Steady flow properties of a mixed solvent through a poly(N-isopropylacrylamide) gel

We investigated the flow properties of mixed solvents of water and N,N-dimethylformamide (DMF) through poly(N-isopropylacrylamide) (PNIPA) gel membranes. The swelling behavior of the gels in the mixed solvents with various compositions was also examined. A simple model to describe the swelling and solvent flow properties for a polymer gel/mixed solvent system was presented. In the swelling measurements, the composition dependence of the degree of swelling (Θ  ) of the PNIPA gels showed a minimum at the DMF fraction (φ′DMF)(φ′DMF) of 0.6. In the solvent flow experiments, pressure gradient acting between two solvent–gel interfaces caused the permeation of the mixed solvents through the gel membranes. Little change in solvent composition by the permeation occurred. The integrated flow rate (F) over time (t) monotonously increased with increasing t. The slope of F versus t curves becomes constant at long times. The inverse of the frictional coefficients for the steady flow plotted against solvent composition is quite similar in shape to the Θ curve. This indicates that the degree of the swelling greatly affects the flow rate of the solvents in the gels.