Effect of relative humidity on the Young's modulus of polyelectrolyte multilayer films and related nonionic polymers

We have measured the Young's modulus and thickness of ultrathin polyelectrolyte multilayer (PEM), polystyrene (PS), and poly(methyl methacrylate) (PMMA) films as a function of relative humidity. We demonstrate that PEMs undergo substantial swelling and plasticization in the presence of ambient water and that both the choice of polyelectrolytes and the pH of the deposition baths influence the response of a PEM film to changes in humidity. These responses were roughly linear in two of the PEM systems tested; however, a third system demonstrated evidence of anti plasticization and an abrupt swelling transition at intermediate humidity. This behavior is attributed to an internal structure rich in hydrogen-bonding sites. Overall, our results suggest that the chemical composition and molecular architecture of PEMs are responsible for differences in the environmental responsiveness of these materials. Both PS and PMMA films exhibited comparatively small but measurable swelling and plasticization in the presence of water; these effects were more pronounced in the case of the more hydrophilic PMMA. Our results offer insight into the material structure and internal chemical interactions that determine the properties and responsiveness of PEM systems.