasing monomer concentr ations in the latex particles, [M](p), and a maximum was observed in d omain 2. At lower values of v(p) the sorption rate decreased in domain 3 and finally became zero as the free liquid monomer phase started to form. Results from batch polymerization suggested that the rate of di ffusion of adsorbed monomer and oligo radicals into the particles was retarded. A simplified form of the Vanzo equation was used to estimate the monomer partitioning. It predicted too high a value of [M],, espe cially in domain 2 of the swelling process. (C) 1998 John Wiley & Sons , Inc.