title: Toward ion-free polyelectrolyte multilayers: cyclic salt annealing.
abstract: Polyelectrolyte multilayers (PEMUs) are made from various combinations of polyanions and polycations. It is now understood that these ultrathin films of polyelectrolyte complex may also incorporate counterions derived from the solutions from which the PEMU was deposited or exchanged into the film postassembly. If these ions are required to compensate nonstoichiometric ratios of polycation and polyanion they cannot leave the film and exert considerable influence on film properties, such as modulus and permeability. These "extrinsic" charges also complicate fundamental studies on PEMUs. We report a method to remove almost all ionic content from a PEMU made of poly(diallyldimethylammonium chloride), PDADMAC, and poly(styrenesulfonate), PSS. In this method, a high salt concentration plasticizes the multilayer past its glass transition, dispersing all the buried excess PDADMA throughout the film. Exposure to a solution of PSS in a lower salt concentration consumes excess PDADMA near the surface without overcompensating with PSS. The process is repeated in a cyclic fashion, removing >95% of the ions charge present in the as-made PEMU.
authors.affiliation: Department of Chemistry and Biochemistry, The Florida State University, Tallahassee, Florida 32306-4390, United States.
authors.forename: Hadi M
authors.initials: HM
authors.lastname: Fares
authors.affiliation: Department of Chemistry and Biochemistry, The Florida State University, Tallahassee, Florida 32306-4390, United States.
authors.forename: Yara E
authors.initials: YE
authors.lastname: Ghoussoub
authors.affiliation: Department of Chemistry and Biochemistry, The Florida State University, Tallahassee, Florida 32306-4390, United States.
authors.forename: Richard L
authors.initials: RL
authors.lastname: Surmaitis
authors.affiliation: Department of Chemistry and Biochemistry, The Florida State University, Tallahassee, Florida 32306-4390, United States.
authors.forename: Joseph B
authors.initials: JB
authors.lastname: Schlenoff