Multiblock Copolymers Containing Highly Sulfonated Poly(arylene

Aug 10, 2012 - The thioether bridges of the SPATS blocks were then selectively oxidized to ... simplify water management and cooling, enable the recov...
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Multiblock Copolymers Containing Highly Sulfonated Poly(arylene sulfone) Blocks for Proton Conducting Electrolyte Membranes Shogo Takamuku and Patric Jannasch* Polymer & Materials Chemistry, Department of Chemistry, Lund University, P.O. Box 124, SE-221 00 Lund, Sweden S Supporting Information *

ABSTRACT: We report on multiblock copolymers consisting of highly sulfonated hydrophilic poly(arylene sulfone) (SPAS) blocks combined with hydrophobic poly(arylene ether sulfone) (PAES) blocks. Thiol-terminated precursor blocks of sulfonated poly(arylene thioether sulfone) (SPATS) were first prepared via polycondensations involving a novel tetrasulfonated dichlorotetraphenyldisulfone monomer, followed by coupling with pentafluorophenyl end-capped PAES precursor blocks under mild conditions to form SPATS−PAES block copolymers. The thioether bridges of the SPATS blocks were then selectively oxidized to obtain the SPAS−PAES copolymers with hydrophilic blocks containing exclusively sulfone bridges. Thus, the SPAS blocks were designed for high chain stiffness and stability toward desulfonation and had an ion exchange capacity (IEC) of 4.2 mequiv g−1. Membranes of the SPAS−PAES copolymers were phase separated on the nanoscale and showed an increased thermal stability and decreased water uptake in relation to the corresponding SPATS−PAES membranes. Meta-connectivity in the sulfonated blocks gave slightly higher water uptake than pure para-connectivity. At 80 °C and 30% relative humidity, the proton conductivity of a SPAS−PAES membrane with an IEC of 1.8 mequiv g−1 reached 5.1 mS cm−1, which was comparable to that of Nafion and far exceeded that of a sulfonated statistical copolymer membrane with a similar IEC. This class of block copolymers possesses very attractive properties and has great prospective to meet the demands of various electrochemical applications.

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INTRODUCTION

oxygen barrier properties which seriously restrict the applicability and commercialization of the PEMFC. This has triggered an extensive research and development of alternative membranes, mainly based on sulfonated high-performance aromatic polymers such as poly(arylene ethers)s, poly(arylene ether sulfone)s (PAESs), poly(arylene ether ketone)s, and polyimides.7−10 These ionic polymers potentially offer high mechanical, chemical, and thermal stability, good barrier properties, and straightforward synthetic pathways. However, the general inability of the ionic groups to efficiently phase separate from the aromatic polymer backbones in the membranes leads to disappointingly low proton conductivities, especially at low RH.11,12 This has led to a variety of synthetic strategies where the ionic groups are concentrated to specific parts or segments in the polymeric structure to induce a higher

Various high-performance membranes with tunable transport and separation properties play an important role in many advanced energy and environmental applications.1−3 For example, the proton-exchange membrane fuel cell (PEMFC) relies on efficiently proton-conducting ionomer membranes with a low permeation of hydrogen and oxygen.4 One of the main barriers to the commercial introduction of PEMFCs for automotive and stationary applications is the lack of membranes which are capable of efficiently conducting protons under low relative humidity (RH) at temperatures at, or above, 100 °C. Operation of PEMFCs under these conditions is expected to enhance the rates of the electrochemical reactions, simplify water management and cooling, enable the recovery of waste heat, and to allow the use of lower quality reformed hydrogen.5,6 State-of-the-art perfluorosulfonic acid (PFSA) membranes such as Nafion have limited morphological and chemical stability, a low mechanical modulus, and poor hydrogen and © 2012 American Chemical Society

Received: June 19, 2012 Revised: August 3, 2012 Published: August 10, 2012 6538

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Scheme 1. Synthetic Pathway to Multiblock Copolymers via K2CO3-Mediated Polycondensations under Mild Conditionsa

Reagents and conditions: (i) K2CO3, DMSO/cyclohexane, 110 °C; (ii) K2CO3, DMAc/toluene, 175 °C; (iii) hexafluorobenzene, K2CO3, NMP, 80 °C; (iv) K2CO3, DMSO/cyclohexane, 110 °C; (v) H+, 80 °C; (vi) H2O2, AcOH/H2SO4, 30 °C. a

under low RH. Unfortunately, the polymer was very brittle and water-soluble even at high molecular weights.28 In order to improve the stability of their materials, Kreuer and co-workers very recently reported on the synthesis and properties of multiblock copolymers containing highly sulfonated poly(phenylene sulfone) blocks.31 However, these polymers were prepared by a nucleophilic substitution reaction involving alkali metal sulfides which is quite challenging, is sensitive to side reactions, and gives limited reaction yields.28,32 In the present paper, we discuss the properties of multiblock copolymers containing highly sulfonated poly(arylene thioether sulfone) (SPATS) blocks prepared by a conventional nucleophilic substitution reaction involving a dithiol and a novel tetrasulfonated dihalide monomer at low temperature (110 °C). SPATS blocks were coupled with hydrophobic PAES blocks, and the thioether bridges were then selectively oxidized to sulfone bridges to form highly sulfonated poly(arylene sulfone) (SPAS) blocks. The SPATS and SPAS blocks have a theoretical IEC of 4.48 and 4.18 mequiv g−1, respectively. The aryl rings in the SPAS blocks are linked by either direct aryl− aryl bonds or by sulfone bridges, which induces a very high macromolecular chain stiffness, as well as a high hydrolytic stability of the sulfonic acid units.29,33 Membranes based on the

level of organization of the ionic phase domains in the membrane.13 This has for example been achieved by directing the sulfonic acid groups to side chains14,15 and by preparing copolymers with precisely sequenced sulfonic acids.16,17 However, the most successful results directed toward improving the proton conductivity of aromatic membranes have been achieved with sulfonated block copolymers. Because of the immiscibility of the ionic and nonionic blocks, these copolymers characteristically self-assemble to form various well-ordered phase structures on the nanometer level.13,18 Block copolymer membranes typically show a higher level of conductivity than statistical copolymers at low RHs because the ionic groups are locally concentrated to larger domains which are able to retain a higher level of percolation. Consequently, there is now a significant interest in durable block copolymers with highly sulfonated blocks for high conductivity at low RH.19−27 Kreuer et al. recently prepared highly sulfonated poly(phenylene sulfone) where each phenylene ring is monosulfonated and exclusively connected by electron-withdrawing sulfone linkages.28−30 The polymer had an ion exchange capacity (IEC) of 4.45 mequiv g−1 and showed excellent thermooxidative and hydrothermal stability in combination with a high proton conductivity at 110−160 °C 6539

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product was precipitated in 2-propanol and washed in deionized water several times before drying at 60 °C overnight (overall yield: 92%). The thioether bridges of the SPATS blocks were subsequently oxidized to obtain the multiblock copolymers with SPAS blocks. These copolymers were designated as xSPASbPAESy. The oxidation to obtain pSPASbPAES1.8 was conducted as follows: pSPATSbPAES2.1 (1.0 g, 4.25 mmol thioether bridge) was placed in acetic acid (20 mL). After adding sulfuric acid (2 mL), hydrogen peroxide (1.8 mL, 21.3 mmol) was slowly added. The reaction was first kept at 30 °C for 2 days and then at 105 °C for 10 min. Copolymer pSPASbPAES1.8 was obtained after filtration, washing in deionized water, and vacuum drying at 60 °C. The multiblock copolymers mSPATSbPAESy and mSPASbPAESy were prepared by the same procedure using mSPATS instead of pSPATS. Polymer and Membrane Characterization. The multiblock copolymers were dissolved in DMSO (5 wt %) and cast at 80 °C for 24 h onto a glass plate which had been cleaned by pretreatment with H2SO4/H2O2 (50% vol/vol). The membranes were detached by immersion in deionized water and acidified in 1 M aqueous HCl at 80 °C for 1 day, followed by repeated rinsing and washing with deionized water. 1 H NMR spectroscopy (Bruker DRX400 spectrometer at 400.13 MHz) was performed in either CDCl3 (δ = 7.28 ppm) or DMSO-d6 (δ = 2.50 ppm) solutions of the samples. Ostwald de Waale viscometers were used to measure the intrinsic viscosity ([η]) of the copolymers in DMSO solutions containing 0.05 M LiBr at copolymer concentrations of 0.1−1.0 g dL−1 at 25 °C. The thermal stability of the copolymer membranes was investigated by thermogravimetric analysis (TGA) between 50 and 600 °C using a Q500 analyzer (TA Instruments). In order to collect the TGA data, the sample was preheated at 150 °C for 10 min to remove any solvent residues (in the H+ form at 1 °C/min under air; in the Na+ form at 10 °C min−1 under N2). The temperature showing 5% weight loss was taken as the degradation temperature (Td). Copolymer membranes (in the Na+ form) were analyzed at 10 °C min−1 under N2 by differential scanning calorimetry (DSC) using TA Instruments Q2000. The data were collected during the second heating scan from 50 to 350 °C, after the first cycle of heating to 300 °C and cooling to 50 °C. The glass transition temperature (Tg) was taken as the midpoint of the transition recorded. AFM images were recorded in tapping mode using Noncontact cantilever, XY stage controller, SPM controller, and XE-100 chamber (Park Systems). The IEC (mequiv g−1) value of the copolymer membranes was determined by an acid−base titration. The water uptake (WU, wt %) was calculated from the weight increase by soaking the membranes in deionized water at temperatures from 20 to 100 °C in 20 °C intervals. The hydration number (λ), the number of water molecules per sulfonic acid group, was calculated by combining water uptake and IEC data. Using the dry (W, g) and the wet (W′, g) membrane weights, WU and λ were calculated as

block copolymers were characterized by, e.g., thermal analysis, water uptake, and proton conductivity measurements.

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EXPERIMENTAL SECTION

Materials. The following reagents and solvents were used as received: 1,3-benzenedithiol (m-BDT, Fluka, 99.0+%); 1,4-benzenedithiol (p-BDT, Alfa Aesar, 97%); 4,4′-bis[(4-chlorophenyl)sulfonyl]1,1′-biphenyl (BCPSBP, Sigma-Aldrich, 98%); dimethyl sulfoxide (DMSO, Acros, 99.9%); cyclohexane (Sigma-Aldrich, HPLC grade); n-butyllithium (n-BuLi, Acros, 2.5 M in hexanes); sulfur dioxide (Sigma-Aldrich, 99.9%); hydrogen peroxide (Acros, 35% solutions in water); acetic acid (Acros, 99.8%); sulfuric acid (Fisher Scientific, 95− 97%); 2-propanol (Fisher Scientific, HPLC grade). Tetrahydrofuran (THF, Fisher Scientific, analytical reagent grade) was dried over molecular sieves predried at 200 °C overnight before use, potassium carbonate (Acros, 99+%) was dried at 120 °C overnight, and dialysis membranes (MWCO 100−500, Spectrum Laboratories) were conditioned in deionized water before use. Tetrasulfonated Monomer Synthesis. BCPSBP was tetrasulfonated using a lithiation−sulfination−oxidation procedure to prepare a novel sulfonated monomer (sBCPSBP). BCPSBP (5.0 g, 9.9 mmol) and THF (200 mL) were added to a reactor fitted with a gas inlet/ outlet, a SO2 gas tube/inlet, a thermometer, and a septum. After degassing the solution during at least seven argon−vacuum cycles, nBuLi solution (17.1 mL, 42.8 mmol) was slowly added after cooling the solution to −70 °C using dry ice in 2-propanol. SO2 gas was introduced 1 h after the addition of n-BuLi, resulting in a bright yellow solution. The solution was allowed to rise to 0 °C after keeping it at −70 °C for 30 min. The sulfinate derivative was collected as a yellow powder by filtration and was dried directly on the glass filter for several hours. H2O2 (13 mL) and H2O (87 mL) were added to the product which was then kept at 40 °C overnight. The sulfonated product was filtered to remove any water-insoluble impurities after boiling the mixture at 110 °C for 60 min. Then, the product was salted out by adding NaCl to the boiling water solution and subsequently recrystallized in a water/2-propanol mixture three times (ca. 1/4 vol/vol) (overall yield: 47%). The 1H NMR spectrum of sBCPSBP is available as Supporting Information. Synthesis of Precursor Blocks. The hydrophilic block, thiol-endcapped sulfonated poly(arylene thioether sulfone) oligomers, SPATS, were prepared via polycondensation. Control of the molecular weight (Mn) was achieved by feeding a precise molar excess of either m-BDT or p-BDT to sBCPSBP (Scheme 1). One example of the preparation of pSPATS oligomer follows: sBCPSBP (1.8586 g, 2.0389 mmol), pBDT (0.3189 g, 2.2418 mmol), K2CO3 (3.1 g, 22 mmol), and DMSO (8 mL) were added to a two-neck 50 mL flask equipped with a magnetic stirrer, a N2 inlet, and a condenser fitted with a CaCl2 filter. The solution was heated to 110 °C and kept for 72 h to complete the polycondensation. The product was then precipitated in 2-propanol, followed by dialysis against deionized water for 2 days. The aqueous solution was filtered, and the product was obtained by evaporating the water (overall yield: 0.98 g, 49%, Mn = 6.0 kDa). The mSPATS oligomer was prepared using the same procedure as above, using mBDT instead of p-BDT. The pentafluoro-end-capped hydrophobic PAES precursor blocks were prepared as previously reported (1H NMR spectra are available as Supporting Information).26 Synthesis of Multiblock Copolymers. The hydrophilic−hydrophobic multiblock copolymers were prepared via a coupling reaction between SPATS and PAES precursor blocks, using the setup already described above. The resulting multiblock copolymers with SPATS blocks were designated xSPATSbPAESy, where x is either p or m for the para- or meta-connectivity, respectively, and y is the IEC value (mequiv g−1) determined by acid−base titrations. pSPATSbPAES2.1 was prepared as follows: pSPATS precursor block (0.978 g, 0.305 mmol end group, 6.4 kDa, K+ form), PAES precursor block (1.022 g, 0.305 mmol end group, 6.7 kDa), and K2CO3 (1.25 g, 9.04 mmol) were added to DMSO (18 mL) and cyclohexane (9 mL). The mixture was then dehydrated at 110 °C for 3 h. After removal of the cyclohexane, the reaction mixture was kept at 110 °C for 24 h. The

W′ − W × 100 W

(1)

1000 × (W ′ − W ) IEC × W × 18

(2)

WU =

λ=

The volume-based IEC, IECv (mequiv cm−3), was calculated from the water volume fraction (WF) of the hydrated membranes as follows:

IECv = IEC × ρp × (1 − WF)

WF =

r= 6540

λ λ+r

(3)

(4)

Vp Vw

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1000 IEC × ρp

(6)

18 ρw

(7)

having meta and para configurations, respectively. The nonionic precursor block was prepared by polycondensation of difluorodiphenyl sulfone (DFDPS) and a precise molar excess of bisphenol S, followed by end-capping to introduce reactive pentafluorophenoxy end groups. The molecular weights were controlled by slightly varying the molar excess of the benzenethiols or the bisphenol S during formation of the hydrophilic and hydrophobic blocks, respectively. The multiblock copolymers with SPATS blocks were then prepared by coupling stoichiometric amounts of the two precursor blocks. Figure 1 shows the 1H NMR spectra of a sulfonated precursor

where ρw (g cm−3) and ρp (g cm−3) are the densities of water and polymer, respectively, and r is the ratio between the partial molar volume of the water (Vw, cm3 mol−1) and polymer (Vp, cm3 mol−1), respectively. The ρp values of the copolymers were estimated by using the correlation between the IEC and the density (see Supporting Information).34 The values of ρw depend slightly on the temperature, and 0.996 (20 °C), 0.990 (40 °C), 0.981 (60 °C), 0.969 (80 °C), and 0.955 g cm−3 (100 °C) were used in the calculations. The proton conductivity (σ, S cm−1) of the membranes was measured both under fully hydrated conditions as a function of temperature and under reduced RH at 80 °C. The former measurement was performed in a sealed cell (size: 1.4 cm × 1.4 cm × 1.0 cm) containing 0.3 mL of water with the membrane fully hydrated. A two-probe method was used in the range −20 to 100 °C with a Novocontrol high-resolution dielectric analyzer V 1.01S in the frequency range 101−107 Hz at 50 mV. In order to investigate the influence of the water content on the conductivity, the effective proton conductivity in the water channels (σ′, S cm−1) was calculated as σ σ′ = (8) WF The effective proton mobility (μ, cm2 s−1 V−1)35,36 was calculated as σ μ= F × [H+] (9) Here, F is Faraday’s constant (9.6485 × 104 C mol−1), and the proton concentration in the wet membrane [H+] (mol cm−3) was calculated as ρw [H+] = (10) λ × 18 The humidity dependence of the proton conductivity from 30 to 90% RH was investigated at 80 °C by a four-probe method with a Gamry potentiostat/galvanostat/ZRA in the frequency range 10−1− 105 Hz and a Fumatech MK3 conductivity cell. The humidity was equilibrated by deionized water in a closed system.

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RESULTS AND DISCUSSION Synthesis of Hydrophilic Oligomers and Multiblock Copolymers. The multiblock copolymers studied in the present work have hydrophilic blocks of highly sulfonated SPAS to provide a combination of high chemical stability, moderate water uptake under immersed conditions, and high proton conductivity under reduced humidity. Placing sulfonic acid groups in ortho sulfone positions has previously been demonstrated to give very high hydrolytic stability.28−31,33 The advantage of using very stiff aromatic backbones as a basis for highly sulfonated polymers with low water uptake has previously been pointed out by Goto et al.37 This can be understood from the low conformational entropy attained by these ionic polymers in the fully hydrated (dissolved) state. The hydrophobic blocks consisted of PAES which was selected for its high Tg and good solubility and film-forming properties. The synthetic pathway to the block copolymers is outlined in Scheme 1. The sBCPSBP monomer was prepared by a metalation−sulfination−oxidation procedure, where all sulfonic acid groups are placed in ortho positions to the sulfone bridges. Sulfonated precursor blocks end-capped with thiol groups were prepared by K2CO3-mediated polycondensations of sBCPSBP and a precise molar excess of either m-BDT or p-BDT. This gave SPATS precursor blocks with arylene dithioether units

Figure 1. 1H NMR spectra of (a) pSPATS precursor block (6.0 kDa), (b) pSPATSbPAES2.1, and (c) pSPASbPAES1.8, recorded using DMSO-d6 solutions.

block and the corresponding multiblock copolymers with the para configuration. As seen in Figure 1a, the shifts arising from the thiol end groups of the oligomer were separated from those of the main chain, and the molecular weight was calculated by comparing the integral of signal d with the combined integral of signal g′ and g″. In general, the calculated molecular weight was found to be lower than the targeted one (Table 1). This may be due to a limited reactivity of sBCPSBP during the polycondensation which took place at 110 °C. Similar results were found for the materials with the meta configuration (1H NMR spectra are available as Supporting Information). 6541

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Table 1. Structure and Property Data of the Multiblock Copolymers and Membranes copolymer

hydrophilic precursor [kDa]

hydrophobic precursor [kDa]

IECa [mequiv g−1]

IECb [mequiv g−1]

IECc [mequiv g−1]

[η]d [dL g−1]

Tge [°C]

Tdf [°C]

Tdg [°C]

WUh [%]

λh

Δlh [%]

Δth [%]

pSPATSbPAES1.3 mSPATSbPAES1.6 pSPATSbPAES2.1 pSPASbPAES1.0 mSPASbPAES1.3 pSPASbPAES1.8

4.9 5.9 6.0 5.3 6.3 6.5

12.9 12.9 6.7 12.9 12.9 6.7

1.25 1.40 2.12 1.22 1.31 2.05

1.29 1.64 2.11 0.93 1.11 1.82

1.32 1.58 2.11 1.00 1.25 1.80

0.39 0.63 0.38 0.38 0.56 0.39

236 234 228 235 231 221

287 284 270 313 309 307

437 397 436 428 421 402

27 62 119 25 60 82

11 22 31 14 27 25

8 8 21 2 4 15

31 31 28 16 20 29

a Theoretical value calculated using block ratios. bDetermined from 1H NMR data. cDetermined by acid−base titration. dMeasured at 25 °C in DMSO solutions containing 0.1 M LiBr. eMeasured by DSC in the Na+ form. fMeasured by TGA in the H+ form. gMeasured by TGA in the Na+ form. hWater uptake, measured under immersed conditions in water at room temperature.

The precursor blocks were successfully coupled under mild conditions in DMSO at 110 °C to avoid side reactions.38,39 The absence of any signals from the end groups at around 7.2 ppm in the 1H NMR spectra of the multiblock copolymers indicated the successful coupling reactions (Figure 1b). The IEC values, calculated by comparing the integrals of signals a and e with that of i, were in very good agreement with the target IECs (Table 1). In addition, the IEC values determined by the titrations were in very good agreement with the values from NMR data. All the copolymers had reasonably high intrinsic viscosities (Table 1) and formed mechanically tough membranes by casting films from DMSO solutions containing 5 wt % copolymer at 120 °C. The SPATS blocks of the copolymers were converted to SPAS blocks by oxidation of the thioether bridges to sulfone bridges using hydrogen peroxide in a mixture of acetic acid and sulfuric acid (Scheme 1).30 Thus, copolymers pSPASbPAES1.8, pSPASbPAES1.0, and mSPASbPAES1.3 were derived from copolymer pSPATSbPAES2.1, pSPATSbPAES1.3, and mSPATSbPAES1.6, respectively. Figure 1c shows the 1H NMR spectrum of a block copolymer after oxidation. As seen, the signal from the dithioarylene units at 7.5 ppm (seen in Figure 1b) completely disappeared, while another signal appeared at around 8.25 ppm to confirm the complete oxidation. In addition, the signals of all the protons in the sulfonated blocks were shifted after the conversion to sulfone bridges. There was no significant change in the intrinsic viscosity of the multiblock copolymers after the conversion, which indicated that the oxidation was completed without any degradation of the molecular weight (Table 1). Morphology and Thermal Properties. The advantageous properties of the multiblock copolymer membranes rely critically on the formation of phase-separated morphologies induced by the immiscibility of the blocks.13,18 Atomic force microscopy (AFM) in the tapping mode of copolymer surfaces indicated a clear phase separation on the ∼30 nm scale (Figure 2). The hydrophilic phase domains were seemingly wellconnected, and a similar surface morphology was observed before and after the oxidation of a given copolymer. The glass transition temperature (Tg) of the copolymers was measured by DSC using membranes in the Na+ form (Figure 3). All the copolymers showed Tgs in the region 220−240 °C which coincided with the Tg of the PAES precursor blocks reported previously.26 No Tg arising from the sulfonated blocks was observed below 300 °C. The thermal stability was analyzed by TGA of the membranes in the H+ form under air at 1 °C min−1 and in the Na+ form under N2 at 10 °C min−1 (Figure 4). As expected, the membranes in the H+ form decomposed at a lower temperature than those in the Na+ form (Table 1).

Figure 2. AFM phase images of multiblock copolymer membrane surfaces before and after oxidation: (a) pSPATSbPAES2.1; (b) pSPASbPAES1.8. The dark and bright regions indicate soft hydrophilic and hard hydrophobic phases, respectively.

Notably, the SPAS multiblock copolymer membranes in the H+ form showed ∼20 °C higher Td values than the corresponding SPATS multiblock copolymer membranes. This result suggested that copolymers with exclusively sulfone bridges have a higher thermal stability than those containing thioether bridges. Water Uptake and Proton Conductivity. The water uptake (wt %) was measured at room temperature under immersed conditions in deionized water. The results depicted in Figure 5a show that pSPATSbPAES2.1 had the highest water uptake because of its high IEC. Membrane pSPASbPAES1.8 showed a 25 wt % lower water uptake in relation to pSPATSbPAES2.1 at 80 °C. At 100 °C, the difference was 30 wt %. This finding may be explained by both a reduction of the IEC after converting from thioether to sulfone bridges and the stiffer chain structure of SPAS in relation to SPATS. A slight decrease in the water uptake was observed for the SPASbPAES copolymers with low to medium IEC values, in relation to the corresponding SPATSbPAES copolymers. When comparing the water uptake of copolymers with different configurations of the main chains, the copolymers with the meta-positioned 6542

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Figure 5. Water uptake (a) and hydration number λ (b) of the multiblock copolymer membranes as a function of temperature in liquid water.

Figure 3. DSC traces of multiblock copolymers in the Na+ form under N2 at 10 °C min−1.

swelling was observed for all the block copolymer membranes, with a lower dimensional change in the in-plane direction than in the through-plane direction (Table 1). This was in accordance with previously published data.19−27 Figure 6a shows the temperature dependence of the proton conductivity of fully hydrated membranes. The data were measured by electrochemical impedance spectroscopy with the membranes in a sealed cell using a two-probe method. As expected, the conductivity increased with the IEC. All the membranes except pSPATSbPAES1.3 and pSPASbPAES1.0 reached conductivities quite comparable to NRE212 in the temperature range of the measurements. Notably, these membranes showed higher conductivities at subzero temperatures than NRE212, most probably because the high ionic content of the hydrophilic blocks depress the freezing of the water.36 This feature is beneficial for fuel cell operation under cold conditions. No significant difference in the conductivity was observed for the membranes before and after oxidation. The humidity dependence of the proton conductivity was measured at 80 °C with the samples in a sample chamber with controlled RH (four-probe method) (Figure 6b). Copolymers with statistically distributed disulfonated units (SPAESy) were prepared as previously reported,40 and their conductivity data were added in order to compare with the sulfonated multiblock copolymers. All the multiblock copolymer membranes displayed a similar humidity dependence of the conductivity as NRE212 and N117, which was less than that of the statistical SPAESy copolymers. Notably, pPASbPAES1.8 showed a level of conductivity quite comparable to N117 in the full RH range and reached 5.0 mS cm−1 at 30% RH. The SPASbPAES membranes did not show any decrease in the conductivity in relation to the SPATSbPAES membranes, despite the reduced IEC after the oxidation. Membrane pSPASbPAES1.8 even showed a slightly increased conductivity (5 mS cm−1 at 30% RH, 80 °C) in relation to pSPATSbPAES2.1. The reason for

Figure 4. TGA traces of multiblock copolymers in the Na+ form under N2 at 10 °C min−1 and in the H+ form under air at 1 °C min−1.

bridges showed slightly higher values. For example, mSPASbPAES1.3 reached a λ value comparable to pSPASbPAES1.8 at room temperature, despite the lower IEC value of the former membrane (Figure 5b). Thus, the pure para-connectivity in the hydrophilic block restricted the water uptake more efficiently because of the lower chain flexibility. As expected, anisotropic 6543

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Normally, the proton conductivity increases with the temperature because of the increase in the water content which increases the local mobility and long-range percolation and also because of the thermal activation of the elementary reactions.43 This is true as long as the water uptake is not excessive and dilution effects have set in. Because the membrane proton conductivity does not address the water content in the membrane, the membrane proton conductivity was normalized on the basis of the water volume fraction to obtain the effective proton conductivity in the water channels, σ′. This parameter indicates the efficiency of the water system present in the membrane for transport of protons (Figure 7a).

Figure 6. Proton conductivity data of multiblock copolymers as a function of temperature under fully hydrated conditions (a) and as a function of RH at 80 °C (b). The data of Nafion (NRE212 and N117) and statistical disulfonated PAESs (SPAESy) copolymers were added for comparison.

the slight increase in the proton conductivity of pSPASbPAES1.8 may be that the effect of the stiff SPAS block on the water uptake was the most pronounced at high IEC values. The conductivity of pSPASbPAES1.8 was found to be very close to that previously measured for a multiblock copolymer having highly sulfonated PAES blocks and an IEC of 1.83 mequiv g−1 (6 mS cm−1 at 30% RH, 80 °C).27 Analysis of the Proton/Water Transport Properties. The use of volume-based parameters has been promoted by Pivovar and Kim in order to investigate the proton/water transport properties of sulfonated copolymers more in-depth.41 The volume-based IEC values (IECv, mequiv cm−3) were calculated by use of the water volume fraction of the present block copolymer membranes (see Supporting Information). The density of the multiblock copolymers was estimated from the relationship between the density and IEC values of sulfonated nonfluorinated PAES copolymers as previously reported32 (see Supporting Information), while that of NRE212 was taken as 1.97 g cm−3.42 In general, the water volume fraction and IECv values of all membranes decreased with increasing temperature as a result of the increasing water uptake. As anticipated from the water uptake results shown in Figure 5, lower water volume fractions were found for the SPAS multiblock copolymers after immersion than for the corresponding SPATS multiblock copolymers at all the investigated temperatures.

Figure 7. Water volume fraction dependence of (a) the effective proton conductivity in the water channels and (b) effective proton mobility of the sulfonated membranes. The corresponding data of NRE212 were added for comparison.

It is however important to notice that σ′ would only be a true conductivity in the ideal case where all the water channels in the membrane were oriented parallel to the electric field. SPASbPAES membranes with low-to-middle IECs showed higher values of σ′ than the corresponding SPATSbPAES membranes at lower water volume fractions. This indicates that the SPASbPAES multiblock copolymer membranes efficiently facilitate proton conductivity at low water contents. On the other hand, at high IECs the value of σ′ was slightly lower for pSPASbPAES1.8 than for pSPATSbPAES2.1 at high water volume fractions. This implies that the latter copolymer can utilize water for proton conduction more efficiently under fully humidified conditions. Figure 7b shows the effective proton mobility in the membrane, μ, derived from the proton conductivity data under fully hydrated conditions. The mobility generally increases if the increase in water content results in an increased proton dissociation and if the water content in the membranes changes the size and shape of the water channels leading to a more effective proton/water transport pathway.35,36 In general, 6544

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the proton mobility increased as a function of temperature for all membranes because of the increasing water contents with temperature, as seen in Figure 7b. As anticipated from the values of σ′ (Figure 7a), the SPASbPAES multiblock copolymer membranes showed significantly higher effective proton mobilities at lower water volume fractions than the corresponding SPATS membranes. However, at high IECs, pSPASbPAES1.8 showed only a slightly higher effective proton mobility than pSPATSbPAES2.1. However, both copolymers showed higher effective proton mobilities than NRE212, which may indicate that they conduct protons more efficiently than NRE212 under fully humidified conditions.

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CONCLUSIONS SPASbPAES multiblock copolymers containing sulfonated blocks exclusively linked by sulfone bridges were prepared by the coupling of precursor blocks and oxidation of the resulting SPATSbPAES multiblock copolymers. The highly sulfonated thiol-terminated precursor blocks were synthesized by using a novel tetrasulfonated dichlorotetraphenyldisulfone monomer. The block copolymers studied in the present work had IEC values in the range 1.0−2.1 mequiv g−1 and sulfonated block lengths of 5.3−6.5 kDa and reached proton conductivites just below that of Nafion 117 at 80 °C and 30% RH. The stiff molecular structure of the SPAS blocks were especially beneficial for membranes with high IEC values and facilitated a high proton conductivity at low RH. This class of block copolymers possesses a very attractive combination of high conductivity, restricted water uptake, and stability toward desulfonation and has great prospective to meet the demands of various electrochemical applications including fuel cells, electrolyzers, and redox flow batteries after optimization of the properties.

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ASSOCIATED CONTENT

S Supporting Information *

1

H NMR spectra of sBCPSBP, sulfonated precursor block with meta configuration, and multiblock copolymers derived thereof, pentafluoro-end-capped hydrophobic PAES precursor blocks; correlation between IEC and density of sulfonated nonfluorinated PAES statistical copolymers; correlation between WF and IECv of the membranes as a function of temperature; correlation between the WF and conductivity diffusion coefficient of the membranes. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]; Fax +46-46-2224012; Tel +46-46-2229860. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the Danish Council for Strategic Research for financial support through Contract 09-065198.

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REFERENCES

(1) Zhang, H. W.; Shen, P. K. Chem. Soc. Rev. 2012, 41, 2382. (2) Chen, D.; Wang, S.; Xiao, M.; Meng, Y. Energy Environ. Sci. 2010, 3, 622. 6545

dx.doi.org/10.1021/ma301245u | Macromolecules 2012, 45, 6538−6546

Macromolecules

Article

(42) Material Safety Data Sheet of Nafion NRE-212, by DuPont. (43) Kreuer, K. D.; Paddison, S. J.; Spohr, E.; Schuster, M. Chem. Rev. 2004, 104, 4637.

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dx.doi.org/10.1021/ma301245u | Macromolecules 2012, 45, 6538−6546