Study On Uv Light Controlled Photo-polymerization And The Alignment Effect To Liquid Crystal Molecules

The alignInent of 1iquid crystal molecu1es on a treated substrae surfae isimPortant and necessary not only fOr preparing liquid crystal devices (LCD), but forresearching the physical properties of liquid crystals. At the present time, the most ofLC aliNents are based on the surfaCe orientaion of liquid crysta1 by rubbedpolymers. However, the rubbing technique is Anown to have several shortcomings formass-production of LCDs such as the generation of electrostatics charges, dustparticles or mechanical defects, Which reduce production yield, especially of activematrix LCDs during the rubbing process. The more sa1ient is that the narrownonunifOrm viewng angle problem, which was induced by the uniaxial characteristicsof liquid crystal was difficult to be solved by rubbing technique. In this thesis, the LCalignment technique for polarized UV exposed photosensitive polymer5 (the opticalalignment or the photo-alignment) and its feasibility were investigated by the numbersin allusion to the rUbbing tecdrical disadvanages.The alignInent characteristics of liquid crystals on the various photosensitivepoIymerized film surfaces were studied wtth using polarized micrograPhy andmeasuring the optical retardation of LC 1ayeL Atomic fOrce microscopy (AFM) wasalso used for observing the morpho1ogy of the photo-polymerized films. It was shownthat the photochemical efficiency of the pre-polmpers containing cinnaxnate grouP ishigher The polymerized percentage is up to 90 % aller exposure of polar1zed UV lightfOr 30 minutes whi1e the LC molecules were uniformly aligfled on this polymersurfaces. and the LC alignment is thermal1y stable until 80"C. However the Prehltangle of 1iquid crystals is only about 0" --0.3' even though using double --exposuremethod. The photochemical efficiency of the pre-polymers containing coumarin groUPis lower. 50% only was participated in photochemical reaction after exposure UV lightfOr 60 minutes, on which fi1m the LC alignment is just able to aPpear. However, thiskind of polyffiers was thermally stable uP to 200"C and was able to induce a largearrange of pretilt angle from 0" to 90". The photosensitive polyimide PI(BTDA-TMMDA) provides the highest photochemical efficiency among these thIee kinds ofpre-polymers. About 80% of the pre-polymer molecules were panicipated -inphotochemicaI reaction after exposure of UV light for 20 minutes. At this case, theliquid cfl'stal molecuIes were uniformly aligned with a preti1t ang]e 0" ~0.6' and theaIignxnent was rather stabIe until l30aC. These results have not been reported in theliteratue.Anchoring strength is an important parameter, which expresses the aligning ability,' of alignent layers to the liquid crystals. The director distribution in liqllid crystallayer and the analytical exPression of Freederickz's transition threshold fOr antiParalleland tWist nematic liquid cry5tal cell under the weak anchoring case were deducedaccording to elastic continuum theory of liquid crysta1s for the first time. The polaranchoring intensity was obtained as a function of temperat'Ur by the aforemenioneddeduction and the research results. It is the improvement to the LC anchoring theoryThe resu1ts from UltraVolet absorption sPectra show that the processes ofphotochemical reaction were determined by a new function rather than Lambert-BeerIaw The absorption coefficients of poIyffiers containing cinnamate group, coumedngrouP and photosensitive polyimide were 0.11, 0.0l and 0.15, respectively Thepo1arized Fourier transformation Infrared absorption spectra (FT-IR showed that theorder pararneters of the polythers containing cinnamate grouP, coumedn grouP andphotosensitive polyimide were 0.02l, 0.025 and 0.0085, resPectvely. .bese resultS hada far-reaching significance fOr researching LC a1ignment mechanism and optimizingthe tecbocs of LC reorientation.