Recently, a variety of “click” or other additive chemistries have been introduced to functionalize polymers after polymerization to target specific applications, for example, membranes, catalysis, or ...drug delivery systems. It is generally assumed that the inclusion of these “click” linking groups has minimal impact on the thermodynamics of the polymer as a whole. In this study, we demonstrate that the introduction of these click-derived units has a profound impact on the Flory–Huggins parameter of polyether derivatives. Using random phase approximation fits for small-angle X-ray scattering data from block copolymer pairs to estimate the Flory–Huggins interaction parameter (χ), we determined that poly(ethylene oxide) (PEO) and poly(allyl glycidyl ether) (PAGE), which differ only by the inclusion of an allyl sidechain, have a χ of 0.030 (at T = 34 °C). While PEO is miscible with poly(lactide) (PLA) at nearly all temperatures, the PLA/PAGE χ determined experimentally is 0.015 (at T = 30 °C). Atomistic molecular dynamics simulations of PEO/PAGE oligomer blends show that upon blending, PEO chains contract and move closer together, while PAGE chains stretch and spread apart, indicating an enthalpic contribution to the χ parameter due to changes in polymer coordination resulting from the conformational asymmetry of PAGE and PEO. These studies demonstrate the large impact that functionalization and side-chain units have on the χ parameter of polymer pairs.
The usual understanding in polymer electrolyte design is that an increase in the polymer dielectric constant results in reduced ion aggregation and therefore increased ionic conductivity. We ...demonstrate here that in a class of polymers with extensive metal–ligand coordination and tunable dielectric properties, the extent of ionic aggregation is delinked from the ionic conductivity. The polymer systems considered here comprise ether, butadiene, and siloxane backbones with grafted imidazole side-chains, with dissolved Li+, Cu2+, or Zn2+ salts. The nature of ion aggregation is probed using a combination of X-ray scattering, electron paramagnetic resonance (in the case where the metal cation is Cu2+), and polymer field theory-based simulations. Polymers with less polar backbones (butadiene and siloxane) show stronger ion aggregation in X-ray scattering compared to those with the more polar ether backbone. The T g-normalized ionic conductivities were however unaffected by the extent of aggregation. The results are explained on the basis of simulations which indicate that polymer backbone polarity does impact the microstructure and the extent of ion aggregation but does not impact percolation, leading to similar ionic conductivity regardless of the extent of ion aggregation. The results emphasize the ability to design for low polymer T g through backbone modulation, separately from controlling ion-polymer interaction dynamics through ligand choice.
Thermopower measurements offer an alternative transport measurement that can characterize the dominant transport orbital and is independent of the number of molecules in the junction. This method is ...now used to explore the effect of chemical structure on the electronic structure and charge transport. We interrogate junctions, using a modified scanning tunneling microscope break junction technique, where: (i) the 1,4-benzenedithiol (BDT) molecule has been modified by the addition of electron-withdrawing or -donating groups such as fluorine, chlorine, and methyl on the benzene ring; and (ii) the thiol end groups on BDT have been replaced by the cyanide end groups. Cyanide end groups were found to radically change transport relative to BDT such that transport is dominated by the lowest unoccupied molecular orbital in 1,4-benzenedicyanide, while substituents on BDT generated small and predictable changes in transmission.
Thermoelectric devices possess enormous potential to reshape the global energy landscape by converting waste heat into electricity, yet their commercial implementation has been limited by their high ...cost to output power ratio. No single "champion" thermoelectric material exists due to a broad range of material-dependent thermal and electrical property optimization challenges. While the advent of nanostructuring provided a general design paradigm for reducing material thermal conductivities, there exists no analogous strategy for homogeneous, precise doping of materials. Here, we demonstrate a nanoscale interface-engineering approach that harnesses the large chemically accessible surface areas of nanomaterials to yield massive, finely-controlled, and stable changes in the Seebeck coefficient, switching a poor nonconventional p-type thermoelectric material, tellurium, into a robust n-type material exhibiting stable properties over months of testing. These remodeled, n-type nanowires display extremely high power factors (~500 µW m
K
) that are orders of magnitude higher than their bulk p-type counterparts.
Human pluripotent stem cells (hPSCs) offer considerable potential for biomedical applications including drug screening and cell replacement therapies. Clinical translation of hPSCs requires large ...quantities of high quality cells, so scalable methods for cell culture are needed. However, current methods are limited by scalability, the use of animal-derived components, and/or low expansion rates. A thermoresponsive 3D hydrogel for scalable hPSC expansion and differentiation into several defined lineages is recently reported. This system would benefit from increased control over material properties to further tune hPSC behavior, and here a scalable 3D biomaterial with the capacity to tune both the chemical and the mechanical properties is demonstrated to promote hPSC expansion under defined conditions. This 3D biomaterial, comprised of hyaluronic acid and poly(N-isopropolyacrylamide), has thermoresponsive properties that readily enable mixing with cells at low temperatures, physical encapsulation within the hydrogel upon elevation at 37 °C, and cell recovery upon cooling and reliquefaction. After optimization, the resulting biomaterial supports hPSC expansion over long cell culture periods while maintaining cell pluripotency. The capacity to modulate the mechanical and chemical properties of the hydrogel provides a new avenue to expand hPSCs for future therapeutic application.
Metal–ligand coordinating polymers utilize labile bonds between polymer-bound ligands and free cations to delocalize and conduct mono and multivalent metal ions in the solid state. These interactions ...simultaneously act as reversible cross-links, leading to delayed terminal relaxation as measured by oscillatory rheology. Well-controlled poly(methyl acrylate)s with imidazole chain ends are synthesized as model polymers to obtain metal–ligand bond lifetimes and to investigate design rules for solid polymer electrolytes. Through changes in identity of the metal species, metal–ligand bond lifetimes are varied over nearly two orders of magnitude. Scaling analysis demonstrates a correlation between the bond lifetime and the ionic conductivity, suggesting a hierarchical conduction mechanism that involves interplay of polymer segmental motion with the dissociation of metal–ligand bonds. This suggests an alternative means to enhance long-range ionic transport that is partially decoupled from efforts to enhance the segmental mobility of ion-conducting polymers.
Controlling crystallinity and molecular packing at nano- and macroscopic length scales in conjugated polymer thin films is vital for improving the performance of polymer-based electronic devices. ...Herein, the inherent amphiphilicity of rigid donor–acceptor copolymers used in high performance polymer electronics is leveraged to allow the formation of highly ordered lyotropic mesophases. By increasing the length and branching of solubilizing chains on cyclopentadithiophene-alt-thiadiazolopyridine-based alternating copolymers, amphiphilicity can be increased, and lyotropic liquid crystalline mesophases are observed in selective solvents. These lyotropic mesophases consist of chain extended polymers exhibiting close, ordered π-stacking. This is evidenced by birefringent solutions and red-shifted absorbance spectra displaying pronounced excitonic coupling. Crystallinity developed in solution can be transferred to the solid state, and thin films of donor–acceptor copolymers cast from lyotropic solutions exhibit improved crystalline order in both the alkyl and π-stacking directions. Because of this improved crystallinity, transistors with active layers cast from lyotropic solutions exhibit a significant improvement in carrier mobility compared to those cast from isotropic solution, reaching a maximum value of 0.61 cm2 V–1 s–1. This approach of rational side chain design bridges the gap from solution structure to solid state structure and is a promising and general approach to allow the expression of lyotropic mesophases in rigid conjugated polymers.
Polymer electrolytes with high Li+-ion conductivity provide a route toward improved safety and performance of Li+-ion batteries. However, most polymer electrolytes suffer from low ionic conduction ...and an even lower Li+-ion contribution to the conductivity (the transport number, t +), with the anion typically transporting over 80% of the charge. Here, we show that subtle and potentially undetected associations within a polymer electrolyte can entrain both the anion and the cation. When removed, the conductivity performance of the electrolyte can be improved by almost 2 orders of magnitude. Importantly, while some of this improvement can be attributed to a decreased glass transition temperature, T g, the removal of the amide functional group reduces interactions between the polymer and the Li+ cations, doubling the Li+ t + to 0.43, as measured using pulsed-field-gradient NMR. This work highlights the importance of strategic synthetic design and emphasizes the dual role of T g and ion binding for the development of polymer electrolytes with increased total ionic conductivity and the Li+ ion contribution to it.
Visualization and statistical regression of compiled data sets are emerging as powerful tools in understanding and screening the design space of materials properties, rapidly providing insights that ...would not be readily gained from studies of individual systems. Here, we describe the curation and analysis of a database of polymer Li+-electrolyte conductivity performance, manually extracted from the published literature. We focus on solid, dry polymer electrolytes without additives. Data were extracted from 65 publications, resulting in 655 unique polymer–anion–salt concentration entries and 5225 individual conductivity data points to create an interactive database: PEDatamine.org. Visualization of the collective data set suggested that individual features, other than the activation energy, are poor predictors of conductivity performance across the wide range of polymer chemistries, Li salts, and salt concentrations examined. The Meyer–Neldel rule suggesting a correlation between the conductivity prefactor and activation energy is shown to hold universally for both Arrhenius and Vogel–Fulcher–Tammann representations. Statistical regression techniques were employed to extract the most important features relevant in determining Li+-ion conductivity. These include polymer molecular weight, glass-transition temperature, existence of electronegative heteroatoms in the monomer, and anion size. However, experimental features can be omitted from the regression model without impacting predictive performance, reinforcing the importance of monomer electronegativity, hydrogen bonding, and anion molecular bulk.
Nanostructured membranes containing structural and proton-conducting domains are of great interest for a wide range of applications requiring high conductivity coupled to high thermal stability. ...Understanding the effect of nanodomain confinement on proton-conducting properties in such materials is essential for designing new, improved membranes. This relationship has been investigated for a lamellae-forming mixture of poly(styrene-b-2-vinyl pyridine) (PS-b-P2VP) with ionic liquid composed of imidazole and bis(trifluoromethylsulfonyl)imide, where the ionic liquid selectively resides in the P2VP domains of the block copolymer. Quasi-elastic neutron scattering and NMR diffusion measurements reveal increased prevalence of a fast proton hopping transport mechanism, which we hypothesize is due to changes in the hydrogen bond structure of the ionic liquid under confinement. This, in combination with unique ion aggregation behavior, leads to a lower activation energy for macroscopic ion transport compared with that in a mixture of ionic liquid with P2VP homopolymer. The proton transference number in both samples is significantly higher than that in the neat ionic liquid, which could be taken advantage of for applications such as proton exchange membrane fuel cells and actuators. These results portend the rational design of nanostructured membranes having improved mechanical properties and conductivity.
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