Empty δ crystal as an intermediate form for the δ toγ transition of syndiotactic polystyrene in supercritical carbon dioxide

The crystal transition behavior of delta form syndiotactic polystyrene (sPS) in supercritical CO2 was investigated in detail by using Fourier transform infrared spectroscopy and differential scanning calorimetry. The empty delta, form sPS was obtained after treating the delta form in supercritical CO2 of 12 MPa in a range from 35 to 80 degrees C. At higher temperatures, the delta form transformed into the delta(e) form transiently and then into the gamma form. The existence of the intermediate delta(e) form lowered the temperature for the delta to gamma transition. At 240 degrees C, supercritical CO2 of 12 MPa transformed the delta crystal form into more stable beta form without passing through the delta(e) form. Increasing the pressure of supercritical CO2 made the delta to delta(e). transition occur easily and decreased the temperature for the delta to beta transition.