Mesomorphism, dielectric permittivity, and ionic conductivity of cholesterol tridecylate doped with few-layer graphite fragments

Abstract

The effect of doping a cholesteric liquid crystal – cholesterol tridecylate (X-20) with few-layer graphite fragments/nanoflakes (GNF) has been studied with the aim to search for carbon nanostructures that able to improve the performance characteristics of helicoidal liquid–crystal materials for electro-optical devices. A nonlinear effect of dopant concentration on the phase transition temperature, thermodynamics, specific ionic conductivity, and the energy of ion transitions between equilibrium positions of X-20/GNF systems has been established. A split SmA → N* transition found upon the addition of GNF, is discussed within the framework of the model of stepwise phase subtransitions (at first, the transition occurs near the nanoparticle surface, and then it takes place in the bulk of the liquid crystal). Comparison of differential scanning calorimetry data with the characteristics of dielectric permittivity and ionic conductivity let us to suggest the generality of the factors that affect both the mesomorphism features during doping of the host liquid crystal matrix with GNF, and the differences in dielectric permittivity, ionic conductivity, and activation energy between the pure X-20 and its doped systems. The observed effects in X-20/GNF systems are explained by the coexistence of two differently directed processes: 1) interaction between GNF particles, leading to their aggregation and formation of percolation networks, 2) dipole–dipole interactions between X-20 molecules and GNF particles, leading to the orientation of X-20 on flat GNFs (anchoring effect). The dominance of one process over another depends on the GNF concentration and temperature. The dipole–dipole interactions of X-20 and GNF increase the order parameter of nematic phase at the macroscopic level, thereby improving the dielectric properties of the host liquid crystal matrix.