It is well-known that particle-polymer interactions strongly control the adsorption and conformations of adsorbed chains. Interfacial layers around nanoparticles consisting of adsorbed and free matrix chains have been extensively studied to reveal their rheological contribution to the behavior of nanocomposites. This work focuses on how chemical heterogeneity of the interfacial layers around the particles governs the microscopic mechanical properties of polymer nanocomposites. Low glass-transition temperature composites consisting of poly(vinyl acetate) coated silica nanoparticles in poly(ethylene oxide) and poly(methyl acrylate) matrices, and of poly(methyl methacrylate) silica nanoparticles in a poly(methyl acrylate) matrix are examined using rheology and X-ray photon correlation spectroscopy. We demonstrate that miscibility between the adsorbed and matrix chains in the interfacial layers led to the observed unusual reinforcement. We suggest that packing of chains in the interfacial regions may also contribute to the reinforcement in the polymer nanocomposites. These features may be used in designing mechanically adaptive composites operating at varying temperature.

The thermomechanical behavior of polymer nanocomposites is mostly governed by interfacial properties which rely on particle-polymer interactions, particle loading, and dispersion state. We recently showed that poly(methyl methacrylate) (PMMA) adsorbed nanoparticles in poly(ethylene oxide) (PEO) matrices displayed an unusual thermal stiffening response. The molecular origin of this unique stiffening behavior resulted from the enhanced PEO mobility within glassy PMMA chains adsorbed on nanoparticles. In addition, dynamic asymmetry and chemical heterogeneities existing in the interfacial layers around particles were shown to improve the reinforcement of composites as a result of good interchain mixing. Here, the role of chain rigidity in this interfacially controlled reinforcement in PEO composites is investigated. We show that particles adsorbed with less rigid polymers improve the mechanical properties of composites.

Dynamics of entangled interfacial polymer layers around nanoparticles determine the linear rheological properties of polymer nanocomposites. In this study, the nonlinear elastic properties of nanocomposites are examined under large-amplitude oscillatory shear (LAOS) flow to reveal the effect of interfacial chemical heterogeneity on the deformation mechanism of polymer-grafted and polymer-adsorbed nanoparticle composites. Adsorbed-poly(methyl methacrylate) (PMMA) layers presented stronger interfacial stiffening and reinforcement than PMMA-grafted layers. Chemical heterogeneities of interfacial layers, provided by polymer-adsorbed and low graft density particles, deformed at smaller strains than the poly(ethylene oxide) (PEO) matrix. Interfaces of loosely bound PMMA and PEO exhibited stiffening at low strains due to the enhanced chain mixing and entanglements. These results demonstrate that chemical and dynamic heterogeneities in interfacial layers have significant importance in designing adaptive polymer nanocomposites for large shear deformation.

Network formation of polymer-grafted nanoparticles in aqueous solutions is an unexplored area in polymer science. In this study, nanoparticles grafted with poly(acrylic acid) (PAA) chains with different grafting densities and similar graft chain lengths at semidilute concentrations are investigated using small-angle neutron scattering (SANS) and rheology experiments. We found that at a low graft density and fully ionized state, stretching of grafted chains accommodates a thick lubricating water layer, which lowers the viscosity. At low graft density and 50% ionization, grafts are in extended conformations as observed from the broad PAA volume fraction profile around the particles in SANS data. This is attributed to the hydrogen bonding between ionized and unionized carboxylic acid groups. The highest viscosity measured at this pH confirms the intraparticle bonding between the grafted chains. Viscosity adjustment with the addition of short poly(N-vinylpyrrolidone) chains suggests that hydrogen bonding is possible within the grafted chains of individual particles at low graft density, whereas interchain networking occurs at high graft density. Molecular dynamics simulation results of model systems confirmed the interparticle bridging at high grafting density. These results demonstrate that hydrogen bonding between polymer-grafted nanoparticles using small molecules can be used to modulate the viscosity of the physical networks in solution.

The effects of nanoscale particle interactions on deposition patterns in the flow coating process are investigated using pH‐responsive poly(acrylic acid) (PAA)‐grafted silica nanoparticles. Interactions between nanoparticles are effectively controlled by grafting densities, PAA brush lengths, and pH, in addition to hydrogen bonding between free polyvinylpyrrolidone (PVP) and PAA. Consequently, various intriguing patterns of randomly distributed dots, polygonal networks, meshes, fork‐like structures along with highly regulated and densely packed stripes parallel to the moving direction of substrates are fabricated. Per se, the flow coating process is shown to form regulated patterns during evaporation by controlling particle–particle interactions with inherent brush properties and external pH.