Influence of Water, Salt, and Temperature on Polyelectrolyte Complexes and Multilayers

Abstract

Polyelectrolyte complexes and multilayers (PECs and PEMs) are made by mixing two oppositely charged polyelectrolytes either by simultaneous mixing or by a layer-by-layer deposition respectively. The properties of PECs and PEMs are very tunable as they are highly responsive to changes in salt and ionic strength, pH, temperature, humidity, molecular mass and composition during and after preparation. This dissertation discusses two studies on the impact of water, salt and temperature on PECs and PEMs. The first study addresses the distribution of water molecules around ion pairs within PEMs. This is important because water existing within PEMs has significant influence on their physical, chemical, and thermal properties. Ionic strength, salt type, and terminating layer are known to influence PEM swelling. However, knowledge of the specifics of water-ion pairing within PEMs, whether that water is affiliated with intrinsic or extrinsic ion pairs, remains lacking. Here, the impact of assembly and post-assembly changes in salt type and ionic strength on water-ion pair interactions in poly(styrene sulfonate)/poly(diallyldimethylammonium) (PSS/PDADMA) PEMs in NaCl and KBr was studied. The second study addresses the influence of environmental factors such as ionic strength, mixing ratio and temperature on the phase behavior of PECs. At room temperature, the PDADMA/PAA PECs exhibit four different phase states: precipitate, co-existing precipitate and coacervate, solid-like gel, and coacervate. As the temperature increased, an upper critical solution temperature (UCST) behavior was identified. All four phases identified at room temperature transitioned to a solution phase at a UCST value which was dependent on the original phase of the PEC. All-atom molecular dynamics (MD) simulations support that precipitates exhibit kinetic trapping, which may contribute to the higher UCST values observed in experiment. Taken together, this study highlights the significance of temperature on the phase behavior of PECs which may play a larger role in stimuli-responsive materials, membraneless organelles, and separations applications.