Stiffening of soft polyelectrolyte architectures by multilayer capping evidenced by viscoelastic analysis of AFM indentation measurements

The mechanical properties of polyelectrolyte multilayer films built up with poly-L-lysine (PLL) and hyaluronan (HA) can be tuned using a chemical cross-linker. The present study is aimed at showing that the viscoelasticity can also be changed by exposing a (PLL/HA)(m) film alternatively to a solution of poly(sodium 4-styrene sulfonate) (PSS) and a solution of poly(allylamine hydrochloride) (PAH), thus forming (PLL/HA)(m)-(PSS/PAH)(n)-PSS films. Force curves have been recorded with an atomic force microscope using a micrometric spherical probe and different approach velocities ranging over 2-3 orders of magnitude. These curves are analyzed in the framework of the Hertzian mechanics corrected for the finite thickness of the film deposited on a hard (glass) substrate, with the indentation limited to 50 nm to preserve material linearity. The force curves cannot be reproduced satisfactorily when the probed films are assumed to be elastic bodies. Better agreement is achieved when the films are depicted as Maxwell bodies, and further improvement is reached when they are represented as a spring in series with a Kelvin unit (referred to as SK model, where S stands for spring and K for Kelvin). The evolution of the SK model parameters with n reveals that the successive depositions of PSS and PAH onto the (PLL/HA)(m) stratum, at a fixed value of m, increase the dynamical stiffness of the films. This effect is attributed to the penetration of PSS into the (PLL/HA)(m) stratum, which is evidenced by infrared spectroscopy. This study thus shows that the deposition of successive polyelectrolyte multilayers can affect the mechanical properties of the underneath multilayers, an effect that has not been described until now.