Competitive adsorption of PAM and HPAM on siliceous material

The present work aims at studying the outcome and kinetics of competitive adsorption between low and high molar mass chains of neutral polyacrylamide (PAM) or partially hydrolyzed polyacrylamide (HPAM) in aqueous solution at the surface of siliceous material. Monodisperse neutral and charged polymers, PAM and HPAM, were synthesized by RAFT/MADIX polymerization and their adsorption, alone or in competitive process, was studied. The analysis of adsorption measurements were carried out quantitatively by size exclusion chromatography which allows for distinguishing the relative adsorption of the chains depending on their molar masses. The effect of the average molar mass, dispersity as well as the kinetics of the displacement of pre-adsorbed polymer chains onto the surface by longer chains was investigated. We evidenced that both PAM and HPAM exhibit the same trend. Close to surface saturation, the high molar mass chains desorb polymer chains of lower molar mass. Moreover, by mixing different monodisperse polymers we were able to model the behavior of the polydisperse systems usually encountered in practical applications. By following adsorption over time, our results show that while low molar mass polymers diffuse rapidly and adsorb first, they are displaced by high molar mass ones once they reach the surface. This result remains unchanged even if the low molar mass chains are added several hours before the higher ones, showing that there is no kinetic limitation to the displacement of low molar mass PAM and HPAM by higher molar mass ones. The understanding and prediction of this complex competitive adsorption behavior over time is of utter importance for the control of the mechanism of action of these polymers notably in industrial processes.