Thermal behaviour of homogeneous ethylene-1-octene copolymers and linear polyethylene at high pressures

The thermal behaviour of a linear polyethylene (LPE) and of homogeneous ethylene-1-octene copolymers (2.1, 5.2 and 8.0 mol% of 1-octene) was investigated at high pressures by use of a high-pressure differential scanning colorimetry (DSC). The heating curves of LPE at pressures above 400 MPa reveal a three-peak pattern (peaks denoted by I, II, III at increasing temperature). It reflects the melting of orthorhombic folded chain crystals (I); the melting of extended chain crystals, superposed on the orthorhembic-hexagonal solid-solid transition (II) and the melting of the hexagonal phase (III). Introduction of a small amount of 1-octene results in a similar multiple-peak pattern but with a weakened high-temperature peak. With increasing comonomer content this three-peak pattern evolves over a two-peak pattern (I, II) into a single-peaked endotherm (I). The influence of the heating rate at elevated pressure as well as the influence of the cooling rate at ambient pressure on the melting behaviour at high pressure are also studied. The data could be fitted well using simple equations making an indicative prediction possible for the crystallisation and melting peak temperatures of homogeneous ethylene-1-octene copolymers as a function of comonomer content and pressure. Besides, the possible occurrence of extended ethylene sequence crystals in homogeneous ethylene copolymers, instead of extended chain crystals (both denoted by II) is discussed. (C) 2000 Elsevier Science Ltd. All rights reserved.