Liquid crystal alignment film, method for preparing the same and use thereof

A method for preparing a liquid crystal alignment film, comprising the steps of:

• (1) coating stearic acid onto a substrate;
• (2) adding a polyamide acid salt solution to deionized water, to allow the polyamide acid salt solution to form a stable film of a monomolecular layer on the surface of the deionized water;
• (3) contacting the substrate coated with a layer of stearic acid horizontally with the monomolecular layer of the polyamide acid salt on the surface of the deionized water, to form a monomolecular polyamide acid salt layer;
• and
• (4) imidizing the monomolecular polyamide acid salt layer to form a monomolecular polyimide film, wherein the polyamide acid salt solution is prepared by mixing a solution of polyamide acid in chloroform and N,N-dimethylacetamide with a solution of octadecyl aliphatic amine in chloroform and N,N-dimethylacetamide,
• wherein the polyamide acid is prepared through the polymerization of aromatic tetracid dianhydride with a monomer selected from 1,4-bis(4-amino-2-trifluoromethylphenoxy)-2-(3′-trifluoromethyl phenyl)benzene, 1,4-bis(4-amino-2-trifluoromethylphenoxy)-2-(3′,5′-trifluoromethyl phenyl)benzene, or 1,4-bis(4-amino-2-trifluoromethylphenoxy)-2-(3′-methylphenyl)benzene in m-cresol, the aromatic tetracid dianhydride being 3,3′,4,4′-diphenylether tetracarboxylic acid dianhydride or 4,4′-hexafluoroisopropylidenyl-phthalic anhydride, and
• wherein the molar of the aromatic tetracid dianhydride to a monomer selected from 1,4-bis(4-amino-2-trifluoromethylphenoxy)-2-(3′-trifluoromethylphenyl)benzene1,4-bis(4-amino-2-trifluoromethyl phenoxy)-2-(3′,5′-trifluoromethylphenyl)benzene, or 1,4-bis(4-amino-2-trifluoromethyl phenoxy)-2-(3′-methylphenyl)benzene is in a range of from 3:4 to 8:9.