Conformational and Dynamic Properties of Poly(ethylene oxide) in BMIM+BF4–: A Microsecond Computer Simulation Study Using ab Initio Force Fields

Structural, energetic and dynamic investigation of poly(ethylene oxide) in imidazolium-based ionic liquids with different cationic structures

In this work, two imidazolium-based ionic liquids (ILs) with different cations including dications (DIL) and monocations (MIL) were blended with poly(ethylene oxide) (PEO). The influence of ILs' structure on the structural and dynamic properties of a PEO/IL system was investigated by molecular dynamics (MD) simulation and density functional theory (DFT) methods. The simulation results show that DIL exhibits weaker interaction with PEO than MIL due to a stronger IL aggregation effect. The intermolecular interaction also makes the PEO chain tend to organize around the imidazolium ring of ILs, which causes the conformational entropy loss. Compared with PEO/MIL, this phenomenon is more significant in PEO/DIL because of the double positive centers of the dication and a longer hydrogen bond lifetime. MD simulation also demonstrates that DIL could act as a "crosslinker" to promote the formation of a physical crosslinking network which has strong dependence on the concentration of IL. The competition between physical crosslinking and plasticizing effects induces non-monotonic variations of relaxation time in PEO/DIL, which is consistent with its unusual change of the glass transition temperature (Tg). Despite stronger hydrogen bonding interactions between PEO and MIL demonstrated by atom-in-molecules (AIM) and reduced density gradient (RDG) analysis, the segmental mobility is slower in PEO/DIL according to the MSD curve. These differences in multiple structural or energetic factors finally lead to different conductive mechanisms and hence obtain different ionic conductivities.

Superionic Bifunctional Polymer Electrolytes for Solid-State Energy Storage and Conversion

Achieving superionic conductivity from solid-state polymer electrolytes is an important task in the development of future energy storage and conversion technologies. Herein, a platform for innovative electrolyte technologies based on a bifunctional polymer, poly(3-hydroxy-4-sulfonated styrene) (PS-3H4S), is presented. By incorporating OH and SO₃ H functional groups at adjacent positions in the styrene repeating unit, "intra-monomer" hydrogen bonds are formed to effectively weaken the electrostatic interactions of the SO₃ ^- moieties in the polymer matrix with embedded ions, promoting rich structural and dynamic heterogeneity in the PS-3H4S electrolyte. Upon the incorporation of an ionic liquid, interconnected rod-like ion channels, which allow the decoupling of ion relaxation from polymer relaxation, are formed in the stiff motif of the polymeric domains passivated by interfacial ionic layers. This results in accelerated proton hopping through the glassy polymer matrix, and proton hopping becomes more pronounced at cryogenic temperatures down to -35 °C. The PS-3H4S/ionic liquid composite electrolytes exhibit a high ionic conductivity of 10^-3 S cm^-1 and high storage modulus of ≈100 MPa at 25 °C, and can be successfully applied in soft actuators and lithium-metal batteries.

Enhancing ion transport in charged block copolymers by stabilizing low symmetry morphology: Electrostatic control of interfaces

Recently, the interest in charged polymers has been rapidly growing due to their uses in energy storage and transfer devices. Yet, polymer electrolyte-based devices are not on the immediate horizon because of the low ionic conductivity. In the present study, we developed a methodology to enhance the ionic conductivity of charged block copolymers comprising ionic liquids through the electrostatic control of the interfacial layers. Unprecedented reentrant phase transitions between lamellar and A15 structures were seen, which cannot be explained by well-established thermodynamic factors. X-ray scattering experiments and molecular dynamics simulations revealed the formation of fascinating, thin ionic shell layers composed of ionic complexes. The ionic liquid cations of these complexes predominantly presented near the micellar interfaces if they had strong binding affinity with the charged polymer chains. Therefore, the interfacial properties and concentration fluctuations of the A15 structures were crucially dependent on the type of tethered acid groups in the polymers. Overall, the stabilization energies of the A15 structures were greater when enriched, attractive electrostatic interactions were present at the micellar interfaces. Contrary to the conventional wisdom that block copolymer interfaces act as "dead zone" to significantly deteriorate ion transport, this study establishes a prospective avenue for advanced polymer electrolyte having tailor-made interfaces.

Phase Behavior of Poly(ethylene oxide) in Room Temperature Ionic Liquids: A Molecular Simulation and Deep Neural Network Study

The phase behavior of polymers in room temperature ionic liquids is a topic of considerable interest. In this work we study the phase diagram of poly(ethylene oxide) in four imidazolium ionic liquids (ILs) using molecular simulation. We develop united atom models for 1-butyl-2,3-dimethylimidazolium ([BMMIM]), 1-ethyl-2,3-dimethylimidazolium ([EMMIM]), and 1-ethyl-3-methylimidazolium ([EMIM]) in an analogous fashion to previously developed models for 1-butyl-3-methylimidazolium ([BMIM]) and tetrafluoroborate ([BF₄]) using symmetry-adapted perturbation theory. At high temperatures we obtain the coexistence concentrations using an interface method where the polymer and IL are simulated in a large elongated box, and an interface between coexisting phases is formed. At lower temperatures we use a deep neural network (DNN) method. The input descriptors for the DNN are the cohesive energy of mixing, the volume change of mixing, and the coordination numbers between cation and polymer, all of which are obtained from simulations of mixed systems at a series of temperatures. The DNN is trained by using the phase-separated systems at high temperatures and a mixed phase at low temperatures. The method predicts a lower critical solution temperature which decreases as the alkyl chain length on the cation is decreased, consistent with experiment. The simulations show that methylation of the cation has little effect on the phase diagram. This is in contrast to what is seen in experiments but could be because the polymer chains in the simulations are too short. At low temperatures the chains display two conformational motifs, namely a crown ether conformation and a ring conformation, each of which can wrap the chain around a single cation. This provides the entropic penalty for mixing and a reason for demixing as the temperature is raised. Such conformations might not be possible for longer chains. The combination of data-driven techniques and molecular simulation shows promise in the study of the phase behavior and physical properties of complex fluids.

Ion transport in small-molecule and polymer electrolytes

Solid-state polymer electrolytes and high-concentration liquid electrolytes, such as water-in-salt electrolytes and ionic liquids, are emerging materials to replace the flammable organic electrolytes widely used in industrial lithium-ion batteries. Extensive efforts have been made to understand the ion transport mechanisms and optimize the ion transport properties. This perspective reviews the current understanding of the ion transport and polymer dynamics in liquid and polymer electrolytes, comparing the similarities and differences in the two types of electrolytes. Combining recent experimental and theoretical findings, we attempt to connect and explain ion transport mechanisms in different types of small-molecule and polymer electrolytes from a theoretical perspective, linking the macroscopic transport coefficients to the microscopic, molecular properties such as the solvation environment of the ions, salt concentration, solvent/polymer molecular weight, ion pairing, and correlated ion motion. We emphasize universal features in the ion transport and polymer dynamics by highlighting the relevant time and length scales. Several outstanding questions and anticipated developments for electrolyte design are discussed, including the negative transference number, control of ion transport through precision synthesis, and development of predictive multiscale modeling approaches.

Solvation and diffusion of poly(vinyl alcohol) chains in a hydrated inorganic ionic liquid

While the behavior of polyelectrolyte chains in aqueous salt solutions has been extensively studied, little is known about polar polymer chains in solvents with extremely high concentrations of inorganic ions, such as those found in ionic liquids (ILs). Here, we report on expansion, solvation and diffusion of poly(vinyl alcohol), PVA, chains in dilute solutions of a hydrated inorganic IL phase change material (PCM), lithium nitrate trihydrate (LNH). This solvent has an extremely high concentration of inorganic ions (≈18 M) with a low concentration of water molecules largely forming solvation shells of Li⁺ and NO₃^- ions, as shown using ATR-FTIR spectroscopy. Diffusion and hydrodynamic size of PVA chains of different molecular weights in this unusual solvent were studied using fluorescence correlation spectroscopy (FCS). A higher scaling exponent obtained from the molecular weight dependences of the diffusion coefficients of PVA chains as well as a lower overlap concentration (c*) of PVA in LNH solutions as measured by FCS suggest an expansion of the polymer coils in this solvent. We argue that enhanced solubility of PVA in LNH solutions is likely a result of increased rigidification of polymer chains due to the binding of solvated Li⁺ ions, which is demonstrated using ⁷Li NMR spectroscopy. We believe that an understanding of solvation and ion-binding capability can offer crucial insight into designing polymer-based shape stabilization matrices for inorganic PCMs.

Rheology of Concentrated Polymer/Ionic Liquid Solutions: An Anomalous Plasticizing Effect and a Universality in Nonlinear Shear Rheology

An anomalous plasticizing effect was observed in polymer/ionic liquid (IL) solutions by applying broad range of rheological techniques. Poly(ethylene oxide)(PEO)/IL solutions exhibit stronger dynamic temperature dependence than pure PEO, which is in conflict with the knowledge that lower-Tg solvent increases the fractional free volume. For poly(methy methacrylate)(PMMA)/IL solutions, the subtle anomaly was detected from the fact that the effective glass transition temperature Tg,eff of PMMA in IL is higher than the prediction of the self-concentration model, while in conventional polymer solutions, Tg,eff follows the original Fox equation. Observations in both solutions reveal retarded segmental dynamics, consistent with a recent simulation result (Macromolecules, 2018, 51, 5336) that polymer chains wrap the IL cations by hydrogen bonding interactions and the segmental unwrapping delays their relaxation. Start-up shear and nonlinear stress relaxation tests of polymer/IL solutions follow a universal nonlinear rheological behavior as polymer melts and solutions, indicating that the segment-cation interaction is not strong enough to influence the nonlinear chain orientation and stretch. The present work may arouse the further theoretical, experimental, and simulation interests in interpreting the effect of complex polymer-IL interaction on the dynamics of polymer/IL solutions.