Polyimide films for liquid crystal displays used in aerospace applications

摘要

Organo-soluble aromatic polyimides with varying chemical structures and molecular weights were synthesized in refluxing m-cresol. Modifications to the dianhydride and diamine monomers were carried out in order to obtian aromatic polyimides that meet the requirements of liquid crystal display (LCD) applications. Solution-cast films exhibited linear optical anisotropy (LOA) that is commonly referred to as uniaxial negative birefringence (UNB), which is characterized by a larger refractive index along the in-plane direction than in the out-of-plane direction. The UNB was critically associated with the backbone linearity and rigidity as well as the intrinsic polarizability of the polyimides. A polyimide synthesized from 2,2'-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride and 2,2'-bis(trifluoromethyl)-4,4'-diaminobphenyl was used to study the effect of molecular weight on the LOA. For films prepared from fixed molecular weight samples, the refractive indices were constant for film thicknesses below 15 umm. The refractive index along the out-of-plane direction decreased while the in-plane refractive index increased as the polyimide molecular weight was increased to a critical level. Films with UNB have been used as negative birefringent compensators in avionic LCDs to improve the display off-angle viewability.