Taking the Plasticizer out of Methacrylic-Acrylic Membranes for K+-Selective Electrodes

Abstract

Methacrylic-acrylic copolymers that can be externally or internally plasticized were used to fabricate potassium ion-selective electrodes (ISE) with crown ethers as the potassium selective ionophores. Copolymers which required external plasticizer have higher methacrylate content and glass transition temperatures (Tg) of approximately +30 °C whereas the internally plasticized (self-plasticized) copolymer is high in acrylate with Tg≈–27°C. Nonlipophilic crown ethers such as benzo-15-crown-5 and benzo-18-crown-6 could be included in the plasticized copolymers and yielded functional ISEs, but as with the plasticized PVC membranes these crown ethers tend to show some leaching which reduced the ISE lifetime; they were also incompatible with the ‘self-plasticized copolymer’, forming opaque membranes and electrodes of no potassium selectivity. In contrast, lipophilic crown ethers such as bis- or mono-nitrobenzo-15-crown-5 were found to be compatible with the self-plasticized copolymers yielding ISEs with good potassium ion selectivity, closely resembling plasticized PVC membrane based electrodes even though these new membranes contained no added plasticizer. This work has shown that it is possible to prepare ISEs based on nonplasticized methacrylic-acrylic polymers using lipophilized crown ethers to ensure compatibility with the polymer and deter leaching.