Metastability studies of syndiotactic polystyrene polymorphism

Polymorphic behavior (i.e., the development of alpha and beta forms) of melt-crystallized syndiotactic polystyrene, sPS, has been studied by structure analysis of FTIR (Fourier transform infrared spectroscopy), WAXD (wide-angle X-ray diffraction), and ED (electron diffraction) as well as thermal analysis of DSC (differential scanning calorimetry). Significant factors that influence the formation of polymorphism were examined and described. Isolated alpha crystals and beta crystals of sPS crystallized at different crystallization temperatures from the melt have been obtained. The equilibrium melting temperatures, T-m(o), of both forms were determined by using linear Hoffman-Weeks (H-W) extrapolation and nonlinear H-W treatment. The T-m(o) (i.e., structural metastability) of beta form in sPS was found to be higher than that of alpha form. The occurrence of phase stability inversion with lamellar size (i.e., morphological metastability) in sPS was recognized. Transformations from alpha to beta phase in the stage of crystal growth or in the heating scanning process have been evidenced. The interlinkage of structural metastability and morphological metastability in sPS polymorphism was examined. The behavior of phase transformation has been successfully interpreted in terms of the stability inversion phase diagram. The formation of the alpha form is indeed a kinetic result of crystallization.