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

The temperature and the cell thickness trends of the scalar order parameter for the dimerized nematic with short-range smectic order of 4,n-nonyloxybe nzoic acid (NOBA) are indicated by FT-FIR spectroscopy. The temperature dep endence of this parameter shows an anomaly, in respect to the dependence ty pical for the classical nematics, at st definite temperature T* in the nema tic range. The scalar order parameter increases nonlinearly at cell thickne ss (d) decreasing between 20 and 6 mu m and goes to saturation when d --> i nfinity, while below 6 mu m down to 2 mu m 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 mu m is comparable to order incr easing induced by a stabilizing electric or magnetic fields; the order para meter decreasing at small cell thickness is an analog of a disordering indu ced by destabilizing fields. The anomalies in the temperature and cell thic kness trends of the scalar order parameter are explained by the dynamics of the hydrogen bonding of the monomers in dimers stimulated by temperature a nd surface action variations. The relation between the surface order parame ter square and the anchoring energy is indicated to be a linear dependence near the nematic-isotopic phase transition temperature. The trend of the an choring energy on the cell thickness is explained by a modification of the Rapini-Papoular surface energy as an effective one, adding one dielectric p art stimulated by phenomena as: existence of a double electric layer, thick ness dependence of the surface charge density and surface polarization. (C) 2000 Elsevier Science B.V. All rights reserved.