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 quantita tively by a model in which the surface transcrystalline region and the bulk phase of the polymer respond in a parallel com bination to the applied stress. The modulus of the trans crystalline region in each polymer is higher than that of the bulk phase. The thickness of a surface region is about 12μ in polypropylene and 15μ in polyethylene. The effects of thermal history and the nature of the molded surface were examined briefly.
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