Organosilane coupling agents are an important class of chemical materials with significant applications in materials science. One end of the organic silane coupling agent molecule is a typical active organic group, such as double bonds, epoxy groups, amino groups, etc; The other end is a hydrolyzable alkoxy group, such as methoxy and ethoxy, which can form Si-0-bridge bonds with inorganic materials through hydrolysis and condensation reactions. So, coupling agents can improve the interfacial bonding between components in composite materials such as glass fiber and polymer composites, and enhance the mechanical properties of the materials. Although the dosage and scope of use of coupling agents have continued to increase in the past 40 years since their introduction, and the variety has also increased, their current applications are still mainly limited to fiber-reinforced or particle filled polymer based composites to improve interfacial bonding. These types of composite materials are commonly used as structural components.
Due to the limited proportion of interfaces in traditional composite materials, the amount of coupling agent used is not large, accounting for only 1-2% of the total volume of the material. The application technology in this area is relatively mature. In recent years, the application of the sol gel method in the research of new materials has attracted widespread attention, especially in the preparation of organic-inorganic hybrid materials and nanocomposites at the molecular level. Compared with traditional composite materials, this type of material has some special properties and is often used as a functional material. Organosilane coupling agents play an important role in such materials due to their unique molecular structure and properties, and their usage has greatly increased. The application of the three most commonly used organosilane coupling agents in the preparation of organic-inorganic hybrid materials by the sol gel method, and the preparation methods and properties of the corresponding hybrid materials are discussed.
1. Methacryloyloxytrimethoxysilane (MPTMS)
One of the applications of organic-inorganic hybrid materials is the incorporation of organic dyes as working materials for solid-state lasers. Taking the material doped with Rhodamine 6G (concentration 50 * 10-4mol/dm3) as an example, the coupling agent accounts for a considerable proportion in the raw material. The vinyl monomer MMA in the raw material can be either self polymerized or copolymerized with MPTMS to form the organic part of the material. The hydrolyzable methoxy group in TEOS and coupling agent molecules forms the inorganic part of the material SiO2 through the sol gel process. The stable Si-C bond in the MTPMS molecule connects the organic and inorganic parts. In addition, GPTMS coupling agent carries epoxy groups, and the diol structure formed by hydrolysis is conducive to the sol and dispersion of organic dyes.
The method of preparing organic-inorganic hybrid materials using MPTMS alone is not entirely the same. One method is to first pre hydrolyze MPTMS with TEOS or other metal alkoxides, and then add polymers such as PMMA dissolved in appropriate solvents to form organic polymer modified inorganic oxide materials. When preparing PMMA modified Al2O3-SiO2, macroscopic phase separation does not occur and transparent materials are formed only when the PMMA content is higher than 20%. Another method is to first copolymerize MPTMS with olefinic organic monomers to form organic polymers with hydrolyzable alkoxy groups. These polymers are then hydrolyzed and condensed with TEOS to form hybrid materials. The hydrolysis process can be catalyzed by inorganic acids or photochemical methods. Here, the MPEMS content in the polymer and the molecular weight of the polymer should be properly controlled. For example, when preparing the hybrid material of PS and SiO2, when the MPEMS monomer unit in the copolymer is less than 22% (mol), phase separation occurs in the sol gel process, and the resulting material is opaque. The unique role of coupling agents is particularly prominent in another preparation method of hybrid materials. In this method, MMA is used to impregnate nanoporous silica gel and make it polymerize in situ to obtain transparent PMMA-SiO2 hybrid materials. There is no chemical bond between PMMA and SiO2. If porous silica gel is treated with coupling agent MPTMS before impregnating MMA, a monolayer of coupling agent and active genes on the coupling agent molecules can be formed on the pore walls.
2. 3-glycidyl ether propyl trimethoxysilane (GPTMS)
The active organic group of GPTMS is epoxy group. The hybrid material obtained by hydrolysis and condensation with silicates and other metal alkoxides can be used as a surface coating for plastics, which can improve surface hardness and abrasion resistance. If 50%~70% (mol) GPTMS, 10%~30% (mol) TEOS, and 10%~20% (mol) metal (such as TI, Zr, Al) alkoxides are used, stable coating solutions can be obtained, and high-performance transparent coatings can be prepared. Aluminum containing coatings have high hardness and abrasion resistance. We have prepared organic modified SiO2 coatings and TiO2-SiO2 coatings on the surface of PMMA, which significantly improve the surface hardness and have an anti reflective effect. The former has a significant anti reflective effect, while the latter increases the hardness.
3. 7-Aminopropyltriethoxysilane (APTES)
The application prospects of semiconductor (such as CdS) quantum dot materials in nonlinear optics are very promising. The moral organic-inorganic hybrid material produced by the sol gel method can be used as its matrix, as described above. APTES, in addition to being a component of the material, is also used to control the size and particle size distribution of quantum dots to achieve specific properties. When preparing porous polydimethylsiloxane (PDMS) films, APTES can be used to control the pore size and adjust the transmittance. When preparing the wear-resistant coating on optical plastic CR-39 as described in the previous section, the adhesion between the coating and the substrate is poor. If APTES is used to pre treat the surface of CR-39, the coating and substrate can be firmly bonded.
