Herein, we report a polymerized ionic liquid diblock copolymer with high hydroxide conductivity and nanoscale morphology. Surprisingly, the conductivity is not only higher (over an order of magnitude) than its random copolymer analog at the same ion and water content, but also higher than its homopolymer analog, which has a higher ion and water content than the block copolymer. These results should have a significant impact on low-cost (platinum-free), long-lasting, solid-state alkaline fuel cells.

A polymerized ionic liquid (PIL) diblock copolymer, poly (MMA-b-MEBIm-Br), was synthesized at various compositions from an ionic liquid monomer, (1-[(2-methacryloyloxy)ethyl]-3-butylimidazolium bromide) (MEBIm-Br), and a nonionic monomer, methyl methacrylate (MMA), via the reverse addition−fragmentation chain transfer (RAFT) polymerization technique. A hydroxide-conducting PIL diblock copolymer, poly(MMA-b-MEBIm-OH), was also prepared via anion exchange metathesis of the bromide-conducting block copolymer. In a former study, the conductivity and morphology of the bromide-and hydroxide-conducting PIL diblock copolymer were examined at one fixed PIL composition: 17.3 mol %. In this study, additional PIL compositions of (6.6, 11.9, and 26.5 mol %) were explored to fully understand the previous unusual conductivity results. Both bromide and hydroxide conductivities were higher in the PIL block copolymer at PIL compositions of 11.9, 17.3, and 26.5 mol % compared to the PIL homopolymer under the same experimental conditions, even though the homopolymer possessed a higher water and ionic content compared to the block copolymers. These unusual results suggest that the confinement of the PIL microdomain within the block copolymer morphology enhances ion transport compared to its predicted value. Morphology factors (or normalized ionic conductivity, f) were as high as >3 at some conditions, which is much higher than the maximum theoretical limit for randomly oriented lamellar domains (f = 2/3). Application of percolation theory revealed a 3−4-fold enhancement of conductivity when comparing the inherent conductivity to the measured PIL homopolymer conductivity. Both morphology factor analysis and percolation theory corroborate with the absolute conductivity results and the hypothesis that PIL domain confinement in PIL block copolymers enhances conductivity over its bulk properties. ■ INTRODUCTIONPolymerized ionic liquid (PIL) block copolymers are a relatively new class of polymer electrolytes, where the cation of the ionic liquid is covalently attached to one of the polymers in a block copolymer and the benefits of both PILs and block copolymers are combined. PILs possess unique properties, such as high solid-state ionic conductivity, high chemical, electrochemical, and thermal stability, and a widely tunable chemical platform, where significant changes in physical properties have been observed with subtle changes in chemistry. 1−5 When PILs are incorporated into the block copolymer, the resulting PIL block copolymer can possess orthogonal properties, such as high modulus (from the nonionic polymer) and high conductivity or transport (from the ionic polymer or PIL) through the self-assembly of two distinct polymers into welldefined nanostructures of long-range order with tunable morphology and domain size. 1,3,6−10 This has promoted the investigation of PIL block copolymers as thin films for organic electronic devices, 11−14 gas permeation membranes for CO 2 separations, 15,16 and solid-state polymer electrolytes for use in...

Nitroxide-mediated polymerization (NMP) affords the synthesis of well-defi ned ABA triblock copolymers with polystyrene external blocks and a charged poly(1-methyl-3-(4-vinylbenzyl)-imidazolium bis(trifl uoromethane sulfonyl)imide central block. Aqueous size-exclusion chromatography (SEC) and 1 H NMR spectroscopy studies confi rm the control of the composition and block lengths for both the central and external blocks. Dynamic mechanical analysis (DMA) reveals a room temperature modulus suitable for fabricating these triblock copolymers into electroactive devices in the presence of an added ionic liquid. Dielectric relaxation spectroscopy (DRS) elucidates the ion-transport properties of the ABA triblock copolymers with varied compositions. The ionic conductivity in these single-ion conductors exhibits Vogel-FulcherTammann (VFT) and Arrhenius temperature dependences, and electrode polarization (EP) analysis determines the number density of simultaneously conducting ions and their mobility. The actuators derived from these triblock copolymer membranes experience similar actuation speeds at an applied voltage of 4 V DC, as compared with benchmark Nafi on membranes. These tailorable ABA block copolymers are promising candidates for ionic-polymer device applications.

The effects of molecular weight and microdomain orientation on the ionic conductivities of poly(styrene-b-methyl methacrylate) diblock copolymer/1-ethyl-3-methylimidazolium bis-(trifluoromethylsulfonyl)imide (SM/IL) mixtures are assessed through complementary experimental and theoretical techniques. Small-angle Xray scattering revealed that SM/IL mixtures have anisotropic lamellar morphologies, preferentially oriented parallel to the casting substrate. A method for quantifying the morphology factor, or microdomain orientation within the Sax−Ottino model, using 2-D SAXS data is presented and applied to SM/IL mixtures. Ionic conductivity increases by up to an order of magnitude with a 2-fold increase in molecular weight, even when accounting for the morphology type, composition, microdomain orientation, and PMMA/IL glass transition temperature. Self-consistent field theory calculations predict a nonuniform solvent distribution in PMMA/IL microdomains, suggesting that polymer mobility and ion transport are reduced near PS−PMMA microdomain interfaces. Thus, the increase in ionic conductivity with increasing block copolymer molecular weight is associated with having fewer PS−PMMA/IL interfaces per unit volume.

Herein, we examine the synergistic impact of both ion clustering and block copolymer morphology on ion conductivity in two polymerized ionic liquid (PIL) diblock copolymers with similar chemistries but different side alkyl spacer chain lengths (ethyl versus undecyl). When saturated in liquid water, water/ion clusters were observed only in the PIL block copolymer with longer alkyl side chains (undecyl) as evidenced by both small-angle neutron scattering and intermediate-angle X-ray scattering, i.e., water/ion clusters form within the PIL microdomain under these conditions. The resulting bromide ion conductivity in the undecyl sample was higher than the ethyl sample (14.0 mS cm(-1)versus 6.1 mS cm(-1) at 50 °C in liquid water) even though both samples had the same block copolymer morphology (lamellar) and the undecyl sample had a lower ion exchange capacity (0.9 meq g(-1)versus 1.4 meq g(-1)). No water/ion clusters were observed in either sample under high humidity or dry conditions. The resulting ion conductivity in the undecyl sample with lamellar morphology was significantly higher in the liquid water saturated state compared to the high humidity state (14.0 mS cm(-1)versus 4.2 mS cm(-1)), whereas there was no difference in ion conductivity in the ethyl sample when comparing these two states. These results show that small chemical changes to ion-containing block copolymers can induce water/ion clusters within block copolymer microdomains and this can subsequently have a significant effect on ion transport.