Session T33: Charged and Ion-Containing Polymers

Coarse-Grained Molecular Dynamics of Polymer Electrolytes Using Drude Oscillators

Ion-polymer interactions in polymer electrolytes must be strong enough to solvate ions and allow for separate cation and anion motion, however, very strong local interactions slow ion dynamics and reduce conductivity. We use generic models and molecular dynamic simulations to understand these effects and suggest strategies to design improved materials. In prior work, we have used an isotropic ion-polymer potential form to reproduce features of ion solvation. However, this does not include polarizability or effects of locally varying dielectric strength due to different polymer and ion densities. In atomistic simulations, polarizability can be included using Drude oscillators, in which an atom is represented by a core bonded to a Drude particle of opposite charge via a strong harmonic spring. We embed Drude oscillators into each polymer bead of our coarse-grained model and study ion-containing polymers with a range of polarizabilities. The calculated dielectric constant from simulations shows a good match with the Clausius–Mossotti equation at low enough polarizability, and the calculated solvation energy agrees with the Born relation. The potential for simulating copolymer systems in this manner will also be discussed.   

Role of chain architecture and ion size on morphology, dynamics, and viscoelasticity of ionomers

Ionomers, polymers with a small fraction of ions in their backbone, form ionic nanodomains and can function as physical crosslinks in polymer networks. The intensity of the physical crosslinks which in turn controls their morphology, dynamics, and rheological response, can be controlled by the charge density and relative size of the ions on the chains. In this regard, the relative position of cation and anion in the form of bridged and pendant ionomers can be used as a design parameter to control the viscoelasticity of these networks. These two families of ionomers differ in ion attachment to their backbone, with bridged featuring both ion types bound to the backbone, while the pendant structure resembles organic cationic ionomers with backbone-bound cations and free anions. In this study, coarse-grained molecular dynamics simulations will be used to study their morphology, dynamics, and rheology. Our results show that bridged ionomers exhibit a percolated ionic network, while pendant ionomers have distinct aggregates. Dynamical heterogeneities are evident in both structures but more pronounced in bridged ionomers at higher temperatures. A direct relationship between ionic bond lifetime and structural relaxation time is established. This linkage indicates the collective ion dynamics in bridged ionomers, whereas pendant ones show initial ion-pair dissociation before escaping their local environment. Similarly, bridged ionomers show higher shear viscosity. Ion size affects their morphology and dynamics by influencing chain spacing and ionic interaction strength. This, in turn, can influence the viscoelastic and mechanical properties of the ionomer. Morphological and rheological results will also be presented for ionomers with different cation sizes.   

Understanding Polymer Design Effects on Hydrated Ionomer Morphology and Hydrophilic Domain Structures

Ionomer membranes are frequently used as proton conducting membranes for fuel cell and water electrolyzer applications [Chemical Reviews 2017, 117 (3), 987-1104] due to their chemical stability, ion conductivity, and mechanical rigidity. The ion transport capabilities and mechanical properties of these membranes are dictated by the morphology of the hydrated ionomer film. The hydrophilic domains of these hydrated ionomers are of particular interest as the structure and connectivity of these domains influence the ion-transport capabilities of the membrane. In this work, we show how desired hydrophilic domain structures can be achieved by tuning the polymer design of the ionomers; and how the morphology of the various ionomer designs studied evolve while varying the extent of hydration. We use coarse-grained molecular dynamics simulations to study ionomers at mesoscopic length scales. These simulations show how the charged sidechain spacing, placement, and length impacts the morphology (i.e., domain sizes and shapes and connectivity of domains) of the polymer backbones, charged sidechains, water molecules, and hydronium beads.    

Relationship between Phase Behavior and Ion Transport in Single-ion Conducting Polymer Blends Electrolyte

Poly(ethylene oxide) (PEO) has been widely studied as a candidate for solid-state electrolytes in next generation lithium metal batteries due to its ability to dissociate lithium salts and enable fast ion transport. However, PEO-based electrolytes possess low cation transference numbers, which results in the formation of steep lithium ion concentration gradients across the electrolyte and limits the maximum charging rate of the battery. Single-ion conducting polymers, wherein the anions are immobilized onto the polymer backbone, inhibit the formation of lithium ion concentration gradients, which greatly increases the cation transference number (t⁺). Nevertheless, these ion-containing polymers typically exhibit low ionic conductivity due to their high glass-transition temperatures. In this presentation, I will introduce a novel class of electrolytes, single-ion conducting polymer blend electrolytes (SIPBE), generated by blending two distinct classes of polymers together. I will present how SIPBEs form miscible blends and possess both high ionic conductivity and high t⁺ values. The phase diagram of new SIPBE systems will be determined through differential scanning calorimetry (DSC) to investigate the effect of polymer molecular weights and salt concentrations on phase behavior. In addition, the miscibility of SIPBE systems can be used to help interpret the ion transport properties measured with electrochemical impedance spectroscopy (EIS). The results will provide insight into thermodynamics and structure-conductivity relationship of the novel SIPBE system.   

Understanding the Impact of Li+ vs Na+ Salt on the Mechanical and Dynamic Properties of Polyzwitterion-supported Ionogel Electrolytes

Polyzwitterion (polyZI) supported ionogel electrolytes have emerged as alternatives for designing mechanically robust electrolytes with high room temperature conductivities. This is due to the PolyZI side chains playing a dual role as noncovalent crosslinking sites and mobile ion dissociation promoters. In this study, we used atomistic molecular dynamic simulations to probe the impact of cation identity (Li⁺vs Na⁺ ) on the elastic moduli and conductivity of single-salt (Li⁺ or Na⁺) and mixed-salt (Li⁺ and Na⁺) ionogels as a function of ZI weight percentage. Our model system is comprised of ZI poly (2-methacryloyloxyethyl phosphorylcholine) (pMPC) loaded with 0.5M X + TFSI/N-butyl-N-methylpyrrolidinium (BMP) TFSI ionic liquid electrolyte (where X ⁺ = Li⁺, Na⁺, or Li ⁺ and Na⁺ ). For both single- and mixed-salt doped ionogel electrolytes, with increasing ZI weight percentage, we observe an increasing association between Li⁺ and Na⁺ ions with the ZI polymer. This observation is accompanied by a decrease in the interaction between Li⁺ and Na⁺ ions with the ionic liquid and salt common anion. Our simulations also reveal a preferential association of Li + - ZI over Na + - ZI in both cases. We also present the probability of ion-pair associations to identify the mechanistic origins of structural and dynamical property differences in both single- and mixed-salt ionogels. Lastly, we show cluster analysis to rationalize the experimentally observed elastic moduli and conductivity results.   

Structure-Property Relationships in Mixed Ionic-Electronic Conductors via Machine Learning-Enhanced Multiscale Modeling

Polymers possessing electronic and ionic coupled functionalities offer unique solutions for biomedical sensors, neuromorphic computing, and all-organic batteries. The rational design of polymers with tailored functionalities is obscured by the interplay of electronic, ionic, and structural degrees of freedom over a wide range of spatiotemporal scales. We recently developed an effective computational approach that combines physics-based and machine learning techniques to incorporate electronic structure information at large spatiotemporal scales. Leveraging this approach, we investigate the structural, ionic, and electronic properties of mixed ionic-electronic conductors based on redox-active nonconjugated polymers. This emerging class of materials for solid-state organic batteries relies on pendant redox-active sites to transport charge carriers. We systematically examine polymer-electrolyte systems and vary the polymer backbone chemistry, electrolyte solution content, and polymer state of charge. As a function of such system parameters, we then explore in detail structural properties, such as redox active site packing and their interaction with the electrolyte, as well as ionic and electronic transport. The derived relationships between chemical structure, morphology, and ionic and electronic transport inform the design of redox-active polymers with improved characteristics for all-organic battery materials.   

Probing the ion binding capabilities of synthetic polyzwitterions

Zwitterionic polymers have shown great promise for a wide range of applications, including ion-separation membranes in water purification devices. Despite significant advances in the characterization of polyzwitterions in different hydrated environments, the mechanism by which local zwitterionic structure and polymer brush architecture affects selective ion capture has yet to be established. In our work, we developed a tunable polymer platform through which we introduce systematic changes to the chemistry of the side chain and directly examine their impact on the polyzwitterion interactions with different ionic species as well as the chain architecture in solution. Specifically, the solution behavior and selective ion-binding capabilities of cysteine-based polyzwitterions are investigated using a combination of dynamic light scattering, turbidity measurements, and ion-coupled plasma mass spectrometry. A deeper understanding of the interplay between chain dimensions, specific ion interactions and zwitterion identity will allow better control of the binding affinity for different ionic species, opening the door to more efficient design of ion-separation membranes.   

Ice Nucleation and Zwitterionic Polymers: Ab Initio Investigations of Molecular Interactions

Ice formation and accumulation on solid surfaces have long presented a formidable challenge across a range of applications. To address this, anti-icing coatings have emerged as a promising solution, effectively reducing ice formation and adhesion. Notably, the recent development of zwitterionic polymer coatings has gained significant attention due to their outstanding anti-icing performance, making them prime candidates for anti-icing applications. In our study, we used Density Functional Theory (DFT) to gain a comprehensive understanding of ice nucleation and its structural properties when interacting with zwitterionic polymers. We delved into various ice clusters, exploring the spectrum of ice structures in the nucleation process. Our focus was on four zwitterionic polymers: poly-sulfobetaine (polySB), structural quasi-isomers of poly-sulfobetaine methacrylates (polySBi), poly-phosphorylcholine (polyMPC), and poly(carboxybetaine acrylamide) (polyCBAA). Our findings reveal the intricate molecular mechanisms responsible for their anti-icing prowess, shedding light on the fundamental interactions between ice and these polymers. This research sets the stage for the development of next-generation anti-icing materials, offering valuable insights to the materials science community.   

Combined effects of pH, salt concentration, and ionizable monomer fraction on the swelling behavior and hysteresis of weak polyelectrolyte brushes

Weak polyelectrolyte brushes (PEBs) are highly tunable pH-responsive coatings, with applications in separations and biological industries. Weak PEBs exhibit significant changes in swelling in response to changes in their charge state, which is regulated by ion exchange between the bulk solution and the brush layer and can be tuned by changing pH and salt concentration. It has been shown, however, that the swelling response of weak PEBs to changes in pH is hysteretic. Therefore, a more thorough understanding of the physical properties that give rise to this hysteretic effect is required to realize the full potential of weak PEBs as pH-responsive coatings. Here, we investigate the combined effects of ionizable monomer fraction (25% to 100%), pH cycling (pH 3 through 10) and salt concentration (1 mM to 1 M) on the swelling behavior and hysteresis in weak basic PEBs using in situ ellipsometry. PEBs consist of random copolymers of basic 2-(dimethylamino)ethyl acrylate (DMAEA) (pKa = 8.4) and neutral 2-hydroxyethyl acrylate (HEA), synthesized by SI-CuCRP. We observe that the swelling and extent of hysteresis in weak PEBs can be controlled by altering both ionizable monomer fraction and salt concentration. Hysteresis generally increases with ionizable monomer fraction, whereas the effects of salt are non-monotonous. The non-monotonic effect of salt concentration is consistent with the change from the osmotic to the salted brush regime as salt concentration is increased.   

Localized Anionic and Hydrophobic Effects, Multiple Hydration States, and Counterions with Large Mobilities Inside Densely Grafted Cationic Brushes

Understanding the hydration behavior of polyelectrolyte (PE) brushes, resolved across an atomistic scale, can be critical for elucidating the new physics governing the properties and behaviors of these brushes. Here we employ all-atom molecular dynamics (MD) simulations for resolving the localized (of unprecedented atomistic-scale resolution) behavior of the cationic PMETAC [Poly(2-(methacryloyloxy)ethyl trimethylammonium chloride] brushes. Despite the well-known cationic nature of the PMETAC brushes, we identify that around the C=O group of the brushes, water molecules demonstrate dipole orientation that mimics the water behavior around strong anions. Additionally, the {N(CH₃)₃}⁺ group, due to the large steric and phobic effect of three methyl groups, triggers hydrophobic hydration enforcing water molecules to attain clathrate-like structures (as evident by a high value of the water structural order parameter). Such enhanced water ordering around the {N(CH₃)₃}⁺ group prevents a strong attraction between the {N(CH₃)₃}⁺ group and the Cl^- ions, thereby ensuring a very large mobility of the Cl^- ions inside the brush layer. Finally, we employ a combination of data analysis and machine learning algorithm (based on identifying representative clusters) to confirm that the water molecules around the {N(CH₃)₃}⁺ group are in two possible hydration states, namely a weak clathrate-like state and a strong clathrate-like state.   

Exploring the cooperative and competitive surface adhesion of polymers containing catechol and cationic residues

Sea animals, such as mussels and sandcastle worms, have evolved a remarkable ability to adhere to wet surfaces through the utilization of a diverse array of flexible proteins. Investigating these natural adhesives holds significant promise for robust adhesion in aquatic environments such as tissue repair. Recent studies on bioadhesives have underscored the prevalence of specific residues within these proteins, notably the substantial occurrence of cationic (e.g., Lysine) and catechol (e.g., DOPA) residues. While DOPA-based polymers have demonstrated robust binding capabilities in dry conditions, their efficacy diminishes significantly when exposed to saline environments. Intriguingly, the substitution of cationic residues into the polymer has been observed to yield robust adhesion even in these challenging wet conditions. This points towards a synergistic interplay between catechol and cationic residues. In this talk, we utilize polymer density functional theory to investigate the adhesion between charged surfaces mediated by polymers with varying composition of these two critical residues. Our findings establish clear design principles crucial for optimizing and tailoring the adhesive properties of these polymers.   

Elucidating The Glass Transitions and Material Properties of Polyelectrolyte Complex Materials

The formulation of functional polymers, like adhesives and coatings, is particularly challenging due to the interplay between performance and processability requirements. Complex coacervation is an entropically driven, associative liquid-liquid phase separation that results in a polymer-rich coacervate, and a polymer-poor supernatant dissolved in an aqueous solution. Salt-driven plasticization allows for the use of complex coacervation as a processing strategy. However, it is not clear whether many of the design rules associated with traditional polymers will still hold for materials based on polyelectrolyte complexation (PECs). To understand this design space, we tested a library of PECs made from oppositely-charged methacrylate copolymers of varying charge density and hydrophobicity. We characterized the resulting solid PEC materials using dynamic mechanical analysis and tensile tests. Our data show that the mechanics, which range from brittle to ductile, are intrinsically tied to PEC water content. We also highlight the effect of temperature, humidity, and salt concentration on the glass transitions of these materials to show how we can use these parameters to process PEC materials and achieve different mechanical responses.   

Sulfonated Ionomer (Bio)Composites for Use in Vanadium Redox Flow Batteries

Sulfonated aromatic polymers, such as sulfonated poly (ether ether ketone) (SPEEK), have emerged as promising proton exchange membranes (PEMs) for vanadium redox flow batteries (VRFBs) due to their high thermal and mechanical stability, low cost, and tunable conductivity via adjusting the sulfonation degree of the polymer. Herein, the impact of both inorganic and organic fillers on the performance properties of SPEEK-based membranes were investigated. Specifically, inorganic silica nanoparticles (SiNPs) functionalized with both cationic (i.e., amine-containing) and anionic (i.e., sulfonic acid-containing) surfaces, as well as the biopolymer lignin, of varying molecular weight, were used to fabricate SPEEK-based ionomer (bio)composites. Membrane performance properties – e.g., proton conductivity and vanadium permeability – of these films were characterized, where SPEEK-lignin composites, with up to 15 mass % lignin, demonstrated significantly enhanced proton selectivity when compared to their neat SPEEK counterparts. Further, to improve our understanding of the interplay of the water transport and polymer swelling kinetics, water diffusion and ionomer swelling kinetics of SPEEK-lignin composites were studied using in situ time-resolved attenuated total reflectance-Fourier transform infrared (tATR-FTIR). Finally, the impact of sulfonic acid group dispersion on membrane properties was investigated, where both random and “blocky” SPEEK membranes were fabricated and compared.   

Ion Transport Kinetics and Energy Barrier in Polymer Nanocomposite with Superionic Ceramic Nanorods

Incorporating superionic ceramic electrolyte fillers into a polymer electrolyte matrix may result in composites with increased ionic conductivity, enhanced electrochemical stability, and favorable device performance. In such a composite electrolyte, the ionic conductivity enhancement mainly originates from two proposed mechanisms: 1) through a fast ion-transport interface layer along the ceramic particle-polymer interface, and 2) through percolated, highly conductive ceramic fillers. It is important to understand the enhancement mechanism, as the electrolyte design principles for these two mechanisms may vary.

In this work, we investigate the ion transport and energy barrier controlling this transport in composites made with a single-ion-conducting (SIC) polymer electrolyte with lithium lanthanum titanate (LLTO) nanorods. We use broadband dielectric spectroscopy (BDS) and pulsed-filed gradient nuclear magnetic resonance (PFG-NMR) to understand the ion transport mechanisms in the composite. The results show that the incorporation of 50 wt% of LLTO nanorod resulted in a two-fold enhancement of the ionic conductivity. The enhancement originates from the interface layer near the LLTO nanorod surface, through enhanced Li ion diffusion and restricted polyanion diffusion. The interfacial zone’s connectivity and thickness are examined. The ion transport energy barrier in the composites is analyzed and quantified as a function of temperature through BDS analysis. Small-angle X-ray scattering (SAXS) is used to investigate the spatial distribution/dispersion/percolation of LLTO in SIC polymer electrolyte matrix. This work sheds light on how to design composite electrolytes to maximize favorable ion transport paths and minimize barriers.   

Liquid to Thermo-responsive Gel Using Liquid-Liquid Printing: Properties and Applications

Poly(2-dimethylaminoethyl methacrylate) (PDMAEMA) with tertiary amine functional groups, exhibits lower critical solution temperature (LCST) behavior and is both thermoresponsive and pH-sensitive. These attributes make it a promising candidate for liquid-to-gel printing processes. PDMAEMA typically forms a soft, brittle gel for T<100^oC.  We exploited physical and electrostatic interactions to generate hollow-tubular-like hydrogels by liquid-liquid printing, taking advantage of a polyelectrolyte complex (PEC) formed between PDMAEMA and sodium poly (styrene sulfonate) (NaPSS). We investigated a range of compositions of PDMAEMA and the PEC system, correlating their behaviors with temperature. The liquid-gel transition was corroborated rheologically, showing that the introduction of PEC, in combination with alginate, substantially elevated the modulus, and transitioned the LCST to an upper critical solution temperature (UCST) response. Following the formation of the liquid membrane and hydrogel membrane, we quantified the diffusion of self-interaction.