Modeling of inter-particle interactions and their effect on macroscopic properties of nanoparticle suspensions

Abstract

In this thesis, a detailed study of polymer nanoparticle interactions is pursued in order to understand the effect of mechanical properties of individual particles on the collective behavior of chemically destabilized colloidal systems during shear-induced aggregation. In this process, micro-clusters are formed by an assembly process that is dominated by the surface properties of the nanoparticles, particularly adhesion. Depending on the material composition and temperature of the system, adhesion force can be controlled, affecting directly the size and morphology of the formed aggregates. In order to investigate the micro-mechanical behavior of the formed clusters during aggregation, particle interaction models were developed and implemented into a CFD-DEM code to simulate the aggregation process under simple shear flow. The interaction models developed in this research have the capability to describe non-contact as well as contact forces present in colloidal systems. Depending on the system temperature, the models can simulate either elastic, elastic-plastic, or viscoplastic deformation between the nanoparticles. Using primary particle mechanical parameters, it was possible to reproduce experimentally observed growth of aggregates with temperature rise by simulating an increase in mechanical adhesion. The experimental system designed to investigate the different deformation mechanisms between the polymer nanoparticles and calibrate the numerical models consisted of a core-shell structure, where the core is composed of polymethyl methacrylate, and the shell is composed of a combination of polymethyl methacrylate and polybutylacrylate. Due to the significantly different glass transition temperature (Tg) of these polymers, the core act as a hard-sphere, while the presence of polybutylacrylate in the shell, gives the surface mechanical softness upon increasing temperature. Particularly, the composition of the shell results in a copolymer with glass transition temperature of 50°C. This composition, and Tg, permits to precisely control the deformation mechanism and adhesion between nanoparticles by tuning the system temperature. As a result, it was observed that the size of the aggregates grow significantly when the temperature rises above the shell Tg, indicating an increase of adhesive force between the nanoparticles. Under these conditions, the surface of the nanoparticles exhibit a transition from plastic to viscous behavior that allows core-shell nanoparticles to bond physically upon contact via a controlled coalescence mechanism. Moreover, it was discovered that when the shear rate increases while the polymer nanoparticles are above its glass transition point, the scaling of steady-state aggregates size as a function of applied shear rate becomes steeper than predicted by available theories. This particular behavior that was never reported before, forms the basis for a novel theory of shear-thinning effect at the surface of polymer nanoparticles.

In this thesis, a detailed study of polymer nanoparticle interactions is pursued in order to understand the effect of mechanical properties of individual particles on the collective behavior of chemically destabilized colloidal systems during shear-induced aggregation. In this process, micro-clusters are formed by an assembly process that is dominated by the surface properties of the nanoparticles, particularly adhesion. Depending on the material composition and temperature of the system, adhesion force can be controlled, affecting directly the size and morphology of the formed aggregates. In order to investigate the micro-mechanical behavior of the formed clusters during aggregation, particle interaction models were developed and implemented into a CFD-DEM code to simulate the aggregation process under simple shear flow. The interaction models developed in this research have the capability to describe non-contact as well as contact forces present in colloidal systems. Depending on the system temperature, the models can simulate either elastic, elastic-plastic, or viscoplastic deformation between the nanoparticles. Using primary particle mechanical parameters, it was possible to reproduce experimentally observed growth of aggregates with temperature rise by simulating an increase in mechanical adhesion. The experimental system designed to investigate the different deformation mechanisms between the polymer nanoparticles and calibrate the numerical models consisted of a core-shell structure, where the core is composed of polymethyl methacrylate, and the shell is composed of a combination of polymethyl methacrylate and polybutylacrylate. Due to the significantly different glass transition temperature (Tg) of these polymers, the core act as a hard-sphere, while the presence of polybutylacrylate in the shell, gives the surface mechanical softness upon increasing temperature. Particularly, the composition of the shell results in a copolymer with glass transition temperature of 50°C. This composition, and Tg, permits to precisely control the deformation mechanism and adhesion between nanoparticles by tuning the system temperature. As a result, it was observed that the size of the aggregates grow significantly when the temperature rises above the shell Tg, indicating an increase of adhesive force between the nanoparticles. Under these conditions, the surface of the nanoparticles exhibit a transition from plastic to viscous behavior that allows core-shell nanoparticles to bond physically upon contact via a controlled coalescence mechanism. Moreover, it was discovered that when the shear rate increases while the polymer nanoparticles are above its glass transition point, the scaling of steady state aggregates size as a function of applied shear rate becomes steeper than predicted by available theories. This particular behavior that was never reported before, forms the basis for a novel theory of shear-thinning effect at the surface of polymer nanoparticles.