THE PHASE-TRANSITION BEHAVIOR OF LINEAR POLYETHYLENES AT HIGH-PRESSURE

The influence of the pressure on the melting behaviour of different li near polyethylene samples with molar masses from 5 to 4000 kg mol(-1) has been investigated by three methods: crystallization and melting at different pressure in the high pressure DSC, crystallization at diffe rent pressure and subsequent melting at normal pressure, crystallizati on at high pressure (500 MPa) and melting at normal pressure but at di fferent heating rates. The dependence of melting temperature and entha lpy change on crystallization pressure are given. The high pressure cr ystallized material seems to be in a metastable state (after decompres sion) with larger enthalpy and higher melting temperature than in the equilibrium state. The high molar mass sample behaves like a single co mponent system, whereas that with low molar mass shows a complex multi component phase separation behaviour. (C) 1997 Elsevier Science B.V.