Comparison of the structure of polyelectrolyte multilayer films exhibiting a linear and an exponential growth regime: An in situ atomic force microscopy study

We report here on the structural characterization of polyelectrolytes multilayer films formed by poly(L-glutamic acid) and poly(L-lysine) (PGA/PLL). The growth of this system is compared to that of poly(styrenesulfonate)/poly(allylamine hydrochloride) (PSS/PAH) multilayers by means of in situ atomic force microscopy (ATM) and by optical waveguide lightmode spectroscopy (OWLS). In contrary to the (PSS/PAH)(i) films that are growing linearly with the number of deposited layer pairs i, optical data evidenced that the (PGA/PLL)(i) films are characterized by an exponential growth. The analysis of the structure of the (PSS/PAH)(i) films reveals a smooth featureless surface covered by small globules. On the other hand, (PGA/PLL)(i) films form extended structures that appear with a vermiculate pattern. We propose a new growth mechanism based on polyelectrolyte diffusion in and out of the film coupled to the formation of polyanion/polycation complexes at the surface of the film in order to explain the whole results.