Effect of pressure on the crystallization kinetics of polyethylene

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Abstract
Volume changes under pressure up to 5000 kg/cm2, at various crystallization temperatures, were investigated for two unfractionated and five fractionated polyethylenes. The shapes of the crystallization curves in the pressure range from 1000 to 2000 kg/cm2 were similar to those at ordinary pressure, and they were scarcely affected by the character of the samples. The n value calculated from the Avrami equation varied from 2 to 3 and was in good agreement with morphological investigations. In the pressure range from 4000 to 5000 kg/cm2, the crystallization was accelerated by the pressure in the first stage of crystallization. The n values varied from 1 to 2, in excellent accord with the presence of extended-chain-type crystals. The molecular weight distribution of the samples played an important role in this pressure range. Fractionated samples with narrow molecular weight distribution were crystallized in two stages over a wide range of temperatures and molecular weights. It is suggested that the two-stage crystallization is closely related to the crystallization mechanism i.e., nucleation and growth, of extended-chain crystals.