Effect of pressure on the melting and crystallization of pseudo binary mixtures of polyethylene and high melting temperature diluents

Abstract

The melting and crystallization behaviour of pseudo binary mixtures of polyethylene (PE)/1,3,5-tribromobenzene and PE/hexamethylbenzene were studied at elevated pressures up to 500 MPa by differential thermal analysis (d.t.a.). The phase diagram of these binary mixtures under high pressure was determined and compared with that of a PE/1,2,4,5-tetrachlorobenzene binary mixture. The pressure dependence of the eutectic temperature T e of the PE/1,3,5-tribromobenzene mixture at 1 atm was 0.33°C/MPa and for PE/hexamethylbenzene 0.31°C/MPa. The eutectic composition does not move at elevated pressure. In both mixtures, the area between the liquidus curve of the diluents and the eutectic line of the hypoeutectic region in the phase diagram increases with pressure but the area between the liquidus curve for PE and the eutectic line in the hypereutectic region decreases with pressure. In the PE/1,3,5-tribromobenzene system, the phase transition from the orthorhombic phase to the hexagonal phase of PE does not occur under high pressure but the transition did occur in the PE/hexamethylbenzene system. The d.t.a. study at 1 atm showed that the formation of an extended chain crystal of PE was largely impeded in the mixture with 1,3,5-tribromobenzene and 1,2,4,5-tetrachlorobenzene.