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 f rom aqueous solution under different added monovalent salt (NaCl) concentra tions was investigated using an ellipsometric technique. The effect of mono valent counterion size on adsorption kinetics of the same copolymer on hydr ophobic surfaces was also part of the investigation. The results, in genera l, indicate that the adsorption process on solid surfaces occurs through th e anchoring of hydrophobic chains due to the short-ranged hydrophobic inter actions. The kinetic data reveal three distinct stages in the adsorption pr ocess: an incubation period, a subsequent fast growth process of the polyme r layer, and a plateau (equilibrium) region. These three stages are found t o be influenced by salt concentration as well as counterion size. The equil ibrium adsorption density increases as a function of salt concentration, an d 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 substrat e is proposed as a possible explanation for this observation. An attempt ha s been made to explain the growth process in terms of an Avrami type orderi ng process. The Avrami analysis indicates that the buildup of polyelectroly te layer structure depends on added salt conditions. Our kinetic data sugge st that the diffusion of the chains to the surface is not the rate-controll ing process for adsorption. A slow birth (nucleation) and fast growth of th e layer seem to be the determining adsorption process.