Physical and potentiometric properties of polyurethane-based cation-selective membranes

Lipophilic salts based on tetraphenylborate derivatives [e.g., potassium te trakis(p-chlorophenyl)borate (KTpClPB), sodium tetraphenylborate (NaTPB), a nd cesium tetrakis(3-methylphenyl)borate are essential ingredients used in the preparation of solvent polymeric cation-selective membranes. The effect s of such lipophilic salts on the physical properties of a polyurethane (PU ) matrix comprising 4,4'-diphenylmethane diisocyanate, 1,4-butanediol, and poly(tetramethylene ether glycol) were examined. Differential scanning calo rimetry measurements revealed that the sodium and potassium salts doped in PU increased the glass-transition temperatures (T-g) of the matrix, while t he film containing cesium salt exhibited slightly decreased T-g. The temper ature dependence of the ionic conductivity for PU60 films doped with KTpClP B is well described by the Arrhenius-type equation, and that doped with NaT PB is described by the Vogel-Tammann-Fulcher (VTF)-type equation. The tempe rature dependence of the ionic conductivity on the VTF-type equation sugges ts that the transport of sodium ions in the PU60 matrix is more strongly co upled to the soft segmental motion, and potassium ions are decoupled from t he polymer host and transported by activated hopping. The effect of added s alt on the internal structure of PU membranes was investigated by measuring the ratio between the free and hydrogen-bonded C=O bands at 1703 and 1730 cm(-1), respectively. The results showed that the ether oxygens in the soft segment, chains are strongly coupled to the potassium or sodium, but much less to cesium. The potentiometric properties of these lipophilic additive doped PU membranes were characterized by incorporating valinomycin and 4-te rt-butylcalix[4]arene-tetraacetic acid tetraethylester as potassium- and so dium-selective ionophores, respectively. Their response behavior could be e xplained by the observed physical characteristics. (C) 2001 John Wiley & So ns, Inc.