Crystallization and melting of polyethylene under high pressure. II. Effect of pressure on melting behavior of various types of crystals

The effect of pressure on the melting point and volume of fusion of polyethylene was studied by high‐pressure dilatometry. Starting materials were crystallized slowly from the melt under pressures of 1500, 3500, 5130 kg/cm2, and 1 atm. It has been shown that the unusual behavior observed at pressures above 4000 kg/cm2 is due to crystallization and melting of two kinds of extended‐chain crystals differing in thermal stability. These are designated as ordinary extended‐chain and highly extended‐chain crystals, respectively. The relation between pressure P and melting temperature Tm of folded‐chain, ordinary extended‐chain, and highly extended‐chain polyethylene was determined precisely. At pressures up to about 3000 kg/cm2, plots of P against Tm for the crystal forms have almost the same curvature and then become parallel. But at pressures above 4000 kg/cm2, ordinary extended‐chain crystals show a linear increase of Tm with a constant slope of about 70 atm/deg. Curve for the highly extended‐chain crystals changes in slope from 70 to 50 atm/deg at pressures between 3500 and 4300 kg/cm2, and then show a sharp increase of Tm with increasing pressure. Experiments show that the meltingpoint curve of the highly extended‐chain crystals overlaps that of the ordinary extended‐chain crystals at pressures below 4000 kg/cm2. Annealing experiments with folded‐chain and ordinary extended‐chain crystals have been made under high pressure. It is suggested that the formation of highly extended‐chain crystals occurs stepwise through the formation and reorganization of ordinary extended‐chain crystals from the original folded‐chain crystals by a mechanism of partial melting and recrystallization at pressures above 4000 kg/cm2.