Session N33: Dynamics of Polymers and Polyelectrolytes II

Charge Asymmetry Suppresses Coarsening Dynamics in Liquid–Liquid Phase Separation of Polymer Coacervates

Liquid–liquid phase separation (LLPS) of charged macromolecules in water typically involves the coarsening of small droplets. In experiments on biocondensate and polyelectrolyte complex coacervation (PEC), the coarsening process is found to be significantly slower than the known spinodal decomposition dynamics for neutral species. The suppressed coarsening in biocondensate formation has recently been attributed to the cellular environments, which are absent for the case of PEC.  One factor that has not been considered is the net charges in the droplets arising from the charge imbalance of the constituent polymers, which can create additional free energy barrier that potentially slows down the coarsening.

 

Using dissipative particle dynamics simulation, we show that charge asymmetry fundamentally alters the LLPS dynamics in PEC. When the small droplets are charge-balanced, the coarsening follows the classical spinodal decomposition; However, when the small droplets carry net  charges, the coarsening dynamics is significantly slower, with a reduced growth exponent that decreases with increasing charge asymmetry. We rationalize this slowing down by showing the free energy barrier in merging two net-charged coacervate droplets.  Solvent quality has effects: Poorer solvent conditions accelerate the coarsening in charge-asymmetric systems, but slightly slows down that in symmetric systems. These results highlight the nontrivial effects of electrostatic interactions on the LLPS dynamics.   

Fusion of block copolymer micelles in ionic liquids

Block copolymer micelles in dilute solutions have been the subject of extensive research aimed at investigating equilibrium structures. However, fundamental understanding of the self-assembly process and relaxation dynamics of block copolymer micelles remains incomplete. This research contributes to the ongoing exploration of micelle self-assembly and relaxations, shedding light on the intricate kinetics of micelle fusion. We explored the relaxation and growth kinetics of 1,2-polybutadiene-block-poly(ethylene oxide) micelles in ionic liquids using dynamic light scattering and liquid-phase transmission electron microscopy. Our observations revealed that the growth of micelles follows a biphasic kinetic mechanism, characterized by a two-step process, where the second step exhibits an order of magnitude lower rate compared to the initial step. When micelles are far from equilibrium, the growth shows second-order kinetics, yet understanding the growth mechanism of micelles near equilibrium remains a complex challenge. These findings enhance our understanding of the dynamic behavior of block copolymer micelles and have potential implications for a wide range of applications, including drug delivery, surfactant design, and nanoreactors.   

Solvent Quality Exponent Inferred from Extensional Relaxation Times of Unentangled Polymer Solutions

Polymer-solvent interactions influence static and dynamic properties of polymer solutions, including molecular weight and concentration-dependent variation in coil size, chain diffusivity, and rheological properties like zero shear viscosity and relaxation time. Therefore, quantitative measurement of solvent quality exponent that provides the power law dependence of equilibrium coil size on molecular weight or the number of Kuhn segments is a critical step in the characterization of macromolecular properties, and making inferences about their phase behavior, rheology, and processability. Traditionally, the solvent parameter is determined by making measurements of how intrinsic viscosity varies with molecular weight, presenting profound challenges in terms of material, time, patience, and equipment needs. Here we propose an alternative procedure based on analyzing extensional relaxation time as a function of concentration for a given polymer. We collate and revisit the recent measurements of extensional relaxation time using analysis of capillary-driven pinching dynamics in experiments carried with diverse techniques like dripping-onto-substrate (DoS) rheometry, capillary breakup extensional rheometer (CaBER), Cambridge Trimaster, and dripping. We include measurements over a wide range of solvents and polymers with varied molecular weights to illustrate how the response is influenced by the solvent quality and the macromolecular properties set by the number, length, and diameter of Kuhn segments.   

Zwitterionic Copolymeric Gel for High-performance Ionic Thermoelectric Generators

Heat generated from various sources is inevitably wasted. Thermoelectric generators (TEGs) are suitable for recovering such waste heat because of their simple structure and operating principle. Herein, a zwitterionic (ZI) polymer-based fully self-healable ionic TEG is presented. By adjusting the position of the positive and negative moieties of the ZI side chain, the movement of free ions contained in the ionogel can be controlled effectively, thus improving the power factor of TEGs. ZI side chains inside the ionogel provide multiple ionic interaction sites and enable fast self-healing characteristics at room temperature. Furthermore, a self-healing electrode composed of liquid metal (LM), waterborne polyurethane (WPU), and polyvinyl alcohol (PVA) is developed for the fully healable TEGs. The overall areal output voltage is effectively improved by connecting 10 legs of p and n thermoelectric gels in series via a self-healing process. The selectively ion-boosted, fully self-healable TEGs developed in this study will open a new horizon for high-performance ionic TEGs.   

Simulations of swollen, self-assembled single-ion-conducting multiblock copolymers

Polymer electrolytes are valued for their improved stability and mechanical properties over traditional organic solvent electrolytes. Unfortunately, polymer electrolytes often suffer from low conductivities due to coupling between ions and slow-moving polymer segments. In this work we study self-assembled single-ion-conducting multiblock copolymers that are swollen with solvents and that show improved lithium-ion conductivity over dry materials. We use a combination of experimental techniques and molecular dynamics simulations to understand the improved conductivity and changes in the self-assembled microstructure from added solvent.   

Ionic Conductivity of Highly-Asymmetric Block Copolymers Based on Polymer Ionic Liquids

Block copolymers (BCPs) based on nonionic polymers linked to polymer ionic liquids (PILs) are a promising class of electrolytes for energy storage and conversion devices. Our prior studies found that the bulk ionic conductivity of certain lamellar-forming materials was significantly depressed relative to simple predictions of the normalized ionic conductivity based on the ionic conductivity of the PIL homopolymer, the composition of the BCP, and the self-assembled morphology of the BCP. We hypothesized that this depression is due to poor connectivity among ionic domains in the lamellar phase, and we tested this hypothesis by examining the ionic conductivity in a series of highly asymmetric BCPs (spherical and cylindrical morphologies) having a majority PIL phase. We found that the normalized ionic conductivity in these materials met or exceeded the predicted values. Notably, the ionic conductivity of some highly asymmetric BCPs exceeded that of the homopolymer PIL, an effect that is tentatively attributed to enhanced decoupling of ion transport and segmental dynamics in the confined BCP domain.   

Improving Lithium Conductivity in Surfactant-Like Multiblock Copolymers through Selective Solvent Swelling

In previous work, we identified a variety of period morphologies in multiblock copolymers comprised of alkyl blocks of a fixed length strictly alternating with polar blocks having one lithium sulfonate group. As a function of alkyl block length and temperature we observed layered, double gyroid, and hexagonally-packed cylindrical morphologies. The addition of DMSO selectively swelled the polar blocks and increased ionic conductivity by 10^4. This work explores the effect of a wide variety of selective solvents on lithium conductivity as measured by electrochemical impedance spectroscopy. X-ray scattering detects morphological changes and IR spectroscopy elucidates the local lithium environment. The experiments are complemented by all atom molecular dynamics simulations. Swelling with propylene carbonate proves to be more effective at increasing lithium conductivity than swelling with ethylene carbonate or diglyme. Insights from these techniques will guide rational solvent selection for enhanced conductivity in polymer electrolytes.   

Investigation of the Adsorption Behavior of Sodium Carboxymethyl Cellulose on Carbon Black Towards the Understanding of Li-ion Battery Slurry Stability

Despite a great interest in Lithium-ion batteries (LIB) as a next-generation energy storage device, a fundamental understanding of the LIB slurry structure remains a challenge. The stability of LIB slurries can be problematic and must be understood for superior electrochemical properties and a long, stable life cycle. The role of polymer binders has been noticed in LIB slurries as they sustain the microstructure through physical or chemical adsorption to the particles during the electrode processing. However, the direct observation of the adsorption is challenging as typical imaging analyses are not available because of the opaque nature of LIB materials.

In this study, we attempt to investigate the adsorption behavior in the model system of sodium carboxymethyl cellulose (CMC) and carbon black (CB) from the perspective of CMC dynamics as the interaction between particle and polymer induces an inevitable conformational change of polymer. Employing broadband dielectric spectroscopy, the dynamic properties of CMC and the change of CMC dielectric responses with the CB were examined in detail. The dynamics of CMC were systemically investigated with varying degrees of substitution (DS) of CMC and CB content. The dynamics of CMC with lower DS have slowed down with more adsorption to CB, resulting in increased conductivity originating from the sturdier CB network.   

Influences of Chain Polarity and Molecular Weight on Ion and Polymer Dynamics in Polymerized Ionic Liquids

The results of a recent molecular dynamics simulation for a lithium ion-containing polymer reveal that lower chain polarities promote ion aggregation, while very high polarity slows down polymer segmental dynamics, both leading to reduced ionic diffusion. In this study, we employ coarse-grained molecular dynamics simulations to explore whether similar effects extend to polymerized ionic liquids. We have developed our Stockmayer fluid model, which treats the polymers as chains of charged and rotating dipolar spheres. This model allows us to systematically adjust the monomers' dipole moments and probe the system's dynamics without the need to model different polymer architectures or introduce additional molecular parameter sets. Moreover, we investigate the effect of molecular weight (chain length) and molecular polarity on the scaling laws of ion transport and viscosity, with a focus on their combined influence on ion aggregation and polymer segmental motion.   

Enhanced Ion Conductivity of Poly(ionic Liquid)-Grafted Nanoparticle-based Hybrid Electrolytes

Poly(ionic liquid) (PIL)-grafted nanoparticles are designed with chains that interact with surrounding free ionic liquid medium, and the arrangement of these particles can help to enhance ion conductivity. Specifically, poly(1-vinylimidazolium bistriflimide)-grafted iron oxide nanoparticles with different chain lengths were synthesized to understand the role of grafting the PIL chains on ionic conductivity. The long-range Coulombic interactions between PIL-grafted chains induce the formation of percolated strings at low particle concentrations in ionic liquid/cosolvent mixtures. The high conductivity of this new hybrid system is attributed to the confined polycationic grafts and cooperative ion motion between the grafted particle networks. The measured property is explained by the ladder mechanism of ion hopping in polyelectrolytes. This study shows the importance of chain confinement and particle percolation for the single-ion conductor design.   

Including stiffness and glass transition temperature differences in a coarse-grained model of salt-doped block copolymer electrolytes

Salt-doped block copolymers are promising solid electrolytes because of their ability to form two distinct microphases which allow for ionic conductivity along with significant mechanical strength. Our prior work focused on understanding how key physical parameters, such as the dielectric strength of the polymers as modeled through scaled Coulomb interactions and an ion-monomer solvation potential, act to set the overall trends in structural and dynamic properties. Our generic coarse-grained model captured the most salient structural and dynamic trends in these systems such as how ion conductivity and domain size change as a function of ion concentration. Here, we also include stiffness and glass transition temperature differences between the two blocks to match these relative differences in salt-doped polystyrene-b-poly(oligo-oxyethylene methyl ether methacrylate) systems. Specifically, by adding angle potentials and tuning the Lennard-Jones and solvation potential strengths, we are able to achieve glass transition temperatures and domain sizes that resemble those observed in experiments, allowing us to better understand local ion mobility in these systems.   

Control of viscoelasticity and ion transport in dynamic polymer networks using lithium salts

Vitrimers are dynamic polymer networks which can offer self-healing and unique physical properties relative to conventional polymers. Here, the properties of polyethylene oxide vitrimers with added salt were characterized with vinylogous urethane (VU) bonds which are catalyzed by the presence of lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) salt. Precise linker lengths ethylene glycol (xEG, x = 2-12) were prepared, and stress relaxation experiments show that the characteristic relaxation time is accelerated by a factor of ~70 due to added salt. As the crosslinking density of the vitrimers increases with shorter linker lengths, stress relaxation times increased with increasing T_g. The characteristic relaxation times do not superimpose when normalized by T_g, while the ionic conductivity collapses onto a universal curve indicating that conductivity is still dominated by segmental dynamics. In addition, ⁷Li solid state NMR and density functional theory (DFT) simulations show that the preference of Li ions to solvate with oxygen atoms along the ethylene glycol chains or with the VU bond site depends on the linker length, such that longer chains have a greater ability to solvate the ions.   

Effect of concentration on solution state redox activity- A bridge between polymer physics and electrochemistry

Redox-active polymers are popular electroactive materials for energy storage due to their unique electronic properties. In solution-state electrochemistry, polymer concentration can be varied. Polymer chains behave differently at different concentrations. We hypothesize that concentration variation from a dilute to a semi-dilute regime can change their redox activity due to change in the electron hopping mechanism. In this study, we investigate the redox activity of phthalimide-based polymers in an organic electrolyte system at different concentrations. These are non-conjugated polymers that have redox active pendant groups attached to their backbone. They gain electrons during charge and reduce electrons during discharge. The electron transfer starts with the diffusion of the redox-active species followed by molecular orientation and then the transfer of electrons to other redox-active groups (self-exchange) or transfer to the current collector (heterogenous). We consider the diffusion cooperative model, where the electron self-exchange occurs with the physical diffusion of the redox-active sites. The molecular orientation occurs due to Brownian motion. An electron hops from one redox unit to another only when they are in their closest proximity. We focus on the variation in the homogenous and heterogenous rate constants, and diffusion coefficient at each concentration. A fixed trend of variation of kinetic parameters and the factors affecting such a trend have been described.

    

Effect of Flow on Charge Transport in Semi-Dilute Redox Active Polymer Solutions

Redox-active polymers (RAPs) are a subset of polyelectrolytes possessing the ability to store charge and undergo redox self-exchange. These materials have garnered attention in the field of redox flow batteries due to their chemical modularity, molecular size, and ability to rapidly charge and discharge. Although their modularity is an opportunity to design at the molecular level for efficient charge transport, this would require a fundamental understanding of how RAP dynamics are coupled to charge transport in redox flow batteries. Previous work has explored the charge transport mechanisms and how they are affected by hydrodynamic interactions in equilibrium. We now seek to understand how flow and subsequently non-equilibrium RAP conformations give rise to charge transport; specifically when chain-to-chain interactions exist.We use Brownian Dynamics simulations paired with a Monte Carlo representation of charge-hopping to model charge transport in flowing solutions, using a 'conformational averaging' technique to account for hydrodynamic interactions. This model is used to show how polymer extension in strong flows give rise to modified charge transport both between chains and within the same chain. Furthermore, this modification depends on the fraction of RAP monomers charged. RAPs with a low fraction of charges show increases in charge transport with flow, while highly charged chains show decreases in charge transport with flow due to the limited amount of accessible hopping sites.