| Abstract
| - 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 viscoelasticitycan also be changed by exposing a (PLL/HA)m film alternatively to a solution of poly(sodium 4-styrenesulfonate) (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 micrometricspherical probe and different approach velocities ranging over 2−3 orders of magnitude. These curves areanalyzed in the framework of the Hertzian mechanics corrected for the finite thickness of the film depositedon a hard (glass) substrate, with the indentation limited to 50 nm to preserve material linearity. The forcecurves cannot be reproduced satisfactorily when the probed films are assumed to be elastic bodies. Betteragreement is achieved when the films are depicted as Maxwell bodies, and further improvement is reachedwhen they are represented as a spring in series with a Kelvin unit (referred to as “SK model”, where S standsfor “spring” and K for “Kelvin”). The evolution of the SK model parameters with n reveals that the successivedepositions of PSS and PAH onto the (PLL/HA)m stratum, at a fixed value of m, increase the dynamicalstiffness of the films. This effect is attributed to the penetration of PSS into the (PLL/HA)m stratum, whichis evidenced by infrared spectroscopy. This study thus shows that the deposition of successive polyelectrolytemultilayers can affect the mechanical properties of the underneath multilayers, an effect that has not beendescribed until now.
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