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The assembly and dynamics of polyelectrolyte complexes (PECs) and polyelectrolyte multilayers (PEMs) are influenced by water content, pH, and salt concentration. However, the influence of divalent salts on the assembly of polyelectrolyte complexes remains unclear. This work showcases that divalent chloride salts directly impact the glass transition temperature and the ion–ion interactions within PECs. Here, poly(diallyldimethylammonium)–poly(styrene sulfonate) (PDADMA–PSS) PECs are assembled in solutions containing MgCl2 and CaCl2 (following the Hofmeister series). These PECs are studied for the cations’ influence on physicochemical properties (glass transition, polymer composition, ion pairing) at varying salt concentrations (0.03 M, 0.10 M, 0.15 M, and 0.20 M). Modulated differential scanning calorimetry (MDSC) experiments demonstrate that PECs assembled with CaCl2 have a significantly higher glass transition temperature when compared to PECs assembled with MgCl2. Neutron activation analysis (NAA) and nuclear magnetic resonance (NMR) spectroscopy demonstrate that this difference is due to strong ion-specific effects influencing the ratio of intrinsic and extrinsic ion pairings in the system. Furthermore, this study demonstrates a universal linear relationship between the thermal transition and the number of water molecules surrounding oppositely charged polyelectrolyte–polyelectrolyte intrinsic ion pairs, even when the salt contains divalent cations. Ion-specific trends have implications on the glass transition and composition of PDADMA–PSS PECs. Divalent salts not only follow the trend of the Hofmeister series but also introduce bridging into the polyelectrolyte complex; however, the structural relaxation of the PEC remains the same. This study offers a bridge between divalent cation behavior on polymer assembly properties and its transition to industrial applications such as controlled drug delivery, sensors, and water purification. 

Polyelectrolyte complexes (PECs) are highly tunable materials that result from the phase separation that occurs upon mixing oppositely charged polymers. Over the years, they have gained interest due to their broad range of applications such as drug delivery systems, protective coatings, food packaging, and surface adhesives. In this review, we summarize the structure, phase transitions, chain dynamics, and rheological and thermal properties of PECs. Although most literature focuses upon the thermodynamics and application of PECs, this review highlights the fundamental role of salt and water on mechanical and thermal properties impacting the PEC's dynamics. A special focus is placed upon experimental results and techniques. Specifically, the review examines phase behaviour and salt partitioning in PECs, as well as different techniques used to measure diffusion coefficients, relaxation times, various superpositioning principles, glass transitions, and water microenvironments in PECs. This review concludes with future areas of opportunity in fundamental studies and best practices in reporting. 

Abstract Quartz crystal microbalance with dissipation (QCM‐D) monitoring is a powerful tool used to sensitively examine the real‐time responses of polymer films to external responses. For example, the technique is commonly used to monitor film growth, material adsorption, thin film swelling, and ion exchange. With its rapidly expanding use, this review is intended to introduce new users to the basic principles of QCM‐D, along with practical challenges and remedies specific to polymer thin films. For both new and experienced users, specific case studies are highlighted including layer‐by‐layer assembly, electrochemical QCM‐D, swelling, sensing, and biological application. Last, the review recommends future directions for research and areas of growth.