Mechanical properties and ionic conductivity of electrospun quaternary ammonium ionomers

Journal of Membrane Science 389 (2012) 478–485

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Mechanical properties and ionic conductivity of electrospun quaternary ammonium ionomers Supacharee Roddecha a , Zexeuan Dong b , Yiquan Wu b , Mitchell Anthamatten a,∗ a b

Department of Chemical Engineering, University of Rochester, 206 Gavett Hall, Rochester, NY 14627, United States Department of Mechanical Engineering, University of Rochester, 235 Hopeman Building, Rochester, NY 14627, United States

a r t i c l e

i n f o

Article history: Received 18 June 2011 Received in revised form 2 November 2011 Accepted 4 November 2011 Available online 11 November 2011 Keywords: Ionomers Electrospinning Anion-exchange membranes Polysulfone

a b s t r a c t Solution processable cationic ionomers are receiving widespread attention for their promising roles as anionic exchange membranes, antimicrobial coatings, and dialysis membranes. The ion conductivity can be improved by increasing the material’s ion-exchange capacity; however this often results in poor mechanical properties. Here we report the synthesis of solution processable polysulfone ionomers with a tunable density of quaternary ammonium functional groups. Electrospinning is explored to create fibrous mats that can be solvent welded and filled with a second material to modulate membrane properties. The tradeoff between hydroxide anion conductivity and mechanical properties in solution cast and electrospun mats is assessed, and electrospinning is shown to improve mechanical properties of the fiber phase with relatively small losses in ion conductivity. © 2011 Elsevier B.V. All rights reserved.

1. Introduction Poly(aryl ether) sulfones are engineering thermoplastics that contain linear aryl, ether, and sulfone repeat units in the polymer backbone. The rigid backbone imparts good mechanical properties, thermal stability, and chemical resistance. Polysulfones are affordable and biocompatible materials that have found industrial and medical applications as advanced membranes [1]. Polysulfone ionomers combine the mechanical, chemical, and thermal stability of the aryl ether sulfone backbone with the hydrophilicity and ion-exchange ability of a charged backbone [2,3]. Quaternized ammonium polysulfones (QAPS’s), for example, can be obtained using the well established and recently popularized chloromethylation method [4]. QAPS membranes are conductive toward hydroxide and carbonate ions and are being developed as solution processable polymer electrolyte membranes for alkaline fuel cells [5–11]. Unlike fuel cells based on the proton exchange membrane, alkaline fuel cells do not require expensive Pt-based catalysts. However, alkaline fuel cell development is limited primarily by the lack of durable anion-exchange membranes with sufficient hydroxide conductivity. Recently, Pan et al. measured the ionic conductivity of solution processable QAPS and reported conductivities on the order of 0.01 S/cm [6]. Increasing the concentration of ion-exchange groups further improves the ionic

∗ Corresponding author. Tel.: +1 585 273 5526; fax: +1 585 273 1348. E-mail address: [email protected] (M. Anthamatten). 0376-7388/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2011.11.016

conductivity of QAPS at the expense of mechanical properties [11,12]. Here, we investigate electrospinning of QAPS ionomers as an approach to overcome the engineering tradeoff between ion conductivity and mechanical properties. Electrospun mats consist of a network of submicron fibers that span macroscopic dimensions. Fibers are created by electric field assisted extrusion of a polymer solution from a spinneret, followed by rapid solvent removal. Below a critical fiber diameter, individual fibers can exhibit significantly higher tensile modulus than in the bulk; and this is attributed to confinement of supramolecular structures within polymer nanofibers during solvent removal [13]. Ion conductivity is influenced by other factors including ion content, the degree of swelling, the mobile ion type, and the spatial arrangement of hydrophilic (ion rich) and hydrophobic domains [14]. We hypothesize that electrospinning of QAPS ionomers will have a larger effect on tensile modulus than on ion conductivity, offering a way to overcome the tradeoff. Our study is further motivated from a membrane processing standpoint. The interstitial spaces between electrospun fibers form continuous pathways in three dimensions and can be used to transport fluids through the network of fibers. Liquid permeability is needed for liquid separation membranes, and the interstitial spaces could be used to introduce an epoxy or thermoset support for electrospun fibers. In this study, quaternary ammonium polysulfones were synthesized with varying ionic content and processed into membranes by both solvent-casting and electrospinning techniques. To our knowledge, this is the first systematic study of electrospinning

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of cation-containing polysulfones. The solution viscosity of the neutral polysulfone and the quaternized polysulfones were studied to understand how chain entanglements and ion clusters affect fiber formation. Tensile testing and ion conductivity measurements were performed on solvent-cast and on electrospun membranes under identical conditions to assess the tradeoff between mechanical and ion transport properties. Finally, to further improve mechanical properties, composite membranes were fabricated by imbibing electrospun mats with a curable siloxane elastomer. 2. Experimental 2.1. Materials Polysulfone (PSU, Udel P-1700) was kindly provided by Solvay Advanced Polymers, average molecular weight (Mw ) is 67,000–72,000 g/mol. Chloromethyl methyl ether (CMME) and trifluoro acetic acid (TFA, 99+%) were purchased from Sigma–Aldrich; trimethylamine (TMA, 45 wt.% in water) from Alfa Aesar; zinc powder, 1,2-dichloroethane, and sodium hydroxide (NaOH) from J.T. Baker; dimethyl formamide (DMF) from EMD; hydrochloric acid (HCl, 36.5–38%) from Mallinckrodt Chemicals; methanol from BDH. All chemicals were used without further purification.

O S O

O

2.3. Electrospinning A DMF solution containing 18 wt.% QAPS-Cl was prepared and injected at a rate of 1.0–1.5 ml/h through a blunt end syringe needle (gauge 22) using a syringe pump (Cole-Parmer, IL, USA). The stainless steel syringe needle was connected to a high voltage power supply (Gamma high voltage research, Inc., FL, USA) and maintained at a potential of 20–27 kV relative to ground. A negative bias potential of 1.5–3 kV was applied to a rectangular aluminum collector positioned 15 cm away from the syringe tip.

CH3 C CH3

O

n

Udel PSU P-1700 CMME, Zn, TFA 25 °C, 1-6 hr

O

O S O

O H2C Cl

CMPS

CH3 C CH3

n

TMA (g) 25 °C, 2 hr

O

O S O

QAPS-Cl

2.2. Synthesis Solvent-cast quaternary amine alkaline polysulfone (QAPS-OH) membranes were prepared following Fig. 1. Chloromethylated polysulfone (CMPS) was prepared according to Lu et al. [5] In a typical procedure, 10 g of polysulfone (Udel P-1700) was dissolved in 200 ml of 1,2-dichloromethane. Then, 0.75 g of zinc powder, 3 ml of trifluoroacetic acid (TFA), and 7.5 ml of chloromethyl methyl ether (CMME) were added. The reaction mixture was stirred at 30 ◦ C for 1–6 h to vary the degree of chloromethyl substitution, and the product was precipitated in methanol and dried at room temperature overnight. 1 H NMR (DMSO, 400 MHz): ı = 1.66 (s, 6H), 4.64 (s, 2H), 7.0 (d, 4H), 7.2–7.5 (multiple d, 6H), 7.55 (s, 1H), 7.9 (d, 4H). Drying the product at high temperature was avoided since heating can result in lower solubility due to thermal crosslinking between reactive chloromethyl groups [7]. To obtain quaternary ammonium chloride polysulfone (QAPSCl), nitrogen gas containing trimethylamine was bubbled through a DMF solution of chloromethylated polysulfone (10%, w/v) for 2 h to ensure complete reaction between chloromethyl groups and TMA. The resulting solution was cast into a film and dried at 50 ◦ C under vacuum overnight to obtain a thin polymer membrane with thickness ∼100 ␮m. Chloride counter ions were then exchanged with hydroxyl ions by immersing cast films into 1 M NaOH for 48 h, followed by washing several times with deionized water. The resulting transparent films (QAPS-OH) were dried at 50 ◦ C in vacuum oven for 72 h. 1 H NMR (DMSO, 400 MHz): ı = 1.66 (s, 6H), 3.1 (d, 9H), 4.6 (s, 2H), 5.95 (s, 1H), 7.0 (d, 4H), 7.2–7.5 (multiple d, 6H), 7.55 (s, 1H), 7.9 (d, 4H).

479

O H2C + Cl N H3C CH3 CH3

CH3 C CH3

n

1M NaOH 25 °C, 48 hr

O QAPS-OH

O S O

O H2C + OH N CH3 H3C CH3

CH3 C CH3

n

Fig. 1. Synthesis scheme of quaternary ammonium alkaline polysulfones (QAPSOH).

Electrospun QAPS-Cl membranes were ion-exchanged with hydroxyl anions by immersion in 0.1 M NaOH for 48 h followed by washing with deionized water. Compared to ion-exchange of solvent-cast films, a weaker NaOH solution was used because the nano-fibrous membranes are more readily ionic exchanged due to their high surface area. Ion-exchange using high NaOH concentrations was found to induce stress-damage to the fibers. 2.4. Characterization Product chemical structures were confirmed using 1 H NMR (Bruker 400 MHz NMR) with d-DMSO as a solvent. Steady-shear viscosity measurements of polymer solutions (1–15 wt.%) were made using a Brookfield viscometer operating at 40 ◦ C in a cone-in-plate geometry with a cone radius of 2.4 cm. Measurements determined the specific viscosity, defined as sp = (0 − s )/s , where 0 is the polymer solution viscosity and s is the pure solvent viscosity. The morphology of the electrospun fibrous membranes was investigated by field effect scanning electron microscope (FESEM, Zeiss Supra 40VP). Ion-exchange capacities (IEC) were measured by soaking the alkaline exchange membranes in weakly acidic aqueous solutions (0.001 M HCl) and measuring the resulting pH change. The pH of the exchange solution increased as hydroxyl groups were released, partially neutralizing the acid solution. Films were exposed to the acidic solution for about 72 h, or until the pH ceased to change.

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Fig. 2.

1

H NMR spectrum of quaternary ammonium alkaline polysulfone (QAPS-OH).

The number of released hydroxyl groups was determined from the solution’s pH change and corresponds to the membranes IEC by IEC =

V (10−pH |before − 10-pH |after ) m

(1)

where V is the volume of the HCl solution and m is the original dry mass of the polymer. Mechanical properties of electrospun and cast films were investigated at room temperature using a tensile testing system (MTS, model QT5) operating at crosshead speed of 0.2 mm/s. Rectangular specimens were cut (2.5 × 0.25 ) according to ASTM D-882. Ionic conductivities of the QAPS-OH membranes were measured under different temperatures and at 90% relative humidity by impedance measurement over the frequency range of 1–104 Hz using a four electrode test cell (BekkTech) connected to a Gamry Instruments PCI4/750 potentiostat. Humidified nitrogen gas was continuously fed to the test cell at a flow rate of 500 cm3 /min. The resistance R of the membranes was taken as the lowest frequency that produced a near zero imaginary response. The conductivity of the membranes was calculated by  = (1/R)(d/w·t)where d is the fixed distance between the measurement electrodes, and w and t are the width and thickness of the sample membrane, respectively. 3. Result and discussion 3.1. Synthesis and chemical characterization Polysulfone was converted to chloromethylated polysulfone (CMPS) by reaction with chloromethyl methyl ether, and the extent of reaction was controlled by varying the stirring time, as indicated in Table 1. NMR confirmed that chloromethyl groups were stoichiometrically converted to quaternary ammonium groups by exposure to gaseous trimethyl amine. Fig. 2 shows a representative NMR of a quaternary ammonium polysulfone after counter ion-exchange with hydroxyl ion (QAPS-OH). The peak positions in the acquired spectra agreed well with those reported by Lu et al. [5] Note that there is no signal at ı = 4.64 ppm, corresponding to chloromethyl protons on CMPS, indicating complete conversion of side groups to the ammonium form. The ion-exchange capacity (IEC) was calculated by comparing the integrated peak intensity of the methylene protons on the ammonium side group (ı = 4.53 ppm) to that of the methyl groups on the polysulfone backbone (ı = 1.66 ppm). Resulting IEC’s from this method are included in Table 1. Interestingly, following ion-exchange the QAPS-OH polymers still exhibited good

solubility in DMF and DMSO with solution properties similar to QAPS-Cl. Samples were named according to their IEC values determined using NMR. The IECs of both solvent-cast and electrospun alkaline membranes were confirmed using acid–base neutralization. Film specimens were immersed into weakly acidic HCl solutions, and IECs were determined by the pH change (see Section 2). The IECs of solvent-cast films, shown in Table 1, agree well with IECs determined from NMR, confirming efficient hydroxide exchange. The same ion-exchange procedure and NMR analysis was conducted on electrospun films and resulted in similarly efficient hydroxide exchange. Table 1 further shows that ion content can be tuned by varying the chloromethylation reaction time. The CMME and catalyst concentration could also be increased to raise the IEC. However, when the IEC exceeded ∼2.5 mmol/g membranes were not dimensionally stable upon exposure to water, and they were very brittle in a dry state. Thus, only samples with low IECs (QAPS-OH-0.6, QAPS-OH-0.9, and QAPS-OH-1.7) were chosen for electrospinning studies. 3.2. Solution viscosity and electrospinning Polymer solutions require sufficient chain overlap and entanglements to generate enough viscosity for quality electrospun fibers. Experiments were conducted on QAPS-Cl solutions to determine the suitable polyelectrolyte concentration to generate bead-free, uniform electrospun fibers. Fig. 3 displays the measured specific

Table 1 IECs of solvent-cast QAPS synthesized using different chloromethylation reaction conditions. Samplea

Stirring time (h)

CMME (ml)

Zinc (g)

TFA (ml)

IEC (mmol/g) 1

QAPS-OH-0.6 QAPS-OH-0.9 QAPS-OH-1.7 QAPS-OH-2.3 QAPS-Cl-2.9 a

1 2.5 4 6 6

7.5 7.5 7.5 10 16

0.75 0.75 0.75 0.75 1

3 3 3 3 4

H NMR

0.58 0.94 1.74 2.26 2.87

pH 0.50 1.00 1.81 2.35 b

Chloromethylation reactions were conducted on 10 g batches of polysulfone starting material. b Ion-exchange could not be performed because of membrane swelling.

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481

Table 2 Scaling exponents and critical concentration for neutral PS and polyelectrolyte QAPS-Cl at various IECs.a

a

Sample

Semi-dilute unentangled exponent (b)

Semi-dilute entangled exponent (c)

Concentrated exponent (d)

Ce (wt.%)

CD (wt.%)

QAPS-Cl-0.6 QAPS-Cl-0.9 QAPS-Cl-1.7

0.54 0.53 0.48

1.24 1.27 1.38

3.61 3.77 3.35

3.0 3.6 2.9

7.1 6.6 7.3

sp ∼ Cb for C* > C > Ce ; sp ∼ Cc for Ce < C < CD ; sp ∼ Cd for CD < C.

viscosities sp plotted against polymer concentration for the neutral polysulfone and three QAPS-Cl polyelectrolytes with different IECs. For the neutral polysulfone solutions, the data fall onto two power law regimes corresponding to exponents of 1.64 and 4.44. The crossover at 9.4 wt.% corresponds to the entanglement concentration Ce [15,16]. Viscosities of the polyelectrolyte solutions are significantly higher than those of neutral polysulfones. In dilute solution, polyelectrolyte chains form extended rod-like structures that possess higher hydrodynamic volumes than neutral chains. As a result, QAPS-Cl polyelectrolytes overlap and form entanglements at lower concentrations than their neutral polysulfones. For polyelectrolytes, theory predicts a power law exponent of 0.5 below Ce and 1.5 above Ce [17]. Least squares fits to the data in Fig. 3 indicate that the QAPS-Cl solutions become entangled at around 3 wt.% (see Table 2). At higher concentrations (C > CD ), the scaling exponent changes again as polymer chain entanglements dominate flow behavior and chain dimensions become independent of concentration. For polyelectrolytes, this regime is predicted to have a scaling exponent of 3.75, and the fitted exponents, shown in Table 2, agree fairly well with the theory [17]. Polymer solutions were electrospun to examine how solution concentration and the presence of entanglements affect the electrospun morphology. Fig. 4 shows the FESEM images of electrospun neutral polysulfone and a representative polyelectrolyte (QAPS-Cl0.6). Other polyelectrolytes showed similar trends. Electrospinning at low concentrations (<6 wt.%) failed to yield polymer fibers—the solution jet could not overcome surface tension, causing it to break up into droplets. At higher concentrations, chain entanglements help to prevent the polymer solution from separating into droplets. At 6 and 10 wt.%, near the onset of the concentrated regime for QAPS-Cl-0.6 (CD ∼ 7 wt.%), both fibers and beads were observed. Defect-free polyelectrolyte fibers were only observed at

QAPS-Cl-1.7 QAPS-Cl-0.9 QAPS-Cl-0.6 neutral PS

5.0

3.3. Mechanical properties

log ηsp

4.5

4.0

3.5

dilute semidilute unentangled semidilute entangled concentrated

3.0

0.0

0.2

0.4

18 wt.%, in the concentrated regime. For the neutral polysulfones, at 18 wt.% beaded fibers were still observed, and uniform, bead-free fibers required concentrations of ∼24 wt.% (images not shown). In summary, at equivalent solution concentrations, the QAPS polyelectrolytes more readily formed fibers than neutral polysulfones, and this can be explained by the viscosity scaling shown in Fig. 4. All electrospun mats and membranes were prepared using a QAPS-Cl solution concentration of 18 wt.%. To improve mechanical and ion transport properties, electrospun mats with three-dimensional connectivity were fabricated by mechanical pressing followed by solvent-annealing. Electrospun ∼100 ␮m thick mats were pressed (13,000 psi. 2 min.) and solventwelded by exposure to DMF vapor (10 min. per side). Fig. 5 shows that the fiber volume density appears higher following compression and that three-dimensional interconnections form between nanofibers during solvent welding. Electrospun mats were further modified to create membranes by filling the interstitial voids with an inert and non-conductive elastomer [18]. Filled membranes could be useful in applications including fuel cells, dialysis, ion selective separations, and protective clothing. Silicone impregnated electrospun membranes were prepared by soaking the compacted and solvent annealed QAPS-Cl electrospun mats in a poly(dimethyl siloxane) prepolymer/curing agent (Sylgard 184). The membrane surfaces were wiped dry prior to thermal curing overnight at 50 ◦ C in a vacuum oven. Fig. 6 displays the morphology of the membrane after silicone impregnation. The images show that the silicone elastomer successfully penetrated and filled the voids between nanofibers. The membrane surface showed both exposed polysulfone ionomer fibers and the silicone matrix material. While silicone materials have high gas permeability, and, therefore, would not be suitable for fuel cell membranes, the demonstrated concept could be extended to include epoxies or other curable resins.

0.6

0.8

1.0

1.2

log C Fig. 3. Dependence of specific viscosity (sp ) on polymer concentration for neutral and charged polysulfone QAPS-Cl solutions. Lines are least squares fits to the data over the ranges shown in the figure.

Tensile stress–strain curves of prepared QAPS-Cl films and electrospun mats are displayed in Fig. 7. The linear relationship between stress and strain observed in the electrospun material at low strain indicates that nonwoven fibers were effectively connected through solvent-welding. At higher levels of strain, both the solvent-cast and electrospun materials begin to yield and subsequently fail. The higher level of noise in the electrospun data is attributed to rearrangement of randomly aligned fibers during extension. Values of Young’s modulus (E), tensile strength (TS), and elongation-at-yield determined from the stress–strain curves are reported in Table 3. A direct comparison of mechanical properties from solvent-cast films and electrospun mats is inappropriate because electrospun mats have voids between fibers, resulting in a much lower density (es ) (see Fig. 8). If the individual fibers are assumed to have the same density as the solvent-cast ionomer membrane (sc ), then the mechanical properties of the electrospun mats can be corrected by multiplying the measured values by the ratio sc /es . Once this correction is applied, then the data (in parentheses in Table 3) indicate that mechanical properties associated with the fiber phase

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Fig. 4. FESEM images of electrospun neutral polysulfone (a–d) and QAPS-Cl-0.6 (e–h) at different polymer concentrations (3, 6, 10, and 18 wt.%).

exceed those of solvent-cast films. Mechanical properties were also measured for selected QAPS-OH samples, and fiber phase mechanical properties again exceeded those of solvent-cast films (data not shown); however properties of QAPS-OH were slightly weaker than those of QAPS-Cl. 3.4. Ion conductivity Electrospun fibers are confined within the mat, but they are randomly oriented. Ion channels are expected to form along the fiber axis (in plane). However, the low packing density of electrospun fibers may also reduce ion conductivity. To deconvolute the effects of reduced volume fraction and fiber alignment, impedance measurements were performed on the following hydroxyl exchanged (QAPS-OH) samples: (i) solvent-cast, (ii) electrospun, and (iii)

Fig. 5. FESEM images of electrospun QAPS-Cl fibers after: (a) electrospinning, (b) mechanically pressing, and (c) solvent vapor (DMF) annealing.

Fig. 6. FESEM images of a silicone impregnated electrospun membrane: (a) surface view and (b) cross section view.

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Fig. 7. Representative tensile stress strain curves for QAPS-Cl membranes at different IECs: (a) solvent-cast films and (b) electrospun, compressed, and solvent annealed mats. The three datasets in (b) are based on samples with different densities, and therefore, cannot be compared to one another.

Table 3 Tensile properties of solvent-cast and electrospun, pressed and solvent annealed QAPS-Cl mats. Sample

QAPS-Cl-0.6 QAPS-Cl-0.9 QAPS-Cl-1.7 a

 (mg/cm3 )

Young’s modulusa (MPa)

Max. stressa (MPa)

Tensile strengtha (MPa)

% Tensile strain

sc

es

Cast

ELS

Cast

ELS

Cast

ELS

Cast

ELS

1062.5 1226.3 1202.8

29.3 18.3 35.5

1651.7 1757.6 1760.0

77.5 (2810.4) 67.8 (4555.4) 176.3 (5975.9)

53.0 50.5 58.1

3.0 (108.8) 2.5 (168.0) 5.0 (169.5)

52.4 49.3 53.1

2.7 (97.9) 2.4 (161.3) 4.9 (166.1)

5.0 4.5 10.4

9.1 5.8 5.6

Values in parenthesis correspond to properties corrected by sc /es .

electrospun followed by mechanical alignment using the tensile test apparatus. Results are shown in Fig. 9. The solvent-cast films show the highest hydroxyl ion conductivity, and the conductivity increases with ion content. Electrospinning significantly decreases the conductivity, mainly due to the presence of non-conducting voids between spun fibers. The unaligned sample QAPS-OH-0.9 had the highest density (es = 374.19 mg/cm3 ) of the electrospun films, and this explains why the uncorrected conductivity (Fig. 8) appears somewhat high. Ion conductivity can also be corrected to account for the lower density of conducting fibers. To do this, the

Fig. 8. Cartoon emphasizing the density difference between solution-cast and electrospun mats. The volume fraction of the ionomer phase is i ≈ es /s .

Fig. 9. Conductivities of ELS and cast QAPS membrane at 80 ◦ C and 90% RH: (a) uncorrected and (b) corrected.

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Fig. 10. Plots illustrating the tradeoff between mechanical properties and ion conductivity for solvent-cast and electrospun mats: (a) Ei  i versus IEC, and (b) TSi  i versus IEC.

following assumptions are made: (1) individual fibers are assumed to have the same density as the bulk ionomer, (2) fiber swelling is negligible, and does not change the ionomer volume fraction significantly, and (3) the interstitial voids are filled through capillary condensation of deionized water ( w ∼ 0.055 ␮S/cm). The void fraction was estimated by comparing the density of electrospun films to that of a solvent-cast film. Provided these assumptions, the inplane conductivity of the ionomer fiber phase ( i ) is related to the mat conductivity ( m ) by m = i i + w w

(2)

where i and w are the volume fractions of the ionomer and water phases, respectively. For each film studied, Eq. (2) can be solved for  i . The corrected results are shown in Fig. 9b, and values are in better agreement with those of the solvent-cast films. This suggests that electrospun fibers conduct hydroxyl ions nearly as effectively as bulk ionomer. The data also show that aligned fibers exhibit equal or better conductivity compared to the solvent-cast ionomer, and thus, fiber alignment supports in-plane ion transport. For fuel cell membranes, increasing ion content generally leads to improvements in ion conductivity at the expense of mechanical properties. For an ideal tradeoff, one would expect that the product of ion conductivity with a mechanical property, such as modulus or tensile strength, should be independent of ion content. Fig. 10 illustrates the engineering tradeoff in solvent-cast films and electrospun mats. The figure shows how two products, Ei ×  i and TSi ×  i , depend on ion content. For solvent-cast films both products weakly increase with IEC, and for electrospun films the products are much more sensitive and strongly increase with IEC. This analysis highlights how the fiber phase mechanical properties are enhanced by electrospinning, whereas the fiber’s ion conductivity is about the same as observe in a solvent-cast film. 4. Conclusions The ability to functionalize commercially relevant polysulfones with quaternary ammonium groups was demonstrated, and products were solution processed into films and electrospun mats. The specific viscosity of ionomer solutions was significantly higher than neutral polysulfone solutions due to overlap and entanglements at lower polymer concentrations. A heightened viscosity enabled electrospinning of ionomers, with variable ionic exchange capacities, into fibrous mats. The high surface area mats could be pressed, solvent-welded and back filled with a curable poly(dimethyl

siloxane) elastomer to form higher density membranes. Mechanical testing revealed an improvement in both the modulus and tensile strength of the fiber phase. Impedance analysis showed conductivities in the fiber phase that were near that of the bulk ionomer phase. These studies demonstrate that electrospinning not only creates a high surface area, low density mat, but also it overcomes the expected engineering tradeoff between conductivity and mechanical properties. Post processing of these and other electrospun membranes may result in applications such as antimicrobial textiles, microbiotic filters, ion selective membrane supports for water desalination, and alkaline conducting membranes. Acknowledgements The authors acknowledge the Royal Thai Government and the University of Rochester’s Department of Chemical Engineering for support of this research. Partial support was also provided by an AFOSR (FA9550-10-1-0067) grant and a Frank J. Horton Research Fellowship. References [1] N.N. Li, A.G. Fane, W.S.W. Ho, T. Matsuura (Eds.), Advanced Membrane Technology and Applications, John Wiley & Sons, Hoboken, 2008. [2] H.B. Park, B.D. Freeman, Z.B. Zhang, M. Sankir, J.E. McGrath, Highly chlorinetolerant polymers for desalination, Angew. Chem. Int. Ed. 47 (2008) 6019–6024. [3] J.H. Pang, H.B. Zhang, X.F. Li, Z.H. Jiang, Novel wholly aromatic sulfonated poly(arylene ether) copolymers containing sulfonic acid groups on the pendants for proton exchange membrane materials, Macromolecules 40 (2007) 9435–9442. [4] L.I. Belen’kii, Y.B. Vol’kenshtein, I.B. Karmanova, New data on the chloromethylation of aromatic and heteroaromatic compounds, Russ. Chem. Rev. 46 (1977) 892–903. [5] S.F. Lu, J. Pan, A.B. Huang, L. Zhuang, J.T. Lu, Alkaline polymer electrolyte fuel cells completely free from noble metal catalysts, Proc. Natl. Acad. Sci. USA. 105 (2008) 20611–20614. [6] J. Pan, S.F. Lu, Y. Li, A.B. Huang, L. Zhuang, J.T. Lu, High-performance alkaline polymer electrolyte for fuel cell applications, Adv. Funct. Mater. 20 (2010) 312–319. [7] J.H. Wang, S.H. Li, S.B. Zhang, Novel hydroxide-conducting polyelectrolyte composed of an poly(arylene ether sulfone) containing pendant quaternary guanidinium groups for alkaline fuel cell applications, Macromolecules 43 (2010) 3890–3896. [8] J.F. Zhou, M. Unlu, J.A. Vega, P.A. Kohl, Anionic polysulfone ionomers and membranes containing fluorenyl groups for anionic fuel cells, J. Power Sources 190 (2009) 285–292. [9] S. Gu, R. Cai, T. Luo, Z.W. Chen, M.W. Sun, Y. Liu, G.H. He, Y.S. Yan, A soluble and highly conductive ionomer for high-performance hydroxide exchange membrane fuel cells, Angew. Chem. Int. Ed. 48 (2009) 6499–6502. [10] G.C. Abuin, P. Nonjola, E.A. Franceschini, F.H. Izraelevitch, M.K. Mathe, H.R. Corti, Characterization of an anionic-exchange membranes for direct methanol alkaline fuel cells, Int. J. Hydrogen Energy 35 (2010) 5849–5854.

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