Studies on the syntheses of polymetalloxanes and their properties as a precursor for amorphous oxide. IV. Properties of SiO2–TiO2 oxide fibers from polytitanosiloxane

From polytitanosiloxanes (PTS), SiO₂–TiO₂ oxide fibers with fairly good tensile strength were prepared, and their mechanical and thermal properties were investigated. The precursor fibers PTS-0.5 and PTS-1.0 were obtained by dry spinning of a highly viscous PTS solution which were formed as the reaction mixture of silicic acid (SA) with bis(2,4-pentanedionato)titanium diisopropoxide (PTP) in the molar ratios (SA/PTP) of 0.5 and 1.0. The precursor fibers PTS-0.5 were too brittle to measure their tensile strength, whereas PTS-1.0 and the heat-treated fibers were found to have tensile strength of 130 (precursor), 540 (500°C), and 450 (900°C) MPa, respectively. Heat-treatment of the fibers PTS-1.0 at above 1000°C forms anatase and rutile of titanium dioxide. The crystallization is resulted from the unreacted PTP which is not incorporated into the polymer network.     

Synthesis of polytitanosiloxanes and their transformation to SiO2–TiO2 ceramic fibers

A one-pot synthesis of polytitanosiloxanes (PTS) and its transformation to SiO₂–TiO₂ ceramic fibers were investigated. PTS was prepared by the hydrolysis of tetraethoxysilane followed by the reaction with bis(2,4-pentanedionato)titanium diisopropoxide in methanol in 33–95 SiO₂ mol %. PTS was considered to be a ladder- or sheet-type polymer consisting of Si? O? Si and Si? O? Ti linkages as a main chain with pendant hydroxyl and 2,4-pentanedionato groups. SiO–TiO ceramic fibers were prepared by the pyrolysis of SiO₂–TiO₂ precursor fibers, which were prepared by the dry spinning of PTS followed by steam treatment. The tensile strength was 610 MPa for the SiO₂–TiO₂ fibers (SiO₂/TiO₂ = 20) after the pyrolysis at 700₀C. © 1994 John Wiley & Sons, Inc.     

Syntheses and Characterization of Polymetallosiloxanes from Silicic Acid and Metal Chlorides

Syntheses and characterization of polymetallosiloxanes by the non-hydrolysis sol-gel process using no metal alkoxides were investigated. The reaction of silicic acid (SA) with MCl₄ (M = Ti, Zr) in the molar ratios SA/MCl₄ = 0.5–3.0 using a tetrahydrofuran-methanol solvent formed polymetallosiloxane (PMS), which was insoluble in organic solvents regardless of the molar ratio. The PMS was isolated as esterified polymetallosiloxane by esterification with isopropyl alcohol for various periods, which were soluble in methanol, acetone, and tetrahydrofuran. The number average molecular weight was 1000–3200 for esterified polytitanosiloxane and 3400–11000 for esterified polyzirconosiloxane. Esterified polymetallosiloxanes had no melting point but decomposition point. The results of analytical data indicated that esterified polymetallosiloxane and/or polymetallosiloxane consisted of the main chain of Si–O–Si and Si–O–M linkage with the pendants of alkoxy, silanol, and chloro group.     

Tautomerism of monochalcogenosilanoic acids CH3Si(O)XH (X = S,Se, Te) in the gas phase and in the polar and aprotic solution: An ab initio computational investigation

Computational investigations by an ab initio molecular orbital method (HF and MP2) with the 6‐311+G(d,p) and 6‐311++G(2df, 2pd) basis sets on the tautomerism of three monochalcogenosilanoic acids CH₃Si(?O)XH (X = S, Se, and Te) in the gas phase and a polar and aprotic solution tetrahydrofuran (THF) was undertaken. Calculated results show that the silanol forms CH₃Si(?X)OH are much more stable than the silanone forms CH₃Si(?O)XH in the gas‐phase, which is different from the monochalcogenocarboxylic acids, where the keto forms CH₃C(?O)XH are dominant. This situation may be attributed to the fact that the Si? O and O? H single bonds in the silanol forms are stronger than the Si? X and X? H single bonds in the silanone forms, respectively, even though the Si?X (X = S, Se, and Te) double bonds are much weaker than the Si?O double bond. These results indicate that the stability of the monochalcogenosilanoic acid tautomers is not determined by the double bond energies, contrary to the earlier explanation based on the incorrect assumption that the Si?S double bond is stronger than the S?O double bond for the tautomeric equilibrium of RSi(?O)SH (R?H, F, Cl, CH₃, OH, NH₂) to shift towards the thione forms [RSi(?S)OH]. The binding with CH₃OCH₃ enhances the preference of the silanol form in the tautomeric equilibrium, and meanwhile significantly lowers the tautomeric barriers by more than 34 kJ/mol in THF solution. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2008     

Structural control of oligomeric methyl silsesquioxane precursors and their thin‐film properties

Oligomeric methyl silsesquioxane (O‐MSSQ) precursors were prepared from methyl trimethoxysilane (MTMS) in a mixed solvent of methyl isobutyl ketone and tetrahydrofuran by variations in the pH and molar ratio of water to MTMS (R₁). The molecular structures of O‐MSSQ were controlled by the reaction conditions. At a fixed pH value, the percentage of the end group, Si? OCH₃, decreased with increasing R₁, but that of Si? OH increased. With the pH increasing, the ratio of Si? OCH₃ groups to Si? OH groups was enhanced, but ratio of the molecular weights was reduced. The molecular weight distribution was progressively broader as the pH value decreased. These results were explained by the effects of R₁ and pH on the hydrolysis and condensation reactions. The prepared O‐MSSQ precursors consisted of mixed cage and network structures. The ratio of cage structures to network structures increased at low pH and high R₁ values. Highly uniform thin films were spin‐coated from the O‐MSSQ precursors, and this was followed by multistep curing. The content of cage structures in O‐MSSQ films decreased with increasing curing temperatures, whereas the network content in O‐MSSQ films showed the opposite trend. Such a structural transformation resulted in significant variations in the physical properties. Both the refractive index and dielectric constant decreased with higher cage/network ratios because of changes in the molar volume. The prepared O‐MSSQ has potential applications as a low dielectric constant material. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 1560–1571, 2002     

Charakterisierung von Organokieselsäurepolymeren durch thermoanalytische Untersuchungen

Four organosilicate polymers synthesized by addition of vinyl- and H-substituted double four-ring silicic acid derivatives were characterized using DSC and simultaneous TG-DTA measurements. Thermooxidative decomposition proceeds in several steps: Oxidation of (Si?H) groups, oxidation of (Si?CH₂?CH₂?Si) bridges, and oxidation of (Si?CH₃) groups, with formation of new (Si?O?Si) bonds.²⁹Si-NMR-spectroscopic measurements confirm this interpretation. Thermoanlytical methods of investigation proved useful to characterize these organosilicate polymers.     

Synthesis,characterization, and properties of polysilaethers containing moiety Si?H bonds in the side chain

A synthetic route to polysilaethers containing moiety Si? H bonds in the side chain (PSEMH) is reported that allows access to hitherto inaccessible oxygen‐interrupt polysilanes. By a Wurtz reductive coupling reaction, an equimolar ratio of dichloromethylsilane to alkali metal yields dichlorodisilane. The alcoholysis of Wurtz coupling resultants is in situ performed, and the polycondensation of hydrolysis occurs simultaneously in the presence of a small amount of N,N‐(dimethylamino)pyridine. The linear polymer is monomodal PSEMH with molecular weights as high as 24,900. The ultraviolet absorption at 292 nm is due to the interactions of the σ_{(Si? Si)} orbital electron delocalization and the p_π(O)–σ*_{π(Si? O)} delocalization along the (SiSiO)_n skeleton. It is redshifted in comparison with those of permethyl polysilaethers analogues and blueshifted in comparison with those of poly(dialkylsilane)s. The fluorescence emissions of the polysilaethers containing moiety Si? H bonds in the side chain are in a narrow range of 300–400 nm. Si? H bonds in polysilaethers play an important role in hydrosilylation reactions. The polysilaethers containing moiety Si? H bonds in the side chains can be used as the starting point for further functionalization via hydrosilylation. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 2476–2482, 2005     

Ab initio study on the mechanism of cycloaddition reaction between H<Subscript>2</Subscript>Si=Si: and formaldehyde

The mechanism of the cycloaddition reaction between singlet H₂Si=Si: and formaldehyde has been investigated with the CCSD(T)//MP2/6-31G* method. From the potential energy profile, it could be predicted that the reaction has three competitive dominant reaction pathways. The reaction rules presented is that the 3p unoccupied orbital of the Si: atom in H₂Si=Si: inserts the π orbital of formaldehyde from the oxygen side, resulting in the formation of an intermediate. Isomerization of the intermediate further generates a four-membered ring silylene (the H₂Si–O in the opposite position). In addition, the [2+2] cycloaddition reaction of the two π-bonds in H₂Si=Si: and formaldehyde also generates another four-membered ring silylene (the H₂Si–O in the syn-position). Because of the unsaturated property of the Si: atom in the two four-membered ring silylenes, the two four-membered ring silylenes could further react with formaldehyde, generating two silicic bis-heterocyclic compounds. Simultaneously, the ring strain of the four-membered ring silylene (the H₂Si–O in the syn-position) makes it isomerize to a twisted four-membered ring product.     

Preparation of polysiloxanes from silicic acid. VII. Effects of the degree of esterification and alkyl groups on condensations of silicic acid esters and formation of fibrous silica

Silicic acid esters were prepared by the reaction of the silicic acid in tetrahydrofuran with various alcohols such as such as methanol, ethanol, 2-propanol, 1-butanol, 2-methyl-2-propanol and 1-octanol, using an esterification apparatus designed especially to allow the preparation of silicic acid esters from alcohols with low boiling points or appreciable steric hindrance. With the intent of obtaining a spinnable ester solution by condensation of silicic acid esters, the effects of the degree of esterification (DE) and alkyl group on gel time and spinnability are investigated. An increase in the DE and the size of the alkyl group led to an increase in the gel time of the esters. Esters solutions with a good spinnability were prepared from n-butyl and isopropyl esters with DEs of 40–50%. In addition, the gel permeation chromatography of condensing esters with low DEs showed a rapid increase in molecular weight. The results showed that spinnability depended on DE and the ester's alkyl group. Fibers could be formed with length of about 10–100 cm from ethyl, isopropyl, and n-butyl esters with DEs 40–50% and with length of 10 cm from octyl ester with DE 26%.     

Dynamic mechanical and dielectric properties of ORMOCER® incorporating functionalized poly(styrene) latexes

Organically modified aluminosilicate hybrid materials incorporating polystyrene and poly(styrene‐co‐hydroxypropyl acrylate) latexes, (3‐glycidyloxypropyl) trimethoxysilane, and aluminum sec‐butoxide [Al(O^sBu)₃] were synthesized by a sol–gel process. The bulk materials obtained were macroscopically homogeneous dispersions with good mechanical properties. Dynamic mechanical and dielectric analyses of these new hybrid materials as a function of the Al(O^sBu)₃ concentration and copolymer composition revealed a series of transitions that represented relaxation processes of the incorporated polymer (glass transition), ?Al? O? Si?, the ?Si? O? Si? part of the network, and segmental motion of unreacted ?Si? (CH₂)₃OCH₂CHCH₂O chains. © 2001 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 39: 860–867, 2001     

Comparisons of glow discharge polymerization between hexamethyldisilazane and trimethylsilyldimethylamine

Glow discharge polymerization between hexamethyldisilazane (HMDSZ) and trimethylsilyldimethylamine (TMSDMA) was compared by means of infrared spectroscopy and ESCA analysis. Infrared spectra pointed out differences in chemical structure between the polymers prepared from the two monomers, although the two polymers were mainly composed of resembling units such as Si? CH₃, Si? CH₂, Si? H, Si? O? Si, and Si? O? C groups: (i) The polymers prepared from TMSDMA contained N → O group, but the polymers from HMDSZ did not contain this group. (ii) Influences of the W/FM parameter (W is the input energy of rf power, F the flow rate of the monomer, and M the molecular weight of the monomer) appeared on decreasing the C? N group and increasing the C?O group in the TMSDMA system, but little influence appeared in the HMDSZ system. ESCA spectra (C_1s, Si_2p, and N_1s core levels) supported the differences between the two polymers elucidated by infrared spectroscopy, and pointed out differences in susceptibility of the Si? N bond to plasma: The N? Si sequence of TMSDMA was completely ruptured in discharge to yield polymers, and the Si? NH? Si sequence of HMDSZ remained in considerable amount.     

Cubic Siloxanes with Both Si−H and Si−OtBu Groups for Site‐Selective Siloxane Bond Formation

Cage‐type siloxanes have attracted increasing attention as building blocks for silica‐based nanomaterials as their corners can be modified with various functional groups. Cubic octasiloxanes incorporating both Si?H and Si?OtBu groups [(tBuO)_nH_8?nSi₈O₁₂; n=1, 2 or 7] have been synthesized by the reaction of octa(hydridosilsesquioxane) (H₈Si₈O₁₂) and tert‐butyl alcohol in the presence of a Et₂NOH catalyst. The Si?H and Si?OtBu groups are useful for site‐selective formation of Si?O?Si linkages without cage structure deterioration. The Si?H group can be selectively hydrolyzed to form a Si?OH group in the presence of Et₂NOH, enabling the formation of the monosilanol compound (tBuO)₇(HO)Si₈O₁₂. The Si?OH group can be used for either intermolecular condensation to form a dimeric cage compound or silylation to introduce new reaction sites. Additionally, the alkoxy groups of (tBuO)₇HSi₈O₁₂ can be treated with organochlorosilanes in the presence of a BiCl₃ catalyst to form Si?O?Si linkages, while the Si?H group remains intact. These results indicate that such bifunctional cage siloxanes allow for stepwise Si?O?Si bond formation to design new siloxane‐based nanomaterials.     

The Novel Formation of Ordered and Varied Silica-Imidazole Complexes from Silicic Acid

Abstract

Fixed quantities of silicic acid in isopropyl alcohol were treated with varying amounts of imidazole in the same solvent, left closed till complexation was complete (～7 days) and the resulting gels on slow evaporation (～30 days) afforded glassy solids, ranging from nodules to tubules, as seen by scanning electron microscope (SEM). In another strategy where soluble oligomers of silica along with monomers can be present and the reaction conditions remaining the same, rectangular cubes of varying sizes were secured. In the view of the significance and potential utility of this novel finding, only duplicate runs, which essentially afforded similar SEM are included here. Elemental analysis established the presence of imidazole in the complexes and their percentage composition was computed using thermo gravimetric analysis (TGA). FTIR showed organics, presence of hydrogen-bonded imidazole, Si?O?Si bonds, and complete absence of Si?O?C bonds. It is suggested that a pattern where hydrogen-bonded imidazole with itself and the polymer, is entombed in the polymerized silicic acid. Domains having such a profile most likely lead to the observed morphology.The present work vastly widens the scope for silicic acid—guest complexation and holds potential to discover a range of silica harboring materials.

GRAPHICAL ABSTRACT      

ELECTRICAL CONDUCTIVITY OF IODINE DOPED POLY(PHTHALOCYANINATOSILOXANE)/PPTA BLEND FIBERS

Poly(phthalocyaninatosiloxane), [Si(Pc)O]n, with the number average degree of polymerization of about 130 was prepared by heating its monomer Si(Pc) (OH)_2, in solid state at 420℃for 42 hrs at 10~(-3) torr dynamic vacuum. The [Si(Pc)O]n powder was iodine doped with I_2-bensene solution for 48 hrs. Pure iodine doped poly(phthalocyaninatosiloxune), {[Si(Pc)O]I_y}n, fibers and {[Si(Pc)O]I_y}n/poly(p-pbenylene terephthalamide) blend fibers were wet-spun with dry nltrogen-sealed Teflon lined device. D.C. electrical conductivity of the fibers was measured by the four-probe method with an automated charge transport measurement system from 80K to room temperature. It was found that the dependence of conductivity, σ, on temperature, T, could fit a group of thermal fluctuation-induced tunnelling (TFIT) equations, and that the dependence of conductivity on volume fraction, φ, of the iodine doped {[Si(Pc)O]I_y}n could fit a group of modified percolation equations. A thrce-dimensioual composite plot of σ-1 / T-φshows that these two groups of equations match each other quite well. It has been pointed out that for the blend fibers their composition is the most important factor for both mechanical and electrical properties.     

Ab initio study of mechanism of forming a silicic <Emphasis Type="Italic">bis</Emphasis>-heterocyclic compound between (CH<Subscript>3</Subscript>)<Subscript>2</Subscript>Si=Si: and ethane

The mechanism of the cycloaddition reaction of forming a silicic bis-heterocyclic compound between singlet state (CH₃)₂Si=Si: and ethene has been investigated with the CCSD(T)//MP2/6-31G* method. From the potential energy profile, it can be predicted that the reaction has one dominant reaction pathway. The presented rule of this dominant reaction pathway that the 3p unoccupied orbital of Si: in (CH₃)₂Si=Si: and the π orbital of ethane forming a π → p donor-acceptor bond, resulting in the formation of a three-membered ring intermediate (INT1); Then, INT1 isomerizes to a four-membered ring silylene (P1), which driven by ring-enlargement effect; Due to sp ³ hybridization of Si: atom in the four-membered ring silylene (P1), P1 further combines with ethene to form a silicic bis-heterocyclic compound (P2).     

Anilinolysis of Picryl Benzoate Derivatives in Methanol: Reactivity,Regioselectivity, Kinetics,and Mechanism

The reaction of picryl benzoate derivatives 1a–g with aniline in methanol proceeds through CO? O and Ar? O bond cleavage pathways. Furthermore, the reactivity of these esters toward anilinolysis is correlated to the energy gap between highest occupied molecular orbital aniline and lowest unoccupied molecular orbital of each ester. The regioselectivity of acyl? oxygen versus aryl? oxygen cleavage is also discussed. The overall rate constants k_tot split into k_{CO? O} (the rate constant of acyl‐oxygen cleavage) and k_{Ar? O} (rate constant of aryl‐oxygen cleavage). The CO? O bond cleavage advances through a stepwise mechanism in which the formation of the tetrahedral intermediate is the rate‐determining step. The Ar? O bond cleavage continues through a S_NAr mechanism in which the departure of the leaving group from the Meisenheimer complex occurs rapidly after its formation in the rate‐determining step.     

Glow discharge polymerization of tetramethylsilane investigated by infrared spectroscopy and ESCA

The influence of operating conditions (W/FM parameter: W input energy of rf power; F flow rate of monomer; M molecular weight of monomer) on glow discharge polymerization of tetramethylsilane (TMS) was investigated by infrared spectroscopy and ESCA. Elemental analyses showed that the compositions of the polymers formed strongly depended on the level of the W/FM parameter, i.e., whether the W/FM value was more or less than 190 MJ/kg. The infrared spectra indicated that these polymers were composed of Si? H, Si? O, Si? C, Si? CH₃, and Si? CH₂ groups, and that there was no significant difference in structural features between polymers prepared at W/FM parameters of more or less than 190 MJ/kg. ESCA spectra (C_1s and Si_2p core-level spectra) showed that the polymers included carbonized carbon, aliphatic carbon Si? C, C? O, Si? O, and SiO₂ species, and that the sum of carbonized and aliphatic carbons reached more than 50%. Raising the W/FM value increased the formation of the carbonized carbon but did not influence the formation of Si units such as Si? C and Si? O groups. From this evidence the rupture of Si? CH₃ bonds in the polymer forming process is emphasized. The magnitude of the W/FM parameter may be related to the detachment of hydrogen from the aliphatic carbon units.     

Hochauflösende Festkörper-NMR-Untersuchungen an Kieselsäuren. III Hochauflösende 29 Si-Festkörper-NMR-Untersuchungen an synthetischen siliciumreichen Kieselsäurehydraten

Solid-State High-Resolution N.M.R. Studies of Silicic Acids. III. Solid-State High-Resolution ²⁹Si-N.M.R. Studies of Synthetic Highly Siliceous Silicic Acid Hydrates Solid-state high-resolution ²⁹Si-N.M.R. experiments were used to study the structure of the silicate layers in three synthetic highly siliceous silicic acid hydrates obtained by cation exchange reactions from the synthetic sodium silicate hydrates ilerite, magadiite and kenyaite. For each of the studied samples two well-separated N.M.R. signals have been recorded which could be assigned to Q³ - (O?Si(OSi?)₃–) and Q⁴-(?Si(OSi?)₄–) units of the silicate layers of the corresponding silicic acid hydrate. On the basis of these results the same model was used to describe the structure of the silicate layers in the studied compounds which has been already used for the interpretation of the N.M.R. results in case of the synthetic sodium silicate hydrates. Compared with the sodium silicate hydrates in the studied silicic acid hydrates a somewhat large Q⁴/Q³-ratio and a high-field shifted Q³signal have been detected. These observations were attributed to structural changes which appeared as a result of the cation exchange reactions.     

Synthesis of New Polysiloxane-Immobilized Ligand System Di(amidomethyl)aminetetraacetic Acid

A new chelating porous polysiloxane-immobilized tetraacetic acid ligand system has been prepared. This material was made by chemical modification of the iminodiacetic acid polysiloxane with thionyl chloride and diethyliminodiacetate, respectively. The polysiloxane functionalized with di(amidomethyl)aminetetraacetic acid of the general formula P-(CH ₂ ) ₃ N(CH ₂ C(O)N) ₂ (CH ₂ COOH) ₄ [where P represents the polysiloxane backbones (Si?O?Si) _n ] was characterized by Thermogravimetric Analysis (TGA) and FTIR spectra. The FTIR results proved that tetraacetic acid groups are successfully grafted onto the polysiloxane surface. This ligand system exhibits high potential for extraction of divalent metal ions (Co⁺², Ni⁺², Cu⁺², and Zn⁺²) from aqueous solution.