Effect of Molecular Weight on Phase Transition of Polyethylene under High Pressure

Abstract

The effect of the molecular weight distribution which makes polyethylene (PE) behave as a multi-component system in the phase transition under high pressure suggested by the authors from thermal study was confirmed by an X-ray method. Fractionated PE crystallized into an extended-chain crystal (ECC) showed the equilibrium successive first-order phase transition of a one-component system. But a PE mixture of two different molecular weights or unfractionated PE crystallized into ECC showed continuous melting of hexagonal solid characteristic of the melting of binary or multi-component systems. PE crystallized into folded-chain crystal (FCC), on the other hand, showed no equilibrium transition. These results confirm our previous suggestion and it is concluded that only monodisperse ECC-PE can be regarded as a thermodynamically pure material.