Preparation of Thermally Imidized Polyimide Nanofiltration Membranes with Macrovoid-Free Structures

Insoluble polyimides (PIs) are ideal materials for organic nanofiltration membranes. However, the membranes have to be prepared using soluble polyamic acid and then converted to PIs via chemical or thermal imidization. The low imidization degree of chemical imidization makes PI membranes easily hydrolyzed in water. On the contrary, a thermally imidized PI has a higher imidization degree and better chemical stability. However, the high imidization temperature (300 degrees C) often induces pore collapsing, membrane brittleness, and low flux. To overcome the drawbacks of thermal imidization, we developed a dual-coagulant bath method to prepare macrovoid-free poly(4,4'-oxydiphenylene pyromellitimide) membranes. The size, distribution, and connectivity of membrane pores were well controlled by tuning the compositions of the dope and the first coagulant. A series of thermally imidized flat-sheet PI membranes with macrovoid-free structures were prepared. These membranes exhibited good mechanical properties and stable nanofiltration performance in N,N-dimethylformamide solvent. This implied that the membranes were qualified for the separation process under harsh organic environments.