Adsorption of polyampholyte copolymers at the solid/liquid interface: the influence of pH and salt on the adsorption behaviour

Polyampholytes are macromolecules that contain oppositely charged groups. W e have studied the adsorption of the polyampholyte diblock copolymer poly(m ethacrylic acid)-block-poly((dimethylamino)ethyl methacrylate), PMAA-b-PDMA EMA, on oxidized silicon surfaces. The amount of polymer adsorbed from aque ous solution of different pH and salt concentration was measured by ellipso metry. The influence of the added salts NaCl, Na2SO4 and CaCl2 was determin ed. In every case adsorption took place, although the polyampholyte and the substrate exhibit the same sign of net charge. For all types of salt, the adsorbed amount shows two maxima close to the isoelectric point (IEP) of th e polymer as a function of pH. Directly at the IEP of the polyampholyte, no adsorption was found. The measured dependences can be explained by the ads orption of one or the other of the two blocks depending on acidity and ioni c strength. Furthermore, the lateral structure of the dried films was inves tigated by scanning force microscopy (SFM).