Session Z30: Dynamics in Charged and Ion-Containing Polymers

Thermotropic Side-Chain Polymer and Small-Molecule Ionic Liquid Crystals with Ion Transport in Nanoscale Domains

Electrochemical energy storage devices are increasingly desired for the electrification of transportation and utilization of renewably generated power.  Safety and performance of batteries is strongly dictated by the performance of the electrolyte.  Polymer and ionic liquid electrolytes offer potential advantages over traditional liquid and inorganic solid-state electrolytes, as they are non-volatile, yet flexible.  Single-ion conducting electrolytes that eliminate ion concentration gradients can offer further advantages: greater electrochemical stability allowing for higher voltage cells, lower interfacial impedance, and higher theoretical charge/discharge rates.  This talk will highlight our recent research on liquid crystalline electrolytes composed of side-chain ionomers or amphiphilic salts with solely non-polar chains, where we aim to understand ion transport in phase segregated, nanoscale ionic domains.  The chemistry of the tethered anion and chains are found to have implications on ionic domain morphology and ion transport properties.   

Molecular Engineering of Ion-Conducting Polymer Membranes: Synthesis, Properties, and Applications

Ion-conducting polymers (e.g., proton and hydroxide) are used as polymer electrolyte membranes, and they are a key component of electrochemical energy conversion and storage technologies such as fuel cells, electrolyzers, and flow batteries. For example, hydrogen fuel cell and water electrolyzer industry also heavily relies on Nafion for a proton-conducting membrane for decades, though it is not ideal proton-conducting membrane material for those applications.

Perfluoroalkyl substrates (PFASs), which include Nafion, have recently received significant pressure from government and environmental activist groups due to their environmental and health hazard of PFAS. Thus, the development of alternative ion-conducting polymers based on hydrocarbon polymers (mostly made of C and H) has attracted new attention within polymer society. Compared to perfluorosulfonated Nafion, hydrocarbon ion-conducting polymers can offer advantages of flexible synthetic tunability, lower gas permeability, and importantly lower production cost. However, their systematic polymer chemistry–morphology structure–property relationships are poorly understood until now.

In this presentation, we describe an overview of recent progress in the development of advanced ion-conducting (H⁺ and OH^–) polymers, key membrane properties, and their state-of-the-art performance in in real world applications of hydrogen fuel cells and water electrolyzers.   

Emergent Viscoelasticity in Polyelectrolyte Complex Coacervates: Relaxation, from Monomer to Entangled Polymer Chains

Polyelectrolyte complex coacervates, PECs, are associated charged polymers with charge pairing between positive, Pol⁺, and negative, Pol^-, repeat units. The Pol⁺Pol^- dynamic physical crosslinks are responsible for increasing the viscosity of PECs on the micro- and macro- scale. Even in liquid-like PECs, often termed coacervates, relaxation times spanning 10 orders of magnitude are found at various length scales. This presentation will discuss the measurements of Pol⁺Pol^- pair relaxation time, about 10^-9 s, to entangled polymer viscosity with an entanglement time of ~50 s. In PECs without added salt the mechanism of segment relaxation is much slower: pair exchange, with a lifetime of about 0.3 s. This lifetime controls the dynamics of the rest of the chain. “Doping” by added salt breaks these Pol⁺Pol^- pairs and accelerates chain motion, while having little influence on the pair lifetime. Therefore, “saloplasticity,” the plasticization of PECs with salt doping, stems from a (completely reversible) reduction in crosslink density and not from a decrease in the Pol⁺Pol^- lifetime by electrostatic screening of charges. A full picture of polymer coils within PECs, obtained by small angle neutron scattering of deuterated polyelectrolyte, enables quantitative analysis according to “sticky reptation” models.   

Unreacted amine groups: indispensable keys to unlock imine bonds in dynamic networks

Several strategic areas rely on the development of polymers, but the growth of plastic waste urges for new materials combining desired mechanical properties and recyclability. A promising solution is the partial/total replacement of permanent crosslinks by dynamic bonds, which allows for designing easily recyclable polymers without loss of their mechanical properties in a desired temperature range. 

Among the possible dynamic bonds, imine functionalities are promising candidates as they undergo rapid exchange and can be formed in the absence of catalysts with water as the only byproduct of the reaction. Imines are formed via condensation of primary amines with carbonyl compounds and can undergo hydrolysis, transamination, and metathesis reactions depending on the concentration of water and unreacted amines. While these mechanisms are well-established in chemistry and biology literature, the utilization of imine bonds as a dynamic moiety in long-chain polymers is recent. Hence, the crucial role of unreacted amines, even in residual amounts, as a mediator of metathesis exchange, is often overlooked in the design and description of these networks. Therefore, we seek to provide a mechanistic description of the dynamic and viscoelastic behavior of telechelic and pendant functionalized model systems based on polydimethylsiloxane (PDMS) bearing imine bonds. We combine linear rheology and broadband dielectric spectroscopy to systematically show that in the absence of unreacted amines, network rearrangement based on imine exchange does not occur at temperatures reasonably lower than polymer degradation. Also, we show that the energy barrier of bond rearrangement is dictated by the concentration of cross-linkers in the system and not by the effective cross-link density.   

Network forming liquids create tunable nanostructures for efficient small molecule transport and intermolecular interactions

Network forming liquids, employing either bulky ions and/or hydrogen bonds exhibit structural and dynamic features with spatiotemporal scales about an order of magnitude smaller and faster than those of traditional polymers. When analyzing their molecular structure, most of these liquids exhibit nanoscale heterogeneity beyond the nearest neighbor solvation shell and in some cases, these larger length scale correlations are reminiscent of the lyotropic liquid crystalline mesophases that make up the phase diagram of many polymers. In this talk, we will discuss the link between nanoscale heterogeneity in network liquids and a liquid’s small molecule transport (e.g., CO2) properties. We will highlight recent x-ray scattering, computation and theory work with water lean solvents for CO2 capture and conversion, novel hydrophobic deep eutectic solvents and phosphine-based ionic liquids, in an effort to demonstrate the connection between microstructure and dynamical properties for some of these systems. Such studies can provide critical insights to guide the design and synthesis of composite or chemically functionalized polymers in many applications including separations, nanochemistry, and advanced structural materials.