Spectral inter-conversion analysis of thermally induced structural changes in polyethylene crystals

Abstract Temperature dependence of conformational disordering in crystals of polyethylene was investigated by the inter-conversion analysis based on the combination of NIR and MIR spectra by means of the partial least-squares (PLS2) regression. Two regression models were built based on the spectral range of 1500–1250 cm −1 and 4400–4200 cm −1 to make the prediction in a wider spectral range. The other models were explored in the relatively narrow two MIR range of 1365–1355 and 1471–1461 cm −1 (due to the hexagonal phase); and three NIR range of 4354–4344 (corresponding to the hexagonal phase), 4327–4317 (relating to the orthorhombic phase) and 4261–4251 cm −1 (previously unconfirmed conformation). Total of twelve PLS2 models were built based on these infrared spectral ranges and used to carry out the spectral inter-conversion analysis. By spectral inter-conversion, the conformational disordering induced by thermal stimulus was clearly differentiated, and the assignment of a band at 4256 cm −1 in the NIR spectral range was confirmed. The qualitative analysis indicates that the temperature dependency of the band at 4256 cm −1 in the NIR range is very similar to that of the bands at 1360 and 4349 cm −1 (which corresponded to the hexagonal phase) indicating they have the common spectral origin. The quantitative comparison of the prediction deviation values between different multivariate spectral models firmly validates the assignment of the band at 4256 cm −1 to the hexagonal sequence formed in the thermally induced conformational disordering.