Anomalous thickness evolution of multilayer films made from poly-L-lysine and mixtures of hyaluronic acid and polystyrene sulfonate

Using a mixture of polyanions or polycations offers a new way to control the properties of polyelectrolyte multilayer (PEM) films. The central issue of PEM films made from blended polyelectrolyte solutions is the relation between the properties of the blended architecture and the properties of the films made from each pure component. Two situations are possible: either (i) the properties of the blended films are intermediate between those corresponding to the single components or (ii) new effects may emerge leading, for instance, to improved mechanical properties. Situation (i) is expected when the chemical natures of both polyelectrolytes from the blended mixture are close, whereas situation (ii) is more probable when the polyelectrolytes from the blend are very different. In this study, we focus on the buildup of PEM films made by the alternate spray deposition of a polyanion blend [a mixture of polystyrene-4-sulfonate (PSS) and hyaluronic acid (HA) in different mass fractions] and a polycation solution of poly-l-lysine (PLL). Whereas (HA-PLL) films exhibit a strong exponential growth with the number of deposition steps, the (PSS-PLL) system is only weakly exponential. We find that when the composition of the polyanion blend ranges from pure (HA-PLL) to pure (PSS-PLL), the films can always be constructed. However, the polyanion composition of the films is far from that of the polyanion solutions used for the buildup. One observes a strong preference for the incorporation of PSS over HA into the films. Moreover, the most striking feature is that the film thickness does not evolve monotonously with the polyanion solution composition but passes through a sharp minimum for a polyanion solution containing 90-95% HA. A possible mechanism for this peculiar finding is proposed.