Free Volume Changes, Crystallization, and Crystal Transition Behavior of Syndiotactic Polystyrene in Supercritical CO2 Revealed by Positron Annihilation Lifetime Spectroscopy

Abstract

The free volume of syndiotactic polystyrene (sPS) conditioned in supercritical CO2 was investigated by using positron annihilation lifetime spectroscopy (PALS). Supercritical CO2 increased the free volume cavity size of amorphous sPS and made the formation of γ crystal occur. The orthopositronium (o-Ps) intensity I3 was correlated well with the crystallinity of γ form sPS, revealed by its gradual decrease with the increase of crystallinity. Furthermore, the treatment atmosphere affected the crystallization behavior of amorphous sPS. The increased free volume cavity size of amorphous sPS treated in supercritical CO2 was larger than those treated in ambient air and supercritical N2. Among these treatment conditions, supercritical CO2 increased the free volume size of amorphous sPS more effectively and thus resulted in the formation of γ crystals. PALS measurements indicated that supercritical CO2 helped remaining a constant high free volume level of the amorphous parts of the semicrystalline sPS. Likely, the crystalline parts were also affected by the scCO2 as a transition from the γ form to the β form was revealed at high temperatures by WAXD.