Enhancement of the orientational order parameter of nematic liquid crystals in thin cells

We report measurements of birefringence (Deltan) of several nematic liquid crystals having transverse as well as longitudinal dipole moments in thin (1.4 to 2.3 mum) and thick (7 to 16 mum) cells. Rubbed polyimide-coated glass plates are used to get planar alignment of the nematic director in these cells. We find significant enhancement (6 to 18%) of Deltan, (alpha S, where S is the orientational order parameter) in thin cells in all compounds with aromatic cores even at temperatures far (similar to 20degreesC) below the nematic-isotropic transition point. The enhancement is larger in compounds having several phenyl rings and lower if the number of phenyl rings is reduced. In a compound that does not have an aromatic core no significant enhancement is observed, implying that the strength of the surface potential depends on the aromaticity of the cores. Assuming a perfect orientational order at the surface, calculations based on the Landau-de Gennes theory show that the thickness averaged enhancement of S is sharply reduced as the temperature is lowered in the nematic phase. The measured order parameter S is further enhanced in thin cells because of the stiffening of the elastic constant which reduces the thermal fluctuations of the nematic director. The combined effect is however too small at low temperatures to account for the experimental data.