Lamellar morphology and equilibrium melting temperature of syndiotactic polystyrene in β-crystalline form

Lamellar morphology and thickness of syndiotactic polystyrene (sPS) samples melt-crystallized at various temperatures were probed using transmission electron microscopy (TEM) and small-angle X-ray scattering (SAXS). In addition, the melting temperature and enthalpy of the crystallized samples were characterized with differential scanning calorimetry. Under appropriate thermal treatments, all the samples investigated in this study were crystallized into β′ crystal modification, as revealed by wide-angle X-ray diffraction. From the SAXS intensity profiles, a scattering peak (or shoulder) associated with lamellar features as well as the presence of anomalous scattering at the zero-scattering vector were evidently observed. The peculiar zero-angle scattering was successfully described by the Debye-Bueche model, and subtraction of its contribution from the raw intensity profiles was carried out to deduce the intensity profile merely associated with the lamellar feature. The lamellar thickness obtained from Lorentz-corrected intensity profiles in this manner agrees with that measured from the TEM images, provided that the two-phase model is applied. On the basis of the Gibbs-Thomson equation, the modest estimations of equilibrium melting temperature and the surface free energy of the fold lamellar surface are 292.7 ± 2.7°C and 20.2 ± 2.6 erg/cm², respectively, when lamellar thicknesses measured by TEM are applied.