KINETICS OF POLYSTYRENE ADSORPTION ONTO GOLD FROM DILUTE THETA SOLUTIONS

A quartz crystal microbalance (QCM) was applied to study the kinetics of adsorption of nearly monodisperse, high molecular weight polystyren e (PS) onto gold from dilute solutions at the THETA condition. An anal ysis of QCM frequency shifts during adsorption, based on the mechanica l resonance theory of Kanazawa et al.,24,25 predicts that the shift is proportional to a linear combination of the adsorbed layer depth and the polymer coverage. A method is proposed to extract the portion of t he shift due to the coverage, which enables one to construct the expec ted isotherm from equilibrium frequency shifts. The effects on the ads orption kinetics of polymer molecular weight and of bulk polymer conce ntration were studied systematically. The data were compared to a prel iminary theory by de Gennes5,6 which assumes that an adsorbed layer re laxes instantaneously during adsorption and that the adsorption rate i s controlled by end-in reptation of chains across the partially develo ped layer. The data deviate from the theory in two ways. Firstly, the adsorption process has a much longer time scale than any realistic est imate based on the end-in reptation mechanism, and this time scale is insensitive to molecular weight, which is at odds with prediction. Sec ondly, the transmission coefficient of a partially formed adsorbed lay er clearly depends on how the layer was formed, suggesting that memory effects play an important role in the layer formation process. In tot o, the kinetic data suggest that long time scale surface rearrangement s insensitive to molecular weight control the adsorption rate in this system.