Reversible crystallization and melting at the lateral surface of isotactic polypropylene crystals

Reversible crystallization and melting in isotactic polypropylene are suggested to occur at the lateral crystal surface by correlation of the amount of reversibility to the structure observed by atomic force microscopy. The degree of reversibility is monitored on heating using temperature-modulated differential scanning calorimetry. It changes with the crystallization conditions on cooling from the melt. In quenched samples the reversing fraction of the heat flow rate is considerably higher than in well-crystallized specimens. On heating, reversible latent heat develops, starting at about 300 K, just above the glass transition, and continues up to the final melting peak. Irreversible processes in the same temperature range have been separated from the reversible effects and involve cold crystallization, annealing, and a phase transition from the quenched mesophase to the stable crystal form. Because of a similar enthalpy-based crystallinity and crystal thickness in the final melting range above approximate to 400 K, the difference in reversibility of crystallization and melting can be explained by the observed differences in crystal morphology. The slowly cooled sample consists of well-organized, laterally extended lamellae, while the quenched sample has a distinctly globular crystal morphology which permits an increased lateral surface area. Differential scanning calorimetry yields the temperature dependence of the crystallinity, and temperature-resolved X-ray scattering is employed to monitor changes of the crystals during heating.