Phase structures of a broad compositional range of polyethylenes by TD-NMR

Abstract

A number of morphological features of polyethylenes across a wide range of densities were studied using time-domain nuclear magnetic resonance (TD-NMR) relaxometry. The populations and T2 relaxation times of both the interphase and amorphous phase exhibit a transition at a density of ca. 935 kg/m3. At a density range of 900–935 kg/m3, the interphase is almost as populous as the crystalline phase. Aging of the samples at ambient temperature was monitored using NMR relaxometry over the course of one month, during which slight decreases in T2 relaxation times of the amorphous phase was found, possibly accompanied by a slight increase of crystallinity, by ca. 0.5 %, and the decrease of the amorphous population by a comparable amount, for the samples with higher crystallinities. The interphase T2 relaxation time was found to exhibit a strong linear correlation with the population of the amorphous phase, similar to what was observed for a series of polypropylenes in a previous study. The heat-treated and aged samples display similar crystallinities as untreated samples at the lower density range, while the former exhibit significantly higher crystallinity than the latter at the higher density range.