Structure Analyses of Charged Polymer Brush Films/Water Interface by Neutron Reflectivity

Abstract

Counter anion-specific hydration state of a cationic polyelectrolyte brush film, and ion-specificity and salt concentration dependence in hydration state of a zwitterionic poly(sulfobetaine) brush film were investigated using neutron reflectivity (NR). The asymmetric chain density profiles in the charged polymer brushes were clearly visualized. The poly(quaternary ammonium cation) brushes with strongly hydrated chloride counter anions produce a more diffuse tail and less swollen bound layer under salt-free deuterium oxide than those with weakly hydrated thiocyanate counter anions. The extent of swelling and the dimensions of swollen chain structure in the poly(sulfobetaine) brushes were dependent on the ion species and salt concentration in the solutions. The anion specificity could be ascribed to ion-specific interactions between the quaternary ammonium cation in sulfobetaine groups and the anions. The insights into the hydration state of the charged polymer brushes described here could be useful to tailor the response of surface-tethered charged polymer films in the presence of various ions.