Adsorption of silane onto cellulose fibers. II. The effect of pH on silane hydrolysis,condensation, and adsorption behavior

The hydrolysis of four alkoxysilane agents, γ‐methacryloxypropyl trimethoxysilane (MPS), γ‐mercaptopropyl trimetoxysilane (MRPS), octyl trimethoxysilane OS), and N‐phenyl‐γ‐aminopropyl trimethoxysilane (PAPS), was carried out in an ethanol/water (80/20) solution under both acid and basic conditions. ¹H, ¹³C, and ²⁹Si NMR spectroscopy were used to provide quantitative analyses of the structural components during hydrolysis and condensation reaction. The analysis revealed that the acid‐catalyzed hydrolysis of silane allows the formation of high amount of silanol groups, reduced the selfcondensation reaction among silanol groups and stabilized the proportion of intermediary hydrolyzed species for several days. However, under basic condition, condensation reactions proceed as soon as the hydrolysis reaction started leading to the rapid consumption of silanol groups through selfcondensation and to the growth of three‐dimensional high molecular structures. The interaction of MPS and MRPS with cellulose fibers and the evolution of their surface properties were then investigated using adsorption isotherms and contact angle measurement. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Hydrolysis and condensation mechanisms of a silane coupling agent studied by 13C and 29Si NMR

The mechanisms of hydrolysis and condensation of a silane coupling agent, γ-methacrylo-xypropyltrimetoxysilane (γ-MPTS) were investigated by the use of both ¹³C and ²⁹Si nuclear magnetic resonance (NMR) spectroscopies. The NMR peaks of the hydrolyzed monomer and condensed polymer were assigned readily. The rate of hydrolysis and condensation of γ-MPTS was dependent on water content. On the basis of the time-dependent behavior of these peaks, the mechanisms of hydrolysis and condensation were clear.     

Control of hydroxyl group content in silica particle synthesized by the sol-precipitation process

This study investigated the control of hydroxyl groups, one of key factors determining the surface properties of silica particles synthesized by the sol-precipitation of tetraethyl orthosilicate (TEOS). Thus, a thermal gravity analysis (TGA) was used to facilitate quantitative measurements of the hydroxyl groups on the silica particles, while BET and FT-IR were used to analyze the specific surface area and functional silane groups on the silica particles, respectively. In the sol-precipitation process, silanes that include various hydroxyl groups are formed as intermediates based on the hydrolysis and condensation of TEOS. Thus, NH₃, as a basic catalyst initiating the nucleophilic substitution of TEOS, was found to accelerate the hydrolysis and increase the hydroxyl group content on the silica particles. Plus, the hydroxyl group content was also increased when increasing the concentrations of TEOS and water as the hydrolysis reactants. However, the hydroxyl group content was reduced when increasing the temperature, due to the promotion of condensation. Based on the weight loss of the particles according to the thermal analysis, the hydroxyl group content on the silica particles varied from 5.6–42.7 OH/nm² under the above reaction conditions.     

NMR spectroscopic studies of dimethyldiethoxy silane hydrolysis and polysiloxane conversion

The hydrolysis rate of alkoxy silane (dimethyldiethoxy silane, DMDES, etc.) is a linear relationship vs. the reaction time under the moisture flux control. The hydrolysis process results in formation of the linear polysiloxane products by a subsequent condensation reaction. An amidized alkoxy silane (3‐(2‐aminoethyl)aminopropylmethyldimethoxy silane, KLM‐602 or γ–aminopropyl‐ triethoxy silane, γ–APS) serves as an internal standard of DMDES hydrolysis kinetics measurements. The hydrolysis kinetics of DMDES with the amidized alkoxy silane and the subsequent linear polysiloxanes conversion have been evaluated and characterized by ²⁹Si‐ and ¹³C‐NMR NMR spectroscopic measurements. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 86: 468–477, 2002     

Kinetics and mechanism of the hydrolysis and alcoholysis of alkoxysilanes

As part of our work with the use of silane coupling agents, we have been investigating the transformations of alkoxysilancs into siloxanes. The influence of a para-substituted phenyl group attached to the silicon was investigated and the rates of acid catalyzed hydrolysis and alcoholysis of these para-substituted phenylalkoxysilanes have been determined. The kinetics and mechanism of the reactions are presented. These reactions are of interest because of their role in the use of silane coupling agents as adhesion promoters, in the preparation of zinc-rich silicate coatings, in the sol-gel process and in the preparation of silicones in general. The hydrolysis reaction was found to be first order in acid, zero order in water and to have a Hammett p of -1.42 when catalyzed by sulfuric acid. These results are consistent with current opinion that the reaction mechanism is SN₁, and involves a positive intermediate, possibly a siliconium intermediate. The alcoholysis reaction was found to be first order in both the silane and the catalysts, and of intermediate order in the alcohol, when catalyzed by carboxylic acids. When catalyzed by dichloroacetic acid, the reaction has a Hammet p value of +0.43. This is consistent with a concerted displacement reaction between the alkoxysilane and the complex involving the alcohol and the carboxylate anion. The intermediate is a negatively charged intermediate, probably a penta-substituted silicon species.     

Effect of SiO2 oligomers on water absorption of cementitious materials

SiO₂ oligomers, a kind of intermedium between silane and SiO₂ nanoparticles, reveal small size, hydrophilic property, and high affinity with cement paste. In cement-based materials, the SiO₂ oligomers transform into SiO₂ nanomaterials. This is first time to report SiO₂ oligomers created by the hydrolysis and condensation reactions of tetraethyl orthosilicate in an acetic acid solution to reduce the water absorption rate and modify the surface microstructure of cement-based materials. Mild acetic acid makes the stable formation of SiO₂ oligomers which promote the reaction with cement phases. Linear, branched and 3D-chain structures of SiO₂ particles can be formed in sequence during the condensation of oligomers. The water absorption rate of cement mortars treated with SiO₂ oligomers was reduced to 46.4%. The SiO₂ oligomers were ideal candidate as a surface-treated agent which resulted in surface densification of cement-based materials.     

Effect of Processing Variables on the Solution Characteristics of γ-Glycidoxypropyltrimethoxysilane (γ-GPS)

The effects of the hydrolysis and condensation processes on the molecular structure of γ-glycidoxypropyltrimethoxysilane (γ-GPS) in aqueous solutions were investigated using Fourier-transform nuclear magnetic resonance (FT-NMR) spectroscopy and FT-Raman spectroscopy. Hydrolysis was characterized by monitoring the production of methanol and the decrease in concentration of SiOCH₃ groups in 1% solutions of deuterium oxide using proton NMR. The production of methanol and loss of methoxy groups in 25% solutions of γ-GPS in water was characterized using Raman spectroscopy. Hydrolysis was found to be a very rapid process, whose rate could be increased or decreased by altering the pH of the solution. NMR spectroscopy showed that hydrolysis was complete in a 1% γ-GPS solution in deuterium oxide after 34 minutes. Raman spectroscopy also showed hydrolysis to be rapid and complete in a 25% solution of γ-GPS in water after 1 hour. Condensation, on the other hand, took a relatively long time to occur. In the NMR spectra, condensation was observed by the broadening of peaks due to the protons on the carbon atom adjacent to the silicon atom. In the Raman spectra, condensation was characterized by the disappearance of the SiOH band near 725 cm^?1 and the development of an SiOSi band near 600 cm^?1. In addition to the proton NMR, Si-29 NMR was used to characterize the silane in 10% solutions of γ-GPS in water. The Si-29 NMR showed oligomer growth with respect to time. The oligomer growth was correlated with mechanical test results.     

Synthesis of silane monomer-modified styrene–acrylate microemulsion coatings by photopolymerization

A relatively concentrated silane monomer-modified styrene–acrylate microemulsion coating with high monomer to surfactant ratio of 7.5:1 has been prepared by microemulsion photopolymerization. The properties and the structure of the microemulsion coatings were investigated using TEM, FTIR and UV–vis measurements. The microemulsions are transparent with high transmittance in the visible range. The particle sizes of the produced latexes are in the range of 34–52 nm with the number average diameter of 40.9 nm and D_w/D_n of 1.16. FTIR spectrum indicates the possible structure of the silane monomer-modified styrene–acrylate copolymer, and confirms the hydrolysis and condensation resulting in siloxane bonds between polymer particles. The microemulsion coatings show enhanced acid, base and water resistance with decrease of surfactant content and increase of silane coupling agent.     

The Influence of pH on the Hydrolysis Process of γ-Methacryloxypropyltrimethoxysilane,Analyzed by FT-IR,and the Silanization of Electrogalvanized Steel

Silanes are commonly used as coupling agents to enhance the adhesion between polymeric and inorganic materials. Once silane hydrolysis has taken place, the condensation of the silane on the substrate surface should follow. Optimum hydrolysis conditions will depend on the type of silane and the process conditions of the solution. The pH is particularly important as it has a significant effect on the hydrolysis process. This paper deals with the hydrolysis process of 1 vol% γ-methacryloxypropyltrimethoxysilane (MPS) in aqueous solution at different pHs (2, 4, 6, 8 and 10). Because the hydrolysis rate is a function of pH, hydrolysis times, ranging from 2 min to 48 h, were studied. Fourier transform infrared spectroscopy was used to evaluate the chemical modifications produced by changing the hydrolysis time. The disappearance of the infrared band due to the Si–O–C groups and the appearance of the bands due to the Si–OH bonds were studied. It was shown that longer times were necessary for complete hydrolysis, under almost neutral pH conditions. On the other hand, the Zn-electrocoated steel was silanized with MPS under an optimum pH and the hydrolysis time conditions and the resulting surfaces were analysed by Reflection–Absorption Infrared Reflectance Spectroscopy.     

Deposition of amine functional silanes onto E-glass fibres,an NMR study

Tris(3-(trimethoxysilyl)propyl)isocyanuratesilane was deposited from anhydrous methanol. The rate of deposition onto E-glass fibres, of this novel silane, was determined by ²⁹Si NMR spectroscopy. During a 16-h treatment period there was no evidence of hydrolysis or condensation of the silane solution. The majority of the silane deposition occurred within the first few minutes of the deposition process. Long deposition times resulted in large amounts of deposited silanes, which were easily dislodged when the fibres were removed from the solution and dried. Given a coherent, highly ordered film it was calculated that 1400 layers of isocyanuratesilane were deposited in a 2% by weight silane solution. It is accepted, however, that silanes are unlikely to deposit as a coherent layer, and condensation will disrupt any previously ordered layers. Solid state NMR indicated that isocyanuratesilane and a ureidosilane may have been bonded to E-glass fibres at a small number of surface SiO sites and that from these locations a highly crosslinked siloxane polymer network was formed. Industrial grade E-glass fibres were shown to have sufficient water adsorbed onto the surface to induce hydrolysis and a high degree of condensation at the E-glass surface.     

Inorganic-organic hybrid polymers from oligobutadiene and mercaptoalkylsilane

The reaction of 3-(trimethoxysilyl)-1-propanethiol with oligobutadiene yielded modified oligomers that can be cross-linked by the sol-gel process. Properties of the materials can be varied in a wide range due to composition, reaction conditions and processing. The influence of the reaction conditions (H⁺-concentration, molar ratio silane:water, dilution, temperature) on hydrolysis and condensation and on the properties of the final material has been studied. These materials are obtained in the form of transparent films, stable in air up to 270°C.     

In situ sol–gel route to novel sulfonated polyimide?SiO2 hybrid proton‐exchange membranes for direct methanol fuel cells

Polyimides (PIs) as high‐performance organic matrices are used in the preparation of PI composites because of their excellent mechanical, thermal and dielectric properties. The sol–gel method is a promising technique for preparing these PI composites due to the mild reaction conditions and the process being controllable. Although sulfonated polyimide (SPI) proton‐exchange membranes have attracted much attention recently, studies on preparing SPI‐based hybrid proton‐exchange membranes for fuel cells have been rare. A series of SPI? SiO₂ hybrid proton‐exchange membranes were prepared from amino‐terminated SPI pre‐polymers, 3‐glycidoxypropyltrimethoxysilane (KH‐560) and tetraethylorthosilicate through a co‐hydrolysis and condensation process using an in situ sol–gel method. The reactive silane KH‐560 was used to react with amino‐terminated SPI to form silane‐capped SPI in order to improve the compatibility between the polymer matrix and the inorganic SiO₂ phase. The microstructure and mechanical, thermal and proton conduction properties were studied in detail. The hybrid membranes were highly uniform without phase separation up to 30 wt% SiO₂. The storage modulus and tensile strength of the hybrid membranes increased with increasing SiO₂ content. The introduction of SiO₂ improved the methanol resistance while retaining good proton conductivity. The hybrid membrane with 30 wt% SiO₂ exhibited a proton conductivity of 10.57 mS cm^?1 at 80 °C and methanol permeability of 2.3 × 10^?6 cm² s^?1 possibly because the crosslinking structure and SiO₂ phases formed in the hybrids could retain water and were helpful to proton transport. Copyright © 2010 Society of Chemical Industry     

Study on the mechanism of the formation of polyhedral oligomeric silsesquioxanes by the 2D correlation infrared spectral

Synthesis of polyhedral oligomeric silsesquioxanes (POSS) by the hydrolytic condensation reaction of trifunctional organosilanes [e.g., RSiCl₃ or RSi(OR)₃] may have achieved success. However, the exact formation mechanism of POSS, especially, an evolution process in the reaction still remains unclear. In this work, a real-time FTIR was carried out to trace the synthesis process of POSS. It was found that linear siloxanes, cyclic siloxanes, and cage-like polysiloxanes were formed during the reaction. With the help of generalized two-dimensional (2D) correlation analysis, we obtained exact sequential change among the three siolxanes, with the linear silicons formed initially. Following this was the emergence of cyclic siloxanes, and finally the cage-like polysiloxanes. Consequently, we not only proved the existence of the linear and cyclic siloxanes but also accurately detected the sequential change of the siloxanes in the process, which exhibited an intact and visual evolution process of POSS formation. Based on this, the reaction mechanism is presented. Finally, the chemical structure of cage-like products was further characterized by ²⁹Si-NMR and GPC measurements. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Effects of water on the preparation,morphology, and properties of polyimide/silica nanocomposite films prepared by sol–gel process

Polyimide/silica (PI/SiO₂) nanocomposite films with 10 wt % of silica content were prepared by sol–gel process under the conditions with and without additional water. The presence of additional water has great effect on the silica particle size and thus on the properties of the prepared PI/SiO₂ films. The results indicated that with additional water, the silica particles formed before the imidization of poly(amic acid) (PAA) and aggregated with the increasing of temperature and degree of the proceeding imidization process. For the nonaqueous process, the hydrolysis condensation reaction of tetraethoxysilane (TEOS) did not occur until the imidization of PAA took place, and no silica particles were found in the unimidized PAA films. The hydrolysis–condensation reaction of TEOS was initiated simultaneously by the trace water released from the imidization reaction, the self‐catalysis mechanism of the approach provide a means of achieving uniformly dispersed silica particles formed in the PI matrix with particle size in the range of 30–70 nm. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 104: 1579–1586, 2007     

Effects of curing,washing, and pre-oxidation on the bonding of monofunctional silanes to polished copper surfaces

Polished Cu surfaces were silylated with trimethylmethoxysilane or trimethylchlorosilane and washed in water to determine the stability of the silane layers. The surfaces were analyzed using X-ray photoelectron spectroscopy (XPS). Methoxysilane was found to remain stably bound during washing, whereas chlorosilane was completely washed away. This confirmed that methoxysilanes were probably bound to surface oxides as hydrogen-bonded Si-O-R species or metal siloxanes, whereas chlorosilanes could only physisorb because they had removed surface oxides and hydroxyl groups. The silane layers were cured at high temperature in vacuum to promote the formation of metal siloxanes from hydrogen-bonded Si-O-R groups. For both silanes, the cured layers were almost completely washed away. The Si-O-R groups in methoxysilane layers were probably converted to less stably bound siloxane dimers during curing. The polished Cu samples were pre-oxidized in an attempt to increase the amount of oxides on the surface and thereby retain chlorosilane. Pre-oxidation converted Cu₂0 to CuO. This did not change the behavior of chlorosilane and reduced the uptake of methoxysilane. Solution and surface reactions are suggested for both silanes. For methoxysilane, silanes or silanols directly hydrogen-bonded to surface oxide or hydroxyl groups are proposed to be the primary species that lead to metal siloxanes.     

Synthesis of phosphorus‐containing polyhedral oligomeric silsesquioxanes via hydrolytic condensation of a modified silane

A simple route to synthesize a new type of phosphorus‐containing polyhedral oligomeric silsesquioxanes (DOPO–POSS) in high yield, by the hydrolytic condensation of a modified silane, is reported. The starting material was a phosphorus‐containing triethoxy silane (DOPO–VTES), which was synthesized by addition reaction between 9,10‐dihydro‐9‐oxa‐10‐phosphaphenanthrene‐10‐oxide (DOPO) and vinyl triethoxy silane (VTES). This product was subjected to hydrolytic condensation using an HCl catalyst in methanol. The new types of phosphorus‐containing POSS were obtained and characterized using ¹H, ¹³C, ²⁹Si‐NMR, MALDI‐TOF MS, XRD, DSC, and FTIR. All of these results suggested that the DOPO–POSS were amorphous mixtures of T₈, T₉(OH), and TGA curve of DOPO–POSS shows that the cage‐like compound has high thermal stability. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Hydrolysis and condensation of silanes in aqueous solutions

Aminosilanes hydrolyze rapidly in aqueous acidic solutions and exhibit good stability; however ¹H NMR measurements indicate that γ-MPS (γ-methacryloxypropyltrimethoxy silane) hydrolyzes slowly and condenses rapidly under similar conditions. The rate constant of these two reactions depends upon the pH of the solution, and the rate constant of the condensation reaction also depends upon the initial γ-MPS concentration. The hydrolysis is faster and the condensation slower at low pH's. These factors explain why an induction period is observed before the adsorption of γ-MPS onto mica takes place and why it does not occur if the induction period is too long. Using the Lentz technique, it is also shown that the main species resulting from the condensation of a concentrated γ-MPS solution is a cyclic trimer or tetramer, in agreement with literature results.     

Enzyme-mediated Synthesis of Silsesquioxanes

In an initial report we demonstrated that some serine proteases were suitable candidates for the hydrolysis and condensation of tetraethoxysilane and phenyltrimethoxysilane under solvent-free conditions. Using trypsin as a model enzyme, we have demonstrated the generality of this method by expanding the number of available silicon-based substrates that can be processed by the enzyme, as well as including pepsin to demonstrate that this phenomenon is not exclusive to a single family of enzymes. A series of time course ²⁹Si NMR experiments using D₂O revealed that the rate of hydrolysis of phenyltrimethoxysilane could be enhanced by a factor of 11–155 times over enzyme-free controls when either trypsin or pepsin were used as catalysts. The following trend was observed when comparing the hydrolysis/condensation rates of a number of different organotrimethoxysilanes: methyltrimethoxysilane>allytrimethoxysilane>ethyltrimethoxysilane>phenyltrimethoxysilane.     

八乙烯基笼型倍半硅氧烷的合成

以乙烯基三甲氧基硅烷为原料、乙酸乙酯为溶剂、盐酸为催化剂,通过水解缩合反应合成了八乙烯基笼型倍半硅氧烷（八乙烯基POSS）,产率为33．29％。通过红外、核磁、质谱等方法对产物的结构进行了表征;探讨了反应条件。结果表明,最佳实验条件为：20mL乙烯基三甲氧基硅烷,200mL乙酸乙酯,30mL盐酸,70mL去离子水,反应温度为20℃,反应时间为4天。     

Hydrolysis and oxidative decomposition of ethyl acetate in sub- and super-critical water

Hydrolysis of ethyl acetate in sub-critical (633 K, 200 bar) and super-critical water (673 K, 240 bar) has been investigated in a tubular flow reactor as a model reaction for the depolymerisation of polyesters. Super-critical reaction conditions enable approximately a 10-fold hydrolysis rate in comparison to sub-critical process. The reaction products ethanol and acetic acid are stable under these conditions. Additionally, oxidative decomposition experiments were carried out using ethyl acetate, ethanol and acetic acid as feed in the presence of air, non-catalysed as well as in the presence of a heterogeneous MnO₂–CuO/Al₂O₃ catalyst (Carulite 150^®). The catalyst caused only slight increase of ethyl acetate conversion in oxidation compared to hydrolysis, but a noticeable increase in CO₂ formation due to destruction of ethanol and acetic acid. In absence of catalyst, the degrees of conversion and selectivities were not affected by addition of oxygen. Fresh and spent catalysts were characterised with standard methods (BET, ICP), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM). The catalyst material was identified as chemically and mechanically stable under the applied reaction conditions, however, a significant sintering accompanied by a deep structural alteration and a slight reduction were observed. Additionally, the stability of reactor material regarding to corrosion was also tested.