Tuning of the Elastic Modulus of Polyelectrolyte Multilayer Films built up from Polyanions Mixture

In this paper, we report on the mechanical characterization of polyelectrolytes multilayer (PEM) films prepared from poly(glutamic acid)-poly(styrenesulfonate) (PGA-PSS) blends, deposited in alternated spray deposition with poly(allylamine hydrochloride) (PAR). The polyanion composition of the blended film was first investigated by Fourier transformed infrared spectroscopy in the attenuated total reflection mode. The monomer molar fraction of PGA, in the film increases almost linearly with x, i.e., the monomer molar fraction of PGA in the sprayed polyanion solution. The mechanical properties of the blended (PAH/PGA(x)-PSS(1-x))(n) film were measured using two methods: (i) the wrinkling metrology method and (ii) by colloidal probe atomic force microscopy. We demonstrate that Young's modulus of the PAH/PGA(x)-PSS(1-x) multilayer films can be systematically controlled by the chemical composition of these films, depending on x. Measurements indicate that increasing the monomer molar fraction of PGA in the blended film results in a decrease in film modulus up to 2 orders of magnitude as compared to the PAH/PSS system. At a monomer molar fraction of PGA in the film around x = 0.7, this system undergoes such a transition. We also show that for a given x the elastic properties of these films are significantly affected by the humidity conditions. For a (PAH/PGA(0.88)-PSS(0.12)) film, Young's modulus of the film varies from several hundred to few MPa by solely altering the relative humidity between 12.5% and 80%.