Preparation of PMDA/ODA polyimide membrane for use as substrate in a thermally stable composite reverse osmosis membrane

In this research, poly(pyromellitic dianhydride-co-4,4′-oxydianiline) (PMDA/ODA PI) has been developed as a substrate for a thermally stable composite reverse osmosis membrane. This substrate membrane was prepared by immersion precipitation of a casting solution composed of 15% polyamic acid (PAA) and 24% ZnCl2 additive. Zinc ions were able to interact with the carboxylic groups of PAA forming an ionic cross-linking structure which facilitated the formation of a membrane with improved surface properties, mechanical strength and permeability. The PAA membranes were chemically imidized by a mixture of acetic anhydride and triethylamine at 100 °C to prevent pore collapse. The conversion of PAA to PI was complete as determined by attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR). The morphologies of the PI membranes were examined by scanning electron microscopy (SEM) and atomic force microscopy (AFM). Composite membranes were developed via interfacial polymerization of m-phenylenediamine (MPDA) and trimesoyl chloride (TMC) on the polyimide membranes. These composite membranes showed 98% rejection to 2.0 g/L NaCl solution with a permeation flux of 1.1 m3 m−2 day−1 at 55.2 bar and room temperature. This composite membrane demonstrated good thermal stability. As the test temperature increased from 25 °C to 95 °C, the permeated flux of 2.0 g/L NaCl solution increased 5.3 times from 0.74 m3 m−2 day−1 to 3.95 m3 m−2 day−1 with a stable rejection rate when the pressure was fixed at 27.6 bar.