Melting behavior and equilibrium melting temperatures of syndiotactic polystyrene in alpha and beta crystalline forms

Syndiotactic polystyrene (sPS) specimens with neat alpha- and beta -form cr ystals were prepared by using press molding under different conditions. The crystal form of the as-molded specimens was verified using wide angle X-ra y diffraction and Fourier transform infrared spectroscopy techniques. Isoth ermal crystallization of alpha- and beta -form sPS was conducted at various temperatures T, and periods t(c). The corresponding melting enthalpies and temperatures upon subsequent heating were determined using differential sc anning calorimetry (DSC). At given T(c)s, the melting temperatures at zero crystallinity were deduced and used for the determination of equilibrium me lting temperature, T-m(0), for both crystal forms. Based on the linear Hoff man-Weeks (HW) plots, the values of T-m(0) for alpha- and beta -form sPS ar e obtained to be 281 and 291 degreesC, respectively. The corresponding thic kening 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 cryst allization. When a recently developed MX plot that is based on a non-linear HW approach is applied, reasonable thickening coefficients but much larger T-m(0) values are derived; being 294 and 320 degreesC for alpha- and beta -form sPS. On the basis of Avrami plot, the beta -form crystals show three- dimensional growth, which is different from that for a-form crystals showin g one-dimensional growth. For specimens with mixed alpha/beta -crystals, th e details of crystal development during the isothermal crystallization at 2 40 degreesC is provided to account for the observation of the triple meltin g peaks which is normally detected by DSC heating scan. (C) 2001 Elsevier S cience Ltd. All rights reserved.