Lee Yook Heng

Ion-transport and diffusion coefficients of non-plasticised methacrylic–acrylic ion-selective membranes

The ion-transport behaviour of methacrylic-acrylic-based polymers for ion-selective electrode (ISE) membranes was investigated by a spectrophotometric method to determine the apparent diffusion coefficient. By observing the degree of deprotonation of the chromoionophore or chromogenic ionophore, the extent of penetration of cations into the polymer films was determined. The transport of the cations into the optode films depended on the stoichiometry of complexation by the ionophores. The apparent diffusion coefficients, estimated from the deprotonation data were of the order of 10(-12) to 10(-11)cm(2)s(-1). These values indicate that the apparent ion mobility in the methacrylic-acrylic ISE membranes is approximately a thousand times lower than that in plasticised PVC ISE membranes. For some ionophores, the value of the apparent diffusion coefficient could be modulated according to the ionophore content in the membrane and the data obtained for a calixarene containing membrane were tested against a model for facilitated diffusion with chained carriers. The data did not fit a model where intramolecular diffusion was limiting, but were consistent with a first-order rate-limiting mechanism involving an intermediate 1:2 complex between ion and ionophore. In this instance, the lowest values for D(app) were thus not necessarily obtained for lowest ionophore loading and in the range examined, a trend of decreasing D(app) with increasing ionophore was noted.

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

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 )b y differential scanning calorimetry indicated that the Tg influenced the amount of plasticiser required for workable ion selective electrode membranes. A Tg below 20C 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 ( N Mw) 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. ©2000 Elsevier Science B.V. All rights reserved.

Methacrylate-acrylate based polymers of low plasticiser content for potassium ion-selective membranes

Polymers prepared from of glycidyl methacrylate, methyl methacrylate and n-butyl acrylate were studied for their possible use as a valinomycin-based potassium ion-selective membrane with low or no plasticiser content. Four polymers were observed to be functional as potassium ion-selective membranes in the presence of valinomycin and lipophilic salt. All polymeric membranes gave Nernstian or near-Nemstian responses over the potassium ion concentration range of 10−5−10−1 M. The potassium selectivity over sodium, log KpotKM values, for all membranes were in the range −3.0 to −4.1 as measured by the mixed solution method with interfering sodium ion at 0.1 M. Interference of Na+ and other cations was also studied by the separate solution method. Terpolymers prepared from methacrylate-acrylate monomers with a high n-butyl acrylate ratio, required little or no o-NPOE as plasticiser to form functional potassium ion-selective membranes with only a slight decrease in potassium ion selectivity. The monomer n-butyl acrylate appeared to act as a built-in plasticiser to lower the glass transition temperature of the polymers and allowed potassium ion-selective behaviour to occur even in the absence of plasticiser.

One‐Step Synthesis of K+‐Selective Methacrylic‐Acrylic Copolymers Containing Grafted Ionophore and Requiring No Plasticizer

A main drawback in the application of ion sensors that are based on plasticized polymeric membranes is the problem of leaching of plasticizer and ionophore. Leaching problems can be critical in solid-state minidevices, e.g., ion-selective electrodes or optodes, where a thin ion sensing polymer film is often empolyed. To resolve such problems, we have designed simple methacrylic-acrylic copolymers that required no plasticizer and which include immobilized polymerizable but hydrophilic ionophores such as 4-acryloylamidobenzo-15-crown-5 (AAB15C5) and 4-acryloylamidobenzo-18-crown-6 (AAB18C6) that would normally be susceptible to leaching from plasticized PVC membranes, were used as model ionophores for the immobilization studies. These crown compounds are attractive since they can be employed as a monomeric unit in the copolymer recipe and thus no additional synthesis or polymer modification is required. Copolymers suitable for ion-selective membrane application were synthesized by introducing more than 80 wt. % of n-butyl acrylate to yield copolymers of Tgs between –20 to –30 °C. The molecular weight distributions of these copolymers were M¯w <50 000 Da and they contained 2 to 6 wt. % of immobilized crown ether ionophores. The ‘self-plasticising’ methacrylic-acrylic membranes formed with these acryloyl crown ethers demonstrated good potentiometric responses to potassium ion. In contrast, AAB15C5 and AAB18C6 entrapped in membranes in a more classical ion selective membrane recipe, even when a plasticizer was included, were still inferior in their response to those with membranes containing immobilized ionophores. The restricted mobility of the immobilized ionophores in these self-plasticizing polymer matrices did not appear to hinder the normal complexation behavior of these ionophores. This was inferred from the possible formation of the usual 2:1 and 1:1 ionophore-cation complexes of AAB15C5 and AAB18C6, respectively, in these new copolymer matrices.

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

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.