Transmission Holographic Gratings Using Siloxane-containing Liquid Crystalline Compounds. Importance of Chemical Structure of Polymer Matrix Components

Abstract

ABSTRACT:Siloxane-containing liquid crystalline compounds were found effective to induce distinct phase separation, when combined with multi-functional acrylates, over a wide concentration range from 6 wt % to 20 wt % in the formation of holographic gratings. Use of ring-opening polymerizable bi-functional epoxides as one of the reactive cross-linking matrix component together with penta-or tetra acrylate initiated by 3,3 0 -carbonylbis(7-diethylaminocoumarin)-diphenyliodonium hexafluorophosphate greatly improved the diffraction efficiency and angular selectivity of the formed grating. Fine gratings with 70% and 78% diffraction efficiency and angular selectivity of about 5were formed with 10 wt % 4-cyano-4 0 -{(5-heptamethyltrisiloxan-1-yl)pentyloxy} biphenyl and 4-cyanophenyl 4-{(5-heptamethyltrisiloxan-1-yl)pentyloxy}benzoate using pentaerythrytol pentaacrylate-neopentylglycol diglycidyl ether-tripropylene glycol diacrylate (4:5:1) as the polymer matrix component. Since holographic storage enables fast data transfer rates by simultaneous writing and reading, an upsurge of interest in optical holography has been noticed in the past decade. Photopolymers and polymer dispersed liquid crystals have been mainly investigated for holographic storage systems. Kawabata reported the holographic recording based on photopolymer system. 1 R. L. Sutherland 2-7 proposed a grating formation via the phase separation of liquid crystal (LC) during the photo-polymerization irradiated by two coherent interfering laser beams. Grating was formed as a refractive index-modulated interference fringe pattern of stratified alternating layers of LC and polymer (holographic polymer dispersed liquid crystal, HPDLC). Principle and application of PDLC systems have been extensively studied. [8][9][10][11][12][13] An important role of LC is the response against electric field in the formed gratings, which makes it possible to be applied as display devices, 14-16 switchable lenses, 17,18 and wave-guided switches. 19 In these systems, control of the morphology of the LC droplets in a photo-polymerized matrix, which depends on the LC content, the cure rate, and compatibility among the components, is the key to develop high performance holographic films with controllable and repeatable electro-optic performances.For HPDLC systems, the principal role of LC is to phase-separate by the diffusion during the polymerization of multi-functional monomer giving polymer matrix, and to give high modulation of the refractive index of the formed gratings. The phase separation is a balance between the rate of diffusion of LC and the rate of gelation of the polymer network dictated by the polymerization kinetics. In designing efficient holographic gratings, much attention has been focused mainly on the elucidation of the effects of functionality of photo-polymerizable monomers. [20][21][22][23] Little attention has been paid to the importance of the chemical structure. In our previous study, it was found that siloxane-containing epoxide monomers were effective fo...

Formation of Stable Holographic Polymer Dispersed Liquid Crystal Grating with High Diffraction Efficiency Assisted by in situ Hydrolysis-Condensation of Trialkoxysilylalkyl Group of Methacrylate Component

“…Recently, we have concentrated on the organic-inorganic hybrid materials [30][31][32][33][34][35][36] to improve the drawbacks of HPDLC system by the incorporation of siloxane components in the grating formation with the intention to assist the efficient phase separation of monomer and LC components with controlled manner by taking advantages of highly incompatible nature of siloxane component against polymer matrix and its flexibility to ease the diffusion of LC. [37][38][39][40][41][42][43] Introduction of polyhedral oligosilsesquioxane structure also improved the performance of the formed grating. 44 Thus, if we consider the structure and reactivity of siloxane compounds in relation with the property, it will be possible to propose new systems to improve the performance of HPDLC gratings.…”