Abstract

Raman spectra of very high signal quality in the skeletal optical region were obtained for linear polyethylene at three different pressures: ambient, 1.03 kbar, and 3.84 kbar. We have employed the factor analysis technique to decompose a series of spectra at temperatures near the melt. We found that there were two non-trivial abstract spectra for data at ambient pressure and 1.03 kbar and three non-trivial abstract spectra for the 3.84 kbar data. The plots of the third reconstruction coefficients vs. temperature gave scattered points in the first two cases and a structured curve in the case of the highest pressure. Based on these observations, we conclude that there are two linearly independent contributions (crystalline and melt) to the two lower pressure data sets and three (crystalline, melt, and intermediate) to the highest pressure data set. We believe this is the first piece of evidence for the existence of the high-pressure phase of polyethylene from spectroscopically derived measurements.

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