Full control of nematic pretilt angle using spatially homogeneous mixtures of two polyimide alignment materials

A mixture of two polyamic acids, one having an alkyl side chain and ordinarily used for vertical liquid crystal alignment, and the other without a side chain and ordinarily used for planar alignment, is deposited on two substrates and baked. It is found that: i) the polymer alignment layer does not phase separate and ii) the pretilt angle 0 of a cell filled with liquid crystal pentylcyanobiphenyl is a function of the relative polymer concentration and can be controlled continuously over the range 0◦ ≤ 0 . 90◦. A model is proposed in which quartic terms in the surface energy are responsible for the smooth variation of 0 with concentration. a. Permanent address: Dept. of Electrical Engineering, Soongsil University, Seoul, Korea