Ionic liquids exhibit many appealing properties, including chemical and thermal stability, vanishing vapor pressure, tunable solvation, high ionic conductivity, and high dielectric constant, that render them appealing as key ingredients in plastic electronics, batteries, fuel cells, gas separation membranes, and actuators. To realize these properties in applications, it is necessary to solidify the material, and/or to confine the ionic liquid within a desired nanostructure. Block copolymers offer unprecedented flexibility to direct self-assembly over lengthscales from 1100 nanometers, while simultaneously providing mechanical integrity. Five different sub-projects will be pursued, that collectively range from very dilute solutions to almost pure polymer, and that are aimed at a fundamental understanding of how polymer/ionic liquid interactions can be utilized to prepare functional materials. Specifically, this research will: (i) extend the concept of the micelle shuttle, whereby copolymer aggregates transfer reversibly and intact between aqueous and ionic liquid phases, to prepare systems that load, transfer, and release cargo at prescribed temperatures, and to prepare nanoemulsions for use in homogenous catalysis; (ii) prepare robust ion gels by tri- and multiblock copolymer self-assembly, whereby gelation and melting occur at tunable temperatures or under optical stimulation; (iii) optimize the use of ion gels as gate dielectric materials in organic transistors, for plastic electronic applications; (iv) create mechanically robust, ordered block copolymer membranes with ionic liquid channels for high ionic conductivity and transport, both isotropic and anisotropic; (v) create supramolecular polymers with relaxation times tunable over a remarkably wide range via hydrogen bonding in ionic liquids. NON-TECHNICAL SUMMARY The goal of this project is to create a new class of functional nanostructured materials by combining ionic liquids with block copolymers. Materials containing polymers and ionic liquids are potentially useful across a wide spectrum of technology platforms, including plastic electronics, actuators, sensors, batteries, fuel cells, and separation membranes. For applications the simultaneous optimization of several properties, such as high ionic transport, mechanical integrity, and facile processing, is essential; this can be achieved through controlling the material structure at the nanoscale. Ionic liquids are already in commercial practice in Europe as green solvents for chemical transformations; block copolymer surfactants should greatly extend the range of reagents, products, and catalysts that can be employed. Graduate students working in this project will be broadly trained in polymer synthesis and advanced characterization techniques, and will have the opportunity to mentor talented undergraduates in research. High school students from the greater Twin Cities, particularly females and underrepresented minorities, will be exposed to polymer science as part of an "Exploring Careers" summer camp administered with the Institute of Technology Center for Educational Programs. Faculty and students from colleges and universities across the US will spend time in the PI's laboratories to utilize one or more of the experimental facilities, through the newly-forged Materials Research Facilities Network.
The goal of this project is to advance a new class of functional nanostructured materials by combining ionic liquids with block copolymers. Ionic liquids exhibit many appealing properties, including chemical and thermal stability, vanishing vapor pressure, tunable solvation, high ionic conductivity, and high dielectric constant, that render them appealing as potential "green" solvents, and as key ingredients in plastic electronics, batteries, fuel cells, gas separation membranes, and actuators. To realize these properties in advanced materials applications, it is necessary to solidify the material, and/or to confine the ionic liquid within a desired nanostructure. Block copolymers offer unprecedented flexibility to direct self-assembly over lengthscales from 1–100 nanometers, while simultaneously providing mechanical integrity. Although ionic liquids have received substantial attention over the last fifteen years, exploration of the properties of polymer/ionic liquid mixtures is still in its infancy. We have developed the "micelle shuttle", whereby micelles composed of poly(butadiene-b-ethylene oxide) (PB-PEO) diblocks were found to transfer between water at modest temperatures (≤ 50 ºC) and hydrophobic ionic liquids (IL) at elevated temperatures. The transfer is quantitative, fully reversible, and the micelle structure remains intact. We envision that this phenomenon could find application in situations where an IL is used as a reaction medium. It is desirable to have simple, efficient, and scalable methods for removing reaction products and byproducts from the IL, or for introducing insoluble reagents, via immiscible aqueous streams. Thus micelles that can reliably, rapidly, and quantitatively transport reagents across the liquid-liquid phase boundary should be quite attractive. We have also pioneered the idea of using IL-filled vesicles as "nanoreactors" in aqueous media. If a catalyst were ensconced in the vesicle interior, the shuttle would facilitate recovery. We have demonstrated that vesicles can be loaded and prepared in the IL, and transferred quantitatively into water. Supramolecular polymers represent a fascinating class of macromolecules, formed by physical associations between smaller, functionalized constituents. Among their appealing features are the possibility for reversible association, triggered by temperature or perhaps by external stimulus, the production of many different materials from a smaller set of building blocks, and facile processing in the less viscous, unassembled state. We designed a versatile system for supramolecular assembly of polyerm netowrks ("gels") in ILs. Two polymer components were utilized: a P2VP-PEA-P2VP triblock, where poly(2-vinyl pyridine) (P2VP) is an H-bond acceptor, and a poly(vinyl phenol) (PVPh) homopolymer, which is an H-bond donor. Poly(ethyl acrylate) (PEA) is readily soluble in the IL, whereas P2VP and PVPh are moderately soluble. The results are quite remarkable. The dynamic moduli, G’ and G", obtained over a range of frequencies at temperatures from 30 to 160 ºC, superpose to give a master curve with a plateau in G’ extending over 10 orders of magnitude. The resulting shift factors also indicate that the viscosity exhibits a comparably strong dependence. The isochronal T dependence of G’ and G" shown in Fig. 5 is also notable for its sharp melting near 140 ºC. We hypothesize that the P2VP–PVPh associations result in a transient network, and that the lifetime of the average association is responsible for the dramatic T dependence. Each P2VP and PVPh block contains about 50 repeat units, and thus a particular sticker might include up to 50 H-bonds. As T is reduced, we estimate that the average number of H-bonds per block increases from <1 in the liquid state at 160 ºC, to ca. 6 at room temperature. dependence of the viscoelastic response. These soft solids can find use in plastic electronics, actuators, electrochemical sensors, and gas separation membranes.
