Well-Defined Imidazolium ABA Triblock Copolymers as Ionic-Liquid-Containing Electroactive Membranes

Solution and Film Self-Assembly Behavior of a Block Copolymer Composed of a Poly(ionic Liquid) and a Stimuli-Responsive Weak Polyelectrolyte

Cu(0)-mediated atom transfer radical polymerization was used to synthesize a poly(ionic liquid), poly[4-vinylbenzyl-3-butylimidazolium bis(trifluoromethylsulfonyl)imide] (PVBBImTf2N), a stimuli-responsive polyelectrolyte, poly[2-(dimethylamino)ethyl methacrylate] (PDMAEMA), and a novel block copolymer formed from these two polymers. The synthesis of the block copolymer, poly[2-(dimethylamino) ethyl methacrylate]-block-[poly(4-vinylbenzyl-3-butylimidazolium bis(trifluoromethylsulfonyl)imide] (PDMAEMA-b-PVBBImTf2N), was examined to evaluate the control of "livingness" polymerization, as indicated by molecular weight, characterizations of degree of polymerization, and 1HNMR spectroscopy. 2D DOSY NMR measurements revealed the successful formation of block copolymer and the connection between the two polymer blocks. PDMAEMA-b-PVBBImTf2N was further characterized for supramolecular interactions in both the bulk and solution states through FTIR and 1H NMR spectroscopies. While the block copolymer demonstrated similar intermolecular behavior to the PIL homopolymer in the bulk state as indicated by FTIR, hydrogen bonding and counterion interactions in solution were observed in polar organic solvent through 1H NMR measurements. The DLS characterization revealed that the PDMAEMA-b-PVBBImTf2N block copolymer forms a network-like aggregated structure due to a combination of hydrogen bonding between the PDMAEMA and PIL group and electrostatic repulsive interactions between PIL blocks. This structure was found to collapse upon the addition of KNO3 while still maintaining hydrogen bonding interactions. AFM-IR analysis demonstrated varied morphologies, with spherical PDMAEMA in PVBBImTf2N matrix morphology exhibited in the region approaching the film center. AFM-IR further revealed signals from silica nano-contaminates, which selectively interacted with the PDMAEMA spheres, demonstrating the potential for the PDMAEMA-b-PVBBImTf2N PIL block copolymer in polymer-inorganic nanoparticle composite applications.

Interfacial Effects in Conductivity Measurements of Block Copolymer Electrolytes

The ionic conductivity in lamellar block copolymer electrolytes is often anisotropic, where the in-plane conductivity exceeds the through-plane conductivity by up to an order of magnitude. In a prior work, we showed significant anisotropy in the ionic conductivity of a lamellar block copolymer based on polystyrene (PS) and a polymer ionic liquid (PIL), and we proposed that the through-film ionic conductivity was depressed by layering of lamellar domains near the electrode surface. In the present work, we first tested that conclusion by measuring the through-plane ionic conductivity of two model PIL-based systems having controlled interfacial profiles using impedance spectroscopy. The measurements were not sensitive to changes in interfacial composition or structure, so anisotropy in the ionic conductivity of PS-block-PIL materials must arise from an in-plane enhancement rather than a through-plane depression. We then examined the origin of this in-plane enhancement with a series of PS-block-PIL materials, a P(S-r-IL) copolymer, and a PIL homopolymer, where impedance spectra were acquired with a top-contact electrode configuration. These studies show that enhanced in-plane ionic conductivities are correlated with the formation of an IL-rich wetting layer at the free surface, which presumably provides a low-resistance path for ion transport between the electrodes. Importantly, the enhanced in-plane ionic conductivities in these PS-block-PIL materials are consistent with simple geometric arguments based on properties of the PIL, while the through-plane values are an order of magnitude lower. Consequently, it is critical to understand how surface and bulk effects contribute to impedance spectroscopy measurements when developing structure-conductivity relations in this class of materials.

Polymer Chemistry for Haptics, Soft Robotics, and Human-Machine Interfaces

Progress in the field of soft devices-i.e., haptics, robotics, and human-machine interfaces (HRHMIs)-has its basis in the science of polymeric materials and chemical synthesis. However, in examining the relevant literature, we find that most developments have been enabled by off-the-shelf materials used either alone or as components of physical blends and composites. In this Progress Report, we take the position that a greater awareness of the capabilities of synthetic chemistry will accelerate the capabilities of HRHMIs. Conversely, an awareness of the applications sought by engineers working in this area may spark the development of new molecular designs and synthetic methodologies by chemists. We highlight several applications of active, stimuli-responsive polymers, which have demonstrated or shown potential use in HRHMIs. These materials share the fact that they are products of state-of-the-art synthetic techniques. The Progress Report is thus organized by the chemistry by which the materials were synthesized, including controlled radical polymerization, metal-mediated cross-coupling polymerization, ring-opening polymerization, various strategies for crosslinking, and hybrid approaches. These methods can afford polymers with multiple properties (i.e. conductivity, stimuli-responsiveness, self-healing and degradable abilities, biocompatibility, adhesiveness, and mechanical robustness) that are of great interest to scientists and engineers concerned with soft devices for human interaction.

Ferrocene-Containing Porous Poly(Ionic Liquid) Membranes: Synthesis and Application as Sacrificial Template for Porous Iron Oxide Films

Herein, the fabrication of iron-containing porous polyelectrolyte membranes (PPMs) via ionic complexation between an imidazolium-based poly(ionic liquid) (PIL) and 1,1-ferrocenedicarboxylic acid is reported. The key parameters to control the microstructure of porous hybrid membranes are investigated in detail. Further aerobic pyrolysis of such porous hybrid membranes at 900 °C can transfer the ferrocene-containing PPMs into freestanding porous iron oxide films. This process points out a sacrificial template function of porous poly(ionic liquid) membranes in the fabrication of porous metal oxide films.

Single- versus Dual-Ion Conductors for Electric Double Layer Gating: Finite Element Modeling and Hall-Effect Measurements

Electric double layer (EDL) gating using a single-ion conductor is compared to a dual-ion conductor using both finite element modeling and Hall-effect measurements. Modified Nernst-Planck Poisson (mNPP) equations are used to calculate the ion density per unit area in a parallel plate capacitor geometry with a bulk ion concentration of 215 ≤ c_bulk ≤ 1782 mol/m³. With electrodes of equal size at a 2 V potential difference, the EDL ion density of the single-ion conductor is ∼7 × 10¹³ ions/cm², which is approximately 50% of the ion density induced in the dual-ion conductor. However, this difference is reduced to 8% when the electrode at which the cationic EDL forms is 10 times smaller than the counter electrode. Thus, for a field-effect transistor gated by a single-ion conductor, it is especially important to have a large gate-to-channel size ratio to achieve strong ion doping. The modeled ion densities are validated by Hall-effect measurements on graphene Hall bars gated by a polyethylene oxide (PEO)-based single-ion conductor. The sheet carrier density, n_S, is ∼2 × 10¹³ cm^-2 at V_g = 2 V, which is 3.5 times smaller than the predicted value and has the same order of magnitude as the n_s measured for a PEO-based, dual-ion conductor on the same graphene. The numerical modeling results can be approximated by a simple analysis of capacitors in series, where the EDLs are modeled as capacitors with thickness estimated by the sum of the Debye screening length and the Stern layer. The series of capacitor estimate agrees with the numerical modeling of the dual-ion conductor to within 10% and the single-ion conductor to within 30% from 0.25 to 2 V (c_bulk = 925 mol/m³); similar agreement is observed in the concentration range of 353-1650 mol/m³ for both single- and dual-ion conductors.

Tetrakis(dialkylamino)phosphonium Polyelectrolytes Prepared by Reversible Addition-Fragmentation Chain Transfer Polymerization

A tetrakis(dialkylamino)phosphonium cation ([P(NR₂)₄]⁺) was appended to a styrenic monomer and explored in reversible addition-fragmentation chain transfer polymerization (RAFT) to conduct random copolymerizations of the cationic monomer with styrene. Well-defined polyelectrolytes with molecular weights up to ∼30 100 and dispersities between ∼1.2 and 1.4 were obtained. Up to 18.9 mol % of the ionic monomer could be incorporated into the polymer with hexafluorophosphate or bis(trifluoromethane)sulfonimide acting as the counterion during polymerization. Differential scanning calorimetry of the hexafluorophosphate polymers revealed glass transition temperatures higher than polystyrene likely due to interactions between the anion and the polymer. Thermogravimetric analysis indicated these materials have high thermal stability with decomposition temperatures approaching 400 °C.

Imidazolium-Containing ABA Triblock Copolymers as Electroactive Devices

Two-step reversible addition-fragmentation chain transfer (RAFT) polymerization and two subsequent postpolymerization modification steps afforded well-defined ABA triblock copolymers featuring mechanically reinforcing polystyrene outer blocks and 1-methylimidazole-neutralized poly(acrylic acid)-based central blocks. Size exclusion chromatography and (1)H NMR spectroscopy confirmed predictable molecular weights and narrow distributions. The ionic liquid (IL) 1-ethyl-3-methylimidazolium trifluoromethanesulfonate ([EMIm][OTf]) was incorporated at 30 wt % into polymeric films. Thermogravimetric analysis, differential scanning calorimetry, and dynamic mechanical analysis determined the thermomechanical properties of the polymers and polymer-IL composites. Atomic force microscopy, small-angle X-ray scattering (SAXS), and transmission electron microscopy (TEM) determined surface and bulk morphologies, and poly(Sty-b-AA(MeIm)-b-Sty) exhibited a change from packed cylindrical to lamellar morphology in SAXS upon IL incorporation. Electrochemical impedance spectroscopy determined the in-plane ionic conductivities of the polymer-IL membranes (σ ∼ 10(-4) S/cm). A device fabricated from poly(Sty-b-AA(MeIm)-b-Sty) with 30 wt % incorporated IL demonstrated mechanical actuation under a low applied voltage of 4 V.

Tuning the Pore Size in Gradient Poly(ionic liquid) Membranes by Small Organic Acids

Highly charged porous polymer membranes with adjustable pore size and gradient pore structure along the membrane cross-section were prepared by ammonia-triggered electrostatic complexation between an imidazolium-based cationic poly(ionic liquid) (PIL) and multivalent benzoic acid derivatives. The PIL and the acid compound were first dissolved homogeneously in DMSO, cast into a thin film onto a glass plate, dried, and finally immersed into an aqueous ammonia solution. The diffusion of ammonia from the top to the bottom into the film neutralized the acid and introduced the gradient pore structure and in situ electrostatic cross-linking to fix the pores. The pore size and its distribution of the membranes were found controllable in terms of the multivalency of the acids, the imidazolium/carboxylate ratio, and the nature of the PIL counteranion.

Polyvinylpyridinium-type gradient porous membranes: synthesis, actuation and intrinsic cell growth inhibition

Gradient porous membranes were prepared from a poly(4-vinylpyridinium) polymer together with carboxylic multi-acid compoundsviaelectrostatic complexation.

3D Printing Phosphonium Ionic Liquid Networks with Mask Projection Microstereolithography

Photopolymerization coupled with mask projection microstereolithography successfully generated various 3D printed phosphonium polymerized ionic liquids (PILs) with low UV light intensity requirements and high digital resolution. Varying phosphonium monomer concentration, diacrylate cross-linking comonomer, and display images enabled precise 3D design and polymeric properties. The resulting cross-linked phosphonium PIL objects exhibited a synergy of high thermal stability, tunable glass transition temperature, optical clarity, and ion conductivity, which are collectively well-suited for emerging electro-active membrane technologies. Ion conductivity measurements on printed objects revealed a systematic progression in conductivity with ionic liquid monomer content, and thermal properties and solvent extraction demonstrated the formation of a polymerized ionic liquid network, with gel fractions exceeding 95%.