MECHANICAL RROPERTIES OF POLYPROPYLENE-STYRENE GRAFT COPOLYMER

Abstract

Static and dynamic mechanical properties of polypropylene-styrene graft copolymer were studied in comparing with those of polypropylene-polystyrene blend and each homopolymer. These properties were interpreted in relation with their fine structure.Yield stress and strength at break of graft copolymer were increased with the increase of styrene content, but Young's modulus did not change remarkably and elongation at break decreased with grafting more than the value of polypropylene homopolymer but was still greater than the value for polystyrene homopolymer and blend.In stress-strain curve, a yield point was observed in graft copolymers and for polypropylene, but not observed in blend and polystyrene.The shape of loss modulus-temperature curve and the peak position of the absorption curve for the graft copolymer are different from that in the blend. Moreover, the curve for the graft copolymer whose powder or small pieces of the film was mixed and melt pressed was different from the curve for the original graft copolymer film, annealed after grafting. The absorption curve for the graft copolymer and blend showed two peaks; one at about 10°C. and another at 110-120°C. The former absorption indicates the αa-absorption of polypropylene which is due to the segmental motion of amorphous chains of the polymer and the latter absorption indicates the main dispersion of polystyrene.The peak temperature of the absorption curve at the higher temperature region for the original graft copolymer and blends agrees with the temperature for polystyrene, but the peak temperature for which was melt pressed and annealed graft copolymer shifted about 10°C. to higher temperature side from the peak temperature for polystyrene. This shift suggests that the molecular mobility of polystyrene branches in the graft copolymer may be restricted by the crystallites of polypropylene. In other words, polystyrene branches exist in the circumstance where crystallites of polypropylene may affect the molecular motions of the polystyrene branches.In the case of blends polystyrene molecules must be excluded from crystallites, and in the original graft copolymer the grafting must take place only on the amorphous chains of polypropylene.