Thermally induced conformational disordering process in high-density polyethylene crystal studied by generalized two-dimensional correlation mid-infrared spectroscopy

Thermally induced conformational changes that occur in high-density polyethylene (HDPE) crystal were studied by mid-infrared (MIR) spectroscopy. Spectral changes of four conformational “defect mode” bands in 1390-1280 cm−1 region were observed during the heating up to the melt. The spectra were analyzed by generalized two-dimensional (2D) correlation technique to elucidate correlations in their responses against temperature. Among the conformational defect bands, two bands at 1368 and 1308 cm−1 have traditionally been assigned to non-planar conformers of gtg′ (kink) and gtg. However, the present study shows the intensity increment of the band at 1368 cm−1 happens at a lower temperature than that of the band at 1308 cm−1. This finding is in favor of the assignment proposed by Cates et al., in which the 1368 cm−1 band is assigned to the gtg conformation excluding the involvement of kink. The spectral correlation among the band at 1368 (gtg), 1353 (double-gauche, gg′), and 1341 cm−1 (end-gauche, eg) has also been studied by 2D correlation analysis. As a result, it was found that the formation of gg′ and eg sequences mostly proceeds at a temperature range higher than 115 °C. The formation of gtg conformer sequence measured by the band at 1368 cm−1 apparently proceeds in two steps: the first at a temperature around 70 °C and the later one occurring at a temperature very close to Tm. The results of this study make correlation relationships clear in the temperature dependency of MIR bands due to conformational disorder sequences.