DescriptionIn this dissertation, two distinct but relevant systems are chosen as representatives of interesting solid-fluid systems. Molecular dynamics (MD) and Monte Carlo techniques are applied to investigate the rheological, mechanical and transport properties of these systems.
Firstly, polyethylene melt embedded with silica nanoparticles is examined to be of our interest. Since it is computationally impractical to model a complex system with a molecular description, a multiscale modeling approach, which combines both atomistic and mesoscale simulations, is employed to efficiently represent and study the polymer nanoparticle systems. Based on a coarse-grained force field for polyethylene, a novel method is developed for determining the solid-fluid interaction at the spherical interface. Our coarse grained model is designed to mimic 4 nm silica nanoparticles in polyethylene melt at 423K. A series of MD simulations are performed to investigate the factors that control the homogeneity of nanofillers inside polymer matrix, also in the presence of nonionic surfactants (short chain alcohols). The effects of nanoparticle filling fraction, polymer chain length, and relative sizes between nanoparticles and polymer chains on the particle dispersion are explored. In addition, a fundamental relationship is pursued between the microstructure and macroscopic properties (transport and rheological) of polymer nanoparticle composites.
In this work another method for determining the solid-fluid interaction parameter is presented: the experimental adsorption isotherms are used to validate the potential parameters. The rapid expansion of silica nanoparticle agglomerates in supercritical carbon dioxide (RESS process) is chosen to be the system of interest. The simulations show that the effective attraction between two identical nanoparticles is most prominent for densely hydroxylated particle surfaces that interact strongly with CO2 via hydrogen bonds, while it is significantly weaker for dehydroxylated particles. We also explore the shearing forces necessary to break an agglomerate in supercritical fluid. The agglomerate experiences deformation followed by elongation, and finally break-up. The calculated diffusion coefficient of CO2 is expected to be smaller than the experimental value, because the nanoparticle agglomerate hinders fluid movement. In the direction of shearing forces, the diffusion of CO2 shows a steep increase after the breakup, confirming the rupture of the agglomerate.