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 À1 and 4300-4362 cm À1 (corresponding to the orthorhombic), and 1328-1374 cm À1 (relating to the defect) and 4235-4295 cm À1 (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 À1 was confirmed. The temperature dependency of the band at 4265 cm À1 in the NIR range was very similar to that of the bands at 1368, 1353, and 1341 cm À1 in the mid-IR range, indicating they had a common spectral origin. The band intensity at 4265 cm À1 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 À1 with respect to the mid-IR bands at 1368, 1353, and 1341 cm À1. The quantitative comparison of the prediction deviation values between different multivariate spectral models further validate the assignment of the band at 4265 cm À1 to the conformational defect sequence formed in the heating process.

An application of concatenated 2D correlation spectroscopy: Exploration of the reversibility of the temperature-induced hydration variation of poly(N-isopropylmethacrylamide) in aqueous solution

“…Wu et al [24] have explored the reversibility of polymer process by using hybrid 2D correlation spectroscopy. Recently, they proposed ''concatenated" 2D correlation spectroscopy, which also has a great advantage in revealing process reversibility [25]. In the present study, we demonstrate the application of this novel technique in analyzing the reversibility of the temperature-induced hydration variation of PNiPMA in water solution.…”

“…4a) is just the same as that of matrix H, while synchronous spectra of the roundtrip data matrix R (Fig. 4c) is the average of those generated from matrices H and C [25].…”

“…The mirror image concatenation completely eliminates the effect of intra-asynchronicity within a matrix. Since matrix H has no interasynchronicity with its mirror image, it is quite reasonable to produce no meaningful cross-peak in the asynchronous spectrum of the mirror image concatenated matrix V [25].…”