(660a) Salt and Temperature Effects in Polyelectrolyte Multilayer Thin Films

Polyelectrolyte multilayers, formed by the ion-pairing of oppositely charged polyelectrolytes at a surface, represent a rich area of thin films linked by noncovalent interactions. These ion-paired polyelectrolytes are sensitive to added salt (ionic strength, salt type, valency) in which small counter-ions compete and screen charged sites. Further, temperature has also been shown to yield a strong effect, in which a multilayer softens abruptly at a critical temperature. These two aspects render polyelectrolyte multilayers as highly stimuli-responsive thin films, although a connection between physical structure and properties is not completely clear.

Here, two studies regarding the effects of salt and temperature on the structure and properties of polyelectrolyte multilayers consisting of poly(diallyldimethylammonium chloride) and (polystyrene sulfonate) (PDAC/PSS) are presented. In the first, the effect of valency on the swelling and contraction of polyelectrolyte multilayer thin films is presented. Depending on the valency of the counteranion or countercation, as well as the charge of the outermost layer, very different behaviors occur. A large part of this work is enabled by quartz crystal microbalance with dissipation (QCM-D), which is an ideal tool for monitoring responsive thin films in situ. The shear modulus, viscosity, and multilayer thickness are quantified as a function of ionic strength and salt type. Results are discussed in the context of multilayer structure and ion-induced restructuring of water within the multilayer. In the second, the effect of salt type and ionic strength on the thermal transition of polyelectrolyte multilayers is presented. It has been previously shown that PDAC/PSS multilayers assembled in the presence of NaCl exhibit a thermal transition akin to (but not the same as) a glass transition. This study examines multivalent ions Ca²⁺ and SO₄^2- in the context of this transition. The results are discussed in the context of the physical meaning of this transition, which is largely driven by reorganization of water around polyelectrolyte sites. Future work will be extended towards polyelectrolyte complexes.