Crystallization Condition of Glassy Syndiotactic Polystyrene in Supercritical CO2

Polymorphism of syndiotactic polystyrene (sPS) induced by supercritical CO2 (scCO2) was investigated as functions of temperature and pressure by using infrared spectroscopy and X-ray diffraction. When a glassy sPS was incubated in scCO2 at 18 MPa, a modification with the trans-trans-gauche-gauche (t2g2) conformation appeared between 40 °C and 80 °C, and the planar zigzag form appeared above 60 °C. Both forms coexisted between 60 °C and 80 °C. Below the critical temperature of CO2 (31.1 °C) no crystallization occurred at any pressure. When the pressure of CO2 is below the critical pressure (7.2 MPa), the t2g2 form did not appear at any temperature but the planar zigzag form appeared above 90 °C. These results indicate that incubation in supercritical CO2 is necessary for the formation of the t2g2 form. On the other hand, the planar zigzag form was obtained irrespective of the physical states of CO2, but depended on the incubation temperature and pressure. This indicates that the conformational stability of the planar zigzag form depends only on the thermal factors. The crystallization temperature into the planar zigzag form decreased with increasing pressure of CO2 due to the plasticizer effect of CO2.