Anomalous anchoring effect of nanopolyimide Langmuir-Blodgett films in a twisted nematic liquid-crystal cell

In a recent paper [Phys. Rev. E 52, 681 (1995)] we showed that in the absence of an external field, the twisted structure of substrates with weak anchoring can induce a deviation of the director at the surfaces from the easy axis and that the anchoring energy can be evaluated by measuring this deviation. To test the prediction, the anchoring energy for nematic 4-pentyl-4'-cyanobiphenyl and polyimide Langmuir-Blodgett (LB) substrates with thicknesses in a range less than 10 nm has been measured. The anchoring energy is found to increase and saturate with the number of LB layers. This result, unlike that expected for the isotropic van der Waals interaction, reveals the anomalous anisotropic interfacial tension between a liquid-crystal slab and polyimide LB films with nanometer thickness. The generalized nonretarded van der Waals energy for the chain-chain interaction is proposed to explain the experimental result.