Synthesis and structure of the grafted layer on silicas chemically modified by aminophosphonic acids

The synthesis and properties of three representatives of a new class of chemically modified silicas containing aminophosphonic acids, namely, (N-methyl-N-propylamino)methylenephosphonic,N-propylaminodi(methylenephosphonic), andN-[2-(propylamino)ethyl]-N,N-di(methylenephosphonic acids), covalently grafted on the silica surface are reported. The modified silicas were obtained by the two-step Kabachnik-Fields reaction in high yields. The concentrations of the grafted groups were determined by elemental analysis and pH-titration. The new materials were characterized by IR and³¹P and¹³C high resolution solid state NMR spectroscopy. The hydration of the modified silicas was found to result in a significant change in the structure of the grafted layer and to increase the mobility of the grafted groups. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 12, pp. 2340–2345, December, 1999.     

Synthesis and structure of grafted layer of silicas modified with alkanesulfonic acid

A convenient method for preparation of silicas modified with alkanesulfonic acid through the reaction of NaHSO₃ with the C₂H₃ groups grafted on the SiO₂ surface was proposed. The influence of the carrier and the structure of the grafted layer on the modification process was studied by DRIFT, MAS NMR, and temperature-programmed desorption mass spectrometry (TPD-MS). It was found that at most 40% of the C₂H₃ groups transform to the acid sites, which mostly have the structure of 2-Si-ethanesulfonic acid. The mechanism of the thermal decomposition of the grafted groups, which are stable up to 240 °C, was proposed. The monomeric structure of the grafted layer leads to partial olygomerization of the C₂H₃ groups through the N=N bonds.     

Properties of silicas chemically modified by monodentate amines studied by conductometry

The protonation of aminoalkyl groups covalently bonded on the silica surface was studied by the conductometric titration method. Porous varieties of silica can adsorb HCl from an aqueous solution. Conductometric titration was proposed for the determination of concentrations and constants of protolytic equilibrium of grafted amino groups. During HCl chemisorption the effect of temperature on the electric conductivity of suspensions of the modified silicas was studied. __________ Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 1789–1793, August, 2005.     

Comparison of surface states of chemically modified silicas in aqueous acetonitrile and methanol

Summary The thermodynamic behaviour of n-alkylbenzenes on chemically modified silicas in reversed-phase liquid chromatography has been examined in acetonitrile-water and methanol-water systems. Plots of the thermodynamic parameters obtained against the organic solvent compositions indicate an interesting trend which implies that the surface states of the chemically modified silicas under the two eluent systems are different.     

Synthesis and characterization of nanographites with chemically modified edges

Nanographites, the structural blocks of activated carbon fibers in which the predominant part of edge carbon atoms forms covalent bonds with a chosen halogen (fluorine or chlorine), were synthesized and studied using X-ray photoelectron spectroscopy, X-ray diffraction analysis, and electron paramagnetic resonance. It was found that the formation of these bonds leads to changes in the density of states at the Fermi level and also in the values of some parameters of the spin system of current carriers of the boundary ??-electron states of nanographites.     

Bifunctional mesoporous silicas with clearly distinguished localization of grafted groups

Bifunctional mesoporous silicas with clearly distinguished localization of grafted groups on the surface of particles and inside their pores were obtained by means of sol-gel synthesis with postsynthetic vapor-phase treatment in vacuum. It was found that the synthesized materials have the hexagonally ordered porous structure typical of MCM-41 type silica.     

Molecular structure and properties of cellulose acetate chemically modified with caprolactone

Cellulose acetate (CA) with a degree of substitution of 1.7 was modified with caprolactone (CL) under various reaction conditions in an internal mixer. Processing temperature changed from 120 to 220 °C, while reaction time varied between 5 and 45 min. The composition and structure of the polymer was analyzed by various methods including FTIR, MALDI-TOF and NMR spectroscopy and its mechanical characteristics were determined by dynamic mechanical analysis and tensile testing. The results indicate that homopolymerization occurs under relatively mild conditions, while grafting requires higher temperatures and longer times. Grafted polycaprolactone (gPCL) chains are attached mainly to positions 2 and 6 of the glucose ring and their length increases with increasing reaction time and temperature, but the chains are always much shorter than those obtained in solution polymerization. Changes in the degree of substitution during grafting are small indicating that homopolymerization proceeds easier than grafting. Grafting seems to be easier in cellulose acetate with a larger degree of substitution in spite of the smaller number of active -OH groups present. Internal plasticization is more efficient than the external plasticizing effect of monomeric caprolactone. Plasticization results in a decrease of stiffness and strength, but deformability increases only slightly.     

Study of modified silicas by inverse gas chromatography part II: Influence of chain length on surface properties of silicas grafted withα−ω diols

Summary The surface properties of silicas modified with – diols having between 4 and 16 carbon atoms were investigated by inverse gas chromatography. It is shown, that the value of the dispersive component of the surface energy does not change monotonously: a minimum value is recorded when the surface is covered by a monolayer of methylene groups. Further, when measuring the adsorption enthalpies of polar probes, an alternation of their H values is observed: grafts having an odd number of carbon atoms systematically show higher H values than the others. A trans-trans configuration of the grafted chains which allows optimum interactions is proposed, suggesting a preferential diesterification reaction of both terminal hydroxyl groups of odd diols.     

Multiwalled carbon nanotubes with chemically grafted polyetherimides

Covalent attachment of a non-fluorinated polyetherimide onto the surface of carboxylic acid-functionalized multiwalled carbon nanotubes (MWNTs) has been achieved via grafting reactions. This confirms for the first time that the grafting reaction occurs at the nanotube surface when the carboxylic acid-functionalized MWNTs react with the polyetherimide with amine-terminated groups, through both amide and imide linkages formed at the interface between the carbon nanotubes and the polyetherimide. Additionally, an increase in the average molecular weight is detected in gel permeation chromatography when the polyetherimide is chemically attached onto the nanotubes. More interestingly, the chemical bonding at the interface provides much better interfacial adhesion and mechanical stress transfer, evidenced by a significant improvement in mechanical properties. As a result of the chemical attachment, the carbon nanotube-reinforced polyetherimide composite films have enhanced electrical conductivity, thermal deformation temperatures, and mechanical properties.     

Synthesis and characterization of functional chemically modified silica fillers

Elaborated methods of synthesis of chemically modified silicas with grafted silicon hydride and amino groups are analyzed. Experimental results on use of modified silicas with bonded amino groups as fillers of carboxyl-containing rubbers and epoxy resins are reported. It was shown that modified silica fillers with grafted silicon hydride groups could be applied for carrying out processes of catalytic solid-phase hydrosilylation of 2-hydroxyethylmethacrylate and some other functional olefins.     

DSC study of polyhydroxyethylmethacrylate filled with modified silicas

Effects of the nature of functional groups (namely, hydroxyl, methyl, silicon hydride, amino, and vinyl) on the surface of pristine and modified silicas on polymerization of 2-hydroxyethylmethacrylate (HEMA) and on structural characteristics of the filled composites have been studied. DSC, FTIR spectroscopy and equilibrium water sorption (ESI) techniques were applied for the composites characterization. Results obtained testify that the chemical nature of the grafted groups has a strong influence on the monomer orientation in the surface layer of the filler. More uniform and cross-linked structures were detected in the composites with particularly methylated silica. Filler with chemically active silicon hydride groups promotes formation of ordered structure with rigid macromolecules. The presence of amino and vinyl groups on the silica surface results in formation of flexible polymer chains with a low cross-linking density or with a low polymerization degree, even at 2?wt% filling degree. Water uptake for composites with vinyl- and amine-containing silicas was low, indicating the close-packing of polymeric molecules in the filled polyHEMA.     

Application of differential scanning calorimetry to study porous structure of hydrothermally modified silicas

The paper presents the studies on determination of physicochemical and structural properties of hydrothermally treated (HTT) silica gel Si-60, using the differential scanning calorimetry (DSC) and N₂ adsorption methods. Modification was performed in overheated water vapours or under the liquid water layer using various sources of thermal energy (in the classical autoclave or in the high-pressure microwave reactor). The characteristics of porous structure were determined on the basis of low-temperature nitrogen adsorption/desorption isotherms and using the calorimetric data (DSC) of thermal effects of phase transition of water introduced inside pores of the studied materials. Great compatibility of the characteristics obtained by means of the above-mentioned methods was proved. As follows from the SEM analysis, during HTT silica is rebuilt resulting in great changes of surface morphology and porous structure of the materials. The range of these changes depends on both the applied system (water vapour/liquid water) and the kind of used energy (classical/microwave).     

Electron-microscope investigation of polish sandomir silicas

Summary Silica was examined under the electron microscope —directly by transmission and indirectly in the form of carbon replicas — and the change in structure resulting from alkaline treatment was examined. It was found that the original silica has a larnellar structure with marked surface relief. After treatment with alkali and washing with water, its surface becomes covered with round particles about 100–200 A in size, a phenomenon which results from the hydrolysis of, alkali silicate and separation of colloidal silicon dioxide.     

Adsorption of water, diethyl ether, and acetonitrile on silicas with grafted perfluorohexyl coatings

The effect of the functionality of modifiers on the adsorption of vapors of compounds capable of strong specific interactions, including water, diethyl ether, and acetonitrile, on silicas with chemically grafted perfluorohexylsilanes (RSi(CH₃)_{3 −m} Cl _m , where m (functionality) is 1, 2, 3 and R is polyfluoroalkyl) was studied by adsorption under static conditions, gas chromatography, and IR spectroscopy. It was found that modification with monofunctional silane results in the most hydrophobic coating and a comparatively low-polarity surface with a strongly decreased contribution of specific interactions compared with other modifiers. Original Russian Text ? T.M. Roshchina, N.K. Shoniya, M.S. Lagutova, A.B. Nikol’skaya, V.Yu. Borovkov, L.M. Kustov, 2009, published in Zhurnal Fizicheskoi Khimii, 2009, Vol. 83, No. 2, pp. 361–369.     

Adsorption of proteins of the surfaces of modified silicas

Sorbents based on porous and nonporous varieties of silica containing chemisorbed primary amines, alcohols, amino-alcohols, amide groups, and phenyl radicals with different types of substituents in the surface layer have been synthesized. It has been established that, depending on the nature of the modifying layer, electrostatic and hydrophobic interactions make the principal contribution to the adsorption of proteins. The supports obtained may be used for the immobilization and Chromatographic separation of biopolymers.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 23, No. 2, pp. 237–241, March–April, 1987.     

Physicochemical and catalytic properties of grafted vanadium species on different mesoporous silicas

Vanadium modified mesoporous silicas type MCM-41, MCM-48, and SBA-15 are obtained by a solid state method. The samples are characterized and compared at the different steps of their preparation by X-ray diffraction, N(2)-physisorption, FTIR, UV-vis, XPS, and TG-TPR. Samples catalytic activity is tested in ethylacetate oxidation. Formation of various vanadium species, mainly isolated and small oligomeric ones, grafted to the support surface silanol groups, is observed. It is found that the state and the properties of the vanadium species depend on the porous characteristics of the silica host matrix. The nature of the catalytic active center in the ethylacetate oxidation is discussed.     

Effect of the surface condition of silicas with grafted monofunctional polyfluoroalkylsilanes on the adsorption of polar molecules

Lyophobized silicas containing relatively small numbers of grafted perfluorohexyl (1.1 nm^?2) and perfluorobutyl (1.7 nm^?2) groups are studied by means of gas chromatography, adsorption under static conditions, and IR spectroscopy. The results are compared to those obtained by us previously for a series of samples with dense polyfluoroalkyl monolayers (≥2.0 nm^?2). Effects related to the influence of the grafting density and the size of fluorine-containing groups on the adsorption of polar compounds and the hydrophobicity of the surface are discussed.