Melting behavior and equilibrium melting temperatures of syndiotactic polystyrene in α and β crystalline forms

Abstract

Syndiotactic polystyrene (sPS) specimens with neat α- and β-form crystals were prepared by using press molding under different conditions. The crystal form of the as-molded specimens was verified using wide angle X-ray diffraction and Fourier transform infrared spectroscopy techniques. Isothermal crystallization of α- and β-form sPS was conducted at various temperatures Tc and periods tc. The corresponding melting enthalpies and temperatures upon subsequent heating were determined using differential scanning calorimetry (DSC). At given Tcs, the melting temperatures at zero crystallinity were deduced and used for the determination of equilibrium melting temperature, Tm0, for both crystal forms. Based on the linear Hoffman–Weeks (HW) plots, the values of Tm0 for α- and β-form sPS are obtained to be 281 and 291°C, respectively. The corresponding thickening coefficients are ca. 1.89 and 1.58, which seem unexpectedly high in consideration of the limited growth of lamellae at the early stage of crystallization. When a recently developed MX plot that is based on a non-linear HW approach is applied, reasonable thickening coefficients but much larger Tm0 values are derived; being 294 and 320°C for α- and β-form sPS. On the basis of Avrami plot, the β-form crystals show three-dimensional growth, which is different from that for α-form crystals showing one-dimensional growth. For specimens with mixed α/β-crystals, the details of crystal development during the isothermal crystallization at 240°C is provided to account for the observation of the triple melting peaks which is normally detected by DSC heating scan.