Regimes of polyelectrolyte dynamics at solid/liquid interfaces.

摘要

This thesis probes the dynamics of polyelectrolyte chains adsorbed at a solid-liquid interface to elucidate the general dynamic behavior of adsorbed chains, and to highlight the unusual dynamic features, particular to polyelectrolytes, which could be exploited in a variety of technological applications. Conventional applications relying on polyelectrolyte adsorption dynamics include papermaking, wastewater treatment, stabilization of colloidal complex fluids, paints, inks, coatings, and pharmaceuticals. New technologies involving adsorbed polyelectrolytes include DNA chips, novel sensors, and biomimetics.; This work employed a model system, comprised of poly(dimethylaminoethyl methacrylate) (DMAEMA) adsorbing onto silica at controlled pH and ionic strength. DMAEMA is a weak polyelectrolyte, whose backbone charge density is pH-dependent. This model system is one, in which adsorption is driven exclusively by electrostatics, and additionally, the polymer remains in solution at conditions when it is not charged. Hence, the DMAMEA-silica system matches the conceptually-simple case of a string of charges interacting with an oppositely charged plane.; In the limit of ultra-low polymer backbone charge, displacement of polyelectrolyte from an oppositely charged surface by monovalent salt was found to be extremely sharp. At reduced pH, with increased backbone charge density and the potential for more segment-surface contacts, the cut-off was more gradual.; Interfacial polyelectrolyte dynamics were examined for the same system in the limit of ultra-low backbone charge, near the salt-induced sharp desorption cut off. Here, several unusual findings are reported and are likely to be broadly generalizable for systems with sparse charge or even a limited number of segment-substrate contact points. The first observation is that of single exponential desorption and self exchange kinetics in this regime. The single exponential behavior suggests that polymer chain displacement from the surface occurs as a single unit, perhaps because of the very low backbone charge. The second finding in this ultra-spare charge limit is that both kinetics are completely surface dominated, with no influence of interfacial mass transport.; The influences of increased numbers of segment surface contacts on self exchange and desorption dynamics were also investigated. A dynamic crossover from simple kinetics (single exponential behavior with self exchange and desorption quantitatively identical and completely controlled at the surface) to a cooperative mode was discovered as the number of polymer-surface contacts was increased. (Abstract shortened by UMI.)