Function of Silicon-Oxygen Bonds in Fabrication of Holographic Gratings

High performance transmission holographic polymer dispersed liquid crystals were fabricated using a mixture of radically photo-polymerizable multi-functional acrylates and ring-opening photo-polymerizable epoxides as monomers by Nd-YAG laser (lambda=532nm) in the presence of diphenyliodonium hexafluorophosphate and 3,3'-carbonylbis(7-diethylaminocoumarin). High diffraction efficiency over 97% was obtained for grating formed from dipentaerythritol penta/hexaacrylate, 1,5-bis(3-glycidoxypropyl)-1,1,3,3,5,5-hexamethyltrisiloxane, 1-vinyl-2-pyrrollidone (reactive diluent), and commercial liquid crystal, E7 (45:36:9:10 in weight percent). 1,5-Bis[2-(1,2-epoxycyclohex-4-yl)ethyl]-1,1,3,3,5,5-hexamethyl-trisiloxane gave a little lower diffraction efficiency (75%), but it gave gratings with reduced angular deviation (0.6 degree for signal beam at 32 degree incident angle) from Bragg profile. Spiroorthoester function was found to be polymerized with the laser. The volume shrinkage was much lower than that prepared with bis(epoxide)s. Gratings were also formed for matrix of trimethylolpropane triacrylate:methacryloxymethyltrimethoxysilane:1-vinyl-2-pyrrolidone in the range from 80:10:10 to 10:80:10 wt% and E7. Gratings with apparently high diffraction efficiency could be fabricated both for methacryloxymethyltrimethoxysilane (cross-linkable by hydrolysis) and methacryloxymethyltrimethylsilane (non cross-linkable by hydrolysis) with high concentration of trimethylolpropane triacrylate. Distinct difference was seen at higher concentration (> 40 wt%) of the methacrylates. Contrary to that gratings with reasonably high and stable diffraction efficiency were successfully fabricated with methacryloxymethyltrimethoxysilane, only low diffraction efficiency was obtained for non cross-linkable methacryloxymethyltrimethylsilane. Importance of hydrophilicity of the spacer group was examined using TL203 as a liquid crystal. Introduction of urethane group or hydroxy group in the spacer dramatically improved the performance of the gratings.