Crystallization and melting of polyethylene under high pressure. I. Crystallization by slow cooling from the melt

Crystallization and melting behavior of linear polyethylene under high pressures up to 6000 kg/cm2 has been investigated with a high‐pressure dilatometer. Crystallization was carried out at a cooling rate of 1°C/min from the melt at each pressure. The samples were characterized by differential scanning calorimetry, density, and electron microscopy. Folded‐chain crystals are formed in the low‐pressure region below 2000 kg/cm2. Crystallization in the intermediate‐pressure region between 2000 and 3500 kg/cm2 gives a mixture of folded‐chain and extended‐chain crystals. The extendedchain crystals are the more stable and predominate at increasing pressure. At high pressures above 4700 kg/cm2, two stages of crystallization and of melting can be observed. The phenomenon suggests that the two kinds of extended‐chain crystals with different thermal stability, i.e., the ordinary extended‐chain crystals and “highly extended‐chain” crystals form through individual crystallization processes from the melt at high pressure.