The current study examined the foaming behavior of poly(vinylpyrrolidone) (PVP)-silica composite nanoparticles. Individually, the two components, PVP and silica nanoparticles, exhibited very little potential to partition at the air-water interface, and as such, stable foams could not be generated. In contrast, combining the two components to form silica-PVP core-shell nanocomposites led to good "foamability" and long-term foam stability. Addition of an electrolyte (NaSO) was shown to have a marked effect on the foam stability. By varying the concentration of electrolyte between 0 and 0.55 M, three regions of foam stability were observed: rapid foam collapse at low electrolyte concentrations, delayed foam collapse at intermediate concentrations, and long-term stability (∼10 days) at the highest electrolyte concentration. The observed transitions in foam stability were better understood by studying the microstructure and physical and mechanical properties of the particle-laden interface. For rapidly collapsing foams the nanocomposite particles were weakly retained at the air-water interface. The interfaces in this case were characterized as being "liquid-like" and the foams collapsed within 100 min. At an intermediate electrolyte concentration (0.1 M), delayed foam collapse over ∼16 h was observed. The particle-laden interface was shown to be pseudo-solid-like as measured under shear and compression. The increased interfacial rigidity was attributed to adhesion between interpenetrating polymer layers. For the most stable foam (prepared in 0.55 M NaSO), the ratio of the viscoelastic moduli, G'/G″, was found to be equal to ∼3, confirming a strongly elastic interfacial layer. Using optical microscopy, enhanced foam stability was assessed and attributed to a change in the mechanism of foam collapse. Bubble-bubble coalescence was found to be significantly retarded by the aggregation of nanocomposite particles, with the long-term destabilization being recognized to result from bubble coarsening. For rapidly destabilizing foams, the contribution from bubble-bubble coalescence was shown to be more significant.

The mutual diffusion process and interphase development taking place at an asymmetrical polymer–polymer interface between two compatible model polymers, poly­(methyl methacrylate) (PMMA) with varying molecular weights and poly­(vinylidene fluoride) (PVDF) in the molten state, were investigated by small-amplitude oscillatory shear measurements. The rheology method, Lodge–McLeish model, and test of the time–temperature superpositon (tTS) principle were employed to probe the thermorheological complexity of this polymer couple. The monomeric friction coefficient of each species in the blend has been examined to vary with composition and temperature and to be close in the present experimental conditions, and the failure of the tTS principle was demonstrated to be subtle. These were attributed to the presence of strong enthalpic interaction between PMMA and PVDF chains that couples the component dynamics. Hence, a quantitative rheological model modified from a primitive Qiu–Bousmina’s model that connected the mobility in the mixed state to the properties of the matrix was proposed to determine the mutual diffusion coefficient (D m). The modified model takes into account the rheological behavior of the interphase for the first time. In turn, viscoelastic properties and thicknesss of the interphase have been able to be quantified on the basis of the modified model. Effects of the annealing factors like welding time, angular frequency, temperature, and the structural properties as well molecular weight and Flory–Huggins parameter (χ) on the kinetics of diffusion and the interphase thickness and its viscoelastic properties were investigated. On one hand, D m was observed to decrease with frequency until leveling off at the terimnal zone, to depend on temperature obeying the Arrhenius law, and to be nearly independent of PMMA molar mass, corroborating the prediction of the fast-mode theory. On the other hand, the generated interphase which reached dozens of micrometers was revealed to own a rheological property approaching its equivalent blend. Scanning electron microscopy coupled with energy dispersive X-ray analysis (SEM-EDX) and transmission electron microscopy­(TEM) were also carried out and confronted to the rheological results. Comparisons between mathematical modeling of concentration profile based on the D m obtained from rheology and the experimental ones of SEM-EDX and TEM were conducted. Thus, a better correlation between theory and experimental results in terms of mutual diffusion and the interphase properties was nicely attained. The obtained data are in good agreement with literatures using other spectroscopical methods.

No abstract

The yielding behaviour of silica nanoparticles partitioned at an air-aqueous interface is reported. Linear viscoelasticity of the particle-laden interface can be retrieved via a timedependent and electrolyte-dependent superposition, and the applicability of the 'soft glassy rheology' (SGR) model is confirmed. With increasing electrolyte concentration a non-ergodic state is achieved with particle dynamics arrested firstly from attraction induced bonding bridges and then from the cage effect of particle jamming, manifesting in a two-step yielding process under large amplitude oscillation strain (LAOS). The Lissajous curves disclose a shear-induced in-cage particle re-displacement within oscillation cycles between the two yielding steps, exhibiting a 'strain softening' transitioning to 'strain stiffening' as the interparticle attraction increases. By varying and the particle spreading concentration, , a variety of phase transitions from fluid-to gel-and glass-like can be unified to construct a state diagram mapping the yielding behaviors from one-step to two-step before finally exhibiting one-step yielding at high and .

Particle-stabilized emulsions and foams are widely encountered, as such there remains a concerted effort to better understand the relationship between the particle network structure surrounding droplets and bubbles, and the rheology of the particle-stabilized interface. Poly(vinylpyrrolidone)-coated silica nanoparticles were used to stabilize foams. The shear rheology of planar particle-laden interfaces were measured using an interfacial shear rheometer and the rheological properties measured as a function of the sub-phase electrolyte concentration and surface pressure. All particle-laden interfaces exhibited a liquid-like to solid-like transition with increasing surface pressure. The surface pressure-dependent interfacial rheology was then correlated to the formed micron-scale structures of the particle-laden interfaces which were imaged using a Brewster angle microscope. With the baseline knowledge established, foams were prepared using the same composite particles and the particle network structure imaged using cryo-SEM. An attempt has been made to correlate the two structures observed at a planar interface and that surrounding a bubble to elucidate the likely rheology of the bubble stabilizing particle network. Independent of the sub-phase electrolyte concentration, the resulting rheology of the bubble stabilizing particle network was strongly elastic and appeared to be in a compression state at the region of the L-S phase transition.

No abstract

International audienceThe focus of this overview is to give a state-of-the-art knowledge on coextrusion and rheology at the interface of multilayer polymers. In this article, we summarize important experimental and fundamental aspects given in the literature in regard to structure-processing-properties relationships. In addition, the modeling of the interfacial phenomena and the role of the interphase on the flow stability in coextrusion are also discussed

The selective capture of mobile radioactive nuclides, such as 137 Cs + , is crucial to the clean-up and remediation of contaminated environments. While remediation remains a challenging task, the current study considers novel organo-clay composites containing potassium copper hexacyanoferrate (KCuHCF) as a viable option for large-scale clean-up. A threestep synthesis has been demonstrated whereby pristine montmorillonite clay was readily modified to incorporate KCuHCF nanoparticles for enhanced and selective Cs + removal from aqueous environments. Alkyldiamine (DT) was used as an organic modifier to intercalate the clay and provided chelating sites to anchor copper onto the clay matrix, from which KCuHCF nanoparticles were subsequently grown in-situ via the coordination of hexacyanoferrate precursors with the immobilized copper ions. The organo-clay-HCF composite particles exhibited a superior Cs + adsorption capacity (qm = 206 mg/g), twice that of the pristine clay. The enhanced performance also extended to high Cs + selectivity in seawater, with the organo-clay-HCF composites demonstrating Cs + selectivity values in excess of 10 5 mL/g, two orders of magnitude greater than the pristine clay. Organo modification of the clay particles reduced the particle wettability, thus facilitating the separation of Cs-loaded composite particles from aqueous environments by collector-less flotation. Batch flotation experiments showed recovery efficiencies of the Cs-loaded composite particles of up to 90%, which was in great contrast to the low recovery of less than 15% for the Cs-loaded pristine montmorillonite. The current study provides a new concept for the treatment of contaminated aqueous environments.