Melting and Solid-Solid Transition of Polyethylene under Pressure

Abstract

The thermal behaviours of unfractionated polyethylene (PE) was observed up to 31.4 kb by differential thermal analysis, and its pressure-temperature phase diagram was obtained. The thermograms showed three signals, denoted I, II and III. Signal I, the melting of folded chain crystal, was observed up to 8 kb. Below 4 kb, signal II corresponded to the melting of orthorhombic solid; between 4∼8 kb, it corresponded to the superposition of the melting of orthorhombic solid and orthorhombic-hexagonal transition; and above 8 kb, it corresponded to the orthorhombic-hexagonal transition alone. Above 4 kb, signal III had a newly-observed flat shape for a specimen crystallized at elevated pressure, but a peak shape for a specimen crystallized at 1 b. The flat signal corresponded to the continuous melting of hexagonal solid. The effect of the molecular weight distribution which makes PE behave as a multi-component system at high pressure is discussed.