A new class of high‐performance n‐type organic thermoelectric materials, self‐doping perylene diimide derivatives with modified side chains, is reported. These materials achieve the highest n‐type ...thermoelectric performance of solution‐processed organic materials reported to date, with power factors as high as 1.4 μW/mK2. These results demonstrate that molecular design is a promising strategy for enhancing organic thermoelectric performance.
Nanostructured mixtures of ionic liquids and polymers are of great interest for a wide variety of electrochemical applications. Understanding the relationship between composition, structure, and ...ionic conductivity for these mixtures is essential for designing new materials. In this work, the effect of nanostructure on ionic conductivity, σ, is investigated for model mixtures of diblock copolymers and ionic liquids that are selective for one of the polymer microphases. It is demonstrated that the concentration dependence of σ is a function of the total volume fraction of ionic liquid and described well by percolation theory. This scaling behavior encourages the design of membranes where the amount of a mechanical component in the block copolymer can be increased to improve the strength of the membrane without sacrificing conductivity. The temperature dependence of σ is a function of the amount of ionic liquid exclusively in the conducting domain. Comparing σ for mixtures of the diblock copolymer poly(styrene-block-2-vinylpyridine) (S2VP) and two different ionic liquids, imidazolium bis(trifluoromethylsulfonyl)imide (ImTFSI) and 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMImTFSI), reveals that the chemistry of the polymer/ionic liquid pair affects both the activation energy for σ and the maximum attainable σ but does not affect how σ scales with ionic liquid concentration. The effect of the morphology on σ is also examined, and it is found that as long as the conducting phase morphology is isotropic and well-connected, σ is not affected by morphology.
The local, nanoscale organization of crystallites in conjugated polymers is often critical to determining the charge transport properties of the system. Block copolymer geometries, which offer ...controlled nanostructures with tethering of chains at interfaces, are an ideal platform to study the local organization of conjugated polymer crystallites. The model conjugated polymer poly(3-(2′-ethyl)hexylthiophene) (P3EHT) features a depressed melting temperature relative to the widely studied poly(3-hexylthiophene) (P3HT), which allows it to robustly form microphase-separated domains that confine the subsequent P3EHT crystallites. Importantly, P3EHT crystallization in confinement is coupled to a rubbery second block via interfacial tethering, mechanical properties, and chain stretching. Here, the impact of thermal processing on the diblock copolymer structure is examined to elucidate the key driving forces controlling the final coupled diblock copolymer and crystalline structures. Surprisingly, the diblock copolymer domain size is significantly impacted by the temperature at which the conjugated domain is crystallized. Decreasing amounts of domain extension are observed with increasing crystallization temperatures. This temperature-dependent domain structure appears to be correlated with the crystallization processes; these processes are inferred from precise changes in the lamellar structure across melting. By carefully tracking the changes in domain structure across melting, this work identifies three distinct regimes. We suggest a structural model of the conjugated block melting processes consisting of (I) excluded-chain relaxation, followed by (II) chain interdigitation during melt-recrystallization, and finally (III) complete melting that is independent of the initial crystallization conditions. These results suggest that P3EHT crystallization processes associated with temperature-dependent chain diffusion and nucleation are primarily responsible for the unexpected temperature-dependent crystallization behavior. They also emphasize that less perfect conjugated polymer crystals may actually be associated with a poorly interdigitated structure. Furthermore, this work demonstrates the utility of leveraging a diblock copolymer structure with a rubbery second block in order to precisely track changes in the crystallite structure.
Ion-conducting polymers are important materials for a variety of electrochemical applications. Perfluorinated ionomers, such as Nafion, are the benchmark materials for proton conduction and are ...widely used in fuel cells and other electrochemical devices including solar-fuel generators, chlor-alkali cells, and redox flow batteries. While the behavior of Nafion in bulk membranes (10 to 100s μm thick) has been studied extensively, understanding its properties under thin-film confinement is limited. Elucidating the behavior of thin Nafion films is particularly important for the optimization of fuel-cell catalyst layers or vapor-operated solar-fuel generators, where a thin film of ionomer is responsible for the transport of ions to and from the active electrocatalytic centers. Using a combination of transport-property measurements and structural characterization, this work demonstrates that confinement of Nafion in thin films induced thickness-dependent proton conductivity and ionic-domain structure. Confining Nafion films to thicknesses below 50 nm on a silicon substrate results in a loss of microphase separation of the hydrophilic and hydrophobic domains, which drastically increases the material’s water uptake while in turn decreasing its ionic conductivity.
Amphiphilic polypeptoids can be designed with specific sequences of hydrophilic and hydrophobic units, which determine their surface properties for antifouling/fouling release purposes. Although the ...sequence-dependent surface structures of polypeptoids have been extensively investigated, e.g., with X-ray spectroscopy, their molecular structures under the aqueous conditions relevant to marine fouling have not been studied. In this work, we applied sum frequency generation (SFG) vibrational spectroscopy to study the surface structures and hydration of a series of amphiphilic polypeptoid coatings with different sequences in air and water. SFG spectra, in agreement with X-ray spectroscopy studies, revealed that the surface coverage of the hydrophilic N-(2-methoxyethyl)glycine (Nme) units in air is affected by both the number and position of the hydrophobic N-(heptafluorobutyl)glycine (NF) units in the peptoid chain and is negatively correlated with the surface concentration of the fluorine element. Our ability to probe the SFG signals of water molecules at the peptoid surface provides new information on the hydrated film properties. From these SFG signals and the time evolution of water contact angles on the polymers, we see that the hydrated film properties are also dependent upon the peptoid sequence. This work indicates that the surface presence of the Nme groups and the ability of the polymers to order and strongly hydrogen bond with interfacial water molecules determine their antifouling properties, whereas the surface restructuring rate upon contact with water affects their fouling release behaviors.
Proton conducting ionomers are widely used for electrochemical applications including fuel-cell devices, flow batteries, and solar-fuels generators. For most applications the presence of interfacial ...interactions can affect the structure and properties of ionomers. Nafion is the most widely used ionomer for electrochemical applications due to their remarkable proton conductivity and stability. While Nafion membranes have been widely studied, the behavior and morphology of this ionomer under operating conditions when confined to a thin-film morphology are still not well understood. Using in situ grazing-incidence small-angle X-ray scattering (GISAXS) techniques, this work demonstrates that the wetting interaction in thin-film interfaces can drastically affect the internal morphology of ionomers and in turn modify its transport properties. Thin films cast on hydrophobic substrates result in parallel orientation of ionomer channels that retard the absorption of water from humidified environments; while films prepared on SiO2 result in isotropic orientation of these domains, thus favoring water sorption and swelling of the polymer. Furthermore, the results presented in this paper demonstrate that upon thermal annealing of Nafion thin films static crystalline domains form within the polymer matrix that restrict further water uptake. The results presented in this study can aid in the rational design of functional composite materials used in fuel-cell catalyst layers and solar-fuels devices.
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.
Mixing simple additives into poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate) (PEDOT:PSS) dispersions can greatly enhance the thermoelectric properties of the cast films with little ...manufacturing cost, but design rules for many of these additives have yet to emerge. We show that controlling stoichiometry in ionic liquid (I.L.) additives can decouple morphological and electronic modifications to PEDOT:PSS and enhance its power factor by over 2 orders of magnitude. Blending I.L. additives with a 1:1 stoichiometry between cationic imidazolium (Im+) derivatives and anionic bis(trifluoromethane)sulfonamide (TFSI–) groups into PEDOT:PSS dispersions raised the film conductivity to ∼1000 S/cm. The Seebeck coefficient, which gives insight into the electronic structure as well as thermoelectric performance, remained unchanged. This behavior mimics that of popular high-boiling solvent additives such as dimethyl sulfoxide and ethylene glycol, which restructure the film morphology to enhance carrier mobility. Blending I.L. additives with a 4:1 stoichiometry between Im+ and TFSI– groups raises the conductivity in a similar manner but also enhances the Seebeck coefficient. This selective Seebeck enhancement proceeds from the interaction of excess Im+ with anionic poly(styrenesulfonate) (PSS–) groups, similar to previous studies using inorganic salts, that results in a shift in charge carrier populations. Inorganic salts by themselves cannot raise the conductivity of PEDOT:PSS to appropriate values since they lack the solvent restructuring effect. These I.L. additives combine the effects of high-boiling solvents and diffuse ions, with the ability to tailor the Seebeck coefficient through ion stoichiometry.
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