Suspension polymerization of thermally expandable core/shell particles

Acrylonitrile (AN)-methacrylonitrile (MAN) copolymer particles with a core/shell structure were prepared by suspension polymerization. The particles were about 10-20 μm in diameter and had a hollow core containing an inert hydrocarbon. The influence of the monomer feed ratio and the polymerization temperature on the particle morphology was studied. One purpose of this study was to determine the boundaries for achieving a core/shell structure with the polymer encapsulating the hydrocarbon. When polymerizing at 62 °C, it was found that an initial AN/MAN feed ratio (fAN) between 0.15 and 0.9 results in core/shell particles with encapsulated hydrocarbon. fAN lower than 0.15 yielded solid particles with no hydrocarbon encapsulated while fAN higher than 0.9 yielded particles built up entirely from agglomerates of smaller primary particles. In contrast, when polymerizing at 80 °C, a much narrower span of fAN (0.5-0.85) yielded particles with hydrocarbon encapsulated. The influence on monomer conversion and the molecular weight of the polymer was also studied.