Abstract
Detailed molecular dynamics (MD) simulations are employed to study how the presence of adsorbed domains and nanoparticle bridging chains affect the structural, conformational, thermodynamic, and dynamic properties of attractive polymer nanocomposite melts in the semi-dilute regime. As a model system we have chosen an unentangled poly(ethylene glycol) (PEG) matrix containing amorphous spherical silica nanoparticles with different diameters and at different concentrations. Emphasis is placed on properties such as the polymer mass density profile around nanoparticles, the compressibility of the system, the mean squared end-to-end distance of PEG chains, their orientational and diffusive dynamics, the single chain form factor, and the scattering functions. Our analysis reveals a significant impact of the adsorbed, interfacial polymer on the microscopic dynamic and conformational properties of the nanocomposite, especially under conditions favoring higher surface-to-volume ratios (e.g., for small nanoparticle sizes at fixed nanoparticle loading, or for higher silica concentrations). Simultaneously, adsorbed polymer chains adopt graft-like conformations, a feature that allows them to considerably extend away from the nanoparticle surface to form bridges with other nanoparticles. These bridges drive the formation of a nanoparticle network whose strength (number of tie chains per nanoparticle) increases substantially with increasing concentration of the polymer matrix in nanoparticles, or with decreasing nanoparticle size at fixed nanoparticle concentration. The presence of hydroxyl groups at the ends of PEG chains plays a key role in the formation of the network. If hydroxyl groups are substituted by methoxy ones, the simulations reveal that the number of bridging chains per nanoparticle decreases dramatically, thus the network formed is less dense and less strong mechanically, and has a smaller impact on the properties of the nanocomposite. Our simulations predict further that the isothermal compressibility and thermal expansion coefficient of PEG-silica nanocomposites are significantly lower than those of pure PEG, with their values decreasing practically linear with increasing concentration of the nanocomposite in nanoparticles.
Highlights
Due to their high surface-to-volume ratios, nanoparticles when embedded in a polymer matrix can induce substantial changes to the physicochemical properties of the material
The molecular dynamics (MD) results presented in this work provide detailed information concerning the structure, conformation, underlying topological network between nanoparticles and tie polymer chains, dynamic, and thermodynamic properties of poly(ethylene glycol) (PEG)-silica nanocomposites, with an emphasis on the role of the adsorbed polymer layer and the chemistry of end-functional groups
The local polymer mass density radially from the surface of the silica nanoparticles is higher than the density of the pure PEG melt at the same thermodynamic conditions, typical for polymers adsorbed onto solid surfaces
Summary
Due to their high surface-to-volume ratios, nanoparticles when embedded in a polymer matrix can induce substantial changes to the physicochemical (dynamical, mechanical, barrier, optical, electrical etc.) properties of the material. Formation of a topological network driven by chains that bridge different nanoparticles and transition to a gel-like state has been reported by Kim et al [17] for poly(ethylene glycol) (PEG) matrices filled with spherical, amorphous silica nanoparticles; the authors studied how such a transition depends on the molecular weight (MW) of the host chains. The local PS density around the silica surface was found to decrease with increasing curvature [40,41] (i.e., for smaller nanoparticles); in addition, polymer chains were found to swell and their dynamics significantly slowed-down compared to the pure melt These findings agree with the results of Mathioudakis et al [39] from a new multiscale simulation approach that helped examine high-MW PS-silica nanocomposites.
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