1Institute of Chemsitry, Chinese Academy of Sciences, Beijing, China 2Department of Chemical Physics, University of Science and Technology of China, Hefei, China
It has been observed previously that, when polyelectrolyte brushes are exposed to external salt solution, it becomes thinner when the salt level is high enough. Before the salt concentration gets to that limit, an enhanced swelling process is discovered in this study with a few polyelectrolyte brush systems, including sodium polystyrene sulfonate (PSSNa), poly([2-(methacryloyloxy)ethyl] trimethylammonium chloride) (PMETAC) and potassium poly(3-sulfopropyl methacrylate) (PSPMA) with different molecular weight and grafting density, by combination of methods including ellipsometry, quartz crystal microbalance with dissipation (QCM-D) and atomic force microscopy (AFM). The swelling enhancement is expressed by the thickening of the brush layer, accompanied by the decrease of refractive index, the increase of the amount of solvent inside the brushes and the increase of the retardation time. A scenario is proposed as that, when the counterions penetrate into the brushes driven by the external salt ions, they disrupt and break up the previously formed multiplets due to the dipole-dipole interaction by the ion-pairs on the polymer chain. The process results in the release of the bound segments and the stretching of the polymer chains.
