Ionic transport in polyelectrolyte-filled cation-exchange membranes

The polyelectrolyte-filled (PF) cation-exchange membranes were prepared by in situ photo-polymerization in pores of the host membranes. It is expected that cationic-gel confined in pores would provide close proximity of exchange-sites leading to better diffusional-transport properties. The host membrane provides mechanical support to cationic-microgel. Poly(propylene), poly(ethersulfone) and poly(vinylidene fluoride) microporous membranes were used as the host membranes. In situ polymerization in pores was carried out by filling pores of the microporous host membrane with polymerizing solution and irradiating with 365 nm UV light. The polymerizing solution filled in pores of the host membranes consisted of required amounts of functional monomer (2-acrylamido-2-methyl-1-propane sulfonic acid), spacer ((3-acryloxypropyl)trimethoxysilane), crosslinker (ethylene glycol dimethacrylate) and UV-initiator (α,α′-dimethoxy-α′-phenyl acetophenone) dissolved in DMF and methanol (1:1) mix solvent. These PF-membranes were found to have ion-exchange capacities in the range of 1.40–1.85 meq g−1. The proton conductivity of PF-membrane was found to be 13×10−2 S cm−1 that is higher than proton conductivity (7.4×10−2 S cm−1) in Nafion-117 membrane. The diffusional-transport properties of the PF-membrane were studied by measuring the self-diffusion coefficient (D) of different valence counterions like Ag+, Na+, Cs+, Ba2+ and Eu3+ ions using radiotracer method. The D values in PF-membranes varied from 0.79×10−6 cm2 s−1 (Na+ ion) to 0.19×10−6 cm2 s−1 (Eu3+). Contrary to this, the D values in Nafion-117 membrane varied from 1.03×10−6 cm2 s−1 (Na+ ion) to 0.045×10−6 cm2 s−1 (Eu3+). The wide variation of the D values in Nafion was attributed to its higher variation in water content in different ionic forms as compared to PF-membranes.