The surface order parameter and anchoring energy in nematics made by hydrogen bonded molecules

Abstract

The temperature and the cell thickness trends of the scalar order parameter for the dimerized nematic with short-range smectic order of 4, n-nonyloxybenzoic acid (NOBA) are indicated by FT-FIR spectroscopy. The temperature dependence of this parameter shows an anomaly, in respect to the dependence typical for the classical nematics, at a definite temperature T ∗ in the nematic range. The scalar order parameter increases nonlinearly at cell thickness ( d) decreasing between 20 and 6 μm and goes to saturation when d→∞, while below 6 μm down to 2 μm it decreases tending to a finite small value. It is indicated that the order parameter increasing of 0.035 with the thickness decreasing from 20 to 6 μm is comparable to order increasing induced by a stabilizing electric or magnetic fields; the order parameter decreasing at small cell thickness is an analog of a disordering induced by destabilizing fields. The anomalies in the temperature and cell thickness trends of the scalar order parameter are explained by the dynamics of the hydrogen bonding of the monomers in dimers stimulated by temperature and surface action variations. The relation between the surface order parameter square and the anchoring energy is indicated to be a linear dependence near the nematic–isotopic phase transition temperature. The trend of the anchoring energy on the cell thickness is explained by a modification of the Rapini–Papoular surface energy as an effective one, adding one dielectric part stimulated by phenomena as: existence of a double electric layer, thickness dependence of the surface charge density and surface polarization.