Annealing effects on the crystalline structures of syndiotactic polystyrene after the crystalline $\beta $ to $\alpha $ form structural transition induced by mechanical strain.

The thermal effects on the polymorphic behavior of syndiotactic polystyrene (sPS) after the crystalline structural transition from $\beta $ to $\alpha $ were investigated. Our group has previously reported that $\beta $ form crystals of sPS could transform into $\alpha $ form crystals by mechanical strain at about 200°C. In this study, we investigated possible crystalline structural transitions of pre-stretched sPS by thermal treatments. More specifically, the samples containing $\beta $ form crystals were stretched at temperatures above the glass transition temperature (T$_{\mathrm{g}})$ before annealing. The crystalline structures in the sPS samples were characterized by Fourier-transform infrared spectroscopy and X-ray diffraction analyses. Before the annealing treatment, the samples stretched at near T$_{\mathrm{g}}$ possessed mesomorphic $\alpha $ form crystals, whereas the samples stretched at higher temperatures had more perfect $\alpha $ form crystals. It was also found that the mesomorphic $\alpha $ form crystals, produced by the mechanical strain at lower temperatures, could transform into perfect $\alpha $ form crystals by annealing, and that the amount of $\alpha $ form crystals slightly increased with the increase in the annealing temperature.