Effects of Surface Properties on Rheological and Interfacial Properties of Pickering Emulsions Prepared by Fumed Silica Suspensions Pre-Adsorbed Poly(N-Isopropylacrylamide)

The hydrophobic fumed silica suspensions physically pre-adsorbed poly(N-isopropylacrylamide) (PNIPAM) in water could prepare oil dispersed in water (O/W) Pickering emulsion by mixing of silicone oil. The resulting Pickering emulsions were characterized by the measurements of volume factions of emulsified silicone oil, adsorbed amounts of the silica suspensions, oil droplet size, and some rheological responses, such as stress-strain sweep curve and dynamic viscoelastic moduli as a function of the added amount of PNIPAM. Moreover, their characteristics were compared with those of the O/W Pickering emulsions prepared by the hydrophilic fumed silica suspensions pre-adsorbed PNIPAM. For the emulsions prepared by the hydrophobic silica suspensions, an increase in the added amount of PNIPAM led to (1) a decrease in the volume fraction of the emulsified oil in the emulsified phase, (2) both the size of oil droplets and the adsorbed amount of the corresponding silica suspensions being almost constant, except for the higher added amounts, and (3) both the storage modulus (G′) and the yield shear strain being constant. The term of 1 is the same for the emulsions prepared by the hydrophilic silica suspensions, whereas both the adsorbed amount of the corresponding silica suspension and the G′ value increase and both the droplet size and the yield shear strain decrease with an increase in the added amount of PNIPAM. The differences between the rheological properties of the emulsions prepared by the hydrophilic silica suspensions and those by the hydrophobic ones are attributed to the hydrophobic interactions of the flocculated silica particles in the Pickering emulsions.     

Dispersion stabilities and rheological properties of fumed silica suspensions

Herein, we have reviewed fumed silica suspensions in dispersing fluids, polymer melts, and polymer solutions, focusing on their dispersion stability and rheological properties as a function of the surface character of fumed silica powders and the silica volume fraction, ?. Hydrophilic fumed silica powders are well dispersed at ? < 0.01 in polar dispersing fluids or polar polymer melts, and their phase states change from sol to gel with increasing ?. Such changes should also be strongly related to the rheological responses of the hydrophilic fumed silica suspensions, which change from Newtonian flow behavior to gel-like elasticity with increasing ?. On the other hand, hydrophobic fumed silica powders are stabilized in both polar and nonpolar dispersing fluids, depending on the interactions between the surface hydrophobic moieties and the dispersing fluids, in addition to those between the residual surface silanol groups and dispersing fluid, except for the particle–particle interactions. Moreover, the effects of the adsorption and desorption of polymers, as well as of non-adsorbing polymers on the dispersion stability and rheological behavior of fumed silica suspensions are discussed, by taking account of their optical microscopic observation and SANS curves.     

Shear-Induced Changes in Rheological Reponses and Neutron Scattering Properties of Hydrophobic Silica Suspensions at Low Silica Concentrations

Rheological responses of hydrophobic fumed silica powders, whose surface silanol groups were modified by hexadecane, suspended in 1,4-dioxane at lower silica concentrations than 6.8 vol% have been investigated as a function of the silica concentration. Transient shear stress behavior before attaining the steady-state shear stress could be classified into three regimes as follows, irrespective of the silica concentration: at the lower shear rates than ca. 0.3 s^?1 a stress overshoot was observed, at the shear rate ranges from 0.3 to 30 s^?1 sustained oscillations in shear stresses were exhibited and these oscillations were first observed for the suspensions at the low particle concentrations, and beyond the shear rate of 30 s^?1 a sigmoid decrease of the shear stress with increasing time, that is, structural breakdown, was observed. At the steady state the silica suspensions showed shear thickening. Small angle neutron scattering (SANS) measurements of the silica suspension under shear flow provided that changes in the SANS intensities were well correlated with the shear thickening behavior. However, shear thinning behavior at higher shear rates did not cause any changes in the SANS intensities.     

Electromechanical Responses of a Crosslinked Polydimethylsiloxane

A high molecular weight polydimethylsiloxane, PDMS, gel was prepared and investigated as an electroactive polymer actuator. Electromechanical properties of the PDMS gels were measured under an oscillatory shear mode at the temperature of 27 °C to determine the effects of crosslink ratio and electric field strength. The storage modulus, G′, of PDMS gel increases linearly with crosslink density but nonlinearly with electric field. The increase in the storage modulus with crosslink density is due to the increase in the number of junction points and strands. With increasing electric field strength, the storage modulus increases as the electric field induces dipole moments generating the electrostatic forces within the matrices. The gel with the crosslink ratio of 0.01 possesses the highest G′ sensitivity of 41% at 2 kV/mm. The temporal response of PDMS gels upon repeated applications of electric field strength of 2 kV/mm was investigated. For the crosslink PDMS (N_c/N_m = 0.01) system, at the electric field of 2 kV/mm, G′ immediately increases and rapidly reaches a steady-state value. With electric field off, G′ decreases and nearly recovers its original value. The crosslinked PDMS (N_c/N_m = 0.01) is nearly a reversible system. Finally, we investigated the bending response of the PDMS films, suspended in silicone oil between copper electrodes. From the deformation data, we estimated the dielectrophoresis force, F_D, to be a linear function of electric field strength.     

Rheological Properties of Silica Particle Suspensions in Mineral Oil

The rheological properties of particles suspended in a non‐polar mineral oil have been investigated as a function of volume fraction of particles, particle size, surface properties and shear rate. Three different types of particles were investigated; glass microspheres, monodisperse silica particles and fumed silica. The suspensions showed shear thinning behavior at higher volume fractions, and the viscosity increased with decreasing particle size. The hydrophobic particles display lass shear thinning effects. The relative viscosity of all the suspensions was well fitted to the Krieger and Dougherty model.     

Rheological properties of polyurethane adhesives containing silica as filler: influence of the nature and surface chemistry of silica

Four silicas, two fumed silicas (one hydrophilic and one hydrophobic) and two precipitated silicas (one hydrophilic and one hydrophobic), were added as filler to solvent‐based polyurethane (PU) adhesive formulations. In general, the addition of silica increased the viscosity, the storage and loss moduli of the PU adhesives but only the hydrophilic fumed silica exhibited pseudoplasticity and thixotropy. The rheological properties imparted by adding filmed silicas to PU adhesive solutions were more noticeable than that of precipitated silicas. Interactions between the hydrophilic fumed silica, the polyurethane and/or the solvent seemed to be responsible for the improved rheological properties of filled PU adhesives.     

SiO2/PEG分散体系动态剪切流变行为

在动态应变条件下, SiO2/PEG200(聚乙二醇, 平均分子量为200)分散体系出现了剪切增稠现象. 剪切流变实验表明, 在两种情况下都出现了剪切增稠: 一种是在不同的恒定频率下应变扫描, 在临界应力γc出现的剪切增稠; 另一种是恒定的应变(γ0=500%)条件下频率扫描, 在临界频率棕c抑10 rad·s-1出现的剪切增稠. 在不同的恒定频率应变扫描条件下, 实验研究了储能模量(G’)和耗能模量(G’’)与应变的关系, 同时初步探讨了应变与不同恒定频率的函数关系. 在线性粘弹性区域内, G’和G’’满足G’∝ω0.57和G’’∝ω0.7指数关系. 在恒定的应变条件下, 发现模量和复数粘度与扫描频率具有强烈的依赖关系, 这些现象可以定性地通过“粒子簇”理论来解释.“粒子簇”理论认为这种剪切增稠的发生是由于形成了亚稳定、流动所导致的“粒子簇”, 使得粘度上升.     

Thermal Destruction of Polydimethyl-Siloxane on a Phosphorus-containing Silica Surface

Thermogravimetry, differential thermal analysis, and IR spectroscopy were used to investigate the process of thermal destruction of adsorbed polydimethylsiloxane (PDMS) in air. The disperse adsorbents were pristine fumed silica and modified fumed silica whose surface contained oxygen compounds of phosphorus. It was shown that under the given experimental conditions the thermal destruction of PDMS on the fumed silica surface was accompanied by the complete transformation of the adsorbed PDMS to SiO₂. In the case of phosphorus-containing silica, the thermal destruction proceeded in a different way. It was found that at 140–300°C depolymerization of the siloxane chains of a certain part of the adsorbed polymer took place with the concurrent removal of volatile products of the reaction. However, the remaining part of the adsorbed PDMS interacted with the modified silica surface to form chemisorbed dimethylsilyl structures. The thermal destruction of the chemisorbed fragments of PDMS in air was initiated at 400°C or above for both types of silica investigated. This revised version was published online in August 2006 with corrections to the Cover Date.     

Rheological Properties of Silica Suspensions in Aqueous Solutions of Block Copolymers and Their Water-Soluble Components

Dynamic moduli of fumed silica suspensions in aqueous solutions of poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO) block copolymers and PEO homopolymers were measured as a function of surface coverage. Since the block copolymers and PEO are adsorbed on the silica surface through hydrogen bonding between the ether oxygen and the silanol group on the silica surface, the interaction between the silanol groups, which is dominant for the aggregation of silica particles, should be prohibited. Dynamic moduli in the silica suspensions were strongly related to the stability of the silica suspensions and the block copolymer, and the longest PEO portion was useful for stabilizing the silica particles. However, the PEO homopolymer did not support stability of the silica particles, suggesting that chain conformation of the PEO portion in the block copolymer is different from that for the PEO homopolymer. Copyright 2001 Academic Press.     

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.     

Generation of chemisorbed benzyl radicals on silica nanoparticles

Functionalized silica nanoparticles (NP) were obtained by esterification of the silanol groups of fumed silica nanoparticles with benzyl alcohol. These particles were characterized by Fourier transform infrared spectroscopy, ¹³C and ²⁹Si NMR spectroscopy, thermogravimetry, total organic carbon, Brunauer–Emmett–Teller analysis, UV-visible spectroscopy, and transmission electron microscopy. NP suspensions in water/acetonitrile mixtures were used as quenchers of benzophenone (BP) phosphorescence in time-resolved experiments at the excitation wavelength of 266 nm. The phosphorescence signals obtained in the presence of the nanoparticles were fitted to biexponential decays. Both decays were accelerated in the presence of increasing amounts of NP. A model, including the reversible adsorption of BP on the NP, which was supported by computer simulations accounts for the observed results. Laser flash-photolysis experiments with excitation at 266 nm of NP suspensions in water/acetonitrile in the presence of BP generated benzyl radicals that were attached to the silica surface. These radicals were detected at their absorption maxima (320 nm) by transient optical techniques.     

二氧化硅分散体系在应力剪切过程中粘弹性及能耗研究

通过动态应力剪切研究了以乙二醇、丙二醇和丁二醇为分散介质的雾化二氧化硅分散体系的粘弹性以及能耗. 研究发现, 随着应力的增大, 体系都经历了线性粘弹区、剪切变稀区以及剪切增稠区. 在线性粘弹区, 储能模量(G′)、耗能模量(G′′)随着应力(σ)的增大保持不变；在剪切变稀区, G′随着σ的增大而减小, 且乙二醇、丙二醇、丁二醇分散体系的减小幅度依次递减, 而G′′基本保持不变；在剪切增稠区, G′、G′′都随着σ的增大而增大. 在所研究的应力范围内, G′′都大于G′, 体系主要体现粘性, 消耗能量为主. 同时还发现在低剪切应力区, 体系所消耗的能量(Ed)都随着最大应变(γ0)成二次方关系增长, 而在剪切增稠区, 当n=2.79、4.93、4.88时, EG/SiO2、PG/SiO2、BG/SiO2的Ed分别随γ0以指数关系增长.     

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.     

Linear Viscoelastic Behavior of Multiphase Dispersions

The linear viscoelastic behavior of polymer-thickened oil-in-water emulsions, polymer-thickened solids-in-liquid suspensions, and their blends is investigated using a controlled-stress rheometer. The emulsions exhibit a predominantly viscous behaviour at low values of oil concentration in that the loss modulus (G") exceeds the storage modulus (G') over most of the frequency range. At high values of oil concentration, the emulsions exhibit a predominantly elastic behavior. The ratio of storage modulus to loss modulus (G'/G") increases with the increase in oil concentration. Emulsions follow the theoretical model of J. F. Palierne (1990, Rheol. Acta 29, 204) only at low values of oil volume fraction (/=G' over most of the frequency range. The ratio G'/G" varies only slightly with the increase in solids volume fraction. The Palierne model describes the linear viscoelastic properties of suspensions accurately only at low values of solids volume fraction. At high values of solids concentration, the Parlierne model underpredicts the linear viscoelastic properties of suspensions and the deviation increases with the increase in solids concentration. The blends of emulsions and suspensions exhibit strong synergistic effects at low to moderate values of frequencies; the plots of blend modulus versus emulsion content exhibit a minimum. However, at high values of frequency, the blend modulus generally falls between the moduli of pure suspension and pure emulsion. The high-frequency modulus data of blends of emulsions and suspensions are successfully correlated in terms of the modulus ratio versus volume fraction of solids, where modulus ratio is defined as the ratio of blend modulus to pure emulsion modulus at the same frequency. Copyright 2000 Academic Press.     

Effects of Fumed Silica on the Viscosity of Acrylic Latex System

Effects of fumed silica on the viscosity of acrylic latex system were studied by recording curves of the shear complex viscosity over shear rate and temperature and by measuring apparent viscosity with a variety of fumed silica concentrations at various steady angular velocities. Results show that the fumed silica obviously affects the shape and value of curves on the amplitude of complex viscosity versus either shear rate or temperature diagrams. Both dynamic and steady shear measurements illustrate a shear thinning phenomena for the acrylic latex filled with fumed silica. Dependence of the apparent viscosity on the fumed silica concentration can be satisfactorily modeled by the Krieger‐Dougherty expression. A fitted parameter that quantitatively describes the sensitivity of the effect of fumed silica on the acrylic latex system increases with angular velocity, but decreases with latex solid content.     

INS and IR study of intermolecular interactions at the fumed silica-polydimethylsiloxane interphase, Part 3. Sicica-siloxane adsorption complexes

A vibration spectra study of the interphase fumed silica andpolydimethylsiloxane (PDMS) particles highlights the microscopic interactionmechanisms in the system. This study involves an extended examination of bothfree silica and PDMS components as well of their adsorption complexes. Thepresent paper, deals with the vibrational analysis of adsorption complexes ofpolymethylsiloxanes and hydrophilic or trimethylsiloxy silylated fumed silica.Vibrations have been investigated involving both experimental andcomputational approaches. Experiments were performed in the framework ofinfrared (FTIR) and inelastic neutron scattering (INS and AWDS) techniques.Spectral interpretation was carried out on the basis of quantum chemical (QCh)semiempirical calculations using AM1 and PM3 parameterisation, and applicationof an original software program for spectra analysis, COSPECO, involvingdirect and inverse spectral problem solutions and spectrum assignments. Indescribing the silica-PDMS interphase, dodecamethylpentasiloxane (Si5) hasbeen used as a linear model of polydimethylsiloxane; its adsorption atdifferent surface coverages on hydrophilic and trimethylsiloxy(TMS)-silylated fumed silicas has been studied. Both, the PDMS and silicacomponents in the adsorption complexes, have been examined using protonatedand deuterated methyl groups of substrate and adsorbate. Additionalinformation was evaluated from the adsorption of longer chain PDMS polymerson hydrophilic silica. The adxorption has a strong impact on the vibrationspectra of both the silica and siloxane. Additional impacts evolve from thedegree of silylation of the silica surface, and the siloxane coverage . The changes in the spectra can be attributed to immobilisation of theadsorbed siloxane on the silica surface, as well as to a `pressure' effect ofthe TMS units of the silica caused by adsorbed PDMS. In terms of interactionenergies, classical H-bonding shows a minor contribution, while Van-der-Waalsforces mainly govern the system; for the latter Si-O dipole interactions alsopay a dominant role.     

Subsurface investigation of the surface modification of polydimethylsiloxane by 172‐nm vacuum ultraviolet irradiation using ToF‐SIMS and VUV spectrometry

We investigate the mechanism of polydimethylsiloxane (PDMS) surface modification by 172‐nm vacuum ultraviolet (VUV) light. Time‐of‐flight secondary ion mass spectrometry and optical spectrometry are used to measure the chemical composition and VUV transmittance of the PDMS before and after surface modification, respectively. For modified samples of bulk PDMS, the VUV transmittance and the depth of the modified region increased with increasing VUV dose. This can be explained by the following self‐reinforcing cycle of (1) modification of PDMS by VUV light to a more silica‐like composition, (2) improvement of the VUV light transparency, and (3) deeper modification. For thin‐film samples of PDMS formed on sapphire substrates, the transmittance at 172 nm also increased with increasing VUV dose and exceeded that of sapphire in the region from 172 to 300 nm. Finally, thin‐film samples of PDMS formed on silicon substrates, which function as a VUV reflector, were also investigated. For these samples, the secondary ion depth profiles for several chemical species in the PDMS were oscillatory, probably due to the interference of the incident and reflected VUV light. These results strongly suggest that the photon energy of the VUV light plays an important role in modifying PDMS.     

Surface‐dependent effect of functional silica fillers on photocuring kinetics of hydrogel materials

Two types of silica: precipitated (P, prepared in non‐polar media, a new type, submicrometer sized) and fumed (F, nanosized), both unmodified and surface modified are investigated as functional fillers for potential applications in nanocomposites with poly(2‐hydroxyethyl methacrylate) matrix. Special attention is paid to the kinetics of composite formation in an in situ photopolymerization process. Silica‐containing formulations polymerize faster; this effect is much stronger for silica P having much larger particle size than silica F. Surface treatment leads to further acceleration of the polymerization in case of silica P but to retardation in case of silica F; the effect of modification of the filler surface on properties of composites is different for each of the silicas. The obtained results are discussed in terms of effects of curvature of silica particles, surface properties, solvation cell, interphase region, viscosity changes, and morphology of the resulting composites. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014 , 52, 3472–3487     

The effects of ultrasonic irradiation on the preparation and properties of Ormosils

Organically modified silicates (Ormosils), derived from silica and polydimethylsiloxane (PDMS), have been prepared by the sol-gel method with ultrasonic irradiation. Tetraethoxysilane (TEOS), PDMS, water and HCl were mixed and exposed to 20 kHz ultrasonic waves under various conditions. Ultrasonic irradiation shortened the gelation time and made it possible to obtain gels without the addition of solvent (e.g. alcohol and tetrahydrofuran (THF)). The reaction mechanism of the ultrasonic irradiated Ormosil solution was investigated by liquid state ²⁹Si NMR. The densities of the gels prepared under ultrasonic irradiation were found to be much higher than those without irradiation. The effect of solvent addition on the properties of gels was also investigated.