Kinetics of Polystyrene Adsorption onto Gold from Dilute θ Solutions

A quartz crystal microbalance (QCM) was applied to study the kinetics of adsorption of nearly monodisperse, high molecular weight polystyrene (PS) onto gold from dilute solutions at the θ condition. An analysis of QCM frequency shifts during adsorption, based on the mechanical 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 the shift due to the coverage, which enables one to construct the expected isotherm from equilibrium frequency shifts. The effects on the adsorption kinetics of polymer molecular weight and of bulk polymer concentration were studied systematically. The data were compared to a preliminary theory by de Gennes^{5, 6} which assumes that an adsorbed layer relaxes instantaneously during adsorption and that the adsorption rate is controlled by end-in reptation of chains across the partially developed layer. The data deviate from the theory in two ways. Firstly, the adsorption process has a much longer time scale than any realistic estimate based on the end-in reptation mechanism, and this time scale is insensitive to molecular weight, which is at odds with prediction. Secondly, the transmission coefficient of a partially formed adsorbed layer clearly depends on how the layer was formed, suggesting that memory effects play an important role in the layer formation process. In toto, the kinetic data suggest that long time scale surface rearrangements insensitive to molecular weight control the adsorption rate in this system.