Expansion characteristics of expandable styrene/methyl methacrylate copolymer particles: The effects of copolymer molecular weight and particle size

Spherical foamable particles were synthesized by suspension polymerization of methyl methacrylate and styrene in the presence of a blowing agent. When the surrounding temperature was high enough to vaporize the blowing agent and soften the polymer shell, the particles were expanded due to pressure difference across the shell. The effects of initiator and crosslinker concentration on the particle size, average molecular weight, and molecular weight distribution of synthesized copolymer, pentane content and expansion properties (density, residual pentane, and dimensional stability of pre-expanded beads) of expandable particles were investigated. The results showed that the expansion behavior of the particles was dependent on molecular weight of the matrix polymer and size of the synthesized particles. In a constant copolymer molecular weight, the pre-expanded bead density decreased with increasing of particle size whereas it increased with molecular weight in a constant particle size. Moreover, the crosslinker improved dimensional stability of the pre-expanded beads