Polyacrylic Acid Brushes: Surface Pressure and Salt-Induced Swelling

Abstract

Polyelectrolyte brushes consisting of polystyrene−poly(acrylic acid) (PS−PAA) diblock copolymers were investigated experimentally using surface pressure isotherms and ellipsometry. The surface pressure π of the block copolymers at the air/water interface was measured as a function of the grafting density σ at various salt concentrations and pH. It is concluded that the scaling behavior of π(σ) 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 π(σ) 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 σ. 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 α in the power law H ∼ Iα is ∼0.1, which is less than that predicted theoretically (1/3).