pH and Salt Response of Mixed Brushes Made of Oppositely Charged Polyelectrolytes Studied by in Situ AFM Force Measurements and Imaging

The response of mixed brushes made of poly­(acrylic acid) and poly­(2-vinyl pyridine) with a mixing ratio of about 60:40 was studied using atomic force microscopy (AFM) force measurements with colloidal probes and AFM imaging with a sharp tip in the pH range between 2.5 and 8 and at varying KCl concentrations up to 1 M. It was found that under all conditions a dense polyelectrolyte complex layer coexists with excess polyelectrolyte chains in varying swelling states depending on pH and salt concentration. The mixed brush thus combines typical features of polyelectrolyte brushes and complexes. So, the increase of the salt concentration not only led to a transition from osmotic to salted brush regime but also to salt-induced softening or partial decomposition of the complex layer. Attractive forces at high salt concentrations indicated the presence of P2VP chains in the swollen layer even at high pH values.