Structure and performance of polyacrylonitrile membranes prepared via thermally induced phase separation

We report the preparation and performance of polyacrylonitrile (PAN) membranes via thermally induced phase separation (TIPS) process using dimethyl sulfone (DMSO2) and glycerol as mixed diluent. A systematic study was carried out to investigate the effects of glycerol content, PAN concentration and cooling rate on the pore shape, pore size, porosity, water flux and mechanical properties of the membranes. Symmetric membranes with uniformly distributed cellular-like pores were fabricated when the mixed diluent contains 20–30 wt.% glycerol. In contrast, sheet- or needle-like pores were formed in the membranes while glycerol content is 10–15 wt.% in the mixed diluent, which is due to the crystallization of DMSO2 prior to the weak liquid–liquid phase separation. The pore size becomes large when the membrane was prepared with high glycerol content, low PAN concentration or small cooling rate. The droplet growth process traced by optical microscopy provides an evidence for the formation of pores with different sizes. Moreover, the water flux of PAN membranes mainly depends on the porosity, which presents the highest value when the ternary system contains 20 wt.% glycerol in the mixed diluent, 10 wt.% PAN concentration, and cools in air bath at 30 °C. Results of tensile tests confirm that the mechanical properties of the membranes have a significant dependence on their microstructure, i.e., membranes with cellular-like pores show greater tensile strength and elongation than those with sheet- or needle-like pores. In conclusion, the successful application of TIPS may provide a new route to the structure modulation and performance enhancement of PAN membranes.