Delong Xie

Criticality for shear-induced gelation of charge-stabilized colloids

Colloidal systems that are well stabilized by electrostatic repulsive forces can be activated by intense shear flow and transformed into solid-like gels, without adding any electrolyte. We have experimentally quantified the critical particle volume fraction for such a transition and found that it is a function of primary particle radius and shear rate. In particular, the values of the critical particle volume fraction obtained under different conditions can be represented through a single power-law function of the Breakage Number (Br), which is defined as the ratio between the shearing energy and the interparticle bonding energy. This finding indicates that, instead of shear rate or stress, the correct parameter quantifying the criticality for shear-induced gelation is Br. In addition, it is shown that the clusters formed in the shear aggregation process exhibit fractal scaling with fractal dimension equal to 2.4 ± 0.04, independent of Br (i.e., of the shear stress, the particle size and the interparticle bonding energy). This is similar to the case of quiescent systems where the Brownian motion-induced aggregations, i.e., diffusion-limited and reaction-limited cluster aggregations, lead to clusters with fractal dimension equal to 1.8 ± 0.05 and 2.1 ± 0.05, respectively, which are independent of the particle type and size and the electrolyte concentration. Moreover, the ratio between the radius of gyration of clusters constructing the gel network and the primary particle radius at criticality decreases as Br increases, following a power-law scaling with exponent of −0.31, which is in good agreement with that for breakup of dense fractal clusters of the same fractal dimension in laminar flow.

Effect of Surfactants on Shear-Induced Gelation and Gel Morphology of Soft Strawberry-like Particles

The role of surfactant type in the aggregation and gelation of strawberry-like particles induced by intense shear without any electrolyte addition is investigated. The particles are composed of a rubbery core, partially covered by a plastic shell, and well stabilized by fixed (sulfate) charges in the end group of the polymer chains originating from the initiator. In the absence of any surfactant, after the system passes through a microchannel at a Peclet number equal to 220 and a particle volume fraction equal to 0.15, not only shear-induced gelation but also partial coalescence among the particles occurs. The same shear-induced aggregation/gelation process has been carried out in the presence of an ionic (sulfonate) surfactant or a nonionic (Tween 20) steric surfactant. It is found that for both surfactants shear-induced gelation does occur at low surfactant surface density but the conversion of the primary particles to the clusters constituting the gel decreases as the surfactant surface density increases. When the surfactant surface density increases above certain critical values, shear-induced gelation and eventually even aggregation do not occur any longer. For the sulfonate surfactant, this was explained in the literature by the non-DLVO, short-range repulsive hydration forces generated by the adsorbed surfactant layer. In this work, it is shown that the steric repulsion generated by the adsorbed Tween 20 layer can also protect particles from aggregation under intense shear. Moreover, the nonionic steric surfactant can also protect the strawberry-like particles from coalescence. This implies a decrease in the fractal dimension of the clusters constituting the gel from 2.76 to 2.45, which cannot be achieved using the ionic sulfonate surfactant.

Effect of primary particle morphology on the structure of gels formed in intense turbulent shear.

We study the effect of primary particle morphology on intense shear-induced gelation without adding electrolytes. The primary particles are composed of a rubbery core grafted with a polystyrene shell. Depending on the shell-to-core mass ratio, the core can be partially covered by the shell, leading to strawberry-like morphology. It is found that at a fixed core mass the fractal dimension of the clusters constructing the gel decreases (i.e., more open cluster structure) as the shell mass increases, until reaching a plateau. The SEM pictures of the gels reveal that the structure variations are due to the occurrence of partial coalescence among particles, which decreases as the shell mass increases. In the region where the fractal dimension reaches a plateau, the coalescence is negligible. The conversion of the primary particles to gels is incomplete and increases as the extent of coalescence decreases. This is related to the fact that the smaller the extent of coalescence, the larger the cluster size. Thus, because of its cubic dependence on the cluster size, the aggregation rate increases as the extent of coalescence decreases, leading to increased conversion. It is therefore evident that the key parameter controlling the gel structure and the particle conversion is the core surface coverage by the shell. To further verify this conclusion, we have carried out the shear-induced gelation of another set of particles with varying core mass. It is found that the only parameter that can well correlate the values of the fractal dimension and particle conversion from the two sets of particles is the core surface coverage.

Synthesis and characterization of core/shell titanium dioxide nanoparticle/polyacrylate nanocomposite colloidal microspheres

Core/shell titanium dioxide (TiO2) nanoparticle/poly(methyl methacrylate-butyl acrylate-methacrylic acid) [P(MMA-BA-MAA)] nanocomposite colloidal microspheres have been successfully synthesized via in situ emulsion polymerization. TiO2 nanoparticles were firstly modified by silane coupling agent, vinyl triethoxysilane (A-151), to increase the dispersibility of TiO2 nanoparticles into the polyacrylate matrix. The A-151-modified TiO2 nanoparticles were characterized by Fourier transform infrared spectra (FTIR), thermogravimetric analysis (TGA), dynamic light scattering (DLS), and transmission electron microscopy (TEM) techniques. The synthesized nanocomposite colloidal microspheres were characterized by TEM, scanning electron microscopy (SEM), energy dispersive X-ray spectrometry (EDS), and differential scanning calorimetry analysis (DSC). The results showed that A-151 coupling molecules was chemically bonded on the TiO2 nanoparticles surface, and the amount of coated A-151 was 3.0 wt%. According to TEM micrographs and DLS results, the dispersibility of modified TiO2 nanoparticles was obviously improved. TEM analysis revealed that an obvious core/shell structure morphology was observed with the core TiO2 particles surrounded by a uniform 10∼15-nm thick polymer shell. SEM–EDS result showed that TiO2 nanoparticles were homogenously monodispersed in the polymer matrix. DSC measurements indicated a glass transition temperature (Tg) enhancement of P(MMA-BA-MAA).

Shear-induced gelation of soft strawberry-like particles in the presence of polymeric P(BA-b-AA) surfactants

The effect of polymeric surfactants, copolymers of n-butyl acrylate (BA) and acrylic acid (AA), on shear-induced gelation of a colloidal system in the absence of additional electrolytes is studied. One random (PBA-co-PAA) and two block copolymer (PBA-b-PAA, with different PAA lengths) surfactants have been synthesized by ATRP and used in this work. The colloidal system is composed of strawberry-like particles with a rubbery core, partially covered by a few grafted plastic patches. In the absence of any surfactant, as the colloidal system passes through a microchannel at a Peclet number of 220 and a particle volume fraction of 0.15, shear-induced gelation occurs and the particles coalesce partially, due to the rubbery core, leading to a fractal dimension of the clusters constituting the gel equal to 2.78. On the other hand, in the presence of any of the three polymeric surfactants, shear-induced gelation occurs only in the range of low surfactant surface density. Meanwhile, the fractal dimension of the clusters decreases with adsorption of the two block surfactants, reaching a plateau value of about 2.58, while for the random surfactant it remains constant and equal to 2.78, like in the absence of any surfactant. This indicates that adsorption of the block surfactants can reduce the particle coalescence, while adsorption of the random surfactant cannot. Moreover, for all three surfactants, as their surface density increases progressively, a transition from solid-like gel to a liquid-like state occurs and finally no shear-induced gelation or even aggregation occurs. Since the three surfactants comprise carboxylic groups, considering also the results in the literature (Zaccone et al., J. Phys. Chem. B, 2008, 112, 1976; 6793), we can reach a general conclusion that carboxylic groups on the particle surface not only stabilize the particles through electrostatic repulsion, but also generate very short-range, strongly repulsive (e.g. hydration, steric) forces, which when high enough protect the particles from intense shear-activated aggregation.

Monitoring coalescence behavior of soft colloidal particles in water by small-angle light scattering

The fractal dimension (Df) of the clusters formed during the aggregation of colloidal systems reflects correctly the coalescence extent among the particles (Gauer et al., Macromolecules 42:9103, 2009). In this work, we propose to use the fast small-angle light scattering (SALS) technique to determine the Df value during the aggregation. It is found that in the diffusion-limited aggregation regime, the Df value can be correctly determined from both the power law regime of the average structure factor of the clusters and the scaling of the zero angle intensity versus the average radius of gyration. The obtained Df value is equal to that estimated from the technique proposed in the above work, based on dynamic light scattering (DLS). In the reaction-limited aggregation (RLCA) regime, due to contamination of small clusters and primary particles, the power law regime of the average structure factor cannot be properly defined for the Df estimation. However, the scaling of the zero angle intensity versus the average radius of gyration is still well defined, thus allowing one to estimate the Df value, i.e., the coalescence extent. Therefore, when the DLS-based technique cannot be applied in the RLCA regime, one can apply the SALS technique to monitor the coalescence extent. Applicability and reliability of the technique have been assessed by applying it to an acrylate copolymer colloid.

Self-association dynamics and morphology of short polymeric PBA-b/co-PAA surfactants

The self-association characteristics of very short and well-defined poly(butyl acrylate)-b-poly(acrylic acid) (PBA-b-PAA) block copolymers in water have been studied. The diblocks are asymmetric with the PBA block longer than the PAA block, giving rise to hollow sphere morphology. This is affirmed by experimental data and theoretical evaluations of the hydrophilic and hydrophobic domain sizes, as well as a value close to 1 for the ratio of the hydrodynamic to the gyration radius of the micelles. Besides, the untypically short PBA blocks (polymerization number around 15) render the micelles dynamic. Indications in support include among others the following: the CMC (critical micellar concentration) values depend, together with the aggregation numbers and the micellar sizes, on the block lengths, as predicted by theory; above the CMC their sizes are concentration-independent, while the micelles disappear below CMC. A comparison was also made with a random PBA-co-PAA copolymer of similar length, which self-associates at an apparent CMC 1 order of magnitude larger than those of the block copolymers, but the size of the formed micelles depends on the concentration.

Optimization of microencapsulation of silane coupling agent by spray drying using response surface methodology

ABSTRACT A microcapsule containing a silane coupling agent was prepared by spray drying. The optimal encapsulating condition was determined by employing a response surface methodology with a central composite design. The effect of emulsion’s solid content and oil concentration, inlet air temperature, and atomizer speed on properties of microcapsule were investigated. The resulting microcapsule containing silane coupling agent and possessing an unbroken surface and a good shape under optimized conditions exhibited an encapsulation efficiency of 82.7%, average size of 21.3 µm, and moisture content of 0.93%. The experimental results agree well with the model prediction, with a relative error below 5.0%. GRAPHICAL ABSTRACT