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 pKa values of the polyacid brushes (pKabulk of PAA brushes is 6.5-6.6 and pKabulk of PMAA brushes is 6.9-7.0), contact angle titration provides effective surface pKa of the brushes (pKasurf of PAA brushes is 4.4+/-0.01 and pKasurf of PMAA brushes is approximately 4.6+/-0.1). The difference between pKabulk and pKasurf suggests that acid groups further from the substrate surface are easier to ionize and have smaller pKa values. Although such behavior of weak polyelectrolyte brushes has been predicted by theoretical simulation, here we provide the first experimental evidence of this behavior.