Dynamic Mechanical Properties of the Transcrystalline Regions in Two Polyolefins

Abstract

The dynamic Young's moduli E of molded films of a polyethylene and a polypropylene were found to decrease with increasing thickness t. An upper limit of E appears to exist when the film thickness is reduced to less than 1 mil. For thicker films, E is inversely proportional to t. Finally, the modulus approaches a lower asymptotic value when t is in excess of 20 mil. The results can be represented quantitatively by a model in which the surface transcrystalline region and the bulk phase of the polymer respond in a parallel combination to the applied stress. The modulus of the transcrystalline region in each polymer is higher than that of the bulk phase. The thickness of a surface region is about 12 μ in polypropylene and 15 μ in polyethylene. The effects of thermal history and the nature of the molded surface were examined briefly.