Analysis of the degree of reversibility of crystallization and melting in poly(ethylene-co-1-octene)

The degree of reversibility of crystallization and melting of poly(ethylene-co-1-octene) containing 7.3 mol % (24 mass %) of 1-octene was measured with temperature-modulated differential scanning calorimetry. The sample was crystallized at different cooling rates after excluding self-nuclei of the crystals. The sample was also examined by X-ray scattering techniques, infrared spectroscopy, and standard differential scanning calorimetry. The cooling rates ranged from 0.1 K min(-1) to quick quenching. The low-temperature enthalpy remained constant in all cases, by increasingly creating a pseudohexagonal crystallinity of lower heat of fusion instead of a smaller amount of orthorhombic crystals. The crystallization during cooling from the nonseeded melt is characterized by a steplike crystal growth, a characteristic not seen on slow, seeded, crystallization. The initial growth is followed by a continuous increase of the crystallinity down to the glass transition. The onset of crystallization shifts to lower temperatures with increasing cooling rates. With increasing cooling rate, the perfection and the size of the initially formed crystals decrease. On slow crystallization orthorhombic crystals grow; on fast crystallization, these are increasingly replaced by the less-ordered, pseudohexagonal crystals. These changes are paralleled by a systematic increase of the reversible crystallization and melting by about 10% when increasing the cooling rate from 1 to 10 K min(-1). For the first time these surprising observations have been dic;cussed on the basis of structural information.