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

The force-elongation curves and key tensile parameters of a set of polyethy lenes were studied over the temperature range from -100 degrees C to their respective melting temperatures, at a fixed strain rate. The polymers chose n possessed a diverse molecular architecture and constitution. They were cr ystallized in such a manner as to generate a wide range in crystallinity le vels and supermolecular structures. Unique to this work are accompanying di latometric 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 tem perature. The yield stresses of all the polymers were very similar to one a nother in the region of the glass transition temperature. However, they div erged at elevated temperatures, depending on the chain structure, linear or branched, and the level of crystallinity. The change in the ultimate prope rties, the draw ratio, lambda(B), after break a nd the true ultimate tensil e strength, with deformation temperature could be correlated with the chang ing 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 depend ence of the yield stress could not be correlated with the dislocation theor y that has been developed to describe yielding. (C) 1998 Chapman & Hall.