Study of the Interlamellar Folded Structure of Polyethylene as Revealed by Melting Point and Crystallinity

A modification of Knox’s1 approach for the estimation of the average number of noncrystalline methylene groups per fold in polyethylene is suggested. The plot of melting point, T(l), from Thomson’s equation against inverse of crystallinity, α, from a simple model which assumes crystallinity deficiency located at the crystal surface yields a family of straight lines that can be represented by T(l)=T∞−K(1−α)/fα. T∞ is here the melting point of an infinitely thick crystal, K is a constant, and f is the average number of noncrystalline backbone units per fold contributing to the amorphous layer. Comparison of experimental data with this model shows that f depends upon crystallization conditions annealing treatment, and chain defect content (branching). The obtained results clearly indicate that f is notably smaller for single crystals than for melt-crystallized material, in agreement with previous results from Kawai.20 The discussion of f data for single crystals supports the view of a regularly folded surface layer with variable amounts of disorder. The analysis of data derived from the results of Fischer and Schmidt22 in the light of this model indicates that the annealing of crystals provokes an increase in f.