Polymerization-structure-property relationships for rubber modified polystyrenes

Rubber modified polystyrenes were prepared by bulk polymerization with various kinds of styrene-butadiene copolymers (SBR) as reinforcing elastomers. Structural characteristics as well as the mechanical properties were determined. An attempt was made to correlate polymerization conditions with structural parameters and mechanical properties.

Viscosity average molecular weight of the polystyrene matrix was determined by viscometry. Gel-sol proportion was obtained from extraction experiments. Degree of crosslinking was obtained from swelling test data. Engineering stress-strain behavior of the products was obtained from uniaxial tension experiments. Izod impact tests were carried out. Rubber morphology was characterized by Transmission Electron Microscope techniques.

It was found that impact improvement is minimal until the molecular weight of the polystyrene matrix reaches about 220,000, Mv. Impact improvement increases with the amount of gel content (rubber plus graft) and with decreasing degree of crosslinking. The rubber morphology also affects the impact strength. Rubber particle sizes below 0.3 micron do not improve impact resistance. Once this critical size is reached, impact strength increases with particle size. The morphology change with rubber concentration was also documented.