Holographic Polymer Dispersed Liquid Crystals: Effect of Partial Matrix Fluorination on Electro-Optical and Morphologica
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Holographic Polymer Dispersed Liquid Crystals: Effect of Partial Matrix Fluorination on Electro-Optical and Morphological Properties Michael D. Schulte1, Stephen J. Clarson1, Lalgudi V. Natarajan2, C. Allan Guymon3, Timothy J. Bunning4 1 Department of MS&E, University of Cincinnati, Cincinnati, OH 45221-0012, U.S.A. 2 Science Applications International Corporation, 4031 Colonel Glenn Hwy, Dayton, OH 45431, U.S.A. 3 Department of Polymer Science, University of Southern Mississippi, Hattiesburg, MS 39406-0076, U.S.A. 4 Air Force Research Laboratory/MLPJ Wright-Patterson Air Force Base, OH 45433-7702, U.S.A. ABSTRACT Holographic polymer dispersed liquid crystal (H-PDLC) films with partially fluorinated matrices were investigated. Electro-optical and morphological studies revealed that fluorinated composites were substantially different from non-fluorinated analogues. The addition of a fluorinated monofunctional acrylate monomer to a pentaacrylate-derived polymer matrix resulted in improved diffraction efficiency. These findings suggest that the partial fluorination of the host polymer decreases the compatibility between the matrix and liquid crystal phase. Morphological differences between fluorinated films and non-fluorinated control specimens were verified using low-voltage, high-resolution scanning electron microscopy (LVHRSEM). INTRODUCTION Polymer dispersed liquid crystals (PDLCs) constitute a unique class of optical composites with nano-scale morphological features1. An amorphous polymer matrix and liquid crystalline (LC) phase serve as the basis for these systems2. In the absence of an electrical field, spatial variations in the refractive indices of the respective phases give rise to the scattering of light. For positive dielectric LCs, a transparent “on” state results in a direction parallel to an applied field when the ordinary refractive index (no) of the LC is very close in value to that of the polymer (np). Holographic PDLCs (H-PDLCs) are volume gratings formed via polymerization induced phase separation (PIPS)3. In this architecture, a refractive index modulation exists throughout the material as a result of alternating layers of liquid crystal and polymer rich planes4. These films are classified as either transmission or reflection gratings depending on the orientation of the planes with respect to the surface of the film. Reflection gratings, with potential applications in display and narrow-notch filters, require high diffraction efficiencies (>95%) and low driving voltages (
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