Structural characterization of sol–gel composites using TEOS/MEMO as precursors

Abstract

An organic/inorganic hybrid SiO₂–matrix composite was prepared through sol–gel process using TEOS and MEMO as precursors. The structures and thermal stability of the composites were investigated by various complementary techniques. The results indicate that both TEOS and MEMO hydrolyze and polymerize in sols to gelatinize and form silica–matrix materials after being baked at 60 °C. Increase of the MEMO concentration in the sols results in reduction of low-molecular compounds (such as H₂O, CH₃OH, etc.) incorporated in baked gels, thus significantly decreasing the thermal weight-loss at temperature lower that 400 °C. On the other hand, the organic functional groups bonded with –Si–O in MEMO polymerize and gelatinize to change the atomic structures and chemical states of Si in the composites. The sol–gel composites induced by TEOS/MEMO gelatinization process become pure silica after thermal treatment over 500 °C, due to decomposition and evaporation of the incorporated organic groups. It is interesting to note that from the sol containing TEOS and MEMO precursors, hydrolyzed product from MEMO precursor preferably dip-deposits compared with that from TEOS. The detailed structure analysis and mechanisms of thermal stability are analyzed in the paper.

Introduction

SiO₂ thin film is known to have excellent properties such as hardness, wear-resistance, anti-corrosion and also optical, dielectric properties, so it has been widely used as functional and protective coatings for various kinds of materials [1], [2], [3], [4]. Sol–gel processes using tetramethoxysilane (TMOS) and tetraethoxysilane (TEOS) as precursors are promising and practical to prepare the silica films due to its low-temperature, cheap processing, and suitable to obtain homogeneous films on a large area substrates. However, the films deposited in the TEOS or TMOS sols are generally brittle due to incorporation and evaporation of the chemicals such as H₂O, CH₃OH and CH₃CH₂OH, etc. after baking or thermal treatment. In this case, many researchers have synthesized organic/inorganic hybrid films using low molecular weight organoalkoxysilanes as the main or additional precursors for the sol–gel reactions. Therefore, organic groups can be introduced into the sol–gel films and blended in the inorganic Si–O network through the Si–C– bond [5], [6], [7], [8], [9], decreasing the brittlement of the films. Furthermore, these organic/inorganic composites are reported to have new and different properties compared with the silica films. Wagh et al. [10], [11] modified the silica aerogels by adding methyltrimethoxysilane (MTMS) into TMOS sols. The results showed that with the MTMS amount increasing, the silica aerogels become hydrophobic but the corresponding transparence decreases. It is also reported that thermal decomposition up to 200 °C of organic (–CH₃) groups was confirmed by infrared spectroscopy (IR) and nuclear magnetic resonance (NMR) spectral analyses of the silica aerogels. Nakagawa and Soga [12] reported that a water repellent and density silica film with high thermal stability can be prepared in a TEOS sol containing (2-perlluorooctyl)ethyltrimethoxysilane (FAS). Yang et al. [13] added trimethylchlorosilane (TMCS) to the silica gels and deposited a sol–gel film with lower dielectric constant (relative to pure SiO₂) due to –CH₃ functional group substituted for –OH group, and the film is stable in the temperature range of 200–500 °C. Kasten et al. [14], [15], [16], [17], [18] reported a sol–gel SNAP (self-assembled nanophase particle) film with a mixture precursor of TMOS/3-glycidoxypropyltri-methoxysilane (GPTMS). This film has excellent corrosion-resistance property due to a smoother surface with fewer defects through incorporation of organic component. These studies indicate that the introduction of organic groups from special silane coupler shows an important influence on the film structures and properties.

3-(Methacryloxypropyl)-trimethoxy silane (MEMO) is also an important sol–gel precursor to prepare inorganic–organic hybrids. The corresponding sol–gel films were also reported to have silanol groups (Si–OH) and Si–O–Si bonds [19]. However, the organic components in MEMO can be also incorporated in the sol–gel films. Moreover, the organic functional groups are bonded strongly with SiOx groups and could polymerize during baking or thermal treatments. From FTIR analysis, some researcher found methacrylate polymerization process together with Si–OH condensation during thermal treatment [20], [21], [22], [23], [24], [25], [26]. In the literature [27], a sol–gel hybrid silicate was prepared from a MEMO-modified TMOS sol, causing changes in the silicate structures and properties. The authors proposed that MEMO introduced into TMOS sol changes the structures of the gels in two different modes: acting as a network modifer or as a network former. The present work is to investigate the structures of organic/inorganic hybrid films by means of various complementary analysis techniques generally and detailedly.