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1. Counter Anion Type Influences the Glass Transition Temperature of Polyelectrolyte Complexes

   https://doi.org/10.1021/acs.macromol.3c02200  

   Lalwani, Suvesh Manoj; Hellikson, Kayla; Batys, Piotr; Lutkenhaus, Jodie L (May 2024, Macromolecules)

   Salt acts as a plasticizer in polyelectrolyte complexes (PECs), which impacts the physical, thermal, and mechanical properties, thus having implications in applications, such as drug delivery, energy storage, and smart coatings. Added salt disrupts polycation−polyanion intrinsic ion pairs, lowering a hydrated PEC’s glass transition temperature (Tg). However, the relative influence of counterion type on the PEC’s Tg is not well understood. Here, the e#ect of anion type (NaCl, NaBr, NaNO3, and NaI) on the Tg of solid-like, hydrated PECs composed of poly(diallydimethylammonium) (PDADMA)−poly(styrenesulfonate) (PSS) is investigated. With increasing the chaotropic nature of the salt anion, the Tg decreases. The relative di#erences are attributed to the doping level, the amount of bound water, the mobility of water molecules within the PECs, and the strength of interactions between the PEs. For all studied salt concentrations and salt types, the Tg followed the scaling of −1/Tg ≈ ln([IP]/[H2O]), in which [IP]/ [H2O] is the ratio of intrinsic pairs to water. The scaling estimates that about 7 to 17% of the intrinsic ion pairs should be weakened for the PEC to partake in a glass transition. Put together, this study highlights that the Tg in PECs is impacted by the salt anion, but the mechanism of the glass transition remains unchanged. 

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2. A Perspective on the Glass Transition and the Dynamics of Polyelectrolyte Multilayers and Complexes

   https://doi.org/10.1021/acs.langmuir.3c00974  

   Li, Hongwei; Lalwani, Suvesh Manoj; Eneh, Chikaodinaka I; Braide, Tamunoemi; Batys, Piotr; Sammalkorpi, Maria; Lutkenhaus, Jodie L (October 2023, Langmuir)

   Polyelectrolyte multilayers (PEMs) or polyelectrolyte complexes (PECs), formed by layer-by-layer assembly or the mixing of oppositely charged polyelectrolytes (PEs) in aqueous solution, respectively, have potential applications in health, energy, and the environment. PEMs and PECs are very tunable because their structure and properties are influenced by factors such as pH, ionic strength, salt type, humidity, and temperature. Therefore, it is increasingly important to understand how these factors affect PECs and PEMs on a molecular level. In this Feature Article, we summarize our contributions to the field in the development of approaches to quantify the swelling, thermal properties, and dynamic mechanical properties of PEMs and PECs. First, the role of water as a plasticizer and in the glass-transition temperature (Tg) in both strong poly(diallyldimethylammonium)/poly(sodium 4-styrenesulfonate) (PDADMA/PSS) and weak poly(allylamine hydrochloride)/poly(acrylic acid) (PAH/PAA) systems is presented. Then, factors influencing the dynamics of PECs and PEMs are discussed. We also reflect on the swelling of PEMs in response to different salts and solvent additives. Last, the nature of water’s microenvironment in PEMs/PECs is discussed. A special emphasis is placed on experimental techniques, along with molecular simulations. Taken together, this review presents an outlook and offers recommendations for future research directions, such as studying the additional effects of hydrogen-bonding hydrophobic interactions. 

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3. Formamide as a Robust Alternative to Water for Plasticizing Polyelectrolyte Complexes

   https://doi.org/10.1021/acs.macromol.4c01691  

   Hassoun, Sarriah; Digby, Zachary A; Abou_Hamad, Nagham; Schlenoff, Joseph B (October 2024, Macromolecules)

   Polyelectrolyte complexes, PECs, are glassy and brittle when dry but may be plasticized with water. Though hydrated PECs contain a high proportion of water, many still exhibit a glass transition in the 0 to 100 oC range. The apparently unique effectiveness of water as a plasticizer of PECs has been an obstacle to further developments in applications and in fundamental studies of PEC properties. In this work it is shown that formamide is an excellent and even superior solvent for plasticizing PECs, substantially decreasing glass transition temperatures relative to those of hydrated PECs when formamide is used as a solvent instead. The affinities of PECs for water and formamide, indicated by the (exothermic) enthalpies of solvent swelling of dry PECs, are comparable. Ion transport dynamics revealed similar lifetimes, about 1 ns, of charge pairs within a PEC solvated with water compared to formamide, despite the differences in their dielectric constants. Ion transport dynamics, which depend on the mobility of pendant groups, have lower cooperativity than those of the polymer backbone. The use of formamide is a significant experimental variable for reducing the glass transition temperature/viscosity of complexed polyelectrolytes and can turn a solidlike hydrated complex into a fluidlike coacervate. 

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4. Divalent cation effects in the glass transition of poly(diallyldimethylammonium)–poly(styrene sulfonate) polyelectrolyte complexes

   https://doi.org/10.1039/d4sm00856a  

   Braide, Tamunoemi; Manoj_Lalwani, Suvesh; Eneh, Chikaodinaka I; Lutkenhaus, Jodie L (December 2024, Soft Matter)

   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. 

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5. Solid–Liquid–Solution Phases in Poly(diallyldimethylammonium)/Poly(acrylic acid) Polyelectrolyte Complexes at Varying Temperatures

   https://doi.org/10.1021/acs.macromol.4c00258  

   Eneh, Chikaodinaka I; Nixon, Kevin; Lalwani, Suvesh Manoj; Sammalkorpi, Maria; Batys, Piotr; Lutkenhaus, Jodie L (March 2024, Macromolecules)

   The coacervation and complexation of oppositely charged polyelectrolytes are dependent on numerous environmental and preparatory factors, but temperature is often overlooked. Temperature eects remain unclear because the temperature dependence of both the dielectric constant and polymer− solvent interaction parameter can yield lower and/or upper critical solution phase behaviors for PECs. Further, secondary interactions, such as hydrogen bonding, can aect the temperature response of a PEC. That is, mixtures of oppositely charged polyelectrolytes can exhibit phase separation upon lowering and/or increasing the mixture’s temperature. Here, the phase behavior of poly- (diallylmethylammonium)/poly(acrylic acid) (PDADMA/PAA) complexes under varying KBr ionic strengths, mixing ratios, and temperatures at a fixed pH (in which PAA hydrogen bonding can occur) is examined. At room temperature, the PDADMA/PAA PECs exhibit four dierent phase states: precipitate, coexisting precipitate and coacervate, solid-like gel, and coacervate. Variabletemperature optical microscopy reveals the upper critical solution temperature (UCST) at which each phase transitioned to a solution state. Interestingly, the UCST value is highly dependent on the original phase of the PEC, in which solid-like precipitates exhibit higher UCST values. Large-scale all-atom molecular dynamics (MD) simulations support that precipitates exhibit kinetic trapping, which may contribute to the higher UCST values observed in the 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. 

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