MULTIPLE MELTING IN SEGMENTED POLYURETHANE BLOCK COPOLYMERS

Multiple melting in crystallizable segmented polyurethane block copolymers has been investigated by simultaneous synchrotron X-ray-differential scanning calorimetry experiments. Results indicate that the origin of multiple endothermic behavior is dependent on the specimen preparation procedure. In melt-crystallized specimens, multiple endotherms are associated with the melting of two distinct hard segment crystal populations; they have different melting points, but have indistinguishable diffraction patterns consistent with a contracted hard segment crystal form. Compression molding leads to hard segment crystals of an extended form. Multiple melting in compression-molded specimens, observed at low heating rates, is associated with melting-recrystallization-melting of the extended crystalline polymorph into a new poorly-organized hard segment structure. This latter structure transforms into the contracted crystalline polymorph if given sufficient time at an appropriate undercooling. The rate of the polymorphic transition to the contracted polymorph depends strongly on the undercooling and does not occur within the time frame of a typical calorimeter experiment.