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

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 T_c and periods t_c. The corresponding melting enthalpies and temperatures upon subsequent heating were determined using differential scanning calorimetry (DSC). At given T_cs, the melting temperatures at zero crystallinity were deduced and used for the determination of equilibrium melting temperature, T_m⁰, for both crystal forms. Based on the linear Hoffman-Weeks (HW) plots, the values of T_m⁰ 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 T_m⁰ 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.