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Elasticity of Polyelectrolyte Nanomembranes: Effects of Solvent Ionic Strength, Molecular Weight, and Thickness

Ashley D. Cramer and James K. Ferri. Department of Chemical and Biomolecular Engineering, Lafayette College, Lafayette College, Easton, PA 18042

The realization of devices with nanometer scales requires a comprehensive understanding of the strength of materials down to nanometer lengths. Polyelectrolyte multilayers are excellent model systems of nanocomposite soft materials with tunable properties. We measured the elastic modulus of ultrathin, asymmetric polyelectrolyte nanomembranes as a function of solvent ionic strength, polyelectrolyte molecular weight, and total membrane thickness.

Nanocomposites of dimyristoyl phosphatidyl glycerol-(poly-allylamine hydrochloride/ poly-styrenesulfonate)n=1-4 were investigated. The surface elastic modulus, Es, is shown to depend on molecular weight, solvent ionic strength, and film thickness. As the molecular weight and the solvent ionic strength increase, Es decreases. Using the film thicknesses and the surface modulii, the bulk modulus is shown to range between 10 - 100 MPa as a function of solvent ionic strength, which is consistent with “salt-induced” softening.