Melting by temperature-modulated calorimetry

Well-crystallized macromolecules melt irreversibly due to the need of molec ular nucleation, while small molecules melt reversibly as long as crystal n uclei are present to assist crystallization. Furthermore, imperfect crystal s of low-molar-mass polymers may have a sufficiently small region of metast ability between crystallization and melting to show a reversing heatflow co mponent due to melting of poor crystals followed by crystallization of impe rfect crystals which have insufficient time to perfect before the modulatio n switches to heating and melts the imperfect crystals. Many metals, in tur n, melt sharply and reversibly as long as nuclei remain after melting for s ubsequent crystallization during the cooling cycle. Their analysis is compl icated, however, due to thermal conductivity limitations of the calorimeter s. Polymers of sufficiently high molar mass, finally, show a small amount o f reversible, local melting that may be linked to partial melting of indivi dual molecules. Experiments by temperature-modulated calorimetry and model calculations are presented. The samples measured included poly(ethylene ter ephthalate), poly(ethylene oxide)s, and indium. Two unsolved problems that arose from this research involve the origin of a high, seemingly stable, re versible heat capacity of polymers in the melting region, and a smoothing o f melting and crystallization into a close-to-elliptical Lissajous figure i n a heat-flow versus sample-temperature plot. (C) 1998 Elsevier Science B.V . All rights reserved.