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

Abstract

Stable transmission holographic polymer dispersed liquid crystal grating with reasonably high diffraction efficiency (72%, induction period 576 s) could be prepared via radical network formation of trimethylolpropane triacrylate (only 10 wt %, photo-cross-linkable, further diluted with 10 wt % 1-vinyl-2-pyrrolidone) and 3-methacryloxypropyltrimethoxysilane (80 wt %, non cross-linkable by radical polymerization, but cross-linkable by hydrolysis-condensation) in the presence of 35 wt % TL 203 as a liquid crystal, assisted by simultaneous hydrolysis-condensation of trimethoxysilyl group. The hydrolysis-condensation was induced by moisture in reagents and proton species produced from 3, 3′-carbonylbis[7′-diethylaminocoumarine] as a photo-sensitizer and diphenyliodonium hexafluorophosphate as a photo-initiator. Grating could not be formed for the same system with 3-methacryloxypropyltrimethylsilane (non hydrolyzable and not cross-linkable).Rather decreased efficiency of 3-methacryloxypropyltriethoxysilane system (13% efficiency, 693 s induction period) was remarkably improved to 75% (75 s induction period) by introducing hydrophilic urethane group into the spacer to facilitate easier hydrolysis. The long grating spacing of 0.9 μm indicates very small volume shrinkage. The maximum diffraction efficiency (85%) was observed at 60–70 wt % 3-N-(2-methacryloxyethoxycarbonyl)aminopropyltriethoxysilane, but with a little larger volume shrinkage.Introduction of aminoalcohol group in the spacer could further shorten the induction period to 18 s, but the diffraction efficiency was rather low (20%).