Factors controlling the preparation of porous polyurethane membranes via thermally induced phase separation

Porous polyurethane (PU) membranes were prepared via thermally induced phase separation (TIPS) technique by employing a home-made mould consisting of a solution container, a membrane-forming platform, a coldplate, a temperature-decreasing system and a temperature-supervising system. The influences of polymer concentration on the surface morphology, pore size, porosity and water vapor transmitting ratio of the porous PU membranes were studied. The bottom surfaces, the surfaces contacting with the membrane-forming platform, of the porous PU membranes are planar and smooth for all the selected polymer concentrations, e.g. φ ₂ from 0.025 to 0.078. The pore sizes are smaller and are estimated in the range of nanometer. While the top surfaces, the free surfaces toward air, possess variable surface morphology and have micrometer-sized pores that are larger than those of the bottom surfaces. This phenomenon is mainly caused by the different coarsening time after spinodal phase separation (polyurethane/1,4-dioxane (DO) solution is an upper critical temperature phase separation system). Preliminary studies showed that the pore sizes and surface morphology of porous PU membranes were also influenced by the temperature of the coldplate and by the composition of solvents, DO/H ₂O, a good solvent and a poor solvent for the PU, respectively.