Synthesis of soluble anion-exchange copolyimides and nanofiltration membrane performances

Novel soluble anion-exchange copolyimides were synthesized by the two-step method. Copolyimides had inherent viscosities between 0.35 and 0.42 dL/g. Ion exchange capacity (IEC) of the polyimides was 0.18-0.33 mequiv/g. These IEC values were well explained with the degree of substitution. All polyimides were soluble in polar solvents such as N-methyl-2-pyrrolidone, N,N-dimethylformamide, and N,N-dimethylacetamide and showed amorphous patterns in X-ray diffraction studies. Soluble copolyimides synthesized in this study had glass transition temperatures between 220 and 265 degrees C. All unmodified polyimides were thermally stable below 560 degrees C. 10% weight loss temperatures were recorded in the range 560-600 degrees C in nitrogen. However, the 2-hydroxypropyl group of anion-exchange copolyimides was degraded at around 260 degrees C. The anion-exchange copolyimide nanofiltration membranes prepared from a cosolvent of diethylene glycol dimethyl ether and N-methyl-2-pyrrolidone showed better salt removal behavior than that of neutral copolyimides nanofiltration membranes.