Role of chain interpenetration in layer-by-layer deposition of polyelectrolytes

The effects of temperature, pH, and salt concentration on the layer-by-layer (LBL) deposition of sodium poly(styrene sulfonate) (PSS)/poly[2-(dimethylamino)ethyl methacrylate] (PDEM) were investigated by use of a quartz crystal microbalance with dissipation (QCM-D). At pH 4, the frequency change (Delta f) gradually decreased to a constant, indicating that the-polyelectrolyte complexes of the layer were not dissolved. As the layer number increased, the -Delta f oscillatedly increased, indicating that the thickness of the multilayer increased. At the same time, the dissipation change (Delta D) oscillatedly increased with the layer number, indicating the chain interpenetration or complexation that led to the alternative swelling-and-shrinking of the outermost layer. For the same layer number, as the temperature increased, the amplitude of Delta D increased, indicating that the chain interpenetration increased. The thickness also increased with temperature. Further increasing the pH to 7 led to a thicker layer, reflected in the larger amplitude of Delta D. At pH 10, the polyelectrolytes no longer formed multilayers on the surface because of the lack of electrostatic interactions. On the other hand, the addition of NaCl also led to a thickness increase. The amplitude in Delta D increased with NaCl concentration, indicating that the chain interpenetration increased. Our experiments indicated that the LBL deposition of polyelectrolytes was dominated by the chain interpenetration. Also, the polyelectrolyte complexes in the layer can redissolve into solution from the surface at a high temperature or a high salt concentration.