The expanding behavior of vinylidene chloride(VDC)-acylonitrile(AN)-styrene(St) terpolymer particles containing 1-fluoro-1

1-dichloroethane(HCFC-141b) was monitored on line by a microscope equipped with a heating apparatus and an automatically photographing system．Effects of the factors such as copolymer composition

expanding temperature

concentration of blowing agent and particle size on the expanding behavior were investigated．It was found that an increase of AN or St units in VDC-AN-St terpolymer led to a decrease of expanding rate of particles．The expanding rate of VDC-AN-St copolymer particles increased with the increase of temperature and concentration of blowing agent

and decreased with the increase of the size of VDC terpolymer particles due to the loss of blowing agent to the surroundings. An “overshoot” phenomenon of particle growth was observed at high expanding rate

which may be attributed to the existence of inertia growth in free expansion of polymer particles．