Phase behavior of nearly symmetric polystyrene-block-polyisoprene copolymers in the presence of CO2 and ethane

The phase behavior of nearly symmetric styrene-isoprene (SI) diblock copoly mers in the presence of compressed CO2 or ethane was studied using small-an gle neutron scattering. Sorption of either fluid depresses the upper order- disorder transition (UODT) by as much as 55 degrees C at fluid densities of less than 0.35 g/cm(3). The solvent-induced transitions are fully reversib le upon fluid desorption. The magnitude of the depression can be controlled by manipulating the mass fraction of fluid sorbed through pressure-mediate d adjustments in solvent density. Consequently, the location of the UODT ca n be tuned over broad ranges, and the state of segregation can be controlle d via isothermal adjustments in total system pressure. The influence of hyd rostatic pressure, inherent to the use of compressed solvents, is small in comparison to the solvent effect over the range of pressures studied.