Cross-Linked, Monovalent Selective Anion Exchange Membrane: Effect of Prealkylation and Co-ions on Selectivity

Tuning of hydrophobicity, water uptake, and size of the ion channel is very crucial for the monovalent anion-selective membrane (MASM) preparation. In this work, we report the preparation of MASM from the cross-linked polyacrylonitrile- co -polyvinylimidazole (PAN- co -PVI) copolymer. The copolymer was in situ quaternized and cross-linked using the polyacrylonitrile- co -polychloromethylstyrene (PAN- co -PCMSt) copolymer as cross-linker. The benzyl chloride moiety (Ar-CH 2 Cl) of PAN- co -PCMSt reacts with imidazolium nitrogen of PAN- co -PVI and provides a positive charge on the membrane matrix, whereas the PAN moiety provides greater compatibility with the PAN- co -PVI copolymer matrix. The faster cross-linking without external prealkylation forms a strong network structure with a low ionic charge on the membrane surface. Judicious extent of prealkylation followed by cross-linking forms a controlled network structure and small-sized ionic channel with moderate water uptake and good mechanical stability. The representative AEM0Q and AEM17Q membranes with 0 and 17% prealkylation exhibited a permselectivity ( P SO 4 2– Cl – ) value of 4.39 and 15.40, respectively, during the separation of 0.01 M NaCl + 0.01 M Na 2 SO 4 by electrodialysis. Upon further increase of prealkylation to 27 and 32%, the water uptake of the membrane increased, which in turn allowed the passage of SO 4 2– along with Cl – and decreased the P SO 4 2– Cl – value to 4.12 and 2.45, respectively. Similar trends were observed during the separation of NaCl + MgSO 4, MgCl 2 + Na 2 SO 4, and MgCl 2 + MgSO 4 mixtures of each 0.01 M concentration.