NMR and Dilatometric Studies of Polyethylene Recrystallization

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Abstract
Polyethylene samples with different thermal history were annealed at temperatures slightly below melting point. The chain immobilization as revealed by NMR shows first a decrease corresponding to a partial melting and a subsequent slow continuous increase as a consequence of progressive recrystallization. Dilatometric measurements demonstrate two different mechanisms of crystal growth.Primary crystallization with an Avrami coefficient n=3 and a long induction period for nuclei formation characterizes the initial solidification from melt during which the bulk of the sample crystallizes. The volume reduction at subsequent crystallization of the remaining amorphous areas completely blocked by spherulites produces a negative pressure and hence a local melting point depression strongly hampering further crystal growth. This effect is still enhanced by increased concentration of rejected impurities, e.g., short chain or branched macromolecules contained in the sample, which being intrinsically noncrystallizable at a given temperature also reduce the maximum attainable crystallinity. After this transition period during which n drastically drops and soon loses any physical meaning the secondary crystallization begins, where the change in density is no longer due to an increase in crystal‐to‐total‐volume ratio but only to the increasing thickness of the initially formed crystals accompanied by a reduction of the surface‐to‐volume ratio. The slow density increase with annealing time is fully accounted for by the better order and density in the growing core of lamellar crystals as compared with the less dense surface layers, and by the time dependence of lamellar thickness at the annealing temperature.