Effect of Compressed CO2 on Phase Transitions and Polymorphism in Syndiotactic Polystyrene

When treated with compressed CO 2, syndiotactic polystyrene (sPS) undergoes a number of solid−solid transitions that do not occur on treatment with liquid solvents. For example, planar mesophase → β, α → β, and γ → β transitions can be brought about under appropriate conditions of temperature and CO 2 pressure. In addition, the transitions of glassy sPS to the planar mesomorphic and to the α form, and the γ → α transition occur at temperatures lower than when the same transitions are effected under ambient pressure. The dissolved CO 2 lowers the glass transition and the cold crystallization temperatures of sPS at the rate of −0.92 and −0.58 °C/atm, respectively. Crystallization kinetics from the sPS−CO 2 solution follow the Avrami equation, but the value of the exponent n is lower than when crystallization is conducted under ambient pressure.