Effect of Crystallization Conditions and Heat Treatment on Polyethylene: Lamellar Thickness, Melting Temperature, and Density

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Abstract
Lamellar thickness, melting temperature, and density were measured for bulk polyethylene crystallized by the application of a fixed pressure to molten polymer at different temperatures. Melting temperature varied from 135.2° to 139.2°C, and density varied from 0.9759 to 0.9842 g/cc. Small-angle x-ray diffraction yields two maxima which apparently are not first and second orders of diffraction. Electron micrographs of fracture surfaces exhibit lamellae, which support the suggestion that the lamellar thickness is the shorter of the two dimensions (469≤L1≤981 Å and 193≤L2≤440 Å) corresponding to the diffraction maxima. However, the functional relationship of both dimensions to melting temperature and density are in accord with existing or easily derived equations. Analysis of these data, together with the result obtained from paraffin data, yields an equilibrium melting temperature of 141.9±1.7°C. Dimensions of the unit cell did not vary with lamellar thickness. Single crystals of polyethylene were collected into mats and heated at a fixed temperature under various pressures. The observed effect of pressure was to limit the fold period attained during heat treatment. The data suggest that the refolding process follows a relationship which describes crystallization from the melt.