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

Abstract

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 ( T g) by differential scanning calorimetry indicated that the T g influenced the amount of plasticiser required for workable ion selective electrode membranes. A T g 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 ( M ̄ w ) 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.