pH-Dependent Growth Laws and Viscoelastic Parameters of Poly-L-Lysine/Hyaluronic Acid Multilayers

The layer-by-layer formation of polyelectrolyte multilayer films based on poly(L-lysine) and hyaluronic acid (PLL/HA) is investigated in situ as a function of the pH value employing the quartz crystal microbalance with dissipation. By the use of the Kelvin-Voigt viscoelastic model, the film thickness, the complex viscosity, and the complex shear modulus are obtained as a function of the single layer number and pH. For all investigated pH values, PLL/HA films show an exponential growth behavior. However, in different pH regimes the exponentiality, represented by the exponential factor of the respective fit function, is more or less pronounced. Toward low or high pH values, PLL interdiffusion causing exponential growth is less relevant for the increase in mass coverage, though in the high (pH > 9) and the low (pH < 5) pH range thicker (PLL/HA) 4 multilayers form. Here, the films are less viscous and less elastic compared to polyelectrolyte multilayers formed at intermediate pH (5 <= pH <= 9) conditions. Film thickness, viscosity, and elasticity do not only change with pH but also with the kind of the terminating layer. This odd-even effect is caused by the diffusion of PLL into and out of the multilayer film.