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Molecular Simulation Studies of Covalently and Ionically Grafted Nanoparticles

Speaker: Bingbing Hong
Series: Final Public Oral Examinations
Location: 307 Hoyt Laboratory
Date/Time: Friday, September 20, 2013, 4:00 p.m. - 5:30 p.m.

Solvent-free covalently- or ionically-grafted nanoparticles (CGNs and IGNs) are a new class of organic-inorganic hybrid composite materials exhibiting fluid-like behaviors around room temperature. Atomistic and coarse-grained molecular dynamics simulations have been performed in this thesis to investigate the thermodynamics, structure, and dynamics of these systems.

Starting from poly(ethylene oxide) oligomers (PEO) melts, we established atomistic  models based on united-atom representations of methylene. The simulations generate densities, viscosities, diffusivities, in good agreement with experimental data.  Coupled with thermodynamic integration methods, the models give good predictions of pressure-composition-density relations for CO2 + PEO oligomers. The dependence of the calculated Henry’s constants on the weight percentage of water falls on a temperature-dependent master curve.

CGNs are modeled by the inclusion of solid-sphere nanoparticles into the atomistic oligomers. The calculated viscosities are of the same order of magnitude as experimental values. Grafted systems have slower local to global dynamics than the ungrafted counterparts – nanocomposites – at high temperatures, but exhibit faster dynamics at lower temperatures. This agrees with the experimental observation that the new materials have liquid-like behavior in the absence of a solvent. To lower the simulated temperatures into the experimental range, we established a coarse-grained CGNs model by matching structural distribution functions to atomistic simulation data.  Coarse-graining of grafted nanoparticles can either accelerate or slowdown the core motions, depending on the length of the grafted chains. This can be qualitatively predicted by a simple transition-state theory.

Similar atomistic models to CGNs were developed for IGNs, and were compared with “generic” coarse-grained IGNs. The elimination of chemical details in the coarsegrained models does not bring in qualitative changes to the structure and dynamics of atomistic IGNs, but saves considerable simulation resources. The coarse-grained IGNs models are later used to investigate the system dynamics through analysis of the dependence on temperature and structural parameters of the transport properties (self-diffusion coefficients, viscosities and conductivities). Further, migration kinetics of oligomeric counterions is analyzed in a manner analogous to unimer exchange between micellar aggregates. The counterion migrations follow the “double-core” mechanism and are kinetically controlled by neighboring-core collisions.