Heat of Fusion of Polymer Crystals: Thermodynamics of Melting

The weight fraction crystallinity (W) of a polymer sample can be determined from the relation W = AH./AH/. However to obtain c f f a more accurate determination of W the term AH. should be cor-c f rected for various excess enthalpy terms. These corrections should include terms related to the nature of the “fold surface,” i. e., the manner in which units in the fold structure are distributed between true folds and cilia. They should also take into account the nature of the crystal, e. g., the possibility of an enthalpy term associated with mosaic block side surfaces in dried-down crystals. The magnitude of the appropriate correction factors (and their affect on W c) has been estimated for various fold/cilia models. From the derived equations it is apparent that the y-intercept of a plot of the form AH./W versus -1 t c (crystal core thickness) is equal to AH° only when dealing with crystals kept in suspension. For dried single crystals the y-intercept is shown to be some function of AH⁰. Since the vast majority of past AH work has been concerned with dried crystals, one must view the reported AH° values with some suspicion.