Nonporous Cation Exchange Membranes with pH-Responsive Properties as Chemical Valves

pH-responsive ion exchange membranes were developed by photoinitiated free radical polymerization of a resin formulation containing poly(ethylene glycol diacrylate) and diurethane dimethacrylate oligomers, dipentaerythritol penta-/hexa-acrylate cross-linker, photoinitiators, and acrylic acid as a stimuli-responsive monomer. The membranes were switched from their protonated, nonconductive state to a deprotonated, ion-conductive state by modulating the pH of the storage solution. This process was analyzed by monitoring the membrane transport properties (ionic resistance and permselectivity) and the water uptake in the different states. It was observed that upon deprotonation at high pH, the higher ionicity of the membrane allowed for cation conduction, which was confirmed by observing a substantial decrease in the area specific resistance of the membrane. The ionic resistance decreased 5 orders of magnitude, from 9.86 ± 0.52 Ω m2 to 3.54 × 10–4 ± 7.78 × 10–5 Ω m2 for a sample with 15 wt % acrylic acid. The increased ionicity of the membrane increased the hydrophilicity of the samples, which was confirmed by observing a higher water uptake for the membranes in their deprotonated state. Additionally, the higher ionicity of the membrane increased the membrane's exclusion of co-ions, as described by the Donnan principle. This phenomenon translated into a higher permselectivity of the membranes in their deprotonated state, which increased from between 0.25 ± 0.03 and 0.57 ± 0.02 to values between 0.76 ± 0.01 and 0.86 ± 0.01. The chemical stability and reversibility of these stimuli-responsive membranes were analyzed by cycling a membrane sample between its protonated and deprotonated states for ten cycles and monitoring the membrane ionic resistance and permselectivity values. It was observed that permselectivity varied between 0.32 ± 0.03 and 0.88 ± 0.01; however, the area specific resistance of the membrane in its protonated state decreased asymptotically with increasing cycle number until it reached a plateau in cycle numbers 5 to 10, likely due to irreversible water swelling.