Metastability studies of syndiotactic polystyrene polymorphism

Polymorphic behavior (i.e., the development of a and β 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 a crystals and β 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 β form in sPS was found to be higher than that of a form. The occurrence of phase stability inversion with lamellar size (i.e., morphological metastability) in sPS was recognized. Transformations from a to β 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 a form is indeed a kinetic result of crystallization.