Energy barrier to self-exchange between PEO adsorbed on silica and in solution

This investigation sought to reveal the dynamic mechanism for self-exchange between an adsorbed homopolymer layer and chains in free solution, employi ng poly(ethylene oxide) (PEO) adsorbed on silica from aqueous solution as a model system. Fluorescence tracer studies of individual populations within a saturated adsorbed layer revealed no dynamic distinctions between those chains which reached the interface early or late during the initial layer p reparation. This observation ruled out the explanation of trapped and mobil e subpopulations for bimodally shaped self-exchange kinetic traces. Studies focusing on chains aged in unsaturated or starved layers revealed faster s elf-exchange rates compared with chains aged in saturated layers. Surface c overage was found to be more important than molecular weight in controlling the exchange rate. Taken together, these results ruled out the number and strength of segment-surface contacts per chains as the determining kinetic factor in exchange dynamics. Interactions between neighboring chains were s hown to be extremely important; however, if classical melt-type entanglemen ts played a role, they did so only at extremely high molecular weights, abo ve 112K. The osmotic barrier posed by loops and tails of the existing layer (present only at moderate and high surfaces coverages) to approaching chai ns from solution was thought to provide a substantial kinetic barrier to th e approach chains from solution.