Factors Controlling Pretilt Angles of Polyimide Alignment Film for Liquid Crystal Displays

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Abstract
Factors controlling pretilt angles of polyimide (PI) alignment film for liquid crystal displays were studied by pretilt angle measurement of PI with various structural features and X-ray photoelectron spectroscopy (XPS). Structural features that resulted in high pretilt angles were: (1) rigid tetracarboxylic acid moiety, (2) diamine moiety containing hexafluoroisopropylidene group and -O- or -CH2- linkage, and (3) sufficient imidization ratio. XPS of PI's that gave high pretilt angles showed higher fluorine content at the surface than in the bulk of the film. The results implied that high pretilt angles result from orientation of the polymer chains within the film, and not simply from a high fluorine content in the material.