Understanding anisotropic transport in self-assembled membranes and maximizing ionic conductivity by microstructure alignment

Magnetic fields can be highly effective in controlling the macroscale orientational order in certain uniaxial surfactant and block copolymer mesophases, leading to improvement in transport properties by reducing microstructural tortuosity. We consider ionic conductivity in model surfactant mesophases subjected to magnetic field alignment and observe significant enhancements of transport in both cylinder and lamellar forming systems, well beyond what is prescribed by microstructural models and continuum theory. Spontaneous defect generation near the order-disorder transition produces pronounced but reversible changes in conductivity on heating and cooling, implying that these pre-transitional defects are equilibrium features of the system. The enhancement and temperature dependence of conductivity closely parallel results obtained in aligned ion-containing liquid crystalline block copolymers and suggest that theoretical treatments overestimate the conductivity of the isotropic state. This is substantiated by a discussion of the underlying assumptions in the theoretical models regarding finite domain sizes and domain connectivity.