Formation and characterization of highly crosslinked anion-exchange membranes

Highly crosslinked/hyperbranched anion-exchange membranes have been prepared by anchoring poly(vinylbenzyl chloride) (PVBCl) within the pores of poly(propylene) microporous base membranes by in situ crosslinking of PVBCl with a diamine 1,4-diazabicyclo[2.2.2]octane (DABCO). The resulting PVBCl-filled precursor membranes were converted to anion-exchange membranes by reacting these with (i) excess of DABCO followed by alkylation with α,α′-dibromo-p-xylene (DBX) (membrane A), and (ii) with excess of tetraethylenepentamine (TEPA) (membrane B). A third membrane C was synthesized by alkylating membrane B with DBX. The chemical analyses indicated that these anion-exchange membranes consist of highly crosslinked/hyperbranched anionic gels within the pores of host microporous membranes. These anion-exchange membranes were characterized in terms of water-uptake capacities, ion-exchange capacities and thermal stability. The physical structures of the membranes were examined by small angle X-rays scattering (SAXS) analysis. The study of SAXS profiles of the dry and water equilibrated membrane A samples indicated that microstructure of anionic gel within the pores of membrane was changed significantly on water equilibration. However, no significant change in the SAXS profile was observed in wet samples of membranes B and C with respect to their dry samples. Thus, the crosslinking generated in membrane A was flexible and very rigid in membranes B and C. The self-diffusion coefficient of I− ions and transport numbers of Cl− ions were measured to examine the effects of crosslinking on transport properties of the membranes.