Polyelectrolyte complexes (PECs) are formed by mixing oppositely charged polyelectrolyte solutions together. They have numerous applications in industry and advanced material areas. PECs are receiving increasing attention because of their stimuli-responsive behaviors to external conditions, such as ionic strength, pH, and temperature. In this dissertation, the phase behavior and thermal properties of PECs are explored. The influence of polycation/polyanion mixing ratio and ionic strength on the phase behavior of PECs in aqueous state are investigated. The PECs consist of poly(diallyldimethylammonium chloride) (PDADMAC) and poly(sodium 4-styrenesulfonate) (PSS). PECs are formed by mixing PDADMA and PSS solutions together rapidly. It then takes minutes to days for the primary PECs to aggregate and precipitate. Turbidity and hydrodynamic size of PECs are recorded at different times. The results mark the colloidally stable, unstable, and solution boundaries and establish a phase diagram based on the mixing ratio and salt concentration. Molecular dynamic (MD) simulations are also performed to investigate the microscopic structure and effective charge distribution of PECs. PECs transit from colloidally stable to unstable when the mixing ratio reaches stoichiometric condition. Salt screening effect becomes stronger with increasing salt concentration, which induces the phase change from unstable to solution states. The thermal properties of PECs containing weak polyelectrolytes, poly(allylamine hydrochloride) (PAH) and poly(acrylic acid) (PAA), are investigated using modulated differential scanning calorimetry (DSC). The properties of PECs are significantly influenced by complexation pH. As pH increases, PAA mol% within PECs decreases and PAA degree of ionization increases. Modulated DSC results show that hydrated PECs undergo a glass-transition-like thermal event. The glass transition temperature (Tvg) decreases with increasing water content or decreasing pH. All Tvg collapse into one master curve when plotted against the ratio of water to intrinsic ion pair (polycation-polyanion). The effect of water-solvent mixtures on Tvg is also examined, including 1-propanol and propanediols. Tvg only depends on water content, implying that organic solvents exhibit no plasticization effect. The results suggest that water works as nontraditional plasticizer to PECs by reducing the electrostatic attractions of intrinsic ion pairs. Thermal behaviors of PDADMAC-PSS complexes are investigated using modulated DSC and all-atom MD simulations. Tvg of hydrated PECs decreases with increasing salt doping level or with increasing water content. The ratio of water to intrinsic ion pair influences Tvg in PDADMAC-PSS system, following a linear relationship. The activation energy obtained from this van't Hoff plot is close to the value of restructuring one hydrogen bond. MD simulations of PDADMA-PSS directly observed the intrinsic and extrinsic ion pairs separately, showing that the Tvg was connected to a decrease in hydrogen bonds between water-PSS at intrinsic ion pairs. The results imply an underlying mechanism for the glass transition: water plasticizes the PECs by weakening intrinsic ion pairing, and water surrounding the intrinsic ion pair facilitates the sliding motion and relaxation of polyelectrolytes within the assembly. This finding impacts current interpretations of relaxation dynamics in charged assemblies and points to water's important contribution at the molecular level.
