Swelling Study of Responsive Polyelectrolyte Brushes Grafted from Mica Substrates: Effect of pH, Salt, and Grafting Density

Poly(acrylic acid) (PAA) brushes covalently linked to mica were prepared using the graft from approach in a two-step process: (i) poly(tert-butyl acrylate) (PtBA) brushes were first synthesized by atom transfer radical polymerization directly from in activated mica substrate (ii) followed by hydrolysis to generate PAA brushes. The hydrolysis reaction was confirmed by water contact angle measurements, polymer thickness measurements, and FTIR. The swelling behavior of the brushes in aqueous solutions was measured by examining the change in brush thickness (L), using atomic force microscopy (AFM), as a function of polymer grafting density (sigma), pH. and salt (NaCl) concentration (C-s). A sharp transition from collapsed to stretched conformation was found at pH 7.5. For pH <= 7, the acrylic acid groups are not dissociated, and no swelling of the polymer layer was observed relative to the dry state, regardless of grafting density and salt concentration. For pH >= 7.5, the brushes behaved as charged polymer brushes exhibiting Pincus and salted-brush regimes that were dependent on the salt concentration. In salt-free solution, the equilibrium thickness scales with surface grafting density according to L proportional to sigma(0.87), and at high salt concentrations, the brushes collapse according to L proportional to sigma C-1.09(s)-0.17. The swelling behavior of PAA brushes was reversible with changes in pH and salt concentration under the studied experimental conditions.