Session B19: Focus Session: Polymers and Ionic Liquids

Block Copolymers and Ionic Liquids: A New Class of Functional Nanocomposites

Block copolymers provide a remarkably versatile platform for achieving desired nanostructures by self-assembly, with lengthscales varying from a few nanometers up to several hundred nanometers. Ionic liquids are an emerging class of solvents, with an appealing set of physical attributes. These include negligible vapor pressure, high chemical and thermal stability, tunable solvation properties, high ionic conductivity, and wide electrochemical windows. For various applications it will be necessary to solidify the ionic liquid into particular spatial arrangements, such as membranes or gels, or to partition the ionic liquid in coexisting phases, such as microemulsions and micelles. One example includes formation of spherical, cylindrical, and vesicular micelles by poly(butadiene-$b$-ethylene oxide) and poly(styrene-$b$-methylmethacrylate) in the common hydrophobic ionic liquids [BMI][PF$_{6}$] and [EMI][TFSI]. This work has been extended to the formation of reversible micelle shuttles between ionic liquids and water, whereby entire micelles transfer from one phase to the other, reversibly, depending on temperature and solvent quality. Formation of ion gels has been achieved by self-assembly of poly(styrene-$b$-ethylene oxide-$b$-styrene) triblocks in ionic liquids, and by the thermoreversible system poly(N-isopropylacrylamide-$b$-ethylene oxide-$b$-N-isopropylacrylamide), using as little as 4{\%} copolymer. Further, these gels have been shown to be remarkably effective as gate dielectrics in organic thin film transistors. The remarkably high capacitance of the ion gels ($>$ 10 $\mu $F/cm$^{2})$ supports a very high carrier density in an organic semiconductor such as poly(3-hexylthiophene), leading to milliamp currents for low applied voltages. Furthermore, the rapid mobility of the ions enables switching speeds approaching 10 kHz, orders of magnitude higher than achievable with other polymer-based dielectrics such as PEO/LiClO$_{4}$. Finally, we have shown that ordered nanostructures of block copolymers plus ionic liquids show the characteristic self-assembly properties of strongly-segregated systems. Prospects for anisotropic ionic conductivity are also being explored.   

Phase Behavior of Block Copolymer Solutions in an Ionic Liquid

Incorporation of ionic liquids into block copolymers is of interest for applications such as high temperature fuel cell membranes. We investigate the lyotropic and thermotropic phase behavior of solutions of poly(styrene-$b$-2-vinyl pyridine) (S2VP) block copolymers in an ionic liquid consisting of imidazole and bis(trifluoromethane)sulfonamide (HTFSI). Using small angle X-ray scattering (SAXS) and static birefringence, we demonstrate that the ionic liquid behaves as a selective solvent, preferentially solvating the poly(2-vinyl pyridine) segment of the block copolymer. At moderate to high concentrations ($\ge $ 40 wt{\%}) of copolymer, we observe lyotropic phase transitions to lamellar and cylindrical (hcp) nanostructures. At low concentrations of S2VP copolymer ($\le $ 30 wt{\%}), we observe poorly-ordered, microphase-separated structures, which do not resemble the face-centered cubic or body-centered cubic spherical micelles observed in block copolymer solutions in molecular solvents. We observe that the order-disorder transition temperature of the series of SVP copolymers does not depend strongly on the concentration of the block copolymer solution in ionic liquid.   

Ordering of Triblock Copolymer Surfactants by Blending with a Room Temperature Ionic Liquid

Well-ordered block copolymer microdomains were obtained by blending Pluronic{\textregistered} PEO-PPO-PEO triblock copolymer surfactants with the room temperature ionic liquid, 1-butyl-3-methylimidazolium hexafluorophosphate. The selective association of the ionic liquid with the PEO blocks increases the segregation strength by increasing the effective interaction parameter between the blocks. The neat copolymer is phase-mixed in the melt whereas the addition of ionic liquid to the copolymer results in phase segregation, forming well-ordered microdomains. The ionic liquid was confirmed to interact with the PEO blocks by a depression in the melting point of the blends with increasing ionic liquid concentration. Further, small angle x-ray scattering experiments show a decrease in the breadth of the first order peak, as well as the appearance of higher order peaks, with increasing ionic liquid concentration. These results confirm the formation of well-ordered microdomains.   

Phase separation induced by polymer-ionic molecule complexation

The miscibility of polymers in ionic solutions has attracted long-standing interest in polymer science. In particular, it has been demonstrated experimentally that phase separation can be driven by complexation of polymers and ionic-molecules. Thermally reversible strong forces such as hydrogen bonding and electrostatic force are often employed to induce the complexation. In this study, we developed a self consistent field theory for polymers which are capable of binding small ionic molecules. Specifically, poly(vinyl alcohol) and borate ion in aqueous solution with sodium chloride are used as a model system. Binding isotherm, phase diagrams, as well as comparisons with experiments, will be presented. The theory provides a closed-loop region for an instability of the homogeneous phase in the phase diagram. Implications of our results to the sol-gel transition arising from the correlation between unoccupied and occupied ion-binding sites of polymers are discussed.   

Morphology and Ion Transport in Mixtures of Polymers and Ionic Liquid

Mixtures of polymers and ionic liquid have been prepared using homopolymers, random copolymers, and block copolymers: poly(methyl methacrylate), poly(methyl methacrylate-\textit{ran}-styrene), and poly(methyl methacrylate-$b$-styrene). The ionic liquid is 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide. These mixtures are investigated using X-ray scattering and electron microscopy. Mixtures of the homopolymer and random copolymer with the ionic liquid are homogeneous and amorphous morphology with excess scattering as content of ionic liquid increases. The block copolymer and ionic liquid mixtures show ordered structures typical of block copolymers that vary with ionic liquid content. The morphologies of the copolymer-ionic liquid mixtures will be correlated with the conductivities.   

Dissolving Polymers in Ionic Liquids.

Dissolution and phase behavior of polymers in ionic liquids have been assessed by solution characterization techniques such as intrinsic viscosity and light scattering (static and dynamic). Elevated viscosity proved the greatest obstacle. As yet, whether principles standard to conventional polymer solutions apply to ionic liquid solutions is uncertain, especially for polymers such as polyelectrolytes and hydrophilic block copolymers that may specifically interact with ionic liquid anions or cations. For flexible polyelectrolytes (polymers releasing counterions into high dielectric solvents), characterization in ionic liquids suggests behaviors more typical of neutral polymer. Coil sizes and conformations are approximately the same as in aqueous buffer. Further, several globular proteins dissolve in a hydrophilic ionic liquid with conformations analogous to those in buffer. General principles of solubility, however, remain unclear, making predictions of which polymer dissolves in which ionic liquid difficult; several otherwise intractable polymers (e.g., cellulose, polyvinyl alcohol) dissolve and can be efficiently functionalized in ionic liquids.   

Polyester Spherulite Crystallization in Ionic Liquids

A series of polyesters have been crystallized in ionic liquids. Spherulites of the polyesters have been grown isothermally from different ionic liquids after cooling the single phase polymer/ionic liquid system from above the polymer melting point temperature. To the authors' best knowledge this is the first reported account of polyester spherulites grown from these non-traditional solvents. The combination of physical properties of the crystallizing system supports the un-restrained branching/splitting volume-filling growth in all radial directions of the suspended crystallizing entity. The morphology of the collected spherulites at various stages of their formation was examined using scanning electron microscopy (SEM). The SEM results provide a clear visual inspection of the early-stage growth forms and the branching/splitting patterns involved in their evolution to the final spherical form.   

Understanding Ion Transport in Polymerized Ionic Liquids using Dielectric Spectroscopy

In order to deduce the mechanism of ion conduction in ion-containing polymers, not only the conductivity needs to be measured but also the number density and mobility of conducting ions must be determined using broadband dielectric spectroscopy, covering broad frequency and temperature ranges. To obtain a transference number of unity, one ionic charge is covalently bonded to the polymer so that only the counterions can contribute to ion conduction. In this study, imidazolium-containing monomer was synthesized and polymerized to make a cationic homopolymer with either tetrafluoroborate or bis(trifluoromethanesulfonyl)imide anionic counterions. These ions can associate into pairs and larger aggregates. The degree of ion pairing can be estimated from the temperature dependence of the dielectric constant and knowledge of the dipole moment of the ion pair, using the 1936 Onsager equation. Using the 1953 Macdonald model makes it possible to determine concentration and mobility of mobile counterions from analysis of electrode polarization in dielectric spectroscopy.   

Enhanced ionic conductivity of polyurethane ionomers by self-solvating cations

We study the effect of different cations on ionic conductivity and dielectric properties of polyurethane ionomeric single-ion conductors with para-phenyl diisocyanate and anionic diols (carboxylate or phosphonate) constituting the hard segments and poly(ethylene glycol) as the soft segment. Bulky cations such as tetra-alkyl ammonium can increase ionic conductivity compared to metallic cations like sodium because bulky cations have lower interaction energy with anions, allowing more dissociation from the anions. In order to increase the conductivity even more, ether oxygens, which are well-known to solvate cations, are incorporated in the alkyl tail of ammonium-type cations. By comparing polyurethane ionomers with sodium, tetramethyl ammonium, and ammonium with ether oxygens in the alkyl tail, we show that the presence of ether oxygen on the ammonium can significantly reduce T$_{g}$ and increase ionic conductivity in our single-ion conductors.   

Weakening Ion Interactions in Ionomers using Ionic Liquid Counterions

Anionic poly(ethylene oxide)-based ionomers are candidate materials for electro-active devices due to the ability of ether oxygens to solvate conducting cations. Conventional alkali metal cations in sulfonated PEO-ionomers are exchanged to ionic liquid counterions and electrical and mechanical properties are measured. Electrode polarization in dielectric spectroscopy is used to determine number density and mobility of conducting counterions. Conductivity and mobility increase with counterion size and exhibit Vogel temperature dependences, meaning counterion motion is coupled with polymer segmental motion. Conducting ion concentrations show Arrhenius temperature dependences, with activation energy reduced as counterion size increases. Oscillatory shear and SAXS suggest ions do not microphase separate, presumably due to ether oxygen solvation of cations. Ionomers with small counterions have higher plateau moduli than larger counterions, suggesting small counterions form more stable quadrupoles. Such studies allow fundamental design of ionic conductors for actuators, as ionic liquids provide larger strains and faster response for electro-active devices.   

Processing of Natural Polymer-nanocomposites using Ionic Liquids as ``Green Solvents''

We report fiber spinning of natural polymers such as cellulose and silk using ionic liquids. Ionic liquids can dissolve cellulose and silk and are less hazardous that the traditional solvents used for dissolving cellulose. We use imidazoluim based ionic liquids as a common solvent to process natural polymers and carbon nanotubes. Cellulose/carbon nanotubes based fibers are spun using wet spinning process. The rheological, mechanical thermal and electrical properties of the fibers are measured. We also characterize the cellulose nanocomposites fibers using ionic liquids by SEM/TEM, X-ray diffraction, TGA and FTIR analysis. Silk and carbon nanotubes fiber processing is also reported using ionic liquids as common solvent.   

Biocompatible Ionic Liquid-Derived Conducting Polymers

A significant and frequently encountered challenge when making an electrical connection to a protein is that its electron-transfer sites are buried within the polypeptide matrix and thus, are not readily accessible to bulk metal electrodes. A further complicating factor is that inorganic (i.e., metallic) electrodes are often incompatible with biological samples. These obstacles might be overcome by the use of conducting oligomers and / or polymers, which are flexible, offering a means to access remote redox centers. These oligomers can be readily modified to include chemical moieties that can connect covalently to sites near redox centers. In addition, conducting polymers can be made to be environmentally responsive (dynamic), processable (conformal coating, soluble) and mechanically durable, thus enabling them to function as an electrical conduit (wire or electrode) to biomolecules. In this work, we describe the design, synthesis and electrochemical properties of thiophene-based ionic liquid monomers and their bulk polymerization by chemical oxidation to yield cationic, aqueous-soluble polymers. Preliminary studies evaluating the electropolymerization of these monomers into nanostructured thin films will also be presented.