Mechanical characterization of polyelectrolyte multilayers using quasi-static nanoindentation

Polyelectrolyte multilayer (PEM) films comprised of the weak polyelectrolyte pair poly(allylamine hydrochloride)/poly(acrylic acid) were characterized in nanoindentation tests near 295 K. Film thickness in ambient air ranged from 80 to 6000 nm, and indentation depth varied from a few nanometers to 3000 nm for the thickest films tested. Elastic modulus and hardness of the films were independent of the contact depth over a range of penetration where artifacts arising from the free surface and the underlying glass substrate were insignificant. Moduli of the dry films were in the range of 10 GPa while the same films immersed in water showed modulus values about 2 orders of magnitude lower. Properties of the dry films were dependent on the selected assembly pH conditions but were insensitive to post-assembly cross-linking; both the modulus and the hardness of the hydrated films were affected significantly by the introduction of covalent cross-links into the PEM structure.