Temperature Regulation of Crown-Mediated Ion Transport through Polymer/Liquid Crystal Composite Membranes: Remarkable Transport Ability of Fluorocarbon-Containing Crown Ethers

Ternary composite membranes composed of polymer (polycarbonate, PC), liquid crystal [N-(4-ethoxy-benzylidene)-4'-butylaniline, EBBA], and crown ethers having a hydrocarbon chain (la, lb, and lc) or a fluorocarbon chain (2b and 2c) have been prepared. The DSC study established that the hydrocarbon-containing crown ethers are dissolved homogeneously in the PC/EBBA membrane, whereas the fluorocarbon-containing crown ethers form microheterogeneous, phase-separated aggregates in the membrane. Transport of K⁺ion through the PC/EBBA/1 membranes occurred below and above tkn (crystal-nematic liquid crystal phase-transition temperature of EBBA, 305 K), and the transport rates were faster above tkn. This indicates that carrier-mediated K⁺transport is directly affected by the fluidity of the membrane phase. The PC/EBBA/2 membranes provided two unexpected transport characteristics: K⁺transport through these membranes is “completely” suppressed below T_KNand (ii) K⁺was transported rapidly above T_KN(21–23 times faster than with the PC/EBBA/la membrane). This unusual transport ability was rationalized, on the basis of the DSC studies and the thermodynamic studies, in terms of “phase-separation” and “desolvation” characteristic of the fluorocarbon-containing crown ethers. The Arrhenius thermodynamic parameters showed a good enthalpy-entropy compensation relationship (r = 0.999) expressed by log E_a(kJ mol^-1) = 5.69 log A + 52.7, but the transport rates were correlated with the increase in the entropy term. This supports the importance of the membrane fludity in efficient ion transport. The PC/EBBA/2b membrane was applied to ion-selective membrane transport and to temperature regulation of ion transport rates. The membrane transport data showed a pronounced K⁺selectively probably because of the formation of a 1:2 metal-crown sandwich complex between K⁺and aggregated 2b. Also, we could observe a reversible, all-or-northing change in the K⁺transport rate that was induced by the on-off-type temperature change. Thus, the PC/EBBA/2 membranes act as an ideal thermocontrolled system for K⁺transport: no transport below T_KNand very efficient transport above T_KN.