Dissociation Behavior of Weak Polyelectrolyte Brushes on a Planar Surface

Poly(acrylic acid) (PAA) brushes and poly(methacrylic acid) (PMAA) brushes on gold substrates were synthesized by surface-initiated atom-transfer radical polymerization of sodium acrylate and sodium methacrylate in water media at room temperature. Fourier transform infrared spectroscopy (FTIR) titration and contact angle titration methods were used in combination to investigate the dissociation behavior of these two brushes. Whereas FTIR titration gives effective bulk pK(a) values of the polyacid brushes (pK(a)(bulk) of PAA brushes is 6.5-6.6 and pK(a)(bulk) of PMAA brushes is 6.9-7.0), contact angle titration provides effective surface pK(a) of the brushes (pK(a)(sruf) of PAA brushes is 4.4 +/- 0.01 and pK(a)(surf) of PMAA brushes is similar to 4.6 +/- 0.1). The difference between pK(a)(bulk) and pK(a)(surf) suggests that acid groups further from the substrate surface are easier to ionize and have smaller pK(a) values. Although such behavior of weak polyelectrolyte brushes has been predicted by theoretical simulation, here we provide the first experimental evidence of this behavior.