Effects of layered silicates on the confined crystalline morphology of poly(hexamethylene terephthalate)

The multiple melting behavior and the associated crystalline forms of poly(hexamethylene terephthalate) (PHT) in PHT-clay hybrids were comprehensively analyzed using differential scanning calorimetry (DSC), wide-angle X-ray diffraction (WAXD) and polarized-light microscopy (PLM). The intercalated morphology of the organoclays (C10A and C-CPC) as evident from transmission electron microscopy (TEM) possesses different orders of layer stacking for various silicates. The samples melt-crystallized at 120 and 130degreesC showed four and three melting peaks respectively upon DSC scanning, displaying various fractions of both alpha and beta crystals of PHT. However, neat PHT exhibited only two melting peaks when melt-crystallized at 130degreesC. Unlike neat PHT, which contains predominantly alpha forms at these temperatures, the clay particles induced mainly beta crystals through heterogeneous nucleation. Additionally, two different forms of spherulites were identified in melt-crystallized samples, with one being a typical Maltese-cross spherulite containing the alpha crystal, and the other being a dendrite-type packed mainly with the beta crystal. In PHT-C-CPC nanocomposites, the presence of alpha-type crystal packing in significant fractions was ascribed to the unique polymer-silicate interactions in the confined space of much ordered layer stacking. In all, the inclusion of layered silicates lends further supports for crucial interpretations of relationships between multiple melting and polymorphisms (unit cells and spherulites) in semi-crystalline polyesters like PHT.