Effects of mechanical strain and heat on the strain-induced crystalline $\beta $ to $\alpha $ structural transition of syndiotactic polystyrene.

The polymorphic behavior of syndiotactic polystyrene (sPS) during the $\beta $ to $\alpha $ form transition was investigated. sPS presents complex polymorphism with five crystalline forms. Quite a few crystalline structural transitions have also been reported, including our recent discovery of the structural transition from $\beta $ to $\alpha $ forms induced by tensile deformation at around 200°C. In this study, we analyzed the individual effects of mechanical strain and heat on the $\beta $ to $\alpha $ crystalline structural transformation caused by the mechanical deformation. sPS film samples containing $\beta $ form crystals were prepared and stretched at 130°C (near the glass transition temperature of sPS), followed by the annealing process of the samples below the melting temperature. X-ray analyses revealed that the stretched sample possessed mesomorphic $\alpha $ forms, indicating that the mechanical strain could invoke the destruction of $\beta $ form crystals by producing mesomorphic $\alpha $ forms. Interestingly, the annealed samples exhibited sharp X-ray reflections typical of $\alpha $ forms, which became even sharper by the increase in the annealing temperature. It was therefore concluded that the heat could induce the structural transitions from mesomorphic $\alpha $ forms to perfect $\alpha $ forms.