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Redispersion of polymer coated colloidal aggregates with changing solvent quality

Posted on:2007-08-31Degree:Ph.DType:Dissertation
University:Carnegie Mellon UniversityCandidate:Hamm, JasonFull Text:PDF
GTID:1441390005963994Subject:Engineering
Abstract/Summary:
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. In this case the swelling of the 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.; 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 and hot-stage fluorescence microscopy to examine the temporal evolution of single polymer-bound particle contacts.; From SALS the redispersion kinetics are found to display one or two regimes where one regime can be described by a single exponential decay, exp(-t/tau 1), and the other by a complex exponential decay, exp{lcub}-(t/tau 2)beta{rcub}. 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 aggregate structure was found to have no impact on the redispersion kinetics. The fluorescence microscopy shows that polymer coated particles undergo complex motions in-plane and out-of-plane prior to the release from a polymer coated surface. The timescales for the release of the particles are on the same order as the SALS experiments.
Keywords/Search Tags:Polymer, Redispersion, Colloidal, Particles, Solvent, SALS, Single exponential decay, Changing
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