The influence of the deformation temperature on the tensile properties of polyethylenes

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The force–elongation curves and key tensile parameters of a set of polyethylenes were studied over the temperature range from −100°C to their respective melting temperatures, at a fixed strain rate. The polymers chosen possessed a diverse molecular architecture and constitution. They were crystallized in such a manner as to generate a wide range in crystallinity levels and supermolecular structures. Unique to this work are accompanying dilatometric studies. These enabled the changing level of crystallinity with temperature to be monitored. The force–elongation curves that were obtained varied in a systematic manner with the chain structure and deformation temperature. The yield stresses of all the polymers were very similar to one another in the region of the glass transition temperature. However, they diverged at elevated temperatures, depending on the chain structure, linear or branched, and the level of crystallinity. The change in the ultimate properties, the draw ratio, λB, after break and the true ultimate tensile strength, with deformation temperature could be correlated with the changing level of crystallinity. The temperature dependence of these properties are strongly dependent on molecular weight and, except for the very highest molecular weights, a maximum is observed. Possible mechanisms that govern the ultimate properties are presented and discussed. The temperature dependence of the yield stress could not be correlated with the dislocation theory that has been developed to describe yielding.

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