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

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 hydrolysiscondensation) 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 0 -carbonylbis[7 0 -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 mm 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% In recent years, holographic polymer dispersed liquid crystal (HPDLC) systems has attracted much interest due to their unique switching property by electric field to make them applicable to information displays, optical shutters, and information storage media. 1-10Photo-polymerizable materials, typically multi-functional acrylates, have been mostly studied as materials for HPDLC because of their advantages of large refractive index modulation, low cost, and easy fabrication and modification.11-17 However, these HPDLC materials still have significant drawbacks such as volume shrinkage, low reliability, and poor long term stability.Since the performance of HPDLC is governed by the stability of precisely formed periodic polymer rich layer and LC rich layer created by interference of two incident laser beams, control of the formation of polymer matrix and phase separation of LC are very important. [18][19][20][21][22][23][24] The principal role of multi-functional acrylate in grating formation is to make the LC phase-separate by the formation of cross-linked polymer matrix. Control of the rate and density of crosslinking in polymer matrix is one of the most important factors, which determines the size, distribution, and shape of phase-separated LC domains, accordingly the final morphology and performance. In order to obtain clear phase separation of LC from polymer matrix to homogeneous droplets, high cross linking density, but...

Improvement of the Performance of Transmission Holographic Grating by Hydrolysis-induced Change of the Property of Polymer Matrix

“…We reported the improved diffraction efficiency by the introduction of triethoxysilyl component in the monomer structure according to ''assisted grating formation'' mechanism. [11][12][13] In this report we have focused on a novel recording system in which hydrolysable trimethylsilyl ether functional group by moisture and proton produced from initiator system was introduced to monomer structure to change the property of the polymer matrix during the recording, which facilitates the efficient phase separation of liquid crystal molecules. …”