Adsorption and exchange of polymers at solid/solution interfaces

• Main Author: Fu, Zengli
• Format: Dissertation
• Language: English
• Published: ProQuest Dissertations & Theses 01-01-1998
• Subjects: Chemical engineering; Plastics; Polymer chemistry; Polymers
• ISBN: 9780591846133; 0591846136

This thesis describes breakthroughs in two areas: experimental methods for polymer adsorption kinetics, and interfacial dynamics for the polyethylene oxide (PEO)/silica/water system. A near-Brewster optical reflectivity experiment was designed and combined with total internal reflectance fluorescence (TIRF) as an integrated probe of adsorption. The former measures the kinetics of the overall mass while the latter distinguishes interfacial populations using a fluorescent label. In addition to competitive adsorption studies, it was also possible to decouple the evolving layer density and thickness. Monodisperse PEO adsorption on silica was high affinity and transport-limited, with the isotherm plateau extending down to free concentrations of 0.01 ppm. Competitive adsorption from mixtures of fluorescein dianion-terminated PEO (F-PEO) and native PEO demonstrated that surface selectivity favors native PEO adsorption, due to the electrostatic repulsion between charged F-PEO chain ends and the surface. The surface selectivity could be tuned through the ionic strength and facilitated determination of the surface-chain end distance. Competitive coadsorption from mixtures of long and short chains employed a noninvasive coumarin label to distinguish between different species. Studies revealed initial mass-transport-limited adsorption of all species and replacement of short chains by longer ones after the surface was nearly saturated. The exchange between short and long chains approached local equilibrium between the layer and the nearby fluid for short chains of relatively low molecular weight (33K) and for relatively young layers (order of 10 minutes old). Increasing the short chain molecular weight or the maturity of the layer led to kinetics that were rate limited by release of the short chains from the surface. Relaxations within adsorbed PEO layers were probed through the self exchange of coumarin tagged and native chains. Results suggest that interfacial relaxations approach completion within 10-20 hours. New evidence for non-equilibrium interfacial behavior at long aging times was presented for flexible PEO chains, in contrast to prior short-time studies which supported local equilibrium. The findings suggest that high numbers of entanglements or segment-surface contacts are at least as important as the role of backbone stiffness in establishing metastable interfacial states.