Synthesis of Imidazolium-Containing ABA Triblock Copolymers: Role

May 23, 2012 - Matthew D. Green,. †. Jae-Hong ... as green reaction media.13−16 ILs are attractive for their ability .... Using the Datasqueeze so...
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Synthesis of Imidazolium-Containing ABA Triblock Copolymers: Role of Charge Placement, Charge Density, and Ionic Liquid Incorporation Matthew D. Green,† Jae-Hong Choi,§ Karen I. Winey,§,∥ and Timothy E. Long*,‡ †

Department of Chemical Engineering and ‡Department of Chemistry, Macromolecules and Interfaces Institute, Virginia Tech, Blacksburg, Virginia 24061, United States § Department of Materials Science and Engineering and ∥Department of Chemical and Biomolecular Engineering, University of Pennsylvania, Philadelphia, Pennsylvania 19104, United States S Supporting Information *

ABSTRACT: Imidazole-based ABA triblock copolymers prepared using nitroxide-mediated polymerization achieved a microphase-separated morphology with ion-conducting central blocks. Difunctional random poly(1-(4-vinylbenzyl)imidazole-co-n-butyl acrylate) (poly(VBIm-conBA)) central blocks provided low glass transition temperature (Tg) phases for ion conduction, and subsequent chain extension for polystyrene external blocks provided mechanical reinforcement. Selective incorporation of 1-ethyl-3-methylimidazolium ethylsulfate ([EMIm][EtSO4]) into the random poly(VBIm-co-nBA) central blocks reduced the Tg of both neutral and charged imidazole-containing central blocks. Mechanical properties for ABA triblock copolymers depended on quaternization, central block composition, and ionic liquid incorporation. Ionic conductivity increased over an order of magnitude due to quaternization of imidazole and additional ionic liquid. Tuning central block composition, charge content, and ionic liquid content provided an avenue to tailor the thermomechanical properties and ionic conductivity of ABA triblock copolymers.

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INTRODUCTION Ionomeric block copolymers are potential membranes for electromechanical transducers, which mechanically deform under an applied voltage due to ion migration and have applications as biomimetic materials, energy harvesting devices, and sensors.1,2 Critical design parameters for the ionomeric membrane include low-resistance to ion motion, moderate moduli (∼100 MPa) at application temperature, electrochemical stability, and polymer morphology that promotes bulk ion diffusion.3,4 Varying phase weight fractions enable various nanoscale morphologies including spheres, cylinders, bicontinuous networks, and lamellae.5 Ionic microdomains facilitate efficient ion-transport if ion-channels or bicontinuous physical networks form.6−10 Other microdomains, such as lamellae and cylinders, require orientation and only allow for efficient diffusion of ions along oriented axes.11 Ionic spheres do not form connected pathways for bulk ion diffusion and are therefore not useful for ion-conducting membranes.12 Ionomeric block copolymers potentially tailor the properties to successfully address the criteria discussed. Ionic liquids (ILs) are salts with melting temperatures below 100 °C that exhibit high ionic conductivity and thermal stability, low volatility, tunable chemical structure, and potential as green reaction media.13−16 ILs are attractive for their ability to satisfy task-specific applications for tailored physical, thermal, chemical, and solution properties.16,17 A second class of IL is polymerizable ILs (PILs),18−22 and PILs are candidates for ion© 2012 American Chemical Society

conducting membranes due to the ionic functionality in each repeating unit.1 Anion selection and functional substituents on PILs tailor the electrochemical properties and enhance the overall ionic conductivity of ILs.19,21,23,24 Specifically, bis(trifluoromethanesulfonate)imide (Tf2N−) and trifluoromethanesulfonate (TfO−) anions resulted in optimal Tg and ionic conductivity.25−28 Elabd, Winey, and coworkers examined the impact of PIL Tg on ionic conductivity and confirmed that reduced Tg increased ionic conductivity.29 Long and coworkers studied the influence of IL uptake on ionic conductivity and mechanical properties of a microphase separated zwitterionic copolymer.30 This study revealed the effect of IL on the occupied microdomain and observed morphological changes in the ionic aggregates while ionic conductivity increased over an order of magnitude upon IL uptake. A major downfall of homopolymer ion conductors is ionic conductivity directly relates to polymer Tg while mechanical properties scale inversely, as Prud’homme and others established earlier.31,32 Thus, lowering T g to improve conductivity also sacrifices modulus. Incorporating mechanically reinforcing microdomains using block copolymers provides mechanical integrity and ion conduction in concert. Elabd, Winey, and coworkers examined the influence of IL Received: January 26, 2012 Revised: April 27, 2012 Published: May 23, 2012 4749

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Bruker Hi-Star two-dimensional detector with a sample to detector distance of 54 and 150 cm. Using the Datasqueeze software,37 2-D scattering patterns were converted to 1-D plots with azimuthal angle integration. The scattering intensity was corrected for the primary beam intensity, and the corrected scattering from an empty cell was subtracted. Samples for transmission electron microscopy (TEM) were sectioned at −90 °C using a Reichert-Jung ultramicrotome with a diamond knife to a nominal thickness of 40−70 nm. Unstained sections of the sample were examined on a JEOL 2010F field emission transmission electron microscope. Images were recorded at an accelerating voltage of 200 kV. Size exclusion chromatography (SEC) in tetrahydrofuran (THF) at 40 °C with a flow rate of 1.0 mL/h, determined molecular weights relative to polystyrene standards using a refractive index detector and a multiangle laser light scattering (MALLS) detector. Refractive index increments (dn/dc) were determined using a Wyatt Optilab T-rEX equipped with a 690 nm laser at 25 °C. Triblock copolymers (0.1−1.0 mg/mL) were dissolved in THF and injected into the Optilab T-rEX using a syringe pump. The dn/dc values were determined with the Astra V software from Wyatt, and used to determine absolute weight-average molecular weights (Mw) from SEC. In situ FTIR spectroscopy was performed with a Mettler-Toledo ReactIR 45 m equipped with a Si Comp probe. Convetional free radical polymerizations were performed at 65 °C under N2 and spectra collected at 2 min intervals over 12 h. Electrochemical impedance spectroscopy (EIS) was performed using an Autloab PGSTAT 302N potientiostat and a four-point electrode sample cell purchased from BekkTeck, Inc. An applied alternating sinewave potential was applied at 0.2 V with frequencies ranging from 0.1 Hz to 1 MHz. The temperature was controlled using an ESPEC BTL433 environmental chamber which controlled the temperature to ±0.1 °C and 10% RH to ±0.1%. Real resistance values were taken as the high x-axis intercept of the Nyquist plot of imaginary impedance vs real impedance. Conductivity was calculated using the relationship σ = l/AR, where l is the distance between electrodes, A is the area, and R is the measured resistance. Synthesis of VBIm. VBIm was prepared according to a previous literature precedent.38 Sodium hydrogen carbonate (NaHCO3) (5.25 g, 62.4 mmol) was added to 100 mL of a binary mixture of water/ acetone (1:1 v:v) in a 250 mL two-neck round bottomed flask equipped with an addition funnel and reflux condenser. To this mixture, imidazole (13.61 g, 0.199 mol) was added and stirred until completely dissolved. VBCl (7.61 g, 49.8 mmol) was added dropwise at 23 °C, and the solution was subsequently heated to 50 °C and stirred for 20 h. Following the reaction, the remaining solid salt was filtered and discarded, and acetone was distilled under reduced pressure at 23 °C. The remaining solution was diluted with 500 mL of diethyl ether and washed with 50 mL of ultrapure water six times. The organic phase was then extracted with 100 mL of 2.0 M HCl three times, and then 200 mL of 4.0 M NaOH was added, producing an opaque heterogeneous solution. This mixture was extracted with 50 mL of diethyl ether three times, the organic phase was dried over anhydrous sodium sulfate, and the ether was removed under reduced pressure at 23 °C. The isolated clear oil formed pure VBIm crystals and dissolved in an equal volume of ethyl acetate and cooled to −20 °C. Synthesis of Bromoethyl Tri(ethylene glycol)methyl Ether. Bromine (9.1 mL, 0.177 mol) was added dropwise to a solution of tri(ethylene glycol)methyl ether (28.4 g, 0.173 mol) and triphenylphosphine (46.1 g, 0.176 mol) in diethyl ether (100 mL). After 1 h, the mixture was filtered over a silica plug and the solvent was removed under reduced pressure. The product was purified using silica gel chromatography (diethyl ether) to give bromoethyl tri(ethylene glycol)methyl ether (32.2 g, 82%) as a colorless liquid. Nitroxide-Mediated Polymerization of nBA. nBA (13.9464 g, 108.8) DEPN2 (67 mg), and DEPN (6.0 mg) were degassed in a round bottomed flask through two freeze−pump−thaw cycles. The solution was heated to 125 °C for 70 min and then cooled to 23 °C. Residual nBA monomer was removed at reduced pressure (0.1 mmHg) at 40 °C.

uptake on the morphology of a poly(styrene-b-methyl methacrylate) diblock copolymer and observed a change from lamellar morphology to cylindrical morphology with addition of IL.11 Mahanthappa, Elabd, and coworkers synthesized a poly(styrene-b-PIL) diblock copolymer and analyzed the influence of varying PIL block weight fraction on the morphology, ionic conductivity, and thermal properties.5 Increased ionic content shifted cylindrical polymer morphology to a combination of cylindrical and lamellar polymer morphology and increased ionic conductivity. Lodge, Frisbie, and coworkers investigated neutral ABA triblock copolymer gels of poly(styrene-b-methyl methacrylate-b-styrene).33 This study emphasized the importance of maintaining block copolymer modulus while maximizing ionic conductivity, as Balsara and coworkers previously demonstrated.34 In Lodge’s work, the polar central block selectively incorporated IL while polystyrene external blocks provided mechanical reinforcement, which introduced an application window where storage modulus remained constant and ionic conductivity increased with temperature. In the present study, we prepared a series of microphase separated ABA triblock copolymers using nitroxide-mediated polymerization. A random poly(1-(4-vinylbenzyl)imidazole-con-butyl acrylate) (poly(VBIm-co-nBA) copolymer central block controlled charge content while polystyrene external blocks provided mechanical reinforcement. Increased VBIm content and quaternization in the random central block increased Tg without effect on the polystyrene external block Tg. Selective incorporation of 1-ethyl-3-methylimidazolium ethylsulfate [EMIm][EtSO4] into the imidazole-containing phase plasticized the central block and reducing the Tg. Neutral triblock copolymers with added IL displayed the highest ionic conductivity and Vogel−Fulcher−Tammann analysis of ionic conductivity showed quality fittings. Tuning the thermomechanical properties and ionic conductivity of ionomeric and ILincorporated triblock copolymers provided a strategy for preparing polymers for ion-conducting membranes.

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EXPERIMENTAL SECTION

4-Vinylbenzyl chloride (VBCl, Sigma, 90%) and imidazole (Sigma, 99%) were used as received. n-Butyl acrylate (nBA) (Sigma, 99%) was distilled under vacuum and stored under an argon atmosphere at −20 °C, styrene (Sigma, 99%) was passed through silica to remove inhibitor prior to use. Ultrapure water was retrieved from a Milli-Q Integral 3 water purification system manufactured by Millipore, Inc. Ntert-Butyl-N-[1-diethylphosphono(2,2-dimethylpropyl)]nitroxide (DEPN) and diethyl 2,5-bis((tert-butyl(1-(diethoxyphosphoryl)-2,2dimethylpropyl)amino)oxy)hexanedioate (DEPN2) were synthesized according to previous literature accounts.35,36 1 H NMR spectroscopy was performed using a 400 MHz Varian Unity at 25 °C in CDCl3 or DMSO-d6. Thermogravimetric analysis (TGA) was performed using a TA Instruments TGA 2950 at a 10 °C/ min heating ramp under a nitrogen atmosphere from 23 to 600 °C. Differential scanning calorimetry (DSC) was performed using a TA Instruments Q1000, scans were performed under N2 from −80 to +200 °C with heating at 10 °C/min and cooling at 100 °C/min; Tg’s were recorded on the second heating cycle. Dynamic mechanical analysis (DMA) was performed with a TA Instruments Q800 from −100 to +150 °C at a 3 °C/min heating ramp in film tension mode, and a single frequency of 1.0 Hz. Small-angle X-ray scattering was performed on the samples at the University of Pennsylvania. The Cu X-rays were generated from a Nonius FR 591 rotating-anode generator operated at 40 kV and 85 mA. The bright, highly collimated beam was obtained via Osmic Max-Flux optics and pinhole collimation in an integral vacuum system. The scattering data were collected with a 4750

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Synthesis of poly(VBIm-b-nBA-b-VBIm). Poly(nBA) (0.4013 g) and VBIm (0.8171 g) were dissolved in DMF (1.6 mL) and degassed through three freeze−pump−thaw cycles. The solution was backfilled with argon and heated to 125 °C for 24 h. The polymer was precipitated in water, redissolved in chloroform, and precipitated into hexanes. The polymer was filtered and dried at reduced pressure (0.5 mmHg) at 60 °C for 18 h. Conventional Free Radical Copolymerization of VBIm and nBA. VBIm (1.0 g, 5.3 mmol), nBA (0.69 g, 5.3 mmol), and AIBN (17.5 mg, 0.10 mmol) were dissolved in DMF (7.5 mL) and purged with argon for 30 min. The solution was then heated to 65 °C and stirred for 24 h. Poly(VBIm-co-nBA) was precipitated in water, filtered, and dried at reduced pressure (0.5 mmHg) at 40 °C for 18 h. Nitroxide-Mediated Copolymerization of nBA and VBIm. VBIm (2.28 g, 12.3 mmol), nBA (30.00 g, 234.0 mmol), DEPN2 (211.7 mg, 0.26 mmol), and DEPN (36.7 mg, 0.12 mmol) were dissolved in DMF (34 mL) and degassed using three freeze−pump− thaw cycles. The solution was backfilled with argon, and heated to 125 °C for 2 h. Residual monomer and DMF were distilled at reduced pressure (0.1 mmHg) and 40 °C, after which the remaining polymer was dissolved in THF and precipitated in hexanes. Poly(VBIm-conBA) was filtered and dried at reduced pressure (0.5 mmHg) at 40 °C for 18 h. Synthesis of Poly(Sty-b-[VBIm-co-nBA]-b-Sty). Poly(VBIm-conBA), (2.02 g, 1.6 × 10−5 mol), DEPN (2.14 mg, 0.0072 mmol), and styrene (4.05 g, 38.8 mmol) were dissolved in DMF (6.5 mL) and degassed using three freeze−pump−thaw cycles. The flask was backfilled with argon, and heated to 125 °C for 4 h. Residual styrene and DMF were distilled at reduced pressure (0.1 mmHg) at 40 °C, and the resulting solid was dissolved in THF and precipitated into hexanes. Poly(Sty-b-[VBIm-co-nBA]-b-Sty) was isolated through filtration, and dried at reduced pressure (0.5 mmHg) at 40 °C for 18 h.

Figure 1. Monomer reactivity ratio determination for conventional free radical copolymerizations of VBIm and nBA using the Mayo− Lewis method.

plotted versus r2 for each copolymerization reaction defined the monomer reactivity ratios rVBIm and rnBA (Figure 1). d[M1] [M1](r1[M1] + [M 2]) = d[M 2] [M 2]([M1] + r2[M 2])

(1)

Nitroxide-mediated polymerization prepared random poly(VBIm-co-nBA) central block precursors with number-average molecular weights of ∼30 000 g/mol and VBIm contents from 0 to 100% (Scheme 1). The Tg increased as VBIm content increased, and agreed with the theoretical Fox equation. Chain extension with styrene provided reinforcing external blocks (Scheme 2). The polystyrene external “A” blocks provided

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RESULTS AND DISCUSSION Poly(VBIm-b-nBA-b-VBIm) ABA triblock copolymers served as our initial composition for the formation of ABA thermoplastic elastomers with an ion-conducting microphase. These triblock compositions are readily accessible using sequential addition to a difunctional, PnBA homopolymer precursor with nitroxidemediated polymerization (see Supporting Information). However, poly(VBIm-b-nBA-b-VBIm) triblock copolymers did not offer a synergy of outstanding mechanical performance and conductivity since the reinforcing phase was also the conducting phase. Thus, random copolymers of VBIm and nBA were prepared to relocate the conducting phase into the central block. Copolymers prepared initially using conventional free radical copolymerization confirmed random incorporation of VBIm into poly(VBIm-co-nBA) copolymers. Previously, Long, Winey, and coworkers showed that VBIm and nBA randomly incorporated into poly(VBIm-co-nBA) copolymers through agreement of Tg’s with the theoretical Fox equation and analysis of copolymer compositions relative to monomer feed ratios using 1H NMR spectroscopy.39 In situ FTIR spectroscopy confirmed random incorporation through the determination of monomer reactivity ratios. Monitoring individual monomer conversion with time using in situ FTIR spectroscopy determined relative monomer consumption rates, and analysis using the Mayo−Lewis method (see Figure 1) according to eq 1 revealed monomer reactivity ratios. In the Mayo−Lewis equation, d[Mi] is the instantaneous concentration of monomer i, [Mi] is the initial concentration of monomer i, and ri is the reactivity ratio toward the other monomer. Assuming arbitrary values of r1 from 0.0 to 2.0 allowed calculation of r2 using the Mayo−Lewis equation for separate copolymerization reactions. The intersection of r1

Scheme 1. Nitroxide-Mediated Polymerization to Prepare Random Poly(VBIm-co-nBA) Central Block Precursors

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dc allowed for absolute weight-average molecular weight determination of poly(Sty-b-[VBIm-co-nBA]-b-Sty) using THF SEC. 1H NMR spectroscopy determined the numberaverage molecular weight of the poly(VBIm-co-nBA) central block precursors and triblock copolymer composition. Tg of ∼100 °C for the polystyrene external blocks further confirmed the ABA triblock structure. TGA revealed lack of weight loss for the triblock copolymers to 280 °C where the onset of weight loss occurred. The n-butyl acrylate repeat unit initiated a twostep degradation at 280 °C through loss of 1-butene.40 Weight loss increased with increased n-butyl acrylate mol % in the poly(Sty-b-[VBIm-co-nBA]-b-Sty) triblock copolymer. DMA revealed that increasing VBIm content increased the Tg of the poly(VBIm-co-nBA) central block (Figure 2). Poly(VBIm-co-nBA) Tg increased 18 °C with an increase from 25 mol % to 50 mol % VBIm content. The polystyrene external block Tg remained constant as the composition of the poly(VBIm-co-nBA) central block varied. Tuning the composition of the central block provided a facile route for tuning the mechanical properties and thermal transitions of the soft, rubbery phase. Quaternization of imidazole in the random poly(VBIm-conBA) central block introduced a permanent positive charge pendant to the polymer backbone (Scheme 3). The charged imidazolium in the random central block increased Tg due to the presence of ionic interactions. Tg increased 30 °C for both charged poly(Sty-b-[EVBIm25%-co-nBA75%]-b-Sty) and charged poly(Sty-b-[EVBIm50%-co-nBA50%]-b-Sty). DMA displayed an increase in Tg for the charged, soft phase that corroborated the Tg’s observed in DSC (Figure 3). The extended glassy plateau caused the polymer to form brittle films at room temperature, and the narrow window between the central block and external block Tg made the rubbery plateau difficult to observe in DMA. Table 1 summarizes the compositions, glass transition temperatures, number-average molecular weights, and molecular weight distributions for the poly(VBIm-co-nBA) precursors, the neutral poly(Sty-b-[VBIm-co-nBA]-b-Sty) triblock copoly-

Scheme 2. Nitroxide-Mediated Polymerization to Prepare Poly(Sty-b-[VBIm-co-nBA]-b-Sty) Triblock Copolymers

mechanical reinforcement to the triblock copolymer, while the random poly(VBIm-co-nBA) central “B” block provided a soft, low Tg phase for ion conduction. Offline measurements of dn/

Figure 2. Influence of poly(VBIm-co-nBA) central block composition on the mechanical properties of poly(Sty-b-[VBIm-co-nBA]-b-Sty). 4752

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at q ∼ 0.09 nm−1 in Figure 4a. Triblock copolymers exhibit limited long-range order when their polydispersity values exceed ∼1.40,41 and domain spacing in block copolymers increases with increasing polydispersity.42 Thus, neutral and charged poly(Sty-b-[VBIm50%-co-nBA50%]-b-Sty) triblock copolymers exhibited microphase separation without long-range order and the primary spacings were larger than the expected values due to the relatively high polydispersity values (∼1.45). A lack of scattering peaks in the small angle region confirms this hypothesis, and thermal annealing did not improve the longrange order in these materials. Interestingly, the charged poly(Sty-b-[EVBIm25%-co-nBA75%]-b-Sty) triblock copolymer showed a second scattering peak at q = ∼1.8 nm−1 that arises from interaggregate scattering between ion-rich domains at a smaller length scale within the microphase separated morphology, Figure 5. Therefore, a proposed morphology for charged poly(Sty-b-[EVBIm25%-co-nBA75%]-b-Sty) based on Xray scattering and electron microscopy results is shown in Figure 6. Addition of [EMIm][EtSO4] effectively plasticized the imidazole-containing central block of the triblock copolymers to mediate the brittle nature of the charged ionomers. The mechanical properties displayed plasticization of the soft central block with reduced Tg and storage modulus of the rubbery plateau for the IL-incorporated neutral poly(styrene-b[VBIm 25%-co-nBA 75% ]-b-styrene) (Figure 7). The lower weight-fraction of polystyrene external blocks after addition of IL lowered the modulus, and plasticization of only the poly(VBIm-co-nBA) central block Tg confirmed phase-specific IL incorporation. The presence of imidazole in the neutral poly(Sty-b-[VBIm-co-nBA]-b-Sty) allowed incorporation of the IL presumably due to the favorable intermolecular actions between imidazole and imidazolium ionic liquid. IL slowly migrated from the neutral triblock copolymers and formed droplets on the membrane surface, however, IL did not migrate from the charged triblock copolymers. The Tg of the soft, central block decreased approximately 20 °C after addition of 20 wt % [EMIm][EtSO4] (Table 2). DMA determined the mechanical properties of IL-containing charged triblock copolymers (Figure 8). Analysis of two separate central block compositions was necessary as addition of IL to charged poly(Sty-b-[EVBIm25%-co-nBA75%]-b-Sty) reduced modulus below suitable values for tensile testing, and charged poly(Sty-b-[EVBIm50%-co-nBA50%]-b-Sty) without added IL formed very brittle films. Charged poly(Sty-b-[EVBIm50%-co-nBA50%]-bSty) with 20 wt % [EMIm][EtSO4] displayed two central block Tg’s below the central block Tg of charged poly(styrene-b[EVBIm25%-co-nBA75%]-b-styrene). Phase separation into ionic aggregates from the nonpolar nBA repeat units occurred to form a three-phase system. Observation of a Tg at −35 °C for poly(nBA) and at 50 °C for the IL-plasticized poly(EVBIm) segments in the central block confirmed the three-phase system. Electrochemical impedance spectroscopy (EIS) determined the ionic conductivity of neutral poly(Sty-b-[VBIm25%-conBA 75% ]-b-Sty) and charged poly(Sty-b-[EVBIm 25% -conBA75%]-b-Sty) in the presence and absence of [EMIm][EtSO4] (Figure 9). Ionic conductivity increased approximately 2 orders of magnitude for poly(Sty-b-[VBIm25%-co-nBA75%]-bSty) upon incorporation of 20 wt % [EMIm][EtSO4]. Similarly, ionic conductivity increased an order of magnitude following quaternization of the imidazole ring to form charged poly(Styb-[EVBIm25%-co-nBA75%]-b-Sty). Addition of [EMIm][EtSO4]

Scheme 3. Quaternization of Imidazole in Neutral Poly(Styb-[VBIm-co-nBA]-b-Sty) with Ethyl Bromide to Prepare Charged Poly(Sty-b-[EVBIm-co-nBA]-b-Sty)

Figure 3. Influence of charge incorporation to the central block on the mechanical properties of poly(Sty-b-[VBIm-co-nBA]-b-Sty).

mers, and charged poly(Sty-b-[EVBIm-co-nBA]-b-Sty) functionalized postpolymerization. Figure 4a shows small-angle X-ray scattering results of neutral and charged poly(Sty-b-[VBIm-co-nBA]-b-Sty). Only charged poly(Sty-b-[EVBIm25%-co-nBA75%]-b-Sty) showed scattering peaks in the small angle region, although the peaks did not coincide with any of the expected block copolymer morphologies. A TEM micrograph (Figure 4b) of the charged poly(Sty-b-[EVBIm25%-co-nBA75%]-b-Sty) exhibited microphase separation with a length scale of ∼70 nm, though without longrange order, and is consistent with the broad scattering features 4753

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Table 1. Composition, Glass Transition Temperatures, Molecular Weights, and Molecular Weight Distributions for ImidazoleContaining Triblock Copolymers VBIma (mol %) poly(VBIm25%-co-nBA75%) poly(Sty-b-[VBIm25%-co-nBA75%]-b-Sty) poly(Sty-b-[EVBIm25%-co-nBA75%]-b-Sty) poly(VBIm50%-co-nBA50%) poly(Sty-b-[VBIm50%-co-nBA50%]-b-Sty) poly(Sty-b-[EVBIm50%-co-nBA50%]-b-Sty)

34.5 19.5 19.5 52.5 24.4 24.4

nBAa (mol %) 65.5 34.8 34.8 47.5 21.3 21.3

styrenea (mol %) 0 45.7 45.7 0 54.3 54.3

Tg1b (oC)

Tg2b (oC)

5 14 44 32 42 72

− 100 107 − 92 110

Mnc (kg/mol) a

45.9 84.4 94.1a 32.3a 70.5 80.7a

Mw/Mnc ND 1.27 ND ND 1.45 ND

a Copolymer compositions determined with 1H NMR spectroscopy. bGlass transition temperatures determined using DSC. cNumber-average molecular weights and distributions determined using THF SEC.

Figure 5. X-ray scattering results in the intermediate angle region (q: 1.0−3.5 nm−1) for the charged poly(Sty-b-[EVBIm25%-co-nBA75%]-bSty) triblock copolymer. Experimental data are shown as open circles, while a model of the liquid packing of monodisperse spherical ionic aggregates is shown as a solid line.43 The model parameters include the radius of the ionic aggregates (R), the radius of closest approach (RCA) that limits the spatial correlation between ionic aggregates, and the number density of ionic aggregates (Np).

Figure 6. Schematic of the proposed morphology of the charged poly(Sty-b-[EVBIm25%-co-nBA75%]-b-Sty) triblock copolymer according to X-ray scattering and TEM results. The center-to-center distance between poly(EVBIm-co-nBA) microdomains is ∼65 nm (Figure 4) and the diameter of closest approach between spherical ionic aggregates is ∼2.6 nm (Figure 5).

Figure 4. (a) X-ray scattering results for neutral and charged poly(Styb-[VBIm-co-nBA]-b-Sty) samples. (b) TEM micrograph of charged poly(Sty-b-[EVBIm25%-co-nBA75%]-b-Sty) triblock copolymer.

to charged poly(Sty-b-[EVBIm25%-co-nBA75%]-b-Sty) increased ionic conductivity another half order of magnitude. Interestingly, neutral poly(Sty-b-[VBIm25%-co-nBA75%]-b-Sty) with IL displayed a higher ionic conductivity above 100 °C than charged poly(Sty-b-[EVBIm25%-co-nBA75%]-b-Sty) with IL. The two triblock copolymers had similar central block Tg’s, and therefore restriction of ion motion related to the electrostatic interactions within charged poly(Sty-b-[EVBIm25%-co-nBA75%]b-Sty) restricted ion conduction in the charged triblock copolymer relative to the neutral triblock copolymer.

Ionic conductivity measurements for neutral poly(Sty-b[VBIm 50% -co-nBA 50% ]-b-Sty) and charged poly(Sty-b[EVBIm50%-co-nBA50%]-b-Sty) in the presence and absence of IL displayed similar results to the VBIm triblock copolymers with 25 mol % (Figure 10). Addition of IL to neutral poly(Styb-[VBIm50%-co-nBA50%]-b-Sty) increased ionic conductivity 3 orders of magnitude, while quaternization increased ionic conductivity 1 order of magnitude. Addition of IL to charged 4754

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Figure 7. Influence of [EMIm][EtSO4] on the mechanical properties of poly(Sty-b-[VBIm25%-co-nBA75%]-b-Sty).

Figure 9. Ionic conductivity for neutral poly(Sty-b-[VBIm25%-conBA75%]-b-Sty) and charged poly(Sty-b-[EVBIm25%-co-nBA75%]-b-Sty) triblock copolymers in the presence and absence of added IL. Solid lines indicate analysis using the VFT equation.

Table 2. Tg’s for Poly(styrene-b-[VBIm-co-nBA]-b-styrene) and Poly(styrene-b-[EVBIm-co-nBA]-b-styrene) in the Presence and Absence of [EMIm][EtSO4] Tg1a (oC)

Tg2a (oC)

0

14

100

20

−11

103

0

44

107

20

−12

105

0

42

92

20

7

102

0

72

110

20

∼6

109

[EMIm][EtSO4] (wt %) poly(Sty-b-[VBIm25%-co-nBA75%]b-Sty) poly(Sty-b-[VBIm25%-co-nBA75%]b-Sty) poly(Sty-b-[EVBIm25%-conBA75%]-b-Sty) poly(Sty-b-[EVBIm25%-conBA75%]-b-Sty) poly(Sty-b-[VBIm50%-co-nBA50%]b-Sty) poly(Sty-b-[VBIm50%-co-nBA50%]b-Sty) poly(Sty-b-[EVBIm50%-conBA50%]-b-Sty) poly(Sty-b-[EVBIm50%-conBA50%]-b-Sty) a

Figure 10. Ionic conductivity for neutral poly(Sty-b-[VBIm50%-conBA50%]-b-Sty) and charged poly(Sty-b-[EVBIm50%-co-nBA50%]-b-Sty) triblock copolymers in the presence and absence of added IL, solid lines indicate analysis using the VFT equation.

Tg’s determined using DSC.

poly(Sty-b-[VBIm50%-co-nBA50%]-b-Sty) with 20 wt % [EMIm][EtSO4] was ∼1 order of magnitude higher than neutral poly(Sty-b-[VBIm25%-co-nBA75%]-b-Sty) with 20 wt % [EMIm][EtSO4]. Fitting temperature-dependent ionic conductivity with the VFT equation, eq 2, revealed critical design criteria and potential barriers to ion conduction in the triblock copolymers (Table 3). In the VFT equation, σ∞ is the infinite temperature conductivity, B is a fitting parameter related to the activation energy of ion conduction, and T0 is the Vogel temperature.44−46 The addition of IL and quaternization reduced the activation energy for ion conduction. Quaternization of the imidazole rings increased T0 following observed trends in Tg. Addition of IL to the charged triblock copolymer reduced T0 also following Tg, and all T0’s were ∼50 K below Tg. Temperature-dependent ionic conductivity for imidazolecontaining triblock copolymers uniformly displayed good agreement with VFT fittings.

Figure 8. Mechanical properties of charged poly(Sty-b-[EVBIm50%-conBA50%]-b-Sty) with 20 wt % [EMIm][EtSO4] relative to charged poly(Sty-b-[EVBIm25%-co-nBA75%]-b-Sty).

poly(Sty-b-[EVBIm50%-co-nBA50%]-b-Sty) increased ionic conductivity ∼1.5 orders of magnitude. The neutral poly(Sty-b[VBIm50%-co-nBA50%]-b-Sty) with 20 wt % [EMIm][EtSO4] displayed a higher ionic conductivity than charged poly(Sty-b[EVBIm50%-co-nBA50%]-b-Sty) with 20 wt % [EMIm][EtSO4] at all temperatures. Finally, the ionic conductivity for neutral

⎛ −B ⎞ σ = σ∞ exp⎜ ⎟ ⎝ T − T0 ⎠

(2)

Increasing the weight fraction of imidazole and imidazolium species in the triblock copolymers resulted in an increase in 4755

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these Tg/T values. An overall trend indicated that ionic conductivity increased 3 orders of magnitude following an increase in imidazole and imidazolium weight fraction of ∼0.3. A constant value of Tg/T limits changes in ionic conductivity from altered chain mobility and directly relates to the concentration of conducting species. However, between individual data points the ionic conductivity did not strictly adhere to the increased weight fraction of conducting species, which suggested a delicate balance between ion mobility, charge content, polymer composition, and the morphology of the triblock copolymers. These characteristics work in concert to influence the ionic conductivity of the imidazolium-containing triblock copolymers. Additionally, ionic conductivity increased approximately an order of magnitude as temperature increased from a Tg/T value of 0.84 to 0.74, as described earlier.

Table 3. VFT-Fitting Parameters for TemperatureDependent Ionic Conductivityb sample poly(Sty-b-[VBIm25%co-nBA75%]-b-Sty) poly(Sty-b-[VBIm25%co-nBA75%]-b-Sty) poly(Sty-b[EVBIm25%-conBA75%]-b-Sty) poly(Sty-b[EVBIm25%-conBA75%]-b-Sty) poly(Sty-b-[VBIm50%co-nBA50%]-b-Sty) poly(Sty-b-[VBIm50%co-nBA50%]-b-Sty) poly(Sty-b[EVBIm50%-conBA50%]-b-Sty) poly(Sty-b[EVBIm50%-conBA50%]-b-Sty)

[EMIm] [EtSO4] (wt %)

B (K)

T0 (K)

σ∞ (S/cm)

Tg, DSCa (K)

0

650

220

1.6 × 10−5

284

20

1390

209

1.2 × 10−1

262

0

930

252

1.7 × 10−3

317

20

830

181

1.2 × 10−3

261

0

540

261

8.2 × 10−6

315

20

410

250

7.2 × 10−3

300

0

160

276

1.5 × 10−5

345

20

500

246

4.9 × 10−3

299

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CONCLUSIONS Synthesis of triblock copolymers incorporating imidazole and imidazolium functionalities provided unique opportunities to design stimuli-responsive materials. VBIm formed near-ideal copolymers with nBA using conventional free radical and nitroxide-mediated polymerization. Nitroxide-mediated polymerization prepared poly(Sty-b-[VBIm-co-nBA]-b-Sty) triblock copolymers that underwent postpolymerization functionalization to introduce IL properties. Varying the VBIm and charge content in the poly(VBIm-co-nBA) central block controlled Tg and block copolymer modulus. Incorporation of [EMIm][EtSO4] influenced the mechanical properties and ionic conductivity of the triblock copolymers. The IL selectively plasticized the central block, reduced Tg, and increased the ionic conductivity. Addition of 20 wt % [EMIm][EtSO4] increased ionic conductivity over ten times higher than quaternization. The temperature-dependent ionic conductivity displayed quality fittings with the VFT equation. Several parameters influence the design of polymeric membranes for electroactive devices. Critical parameters elucidated in this investigation include: weight fraction of the mechanically reinforcing phase, conducting phase Tg, charge content in the conducting phase, and IL incorporation. Balancing these design parameters in concert provides a roadmap for tuning the thermomechanical properties, ionic conductivity, and morphology of ionomeric triblock copolymers for ion-conducting membranes.

Tg’s determined using DSC. bB = VFT activation energy for ion conduction; T0 = Vogel temperature; σ∞ = infinite temperature conductivity. a

ionic conductivity attributed to the added charge density (Figure 11). Imidazole and imidazolium weight fraction

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Figure 11. Ionic conductivity with increasing weight fraction of imidazole and imidazolium species. Consistent values of Tg/T (Tg/T = 0.74 (circles) and Tg/T = 0.84 (squares)) reduce changes in charge mobility.

ASSOCIATED CONTENT

S Supporting Information *

Synthetic strategy for the preparation of poly(VBIm-b-nBA-bVBIm) and subsequent characterization of the triblock copolymers using DMA, SAXS, and TEM. This material is available free of charge via the Internet at http://pubs.acs.org.

increases in Figure 11 in the order: neutral poly(Sty-b[VBIm25%-co-nBA75%]-b-Sty) < neutral poly(Sty-b-[VBIm50%co-nBA 50% ]-b-Sty) < charged poly(Sty-b-[EVBIm 25% -conBA75%]-b-Sty) < neutral poly(Sty-b-[VBIm25%-co-nBA75%]-bSty) with 20 wt % [EMIm][EtSO4] < charged poly(Sty-b[EVBIm50%-co-nBA50%]-b-Sty) < neutral poly(Sty-b-[VBIm50%co-nBA50%]-b-Sty) with 20 wt % [EMIm][EtSO4] < charged poly(Sty-b-[EVBIm 25% -co-nBA 75% ]-b-Sty) with 20 wt % [EMIm][EtSO 4 ] < charged poly(Sty-b-[EVBIm 50% -conBA50%]-b-Sty) with 20 wt % [EMIm][EtSO4]. The line labeled Tg/T = 0.74 does not contain charged poly(Sty-b[EVBIm50%-co-nBA50%]-b-Sty), and the line labeled Tg/T = 0.84 contains neither charged poly(Sty-b-[EVBIm25%-co-nBA75%]-bSty) with 20 wt % [EMIm][EtSO4] nor neutral poly(Sty-b[EVBIm50%-co-nBA50%]-b-Sty) with 20 wt % [EMIm][EtSO4] as the closest conductivity measurements did not correspond to

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors wish to thank Dr. Philippe Bissel for help with monomer synthesis, Dr. Rebecca Brown for help with DEPN2 synthesis and providing poly(nBA), and Dr. Erin Murphy for help with AFM. This material is based upon work supported by 4756

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the U.S. Army Research Office under Grant Number W911NF07-1-0452 Ionic Liquids in Electro-Active Devices (ILEAD) MURI. This material is based upon work supported in part by the Macromolecular Interfaces with Life Sciences (MILES) Integrative Graduate Education and Research Traineeship (IGERT) of the National Science Foundation under Agreement No. DGE-0333378. This material is based upon work supported by the Army Research Office (ARO) under Award No. W911NF-10-1-0307. We acknowledge the Institute for Critical Technology and Applied Science (ICTAS) for funding the acquisition of instrumentation used in this research.

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dx.doi.org/10.1021/ma300185b | Macromolecules 2012, 45, 4749−4757