Relationship between impact strength and spherulite growth in linear polyethylene

Abstract

AbstractSamples of Ziegler‐type polyethylene containing no additives were placed in a temperature‐programmed oven, melted, and subsequently cooled at constant rates. Average spherulite size, based on the measurement of fifty spherulites, and impact strength by a falling ball method were determined for different cooling rates and for samples having weight‐average molecular weights of 125,000 175,000, and 300,000. It was found that both the formation of a brittle structure and the growth of spherulites occur over the relatively narrow temperature range in which the molten polymer passes from the liquid to the solid state. The rate of spherulite growth is initially proportional to the square root of time, viz., diffusion controlled, but subsequently becomes linear with time. The spherulite growth rate decreases with increasing molecular weight. There is no change in spherulite size after aging six months at room temperature. The impact strength decreases with increasing average spherulite size; however, the lower the average molecular weight, the greater will be the impact strength for a given average spherulite size. The loss in impact strength is attributed to the rupture of amorphous chain strained by the formation and growth of spherulites.