Analysis of polar orientational order controlled by nanometer-thick polyimide Langmuir-Blodgett films

Orientational ordering processes of liquid crystalline 4-n-pentyl-4′-cyanobiphenyl (5CB) were studied by the optical second-harmonic generation measurements, in-situ, during evaporation onto silica substrates coated with n-layer polyimide Langmuir-Blodgett films (n = 1, 3, 5, or 11). It was found that the polar orientational order, S1 = 〈cos θ〉, of 5CB decreases as the thickness of polyimide layer increases from 0.4 nm to 2 nm, and then it saturates. It was shown that the electrostatic potential energy calculated with consideration of mirror-image charges at the interface accounts for the orientational order dependence on the polyimide layer thickness.