Effective preparation of crack-free silica aerogels via ambient drying

Effective ambient-drying techniques for synthesizing crack-free silica aerogel bulks from the industrial waterglass have been developed. Silica wet gels were obtained from aqueous colloidal silica sols prepared by ion-exchange of waterglass solution (4–10 wt% SiO₂). Crack-free monolithic silica aerogel disks (diameter of 22 mm and thickness of 7 mm) were produced via solvent exchange/surface modification of the wet gels using isopropanol/trimethylchlorosilane/n-Hexane solution, followed by ambient drying. The effects of the silica content in sol and the molar ratio of trimethylchlorosilane/pore water on the morphology and property of final aerogel products were also investigated. The porosity, density, and specific surface area of silica aerogels were in the range of 92–94%, 0.13–0.16 g/cm³, and ∼675 m²/g, respectively. The degree of springback during the ambient drying processing of modified silica gels was 94%.     

Optimization in the immobilization of penicillin G acylase by entrapment in xerogel particles with magnetic properties

Biocatalysis presents a sound alternative to chemical synthesis in the field of drug production, given the highly selective nature of biological catalysts. Penicillin G Acylase (PGA) from E. coli is currently used to hydrolyze penicillin G (PG) and catalyzes the synthesis of β-lactam antibiotics. In this work, particular emphasis is given to recent developments in penicillin G acylase immobilization, by entrapment simultaneously with nano-magnetic particles in a silica matrix. The sol–gel biocatalytic particles were prepared either by a conventional method (crushed powder) or by a more recent approach, based in an emulsion system using 150 mM AOT/isooctane, which allowed for the formation of spherical micro- and nanobeads. The effects on PGA activity of different sol–gel precursors, additives, enzyme concentration, aging, drying conditions and mechanical stability were evaluated. After these optimization studies, a mechanically stable carrier based on porous xerogels silica matrixes, starting from tetramethoxysilane (TMOS) with 65–67% PGA activity yield in these carriers allowed an immobilization yield of 74 mg protein g_{dry sol–gel}⁻¹ and 930 Ug_{dry sol–gel}⁻¹ for specific activity were obtained.     

Study on the influence of teos–diol molar ratio on their chemical interaction during the gelation process

Hybrid organic–inorganic materials, silica–diol, were synthesized by the sol–gel process from mixtures of tetraethylorthosilicate (TEOS) and diols: ethylene glycol (HO–CH₂–CH₂–OH) and 1,3 propane diol (HO–CH₂–CH₂–CH₂–OH), in acid catalysis. The gels have been synthesized for a molar ratio H₂O:TEOS = 4:1 and different molar ratios diol/TEOS: 0.25; 0.5; 0.75; 1.0; 1.25 and 1.5. The resulting gels were studied by thermal analysis and FT-IR spectroscopy, in order to evidence the interaction of diols with silica matrix. Thermal analysis indicated that the condensation degree increases with the molar ratio diol/TEOS until a certain value. The thermal decomposition of the organic chains bonded within the silica network in the temperature range 250–320 °C, leaded to a silica matrix with modified morphology. The adsorption–desorption isotherms type is different for the samples with and without diol. Thus, the specific surface areas have values <11 m²/g for the samples without diol and >200 m²/g for the samples with diols, depending on the annealing temperature.     

Effect of laponite clay particles on thermal and rheological properties of Pluronic triblock copolymer

The high concentration 17 wt% triblock copolymer poly(ethylene oxide)₁₀₀–poly(propylene oxide)₆₅–poly(ethylene oxide)₁₀₀ Pluronic F127 aqueous solutions with the addition of laponite is investigated as a novel temperature-sensitive hydrogel system. The critical micelle temperature (cmt) and the sol-to-gel transition were characterized by rheological experiments and differential scanning calorimetry. Experimental results showed that laponite particles have no significant influence on the cmt. On the other hand, viscoelastic measurements have highlighted an increase of the sol-to-gel transition temperature for mixtures with 2 and 3 wt% of laponite particles. This additive can be used to adjust the gelation temperature close to physiological temperature in medical applications.     

Thermal degradation behavior of a carboxylic acid-terminated amphiphilic block copolymer poly(ethylene)-b-poly(ethylene oxide)

The thermal degradation of an amphiphilic block copolymer poly(ethylene)-b-poly(ethylene oxide)-carboxylic acid terminated (PE-b-80%PEO–CH₂COOH) and its salt obtained as intermediary product from chemical oxidation of the end group of poly(ethylene)-b-poly(ethylene oxide) (PE-b-80%PEO) has been studied using a thermogravimetric mass spectrometry (TG/MS) coupled system. The isothermal fragmentation of PE-b-80%PEO–CH₂COOH showed a more complex fragmentation pattern than PE-b-80%PEO owing to the simultaneous occurrence of the polyether block and the carboxylic end group fragmentations. This led to the appearance of four overlapping ion current peaks of fragments with m/z 44 and two peaks relative to m/z 18 at different times by acid-terminated copolymer. For the PE-b-80%PEO copolymer, two ion current peaks associated to m/z 44 and one large peak relative to m/z 18 fragments were detected. The intermediary product (PE-b-80%PEO–CH₂COO⁻ K⁺) showed differences related to the fragmentation behavior. It has more defined ion current signals and presented characteristic peaks attributed to m/z 43 fragment at the very beginning of the thermal degradation process, which it not detected in the acid copolymer.     

A self-diffusion study in aqueous solution and lyotropic mesophases of amphiphilic block copolymers

 Water-soluble poly(ethylene oxide)–poly(propylene oxide)–poly(ethylene oxide) (PEO–PPO–PPO) triblock copolymers are high-molecular-weight nonionic copolymers and form micellar solutions and liquid-crystalline mesophases in water. We studied the temperature dependence of polymer and water self-diffusion in solutions and lyotropic mesophases of the PEO₁₃ PPO₃₀ PEO₁₃/water and PEO₂₁ PPO₄₇ PEO₂₁/water binary systems. The self-diffusion measurements were performed by means of the pulsed field gradient spin-echo NMR method. The analysis of the water mobility was realised using “the obstruction factor” and “the two-site model”, which consider the reduction of the water self-diffusion due to the microstructure of the lyotropic aggregates and to the presence of one part of the solvent bound to the polymer aggregate surfaces. We calculated the water obstruction factors and the hydration numbers as a function both of the polymer composition and of the temperature. The results are compared with the data obtained in mesophases formed by classical surfactants. Received: 16 September 1999 Accepted in revised form: 24 November 1999     

Europium-Containing Organic Gels and Organic and Carbon Aerogels. Preparation and Initial Applications in Catalysis

Summary. We prepared organic gels and organic and carbon aerogels doped with europium through sol–gel processes. Eu-gels were prepared by sol–gel polymerization of the potassium salt of 2,4-dihydroxybenzoic acid with formaldehyde followed by ion-exchange with Eu(OTf)₃. Eu–organic aerogels were obtained after CO₂ supercritical drying of the gels and Eu–carbon aerogels were obtained by pyrolysing the organic aerogels. The Eu-gels containing 12% europium proved to be efficient as recoverable catalyst in Michael additions. Deceased on February 20, 2006     

Obtaining of Ni<Subscript>0.65</Subscript>Zn<Subscript>0.35</Subscript>Fe<Subscript>2</Subscript>O<Subscript>4</Subscript>/SiO<Subscript>2</Subscript> nanocomposites by thermal decomposition of complex compounds embedded in silica matrix

This article presents the results of our investigation on the obtaining of Ni_0.65Zn_0.35Fe₂O₄ ferrite nanoparticles embedded in a SiO₂ matrix using a modified sol–gel synthesis method, starting from tetraethylorthosilicate (TEOS), metal (Fe^III,Ni^II,Zn^II) nitrates and ethylene glycol (EG). This method consists in the formation of carboxylate type complexes, inside the silica matrix, used as forerunners for the ferrite/silica nanocomposites. We prepared gels with different compositions, in order to obtain, through a suitable thermal treatment, the nanocomposites (Ni_0.65Zn_0.35Fe₂O₄)_x–(SiO₂)_100–x (where x=10, 20, 30, 40, 50, 60 mass%). The synthesized gels were studied by differential thermal analysis (DTA), thermogravimetry (TG) and FTIR spectroscopy. The formation of Ni–Zn ferrite in the silica matrix and the behavior in an external magnetic field were studied by X-ray diffraction (XRD) and quasi-static magnetic measurements (50 Hz).     

Thermal behaviour study of some sol–gel iron-silica nanocomposites

Two series of nanocomposites from the Fe_xO_y–SiO₂ system, containing 20 mass% iron oxide were prepared by the alkoxide route of the sol–gel method, in the absence and presence of catalyst. The silica gel has been obtained using tetraethoxysilane. The iron(III) nitrate nonahydrate has been used as iron oxides source. The samples have been prepared in identical conditions, differing only by the gelation times, induced by different surface of evaporation/volume (S/V) ratios of sol let to gelify. Thermal analysis data have established the thermal treatments conditions of the prepared samples and were correlated with X-ray diffraction, IR spectroscopy and TEM results, in order to accomplish a complete structural characterization. The correlation between the structural modifications of the Fe_xO_y–SiO₂ nanocomposites and different conditions of drying has been established.     

Effect of the functionality of junctions on the elasticity of polymacromonomer networks: Computer simulation

The elastic properties of polymer networks formed via the radical polymerization of macromonomers with two polymerizable end groups are studied via computer simulation. It is shown that variation in the average functionality of network junctions, f _avg, in a wide range (∼5–55) leads to a significant change in the shear modulus of the network. According to experiments with real networks (gels of poly(ethylene oxide) macromonomers), the shear modulus increases as f _avg increases. This effect is not due only to a decrease in the fluctuations of positions of network junctions. The main cause of the increase in the modulus is that the modulus component due to interaction between polymer chains (entanglements) increases as the functionality of junctions in the investigated networks increases. The conclusion is made that these networks gain entanglements during the formation of network junctions with high functionality rather than inherit them from the solution of macromonomer chains.     

Enhanced electrochemical properties of polyethylene oxide-based composite solid polymer electrolytes with porous inorganic–organic hybrid polyphosphazene nanotubes as fillers

Solid composite polymer electrolytes consisting of polyethylene oxide (PEO), LiClO₄, and porous inorganic–organic hybrid poly (cyclotriphosphazene-co-4, 4′-sulfonyldiphenol) (PZS) nanotubes were prepared using the solvent casting method. Differential scanning calorimetry and scanning electron microscopy were used to determine the characteristics of the composite polymer electrolytes. The ionic conductivity, lithium ion transference number, and electrochemical stability window can be enhanced after the addition of PZS nanotubes. The electrochemical impedance showed that the conductivity was improved significantly. Maximum ionic conductivity values of 1.5 × 10⁻⁵ S cm⁻¹ at ambient temperature and 7.8 × 10⁻⁴ S cm⁻¹ at 80 °C were obtained with 10 wt.% content of PZS nanotubes, and the lithium ion transference number was 0.35. The good electrochemical properties of the solid-state composite polymer electrolytes suggested that the porous inorganic–organic hybrid polyphosphazene nanotubes had a promising use as fillers in SPEs and the PEO₁₀–LiClO₄–PZS nanotube solid composite polymer electrolyte might be used as a candidate material for lithium polymer batteries.     

Morphology and photoluminescence study of titania nanoparticles

Titania nanoparticles are prepared by sol–gel chemistry with a poly(ethylene oxide) methyl ether methacrylate-block-poly(dimethylsiloxane)-block-poly(ethylene oxide) methyl ether methacrylate triblock copolymer acting as the templating agent. The sol–gel components—hydrochloric acid, titanium tetraisopropoxide, and triblock copolymer—are varied to investigate their effect on the resulting titania morphology. An increased titania precursor or polymer content yields smaller primary titania structures. Microbeam grazing incidence small-angle X-ray scattering measurements, which are analyzed with a unified fit model, reveal information about the titania structure sizes. These small structures could not be observed via the used microscopy techniques. The interplay among the sol–gel components via our triblock copolymer results in different sized titania nanoparticles with higher packing densities. Smaller sized titania particles, (∼13–20 nm in diameter) in the range of exciton diffusion length, are formed by 2% by weight polymer and show good crystallinity with less surface defects and high oxygen vacancies.     

Structure of fumed silica gels in dodecane: enhanced network by oscillatory shear

The structure and viscoelastic properties of fumed silica gels in dodecane were studied by means of dynamic rheology. With increasing the specific surface area of fumed silica nanoparticles, the plateau elastic modulus (G′), which is frequency-independent and shows the characteristic of a network of the fumed silica gels, decreases. Such networks of fumed silica gels show a significant temperature-dependent behavior and a transition temperature (T _c) related with the restructuring of nanoparticle chain aggregates of fumed silica in gels. Under oscillatory shear, the fumed silica gels experience disorganization and reorganization and present strong structural recovery ability after adjusting oscillatory shear (AOS) at small strain amplitudes (1–10%), and a more perfect network structure than that in origin gels can be induced. Elevated temperature (above T _c) improves the network structure to be more compact and stronger than that at a lower temperature, as a result, the deformation resistance during the AOS period and the structural recovery after AOS are enhanced. These results indicate that the network structure and viscoelastic properties of fumed silica gels can be tailored and optimized by performing small-amplitude oscillatory shear at a properly selected temperature.     

The interaction between TEOS and some polyols

Hybrid organic-inorganic materials, silica – polyols (ethylene-glycol – EG; 1,2 propane diol – 1,2PG; 1,3 propane diol – 1,3PG and glycerol – GL), were prepared by a sol-gel process starting from tetraethylorthosilicate (TEOS) and polyols, in acid catalysis. The resulting materials were studied by thermal analysis (in air and nitrogen), FTIR and solid state ²⁹Si-NMR spectroscopy. These techniques evidenced the presence of polyols in the silica matrix both hydrogen bounded and chemically bounded in the silica network. The thermal analysis proves to be the most appropriate technique to evidence the organic chains linked in the matrix network and to follow the thermal evolution of the gels to the SiO₂ matrix.     

Synthesis of silica nanoparticles by modified sol–gel process: the effect of mixing modes of the reactants and drying techniques

A modified preparation of silica nanoparticles via sol–gel process was described. The ability to control the particle size and distribution was found highly dependent on mixing modes of the reactants and drying techniques. The mixture of tetraethoxysilane and ethanol followed by addition of water (Mode-A) produced monodispersed powder with an average particle size of 10.6 ± 1.40 nm with a narrow size distribution. The freeze drying technique (FD) further improved the quality of powder. In addition, the freeze dried samples have shown unique TGA decomposition steps which might be related to the well-defined structure of silica nanoparticles as compared to the heat dried samples. DSC analysis showed that FD preserved the silica surface with low shrinkage and generated remarkably well-order, narrow and bigger pore size and pore volume and also large endothermic enthalpies (ΔH _FD = −688 J g⁻¹ vs. ΔH _HD = −617 J g⁻¹) that lead to easy escape of physically adsorbed water from the pore at lower temperature.     

Further Insights into the Porous Structure of TEOS Derived Silica Gels

The porous structure of TEOS derived silica gels was studied using nitrogen adsorption at 77 K. Silica gels were prepared using TEOS, H₂O and ethanol for different molar ratios. No catalyst was used in this study. Silica gels were also heat treated up to 1000°C. The nitrogen sorption isotherms were analyzed by two models: Fractal and Percolation Theories. Using the fractal analysis approach, the surface roughness of the porous structure of silica gels was determined. The surface fractal dimension depends on the hydrolysis conditions and heat treatment. The surface fractal dimension decreases with increasing H₂O/TEOS molar ratio or heating temperature. For the silica gels studied, the surface fractal dimension changed from 2.6 to 2.5 after heating the gels, and from 2.4 to 2.6 with decreasing H₂O/TEOS ratio.Using the Percolation theory, we have determined the connectivity of the porous structure of silica gels. The extent of sorption hysteresis of the nitrogen isotherms reflects the connectivity of the pore network. The mean coordination number (connectivity) Z, and the linear dimension of the network, L, have been calculated from the hysteresis of the isotherms. For the as-prepared silica gels, Z was about 8 and L close to 2. On heating the gels, Z decreases to 4 and L increases to 7, results which are in accordance with the collapse of the porous network.     

Thermogravimetry applied to characterization of SBA-15 nanostructured material

Nanoporous silica with narrow pore size distribution has attracted increasing attention as a novel material for separations and reactions involving large molecules. SBA-15 has been synthesized in an acidic medium using a triblock copolymer as template. In this work, the SBA-15 was synthesized by the hydrothermal treatment at 373 K for 48 h, of a gel with the following overall molar composition: 1.0TEOS:0.017P123:5.7HCl:193H₂O, where TEOS is tetraethyl orthosilicate and P123 is poly(ethylene oxide, propylene oxide and 1,4-dioxane). The obtained material was characterized by thermogravimetry, X-ray diffraction, infrared spectroscopy and BET surface area. A kinetic study using the model free model was accomplished in the stage of decomposition of the template (P123). The obtained value of the apparent activation energy was ca. 131 kJ mol^–1.     

Structural characterization of hierarchically porous alumina aerogel and xerogel monoliths

Detailed nanostructures have been investigated for hierarchically porous alumina aerogels and xerogels prepared from ionic precursors via sol–gel reaction. Starting from AlCl₃·6H₂O and poly(ethylene oxide) (PEO) dissolved in a H₂O/EtOH mixed solvent, monolithic wet gels were synthesized using propylene oxide (PO) as a gelation initiator. Hierarchically porous alumina xerogels and aerogels were obtained after evaporative drying and supercritical drying, respectively. Macroporous structures are formed as a result of phase separation, while interstices between the secondary particles in the micrometer-sized gel skeletons work as mesoporous structures. Alumina xerogels exhibit considerable shrinkage during the evaporative drying process, resulting in relatively small mesopores (from 5.4 to 6.2 nm) regardless of the starting composition. For shrinkage-free alumina aerogels, on the other hand, the median mesopore size changes from 13.9 to 33.1 nm depending on the starting composition; the increases in PEO content and H₂O/EtOH volume ratio both contribute to producing smaller mesopores. Small-angle X-ray scattering (SAXS) analysis reveals that variation of median mesopore size can be ascribed to the change in agglomeration state of primary particles. As PEO content and H₂O/EtOH ratio increase, secondary particles become small, which results in relatively small mesopores. The results indicate that the agglomeration state of alumina primary particles is influenced by the presence of weakly interacting phase separation inducers such as PEO.     

Formation and structure of ionomer complexes from grafted polyelectrolytes

We discuss the structure and formation of Ionomer Complexes formed upon mixing a grafted block copolymer (poly(acrylic acid)-b-poly(acrylate methoxy poly(ethylene oxide)), PAA₂₁-b-PAPEO₁₄) with a linear polyelectrolyte (poly(N-methyl 2-vinyl pyridinium iodide), P2MVPI), called grafted block ionomer complexes (GBICs), and a chemically identical grafted copolymer (poly(acrylic acid)-co-poly(acrylate methoxy poly(ethylene oxide)), PAA₂₈-co-PAPEO₂₂) with a linear polyelectrolyte, called grafted ionomer complexes (GICs). Light scattering measurements show that GBICs are much bigger (~70–100 nm) and GICs are much smaller or comparable in size (6–22 nm) to regular complex coacervate core micelles (C3Ms). The mechanism of GICs formation is different from the formation of regular C3Ms and GBICs, and their size depends on the length of the homopolyelectrolyte. The sizes of GBICs and GICs slightly decrease with temperature increasing from 20 to 65 °C. This effect is stronger for GBICs than for GICs, is reversible for GICs and GBIC-PAPEO₁₄/P2MVPI₂₂₈, and shows some hysteresis for GBIC-PAPEO₁₄/P2MVPI₄₃. Self-consistent field (SCF) calculations for assembly of a grafted block copolymer (having clearly separated charged and grafted blocks) with an oppositely charged linear polyelectrolyte of length comparable to the charged copolymer block predict formation of relatively small spherical micelles (~6 nm), with a composition close to complete charge neutralization. The formation of micellar assemblies is suppressed if charged and grafted monomers are evenly distributed along the backbone, i.e., in case of a grafted copolymer. The very large difference between the sizes found experimentally for GBICs and the sizes predicted from SCF calculations supports the view that there is some secondary association mechanism. A possible mechanism is discussed.