Phase behaviour of n-CB liquid crystals confined to controlled pore glasses

Abstract

In this review, we present the phase behaviour of liquid crystals (LCs) confined to Controlled-Pore Glasses (CPG) of various pore sizes. We consider either non-treated or silane-treated CPG pore surfaces, which impose parallel and homeotropic alignment to n-cyanobiphenyl (n-CB) LCs, respectively. The resulting nematic or smectic ordering depends on the characteristic void radii R and is well described in terms of the Landau-de Gennes free energy. Experimental data for different n-CB LCs, 5CB, 8CB, and 12CB, demonstrate phase transitions in orientation and translational degrees of freedom. In the bulk, 5CB and 8CB exhibit on cooling from the isotropic (I) phase the 1st order isotropic-nematic (I−N) phase transitions. In addition, 8CB also exhibits on further cooling a 2nd order nematic-smectic A (N−SmA) phase transition. 12CB, on the other hand, only has a 1st order isotropic-smectic A (I−SmA) phase transition. The bulk LCs exhibit a variety of different phase structures. When LCs are confined to porous media, their phase structures can be modified and even glass transition is possible.For LCs confined to CPG, the phase transition temperatures are shifted, depending on the pore size and the pore surface treatment. The Landau-de Gennes model predicts that the present phase transition shifts display inversely linear and inversely quadratic contributions in terms of the characteristic pore sizes. Furthermore, for strong enough confinement the phase transitions are replaced by gradual evolution of LCs order. These predictions are in line with nuclear magnetic resonance, small- angle X-ray scattering, and high-resolution calorimetry experimental observations.