Anna B. Wojcik

Transparent inorganic/organic copolymers by the Sol-Gel process: Copolymers of tetraethyl orthosilicate (TEOS), vinyl triethoxysilane (VTES) and (meth)acrylate monomers

Vinyl triethoxysilane (VTES) and (meth)acrylate monomers were mixed to create covalently bonded inorganic/organic copolymers, with and without tetraethyl orthosilicate (TEOS). Vinyl groups underwent free radical polymerization along with copolymerization with (meth)acryl groups of the monomer. For the two-monomer system, (meth)acrylate monomer and VTES, polymerization resulted in mechanically strong copolymers with flexure strengths greater than 40 MPa. Three-component materials obtained by polymerization of (meth)acrylate monomer, VTES and TEOS were homogeneous, highly transparent, with flexure strengths similar to those for silica xerogels, about 20 Mpa.

Transparent Organic/Inorganic Hybrid Gels: A Classification Scheme

An attempt is made to classify transparent organic/inorganic materials synthesized by the sol–gel process. The chemical structures of the hybrids are the main criteria for this classification. Three main types of organic/inorganic gels are distinguished and their basic physico-chemical characteristics are outlined.

High molecular weight poly(ethylene oxide)/silica hybrids by the sol-gel process

We prepared inorganic/organic hybrids by the sol-gel process, using tetraethyl orthosilicate (TEOS) in an aqueous solution of poly(ethylene oxide) (PEO) under acidic conditions (HCl). The hybrids represent so-called sequential interpenetrating networks with PEO (polymer I) and silica (polymer II). The PEO content was between 0.25 and 1.0 g of PEO for every 50 ml of TEOS. The PEO molecular weight was either 100 000 or 300 000. The water-to-alkoxy ratio (R) was either R = 3 or R = 8. Among these variables, the water-to-alkoxy ratio had the largest influence on physical properties, such as density and hardness. The types of behavior were divided into two categories, one for the gels prepared with less than stoichiometric water (R 4).

Transparent inorganic/organic copolymers by the sol-gel process: Thermal behavior of copolymers of tetraethyl orthosilicate (TEOS), vinyl triethoxysilane (VTES) and (meth)acrylate monomers

Three types of inorganic/organic copolymers have been prepared in a one-step sol-gel process, and their thermal stabilities have been studied. The one-step sol gel process was carried out in mixtures of three monomeric components: HEMA (hydroxyethyl methacrylate)-VTES (vinyl triethoxysilane)-TEOS (tetraethyl orthsilicate), HDDA (hexanediol diacrylate)-VTES-TEOS and GPTA (glycerol propoxy triacrylate)-VTES-TEOS. Copolymers with HEMA, which is able to form a linear organic polymer, were the least thermally stable materials. They lost the highest proportion of weight during heat treatment to 600°C and exhibited the lowest decomposition temperatures. Copolymers with HDDA and GPTA, which are able to form crosslinked organic polymers, had higher decomposition temperatures, and their weight loss during heat treatment to 600°C was small. The skeletal densities of all copolymers increased slightly during heat treatment.

Organic-inorganic gels based on silica and multifunctional acrylates

Rigid, microporous silica rods, prepared previously by the sol-gel process, were infiltrated with three triacrylate monomers to give sequential interpenetrating networks (IPNs). The three commercially available triacrylate monomers are glycerol propoxy triacrylate (GPTA), trimethylolpropane triacrylate (TMPTA) and pentaerythritol triacrylate (PETA). Each solution containing a photoinitiator was allowed to infiltrate the silica rod, after which the sample was UV irradiated to polymerize the triacrylate monomer. Emphasis was placed on establishing the infiltration and polymerization conditions leading to optically transparent composites. For comparison, one of the triacrylates (GPTA) was introduced into a hydrolyzing sol-gel formulation containing TEOS, ethanol, water and acid catalyst, to effect a simultaneous IPN. The sequential IPN and the simultaneous IPN samples gave similar transmission curves, despite differences in skeletal density.

Organically modified silicate coatings for optical fibers

Three kinds of UV-curable organically modified silicates have been prepared to be used as protective coatings for optical fibers. The synthesis involves the reaction of the thiol group of 3-mercaptopropyl-trimethoxysilane with a C equals C bond in one of the acrylic groups of three commercially available aliphatic triacrylates. The methoxysilyl groups of the synthesized diacrylate methoxysilanes were subjected to hydrolysis and condensation to form Si-O-Si units. Transparent, viscous, solvent-free resins were obtained that hardened in seconds when exposed to UV radiation. The coating derived from the reaction with glycerol propoxy triacrylate (GPTA) proved to adhere the best of the three to both plastic and glass substrates. It was then tested as a protective coating for silica fibers. Reliability tests were carried out including bending strength and fatigue tests at pH 7 and 10. The results show improved water resistance of the coated fiber in neutral conditions.

Rhodamine 6G-doped inorganic/organic gels for laser and sensor applications

Rhodamine 6G-doped hybrid gels were prepared by a sol-gel route. The inorganic component tetraethyl orthosilicate (TEOS) and the organic component glycerol propoxy triacrylate (GPTA) were simultaneously polymerized to create interpenetrating silica-GPTA networks. Crack-free, transparent dried gels in the shape of rods and fibers were obtained. Lasing properties of the hybrid rod-shaped monoliths were determined as a function of rhodamine concentration. In addition, gels in the shape of fibers were evaluated as fiber lasers for detecting ammonia vapor.

Transparent poly(vinyl acetate)-silica gels by a sol-gel process

Rod shaped silica-poly(vinyl acetate) (PVAc) gels have been prepared by a sol gel process. In situ polymerization of tetraethoxysilane (TEOS) was accomplished in the presence of low molecular weight PVAc by dissolving various amounts of PVAc in a mixture of TEOS, ethanol, water and hydrochloric acid (HCl). Gelation of this mixture was carried out between room temperature and slightly above. Silica-PVAc rods recovered from cylindrical molds were homogeneous and transparent. Gels with weight percents of PVAc ranging from 2% to 50% were prepared. Silica-PVAc gels have higher flexure strengths, less brittle character and improved water durability in comparison with pure sol- gel silica.

Mechanical behavior of silica optical fibers coated with low-index low-surface energy perfluorinated polymer

Two formulations based on perfluorinated polymer were prepared for use as UV-curable optical cladding for silica fibers. In the first formulation an adhesion promoting agent based on fluoroacrylate resin was synthesized and mixed with the experimental product Defensa 7702++ in order to promote wetting and chemical adhesion to the silica fibers. In the second formulation, wetting and physical adhesion between the liquid coating and the silica fibers were achieved by increasing the viscosity of the starting coating by addition of unsaturated perfluorinated polymer into Defensa. Both formulations were used as primary coatings on dual coated silica optical fibers. The mechanical behavior of the formulations was characterized by the strip test, the pull-out test and zero stress aging in 90 degrees Celsius pH 7 buffer. The results show that both formulations exhibit better wetting-adhesion characteristics than unmodified starting coating and that the strength degradation during zero- stress aging was lower for the fiber coated with the formulation of higher viscosity.

Transparent organic/inorganic hybrid sol-gel materials based on perfluorinated polymers and silica

Two types of hybrid gels based on silica and perfluorinated polymers have been prepared. The first type involves a perfluorinated polymer containing acrylate groups. Perfluoropolyether diol diacrylate (PFDA) was functionalized by reacting it with (3-mercapto-propyl) trimethoxysilane by a Michael addition. The resulting silyl derivative (PFDAS) was able to copolymerize with a silica precursor, tetraethylorthosilicate (TEOS), resulting in perfluorinated polymer/silica hybrid gels. For the second type, perfluoroalkylsilane (FAS), vinyltriethoxysilane (VTES), and TEOS were polymerized in one step. In both cases, the gels were transparent, crack-free and water repellent. Since the inorganic and organic components are covalently bonded to each other, these materials can be classified as organic/inorganic copolymers.