J. L. Oteo

A FT-IR Study of the Hydrolysis of Tetraethylorthosilicate (TEOS).

Abstract The hydrolysis and polymerization of a non-catalyzed silica sol was investigated by Fourier Transform Infrared Spectroscopy (FT-IR) and deconvolution of the infrared spectra. The hydrolysis was followed by the 1168 and 812 cm−1 bands which have been observed that decrease continously with the reaction time, and they dissapear showing the complete hydrolysis. The bands located at 1200 and 1147 cm−1 are assigned to polymerization of Si-OH groups forming Si-O-Si bonds in cyclic or linear structures respectively. Both bands increase with time and are present simultaneously in the spectra showing that both kind of cross-linking of Si-OH groups are taking place in the sol to form the gel. In the gel the presence of Si-OH groups and Si-O− free broken bonds have been detected by means of the bonds located at 960 and 920 cm−1 respectively.

Characterization and sintering behaviour of submicrometre titanium dioxide spherical particles obtained by gas-phase hydrolysis of titanium tetrabutoxide

Spherical fine (∼0.7 μm) titania powders were prepared by vapour-phase hydrolysis of a titanium tetrabutoxide/butanol solution. Powder X-ray diffraction showed that as-prepared powders were amorphous and crystallized to anatase when calcined at 450°C. Although the spherical titania particles shrank on calcination and retained the spherical shape, the primary particles grew to a notable extent after calcining. The individual calcined titania spheres were constituted by microporous agglomerates of about 13 nm primary anatase particles. When isopressed at 200 MPa, the titania spheres were crushed to form dense green bodies (∼55% theoretical density). These green compacts gave dense bodies (>99%) of rutile when sintered at 1030°C for 2 h with a submicrometre and quite uniform microstructure.

DSC and FT-IR analysis of the drying process of titanium alkoxide derived precipitates

Abstract The drying process of titanium alkoxide-derived precipitates has been studied by means of FT-IR spectroscopy and DSC analysis. From the FT-IR spectrum, it has been possible to observe that unhydrolyzed alkoxide molecules continue the hydrolysis process by using water molecules retained inside the structure of the precipitate or formed by condensation of the Ti–OH groups. However, the quantification of the water content inside the precipitate was carried out by DSC analysis. This technique also allowed to determine two kinds of water molecules: free or interstitial water and coordinatively bound water. These two kinds of water gave two endothermic peaks at ca. 160–180° and 305°C, respectively. The transformation of the amorphous precipitate to a crystalline one is detected by an exothermic peak at ca. 314°C. The elimination of the coordinatively bound water is associated with this crystalline transformation.

Synthesis of inorganic-organic hybrid materials from TEOS, TBT and PDMS

Inorganic-organic hybrid materials have been synthesized by reaction of tetraethoxysilane (TEOS), titanium tetrabutoxide (TBT) and silanol-terminated polydimethylsiloxane (PDMS). The hydrolysis and polymerization reactions of TEOS and PDMS in presence of TBT have been followed by means of FT-IR spectroscopy. Hydrolysis reactions have been characterized by Si—O—C and Ti—O—C bonds and polymerization reactions by Si—O—Si, Si—O—Ti and Ti—O—Ti bonds. The instantaneous hydrolysis of TEOS, the condensation reactions between Si—OH groups forming crosslinked and linear Si—O—Si structures and the copolimerization reactions between Si—OH groups and PDMS molecules have been observed. The reaction between Ti—OH and Si—OH groups or PDMS gives Si—O—Ti bonds. Si—O—Ti bonds are formed during the addition step of TBT and they show a decrease along the reaction time up to gelling. By another hand, the concentration of crosslinked and linear Si—O—Si structures depend on the TBT concentration. High TBT concentration favors the formation of crosslinked chains. TBT influences the hydrolysis and copolymerization reactions between hydrolyzed TEOS and PDMS molecules.

FT-IR Study of the Hydrolysis and Polymerization of Tetraethyl Orthosilicate and Polydimethyl Siloxane in the Presence of Tetrabutyl Orthotitanate

Abstract In this work, we have used FT‐IR spectroscopy to study the hydrolysis and polymerization reactions of tetraethyl orthosilicate (TEOS) and polydimethyl‐siloxane (PDMS) in the presence of tetrabutyl orthotitanate (TBOT). These reactions are used for obtaining SiO2–PDMS–TiO2 organically modified silicates (Ormosils). In order to obtain semi‐quantitative information about such reactions, a deconvolution procedure of the FT‐IR spectra has been done by use of a computer program. Hydrolysis reactions have been characterized by Me–O–C (Me = Si, Ti) bonds, and polymerization reactions by Me–O–Me bonds. Instantaneous hydrolysis of TEOS has been observed, together with condensation reactions between Si–OH groups, which give crosslinked and linear Si–O–Si structures. The TBOT is also hydrolyzed, but the high acid concentration inhibits condensation reactions between Ti–OH groups. The PDMS also condenses mainly with Si–OH groups and probably with Ti–OH, finally forming Me–O–PDMS bonds. The formation of Si–O–Si crosslinked structures and also Me–O–PDMS structures continues until the end of reaction. The gelling time is dependent on TBOT concentration in the reaction medium and, therefore, polycondensation reactions are dependent on TBOT concentration.

Inverse gas chromatography: a new approach to the estimation of specific interactions

Inverse gas chromatography (IGC) has proved to be a powerful tool for characterisation of solid surfaces, particularly those that cannot be easily studied by other methods, like fibres or powders. One of the magnitudes to be measured by this method is the London component of the surface free energy, which denotes the ability of the surface to interact with external material through Van der Waals forces. This value, is determined by measuring the retention volumes of the series of n-alkanes. The other interesting magnitude is the polar character of the surface energy. Several methods are described in the literature to evaluate this polarity or specific component of the surface energy, not all giving concordant results. In this paper a method is proposed for the evaluation of this specific component of the surface energy, based on the fact, that the shift of Kovats index of polar compounds in a column, taking as reference a non polar column, is directly in relation with the polarity of the column. Some data are given of interactions obtained with some of the polar probes with previously described methods and the new one. The study material is several carbon fibres, cured and uncured epoxy resins, two modified polyethylene samples and a commercial SP100 column.

A DSC study of the drying process of TEOS derived wet silica gels

Abstract The drying process of wet silica gels obtained from TEOS has been studied by means of FT-IR, DSC and specific surface area measurements. FT-IR spectroscopic results have not given any important information about the drying process of silica gels. However, DSC results have shown a dependence of the DSC peak area on drying time. Three different water alkoxide ratio were studied, and the DSC peak area was higher when the water alkoxide ratio increased. A more open xerogel structure was found for the high water alkoxide ratio showing a high specific surface area and therefore lower drying times. For the silica gels obtained with 200 1 and 100 1 water alkoxide molar ratio, a complete drying time was observed for 25 and 28 days, respectively. As the drying time increases the gel structure becomes more closed, being necessary higher temperatures for removing water or alcohol from the gel.

Synthesis and Characterization of Silicon Oxycarbide Derived Nanocomposites Obtained through Ceramic Processing of TEOS/PDMS Preceramic Materials

Bulk silicon oxycarbide derived ceramic nanocomposites have been prepared by the application of the conventional ceramic processing to preceramic materials. Tetraethylortosilicate/ polydimethylsiloxane preceramic materials obtained by sol-gel process were thermally treated and attrition milled to 4 micrometers. Subsequently, the preceramic powders were pyrolized at 1100 °C to obtain silicon oxycarbide powders that were pressed and sintered at 1550 °C up to 16 hours. Silicon oxycarbide glasses obtained at 1100 °C from pyrolysis of preceramic materials consist of a Si-O-C network and a carbon like graphite phase well dispersed. At annealing temperatures higher than 1100°C silicon oxycarbide glasses undergo a rearrangement which involves a phase separation to silica and silicon carbide and a segregation of carbon like graphite phase. At these temperatures the material can be considered as a glassy matrix nanocomposite. At temperatures higher than 1500 °C the carbothermal reduction occurs with the consumption of both silica and free carbon phase. However, the nanocomposite structure is maintained but with different constituents. The silicon oxycarbide glasses obtained at 1100 °C are amorphous. However, as a result of all involving processes taken place during the ceramic process, the nanocomposites formed at 1550 °C comprise a silica matrix and nanodomains of carbon like graphite and silicon carbide both of them displaying an incipient crystallization. Structure and crystalline size evolution, from preceramic materials to silicon oxycarbide derived nanocomposites, have been determined by FT-IR and Raman spectroscopies, XRD and 29Si-MASNMR.

Hydrolysis of Titanium Tetrabutoxide. Study by FT-IR Spectroscopy

Abstract In this work we have studied the hydrolysis of titanium tetrabutoxide (TTB) in butyl alcohol. The hydrolysis was carried out by adding H2O for a molar ratio water/alkoxide of 4. The hydrolysis has been followed by FT-IR spectroscopy, and the evolution of the Ti-O-C bands of the TTB molecule have been analysed. These bands are located at 1130, 1100 and 1039 cm−1 and their intensities decrease with the hydrolysis time. However, these bands do not disappear for several days of hydrolysis confirming the difficulty to obtain a completely hydrolysed TTB molecule. Two IR bands have been assigned to Ti-OH groups and adsorbed H2O molecules. These bands appear at 3660 and 3525 cm−1 respectively and they are firstly observed when a white precipitate appears in the solution. This work shows that this study can be rapidly carried out by FT-IR spectroscopy and can also be applied to the well-established sol-gel process.

FT-IR and porosity study of Si-B-C-O materials obtained from TEOS-TEB-PDMS derived gel precursors

Mixed vitreous oxycarbide glasses containing silicon and boron were prepared by pyrolysis of hybrid precursors in N2 at 1100°C. Four different precursors were synthesized through the sol-gel process using TEOS, TEB and PDMS. Poly-condensation between the Si(OH) groups, from the hydrolysed TEOS, and the PDMS was observed by FT-IR spectroscopy. Boron seems not to incorporate into the network until pyrolysis of the precursors that form the oxycarbides. The oxycarbides were porous made of spherical interconnected particles. The porosity and mean pore size of the glasses increased with increasing TEB content.