Lamellar Thickness Dependence of Crystal Modification Selection in the Syndiotactic Polystyrene γ-to-α/β Phase Transition Process.

The syndiotactic polystyrene (sPS) γ-to-α/β phase transition was investigated using temperature-dependent synchrotron wide-angle X-ray diffraction (WAXD) and small-angle X-ray scattering (SAXS) measurement. The sPS γ form samples, which were obtained from acetone solvent induced crystallization followed by annealing at various temperatures (Ta), exhibited a regular manner on the selection of the α or β crystal modification during the phase transition process. As the Ta was increased from 25 to 150 °C, the weight fraction of β form in bulk sPS decreased from 55% to 0%, but that of the α form increased from 45% to 100%. In other words, the α form became the dominating crystalline modification at higher Ta. The lamellar thickness (dc) of the γ form at the initial state (before heating) ranged from 4.4 to 5.8 nm, and the weight crystallinity (Xc) was between 0.52 and 0.64. However, the dc values of the γ form at the final state (before γ-to-α/β phase transition) were very close for the samples pretreated at different Tas, which were in the range 7.0-7.4 nm. Therefore, the most plausible interpretation for the crystal modification selection during the γ-to-α/β phase transition was that the nucleation of β form was suppressed totally in the γ form with higher initial dc value (>5.8 nm). With the elevated Ta and increase of initial dc, the promoted nucleation of α form in the γ form resulted in the enhanced growth of the α crystalline phase during the γ-to-α/β phase transition. As a result, after the phase transition the crystalline phase in bulk sPS had a gradually increased weight fraction of the α form and turned out to be pure α form when Ta was increased from 25 to 150 °C. [ABSTRACT FROM AUTHOR]