Silica Volume Fraction Research Articles

Rheological properties of colloidal gels formed from fumed silica suspensions in the presence of cationic surfactants

Rheological responses of colloidal gels formed from fumed silica suspensions in aqueous KOH solution at pH 11 by the addition of cationic surfactants, such as dodecyltrimethylammonium chloride (C 12TAC) and hexadodecyltrimethylammonium chloride (C 16TAC) have been investigated as functions of silica and surfactant concentrations. Stable and aggregated fumed silica suspensions with negative charges cause gelling by adding the cationic surfactants through electrical neutralization of their micelles. The resulting critical strain and storage modulus of the gelled silica suspension increase with an increase in the surfactant concentration, irrespective of the cationic surfactant. This means that the higher the surfactant concentration is, the more effective the electrical neutralization interaction through the micelle of the cationic surfactant is. Moreover, the resulting gels can be classified into the strong-link gel and the weak-link one in the presence of C 12TAC and C 16TAC, respectively, from a comparison of the silica volume fraction dependences of critical strain and storage modulus with the fractal gel model.

Effect of Steel Fiber on the Mechanical Properties of Cement-Based Composites Containing Silica Fume

An experimental program was carried out to evaluate the mechanical properties of cement-based composites. Test variables included water to cementitious ratio, dosage of silica fume and volume fraction of steel fiber. Compressive strength test, direct tensile strength test, splitting tensile strength test, abrasion resistance test and drop weight test were performed and the results were analyzed statistically. According to the results of this study, the designed direct tensile testing method was a suitable method to estimate the tensile strength of fiber cement-based composites. Addition of fibers provided better performance for the cement-based composites, while silica fume in the composites would help obtaining uniform fiber dispersion in the matrix and improve strength and the bonding between fiber and matrix resulting from extra dense calciumsilicate-hydrate gel. The combination of steel fibers and silica fume can greatly increase the mechanical properties of cement-based composites. Besides, a multiple regression analysis was conducted to correlate compressive strength, direct tensile strength, abrasion coefficient and impact number with w/cm ratio, silica fume content and steel fiber content and a fairly agreement between test data and estimated values was found.

Experimental study on the rate dependent strength of ice‐silica mixture with silica volume fractions up to 0.63

We conducted deformation experiments of ice‐1 μm silica beads mixture to clarify the effects of silica beads volume fraction and temperature on the strength. The silica beads volume fraction was changed from 0 to 0.63 to simulate the surfaces of icy bodies. Unconfined uniaxial compression tests were made in a cold room at the temperatures from −10°C to −25°C and the constant strain rates ranged from 2.9 × 10−3 to 8.5 × 10−7 s−1. We determined the rate dependent strength of the mixture written by = A · σmaxn from the relationship between the maximum stress, σmax, on the stress‐strain curve and the applied strain rate, . At −10°C, the mixtures with silica volume fractions of 0.004–0.04 had almost the same strength with pure ice and the stress exponent, n, is about 3. On the other hands, at the silica volume fractions more than 0.15, the mixture became harder as the beads were more included, and it had the same stress exponent, about 6. This high stress exponent might be caused by crack generation. Also, we found that the A for silica volume fractions more than 0.15 was written by an exponential equation related to the silica volume fraction, ϕ, A = 6.86 × 10−8exp(−6.35ϕ). Furthermore, we found that the n of ϕ = 0.15 was independent of the temperature, and the brittle‐ductile boundary of ϕ = 0.29 and 0.63 was more than 30°C higher than that of pure ice.

Phase transition and proton transport characteristics in CsH2PO4/SiO2 composites

Abstract The stabilization of superprotonic phase in neat CsH 2 PO 4 and CsH 2 PO 4 /SiO 2 composites as well as the anomalous phase transformation with a large hysteresis was investigated through proton conductivity, thermal analysis and Raman spectroscopy. The reversibility of the transition to the superprotonic phase and the phase transformation between monoclinic phase and cubic phase in neat CsH 2 PO 4 at around T c = 230 °C was confirmed under humidified and sealed conditions. In CsH 2 PO 4 /SiO 2 composites, a large asymmetric thermal hysteresis in the conductivity appeared, i.e. significant supercooling in the superprotonic phase was induced in silica matrices. A differential thermal analysis revealed that the temperature of a reverse transition from the cubic phase (superprotonic phase) to the monoclinic phase decreased in the composites. This effect became significant with an increase in silica volume fraction. The stabilization of superprotonic phase (cubic phase) in the composites will be induced by shear elastic forces at the interface between CsH 2 PO 4 and silica particles. The main origin of the anomalous asymmetric thermal hysteresis in proton conductivity is the phase stability arising from the shear elastic forces and a proton-conducting network in silica matrices.

Time-Resolved in Situ Small-Angle X-ray Scattering Study of Silica Particle Formation in Nonionic Water-in-Oil Microemulsions

The formation of silica particles by the ammonia-catalyzed hydrolysis of tetraethyl orthosilicate (TEOS) in the polyoxyethylene (5) nonylphenyl ether (NP-5)/cyclohexane/water microemulsion system was investigated by time-resolved small-angle X-ray scattering (SAXS). The SAXS data could be modeled as a combination of two species where one describes the silica-particle containing microemulsion droplets and the other the reverse droplets. The analysis allowed the determination of the evolution of the system of particles of silica and reverse droplets. A model of nucleation and growth of the silica particles is confirmed and the volume fraction versus time data for the silica particles is in agreement with first order kinetics with respect to TEOS concentration. Moreover to describe the long time evolution of the system, a correlation among the silica particles has been taken into account by introducing a structure factor with a local silica volume fraction eta = 0.1. This high local density is 2 orders of magnitude larger than the global silica fraction and can be explained in terms of depleting interaction.

Structure and rheological properties of model microemulsion networks filled with nanoparticles

Model microemulsion networks of oil droplets stabilized by non-ionic surfactant and telechelic polymer C18 -PEO(10k)- C18 have been studied for two droplet-to-polymer size ratios. The rheological properties of the networks have been measured as a function of network connectivity and can be described in terms of simple percolation laws. The network structure has been characterised by Small Angle Neutron Scattering (SANS). A Reverse Monte Carlo (RMC) approach is used to demonstrate the interplay of attraction and repulsion induced by the copolymer. These model networks are then used as matrix for the incorporation of silica nanoparticles (R = 10 nm), individual dispersion being checked by scattering. A strong impact on the rheological properties is found for silica volume fractions up to 9%. q(A-1).

Nanostructure Evolution: From Aggregated to Spherical SiO2 Particles Made in Diffusion Flames

AbstractThe formation of nanostructured silica particles by oxidation of hexamethyldisiloxane (HMDSO) in co‐annular diffusion flames is investigated by in‐situ small‐angle X‐ray scattering (SAXS). This enabled the nonintrusive monitoring of the mass fractal dimension, the aggregate size, and the number of primary particles per aggregate, along with the silica volume fraction, the primary particle diameter, the geometric standard deviation, and the number density along the flame axis. Parallel to this, thermophoretic sampling (TS) of the particles and analysis by transmission electron microscopy (TEM) allowed for direct comparison of particle morphology to that obtained from the above SAXS analysis, which were compared also to the ultra‐small‐angle X‐ray scattering (USAXS) data for product particles collected from the filter. The flame temperature was measured by in‐situ Fourier transform infrared (FTIR) spectroscopy. By increasing the oxygen flow rate, the laminar diffusion flame changed to a turbulent, premixed‐like flame. Non‐aggregated, spherical particles were formed in the former, while aggregates were formed in the latter flame. The relatively long high‐temperature particle residence times in the laminar diffusion flame facilitated sintering of the aggregates formed earlier into compact spherical particles at later stages of the flame.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)

Polysulfone/silica nanoparticle mixed-matrix membranes for gas separation

Mixed-matrix membranes (MMMs) are based on polymeric membranes filled with inorganic particles as a means to improve their gas separation performance. In this study, MMMs were prepared from polysulfone (PSf) containing embedded nonporous fumed silica nanoparticles and the gas permeation properties of the resulting membranes were investigated. Physical properties such as film density, thermal degradation and glass transition temperature of PSf/silica MMMs were characterized. The distribution of the silica nanoparticles in PSf was observed by transmission electron microscopy (TEM) and scanning electron microscopy (SEM). Furthermore, the interface between the polymer and silica agglomerates was studied in relation with the gas transport properties. The gas permeabilities of hydrogen, helium, oxygen, nitrogen, methane, and carbon dioxide were measured as a function of silica volume fraction and diffusion and solubility coefficients were determined by the time-lag method. The effect of silica nanoparticles in PSf membranes on gas permeability is in contrast with predictions based on the Maxwell model. The O 2 permeability is approximately four times higher and CH 4 permeability is over five times greater than in a pure PSf membrane. However, the performance comprising permeability versus selectivity of PSf/silica MMMs for O 2/N 2 and CO 2/CH 4 follows a similar slope to that of the trade-off upper bound with increasing silica content.

Influence of Monovalent Electrolytes on Rheological Properties of Gelled Colloidal Silica Suspensions

Rheological responses of the gelled (G) Snowtex 20 silica suspensions in the presence of LiCl, NaCl, and KCl have been investigated as functions of concentrations of silica and salt at pH 9.8 in terms of the Hofmeister series effect. The primary silica particle is isolated, and it is coagulated to form a gel above at 0.1, 0.3, and 0.5 M concentrations of LiCl, NaCl, and KCl, respectively, when the silica volume fraction φ is beyond 1.0%. The resulting G silica suspensions are classified into a strong‐link gel and their power‐law dependences of the critical strain as well as the storage modulus on the silica volume fraction were compared with the predictions by the fractal gel model. The resulting power‐law exponents of the critical strain are negative, this is in agreement with that predicted by the fractal model, and their magnitudes decrease with an increase in salt concentration, irrespective of the salt. Moreover, the magnitude of the power‐law exponent for the critical strain is well related to the Hofmeister series effect, namely it decreases in the order Li+, Na+, and K+, and the least hydrated K+ adsorbs in great enough in amount to create a less flexible network structure in the G silica suspension due to stronger attraction between the silica particles. Moreover, the stronger attraction, on the other hand, should be responsible for both the larger storage modulus and the higher power‐law exponent of the silica volume fraction dependence on the storage modulus. Therefore, the Hofmeister series effect is useful to understand the rheological properties of the G Snowtex 20 silica suspensions.

Aging effects of precipitated silica in poly(dimethylsiloxane)

We report the transformation from gels to viscous fluids within a time period of 101–103h in mixtures of precipitated silica in silicone oil. Scaling behavior in elastic and viscous moduli versus frequency, similar to that reported by Trappe and Weitz [Phys. Rev. Lett. 85, 449–452 (2000)], has been observed for suspensions of different ages, silica volume fractions, and silicone oil molecular weights; this shows self-similarity in the stress-bearing silica network in these samples. The elastic plateau modulus, extracted from the scaling of the moduli for each suspension, decays exponentially with time, but the initial plateau modulus depends only on the silica volume fraction and is independent of the properties of the base fluids. The aging time constant is larger if thinner silicone oil or a higher silica volume fraction is used. The aging rate is slower for silica in methyl-terminated silicone oil than that in hydroxyl-terminated silicone oil by about an order of magnitude. We attribute the aging effects to the adsorption of poly(dimethylsiloxane) on silica and use a model based on the “poisons” model by Cohen-Addad and de Gennes [C. R. Ocad. Sci. Paris, Série II 319, 25–30 (1994)] and on the simulation work by Tsige and co-workers [J. Chem. Phys. 118, 5132–5142 (2003)] to explain our observations. No aging effect has been observed for silica-mineral oil mixtures.

Surface hardness and transparency of poly(methyl methacrylate)-silica coat film derived from perhydropolysilazane

To prepare hard and transparent poly (methyl methacrylate)-silica coat film on glass or polycarbonate substrates, poly(methyl methacrylate-co-2-hydroxyethyl methacrylate) random copolymers and perhydropolysilazane (PHPS) were blended in solution. Then the solution was cast on the substrates. The grafting of PHPS onto 2-hydroxyethyl methacrylate unit was analyzed by 1 H NMR spectroscopy. Surface hardness and transparency of the coat film were measured by nano-indentation method and UV-Vis spectroscopy, respectively. Surface hardness of coat film depended on the volume fraction of silica in the coat film, and reached 2.7 GPa when the volume fraction of silica was 76.4%. Transparency of the coat films prepared with PHPS was almost 100%, indicating that the coat film prepared with PHPS was highly transparent not only on glass substrate but also on the polycarbonate substrate.

Rheological Properties of Fumed Silica Suspensions in the Presence of Potassium Chloride

Changes in rheological properties of fumed silica particles dispersed in aqueous KOH solution at pH 11 by the addition of KCl have been studied as functions of the silica and KCl concentrations. Since the fumed silica particles are stable in the state of aggregates of the primary particles, coagulation of such aggregates by addition of KCl should cause a gel. Thus, the resulting gel can be regarded as the weak-link gel according to the fractal gel model. States of the fumed silica suspensions in the presence of KCl were roughly classified into four phases, such as sol (S), pre-gelled (PG), gelled (G), and two-phase separated (TP) silica suspension from visual observation. The PG and G silica suspensions were conducted by strain and frequency sweep measurements of dynamic moduli, whereas the S silica suspensions were mainly examined by steady-state viscosity measurements. The critical strains obtained from the strain sweep measurements of storage modulus G' were almost independent of the silica volume fraction φ at lower KCl concentrations than 0.4 M, whereas they increased with the positive power-law of φ at higher KCl concentrations than 0.5 M. The positive power-law exponent is in agreement with the prediction for the weak-link gel. The PG and G silica suspensions show that the G' values in the linear response regions indicate little frequency dependence. The values of G' at the fixed KCl concentration exhibit a power-law behavior and the resulting power-law exponent increases with an increase in KCl concentration.

Nanostructure and Mechanical Properties of Polybutylacrylate Filled with Grafted Silica Particles

We investigate the nanostructure and the linear rheological properties of polybutylacrylate (PBA) filled with Stöber silica particles grafted with PBA chains. The silica volume fractions range from 1.8 to 4.7%. The nanostructure of these suspensions is investigated by small-angle neutron scattering (SANS), and we determine their spectromechanical behavior in the linear region. SANS measurements performed on low volume fraction composites show that the grafted silica particles are spherical, slightly polydisperse, and do not form aggregates during the synthesis process. These composites thus constitute model filled polymers. The rheological results show that introducing grafted silica particles in a polymer matrix results in the appearance of a secondary process at low frequency: for the lowest volume fractions, we observe a secondary relaxation that we attribute to the diffusion of the particles in the polymeric matrix. By increasing the silica volume fraction up to a critical value, we obtain gellike behavior at low frequency as well as the appearance of a structure factor on the scattering intensity curves obtained by SANS. Further increasing the silica particle concentration leads to composites exhibiting solidlike low-frequency behavior and to an enhanced structure peak on the SANS diagrams. This quantitative correlation between the progressive appearance of a solidlike rheological behavior, on one hand, and a structure factor, on the other hand, supports the idea that the viscoelastic behavior of filled polymers is governed by the spatial organization of the fillers in the matrix.

Surface-functionalized nanoparticles with liquid-like behavior: The role of the constituent components

Ionically modified silica nanoparticles with large counter anions (sulfonate, isostearate) at two silica volume fractions (13 and 27%) form a viscous fluid and a glass but not crystalline solids. Dielectric spectroscopy, Brillouin scattering and shear rheometry were employed to investigate these new nanoparticle-based fluids. The glass transition temperature and hence the local dynamics are governed by the large counter anions, whereas the flow properties can be controlled by the spatial correlation between the nanoparticles, e.g. by tuning the volume fraction of hard cores and local interactions between segments in the soft corona. Liquid-like ordering of the cores was revealed by X-ray scattering and found to influence significantly the macroscopic flow properties of these salts.

Rheological behavior of fumed silica suspension in polyethylene glycol

The rheological behavior of fumed silica suspensions in polyethylene glycol(PEG) was studied at steady and oscillatory shear stress using AR 2000 stress controlled rheometer. The systems show reversible shear thickening behavior and the shear-thickening behavior can be explained by the clustering mechanism. The viscosity and the degree of shear-thickening of the systems strongly depend on the mass fraction of the silica, the molecular weigh of PEG and the frequency used in the rheological measurement. The silica volume fraction of the systems is 1.16%–3.62%, corresponding to the mass fraction of 4%–9%. The shear-thickening taking place in the low volume fraction may contribute to the fractal nature of the silica. At oscillatory shear stress, when the shear stress is less than the critical stress, the storage modulus decreases significantly, meanwhile the loss modulus and the complex viscosity almost remain unchanged; when the shear stress is larger than the critical stress, the storage modulus, the loss modulus and the complex viscosity increase with the increase of shear stress. The loss modulus is larger than the storage modulus in the range of stress studied and both moduli depend on frequency.

Mechanical and Transport Properties of Colloidal Silica-Unsaturated Polyester Composites

Colloidal silica-reinforced polyester composites with different volume fractions of colloidal silica are investigated for morphology, tensile strength, and flexural strength. Morphology is studied using scanning electron microscopy. Mechanical properties are studied with respect to varying filler content i.e., from 0.5 to 2.5% (by wt.) and it is found that the tensile and flexural strengths increase initially with increasing filler content and then start to decreasing with the increase in filler content. There are several explanations for these effects, which are discussed in terms of reinforcing mechanisms. In addition, the effect of various aggressive solvents is studied. It is found that the chemical absorption increases with increasing filler content. The kinetics of swelling and sorption has also been studied for carbon tetrachloride, toluene, xylene, and formaldehyde at 30 C. Diffusivity coefficient decreases and sorption coefficient increases with increasing filler content. In addition, the mode of transport is observed to be non-Fickian. The effect of solvents on the diffusion behavior of the composites is also studied. It is found that the equilibrium degree of swelling increases with the increase in the solubility parameter of the solvent and is maximum in the solvent, which has the solubility parameter value near to that of the polyester resin. As solubility parameter value deviates more from that of the resin, the equilibrium absorption value decreases.

Polymer–filler interactions in sol–gel derived polymer/silica hybrid nanocomposites

AbstractThe effect of polymer–filler interaction on solvent swelling and dynamic mechanical properties of the sol–gel derived acrylic rubber (ACM)/silica, epoxidized natural rubber (ENR)/silica, and poly (vinyl alcohol) (PVA)/silica hybrid nanocomposites has been described for the first time. Tetraethoxysilane (TEOS) at three different concentrations (10, 30, and 50 wt %) was used as the precursor for in situ silica generation. Equilibrium swelling of the hybrid nanocomposites in respective solvents at ambient condition showed highest volume fraction of the polymer in the swollen gel in PVA/silica system and least in ACM/silica, with ENR/silica recording an intermediate value. The Kraus constant (C) also followed a similar trend. In dynamic mechanical analysis, the storage modulus dropped at higher strain (>1%), which indicated disengagement of polymer segments from the filler surfaces. This drop was maximum in ACM/silica, intermediate in ENR/silica, and minimum in PVA/silica, both at 50 and 70 °C. The drop in modulus with theoretical volume fraction of silica (ϕ) was interpreted with the help of a Power law model ΔE′ = a1ϕ, where a1 was a constant and b1 was primarily a filler attachment parameter. Strain dependence of loss modulus was observed in ACM/silica hybrid nanocomposites, while ENR/silica and PVA/silica nanocomposites showed almost strain‐independent behavior. The storage modulus showed sharp increase with increasing frequency in ACM/silica system, while that was lower in both ENR/silica (at higher frequency) and PVA/silica systems (in the entire frequency spectrum). The increase in modulus with ϕ also followed similar model ΔE′ = a2ϕ proposed in the strain sweep mode. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 2399–2412, 2005

PolyHIPE: IPNs, hybrids, nanoscale porosity, silica monoliths and ICP-based sensors

Typical polyHIPE (porous polymers from high internal phase emulsions) have a cellular structure with volume fractions from 0.2 to 0.04, cell diameters from 15 to 25 μm and intercellular pore diameters from 0.5 to 10 μm. Unique interpenetrating polymer networks synthesized within the polyHIPE exhibited enhanced mechanical properties and an extended temperature range for damping. Hybrid polyHIPE that combine an inorganic polysilsesquioxane network with an organic polystyrene network exhibited superior high temperature mechanical properties and enhanced thermal stability. A nanoscale porosity in the cell walls, produced through the addition of a porogen to the HIPE, reduced the density and significantly enhanced the specific surface area. Porous silica monoliths with silica volume fractions of as low as 0.02 were produced through the pyrolysis of hybrid polyHIPE. PolyHIPE coated with an intrinsically conducting polymer exhibited reversible and repeatable changes in conductivity on exposure to acetone vapor, demonstrating their potential as sensor materials.

Structure and rheological properties of soft–hard nanocomposites: influence of aggregation and interfacial modification

A study of the reinforcement effect of a soft polymer matrix by hard nanometric filler particles is presented. In the main part of this article, the structure of the silica filler in the matrix is studied by small angle neutron scattering (SANS), and stress–strain isotherms are measured to characterize the rheological properties of the composites. Our analysis allows us to quantify the degree of aggregation of the silica in the matrix, which is studied as a function of pH (4–10), silica volume fraction (3–15%) and silica bead size (average radius 78 and 96 Å). Rheological properties of the samples are represented in terms of the strain-dependent reinforcement factor, which highlights the contribution of the filler. Combining the structural information with a quantitative analysis of the reinforcement factor, the aggregate size and compacity (10–40%) as a function of volume fraction and pH can be deduced. In a second, more explorative study, the grafting of polymer chains on nanosilica beads for future reinforcement applications is followed by SANS. The structure of the silica and the polymer are measured separately by contrast variation, using deuterated material. The aggregation of the silica beads in solution is found to decrease during polymerization, reaching a rather low final aggregation number (less than ten).

Rheological studies of fumed silica–polydimethylsiloxane suspensions

In this study, dynamic mechanical measurements and transmission electron microscopy are used to examine the behavior of fumed silica/polydimethylsiloxane (PDMS) suspensions. The work focuses on three aspects: (i) the modulation of the interactions between silica and PDMS through a controlled silylation of the silica surface; (ii) the effect of the process used to graft the silica surface; (iii) the effect of the silica volume fraction used in the dispersions. A relation between the silica grafting ratio, the aggregate size and the rheological properties has been established and the results have interpreted through a model of fractal clusters.