pH-Responsive Swelling of Poly(acrylic acid) Brushes Synthesized by the Grafting Onto Route

The swelling of poly(acrylic acid) brushes in aqueous solutions is studied by neutron reflectivity. The brushes are synthesized on silicon wafers using a grafting onto approach and their stretching is investigated at various pH, grafting densities, and chain lengths. Neutron reflectivity provides the average thickness and, more interestingly, the density profile of the brushes. The profiles obtained experimentally are in good agreement with theory. The swelling ratio of the poly(acrylic acid) brushes is compared with that of other polymer brushes formerly investigated using classic scaling laws. Owing to a new representation with the normalized thickness, it is demonstrated that there is a general behavior for all polymer brushes with a master curve for neutral polymer brushes and for polyelectrolyte brushes.