Polyacrylic acid brushes: Surface pressure and salt-induced swelling

Polyelectrolyte brushes consisting of polystyrene-poly(acrylic acid) (PS-PAA) diblock copolymers were investigated experimentally using surface pressure isotherms and ellipsometry. The surface pressure pi of the block copolymers at the air/water interface was measured as a function of the grafting density sigma at various salt concentrations and pH. It is concluded that the scaling behavior of pi>(*) over bar *(sigma) of long PAA chains at high ionic strengths and low pH agrees with predictions of analytical mean-field models. The theoretical predicted scaling behavior of pi>(*) over bar *(sigma) for annealed brushes at low ionic strength and low pH is not observed because of adsorption of the polyacid chains to the air/water interface. The thickness of PAA brushes on hydrophobically modified Si wafers was measured with ellipsometry as a function of pH, total ionic strength I, and sigma. It is observed that at a given pH the brush thickness behaves nonmonotonically as a function of I (i.e., it initially increases and subsequently decreases with increasing I). This nonmonotonic behavior agrees with theoretical predictions for annealed brushes. The experimentally observed scaling exponent alpha in the power law H similar to I-alpha is similar to0.1, which is less than that predicted theoretically (1/3).