Methacrylic–acrylic polymers in ion-selective membranes: achieving the right polymer recipe Original Research Article

The polymer characteristics of the methacrylic–acrylic copolymers which have been successful in producing membranes for potassium ion-selective electrodes were investigated. Measurement of copolymer glass transition temperature (Tg) by differential scanning calorimetry indicated that the Tg influenced the amount of plasticiser required for workable ion selective electrode membranes. A Tg below −20°C was required, which could be achieved with or without the use of a plasticiser; without using plasticiser the copolymer should contain more than 80 wt.% of n-butyl acrylate. Under the conditions for the free radical solution polymerisation used, proton NMR spectroscopy studies on the copolymers showed that the incorporation of n-butyl acrylate into the copolymer was lower than expected when the methacrylate content was high. However, when the amount of methacrylate used in the feed was low, n-butyl acrylate incorporation could reach almost 100% (relative to the n-butyl acrylate in the feed). Therefore, for an efficient incorporation of n-butyl acrylate into the copolymer, the methacrylate content must be kept below 30 wt.%. A high concentration of both methacrylate and acrylate monomers should also be used during polymerisation to ensure that the copolymer produced has a molecular weight distribution (M̄w) of greater than 80 000: this is required to provide physical strength to the ion-selective membrane. Potentiometric studies on some of the high acrylate membranes using valinomycin as potassium-ion selective ionophore showed that these non-plasticised membranes gave performance similar to that of a plasticised poly(vinyl chloride) membrane using the same ionophore and could be readily deployed more widely due to their ease of producing and ionophore incorporation.