Polyelectrolyte Brushes: Water Content, Zeta Potential and Mechanical Properties

Polymer brushes of poly[2-(methacryloyloxy)ethyl]trimethylammonium chloride (PMETAC) and poly(sulfo propyl methacrylate) (PSPM) were synthesized by Atomic Transfer Radical Polymerization from planar and colloidal surfaces. Polymer brush growth was followed by QCMD and the water content determined by combined QCMD and elipsometry. From the water content the percentage of water lost during the brush collapse with the ionic strength could be obtained.Highly charged PSPM brushes were indented by Atomic Force Microscopy at different ionic strengths. The force response was fitted to a phenomenological equation analogous to the equation of state of a compressible fluid. Internal energy and brush compressibility were obtained as a function of ionic strength.Spherical brushes of PMETAC and PSPM display an invariance of the zeta potential with ionic strength in the range from 20 mM to 200 mM NaCl, the zeta potential remains almost constant. This invariance can be explained applying a hairy surface approach.