Stimuli-Responsive Interfaces Using Random Polyampholyte Brushes

We study the pH-responsive behavior of random polyampholyte brushes synthesized by surface-initiated Atom Transfer Radical Polymerization. We define the annealed polyampholyte as the random copolymer that contains weak acid (methacrylic acid) and weak base (2-(dimethylainino)ethyl methacrylate) units. The semi-annealed polyampholyte is defined as the random copolymer that contains the methacrylic acid and a strong base unit (the quaternized amine). The structure of the polyampholyte brushes in aqueous solutions at different pH is determined using neutron reflectivity. The monomer density profile and therefore the brush thickness are obtained. Polymer brushes with various grafting densities are compared. We demonstrate that the structure of the polyampholyte brush depends strongly on the net charge of the chains. With an excess of charge, the brush is as stretched as a polyelectrolyte brush, and the swelling behavior is in good agreement with the scaling laws. At the isoelectric point corresponding to a zero net charge, the polyampholyte effect dominates, resulting in the collapse of the chains. We find that weakly dense polyampholytes have the greatest amplitude of stretching-collapse. We also compare the responsive behavior of the random polyampholyte brushes with that of diblock polyampholyte brushes and mixed polyelectrolyte brushes.