, Volume 3, Issue 5, pp 219-227
Date: 14 Feb 2007

A novel process for holographic polymer dispersed liquid crystal system via simultaneous photo-polymerization and siloxane network formation

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Abstract

Formation of transmission holographic polymer dispersed liquid crystal gratings was studied for matrix components of trimethylolpropane triacrylate:trimethoxysilylmethyl methacrylate:1-vinyl-2-pyrrolidone (reactive diluent) in the range from 80:10:10,wt% to 10:80:10,wt% and E7 as nematic liquid crystal under the irradiation with Nd-YAG laser (532,nm).

The optimum concentration of E7 in the recording solution was 35,wt% (65,wt% of matrix components) in presence of small amounts of radical photo-initiator system (Rose Bengal 0.05,wt%, NPG 0.1,wt%). When the concentration of methacrylate in matrix components was low (< 30%), gratings with apparently high diffraction efficiency could be fabricated both for trimethoxysilylmethyl methacrylate (cross-linkable by hydrolysis) and trimethylsilylmethyl methacrylate (non-cross-linkable by hydrolysis), although transiently high initial diffraction efficiency was observed by the non-equilibrium initial photo-polymerization of cross-linking components. Distinct difference was seen at higher concentration (> 50,wt%) of the methacrylates. Contrary to that gratings with reasonably high and stable diffraction efficiency were successfully fabricated with trimethoxysilylmethyl methacrylate, gratings with only low diffraction efficiency were obtained for non-cross-linkable trimethylsilylmethyl methacrylate.

With higher concentration of photo-sensitizer and photo-initiator (0.2,wt%; 1.0,wt%), and shorter irradiation time, gratings with high diffraction efficiency could be fabricated only for the photo-initiator system of 3,3′-carbonylbis(7-diethylaminocoumarin) and diphenyliodonium hexafluorophosphate with shorter induction period (∼174,s).

Grating with diffraction efficiency of 72% was obtained with trimethoxysilylpropyl acrylate (80,wt% in the matrix component) and 35,wt% E7. By increasing the concentration of diphenyliodonium hexafluorophosphate to 2,wt%, diffraction efficiency increased to 85% and induction period was shortened to 129,s, and low volume shrinkage of 8% was attained via simultaneous radical cross-linking of trimethylolpropane triacrylate and siloxane network formation of trimethoxysilyl groups of trimethoxysilylmethyl acrylate by atmospheric moisture catalyzed by cationic species produced from the initiator system.

In SEM morphology, although gratings formed with high concentration of trimethoxysilylpropyl acrylate had some cracks in polymer matrix, the largest grating spacing indicating the lowest volume shrinkage and very regular and well-defined gratings were observed.