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.
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