ALIGNMENT AND ORDERING MECHANISMS AT A LIQUID-CRYSTAL SOLID INTERFACE

The role of surface coupling agents on the aligning and ordering mechanisms at a liquid crystal-solid interface are examined with deuterium nuclear magnetic resonance. The cylindrical channels of alumina membranes 0.2 mum in diameter are chemically modified using an aliphatic acid (CnH2n+1COOH) as a surface coupling agent and filled with the liquid crystal compound 4'-pentyl-4-cyanobiphenyl deuteriated in the alpha position of the hydrocarbon chain (5CB-alphad2). The preferred anchoring direction at the cavity wall and its strength are found to depend on the length of the aliphatic chain of the surface coupling agent which determine the nematic director field in the pores. The planar polar configuration with homeotropic anchoring conditions is stable for agents with n greater-than-or-equal-to 7 while chain lengths n less-than-or-equal-to 6 support a uniform axial configuration with planar anchoring at the cavity wall. The pretransitional orientational ordering at the cavity wall above the clearing temperature is strongly reflected in the spectra. The radical changes in the quadrupole splitting as the length of the aliphatic chain of the surface coupling agent is varied indicates strong coupling between the 5CB molecules and the n=15 surface, while shorter chain lengths reveal substantially reduced degrees of coupling.