Grafting γ-aminopropyl triethoxysilane onto silica: consequence on polyacrylic acid adsorption

A set of silica samples with various surface density of amine functions was prepared by grafting γ-aminopropyl triethoxysilane from carbon tetrachloride solutions. The surface concentration of APS grafted molecules was determined by a solution depletion method, whereas the irreversibility of grafting was checked by infrared measurements in solution as well as at the solid surface. The adsorption of a polymer and its representative monomer (polyacrylic acid and propionic acid, respectively) on these grafted samples were studied as a function of grafting density, by determining adsorption isotherms and adsorption enthalpies (by microcalorimetry). The surface concentrations of both molecules are strongly enhanced at the saturation plateau when the grafting density is increased. At high grafting density and intermediate coverage of the surface by the polymer, non-equilibrium phenomena are observed. At saturation, as shown by adsorption enthalpy measurements, the fraction of polymer units belonging to trains seems weakly modified. A mechanism of polymer adsorption is proposed.