Elastic factors controlling sorption and transport properties of polyethylene

Abstract The const rainment of the noncrystalline regions in polyethylene samples with various thermal and mechanical histories has been determined from measurements of the degree to which the activity of para ‐xylene sorbed in the polymer is enhanced relative to its hypothetical value in the absence of crystallites. With increasing temperature, the enhancement decreases smoothly and vanishes at the polymer melting point (∼135°), while the degree of crystallinity of the polymer remains virtually constant below 100°C. This dependence of the solvent activity enhancement on temperature rather than on degree of crystallinity coupled with the observation that the behavior is reversible below 100°C. suggests that the tension on the intercrystalline tie chains is in equilibrium with the free energy driving force of crystallization. Thermodynamic analysis provides a means of calculating the fraction of the noncrystalline polymer incorporated in tie chains. This elastically effective fraction increases from 0.25 to 0.35 with the rate at which the polymer is cooled from the melt, and approaches unity for highly oriented, cold‐drawn samples. The elastically ineffective fraction of the noncrystalline polymer presumably consists of chain folds, loops, ends, and short molecules.