Influence of seed polymer molecular weight on polymerization kinetics and particle morphology of structured styrene-butadiene latexes

Heterogeneous film-forming latexes were prepared using two-stage, seeded em ulsion polymerization, The polymerization was performed in a calorimetric r eactor with a control unit that monitored the reaction rate and controlled the charging rate of the monomers. Three types of styrene seed latexes were prepared at 70 degrees C. The first was an unmodified polystyrene IFS) lat ex. The second had the molecular weight lowered by the use of carbon tetrac hloride (CCl4) as a chain-transfer agent, added at the start of the polymer ization. For the third one, divinylbenzene (DVB) was used as a comonomer. D VB was added under starved conditions near the end of the polymerization to achieve crosslinked particle shells and to introduce double bonds as possi ble grafting sites. The second polymerization step was performed at 80 degr ees C as a batch operation in a 200-mL calorimeter reactor. The second-stag e polymer was poly(styrene-co-butadiene-co-methacrylic acid) (S/B/MAA). A f ixed S/B ratio was used together with varying small amounts of MAA. Particl e morphology and particle-size distributions were examined after the second stage using TEM after staining nifh osmium tetroxide. The particle morphol ogy was found to depend on both the seed composition and the amount of MAA used in the second stage. Molecular weight and crosslinking of the DVB-cont aining seed influenced the internal particle viscosity, which gave differen ces in the polymerization rate and the particle morphology. Crosslinking of the second-stage polymer decreased the monomer concentration in the partic les, which could be detected as a change in the slope the pressure/conversi on curve. This phenomenon was used to indicate the critical conversion for crosslinking of the second-stage polymer. (C) 2000 John Wiley & Sons, Inc.