Adsorption and exchange dynamics in aging hydroxyethylcellulose layers on silica

The adsorption kinetics of hydroxyethylcellulose (HEC) on silica and relaxa tions in adsorbed HEC layers mere probed using total internal reflectance f luorescence and near-Brewster reflectivity. Like many random-coil polymers, HEC was found to adsorb at the transport-limited rate. Relaxations occurre d at nearly constant interfacial mass when HEC layers were exposed to aqueo us solvent, causing the subsequent exchange of chains between the er and th e free solution to become increasingly hindered. Eventually, on the time sc ale of a day, layers became immobilized and unable to accommodate chains fr om free solution. A continued fluorescence decay, beyond time scales that c ould be probed with self exchange, suggested further relaxations of the ads orbed HEC. The polydisperse HEC system (with an average molecular weight ne ar 450,000) behaved qualitatively similar to molecular weight standard poly ethylene oxide (PEO) layers on silica. For instance, relaxations in PEO lay ers occurred an a time scale of 10-20 h, like the HEC layers. Young layers of the latter how ever, exhibited self-exchange kinetics that were an order of magnitude slower than PEO layers of similar age. This difference in ads orbed layer dynamics was attributed to HEC's stiffer backbone, compared wit h flexible PEG. (C) 2000 Academic Press.