Structural and mechanical properties of polyelectrolyte multilayer films studied by AFM

Mechanical properties of polyelectrolyte multilayer thin films were studied by nanoindentation experiments using an atomic force microscope. We obtained force-distance measurements for a model polycation/polyanion multilayer system, poly(allylamine hydrochloride) (PAH), and an azobenzene-containing polyelectrolyte (P-Azo) prepared at varying charge densities. The relative Young's modulus of the films was determined as a function of the ionization fraction of the multilayer films, prepared to have identical thickness. Multilayer films assembled with polyelectrolytes of high charge density exhibited an elastic modulus that was significantly larger (nearly 2 orders of magnitude) than those prepared with low charge density. An estimate of the relative loop length between ionic cross-links in the multilayer films is then ascertained by analogy to previously studied covalently cross-linked polymer networks. The modulus values in our films ranged from 10(-2) to 10(-4) kPa, and this implies loop lengths of 1-50 segments. Atomic force microscopy force-distance measurements were also used to compare the relative adhesion values between polycation and polyanion layers in films constructed with varying charge densities. This was done by coating an AFM tip with multilayers and indenting into a surface containing the same multilayer film but capped with the oppositely charged surface polyelectrolyte layer. Adhesion values were typically between 0.5 and 6.7 nN and were found to depend on the ionic cross-link density of the PAH/P-Azo film, in which the highly ionically cross-linked samples exhibited the largest adhesion.