Decoupling Mechanical and Conductive Dynamics of Polymeric Ionic Liquids via a Trivalent Anion Additive

The mechanical and conductive properties of a polymeric ionic liquid (PIL) are decoupled through the addition of a fraction of trivalent anions to a chloride single-ion conductor. Trivalent phosphate ions strongly coordinate with polymer-bound imidazoliums, producing an increase in both the ionic conductivity and the polymer viscosity. Both the viscosity and the ionic conductivity increase with phosphate content, and the conductivity is superior to that of the neat PIL at larger trivalent anion concentrations. The interaggregate spacing (determined by X-ray scattering), glass transition temperature (measured by calorimetry), and free volume (estimated by rheology) are each sensitive to the presence of trivalent ions but not to changes in the phosphate concentration. Thus, the presence of a fraction of trivalent anions qualitatively changes the structure and interaction of ions, resulting in modified macroscopic properties of the PIL. We hypothesize that this step change in properties upon introducing phosphate ions is due to a densification of ion aggregates by the trivalent ion, which strongly binds to imidazolium ions. This provides a new mechanism for creating PILs with tailored conductive and rheological behavior

Visualizing ion transport in polymers via ion-chromic indicators

There is growing interest in polymers with high ionic conductivity for applications including batteries, fuel cells, and separation membranes. However, measuring ion diffusion in polymers can be challenging, requiring complex procedures and instrumentation. Here, a simple strategy to study ion diffusion in polymers is presented that utilizes ion-chromic spiropyan as an indicator to measure the diffusion of LiTFSI, KTFSI, and NaTFSI within poly(ethylene oxide)-based polymer networks. These systems are selected, as these are common ions and polymers used in energy storage applications, however, the approach described is not specific to materials for energy storage. Specifically, to enabling the study of ion diffusion, these salts cause the spiropyran to undergo an isomerization reaction, which results in a significant color change. This colorimetric response enables the determination of the diffusion coefficients of these ions within films of these polymers simply by optically tracking the spatial-temporal evolution of the isomerization product within the film and fitting the data to the relevant diffusion equations. The simplicity of the method makes it amenable to the study of ion diffusion in polymers under a range of conditions, including various temperatures and under macroscopic deformation

Molecular-Weight Dependence of Center-of-Mass Chain Diffusion in Polymerized Ionic Liquid Melts

Polymerized ionic liquids (PILs) with flexible polymer chains and weakly interacting ionic liquid (IL) groups have received great attention for their desirable properties in electrochemical applications such as ionic conductivity. Less is known about their dynamic properties such as center-of-mass chain diffusion and how it depends on molecular weight in the presence of IL groups. In this work, a series of acrylic PILs with imidazolium cations and bis(trifluoromethanesulfonyl)imide (TFSI) anions (TFSI-f-PILN) were synthesized via reversible addition–fragmentation chain-transfer polymerization with degrees of polymerization N ranging from 40 to 236. A fluorescent acrylic monomer with the 7-nitrobenzofurazan group was copolymerized at trace levels as a probe of chain motion, and the diffusion coefficient (D) of TFSI-f-PILN was determined by fluorescence recovery after photo bleaching at Tg + 45 K. Within the uncertainty of 3–20%, a scaling relationship of D ∼ N–2 was observed which is the same as the scaling of linear neutral polymers. Wide-angle X-ray scattering exhibited no peak at ∼5 nm–1, indicating no long-range imidazolium-TFSI ionic correlations. Our results indicate that the molecular weight dependence of center-of-mass diffusion is not affected by electrostatic interactions of IL groups. No transition from a Rouse regime (D ∼ N–1) to reptation regime (D ∼ N–2) was observed within the studied N range

Ion Transport in Dynamic Polymer Networks Based on Metal–Ligand Coordination: Effect of Cross-Linker Concentration

The development of high-performance ion conducting polymers requires a comprehensive multiscale understanding of the connection between ion–polymer associations, ionic conductivity, and polymer mechanics. We present polymer networks based on dynamic metal–ligand coordination as model systems to illustrate this relationship. The molecular design of these materials allows for precise and independent control over the nature and concentration of ligand and metal, which are molecular properties critical for bulk ion conduction and polymer mechanics. The model system investigated, inspired by polymerized ionic liquids, is composed of poly­(ethylene oxide) with tethered imidazole moieties that facilitate dissociation upon incorporation of nickel­(II) bis­(trifluoro­methylsulfonyl)­imide. Nickel–imidazole interactions physically cross-link the polymer, increase the number of elastically active strands, and dramatically enhance the modulus. In addition, a maximum in ionic conductivity is observed due to the competing effects of increasing ion concentration and decreasing ion mobility upon network formation. The simultaneous enhancement of conducting and mechanical properties within a specific concentration regime demonstrates a promising pathway for the development of mechanically robust ion conducting polymers

Harvesting Waste Heat in Unipolar Ion Conducting Polymers

The Seebeck effect in unipolar ion-conducting, solid-state polymers is characterized. The high Seebeck coefficient and sign in polymer ion conductors is explained via analysis of thermogalvanic multicomponent transport. A solid-state, water-processeable, flexible device based on these materials is demonstrated, showcasing the promise of polymers as thermogalvanic materials. Thermogalvanic materials based on ion-conducting polymer membranes show great promise in the harvesting of waste heat

Molecular Design of Multimodal Viscoelastic Spectra Using Vitrimers

Imparting multiple, distinct dynamic processes at precise time scales in polymers is a grand challenge in soft materials design with implications for applications including electrolytes, adhesives, tissue engineering, and additive manufacturing. Many competing factors, including the polymer architecture, molecular weight, backbone chemistry, and presence of a solvent, affect the local and global dynamics and in many cases are interrelated. One approach to imparting distinct dynamic processes is through the incorporation of dynamic bonds with widely varying kinetics of bond exchange. Here, statistically cross-linked polymer networks are synthesized with mixed fast and slow dynamic bonds with 3 orders of magnitude different exchange kinetics. Oscillatory shear rheology shows that the single component networks (either fast or slow) exhibit a single relaxation peak while mixing fast and slow cross-linkers in one network produces two peaks in the relaxation spectrum. This is in stark contrast to telechelic networks with the same mixture of dynamic bonds, where only one mixed mode is observed, and here we provide molecular design guidelines for having each dynamic bond contribute a distinct relaxation mode. By comparing the polymer architecture and the difference in the number of dynamic bonds per chain, we have elucidated the role of network architecture in imparting multimodal behavior in dynamic networks. A highly tunable and recyclable material has been developed with control of rubbery plateau modulus (through cross-link density), relaxation peak locations and ratio (through cross-linker selection and molar fractions), and tan δ (through the relationships of the rubbery plateau and relaxation peak locations)

Structure–Conductivity Relationships of Block Copolymer Membranes Based on Hydrated Protic Polymerized Ionic Liquids: Effect of Domain Spacing

Elucidating the relationship between chemical structure, morphology, and ionic conductivity is essential for designing novel high-performance materials for electrochemical applications. In this work, the effect of lamellar domain spacing (<i>d</i>) on ionic conductivity (σ) is investigated for a model system of hydrated diblock copolymer based on a protic polymerized ionic liquid. We present a strategy that allows for the synthesis of a well-defined series of narrowly dispersed PS-<i>b</i>-PIL with constant volume fraction of ionic liquid moieties (<i>f</i>_IL ≈ 0.39) and with two types of mobile charge carriers: trifluoroacetate anions and protons. These materials self-assemble into ordered lamellar morphologies with variable domain spacing (ca. 20–70 nm) as demonstrated by small-angle X-ray scattering. PS-<i>b-</i>PIL membranes exhibit ionic conductivities above 10^–4 S/cm at room temperature, which are independent of domain spacing consistent with their nearly identical water content. The conductivity scaling relationship demonstrated in this paper suggests that a mechanically robust membrane can be designed without compromising its ability to transport ions. In addition, PIL-based membranes exhibit low water uptake (λ ≈ 10) in comparison with many proton-conducting systems reported elsewhere. The low water content of the materials described herein makes them promising candidates for electrochemical devices operating in aqueous electrolytes at low current densities where moderate ion conduction and low product crossover are required

Anisotropic Thermal Transport in Thermoelectric Composites of Conjugated Polyelectrolytes/Single-Walled Carbon Nanotubes

We report a method to determine the thermal conductivities of polymer composites with single-walled carbon nanotubes (SWNTs) using time-domain thermoreflectance. Both through-plane and in-plane thermal conductivities were determined. Two types of CPEs used in these studies are of the same conjugated backbone but with either cationic (CPE-PyrBIm₄) or anionic (CPE-Na) pendant functionalities. The CPE-Na/SWNT composites are p-type conductors, whereas the CPE-PyrBIm₄/SWNT counterparts exhibit n-type charge transport. The CPE/SWNT films were prepared through a filtration method that preferentially aligns the SWNTs in the in-plane direction. Attaching the composites onto glass substrates with a precoated heat transducer allows one to measure the through-plane thermal conductivity of materials with rough surfaces. The in-plane thermal conductivity can be measured by embedding thick samples into epoxy followed by microtoming to expose the relatively smooth cross sections. The thermal conductivity along the in-plane direction is found to be higher than that along the through-plane direction. Indeed, the anisotropy factor of thermal conductivity in these composites is approximately an order of magnitude, favoring in-plane direction

Colloidal Semiconductor Quantum Dots with Tunable Surface Composition

Colloidal CdS quantum dots (QDs) were synthesized with tunable surface composition. Surface stoichiometry was controlled by applying reactive secondary phosphine sulfide precursors in a layer-by-layer approach. The surface composition was observed to greatly affect photoluminescence properties. Band edge emission was quenched in sulfur terminated CdS QDs and fully recovered when QDs were cadmium terminated. Calculations suggest that electronic states inside the band gap arising from surface sulfur atoms could trap charges, thus inhibiting radiative recombination and facilitating nonradiative relaxation

Control of Lithium Salt Partitioning, Coordination, and Solvation in Vitrimer Electrolytes

Vitrimers are an important class of materials offering advantages over conventional thermosets due to their self-healing properties and reprocessability. Vitrimers are ideal candidate materials for solid polymer electrolytes because their viscoelasticity and conductivity can be independently tuned by salt addition in distinct ways from linear polymer electrolytes while further providing resistance to lithium dendrite propagation. In this work, the chemical and physical properties of vinylogous urethane (VU) vitrimers were characterized by using a combination of experiments and simulations to develop molecular design rules for controlling material properties. A series of VU vitrimers containing lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) salt were synthesized by precisely controlling the VU cross-linking density using defined linker lengths of ethylene glycol (xEG, x = 2, 3, 4, 6, or 12), thereby enabling control over the dynamic bond-to-EG ratio. Viscoelastic measurements show that the characteristic relaxation time τ* of VU vitrimers containing salt decreased by a factor of ∼70 relative to neutral vitrimers due to Li-ion coordination and catalysis of VU bond exchange. Stress relaxation times and shear moduli decrease with lower cross-linking densities in VU vitrimers. Solid-state 7Li NMR further reveals that VU vitrimers with longer linker lengths prefer lithium-ethylene oxide (Li-EO) solvation, whereas shorter linkers cannot sufficiently solvate the cation, and Li-VU coordination is preferred. Density functional theory (DFT) simulations were used to elucidate the dominant binding mode of Li-ion interaction as a function of linker length. The preferential partitioning of Li at the VU site leads to an order of magnitude decrease in stress relaxation times with a negligible impact on the conductivity after normalizing to the glass transition temperature Tg. Interestingly, our results show universal behavior for Tg-normalized ionic conductivity data regardless of linker length. Overall, this work provides new avenues for orthogonal tuning of bulk dynamics, recyclability, and conductivity in vitrimer electrolytes