Abstract
By means of stepwise crystallization or heat treatment of high-pressure polyethylene samples with several melting peaks can be produced. The melting curves truly reflect the thermal pre-history of the sample, and during melting a distinct “thermal memory effect” can be observed. The stepwise heat treatment produces individually and independently melting crystallites. The thermal and mechanical stability of crystallites formed by stepwise heat treatment has been studied. The crystallites are stable below the temperature of heat treatment, and also possess a remarkable mechanical stability, retaining the original thermal properties even under very high specific elongation. The change in the character of melting curves as a function of elongation is in unanimous correlation with the structural changes taking place under stretching. The memory effect, strongly expressed in the case of high-pressure polyethylene, can be traced back to the irregular, branched chain structure.
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