Melting and annealing of polyethylene single crystals

Abstract

Melting and annealing of irradiated and unirradiated polyethylene single crystals have been studied with a differential scanning calorimeter. Crystals grown at 85°C in xylene exhibited multiple melting peaks. These peaks have been related to the lamellar thickening process. When irradiated, these crystals melted at 127°C with a single peak indicating little change in lamellar morphology during the heating process. Isothermal annealing of 85°C crystals between 118°C and 126°C caused an initial lowering in the apparent heat of fusion followed by a gradual increase approximately linear with log time. In addition, the lamellar thickness (as derived from melting points) at each temperature increased linearly with log time. The annealing process in this temperature region was attributed primarily to lamellar thickening. Above 126°C, annealing occurred also by recrystallization of melted lamellae. At 114°C and below, no changes in apparent heat of fusion or lamellar thickening were noted regardless of annealing time. Annealing of irradiated samples was studied as was the effect of irradiation on the apparent heat of fusion and melting point of recrystallized polyethylene. Under all circumstances the apparent heat of fusion was substantially less than the thermodynamic value for ideal crystals.