Adsorption kinetics of a hydrophobic-hydrophilic diblock polyelectrolyte at the solid-aqueous solution interface:: A slow birth and fast growth process

The adsorption kinetics of a diblock copolymer poly(tert-butyl methacrylate)-b-poly(glycidyl methacrylate sodium sulfonate) on hydrophobic substrate from aqueous solution under different added monovalent salt (NaCl) concentrations was investigated using an ellipsometric technique. The effect of monovalent counterion size on adsorption kinetics of the same copolymer on hydrophobic surfaces was also part of the investigation. The results, in general, indicate that the adsorption process on solid surfaces occurs through the anchoring of hydrophobic chains due to the short-ranged hydrophobic interactions. The kinetic data reveal three distinct stages in the adsorption process: an incubation period, a subsequent fast growth process of the polymer layer, and a plateau (equilibrium) region. These three stages are found to be influenced by salt concentration as well as counterion size. The equilibrium adsorption density increases as a function of salt concentration, and the dependence is found to be different from the theoretical predictions. The incubation time increases with salt concentration according to a power la cv dependence, and a simple bound ionic layer formation on the substrate is proposed as a possible explanation for this observation. An attempt has been made to explain the growth process in terms of an Avrami type ordering process. The Avrami analysis indicates that the buildup of polyelectrolyte layer structure depends on added salt conditions. Our kinetic data suggest that the diffusion of the chains to the surface is not the rate-controlling process for adsorption. A slow birth (nucleation) and fast growth of the layer seem to be the determining adsorption process.