Effect of molecular weight on high pressure crystallization of linear polyethylene. I. Kinetics and gross morphology

Ten linear polyethylenes ranging from M_w = 4.9 × 10⁴ to 4.6 × 10⁶ were crystallized in a dilatometer at 0.51 GPa and 242°C and then cooled slowly. Volume vs time data were used to follow the kinetics of the crystallization. The dilatometer data for the isothermal part of the crystallization were fitted to the Avrami equation. The time exponent was independent of molecular weight and the average was n = 2.2. Electron microscopy of fracture surfaces showed that all of the polyethylenes crystallized in extended chain morphology. The crystalline order and maximum extended chain crystallite thickness decreased with increasing molecular weight. The dominant morphological feature of the crystallized high molecular weight samples was a strand-like network superstructure. Attempts to stabilize the hexagonal structure formed in the isothermal part of the crystallization failed, and all specimens had only the usual orthorhombic crystal structure.