Very strong azimuthal anchoring of nematic liquid crystals on uv-aligned polyimide layers

The azimuthal anchoring energy of the nematic liquid crystal 4-n-pentyl-4 '-cyanobiphenyl (5CB) on a uv-aligned polyimide substrate with in-plane order parameter S '=0.2 is measured. The measurements are performed at temperature T=24 degrees C using simultaneously a high accuracy reflectometric method and a high accuracy transmitted light method. With both the methods, we observe an apparent surface director rotation opposite to the orienting torque that would correspond to a negative extrapolation length. It is shown that this unusual behavior is due to the relatively high birefringence of the uv-aligned polyimide layers. Taking into account for this birefringence, we find a small but positive extrapolation length. The experimental results are interpreted in terms of a simple mesoscopic model where the nematic molecules are assumed to be rigidly attached on the polymer surface and the measured extrapolation length is entirely due to the order parameter variation in a thin interfacial layer where the nematic order parameter passes from the surface value to the bulk value within a few nematic correlation lengths. Assuming the surface order parameter is S-0=0.37, the correlation length of the nematic liquid crystal is estimated to be xi(c)'=2.4 +/- 1 nm. The corresponding thermodynamic extrapolation length is d(e)=2.8 +/- 1.2 nm that corresponds to a very strong azimuthal anchoring.