Effect of cosolvent on solid phase transitions of α- to β-form crystal of syndiotactic polystyrene occurred in supercritical Co2 containing solvent

Abstract

The solid phase transition mechanism of α- to β-form crystal upon specific treating with supercritical CO2 + cosolvent on original pure α and mixed (α+β) form syndiotactic polystyrene (sPS) was investigated, using wide angle X-ray diffraction and differential scanning calorimetry measurements as a function of temperature, pressure, and cosolvent content. As in the supercritical CO2, sPS in supercritical CO2 + cosolvent underwent solid phase transitions from α- to β-form, and higher temperature or higher pressure favored this transformation. Due to the higher dipole moment of acetone, small amounts of acetone used as cosolvent with CO2 made the transition of α- to β-form occur at lower temperature and pressure than in supercritical CO2, and made the α-form crystal completely transform to β-form in the original mixed (α+β) form, whereas ethanol did not. The original β-form crystal in the original mixed (α+β) form sample acted as the nucleus of new β-form crystal in the presence of cosolvent as it did in supercritical CO2, when compared with the original pure α-form sample. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 1625–1636, 2007