High pressure miscibility and phase separation of syndiotactic polystyrene in toluene and toluene + CO2 mixtures have been studied in a variable volume view cell at pressures in the range from 10 to 50MPa. Instead of a solid-fluid phase separation which is typical for semicrystalline polymer solutions [1,2], a sol-gel phase separation was observed for syndiotactic polystyrene in toluene and toluene + CO2 mixture. The sol-gel boundaries were determined for different polymer concentrations in the range from 1 to 13 wt % in pure toluene. In toluene + CO2 mixtures, the concentration of polymer in solution was fixed at around 4.5 wt %, while the CO2 content in the fluid mixture was varied from 0 to 16.4 % by weight. The locations of the phase boundaries during cooling and heating were both determined at different pressures. The phase boundaries were found to shift to higher temperatures with increasing polymer concentration. The sol-gel boundary is a sharp boundary with a positive .

The gels formed under high pressures were collected as wet gels after cooling and depressurization and characterized by DSC using a high pressure DSC capsule. For the wet gel, DSC scans show a gel melting peak at around 110 °C observed in a heating run and a gel formation peak at around 45 °C observed in a cooling run that follows. These temperatures are comparable with the phase transition temperatures which were determined in the view cell. Once the gel is dried, the DSC scans still show a gel melting peak in a higher temperature range in a heating run while no gel formation peak is observed in a cooling run. The wet and dry gels were characterized by FTIR also. The helical ä phase structure was observed in both the wet and the dry gels assigned by the presence of absorbance of helix bands in the region 500-600 cm-1 and a doublet at 934/943 cm-1 in these spectra that are characteristic of the ä phase [3]. These results indicate that there is still some toluene entrapped in the polymer matrix to form a clathrate phase even in a dry gel. To remove the entrapped solvent from polymer matrix, supercritical CO2-extraction at 40 °C and 22 MPa was employed. High-porosity aerogels with low density were obtained after extraction from both the wet and dry gels.

References: 1. J. Fang, and E. Kiran, J. Supercrit. Fluids. 38 (2006) 132 2. W. Zhang, C. Dindar, Z. Bayraktar, E. Kiran, J. Appl. Polym. Sci. 89 (2003) 2201 3. S. Moyses, and S. J. Spells, Macromolecules 32 (1999) 2684