Raman spectroscopic observation of conformational changes during thermal exposure and its effects on melting and crystallization behavior of high-density polyethylene

In the present study, we used Raman spectroscopy to reveal conformational changes of high-density polyethylene (HDPE) during long-time thermal exposure in air at 110 °C. In the early stage, we observed an increase in crystallinity complemented by a decrease in the number of non-crystalline short trans sequences owing to structural rearrangement, which is the typical effect of annealing. Deterioration of the mechanical properties, such as a decrease in the strain at break, gradually proceeded, even before chemicrystallization. This will be ascribed to conformational reordering of the crystalline structure, i.e., a gradual increase in crystallinity accompanied by a decrease in both non-crystalline short and long trans sequences. During chemicrystallization, a further increase of crystallinity accompanied by elimination of the non-crystalline short and long trans sequences resulted in embrittlement of the HDPE specimen. Conformational changes in the HDPE specimen after thermal exposure were monitored in situ during heating and cooling. The trans sequence formed during thermal exposure was highly stabilized, resulting in higher crystallinity and trans fractions up to the vicinity of the melting temperature. The detection of non-crystalline short trans sequences can be used to predict the degree of deterioration of thermally exposed HDPE owing to the strong correlation between the decrease in the number of non-crystalline short trans sequences and the strain at break.