Structural and Optical Aspects of Sol-Gel Optical Composites
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XIAOCHUN LI AND TERENCE A. KING Laser Photonics Group, Department of Physics and Astronomy, University of Manchester, Manchester M13 9PL, UK
Schuster Laboratory,
ABSTRACT Sol-gel derived optical composites have been prepared by both the in-situ polymerisation and ORMOSIL methods and characterised by FT-Raman and near infrared (NIR) spectroscopies regarding their chemical structures, optical properties and molecular interactions. A model based on intermolecular and intramolecular hydrogen bondings has been proposed to interpret the observed spectral results.
INTRODUCTION Sol-gel derived optical materials based solely on inorganic systems such as tetramethoxysilane (TMOS)-derived gel silica can be modified by introducing an organic into the inorganic matrix. This has led to the development of organic-inorganic composites with changes in their optical and mechanical properties. There are broadly two different ways to do this. Method 1 involves impregnating a porous gel glass with an organic monomer and polymerising it in situ [1]. This leads to composite materials in which both the organic and inorganic phases physically exist and can be chemically identified. Method 2 is the use of organosubstituted silico esters in a sol-gel process [2]. This leads to organically modified silicates (ORMOSILs) in which the organic components are chemically bonded to the inorganic backbone and become part of an integrated network structure. In this paper, the composites prepared by both the in-situ polymerisation and ORMOSIL methods are investigated by FT-Raman and near infrared (NIR) spectroscopies regarding their chemical structures, optical properties and the possible molecular interactions within the composites.
EXPERIMENTAL
In-situ polymerisation A gel silica glass, prepared from TMOS by Geltech Inc. (USA) and partially densified at 600'C, was initially dehydrated in vacuo for 24 hours at room temperature, and impregnated by immersion in a 0.125% benzoyl peroxide/methyl methacrylate (MMA) solution overnight in a sealed container. The container was then unsealed, transferred to a pressurised reaction vessel under 30 lbs/in 2 .N2 pressure and kept at 60'C for 3 days for polymerisation to proceed. The sample was then detached from the container and polished to obtain smooth surfaces.
541 Mat. Res. Soc. Symp. Proc. Vol. 346. 01994 Materials Research Society
ORMOSIL preparation 3-(trimethoxysilyl)propyl methacrylate (TMSPM) and tetramethoxysilane (TMOS) were reacted at room temperature with acid water (HC1) using methanol as the solvent. The molar ratios of TMSPM, TMOS, H 20, HCl and CH 3OH were 1:1:10:6.5x10-3:2. Discs were cast, gelled and dried below 100°C. Sample surfaces were then ground and polished.
Spectroscopic measurements Dehydration was carried out in vacuo overnight at ambient temperature before the sample was spectroscopically studied under an atmosphere of high-purity nitrogen. Subsequent rehydration was performed by letting air into the sample cell under ambient conditions. Raman experiments were performed usi
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