Dynamic mechanical studies of irradiated polypropylene

The internal friction and the dynamic modulus of isotactic polypropylene samples subject to varying degrees of pile irradiation have been studied at low audio frequencies and over the temperature range from 100°K. to over 400°K. The radiation dosages used varied from 5.4 × 1017 to 14 × 1018 nvt. For irradiation doses up to about 3 × 1018 nvt, one observed change is a decrease in the temperature at which the final upswing in damping and drop in modulus occurs, presumably as a result of defects produced in the crystallites by the irradiation. As the irradiation dose increases above 3 × 1018 nvt, all evidence of crystallinity is lost, and the internal friction peak which occurs in the neighborhood of 300°K. and which is associated with the glass transition of the amorphous regions moves to higher temperatures as a result of the increasing chainstiffening effect due to crosslinking. The secondary loss peak at 250°K. attributed to the motion of a small number of segments in the amorphous regions is found to increase in magnitude with increasing dose. The results of density determinations made before and after irradiation and also after subsequent exposure of the irradiated samples to melting temperatures and of solvent extraction studies of the irradiated samples confirm the simultaneous occurrence of crosslinking and of crystalline degradation upon pile irradiation of polypropylene samples.