Direction and pressure response of osmotic pressure in binary polymer solutions

Abstract

Using a molecular equation-of-state (EOS) theory, I discuss the direction and pressure response of osmotic pressure П for a composite system with a pure solvent and its non-ideal solution of a polymeric component separated by a semi-permeable membrane. A perturbed hard sphere chain model is chosen here for the EOS to estimate П in a general way while pressure is applied. It is shown that П is a phase behavior in connection with the phase stability of the solution. In case of an incompatible pair upon forcing a single phase, there exists a region inside the binodal line for negative П possessing a spinodal point as the threshold of П=0. It is the potential energy term that contributes to the negative П and drives solvent molecules away from polymers. Those actions are reversed when phase separation occurs on the solution side. In addition, it is revealed that the solution responds to pressure with either increase or decrease in П upon pressurization, which depends on the pressure coefficients of spinodals and solvent compressibility.