Photochromic Liquid Hydrogels as Hosts for Holographic Materials
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system could be coupled to longer holographic lifetimes and capacities, then technology for large data storage and rapid information retrieval could be realized in a system more compact than current CD-ROM technology. In this project, we are investigating the feasibility of increasing holographic capability by coupling a spiropyran dye to a liquid crystal (LC) system which offers increased birefringence in the writing of the hologram. Given that a key issue is the magnitude of the real component of the refractive index, increasing the birefringence may be a useful approach. In writing the hologram, the LC's go from isotropic to an ordered dispersion, a property which can be linked to those of bR to improve holographic lifetime. An attractive aspect of biopolymers is their liquid crystalline properties. This work exploits a model LC biopolymer with which we have had extensive experience, namely poly(y -benzyl-L-glutamate), "PBLG" 2. 3. Polymer gels with LC order have been prepared by the crosslinking of a LC solution of PBLG. These gels with cholesteric LC order have been prepared, and they show cholesteric-isotropic reversible transition accompanied by the helix-coil transition of PBLG molecules 4 . These gels have been used to orient dye molecules introduced either by doping or by covalent linkages5 . This overall project will investigate the feasibility of a PBLG 'host' to increase the birefringence of the written hologram and thusly refine its holographic potential (vis a vis its lifetime). The experimental work, reported herein, has first focused on characterizing the ordering behaviors of the LC host prior to introduction of the photochromic dye. EXPERIMENTAL Materials All PBLG's were obtained from Sigma Chemical Co., St. Louis, MO and used as received. Three molecular weight ranges with nominal weight averages of 26kD, 11 8kD, and 236kD were investigated in this study. All solvents were obtained from Fisher Scientific, Inc., and used as received. In-Plane Electric Field Alignment of PBLG The cell design for in-plane poling uses teflon blocks machined to include rectangular reservoirs with electrodes imbedded at either end and providing electrode gaps of 1.2, 2.8, and 6.2 cm. A power supply (Del Electronics Corp., Valhalla, NY, Model RHVS60-300P) capable of delivering 60kV DC enables preparation of large films at high field strengths. Electrodes were placed so that their upper and lower edges were above and below the solution layer. The entire apparatus is contained within a glass housing. This cell design provides for an optically clearer film of more uniform thickness by reducing the problem of electroconvection via control over the solvent atmosphere within the cell and the rate of solvent evaporation. In addition, the teflon material exhibits good chemical resistance. Characterization of the Aligned PBLG Hosts by Infrared Spectroscopy Films were characterized for alignment using FTIR using the method of Marcher et al. 6. Infrared spectra were taken on a Perkin-Elmer (Norwalk, CT) 1600 Series FTIR equipped
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