Kouichi Hatada

Effect of tie molecules on the craze strength of polypropylene

The fracture mechanism of crazing in polypropylene (PP), the molecular weight of which was controlled by several different methods, was examined. The craze strength was improved with increasing number-average molecular weight in all specimens regardless of the weight-average molecular weight. Examination of the number of tie molecules between neighbouring lamellae suggested that craze fracture was caused by the failure of tie molecules. It may be expected from these results that PP of high fluidity can be produced without a drop in strength if the weight-average molecular weight approaches the number-average molecular weight under the retention of the number-average molecular weight.

Stability of plastic deformation in polyolefins

The difference in the stability of plastic deformation between polypropylene (PP) and polyethylene (PE) was analysed by a theory of instability of plastic deformation around the preformed voids under a dilational stress. It could be concluded that the unstable plastic deformation of PP compared with that of PE is the result of a high ratio of shear yield stress to shear modulus related to the difference in the microstructural changes during deformation.