Spectral Inter-Conversion Analysis of Conformational Defects in Polyethylene Crystals

Abstract

Conformational discrimination of polyethylene crystals during a heating process was carried out using a novel spectral inter-conversion analysis based on the combination of near-infrared (NIR) and mid-infrared (mid-IR) spectroscopy. Two mid-IR and two NIR spectral ranges relating to different conformational sequences were explored at 710–752 cm−1 and 4300–4362 cm−1 (corresponding to the orthorhombic), and 1328–1374 cm−1 (relating to the defect) and 4235–4295 cm−1 (corresponding to the unconfirmed conformation). Five partial least squares-2 (PLS-2) models were built based on these four infrared spectral ranges and were used to make the basis for spectral inter-conversion analysis. By comparing the conformational disordering induced by thermal stimulus, the attribution of an obscure NIR band at 4265 cm−1 was confirmed. The temperature dependency of the band at 4265 cm−1 in the NIR range was very similar to that of the bands at 1368, 1353, and 1341 cm−1 in the mid-IR range, indicating they had a common spectral origin. The band intensity at 4265 cm−1 was also extracted from the predicted spectra and compared with the intensity of the measured spectra. The excellent matching in the trend of intensity changes with temperature further demonstrated the same vibrational origin of the NIR band at 4265 cm−1 with respect to the mid-IR bands at 1368, 1353, and 1341 cm−1. The quantitative comparison of the prediction deviation values between different multivariate spectral models further validate the assignment of the band at 4265 cm−1 to the conformational defect sequence formed in the heating process.