Amorphous-phase miscibility and crystal phases in blends of polymorphic poly(hexamethylene terephthalate) with monomorphic poly(pentamethylene terephthalate)

Annealing or heat scan-induced lamellar thickening and factors influencing crystal unit cells in polymorphic poly(hexamethylene terephthalate) (PHT) were probed using polarized-light optical microscopy (POM), differential scanning calorimetry (DSC), wide-angle X-ray diffraction (WAXD), synchrotron small-angle X-ray scattering (SAXS). The DSC and WAXD results show that post-scanning or annealing (up to 140°C) on 110°C-crystallized PHT neither transform crystal cell types nor does thin lamella into thick lamella. Rapid lamellar thickening in PHT could take place during DSC scanning, which was proven by comparing SAXS data. T_max (final temperature of heating); within a range of 180 up to 220°C) does not influence the polymorphism or multiple melting peaks in PHT. All evidence suggests that kinetic factors are less influential on the polymorphism in PHT. Further, polymorphic poly(hexamethylene terephthalate) (PHT) was blended with monomorphic poly(pentamethylene terephthalate) (PPT) to form a crystalline/ crystalline blend system. The semicrystalline PPT and PHT are miscible in the melt state or quenched amorphous phase. The miscibility, via weak intermolecular interactions, in the amorphous phase of the PHT/PPT blends exerts almost no influence on the crystalline domains, where PPT does not interfere with the formation of α or β crystal forms in PHT, and vise versa, PHT does not interfere with the sole α-crystal form in PPT.