Multiscale Structure, Interfacial Cohesion, Adsorbed Layers, and Thermodynamics in Dense Polymer-Nanoparticle Mixtures

We establish the existence and size of adsorbed polymer layers in miscible dense nanocomposites and their consequences on microstructure and the bulk modulus. Using contrast-matching small-angle neutron scattering to characterize all partial collective structure factors of polymers, particles, and their interface, we demonstrate qualitative failure of the random phase approximation, accuracy of the polymer reference site interaction model theory, ability to deduce the adsorbed polymer layer thickness, and high sensitivity of the nanocomposite bulk modulus to interfacial cohesionclose181

Role of Polymer Segment-Particle Surface Interactions in Controlling Nanoparticle Dispersions in Concentrated Polymer Solutions

The microstructure of particles suspended in concentrated polymer solutions is examined with small-angle X-ray scattering and small-angle neutron scattering. Of interest are changes to long wavelength particle density fluctuations in ternary mixtures of silica nanoparticles suspended in concentrated solutions of poly(ethylene glycol). The results are understood in terms of application of the pseudo-two-component polymer reference interaction site model (PRISM) theory modified to account for solvent addition via effective contact strength of interfacial attraction, ?? pc, in an implicit manner. The combined experimental-theoretical study emphasizes the complex interactions between solvent, polymer, and particle surface that control particle miscibility but also demonstrate that these factors can all be understood in terms of variations of ?? pc.close151

Molecular Weight Effects on Particle and Polymer Microstructure in Concentrated Polymer Solutions

Microstructure of particles and polymers in concentrated polymer solutions are examined. Silica nanoparticles with diameters of 44 nm are suspended in concentrated poly(ethylene glycol) (PEG) solutions working with PEG molecular weights from 300 to 20 000. Using time-domain NMR, we explore the effects of PEG adsorption on the silica particles on the mobility of polymer segments. Our results reveal that polymer mobility varies from the particle surface to the bulk with different relaxation times; polymers are glassy near the particle surface and mobile in the bulk. While polymer segments adsorb to the particle surfaces to form a glassy layer in a molecular weight independent manner, the number of polymer segments with intermediate mobility between that of the glassy state and the bulk increases with molecular weight Systematic variations in the nanoparticle and polymer microstructure in concentrated polymer solutions are explored using contrast matching small angle neutron scattering techniques demonstrating that the thickness of adsorption layer of polymer segments increases with increasing PEG molecular weights supporting NMR observations. However, the particle microstructure evolves in a manner suggesting attractive interactions increase with molecular weight We compare experimental observations and predictions by the polymer reference interaction site model (PRISM) revealing the limits of the theory when applied to the silica concentrated PEG solution system.. While PRISM predicts experimental observations in polymer and particle microstructure quantitatively at low molecular weight, it fails as molecular weight increases, suggesting that polymer configurations are not able to sample equilibrium configurationsclose6

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Particle Restabilization in Silica/PEG/Ethanol Suspensions: How Strongly do Polymers Need To Adsorb To Stabilize Against Aggregation?

We study the effects of increasing the concentration of a low molecular weight polyethylene glycol on the stability of 44 nm diameter silica nanoparticles suspended in ethanol. Polymer concentration, c(p), is increased from zero to that characterizing the polymer melt. Particle stability is accessed through measurement of the particle second-virial coefficient, (B) over bar (2), performed by light scattering and ultrasmall angle X-ray scattering (USAXS). The results show that at low polymer concentration, c(p) (2) values are positive, indicating repulsive interactions between particles. (B) over bar (2) decreases at intermediate concentrations (3 wt % (2) increases and stabilizes at a value expected for hard spheres with a diameter near 44 nm, indicating the particles are thermodynamically stable. At intermediate polymer concentrations, rates of aggregation are determined by measuring time-dependent changes in the suspension turbidity, revealing that aggregation is slowed by the necessity of the particles diffusing over a repulsive barrier in the pair potential. The magnitude of the barrier passes through a minimum at c(p) approximate to 12 wt % where it has a value of similar to 12kT. These results are understood in terms of a reduction of electrostatic repulsion and van der Waals attractions with increasing cp. Depletion attractions are found to play a minor role in particle stability. A model is presented suggesting displacement of weakly adsorbed polymer leads to slow aggregation at intermediate concentration, and we conclude that a general model of depletion restabilization may involve increased strength of polymer adsorption with increasing polymer concentrationclose171