Real Time Study of Reflective H-PDLC Gratings
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ABSTRACT The formation of holographic reflection gratings in polymer dispersed liquid crystals(H-PDLC) was investigated in real time both in reflection and transmission modes. Real time study is useful for obtaining information on the speed of grating formation, phase separation dynamics, host polymer shrinkage, polarization dependence of the reading beam, bleaching of the photointiator dye and scattering of the writing beam during grating formation. The real time studies showed that the formation of the grating is fast and reaches a maximum in a few seconds. The diffraction efficiency (DE) VS time curve shows an initial increase followed by a sharp decrease and a recovery to higher efficiencies. Variation of monomer functionality in the pre-polymer syrup resulted in a decrease of reflection efficiency as the functionality is lowered. Varying the writing power of the laser beam gave higher efficiencies at powers exceeding 10 mW/cm2. Liquid crystal (LC) loadings of >16% are needed to obtain measurable efficiencies. Varying the polarization of the probe beam (S vs P) revealed differences in the temporal evolution of DE. Morphology studies are indicative of the differences in the phase separation due to monomer functionality, LC concentration and also laser power. The growth dynamics of H-PDLC gratings are very different from the well known Du Pont photopolymer films which may be due to the phase separation process accompanying gelation in H-PDLC systems. INTRODUCTION Reflective display technologies using liquid crystals are currently the subject of intensive research. The well known technologies are based on cholesteric LCD's, conventional TN and STN LC's with back lighting and LC polymer dispersions [1-2]. Though each of these have their own merits, the power consumption in using a backlight is a disadvantage for large scale commercial display market. A recent development in this field is based on holographically formed polymer dispersed liquid crystals where switchable gratings are formed by curing a pre-polymer syrup in the interference fringes of two coherent, counter propagating laser beams in the visible region[3-6]. Fast curing occurs in the bright interference fringes with monomer flowing from the dark region into the bright, at the same time diffusion of the nematic liquid crystal into the dark areas takes place as a result of lowering of the misciblity gap due to polymerization. Phase
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Mat. Res. Soc. Symp. Proc. Vol. 597 © 2000 Materials Research Society
separation of the nematic LCs into droplets occurs in the darker region. The resulting films have periodic phase separated LC domains separated by dense polymer. The phase separated droplets are sub micron in size and there is very little scattering of the film. In the field off state, there is random orientation of the droplet directors which leads to index mismatch between the polymer rich and liquid crystal rich regions. In the off state, diffraction of light due to the presence of a holographic grating is observed. When an electric field is appli
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