REPULSIVE WALL EFFECTS LEADING TO GRADUATED CORE SHELL SUPRAMOLECULAR STRUCTURE IN POLYSTYRENE LATEXES

The factors controlling the core-shell supramolecular structure inside latex particles were investigated in a seeded styrene emulsion polymerization by small-angle neutron scattering (SANS). Seed latexes of deuterated polystyrene (polymer I) were swollen with an equal weight of protonated styrene monomer (monomer II) and equilibrated. Latex particles of about 700-800 Å in diameter contained polymers with weight-average molecular weights ranging from 2 x 104 to 8 x 105 g/mol. SANS measurements were done on the latex dispersions in the swollen state, at intermediate conversion, and after the complete conversion of the second-stage polymerization. The existence of segregation was identified as highest in the swollen state and decreasing somewhat as polymerization proceeds. The particle-water interface can be described as a repulsive wall in the de Gennes and Cassasa sense, leading to a concentration gradient of polymer I. This produces a polymer-rich core surrounded by monomer-rich shell. Both SANS results and the theoretical segment density profile indicate that the extent of segregation depends on the relative ratio of the chain radius of gyration to the particle radius, going through a maximum when this ratio becomes about 0.2. The kinetics of emulsion polymerization should be modified to allow for the repulsive wall effect and concomitant segregation. © 1990, American Chemical Society. All rights reserved.