Empty δ Crystal as an Intermediate Form for the δ to γ Transition of Syndiotactic Polystyrene in Supercritical Carbon Dioxide

The crystal transition behavior of δ form syndiotactic polystyrene (sPS) in supercritical CO 2 was investigated in detail by using Fourier transform infrared spectroscopy and differential scanning calorimetry. The empty δ e form sPS was obtained after treating the δ form in supercritical CO 2 of 12 MPa in a range from 35 to 80 °C. At higher temperatures, the δ form transformed into the δ e form transiently and then into the γ form. The existence of the intermediate δ e form lowered the temperature for the δ to γ transition. At 240 °C, supercritical CO 2 of 12 MPa transformed the δ crystal form into more stable β form without passing through the δ e form. Increasing the pressure of supercritical CO 2 made the δ to δ e transition occur easily and decreased the temperature for the δ to β transition.