Sol–gel transition and rheological properties of silica nanoparticle dispersions

Possible variants of the rheological behavior of silica model dispersions have been analyzed. Different types of interaction between the particles and a dispersion medium make it possible to obtain different systems from low-viscosity sols to gels. Proton-donor (water) and aprotic (dimethyl sulfoxide) media have been used for comparison. Dispersions in the aprotic medium behave as non-Newtonian viscous fluids exhibiting shear thinning or shear thickening depending on deformation rate. Aqueous dispersions are viscoelastic and viscoplastic objects that exhibit the shear thickening at stresses higher than the yield stress. The introduction of small amounts of poly(ethylene oxide) into the organic dispersion medium initiates gelation. An increase in the polymer content in the dispersion medium above the concentration corresponding to the formation of a macromolecular network promotes an increase in stiffness and strength of the gels. The rheological behavior of gels is influenced by the polymer molecular mass and its affinity for a solvent.     

Shear rheology of micro-fibrillar cellulose aqueous suspensions

Micro-fibrillar cellulose aqueous suspensions with different fiber lengths were prepared by mechanical refining of softwood pulp fiber suspensions at different specific refining energies. Effects of refining energy level, micro-fiber concentration and temperature on the rheological properties of these aqueous suspensions were studied. These microfibers form a three-dimensional network, which displays typical shear-thinning behavior with little thixotropic tendency, at concentrations as low as 0.5 wt%. A viscoelastic analysis showed that these micro-fibrillar cellulose suspensions at different concentrations (from 0.5 to 2 wt%) exhibit a viscoelastic gel-like behavior [G′ > G″ over an extended range of frequencies (ω) and a weak dependency of G′ on ω] at 25 °C. The storage modulus, G′, at 1 rad/s increased strongly upon increasing concentration from 0.5 to 2 wt% following a power law with an exponent of 3.2. However, increasing the temperature decreases the storage modulus, G′, due to weakening or disruption of intermolecular interactions at elevated temperatures. The viscoelastic behavior changes to liquid-like, with G″ > G′ at the investigated frequency range, for the suspensions at 85 °C.     

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.     

不同分子量可德胶水悬浮液的粘弹性研究

采用动态粘弹性测量研究了不同分子量的生物大分子可德胶 (Curdlan)水悬浮液 (ASC)的流变学特性 .室温下观察到ASC具有弹性 (Solid like)行为 ,储能模量G′在测量范围内轻微依赖频率 ,而损耗模量G″和损耗角正切tanδ存在最小值 .ASC粘弹性随可德胶分子量和浓度的增加而增强 .ASC的流动特性符合Herschel Bulkley模型 ;其弹性行为可以通过渗流理论的标度弹性模型来描述 .网络结构是由于可德胶颗粒聚集或絮凝而形成的 ,当可德胶含量超过临界浓度cs=0 3 %时 ,弹性模量G′与可德胶浓度存在标度关系G′=Goεt,其中标度指数t=2 5 4.     

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.     

Effects of liquid crystalline and shear alignment on the optical properties of cellulose nanocrystal films

Rheo-optics, microspectrophotometry, and optical contrast measurements were used to gain new insights into the interrelated effects of liquid crystalline phase behavior, flow alignment, and microstructural relaxation on cellulose nanocrystal (CNC) films’ alignment and optical properties. Optical contrast measurements were found to be an effective and facile way of determining changes in anisotropy directly from cross-polarized microscopy images. This method was used to continuously measure microstructural relaxation after the cessation of shear as well as the anisotropy of dried CNC films. Aqueous liquid crystalline CNC dispersions showed greater alignment after shear than isotropic or biphasic dispersions. However, CNC gels exhibited lower alignment at equivalent shear rates. The combination of greater initial alignment and slower relaxation of sheared liquid crystalline dispersions resulted in the most optically anisotropic films. Depending on their thickness, the CNC films were optically transparent in the visible regime or exhibited tunable interference colors. The results of this work highlight the tunability of CNC dispersion processing for producing color filters and other optical materials with controlled properties.     

Rheological behavior and particle suspension capability of guar gum: sodium tetraborate decahydrate gels containing cellulose nanofibrils

Guar gum (GG) fracturing fluids were studied by incorporating cellulose nanofibrils (CNFs) in anhydrous borax crosslinked guar gum gels. To fully understand the impact of CNF on the proppant suspension capability of developed fracturing fluids, their shear rate-dependent viscosity and viscoelasticity were investigated. The shear rate dependencies of fluids was fitted to the Carreau model. The zero shear rate viscosity and elasticity of fracturing fluids increased significantly by incorporating CNF in guar gum gels. On the other hand, the viscosity at high shear rates (>100 s^?1) decreased as desired. The proppant settling velocities through fracturing fluids were evaluated by modeling the terminal falling velocity of proppants moving through a Carreau model fluid. The experimental results of the rheological behavior and the modeling results of the proppant settling rate indicated that the fracturing fluids containing CNF had better suspension capabilities. In addition, the lower viscosities of CNF formulated GG gels at higher shear rates will make them more pumpable.     

Properties of carrageenan gels with immobilized lysozyme

Absract  Rheological properties of 3% carrageenan gels formed in 0.4 M sodium chloride solution in the presence of lysozyme are studied in detail. It is shown that the addition of protein results in an increase in the gel-sol transition temperature by 2°C (transition temperature of 3% gel is 48°C, lysozyme concentration is 0.5 mg/ml). Based on the frequency dependences of dynamic moduli at various temperatures, it is revealed that systems possess viscoelastic properties at low frequencies. Within a wide frequency range up to gel-sol transition temperature, systems become elastoviscous and, at higher frequencies, they demonstrate forced transition to glassy state. It is shown that carrageenan inhibits enzyme activity of lysozyme. The interaction between enzyme and carrageenan leads to changes in lysozyme conformations, i.e., the content of α-helices increases and that of turns decreases. It is demonstrated for the first time that, in the presence of a so-called nonspecific (for the gelation of carrageenan) sodium ion, it is possible to prepare gels with the necessary structure and rheological properties and gel-sol transition temperature. These gels can release a lysozyme under the conditions of transmucosal prolonged delivery. Original Russian Text ? G.P. Yampol’skaya, A.A. Elenskii, N.V. Pan’kina, B.N. Tarasevich, V.G. Kulichikhin, 2009, published in Kolloidnyi Zhurnal, 2009, Vol. 71, No. 2, pp. 275–284.     

Rheological properties of mixed gels made of microparticulated whey proteins and β-lactoglobulin

Microparticulated whey proteins (MWP) in suspension or included in a globular protein network revealed that the degree of flocculation was important in the final structure of the material. The MWP dispersions developed an antithixotropic character when low shear rate was applied to the system even at low temperature. Similar results were obtained on globular protein solutions and could be interpreted in terms of an orthokinetic aggregation. MWP participated to the formation and reinforcement of heat-induced β-lactoglobulin (BLG) gels. The mechanical properties of mixed gels demonstrated synergestic effects at low MWP massic fraction (BLG:MWP ratio higher than 2:1). A possible interpretation would be that both interaction of MWP with the network and flocculation of the MWP inside the pores could occur. However, increasing the fraction of MWP induced heterogeneities in the protein network. In particular, micro-phase separation due to steric hindrance effects or thermodynamic incompatibility between the two components could occur and decrease viscoelastic properties of mixed gels.     

Viscoelastic behavior of epoxy prepolymer/organophilic montmorillonite dispersions

Dispersions of a bisphenol A‐based epoxy resin with an organophilic montmorillonite (Nanofil 919) were studied by X‐ray diffraction and oscillatory shear rheometry. X‐ray studies reveal that the clay is intercalated by the epoxy and forms stable dispersions. The viscoelastic behavior of the nanodispersions was measured as a function of the Nanofil concentration and temperature. An increase in both G′ and G″ moduli was detected as the concentration increases. Furthermore, a transition from a liquid‐like behavior, at low temperatures, to a solid‐like behavior, at higher temperatures, was observed for all the samples. This transition is accounted for the formation of a percolated structure of interconnected tactoids through hydrophobic interactions. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 1837–1844, 2008     

Studying the Interaction of Xanthan Gum and Pectin with Some Functional Carbohydrates on the Rheological Attributes of a Low-Fat Spread

Effects of xanthan gum (XG) (0.1 wt%) and pectin (PE) (0.5 wt%) alone and in combination with different concentrations (0.2 and 0.4 wt%) of locust bean gum (LBG), modified starch (MS), and Na-alginate (ALG) on some of the rheological characteristics of low-fat spreads, including flow behavior curves, rheological modeling, apparent viscosity, rheological modules (storage modulus (G′) and loss modulus (G″)), and delta degree (G″/G′) were studied. Results showed the power-law model was better than the Herschel–Bulkley model to describe the flow curve of dispersions. The k-value in the power-law model increased with increase in biopolymers concentration in solution. All samples exhibited shear-thinning flow behavior with a low yield stress. Dynamic oscillatory shear test showed that the spreads had a viscoelastic solid behavior with a gel-like structure. The G′ value was increased by increasing frequency from 0.03 to 15 Hz, while the G″ and G″/G′ values decreased. Also, MS in combination with XG and PE led to increase the G′ values of spreads in comparison with ALG and LBG. Moreover, microstructural and stability observations revealed that the spreads prepared with 0.1% XG-0.2% LBG significantly had the highest oiling out.     

Dynamic behavior of ultrahigh molecular weight poly(methyl methacrylate) in semidilute solutions

The paper presents some rheological investigations on ultrahigh molecular weight (u.h.m.w.) (M_w > 10⁷) poly(methyl methacrylate) in semidilute solutions. The main interest was to study the viscoelastic behavior of the semidilute solutions at different concentrations and temperatures. In the 60‐600 rad/s frequency range, the experimental data show a predominantly elastic response (G′ > G″) for the long poly(methyl methacrylate) chains in toluene.     

Viscoelastic and shear viscosity studies of colloidal silica particles dispersed in monoethylene glycol (MEG), diethylene glycol (DEG), and dodecane stabilized by dodecyl hexaethylene glycol monoether (C12E6)

Silica dispersions stabilized by a nonionic surfactant, dodecyl hexaethylene glycol monoether (C 12E 6), were studied using rheological measurements. The viscosity-shear rate flow behavior of silica in monoethylene glycol (MEG) is shear thinning at low shear rates, leading to a Newtonian plateau at high shear rates for all dispersions studied. All rheological properties showed an increase above a critical surfactant concentration. The dispersions were stable at low levels of C 12E 6 concentrations because of electrostatic repulsions as deduced from the zeta potentials of silica that were on the order of about -30 to -65 mV in monoethylene glycol (MEG). Instability on further addition of C 12E 6 to the silica particles, a phenomenon normally obtained with high-molecular-weight polymers, was observed in MEG. Viscoelatic measurements of silica in monoethylene glycol at various surfactant concentrations showed a predominantly viscous response at low frequency and a predominantly elastic response at high frequencies, indicative of weak flocculation. Instability is explained in terms of hydrophobic and bridging interactions. Restabilization observed at high surfactant concentration was due to the steric repulsion of ethoxy groups of micellar aggregates adsorbed on silica particles. The study also revealed that the presence of trace water introduced charge repulsion that moderated rheological measurements in glycol media and introduced the charge reversal of silica particles in dodecane.     

Linking Semiconductor Nanocrystals into Gel Networks through All‐Inorganic Bridges

For colloidal semiconductor nanocrystals (NCs), replacement of insulating organic capping ligands with chemically diverse inorganic clusters enables the development of functional solids in which adjacent NCs are strongly coupled. Yet controlled assembly methods are lacking to direct the arrangement of charged, inorganic cluster‐capped NCs into open networks. Herein, we introduce coordination bonds between the clusters capping the NCs thus linking the NCs into highly open gel networks. As linking cations (Pt²⁺) are added to dilute (under 1 vol %) chalcogenidometallate‐capped CdSe NC dispersions, the NCs first form clusters, then gels with viscoelastic properties. The phase behavior of the gels for variable [Pt²⁺] suggests they may represent nanoscale analogues of bridged particle gels, which have been observed to form in certain polymer colloidal suspensions.     

Specific swelling behaviors of alkali‐metal poly(styrene sulfonate) gels in aqueous solvent mixtures

Counterion‐ and solvent‐specific swelling behaviors were investigated for alkali‐metal poly(styrene sulfonate) (PSSM) gels having different degrees of sulfonation in aqueous organic solvent mixtures [water plus methanol, ethanol, 2‐propyl alcohol, t‐butyl alcohol, dimethyl sulfoxide (DMSO), acetone, acetonitrile, tetrahydrofuran, or dioxane]. With an increasing organic solvent concentration, most gel systems, except for DMSO, showed a volume phase transition. The transition abruptly occurred without significant deswelling in the lower solvent concentration region. Such swelling behavior contrasted with that of other common charged gel systems, including alkali‐metal polyacrylate (PAAM) gels, which showed gel collapse after gradual deswelling with an increasing organic solvent concentration. The dielectric constant at the critical transition point (D_cr) for most mixed solvent systems decreased in the order of PSSK ≥ PSSCs ≥ PSSNa > PSSLi; that is, larger counterion systems were favorable for the transition. The counterion specificity also contrasted with our previous results for PAAM gels: PAANa > PAAK > PAALi ～ PAACs. On the other hand, the solvent specificity for the PSSM gels was similar to that for the PAAM gels; the higher the dielectric constant was of the organic solvent, the higher the D_cr value was at which the transition occurred. These specificities were examined on the basis of the solvation properties of the counterions and polymer charged groups and the solvent properties such as the Gutmann–Mayer donor number and acceptor number. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 1166–1175, 2007     

Strain hardening and fracture of heat-set fractal globular protein gels

Non-linear mechanical behavior at large shear deformation was been investigated for heat-set beta-lactoglobulin gels at pH 7 and 0.1 M NaCl using both oscillatory shear and shear flow. These gels have a self-similar structure at length scales smaller than the correlation length of the gel with fractal dimension d(f)=2. Strain hardening is observed that can be well described using the model proposed by Gisler et al. [T.C. Gisler, R.C. Ball, D.A. Weitz, Phys. Rev. Let. 82 (1999) 1064] for fractal colloidal gels. The increase of the shear modulus normalized by the low strain value (G(0)) is independent of G(0). For weak gels the elasticity increases up to a factor of ten, while for strong gels the increase is very small. At higher deformation irreversible fracture occurs, which leads eventually to macroscopic failure of the gel. For weak gels formed at low concentrations the deformation at failure is about 2, independent of the shear modulus. For strong gels fracture occurs at approximately constant stress (2 x 10(3) Pa).     

Structure and flow behavior of crosslinked ethyl acrylate–methacrylic acid copolymer dispersion particles

 Changes in viscosity, pH and static light scattering behavior on alkali addition of model dispersions of statistical copolymers of ethyl acrylate with 15 and 50 wt% of methacrylic acid prepared by semicontinuous emuslion copolymerization and crosslinked with various amount of N,N′-methylenebisacrylamide were investigated. It was found that about 1% of crosslinking agent was sufficient to prevent unpredictable disintegration of dispersion particles and gauranteed reproducible flow behavior of alkalinized dispersions. The viscosity of alkalinized dispersions of crosslinked particles at low concentration is controlled by the effective hydrodynamic volume of swelled particles which decreases with the crosslinking degree of copolymer. On the other hand, at higher particle concentration close to critical concentration at which the space is filled up by swelled particles the influence of particle interaction prevails and due to lower deformability of crosslinked particles viscosity increases with a crosslinking degree. In comparison with similar dispersions prepared without a crosslinking agent these results indicated much uniform structure of dispersion particles and suppression of influence of random crosslinking due to chain transfer reactions. The study shows that dispersions of crosslinked copolymers of this type could be considered as thickners of latex binders with stable and reproducible thickening properties. Received: 23 February 1998 Accepted: 11 June 1998     

Characterization of MWCNT/TPU systems by large amplitude oscillation shear

Interest in polymeric materials with dispersed nanotubes has increased in recent years. There are several methods to characterize this kind of dispersions that may be based on evaluating the percolation concentration, the “goodness” of the dispersion, or the matrix-nanotube interphase. Among other techniques, rheology and conductivity are used to this aim. Commonly, the oscillatory rheology measurements are performed within the linear viscoelastic range, which is achieved by operating at small amplitude oscillation shear. Nevertheless, these measurements do not fully describe the behavior of the dispersion structure. In this work, we propose the use of medium amplitude oscillation shear and large amplitude oscillation shear to characterize the dispersion/structure of a thermoplastic polyurethane matrix filled with multiwalled carbon nanotubes. The Ewoldt framework mathematical approach is used to analyze the non-linear stress response. That approach allows obtaining physically grounded magnitudes from the experimental data. These magnitudes allow for a better understanding of the effects of the filler content.     

Preparation and rheological characterization of poly(n-butyl methacrylate)/montmorillonite composites

Intercalated nanocomposites constituted of poly(butyl methacrylate) (PBMA) as the matrix and an organically modified montmorillonite as the nanosize filler were prepared and rheologically characterized in detail. The rheological behavior of the composites showed dependence on both temperature and clay content. For composites of low clay contents, the steady shear viscosity showed a Newtonian plateau in the low shear rate region at low temperatures and the plateau was replaced by a shear-thinning curve when the temperature was raised. For composites of higher clay contents, strong shear-thinning behavior were observed at all shear rates and all temperatures. The viscoelastic data of the composites showed unusual terminal behavior of a decreasing terminal slope at low frequencies with increasing temperature and clay loading. X-ray diffraction spectra showed that annealing process at higher temperatures shifted the Bragg reflection peaks to a lower angle and broadened the peaks, which provided the evidence for the existence of a temperature-induced solid-like structure that was responsible for the shear thinning and the unusual terminal viscoelastic behavior.     

Dopamine Self-Polymerization as a Simple and Powerful Tool to Modulate the Viscoelastic Mechanical Properties of Peptide-Based Gels

Dopamine is a small versatile molecule used for various biotechnological and biomedical applications. This neurotransmitter, in addition to its biological role, can undergo oxidative self-polymerization to yield polydopamine, a robust universal coating material. Herein, we harness dopamine self-polymerization to modulate the viscoelastic mechanical properties of peptide-based gels, expanding their ever-growing application potential. By combining rapid peptide assembly with slower dopamine auto-polymerization, a double network gel is formed, where the fibrillar peptide gel network serves as a scaffold for polydopamine deposition, allowing polydopamine to interpenetrate the gel network as well as establishing crosslinks within the matrix. We have shown that triggering the assembly of a lysine-rich peptide gelator in the presence of dopamine can increase the mechanical rigidity of the resultant gel by a factor of 90 in some cases, while retaining the gel’s shear thin-recovery behavior. We further investigate how factors such as polymerization time, dopamine concentration and peptide concentration alter the mechanical properties of the resultant gel. The hybrid peptide–dopamine gel systems were characterized using rheological measurements, circular dichroism spectroscopy and transmission electron microscopy. Overall, triggering peptide gelation in the presence of dopamine represents a simple yet powerful approach to modulate the viscoelastic mechanical properties of peptide-based gels.