Ionic Conductivity of Nanostructured Block Copolymer/Ionic Liquid Membranes

Effects of a Nanophase-Separated Structure on Mechanical Properties and Proton Conductivity of Acid-Infiltrated Block Polymer Electrolyte Membranes under Non-Humidification

Acid-infiltrated block polymer electrolyte membranes adopting a spherical or lamellar nanophase-separated structure were prepared by infiltrating sulfuric acid (H₂SO₄) into polystyrene-b-poly(4-vinylpyridine)-b-polystyrene (S-P-S) triblock copolymers to investigate the effects of its nanophase-separated structure on mechanical properties and proton conductivities under non-humidification. Lamellae-forming S-P-S/H₂SO₄ membranes with a continuous hard phase generally exhibited higher tensile strength than sphere-forming S-P-S/H₂SO₄ membranes with a discontinuous hard phase even if the same amount of Sa was infiltrated into each neat S-P-S film. Meanwhile, the conductivities of lamellae-forming S-P-S/H₂SO₄ membranes under non-humidification were comparable or superior to those of sphere-forming S-P-S/H₂SO₄ membranes, even though they were infiltrated by the same weight fraction of H₂SO₄. This result is attributed to the conductivities of S-P-S/H₂SO₄ membranes being greatly influenced by the acid/base stoichiometry associated with acid-base complex formation rather than the nanophase-separated structure adopted in the membranes. Namely, there are more free H₂SO₄ moieties that can release free protons contributing to the conductivity in lamellae-forming S-P-S/H₂SO₄ membranes than sphere-forming S-P-S/H₂SO₄, even when the same amount of H₂SO₄ was infiltrated into the S-P-S.

Effects of the Structure of Benzenesulfonate-Based Draw Solutes on the Forward Osmosis Process

A series of phosphonium-based ionic liquids (ILs) based on benzenesulfonate derivatives (tetrabutylphosphonium benzenesulfonate ([TBP][BS]), tetrabutylphosphonium 4-methylbenzenesulfonate ([TBP][MBS]), tetrabutylphosphonium 2,4-dimethylbenzenesulfonate ([TBP][DMBS]), and tetrabutylphosphonium 2,4,6-trimethylbenzenesulfonate ([TBP][TMBS])) were synthesized via anion exchange with tetrabutylphosphonium bromide ([TBP][Br]). Then, we characterized the ILs and investigated their suitability as draw solutes for forward osmosis (FO), focusing on their thermoresponsive properties, conductivities, and osmotic pressures. We found that aqueous [TBP][BS] was not thermoresponsive, but 20 wt% aqueous [TBP][MBS], [TBP][DMBS], and [TBP][TMBS] had lower critical solution temperatures (LCSTs) of approximately 41, 25, and 21 °C, respectively, enabling their easy recovery using waste heat. Based on these findings, 20 wt% aqueous [TBP][DMBS] was tested for its FO performance, and the water and reverse solute fluxes were found to be approximately 9.29 LMH and 1.37 gMH, respectively, in the active layer facing the draw solution (AL-DS) mode and 4.64 LMH and 0.37 gMH, respectively, in the active layer facing the feed solution (AL-FS) mode. Thus, these tetrabutylphosphonium benzenesulfonate-based LCST-type ILs are suitable for drawing solutes for FO process.

Acidity effects of medium fluids on anhydrous proton conductivity of acid-swollen block polymer electrolyte membranes

Proton-conductive polymer electrolyte membranes (PEMs) were prepared by infiltrating sulfuric acid (Sa) or phosphoric acid (Pa) into a polystyrene-b-poly(4-vinylpyridine)-b-polystyrene (S–P–S) triblock copolymer. When the molar ratio of acid to pyridyl groups in S–P–S, i.e., the acid doping level (ADL), is below unity, the P-block/acid phase in the PEMs exhibited a moderately high glass transition temperature (T_g) of ∼140 °C because of consumption of acids for forming the acid–base complexes between the pyridyl groups and the acids, also resulting in almost no free protons in the PEMs; therefore, the PEMs were totally glassy and exhibited almost no anhydrous conductivity. In contrast, when ADL is larger than unity, the T_gs of the phase composed of acid and P blocks were lower than room temperature, due to the excessive molar amount of acid serving as a plasticizer. Such swollen PEMs with excessive amounts of acid releasing free protons were soft and exhibited high conductivities even without humidification. In particular, an S–P–S/Sa membrane with ADL of 4.6 exhibited a very high anhydrous conductivity of 1.4 × 10⁻¹ S cm⁻¹ at 95 °C, which is comparable to that of humidified Nafion membranes. Furthermore, S–P–S/Sa membranes with lower T_gs exhibited higher conductivities than S–P–S/Pa membranes, whereas the temperature dependence of the conductivities for S–P–S/Pa is stronger than that for S–P–S/Sa, suggesting Pa with a lower acidity would not be effectively dissociated into a dihydrogen phosphate anion and a free proton in the PEMs at lower temperatures.

Sulfuric acid-swollen block polymer membranes exhibit anhydrous conductivities of ∼0.1 S cm⁻¹ that is higher than those of phosphoric acid-swollen membranes, whereas temperature dependence of conductivities of the latter is stronger than the former.

Controlling the Thermally Induced Phase Separation of Polymer/Ionic Liquid Blended Films with Concentrated-Polymer-Brush-Decorated Hybrid Particles

The development of quasi-solid electrolytes for electrical devices operating at high voltages is important for addressing future energy storage requirements. Here, we report a new method to fabricate quasi-solid electrolytes through the thermally induced phase separation of a polymer/ionic liquid (polymer/IL) blend. In a polymer/IL blend that exhibits lower critical solution temperature-type phase separation, we demonstrate that the addition of silica particles decorated with concentrated polymer brushes (CPB-SiPs) can prevent macroscopic phase separation after heating, resulting in a quasi-solid electrolyte with a continuous IL phase. This is due to the adsorption of CPB-SiPs onto the polymer/IL interface in the phase-separated structure. We also reveal a relationship between the molecular weight of the CPB and the phase-separated structure. Namely, a quasi-solid film with a bicontinuous phase-separated structure is formed only when polymers with an appropriate molecular weight are grafted on the CPB-SiPs. The resulting quasi-solid film exhibits a relatively high ionic conductivity, owing to the existence of a continuous ion-conductive phase solely consisting of IL. In addition, we fabricated a quasi-solid electrolyte with the blended film and successfully applied it to an electric double-layer capacitor operating at a high voltage, owing to the wider potential window of the IL employed herein.

Design rules for glass formation from model molecules designed by a neural-network-biased genetic algorithm

The glass transition - an apparent amorphous solidification process - is a central feature of the physical properties of soft materials such as polymers and colloids. A key element of this phenomenon is the observation of a broad spectrum of deviations from an Arrhenius temperature of dynamics in glass-forming liquids, with the extent of deviation quantified by the "fragility" of glass formation. The underlying origin of "fragile" glass formation and its dependence on molecular structure remain major open questions in condensed matter physics and soft materials science. Here we employ molecular dynamics simulations, together with a neural-network-biased genetic algorithm, to design and study model rigid molecules spanning a broad range of fragilities of glass formation. Results indicate that fragility of glass formation can be controlled by tuning molecular asphericity, with extended molecules tending to exhibit low fragilities and compact molecules tending toward higher fragilities. The glass transition temperature itself, on the other hand, correlates well with high-temperature activation behavior and with density. These results point the way towards rational design of glass-forming liquids spanning a range of dynamical behavior, both via these physical insights and via future extensions of this evolutionary design strategy to real chemistries. Finally, we show that results compare well with predictions of the nonlinear Langevin theory of liquid dynamics, which is a precursor of the more recently developed elastically collective nonlinear Langevin equation theory of Mirigian and Schweizer, identifying this framework as a promising basis for molecular design of the glass transition.

Role of Defects in Ion Transport in Block Copolymer Electrolytes

Ion conducting block copolymers can overcome traditional limitations of homopolymer electrolytes by phase separating into nanoarchitectures that can be simultaneously optimized for two or more orthogonal material properties such as high ionic conductivity and mechanical stability. A key challenge in understanding the ion transport properties of these materials is the difficulty of extracting structure-function relationships without having complete knowledge of all nanoscale transport pathways in bulk samples. Here we demonstrate a method for deriving structure-transport relationships for ion conducting block copolymers using thin films and interdigitated electrodes. Well-defined and directly imaged structure in films of poly(styrene)-block-poly(2-vinylpyridine) is controlled using techniques of directed self-assembly then the poly(2-vinylpyridine) is selectively converted into an ion conductor. The ion conductivity is found to be directly proportional to the total number of connected paths between electrodes and the path length. A single defect such as a dislocation anywhere in the path of an ion conducting route disconnects and precludes that pathway from contributing to the conductivity and results in an increase in the dielectric parameter of the film. When all the ion conduction pathways are blocked between electrodes, the conductivity is negligible, 4 orders of magnitude lower compared to a completely connected morphology and the dielectric parameter increases by a factor of 50. These results have profound implications for the interpretation, design, and processing of block copolymer electrolytes for applications as ion conducting membranes.

Block copolymers for supercapacitors, dielectric capacitors and batteries

Block copolymer-based energy storage emerges as an active interdisciplinary research field. This topical review presents a survey of the recent advances in block copolymers for energy storage. In the first section, we introduce the background of electrochemical energy storage and block copolymer thermodynamics. In the second section, we discuss the current understandings of block copolymer chemistry, processing, pore size, and ionic conductivity. In the third section, we summarize the design principles and state-of-the-art applications of block copolymers in three energy storage devices, namely, supercapacitors, dielectric capacitors, and batteries. Lastly, we present our perspectives on future possible breakthroughs and associated challenges that are essential to propel the development of advanced block copolymers for energy storage. We expect the review to encourage innovative studies on integrating block copolymers into energy storage applications.

Control of Phase Separation in Polystyrene/Ionic Liquid-Blended Films by Polymer Brush-Grafted Particles

Immiscible composite materials with controlled phase-separated structures are important in areas ranging from catalysis to battery. We succeeded in controlling the phase-separated structures of immiscible blends of polystyrene (PS) and two ionic liquids (ILs), namely, N, N-diethyl- N-(2-methoxyethyl)- N-methylammonium bis(trifluoromethylsulfonyl)imide (DEME-TFSI) and 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, by adding precisely designed concentrated polymer brush-grafted (CPB-grafted) silica nanoparticles (CPB-SiPs) prepared by surface-initiated atom-transfer radical polymerization. We discuss relationships between chemical species and molecular weights of the CPB and phase-separated structures. When the CPB was composed of a PS homopolymer of an appropriate molecular weight, the IL phase formed a continuous structure and a quasi-solid-blended film was successfully fabricated because the CPB-SiPs were adsorbed at the PS/IL interface and prevented macroscopic phase separation. We propose that CPB-SiP adsorption and the fabrication of quasi-solid films are governed by the degree of penetration of the matrix PS chains into the CPB and deformability of the CPB-SiPs. We found that the DEME-TFSI domain size can be controlled by the CPB-SiP content and that only 1 wt % of the CPB-SiPs was needed to fabricate a quasi-solid film. In addition, we investigated the ionic properties of the quasi-solid PS/DEME-TFSI-blended film. Owing to continuous ion channels composed only of DEME-TFSI, the film exhibited an ionic conductivity of 0.1 mS/cm, which is relatively high compared to previously reported quasi-solid electrolytes. Finally, we demonstrated that an electric double-layer capacitor fabricated using this film as the electrolyte exhibited high charge/discharge cycling stability and reversibility.

Star-Shaped Block Copolymers: Effective Polymer Gelators of High-Performance Gel Electrolytes for Electrochemical Devices

Ion gels composed of copolymers and ionic liquids (ILs) have attracted great interest as polymer gel electrolytes for various electrochemical applications. Here, we present highly robust ion gels based on a six-arm star-shaped block copolymer of (poly(methyl methacrylate)- b-polystyrene)₆ ((MS)₆) and an ionic liquid of 1-ethyl-3-methylimidazolium bis(trifluoromethyl sulfonyl)imide ([EMI][TFSI]). Compared to typical ion gels based on linear polystyrene- b-poly(methyl methacrylate)- b-polystyrene (SMS), the (MS)₆-based gels show mechanical moduli of more than twice under various strains (e.g., stretching, compression, and shear). In addition, the outstanding mechanical property is maintained even up to 180 °C without a gel-sol transition. To demonstrate that (MS)₆-based ion gels can serve as effective gel electrolytes for electrochemical applications, tris(2,2'-bipyridyl)ruthenium(II) (Ru(bpy)₃²⁺), a representative electrochemiluminescent (ECL) luminophore, is incorporated into the gels. In particular, flexible ECL devices based on (MS)₆ gels exhibit high durability against bending deformation compared to devices with gels based on linear SMS having a similar molecular weight and a composition. This result implies that star-shaped block copolymers are effective gelators for achieving flexible/wearable electrochemical electronics.

Ionic Correlations in Random Ionomers

Understanding the electrostatic interactions in ion-containing polymers is crucial to better design shape memory polymers and ion-conducting membranes for multiple energy storage and conversion applications. In molten polymers, the dielectric permittivity is low, generating strong ionic correlations that lead to clustering of the charges. Here, we investigate the influence of electrostatic interactions on the nanostructure of randomly charged polymers (ionomers) using coarse-grained molecular dynamics simulations. Densely packed branched structures rich in charged species are found as the strength of the electrostatic interactions increases. Polydispersity in charge fraction and composition combined with ion correlations leads to percolated nanostructures with long-range fluctuations. We identify the percolation point at which the ionic branched nanostructures percolate and offer a rigorous investigation of the statistics of the shape of the aggregates. The extra degree of freedom introduced by the charge polydispersity leads to bicontinuous structures with a broad range of compositions, similar to neutral A-B random copolymers, as well as to desirable percolated ionic structure in randomly charged-neutral diblock copolymers. These findings provide insight into the design of conducting and robust nanostructures in ion-containing polymers.

Fabrication of size-controlled nanoring arrays by selective incorporation of ionic liquids in diblock copolymer micellar cores

We report the synthesis of arrayed nanorings with tunable physical dimensions from thin films of polystyrene-block-poly(4-vinylpyridine) (PS-P4VP) micelles. For accurate control of the inner and outer diameters of the nanorings, we added imidazolium-based ionic liquids (ILs) into the micellar solution, which were eventually incorporated into the micellar cores. We observed the structural changes of the micellar cores coated on a substrate due to the presence of ILs. The spin-coated micellar cores were treated with an acidic precursor solution and generated toroid nanostructures, of which size depended on the amount of IL loaded into the micelles. We then treated the transformed micellar films with oxygen plasma to produce arrays of various metal and oxide nanorings on a substrate. The spacings and diameters of nanorings were governed by the molecular weight of the PS-P4VP and the amount of IL used. We also demonstrated that arrayed Pt nanorings enabled the fabrication of reduced graphene oxide anti-nanoring arrays via a catalytic tailoring process.

Controlling Microstructure-Transport Interplay in Highly Phase-Separated Perfluorosulfonated Aromatic Multiblock Ionomers via Molecular Architecture Design

Proton-conducting multiblock polysulfones bearing perfluorosulfonic acid side chains were designed to encode nanoscale phase-separation, well-defined hydrophilic/hydrophobic interfaces, and optimized transport properties. Herein, we show that the superacid side chains yield highly ordered morphologies that can be tailored by best compromising ion-exchange capacity and block lengths. The obtained microstructures were extensively characterized by small-angle neutron scattering (SANS) over an extended range of hydration. Peculiar swelling behaviors were evidenced at two different scales and attributed to the dilution of locally flat polymer particles. We evidence the direct correlation between the quality of interfaces, the topology and connectivity of ionic nanodomains, the block superstructure long-range organization, and the transport properties. In particular, we found that the proton conductivity linearly depends on the microscopic expansion of both ionic and block domains. These findings indicate that neat nanoscale phase-separation and block-induced long-range connectivity can be optimized by designing aromatic ionomers with controlled architectures to improve the performances of polymer electrolyte membranes.

Role of Tethered Ion Placement on Polymerized Ionic Liquid Structure and Conductivity: Pendant versus Backbone Charge Placement

The role of ion placement was systematically investigated in imidazolium bis(trifluoromethane)sulfonimide (ImTFSI) polymerized ionic liquids (PILs) containing pendant charges and charges in the backbone (sometimes called ionenes). The backbone PILs were synthesized via a facile step growth route, and pendant PILs were synthesized via RAFT. Both PILs were designed to have nearly identical charge density, and the conductivity was found to be substantially enhanced in the backbone PIL systems even after accounting for differences in the glass transition temperature (T_g). Wide-angle X-ray scattering (WAXS) revealed an invariance in the location of the amorphous halo between the two systems, while the anion-anion correlation peak was shifted to lower scattering wavevector (q) in the backbone PILs. This indicates an increase in the correlation length of ions and is consistent with charge transport along a more correlated pathway following the polymer backbone. Due to the linear nature of the backbone PILs, crystallization was observed and correlated with changes in conductivity. Upon crystallization, the conductivity dropped, and eventually, two populations of mobile ions were observed and attributed to ions in the amorphous and near-crystallite regions. The present work demonstrates the important role of ion placement on local structure and conductivity as well as the ability of backbone PILs to be used as controllable optical or dielectric materials based on crystallization or processing history.

Poly(1-Vinyl-1,2,4-triazolium) Poly(Ionic Liquid)s: Synthesis and the Unique Behavior in Loading Metal Ions

Herein, the synthesis of a series of poly(4-alkyl-1-vinyl-1,2,4-triazolium) poly(ionic liquid)s is reported either via straightforward free radical polymerization of their corresponding ionic liquid monomers or via anion metathesis of the polymer precursors bearing halide as counter anion. The ionic liquid monomers are first prepared via N-alkylation reaction of commercially available 1-vinyl-1,2,4-triazole with alkyl iodides, followed by anion metathesis with targeted fluorinated anions. The thermal properties and solubilities of these poly(ionic liquid)s have been systematically investigated. Interestingly, it is found that the poly(4-ethyl-1-vinyl-1,2,4-triazolium) poly(ionic liquid) exhibited an improved loading capacity of transition metal ions in comparison with its imidazolium counterpart.

Anhydrous Proton Conducting Polymer Electrolyte Membranes via Polymerization-Induced Microphase Separation

Solid-state polymer electrolyte membranes (PEMs) exhibiting high ionic conductivity coupled with mechanical robustness and high thermal stability are vital for the design of next-generation lithium-ion batteries and high-temperature fuel cells. We present the in situ preparation of nanostructured PEMs incorporating a protic ionic liquid (IL) into one of the domains of a microphase-separated block copolymer created via polymerization-induced microphase separation. This facile, one-pot synthetic strategy transforms a homogeneous liquid precursor consisting of a poly(ethylene oxide) (PEO) macro-chain-transfer agent, styrene and divinylbenzene monomers, and protic IL into a robust and transparent monolith. The resulting PEMs exhibit a bicontinuous morphology comprising PEO/protic IL conducting pathways and highly cross-linked polystyrene (PS) domains. The cross-linked PS mechanical scaffold imparts thermal and mechanical stability to the PEMs, with an elastic modulus approaching 10 MPa at 180 °C, without sacrificing the ionic conductivity of the system. Crucially, the long-range continuity of the PEO/protic IL conducting nanochannels results in an outstanding ionic conductivity of 14 mS/cm at 180 °C. We posit that proton conduction in the protic IL occurs via the vehicular mechanism and the PEMs exhibit an average proton transference number of 0.7. This approach is very promising for the development of high-temperature, robust PEMs with excellent proton conductivities.

Morphology-enhanced conductivity in dry ionic liquids

The size polarity and tail stiffness of amphiphilic ionic liquid molecules can be tailored to obtain 3D continuous ionic channels possessing isotropic conductivities.

Effects of Block Length and Membrane Processing Conditions on the Morphology and Properties of Perfluorosulfonated Poly(arylene ether sulfone) Multiblock Copolymer Membranes for PEMFC

Perfluorosulfonated poly(arylene ether sulfone) multiblock copolymers have been shown to be promising as proton exchange membranes. The commonly used approach for preparation of the membrane is solvent casting; the properties of the resulting membranes are very dependent on the membrane processing conditions. In this paper, we study the effects of block length, selectivity of the solvent, and thermal treatment on the membrane properties such as morphology, water uptake, and ionic conductivity. DiMethylSulfOxide (DMSO), and DiMethylAcetamide (DMAc) were selected as casting solvents based on the Flory-Huggins parameter calculated by inversion gas chromatography (IGC). It was found that the solvent selectivity has a mild impact on the mean size of the ionic domains and the expansion upon swelling, while it dramatically affects the supramolecular ordering of the blocks. The membranes cast from DMSO exhibit more interconnected ionic clusters yielding higher conductivities and water uptake as compared to membranes cast from DMAc. A 10-fold increase in proton conductivity was achieved after thermal annealing of membranes at 150 °C, and the ionomers with longer block lengths show conductivities similar to Nafion at 80 °C and low relative humidity (30%).

Synthesis of random terpolymers bearing multidentate imidazole units and their use in functionalization of cadmium sulfide nanowires

This work reports on a new synthesis method for random ternary copolymers that are shown to tether a molecular dye payload to cadmium sulfide nanowires in aqueous solution.