| Abstract: | The kinetics of isothermal crystallization of polyethylene under high pressures ranging from 840 to 5300 kg/cm2 has been studied dilatometrically. The crystallization rate estimated from the half-time of the overall transformation increases markedly with pressure. The Avrami exponent n becomes smaller with increasing pressure. Values of n ≈ 2 for the crystallization at 840 and 1950 kg/cm2, and n ≈ 1 at 5100 and 5300 kg/cm2 were obtained. Differential scanning calorimetry and electron microscopy data are presented. It is concluded that extended-chain crystals grow rapidly, predominantly in one dimension, at high pressure. Relations between log k and Tm/T(ΔT) and Tm2/T(ΔT)2 are nearly linear. Here, k is the crystallization rate constant from an Avrami equation, ΔT = Tm – T, Tm is the melting point, and T is the temperature of crystallization. From the dependence of the slope of the straight line on the crystallization pressure it is concluded that the surface energy of crystal nuclei decreases with increasing pressure. |
