Reversible molecular memory and pH-switchable swelling transitions in polyelectrolyte multilayers

The systematic design of materials that alter their macroscopic properties in response to environmental stimuli continues to provide a basis for stimuli-responsive applications in biomaterials and microelectronics. We report discrete volume phase transitions in ultrathin polyelectrolyte multilayers (PEMs) driven by changes in environmental pH of the post-assembled film. These films exhibit a history-dependent swelling behavior and molecular conformational memory. We illustrate that the same two polymers-poly(allylamine hydrochloride)/poly(styrenesulfonic acid) (PAH/SPS)-can be incorporated into a multilayer film with specifically designed molecular architectures (by virtue of assembly pH conditions) that enable them to be either virtually insensitive or highly responsive to small changes in post-assembly pH. We thus provide a general design strategy for PEMs that exhibit multiple conformational states and related phenomena. We further demonstrate the ability to spatially regulate the affinity of molecular species to PEMS-specifically, by virtue of pH-triggered conformational changes in the PEM. Finally, we show the potential of these materials for sustained molecular release.