Influence of Salt Doping on the In Situ Synthesis of Zeolitic Imidazolate Framework-8 in Poly(diallyldimethylammonium chloride)/Poly(sodium-p-styrenesulfonate) Polyelectrolytes Complexes

Polyelectrolyte complex membranes display relatively high ion transport properties due to their large free volume and electrostatic nature, which make them suitable for the in situ synthesis of various compounds. Here, poly(diallyldimethylammonium chloride) and poly(sodium-p-styrenesulfonate) were used to fabricate polyelectrolyte complex membranes which were further modified by the in situ synthesis of zeolitic imidazolate framework-8 (ZIF-8). Polyelectrolyte complexes prepared from different polycation : polyanion molar ratios (2 : 1, 1 : 1, and 1 : 2) were sequentially dipped in a solution of zinc acetate and then in a 2‑methylimidazole solution to induce the formation of ZIF-8 crystals. Scanning electron microscopy results revealed that the ZIF-8 crystals could be synthesized only in the 1 : 2 ratio polyelectrolyte complex which presents an excess of polyanion and that the synthesis was restricted to the polyelectrolyte complex’s surface. Interestingly, using a “salt doping” approach with increasing sodium chloride concentrations from 0 to 1.5 M, zinc ion diffusion could be enhanced, leading to an improved ZIF-8 crystal formation in the polyelectrolyte complex membranes. These results are of interest for the development of functional membranes with asymmetric particle distribution, found for example in pervaporation membranes or in alkaline fuel cell separation membranes.