Abstract
While steric stabilization of colloidal suspensions has been extensively studied, redispersion from the aggregated state for polymer-coated particles in low shear conditions has not. Many colloidal aggregates can be redispersed by high shear agitation, but we are motivated specifically by those systems where high shear is technologically infeasible (such as particulate drug dispersion from pharmaceutical tablets in the gastrointestinal tract). In such systems, the driving force for redispersion derives entirely from surface chemistry. Swelling of interfacial polymer films in the gaps between aggregated particles provides the driving force for redispersion. Sterically stabilized suspensions can be caused to reversibly aggregate under poor solvent conditions for the adsorbed polymer and to redisperse as good solvent quality conditions are restored.
In this thesis, we investigate the aggregation and redispersion of colloids that are sterically stabilized by adsorbed polymers. We examine four specific questions: 1) How fast do aggregates of polymer coated particles redisperse when the solvent quality is changed from a poor solvent to a good solvent? 2) How does the structure of the adsorbed polymer film control redispersion kinetics? 3) Are the redispersion kinetics dictated solely by the conditions of the polymer films, or does the overall structure of the aggregate play a role? 4) What qualitative generalities exist in redispersion kinetics behaviors across varying polymer solution compositions and conditions?
We examine the dynamics of aggregation and redispersion for colloidal polystyrene with adsorbed polymer layers that have a lower critical solution temperature (LCST). Two different types of polymer systems are used: a triblock co-polymer, poly(ethylene oxide)-block-poly(propylene oxide)-block-poly(ethylene oxide) (Pluronic F108) and a homopolymer, poly(N-isopropylacrylamide) (PNIPAM). The investigation involves changing the solvent quality of the stabilizing polymer by controlling the temperature above and below the theta temperature. Each particle-polymer system is investigated in various solvent systems by changing the electrolyte. The electrolyte in each system contributes to modifying the theta temperatures at which particle interactions change between attractive and repulsive. We use small angle light scattering (SALS) to monitor the kinetics for the aggregation and redispersion. The effects of the polymer surface coverage, free polymer concentration, degree of cooling, and the aggregate structure are examined to determine their role on the redispersion kinetics.
The redispersion kinetics are found to display one or two regimes where one regime can be described by a single exponential decay, exp(-t/τ1), and the other by a complex exponential decay, exp{-(t/τ2)β}. The redispersion in both polymer systems shows a decrease in the time constant for the single exponential decay regime and a tendency for more of the total redispersion to occur in the single exponential decay regime with increasing polymer concentration. The overall time for redispersion is relatively constant within each experimental system except at the lower concentrations with PNIPAM which redisperses much more slowly. The onset temperature of each regime is also probed by cooling to specific temperatures below the theta temperature. The complex exponential decay regime begins at higher onset temperatures (better solvent quality) than the single exponential decay regime. The aggregate structure has no impact on the redispersion kinetics.
We have developed hot-stage fluorescence microscopy to examine the temporal evolution of single polymer-bound particle contacts. The microscopy shows that PNIPAM coated particles that are attached to PNIPAM coated glass surfaces undergo complex motions in-plane and out-of-plane prior to the release from the surface. The timescales for the release of the particles are on the same order as the SALS experiments. The presence of lateral traps in these experiments suggests that tangential forces between the particles in the colloidal aggregates are important in the redispersion process.
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