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

Two substrates were coated with a mixture containing two polyamic acids. One of the polyamic acids has an alkyl side chain and ordinarily is used for vertical liquid crystal alignment. The other polyamic acid does not have a side chain and ordinarily is used for planar liquid crystal alignment. The substrates were baked in order to promote imidization. It was observed that over a wide concentration range of mixtures the two polymer species comprising the alignment layer do not phase separate. A cell then was constructed and filled with the liquid crystal pentylcyanobiphenyl. It was found that the nematic pretilt angle θ₀ is a function of the relative polymer concentration and can be controlled over the range from near 0° to ∼70° with respect to the vertical orientation. A model is proposed in which quartic terms in the surface energy are responsible for the variation of θ₀ with concentration.