Equilibrium Crystallization Temperature of Syndiotactic Polystyrene γ Form

The crystallization behavior of syndiotactic polystyrene ( s PS) γ form undergoing annealing at various temperatures was investigated using the thermodynamic phase diagram based on Strobl’s crystallization theory. On the basis of the differential scanning calorimetric results, it was observed that γ form melt-recrystallization occurred at a higher temperature with the increasing lamellar thickness, which resulted from the pre-annealing at the elevating temperature after acetone induced crystallization. Further temperature dependent small-angle X-ray scattering (SAXS) measurement revealed the evolution of the γ form lamellae upon heating until phase transition, involving three different regimes: lamellae stable region (25−90 °C), melt-recrystallization region (90−185 °C) and pre-phase transition region (185−195 °C). As a result, recrystallization line, equilibrium recrystallization line and melting line were developed for the s PS γ form crystallization process. Since the melt of γ form involved a γ -to- α / β form phase transition, the melting line was also denoted as the phase transition line in this special case. Therefore, the equilibrium crystallization temperature and melting (phase transition) temperatures were determined at around 390 and 220 °C on the basis of the thermodynamic phase diagram of the s PS γ form.