Silicate Ester Research Articles

UV-heat dually curable epoxy resin composites with low filler contents and high curing speed

Epoxy resin composites are a type of polymer-based materials with wide application scopes. A mass of fillers was added to improve thermal properties which may degrades mechanical properties at the same time. In addition, the common heat-curing process takes several hours to completely cure. To improve the performance of epoxy resin composites of low-content fillers, we prepare UV-heat dually curable epoxy resin with silicate ester. The overall curing time of UV-heat curing process is about half of the traditional thermal-curing process. The breakdown voltage of epoxy resin modified by silicate ester is up to 181 MV/m, which is increased by 50 % compared to pure epoxy resin. The silicate ester curing agent can react with both epoxy resin and silica fillers and can thus improve the dispersion of the fillers in the matrix and enhance the interaction between fillers and the resin. As a result, the coefficient of thermal expansion (CTE) of the composites was greatly reduced. The CTE of silicate ester-modified epoxy resin composites is 93 ppm/K and 137 ppm/K over the temperature range below and above its Tg, respectively, which is 25 % lower than those of normal epoxy resin composites.

Silicon Hybridization for the Preparation of Room-Temperature Curing and High-Temperature-Resistant Epoxy Resin.

Specialized epoxy resin, capable of achieving room-temperature profound curing and sustaining prolonged exposure to high-temperature environments, stands as a pivotal material in modern high-end manufacturing sectors including aerospace, marine equipment fabrication, machinery production, and the electronics industry. Herein, a silicon-hybridized epoxy resin, amenable to room-temperature curing and designed for high-temperature applications, was synthesized using a sol-gel methodology with silicate esters and silane coupling agents serving as silicon sources. Resin characterization indicates a uniform distribution of silicon elements in the obtained silicon hybrid epoxy resin. In comparison to the non-hybridized epoxy resin, notable improvements are observed in room-temperature curing performance, heat resistance, and mechanical strength.

AgF-mediated Hofmann-type reaction of N-tosyl cyclic imides with silicate esters

A facile and efficient AgF-mediated Hofmann-type reaction of N-tosyl cyclic imides with silicate esters was developed. The N-tosyl cyclic imides undergo a Hofmann-type rearrangement/alkoxylation reaction in which the silicate ester acted as the alkoxy source. This reaction proceeds under simple and mild conditions to generate a series of anthranilic acid-derived carbamates in moderate to good yields.

Hydrazone bond-oriented molecularly imprinted nanocomposites for the selective separation of protein via the well-defined recognition sites.

The development of hydrazone bond-oriented epitope imprinting strategy is reported to synthesize the polymeric binders for the selective recognition of a protein-β2-microglobulin through either its N- or C-terminal epitope. The dynamic reversibility of hydrazone bond facilitated not only the oriented assembly of the template peptide hydrazides onto the substrate but also the efficient removal of them from the imprinted cavities. The well-defined surface imprinted layer was successfully constructed through the precise control over the polymerization of silicate esters. Binding performance of the C-terminal peptide imprinted nanocomposite was significantly improved after tuning the non-covalent interactions using the sequence-matching aromatic co-monomers. The dissociation constant (Kd) between the optimized nanocomposite and epitope peptide was 0.5µmol L-1. The nanomaterial was utilized for the selective extraction and determination of β2-microglobulin from human urine by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) and HPLC-UV with satisfied recoveries of 93.1-112.3% in a concentration range 1.0-50.0μg⋅mL-1.

Hexafluorosilicic Acid (FSA): from Hazardous Waste to Precious Resource in Obtaining High Value-Added Mesostructured Silica

In this work, industrial waste hexafluorosilicic acid (H2SiF6 or FSA) has been proven to be a low-cost alternative to silicate esters for the synthesis of high-quality MCM-41 (high surface area, hi...

Strengthened cellulosic gels by the chemical gelation of cellulose via crosslinking with TEOS

Physical and chemical gels of cellulose were obtained by (1) coagulation of dimethyl sulfoxide (LiCl/DMSO) cellulose solution in ethanol and (2) crosslinking by tetraethyl orthosilicate (TEOS) in non-aqueous medium under catalysis with ethylenediamine (EDA). Their structure and properties were studied. The highest storage modulus was 1.08 MPa (physical) and 4.04 MPa (chemical), respectively. The surface area (SBET) of cellulose chemical gels, which were composed of three-dimensional networks of long silica-modified fibrils, was as high as 296.8 m2/g. It is noticeable that it is not simple self-condensation of organic silicate that occurred but instead the ostensive silicic ester of cellulose was formed. This novel cellulose derivative and its chemical gelation may lead to a new category of cellulose-based organic–inorganic hybrid materials.

Semisynthetic Antimycobacterial C-3 Silicate and C-3/C-21 Ester Derivatives of Fusidic Acid: Pharmacological Evaluation and Stability Studies in Liver Microsomes, Rat Plasma, and Mycobacterium tuberculosis culture.

Fusidic acid (FA), a natural product fusidane triterpene-based antibiotic with unique structural features, is active in vitro against Mycobacterium tuberculosis, the causative agent of tuberculosis (TB). While possessing good pharmacokinetics in man, FA is rapidly metabolized in rodents, thus complicating proof-of-concept studies in this model. Toward the repositioning of FA as an anti-TB agent, we herein describe the synthesis, activity, and metabolism of FA and semisynthesized ester derivatives in rat liver microsomes, rat plasma, and mycobacterial cell culture. FA and derivative molecules with a free C-3 OH underwent species-specific metabolism to the corresponding 3-OH epimer, 3-epifusidic acid (3-epiFA). FA was also metabolized in rat plasma to form FA lactone. These additional routes of metabolism may contribute to the more rapid clearance of FA observed in rodents. C-3 alkyl and aryl esters functioned as classic prodrugs of FA, being hydrolyzed to FA in microsomes, plasma, and Mycobacterium tuberculosis culture. In contrast, C-3 silicate esters and C-21 esters were inert to hydrolysis and so did not act as prodrugs. The antimycobacterial activity of the C-3 silicate esters was comparable to that of FA, and these compounds were stable in microsomes and plasma, identifying them as potential candidates for evaluation in a rodent model of tuberculosis.

Alkali Metal Salt as Catalyst for Direct Synthesis of Carbamate from Carbon Dioxide

Carbamates are an important motif in agricultural chemistry, medicinal chemistry, and polyurethane preparation. Potassium carbonate (K2CO3) is a common, nontoxic, and inexpensive chemical and is widely used in organic synthesis. Herein we report the use of K2CO3 as a catalyst for direct synthesis of carbamate from amine, silicate ester, and carbon dioxide. Especially, both alkyl amines and aromatic amines could be activated, largely expanding the substrate scope compared with the use of alcohol as the coupling reagent. Anion-assisted deprotonation of amine was the key step in the catalytic process, and the alkali metal served as a Lewis acid. Isolated yield of the corresponding carbamate was up to 97%. The use of K2CO3 as the catalyst decreased 49 kJ mol–1 activation energy of the reaction compared with our previously reported ionic liquid catalyst from Arrhenius analyses.

A Simple Zinc Catalyst for Carbamate Synthesis Directly from CO2.

Several zinc salts were employed as catalysts for the synthesis of carbamates directly from aromatic amines, CO2 , and silicate esters. Zn(OAc)2 offered the best performance among the salts tested. The addition of an N-donor ligand such as 1,10-phenanthroline increased the yield. The best catalytic performance of Zn(OAc)2 can be explained by carboxylate-assisted proton activation. The interaction between the substrate and the catalyst can be observed by chemical shifts in 1 H and 15 N NMR spectra. Isocyanate was a key intermediate, which was generated from amine and CO2 . Silicate ester was finally converted to siloxane, which was determined by 29 Si NMR. The commercially available catalyst system could be reused. The yield of isolated carbamate could reach up to 96 % with various substrates, and the catalytic reaction was amine-selective in the presence of other functional groups.

Direct synthesis of carbamate from CO2 using a task-specific ionic liquid catalyst

An easily synthesized ionic liquid was used as an organocatalyst to synthesize carbamates directly from amine, CO2, and silicate esters.

Fe-Catalyzed Aerobic Oxidative C–CN Bond Cleavage of Arylacetonitriles Leading to Various Esters

Fe-catalyzed aerobic oxidative esterifications of arylacetonitriles with alcohols, tri alkoxsilanes, silicate esters, or borate esters have been developed. The acyl groups which were in situ generated via chemoselective C(CO)-CN bond cleavage were directly used as electrophiles, leading to corresponding aryl esters in good to excellent yields under molecular oxygen when attacked by alcohols or alcohol surrogates. Dioxygen serves as both oxidant and reactant in this protocol. The reaction has a very broad substrate scope. Cheap starting materials as well as environmentally benign and inexpensive iron catalyst and ideal oxidant O2 feature this transformation and make it a practical and sustainable protocol to afford esters.

Hollow Silica as an Optically Transparent and Thermally Insulating Polymer Additive.

We present an improved synthesis route to hollow silica particles starting from tetramethyl orthosilicate (TMOS) instead of the traditionally used ethyl ester. The silica was first deposited onto polystyrene (PS) particles that were later removed. The here introduced, apparently minor modification in synthesis, however, allowed for a very high purity material. The improved, low density hollow silica particles were successfully implemented into polymer films and permitted maintaining optical transparency while significantly improving the heat barrier properties of the composite. Mechanistic investigations revealed the dominant role of here used methanol as a cosolvent and its role in controlling the hydrolysis rate of the silicic ester, and subsequent formation of hollow silica particles. Systematic experiments using various reaction parameters revealed a transition between regions of inhomogeneous material production at fast hydrolysis rate and reliable silica deposition on the surface of PS as a core-shell structured particle. The shell-thickness was controlled from 6.2 to 17.4 nm by increasing TMOS concentration and the diameter from 95 to 430 nm through use of the different sizes of PS particles. Hollow silica particle with the shell-thickness about 6.2 nm displayed a high light transmittance intensity up to 95% at 680 nm (length of light path ∼ 1 cm). Polyethersulfone (PES)/hollow silica composite films (35 ± 5 μm thick) exhibited a much lower thermal conductivity (0.03 ± 0.005 W m·K(-1)) than pure polymer films. This indicates that the prepared hollow silica is able to be used for cost and energy effective optical devices requiring thermal insulation.

Kinetic Studies of Reactions of Hydroxyporphyrins on Silicate Surface

Construction of the monolayer film of semiconductive and redox-active porphyrins on the metal oxides such as silicate or titanates attracts interests for possible applications of the process in fabricating organic electronic devices, organic catalysts, and organic-inorganic hybrid materials. Porphyrins bearing linker functional groups such as hydroxy or bromo groups in the peripheral meso-phenyl groups were reacted with the surface silanol groups of silicate glass at 80 to 240 oC in a solid-solid reaction to obtain the monolayer film of porphyrin on the silicate glass. The rates of the reaction were determined by visible spectroscopic studies and they decreased in the order: primary alcohol > primary bromide ~ tertiary alcohol. In addition to the structures of the linker groups, melting points of porphyrins also affect the reactivity of the porphyrin. Porphyrins with long alkyl chains reacted faster than those without long alkyl chains, because the long alkyl groups would facilitate molecular motion to reach the transition state of the reaction. Hydrolysis of the porphyrin monolayer prepared from primary alcohol in 1 M aq. HCl at 50 oC proceeded in a similar rate to that prepared from tertiary alcohol. Base catalyzed hydrolysis in 1 M aq. NH3 at 30 oC of the porphyrin monolayer prepared from tertiary alcohol proceeded 20 times slower than that prepared from the primary alcohol. In the literature, it is well established that the hydrolysis of the silicate ester bond proceeds via the Si-O bond cleavage, since the silicon atom can form a penta-coodinated structure, and much more reactive than carbon atom toward a substitution reaction. Hydrolysis of the porphyrin monolayer prepared from the tertiary alcohol in H2 18O afforded 18O-labelled tertiary alcohol, indicating that there was a covalent bond between the porphyrin and silicate glass, and the C-O bond was cleaved in the acid catalyzed hydrolysis reaction of the tertiary alkyl silicate ester on the silicate glass.

Silicate Esters of Paclitaxel and Docetaxel: Synthesis, Hydrophobicity, Hydrolytic Stability, Cytotoxicity, and Prodrug Potential

We report here the synthesis and selected properties of various silicate ester derivatives (tetraalkoxysilanes) of the taxanes paclitaxel (PTX) and docetaxel (DTX) [i.e., PTX-OSi(OR)3 and DTX-OSi(OR)3]. Both the hydrophobicity and hydrolytic lability of these silicates can be (independently) controlled by choice of the alkyl group (R). The synthesis, structural characterization, hydrolytic reactivity, and in vitro cytotoxicity against the MDA-MB-231 breast cancer cell line of most of these derivatives are described. We envision that the greater hydrophobicity of these silicates (vis-à-vis PTX or DTX itself) should be advantageous from the perspective of preparation of stable aqueous dispersions of amphiphilic block-copolymer-based nanoparticle formulations.

Control of Thin Liquid Film Morphology During Solvent-Assisted Film Deposition

Liquid films of different silicate esters were deposited from volatile solvents on hydroxylated and hydrogen-passivated silicon surfaces. We show that adsorption of silicate ester molecules and the resulting structural morphology of the liquid films not only are determined by attractive van der Waals forces with contributions from electrostatic interactions between the silicone ester moieties and oxide surface sites but also can be tuned by modifying the substrate surface or by changing the liquid-solvent interactions. Our results also show the importance of the conformational properties of liquid molecules and their rearrangements at the liquid/solid interface for controlled solvent-assisted film deposition.

Investigations of stone consolidants by neutron imaging

The chemical preservation and structural reintegration of natural stones applied in historical buildings is carried out by the use of different stone strengtheners. As these agents contain hydrogen, they offer good properties for neutron imaging. The main interest in the restoration process is the development of a suitable stone consolidant. In cooperation with the St. Stephans Cathedral and the geologists at Vienna University of Technology, we are investigating the penetration depth and distribution of different stone consolidants. These studies are being carried out with different stone samples, mostly porous natural building stones, limestones and sandstones. The two strengtheners used in this study are ethyl silicate ester (Wacker OH100) and dissolved polymethylmetacrylate (PMMA, Paraloid B72). Neutron radiography and neutron tomography can be used successfully to visualize the distribution of consolidants both in two and three dimensions.

Photo-responsive Polymers and their Applications to Optical Memory

We synthesized photo-responsive azo polymers from novel malonic ester and silicate ester monomers having two symmetrical azobenzene groups in the side chain. The possibility of their applications as reversible data storage media was investigated utilizing optical data recording techniques. As a result, the azo polymers had very excellent stability and resolution of the stored data as well as the sensitivity to pump beam. The polymeric thin films also could be used as erasable digital data materials for a practical use by an Ar ion pulse laser (pulse width: 10 μs). In addition, the polymeric thick films (500–1000 μm) from new monomer and methyl methacrylate showed good angular selectivity for holographic memory by using optical two wave mixing technique.

Evaluation of consolidating and water repellent treatments applied to the miocene sandstone used in Tunisian Heritage Monuments

Summary The research reported in the present paper focused on the behaviour of the Miocene sandstone used to build the Roman aqueduct at Zaghouan-Carthage and other Tunisian Heritage monuments, after application of water repellent and consolidating treatments as a preliminary to restoration. Commercial consolidants and water repellents were used in the experiments: two ethyl silicate consolidants, Tegovakon (TV) and Keim-Silex OH (KSOH); two (siloxane) water repellents, Tegosivin THE 328 (THE) and Tegosivin HLJ00 (THE); and a dual (consolidate and water repellent) action substance, Keim- Silex H (KSH) (silicate acid ester base with siloxane). A mixed treatment consisting of successive coats of TV and THL (TVHL) was also applied. These organosilicate consolidants and water repellents acted on the porous structure of the sandstone, reducing total porosity and water vapour permeability. The water repellent THE was found to affect these properties least, with a pore size distribution that resembled the distribution in the untreated sandstone most closely. Water repellents diminish water absorption and consolidants increase mechanical strength. The TV-THL mix, which yielded results similar to those obtained with water repellents alone, was unable to prevent the substantial scaling that occurs during (RILEM) salt crystallisation-induced artificial ageing. The best results were found with the dual action consolidant! water repellent product (KSH), which improved the mechanical properties while affording protection from the decay caused by salts in artificial ageing trials. This substance was found to reduce water vapour permeability, however

The Synthesis, Condensation, and Luminescence of Stilbenoid Oligomers with Alkoxysilane End Groups

ABSTRACTA synthetic route to highly luminescent organic semiconductors with curable alkoxysilyl groups is described. Monodisperse oligo(phenylenevinylene)s are rigidly connected to di- and triethoxysilanes via Heck reactions or via cross-metathesis. Hydrolysis of the silicic esters yields silanols condensing to linear and cyclic oligo-OPV-siloxanes or to three-dimensional networks, thus allowing the transformation of small molecules to fluorescent materials with well-defined chromophores. Transparent films are obtained by casting of soluble cyclosiloxanes and from OPV-silanetriols, the latter can be cured to insoluble networks.

Synthesis of New Poly(Silicate Esters) with Disperse Red 1 and Their Application as Optical Data Storage Materials

New silicate ester monomer was synthesized by the reaction of 3-methacryloxypropylmeth-yldichlorosilane with disperse red 1 in methylene chloride. The monomer was polymerized with a radical initiator to produce the corresponding homopolymer and copolymers. These polymers were found to be excellent as reversible optical information recording media for data storage and retrieval through a trans-cis isomerization of the azobenzene groups by Ar laser irradiation.