Combining Experimental and Theoretical Tools to Probe Radio-Oxidation Products in Polyethylene

Abstract

Polyethylene is one of the most used polymers in a variety of sectors. A typical technique used to assess aging is infrared spectroscopy. Under oxidation, the region of the spectrum that is most studied is the one containing the carbonyl signature. However, various carbonyl groups contribute to the carbonyl peak: ketones, aldehydes, esters, lactones, carboxylic acids, and more. A usual procedure to quantify each of them is the deconvolution of experimental peaks based on experimental assignments of infrared bands. In this paper, we complement this procedure, applied to two polyethylene types, with extended density functional theory (DFT) calculations of infrared spectra, using a polyethylene model mimicking the main features of a semicrystalline polymer. We compare theoretical frequencies and infrared intensities with parameters extracted from the literature that are used to, eventually, estimate concentrations. We provide an alternative estimation entirely based on theoretical data, showing that DFT can be a valuable tool to analyze, or at least complement, experimental data to assess polymer aging. The comparison of different deconvolution procedures raises the question of the contribution of conjugated ketones in the global carbonyl buildup, as well as that of ketones/alcohols pairs, or the relative concentration of esters and aldehydes.