Electro-optical and Morphological Properties of Bragg Transmission Gratings Written In Holographic Polymer Dispersed Liq
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Electro-optical and Morphological Properties of Bragg Transmission Gratings Written In Holographic Polymer Dispersed Liquid Crystals by Thiol-ene Photopolymerization L. V. Natarajan,1 V. P. Tondiglia,1 R. L. Sutherland1, D. Tomlin2 and T. J. Bunning Air Force Research Laboratory, Materials and Manufacturing Directorate/MLPJ, WrightPatterson AFB, OH 45433 1. Science Applications International Corporation, 4031 Col Glen Highway, Dayton, OH 45431 2. Universal Energy Systems, Springfield Pike, Dayton, OH 45431
ABSTRACT Holographic Bragg transmission gratings formed via the anisotropic phase separation of nanosized LC droplets (H-PDLC) were fabricated using thiol-ene photopolymerization. Using coherent UV laser light and a single prism, electrically switchable transmission gratings in the Bragg regime were written. The performance of the thiol-ene-based gratings were compared with those of multifunctional acrylate-based gratings written under similar conditions. Optical and electro-optical measurements suggest that thiol-ene polymers offer promise as hosts for improved H-PDLC performance. Interesting differences in the diffraction efficiencies for s- and p-probe beams are noted for the two matrices. Morphology studies by TEM and SEM exhibit striking differences in droplet shape and uniformity. These differences are speculated to be due to differences in polymer MW growth due to the step-growth propagation mechanism for thiol-enes as compared to the chain-growth propagation mechanism in multifunctional acrylates. The response times of the thiol-ene gratings were ten times slower than those of acrylates. INTRODUCTION Polymer-dispersed liquid crystals have been examined for a number of years as electrically switchable two-phase composite system.1 Generically, phase separation of a low molar mass liquid crystal in a polymer matrix will lead to micron-sized LC domains separated by a crosslinked or elastic polymer matrix. Application of an electric field will modulate the director orientation in the LC domains thereby modulating the refractive index mismatch between the two phases. The size scale of these systems has been optimized to enhance multiple scattering events thereby generating the maximum scattering contrast. Over the past few years, however, there has been a growing interest in nanoscale PDLC systems, ones in which the LC domains are 2 µm also using this chemistry.9,10 In this paper, we show results from Bragg transmission gratings written utilizing thiol-ene polymers using coherent UV laser light. We compare the properties of an acrylate-based grating written under similar conditions. EXPERIMENTAL PROCEDURES Using the commercial available thiol-ene formulation NOA 65, only weak transmission gratings with DE < 20% were obtainable. The addition of Daracur 4265 UV initiator (1%) greatly enhances the DE (>60%) for the same UV power. Thin sample cells (glass or ITO-glass) were made using a 8 µm spacers. The pre-polymer formulation was made up of 70% by weight of NOA-65 and28% nematic liquid crystal BL037 (Me
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