Tuning the acid recovery performance of poly(4-vinylpyridine)-filled membranes by the introduction of hydrophobic groups

Two series of anion-exchange membranes have been produced by photo-initiated copolymerization of 4-vinylpyridine and divinylbenzene (DVB) within the pores of polypropylene microfiltration membranes followed by quaternization of the pyridine nitrogen atoms. The membranes were tested for use in acid recovery applications by performing diffusion dialysis of acid/salt solutions (HCl/NaCl/MgCl2) and determining dialysis coefficients and acid/salt selectivity. The nature of the water present in the membranes was examined by differential scanning calorimetry (DSC). The first series was comprised of membranes with varying DVB content and all were quaternized with a methyl group. When compared to similar protonated membranes, this series showed a decrease in fixed charge concentration, an increase in dialysis coefficients, an increase in selectivity of HCl over NaCl and a slight decrease in selectivity of HCl over MgCl2. The second series had different quaternizing groups. It was found for this series that increasing the size of the N-alkyl substituent resulted in an increase in the membrane fixed charge concentration, a decrease in dialysis coefficients and a dramatic increase in selectivity of acid over salts. The results are interpreted in terms of the effects of the hydrophobic alkyl groups on the water structure in the membranes. The acid recovery performance of the N-alkylated membranes was shown to be better than a commercial diffusion dialysis membrane.