Melting by temperature-modulated calorimetry

Well-crystallized macromolecules melt irreversibly due to the need of molecular nucleation, while small molecules melt reversibly as long as crystal nuclei are present to assist crystallization. Furthermore, imperfect crystals of low-molar-mass polymers may have a sufficiently small region of metastability between crystallization and melting to show a reversing heatflow component due to melting of poor crystals followed by crystallization of imperfect crystals which have insufficient time to perfect before the modulation switches to heating and melts the imperfect crystals. Many metals, in turn, melt sharply and reversibly as long as nuclei remain after melting for subsequent crystallization during the cooling cycle. Their analysis is complicated, however, due to thermal conductivity limitations of the calorimeters. Polymers of sufficiently high molar mass, finally, show a small amount of reversible, local melting that may be linked to partial melting of individual molecules. Experiments by temperature-modulated calorimetry and model calculations are presented. The samples measured included poly(ethylene terephthalate), poly(ethylene oxide)s, and indium. Two unsolved problems that arose from this research involve the origin of a high, seemingly stable, reversible heat capacity of polymers in the melting region, and a smoothing of melting and crystallization into a close-to-elliptical Lissajous figure in a heat-flow versus sample-temperature plot. (C) 1998 Elsevier Science B.V. All rights reserved.