Polarized UV light induced alignment for liquid crystal displays on well-defined polyimide films.

摘要

The surface alignment of liquid crystals plays an essential role in determining the behavior of the bulk phase. Liquid crystal displays (LCDs) require uniform alignment on the confining substrates for their quality and function. The conventional rubbing method has some drawbacks, such as particle contamination and static charge buildup. As an alternative to mechanical rubbing, photoalignment uses polarized UV light (PUV) exposure of polymer films to obtain uniform alignment. This noncontact process avoids the drawbacks of rubbing and is particularly useful in multidomain applications.; Photoalignment was first observed on polyvinylcinnamate (PVC) film which is not commercially viable because of low thermal stability. We discovered photoalignment on commercial polyimides which are specially designed for liquid crystal alignment. The unique properties of the polyimides, such as good mechanical strength, thermal stability, chemically inert to liquid crystals and processing solvents, etc., make them very suitable as LCD alignment layers. In this dissertation, I report photoalignment on polyimides, explore the generation of pretilt angle, and develop models to explain the photoalignment mechanism.; I found that polarized UV exposure of polyimides results in alignment perpendicular to the direction of UV polarization. The liquid crystal anchoring strength on photoaligned polyimides is strong and comparable with that achieved by rubbing. The UV and IR spectroscopic studies of polyimide films showed that the photoreaction induced by UV exposure is mainly bond cleavage followed by reduction of the carbonyl, phenyl and C-N groups associated with the imide ring. Anisotropic absorption of polarized UV results in an anisotropic distribution in the reaction products of the resulting film. The interactions between liquid crystals and the anisotropic surface leads to preferential alignment of the liquid crystal.; Generation of pretilt angles is crucial for photoalignment to be viable in liquid crystal applications. Unlike rubbing, photoalignment induced by normally incident PUV exposure has zero pretilt angle. I investigated oblique exposure with polarized UV to generate uniform pretilt. For oblique exposure, photogenerated pretilt angle is strongly influenced by the polyimide curing temperature and UV exposure conditions. The tilt angle increases with increasing curing temperature for both uniform and random alignment, suggesting that the degree of imidization is the main factor controlling the pretilt angle. Normal PUV exposure breaks the in-plane symmetry resulting in an anisotropic surface which aligns the liquid crystals in a preferential direction. The polarization of oblique PUV exposure makes an angle with the surface and the photoreaction for on-axis transition moments is much easier than that of off-axis. Consequently the tilt degeneracy is broken and the liquid crystals tilt in a preferred direction.; As a practical example of the application of photoalignment using polyimides, I used a one mask process to fabricate a four-domain twisted nematic cell with improved viewing characteristics. In related studies, I investigated the effect of UV exposure on the alignment of the rubbed polyimide films. I found that polarized UV exposure of weakly rubbed polyimide substrates strongly influences the rubbing alignment direction, but has less effect on strongly rubbed substrates.