A differential thermal analysis study of the effects of thermal history on polyethylene

The melting and crystallization behavior of polyethylene samples has been investigated by differential thermal analysis (DTA), with special emphasis on the effects of thermal conditioning on the crystalline state of the sample. Techniques are outlined for impressing a wide variety of thermal histories on the sample in the DTA cell. In particular, the use of an annealing procedure is shown to provide a significant evaluation of the extrapolated value of the equilibrium melting temperature, combined with a rapidity not possible by the dilatometric method. Theoretical implications of the various procedures and results are briefly discussed. Critical examination of the fine structure observed in the melting thermograms leads to the conclusion that under certain conditions crystallites originating from both heterogeneous and homogeneous nucleation can be detected. The temperature rise associated with very fast crystallization is considered to influence the subsequent melting temperature more significantly than the effects of re‐crystallization during the actual melting cycle. Some illustrations are given of the general usefulness of the conditioning procedures in clarifying phenomena observed with various samples.