| Abstract: | Quantitative data on solvent evaporation rates and equilibrium phase separation compositions corresponding to different film casting conditions are given, and the results are discussed with reference to reverse osmosis data on membrane performance. A typical evaporation rate curve plotted on a semilog scale consists of an initial linear part from whose slope evaporation rate constants b have been determined. For a given casting solution composition and temperature, an optimum value of b exists for best membrane performance, and identical values of b result in same membrane performance. Treating the casting solution as a polymer–solvent–nonsolvent ternary system, the data on equilibrium phase separation compositions have been plotted in a triangular diagram for solution temperatures of ?10°, 0°, and 24°C. The data indicate that even small changes in solution temperature can bring about significant changes in solution structure and hence membrane performance, and useful conclusions on solution structure can be drawn from such phase equilibrium diagrams. The data and correlations presented here offer a quantitative confirmation of the governing significance of the solution structure-evaporation rate concept relating to the mechanism of phase separation and pore formation in the surface region during the process of making reverse osmosis membranes. |
