Highly Fluorinated Sulfonated Poly(arylene ether sulfone) Copolymers

Article pubs.acs.org/IECR

Highly Fluorinated Sulfonated Poly(arylene ether sulfone) Copolymers: Synthesis and Evaluation of Proton Exchange Membrane Properties Aruna K. Mohanty,† Ershad A. Mistri,† Susanta Banerjee,*,† Hartmut Komber,‡ and Brigitte Voit‡ †

Materials Science Centre, Indian Institute of Technology, Kharagpur 721302, India Leibniz-Institut für Polymerforschung Dresden e.V., Hohe Strasse 6, 01069 Dresden, Germany

‡

ABSTRACT: This article reports the synthesis and characterization of several new fluorinated sulfonated poly(arylene ether sulfone) (6FBPAQSH-XX). The structures of the prepared copolymers were confirmed by FTIR and NMR spectroscopy. The copolymers showed high molecular weight, good solubility, and film-forming capabilities. The copolymers exhibited high thermal and oxidative stability and good mechanical properties depending upon their chemical compositions. Transmission electron microscopy (TEM) showed cluster like microstructures for the copolymer membranes suggesting good phase separated morphology. The cluster size increased with increasing mole percentage of sulfonate comonomer and was in the range of 5−65 nm. The copolymers showed good dimensional stability with very low water uptake and swelling ratios. Ion exchange capacity (IECW) of the copolymer films was determined by titration methods and was also calculated from the monomer composition obtained by 1H NMR spectroscopy. The proton conductivities of acid form membrane increased with IECW value and temperature, with maximum to 108 mS/cm at 90 °C for 6FBPAQSH-60 copolymer.

1. INTRODUCTION In recent years, proton exchange membranes (PEMs) have drawn great interest in terms of their application as solid electrolyte in fuel cells, because fuel cells are considered as one of the promising clean energy sources with very high energy conversion efficiency.1−4 In this regard, Nafion membranes are the most widely used membrane materials because of their excellent oxidative and chemical stability along with high proton conductivity.3 However, Nafion membranes have their inherent drawbacks such as low operational temperature (fuel cell temperature < 80 °C), high fuel crossover, and high cost due to their complex synthetic processes.5 This has motivated more intense research over the past decade for alternative lower cost polymer membrane electrolytes having high proton conductivity, high chemical and mechanical stability, and low permeability to fuel and oxidants. In past few years, various classes of hydrocarbon-based sulfonated polymers have been synthesized and evaluated in terms of their applicability as polymer electrolyte. Examples are poly(arylene ether sulfone)s,6−18 poly(arylene thioether sulfone)s,19 poly(ether ketone)s,20−22 poly(arylene ether ether ketone ketone)s,23−25 poly(aryl ether nitrile)s,26,27 poly(phthalazinone arylene ether)s,28,29 polyimides,30,31 and poly(benzimidazole)s.32,33 The conductivity among these polymer electrolytes is because of two broad factors: (1) concentration of polar acid groups (labile protons) and (2) phase separated morphological properties of polymer electrolyte membranes. Generally, an increase in concentration of polar groups results in high conductivity. However, beyond a certain threshold concentration of polar groups, the polymeric membrane loses its mechanical properties and dimensional stability (high swelling, sometimes leading to hydrogel formation), and becomes sometimes water-soluble that is undesired for fuel cell application. Thus, at optimal IECW value, © 2013 American Chemical Society

it is always expected for a membrane to show high conductivity with low swelling and better management of absorbed water content. In these limited parameters, the second factor, morphology of the membrane electrolyte, plays an important role for achieving a good conductivity while maintaining the good mechanical properties and dimensional stability. Continuous phase separated morphology in the membranes generally results in improved proton diffusion leading to good proton conductivity. Fluorinated PEMs have a natural advantage over traditional hydrocarbon-based PEMs in terms of better phase separated morphology while having good conductivity.10,14,16 Sankir et al.27 reported that the 4,4′-hexafluoroisopropylidene diphenol (6FBPA)-based fluorinated polymer membrane showed high fluorine content (from X-ray photoelectron spectrometer (XPS) analysis) at the polymer air interface due to the selfassembling ability of fluorinated monomer. The increasing incorporation of 6FBPA monomers resulted in a decrease in water uptake and better management of water. Durability of the membrane electrode assemblies (MEAs) is also a critical parameter for applicability of PEM in fuel cells. In comparison to the hydrocarbon-based membrane electrolyte, fluorinated polymer electrolyte provides the advantage of better compatibility with Nafion binder in the preparation of MEAs with low cell resistance and improved durability.10 Also, the oxidative stability of the sulfonated PEMs increases with increasing fluorine containing groups and decreasing ion exchange capacity (IECW) and water-absorbing capability.3 The Guiver group23 used 3,3′-disodiumsulfonyl-4,4′-dichlorodiphenylsulReceived: Revised: Accepted: Published: 2772

December 18, 2012 January 31, 2013 February 4, 2013 February 4, 2013 dx.doi.org/10.1021/ie303380a | Ind. Eng. Chem. Res. 2013, 52, 2772−2783

Industrial & Engineering Chemistry Research

Article

Scheme 1. Synthesis of 6FBPAQSH-XX Copolymers

binder used in future preparation of MEAs. Moreover, SDCDPS with features of localized sulfonic acid density and rigid QBF moiety are incorporated into the backbone structure of polymer to increase the statistical length of the nonsulfonated segment, which may help to obtain good mechanical property, high hot water stability, and high proton conductivity. The structure of the 6FBPAQS-XX copolymers is characterized by NMR and FTIR spectroscopy. The physical properties and proton conductivities of the membranes are also investigated and compared to our BPAQSH-XX series.

fone with features of localized sulfonic acid density and 4,4′difluorobenzophenone to increase the statistical length of nonsulfonated segment for better mechanical strength and high proton conductivity. In addition, some research groups have reported that introduction of bulky pendant groups and/or bulky rigid structural moiety into sulfonated aromatic polymer backbone resulted in increasing free volume and inhibiting chain packing of the polymers, which could further improve the solubility and proton conductivity of the polymers.12,24 Thus, taking clues from the above cited literature, we have recently reported novel 4,4′-isopropylidene diphenol (BPA)-based partially fluorinated sulfonated poly(arylene ether sulfone)s using 4,4′-bis(4′-fluoro-3′-trifluoromethyl benzyl) biphenyl (QBF), which showed high oxidative stability, high thermal stability, and good proton conductivity.17 In this work, to know the effect of increased fluorine incorporation on the PEM properties, we have replaced BPA with 6FBPA in the copolymers. The present work reports the synthesis of a series of 6FBPAQSH-XX copolymers with varying degree of sulfonation through polycondensation of 4,4′-bis(4′-fluoro-3′trifluoromethyl benzyl) biphenyl (QBF), 3,3′-disodiumsulfonyl-4,4′-dichlorodiphenylsulfone (SDCDPS), and 4,4′-hexafluoroisopropylidene diphenol (6FBPA). To get good microphase separated structures, hydrophilic sulfonic acid groups were introduced through SDCDPS monomer; meanwhile, hydrophobic trifluoromethyl groups containing QBF and 6FBPA were also incorporated to the polymer backbone, which should promote thermal stability, oxidative stability, and, most importantly, should increase the hydrophobicity of nonsulfonated backbone segments of sulfonated copolymers. The 6FBPAQSH-XX copolymers are expected to have good solubility due to the synergetic effects of the pendant −CF3 groups and noncoplanar kink structure of 6FBPA moieties in the polymer backbone, which disrupts the regularity of the molecular chains and hinders the dense chain stacking. It is anticipated that the high fluorine content of 6FBPAQSH-XX copolymers will bring good compatibility with Nafion-based

2. EXPERIMENTAL SECTION 2.1. Materials. The bishydroxy monomer 4,4′-hexafluoroisopropylidene diphenol (6FBPA) (95% purity) obtained from Fluorochem was used as received. The sulfonated monomer 3,3′-disodiumsulfonyl-4,4′-dichlorodiphenylsulfone (SDCDPS) was purchased from Chemos GmbH and was used as received. N-Methyl-2-pyrrolidone (E. Merck) was purified by stirring with NaOH and distilled twice from P2O5 under reduced pressure. Toluene (Merck) was refluxed over Na metal to remove water and was freshly distilled before use. The perfluoroalkylated monomer, 4,4′-bis(4′-fluoro-3′-trifluoromethyl benzyl) biphenyl (QBF), was prepared according to the procedure reported in our previous work.34 2.2. Synthesis of Sulfonated Poly(arylene ether sulfone)s. The detailed synthesis and structures of the polymers prepared are shown in Scheme 1. The mole percentage of SDCDPS used in the polymerization of particular copolymer is indicated by −XX. The salt (−Na) form of polymer is represented by 6FBPAQS-XX, whereas the acid (−H) form of polymer is represented by 6FBPAQSH-XX. Meanwhile, nonsulfonated polymer 6BPAQSH-0 will be referred to as 6FBPAQ throughout this article for ease in understanding. Polymerization reactions were carried out in a 50 mL, three-necked round-bottomed flask equipped with a nitrogen inlet, a stir bar, and a Dean−Stark trap fitted with condenser. The reactions were conducted under constant flow 2773

dx.doi.org/10.1021/ie303380a | Ind. Eng. Chem. Res. 2013, 52, 2772−2783


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run. Thermogravimetric (TGA) measurements were done on a TGA Q5000 of TA Instruments at a heating rate of 10 K/min to determine the decomposition temperature under synthetic air. For TGA analysis, the samples were preheated under vacuum at 150 °C for at least 6 h to remove moisture. Inherent viscosities of the polymer solutions of 0.5 g/dL concentration in NMP were measured using an Ubbelohde viscometer at (30 ± 1) °C. Stress−strain behavior of the thin polymer films (10 mm × 25 mm) was measured at room temperature using UTM-Instron, Plus-8800 at a strain rate of 5% of sample length per minute. Transmission electron microscopy (TEM) was undertaken of ultra microtome membranes using a TEM instrument (FEI-TECNAI G2 20S-TWIN). To stain the ionic domains, at first membrane samples in proton form were converted into Pb2+ form by immersing them in 0.5 M lead acetate aqueous solution overnight, and then they were thoroughly washed with deionized water and dried at room temperature for 24 h before measurement. The ion content of the copolymer is represented as ion exchange capacity (IECW) and is calculated using the equation IEC W = (1000/ MWrepeat unit) × DS × 2, where DS is the degree of sulfonation. The IECW,titr values, water uptake, swelling ratio, oxidative, and hydrolytic stability of the membranes were determined according to the reported protocol.17 The in-plane proton conductivities of the polymer membranes were determined using AC impedance spectroscopy (HIOKI 3532-50 LCR HiTESTER) over a frequency range of 100 Hz to 2 MHz using homemade conductivity cell.13,17 The membranes (2 cm × 1 cm) were first equilibrated in dionized water for at least 3 days, and then an in-plane measurement was carried out in dionized water by increasing the temperature from 30 to 90 °C at a heating rate 1−2 °C/min.

of nitrogen. A representative (6FBPAQSH-20) polymerization procedure is as follows: The flask was charged with QBF (0.6456 g, 1.35 mmol), SDCDPS (0.1657 g, 0.34 mmol), 6FBPA (0.5672 g, 1.69 mmol), K2CO3 (0.5129 g, 3.71 mmol), NMP (10 mL), and toluene (5 mL). The mixture was then heated to reflux (160 °C, oil bath temperature) for 2−3 h to remove the water azeotropically with toluene. After removal of the toluene from the Dean−Stark trap, the reaction temperature was increased to 180 °C and maintained for another 20 h until the reaction solution became viscous. After being cooled to room temperature and dilution with DMAc, the polymer solution was precipitated in isopropanol. These products then were washed several times in deionized water to remove any inorganic impurities, filtered, and dried under vacuum at 100 °C for at least 24 h. Film casting and membrane acidification process was followed according to the reported protocol.17 The thicknesses of the membranes (in the range 65−80 μm) were measured as the average of the five different measurements of the same membrane using a Mitutoyo Digimatic micrometer with an accuracy of 0.001 mm, and in each case standard deviation was within 3 μm of the average thickness of the membrane. NMR data of the acid form, 1H NMR (DMSO-d6, 90 °C): 8.40 (3), 8.05 (15, 17), 7.85 (5, 20, 21), 7.47 (9 next to Q unit), 7.38 (9 next to S unit), 7.31 (18), 7.22 (8 next to Q unit), 7.13 (8 next to S unit), 7.10 ppm (6). 13C NMR (DMSO-d6, 50 °C): 157.1−156.5 (1, 7), 152.8 (13), 140.0 and 139.8 (2), 138.8 (22), 137.1 (19), 135.8 (16), 135.2 (4), 132.4 (17), 131.6 (9 next to Q unit), 131.3 (9 next to S unit), 129.9 (11), 128.3 (3), 127.7, 127.6, 127.3, and 127.2 (10 for 6FBPA located between S/S, S/Q, Q/S, and Q/Q, respectively), 127.2 and 127.15 (20, 21), 125.0 (15), 123.9 (q, 12), 123.2 (q, 23), 121.1 (18), 120.9 (q, 14), 120.7 and 120.6 (6), 119.3 (8 next to S unit), 118.3 (8 next to Q unit), 63.3 ppm (septet, 11). 19F NMR (DMSO-d6, 90 °C): −60.4 (23), −63.4 ppm (12). 2.3. Measurements. Elemental carbon, hydrogen, and sulfur of the copolymers were analyzed via a Euro Elemental Analyzer instrument (Euro Vector, EA, Italy). 1H (500.13 MHz), 13C (125.76 MHz), and 19F (470.59 MHz) NMR spectra of the copolymers in acid form (membrane material) were recorded on an Avance III 500 NMR spectrometer (Bruker, Germany) at 50 °C (13C) and 90 °C (1H, 19F), respectively. To ensure full 1H relaxation, a pulse delay of 60 s was applied for the 1H NMR measurements. DMSO-d6 was used as solvent and internal reference (δ(1H) = 2.50 ppm; δ(13C) = 39.6 ppm). The 19F NMR spectra were referenced on external C6F6. The signal assignments were confirmed by 1 H−1H and 1H−13C correlated spectra. FTIR spectra of the copolymers were recorded from a BRUKER (TENSOR 27) spectrophotometer using KBr pellets at room temperature and humid-free atmosphere. The molecular weights of polymers were measured by size-exclusion chromatography (SEC) using DMF with 5 g/L LiBr as eluent. Sample concentration was 1 mg/mL. The apparatus consists of a Gynkotek HPLC pump, a Agilent Autosampler 1200, and linear columns [All GRAM, Polymer Standards Service (PSS)] consisting of a precolumn 10 μm/8 mm × 50 mm, a column 10 A°/8 mm × 300 mm, and two columns 3000 A°/8 mm × 300 mm. A refractive-index (RI) detector of Knauer made was used as a detector. For calibration, linear PMMA with molecular weight between 500 and 1 000 000 Da was used. DSC measurements were made on DSC Q1000 of TA Instruments at a heating rate of 10 K/min under nitrogen. Glass transition temperatures (Tg) were taken as the middle point of the step transition in the second heating

3. RESULTS AND DISCUSSION 3.1. Synthesis and Characterization of 6FBPAQS-XX Copolymers. Polymerizations of 6FBPA with stoichiometric amounts of bis-fluoro monomer, QBF, and disulfonated monomer, SDCDPS, were carried out in the presence of excess K2CO3 in NMP as solvent (15−20% solid content) according to the reported protocol.17 In QBF, trifluoromethyl group ortho to leaving group (−F atom) activates the fluoro displacement by phenoxide due to the stabilization of negative charge at the 2- or 4-position by its negative inductive effect.35,36 The steric congestion due to a bulky trifluoromethyl group may also facilitate the formation of a stable Meisenheimer complex with the release of steric strain.37 Toluene was used for azeotropic removal of water. During the initial stage of the polymerization, the reaction temperature was maintained at 140−150 °C, and the water generated was effectively removed along with toluene through a Dean−Stark trap. Upon completion of bisphenoxide formation, the reaction temperature was raised to 180 °C for effective nucleophilic displacement reaction. In comparison to our previous BPAQSXX series, the present 6FBPAQS-XX series required a little longer polymerization time because of the lower nucleophilicity of bisphenoxide from 6FBPA due to the presence of deactivating hexafluoroisopropylidine group. High molar mass polymers were obtained after 20 h as judged by the remarkable increase of the viscosity of the reaction medium. During the reaction, trifluoromethyl groups in 6FBPA and QBF compensate for the lower solubility of the sulfonated monomer SDCDPS in NMP, which helps to keep the polymer in solution even at high molar masses. The resulting whitish viscous 2774



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Table 1. Composition and Properties of 6FBPAQSH-XX Copolymers and Comparison with BPAQSH-XX Copolymers17 IECW (mequiv/g) polymer-XXa

DSb NMR

theorc

titr.

NMRd

WUWe (%)

IECV,f (wet) mequiv/cm3

σ (mS/cm) at 30 °C

Eag (kJ/mol)

6FBPAQSH-20 6FBPAQSH-30 6FBPAQSH-40 6FBPAQSH-50 6FBPAQSH-60 BPAQSH-20 BPAQSH-30 BPAQSH-40 BPAQSH-50 BPAQSH-60

0.16 0.29 0.35 0.44 0.53 0.19 0.29 0.40 0.49 0.56

0.53 0.79 1.07 1.35 1.63 0.61 0.93 1.25 1.57 1.91

0.48 0.76 0.94 1.22 1.47 0.58 0.84 1.21 1.46 1.70

0.42 0.77 0.93 1.19 1.44 0.59 0.88 1.25 1.53 1.78

2 5 8 13 23 4 7 12 22 40

0.73 1.03 1.31 1.51 1.60 0.74 1.06 1.34 1.50 1.54

4 5 19 38 54 9 13 16 31 40

15.02 15.55 12.89 11.88 10.71 17.26 16.44 15.72 13.96 11.34

a

XX represents the feed SDCDPS monomer mole %. bDegree of sulfonation: The theoretical value is calculated from the monomer feed ratio, the NMR value corresponds to the molar content of SDPS (corresponding to SDCDPS monomer) units in the polymer determined from 1H NMR spectra. cIECW, theo = (1000/MWrepeat unit) × DSTheo × 2, where DSTheo is calculated from monomer feed ratio. dIECW, NMR = (1000/MWrepeat unit) ×x DSNMR × 2. eWUW (%) = [(Wwet − Wdry)/Wdry × 100], where Wwet and Wdry are the weights of the wet and dry membranes, respectively. f IECV(wet) = (IECW, Theo × dM)/(1 + 0.01 × WUW(%) × dM), where dM is the density of the polymers. gActivation energy.

ring stretching, and 1248−1131 cm−1 to C−F stretching frequencies.34,38 The aromatic CC band at 1605 cm−1, corresponding to disubstitution on aromatic phenyl for 6FBPAQ, was split into two bands at 1613 and 1587 cm−1 for the 6FBPAQS-XX series copolymers due to increased mole percentage of trisubstituted (because of sulfonation) phenyl ring. The intensity of the 1587 cm−1 band, corresponding to trisubstituted phenyl rings, increased with an increasing sulfonated monomer ratio. Characteristic bands for the aromatic sodium sulfonate symmetric and asymmetric stretching vibrations were observed at 1028 and 1095 cm−1 for all resulting copolymers.13,39 In contrast to the report from the other research group that sulfonic acid groups may dissociate from their parent structure during high-temperature reactions,40 the intensity of the characteristic sulfonate absorption bands (1028 and 1095 cm−1) in 6FBPAQS-XX series of copolymers increases with increasing SDCDPS content, which confirms successful introduction of sulfonate groups into the polymers. When carefully noted, these peaks showed a slight shift in peak position (although not significant) toward high frequency with a simultaneous increase in the intensity with increasing DS. This phenomenon is clearly indicative of increased hydrogenbond interaction between the sulfonate groups and bound water molecules, thus implying higher hydrophilic character of the samples with increasing DS. For 13C NMR spectroscopic characterization, sample 6FBPAQSH-50 with a balanced content of QB (corresponds to QFB monomer) and SDPS (corresponds to SDCDPS monomer) units was selected. Figure 2 depicts its 13C NMR spectrum recorded at 50 °C to reduce signal broadening. A complete signal assignment was possible also using 2D NMR methods confirming the presence of all expected structural units. The two low-field shifted signals groups at 157 (C1, C7) and 153 ppm (C13) represent the carbons involved in the ether linkages. Signals indicating phenolic end groups were not observed. Two effects result in signal splitting in the 13C NMR spectrum: The presence of 19F nuclei in the trifluoromethyl groups results in 13C−19F scalar couplings to the neighboring carbons and thus in signal splitting in quartets (C12, C14, C23) and in a septet (C11). Smaller couplings are not resolved. Signal splitting for other carbons is caused by the monomer sequence in the copolymer. Thus, three types of 6FBPA-centered triads occur: QB-6FBPA-QB, QB-6FBPA-SDPS, and SDPS-6FBPA-

reaction mixtures were diluted with DMAc and were isolated as fibers after precipitation in isopropanol. The fibers showed an increasing tendency to swell in isopropanol with increasing concentration of sulfonic acid groups. The products were washed several times with hot distilled water for the removal of inorganic impurities present in the system, and the isolated products were dried in vacuum at 100 °C for at least 24 h. Polymerization compositions and the most important properties of polymers are summarized in Table 1. The inherent viscosity of the 6FBPAQS-XX copolymers was evaluated from their solution in NMP (0.5 g/dL), and the values are in the range 0.82−1.87 dL/g. High viscosity values of the copolymers indicated the formation of high molar masses. This was also confirmed from their number average molecular weight values (42.4−52.9 kDa) obtained from SEC measurements. All of the 6FBPAQS-XX copolymers showed narrow polydispersity indices (PDI) in the range 2.04−2.32. The results of elemental (C, H, and S) analyses of all of the poly(arylene ether sulfone)s support the copolymer structures. The FTIR spectra (Figure 1) for 6FBPAQS-XX series copolymers (salt form) showed characteristic absorption bands at 1051 cm−1 (Ph−O−Ph symmetric stretching) and 1334 cm−1 (Ph−O−Ph asymmetric stretching) that are generated due to aryl ether formation. In addition, all of the copolymers exhibited absorption band at 1488 and 1512 cm−1 to aromatic

Figure 1. FT-IR spectra of 6FBPAQS-XX copolymers as a function of degree of sulfonation. 2775



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Figure 2. 13C NMR spectrum of 6FBPAQSH-50 copolymer with signal assignment (50 °C, DMSO-d6). Symbols were used to mark QB-6FBPA (*) and SDPS-6FBPA (#) dyad signals. The formula depicts a structural fragment with atom numbering.

SDPS with a statistical probability of (1 − n)2, 2(1 − n)n, and n2, respectively (n = molar content of SDPS). This effect results in four signals as observed for C7 and C10 of 6FBPA unit. Longer sequences result in the splitting observed for C2, C4, and C6 of SDPS unit. The splitting in two signals as observed for C8 and C9 but also for H8 and H9 in the 1H NMR spectra (Figure 3) reflects the QB-6FBPA and SDPS-6FBPA dyads.

signal regions I−III as indicated in Figure 3. For all copolymers, the composition points to a slightly lower SDPS content than expected from the monomer feed (Table 1). The solubility of the copolymers was tested in different solvents, N-methyl pyrrolidone (NMP), dimethyl acetamide (DMAc), dimethyl formamide (DMF), dimethyl sulfoxide (DMSO), tetrahydrofuran (THF), methanol, and water, keeping the polymer concentration at about 10% (w/v). The copolymers were soluble in different dipolar aprotic solvents such as NMP, DMAc, DMF, and DMSO and were insoluble in THF even on heating. The copolymers were insoluble in THF, methanol, and water at all temperatures. This water insolubility designates a high hydrolytic stability at room temperature. On the other hand, good solubility of the resulting 6FBPAQSH-XX copolymers in high polar aprotic solvent makes the membrane fabrications possible by the solvent casting method. 3.2. Thermal Properties. Thermogravimetric analysis (TGA) of the homopolymer and copolymers was carried out at a heating rate of 10 °C/min under synthetic air, and the thermograms are shown in Figure 4. Unlike the nonsulfonated 6FBPAQ polymer, which showed single step degradation, the sulfonated copolymers in acid form exhibited two distinct thermal degradation steps. The first step at around 260 °C was

Figure 3. 1H NMR spectra of 6FBPAQSH-XX copolymers with signal assignment and indicated integral regions used for calculation of polymer composition (90 °C, DMSO-d6). Symbols were used to mark QB-6FBPA (*) and SDPS-6FBPA (#) dyad signals.

Their content follows (1 − n) and n, respectively. In fact, the intensity ratio of H9*/H9# perfectly matches the molar ratio of both monomers in the copolymers as determined from 1H signal integrals. The changing QB/SDPS ratio can be well followed by intensity changes for H18(QB) and H3(SDPS). The copolymer compositions were calculated from the integrals of

Figure 4. TGA thermograms of the 6FBPAQSH-XX copolymer membranes. 2776



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MPa, modulus 0.26 GPa).7 However, the elongations at break (5−7%) of these membranes were much lower than that of Nafion 117 membrane (>200%). Here, it may be interesting to note that where Nafion (with EW of 1100) having 3−12% crystallinity was reported to show continuous increase in the tensile strength before breaking, 42 almost all of the 6FBPAQSH-XX membranes exhibited no significant change in tensile strength after yield point. It may be explained on the basis of the morphology for Nafion where the alignment of chain aggregates increased with tensile stress, whereas 6FBPAQSH-XX copolymer membranes, due to the amorphous character, showed marginal change in tensile strength during applied stress. In contradiction to higher tensile strength and Young’s modulus of 6FBPAQSH-XX series of copolymers, in comparison to our previous BPAQSH-XX series, 1 7 6FBPAQSH-XX copolymer membranes showed inferior elongation break. This may be due to the replacement of the BPA moiety with the relatively rigid and hydrophobic 6FBPA moiety. BPAQSH-XX copolymer membranes absorb more water (bound water) than do 6FBPAQSH-XX copolymer membranes that weakened the intermolecular interactions due to the plasticizing effect, thus showing lower tensile strength but higher elongation at break.42 In addition, morphology plays a great role in deciding the mechanical property of the sulfonated membranes. 6FBPAQSH-XX copolymers exhibited a different morphology as compared to BPAQSH-XX membranes, which is discussed in greater detail in the following section. It is obvious that 6FBPAQSH-XX copolymers membranes (acid form) exhibited relatively poor mechanical property from their salt form membranes. It may also be noted that there was no obvious effect of DS on the mechanical properties of 6FBPAQSH-XX polymers. This might be due to the fact the mechanical properties of membrane generally are influenced by a number of factors such as chemical structure, molecular weight, solvent used for casting of membranes, curing process of the membranes, and uniform membrane thickness. In summary, the stress−strain behavior of the 6FBPAQSH-XX copolymer membranes indicates they are mechanically strong enough for usage as PEM. The oxidative stability of 6FBPAQSH-XX was investigated by immersing the films (5 mm × 5 mm) into Fenton’s reagent (2 ppm FeSO4 in 3% H2O2) at 80 °C. The oxidative stability was defined in terms of the time expended for the sample to start to break in the solution (t). As reported earlier,17 in this series also the oxidative stability of the membranes decreased with increasing degree of sulfonation. Naturally, 6FBPAQSH20 showed the highest oxidative stability (t = 30.7 h), whereas 6FBPAQSH-60 showed the lowest oxidative stability (t = 4.1 h). The remaining copolymers 6FBPAQSH-30, 6FBPAQSH40, and 6FBPAQSH-50 showed their first breaking time between the above-mentioned time as t = 26.5, 4.8, and 4.3 h, respectively. The good mechanical strength and dimensional stability after 1 h of treatment indicated high oxidative stabilities, comparable to various other PEMs having sulfone17 and ketone20 linkages and superior to the reported fluorene based sulfonated poly(arylene ether sulfone)s.12 It is believed that the improved oxidative stability of 6FBPAQSH-XX membranes in comparison to their counterpart in BPAQSHXX membranes is due to the less hydrated structure of 6FBPAQSH-XX (low IECW value), and that is because of the relatively high fluorine content. The high oxidative stability of 6FBPAQSH-XX membranes was also due to their purely aromatic structure. In addition, it is reasoned that the presence

attributed to the loss of sulfonic groups, while the second degradation step at about 530 °C was related to degradation of the 6FBPAQSH-XX copolymers main-chain. However, the main-chain degradation temperature of 6FBPAQSH-XX copolymers was lower than that of 6FBPAQ as a result of probable contribution from catalytic effect of SO3H, increasing the rate of polymer decomposition.41 The 5% weight loss temperature of copolymers depends on DS and was within 300−407 °C, indicating their high thermal stability. The polymers were investigated by differential scanning calorimetry (DSC) analysis to determine glass transition temperature (Tg). Similar to our previous report,17 the Tg of the copolymers increased with an increase of DS. The nonsulfonated homopolymer showed Tg at 209 °C, whereas the sulfonated copolymers 6FBPAQSH-20 and -30 showed Tg’s at 266 and 297 °C, respectively. This increase in the Tg is attributed to the increased hindrance to internal rotation as a result of ionomer effect and bulkiness due to −SO3H moieties.24 The DSC curve of the remaining copolymers with DS above 30 showed no obvious signature for glass transition temperatures up to 350 °C. 3.3. Mechanical Properties, Oxidative Stability, and Hydrolytic Stability. Good mechanical property is one of the important requirements for the PEMs to be successfully fabricated into MEAs for fuel cells. The stress−strain plots for both −Na form and −H form 6FBPAQSH-XX membranes are shown in Figure 5. The membranes showed high tensile strength (35−52 MPa) and high modulus (1.17−1.52 GPa) in comparison to Nafion 117 membrane (tensile strength 25.65

Figure 5. Stress−strain plot of the copolymer membranes in (a) 6FBPAQS-XX (−Na form) and (b) 6FBPAQSH-XX (−H form). 2777



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Figure 6. TEM micrograph of lead-stained 6FBPAQS-XX copolymers: (a) XX = 20, (b) XX = 30, (c) XX = 40, (d) XX = 50, and (e) XX = 60.

successfully incorporated into the polymer backbones via sulfonated monomer copolymerization without any side reactions. However, IECW value from the NMR spectra was a little lower as compared to the IECW from the feed ratio. The accuracy of the IECw values calculated from the molar content of sulfonated SDPS units as obtained from 1H NMR spectra depends on the accuracy in the determination of signal integrals. Integrals of small signals as that of H3 (=integral I) used to determine the SDPS content are more error-prone than those of large signals. Probably, this is an additional reason for the differences in IECW values given in Table 1. Nevertheless, the washing away of sulfonated oligomers during purification process is a further reason of lowered IECW as compared to the IECW expected from monomer feed. The microstructure of the membranes has significant effects on the proton transport properties and mechanical property. To get a clear understanding of the proton conductivity behavior of membranes, we investigated the microstructure of the 6FBPAQSH-XX membranes by TEM analysis, which is shown in Figure 6. To examine phase separation and ionic aggregation, membranes were stained with lead acetate; thus, dark areas correspond to regions of high ionicity and brighter areas to hydrophobic regions. 6FBPAQSH-20 and 6FBPAQSH-30 showed small isolated ionic domains dispersed throughout the hydrophobic region. The TEM micrograph of 6FBPAQSH-40 exhibited a clear microphase separated structure with somewhat larger ionic clusters (12−20 nm). 6FBPAQSH-50 and 6FBPAQSH-60 exhibited excellent phase separated morphology with a large amount of medium-sized ionic clusters (15−26 nm) along with a certain amount of bigger ionic clusters (30−65 nm). Thus, the TEM images of higher sulfonic acid containing membranes suggested that the sulfonic acid groups might aggregate into hydrophilic clusters. With increasing DS, the number of isolated ionic domains gradually increased, until finally the ionic clusters come into close proximity with each other, giving rise to bigger size ionic clusters (good for water keeping), which could provide much better proton transport pathways or ionic transport channels. In

of sulfonic acid groups in already electron-deficient aromatic moieties and the trifluoromethyl group in aromatic ring ortho to the ether linkage greatly minimizes the probability of hydrolysis of polymer main chain. In addition, the hydrolytic stability of the 6FBPAQSH-XX membranes was also evaluated by immersing them in water at 100 °C for 24 h. During this accelerated test, none of the membranes showed any changes in their appearance, weight, and IECW,Titr. values and support excellent hydrolytic stability of the 6FBPAQSH-XX membranes. 3.4. IECW, Microstructure, Water Uptake, and Swelling Ratio. IECW influences the water uptake and proton conductivity of PEM and was measured from the mathematical expression in the measurement section. As observed, the IECW value is directly proportional to the degree of sulfonation and inversely proportional to the molecular weight of the polymer repeat unit structure, which in turn depends on the molecular weight of the different monomers used for polymer preparation. When BPA in our previous BPAQSH-XX series was replaced by heavier 6FBPA comonomer to obtain 6FBPAQSH-XX series, elemental fluorine content increased from 7.2−13.9% to 21.6−26.9%, and IECW value decreased from 1.91−0.61 to 1.63−0.53 mequiv/g (13−15% decrease) (Table 1). This was part of our objective to determine the effect of increased fluorine content on IECW and consequent related PEM properties such as proton conductivity, water uptake, and swelling properties of the resulted membranes. So at first, −Na forms of the membranes were cast from DMAc solution and subsequently converted to −H form in 1.5 M H2SO4. Next, the IECW of the flexible acid-form membranes was determined by classical acid−base titration and is listed in Table 1. The experimental IECW values were in the range from 0.48 to 1.47 mequiv/g, almost close to the theoretical data derived from monomer feed ratios, which indicated that the −Na form of copolymers was fully converted to the corresponding -−H form after the proton-exchange process. These values were also in agreement with the IECW values calculated from NMR composition data, indicating that the sulfonate groups were 2778



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contrast, the analogous BPAQSH-XX membranes showed a phase segregated cocontinuous morphology of softer region (black) and harder region (bright or gray), fibril-like cylindrical structures along the thickness of the membrane.17 The highly hydrophobic character due to 6FBPA and QBF moieties might help in obtaining ionic clusters of microphase separated morphology in 6FBPAQSH-XX membranes. We studied the water uptake and swelling behavior of the copolymers as they influence the proton conductivity and mechanical stability of the PEMs. In sulfonated poly(arylene ether ketone) or sulfonated poly(aryl ether sulfone), hydrophilic sulfonic acid clusters are distributed in continuous hydrophobic domains, and these hydrophilic regions are mainly responsible for water uptake. These hydrophilic regions imbibe water and increase the cluster sizes into interconnecting channels for protons. In other words, the microstructure concerning hydrophobic and hydrophilic domains greatly changes in the presence of water, which in turn affects the proton conductivity and mechanical properties of PEMs. Controlling the water uptake at low sulfonation degree brings several advantages on the basis of the water management and its related PEM properties. The water uptake (WUW) and swelling ratio of the membranes were determined by measuring the changes in weight and length at desired temperature. The value of WUW was taken when three constant WUW values were obtained for a particular temperature. Similarly, the number of water molecules per sulfonic acid group (λ) was calculated using the equation: λ = (WUW (%) × 10)/ (IECW, theo × MW, H2O), where MW, H2O = 18 g/mol.17 The volume-based water uptake (WUV) was calculated from the multiplication of weight-based water uptake and density of the copolymer membranes. The density was calculated from dimension and weight of dry membrane. The density values were 1.42 (±0.013), 1.39 (±0.016), 1.36 (±0.009), 1.31 (±0.013), and 1.26 (±0.035) for increasing DS in 6FBPAQSHXX membranes. To match with the realistic hydrated condition of fuel cell, volumetric IEC (IECV, mequiv/cm3), that is defined as molar concentration of sulfonic acid groups per unit volume containing absorbed water, was also calculated for both 6FBPAQSH-XX and BPAQSH-XX series membranes, and compared in Table 1. Similar to our previous BPAQSH-XX series,17 the water uptake and swelling ratio of the 6FBPAQSHXX membranes increased with increasing temperature and IEC value. However, as expected, the water uptake for 6FBPAQSHXX membranes was relatively lower in the range 2−23% at 30 °C and 4−38% at 80 °C. In comparison to our previous BPAQSH-XX series, the 6FBPAQSH-XX membranes showed very low swelling at all temperatures such as 1.3%, 2.0%, 2.8%, 3.6%, and 5.1% at 30 °C, and 1.9%, 2.6%, 3.9%, 5.3%, and 7.6% at 80 °C. Figure 7 shows the effect of IECW and IECV (dry) on water uptake of 6FBPAQSH-XX and BPAQSH-XX copolymers as a function of temperature (30 and 80 °C). The figure shows deflection at IECW = 1.07 mequiv/g [IECV (dry) = 1.45 mequiv/cc] for 6FBPAQSH-XX series and IECW = 1.25 mequiv/g [IECV (dry) = 1.53 mequiv/cc] for BPAQSH-XX series. 6FBPAQSH-20, 6FBPAQSH-30, and 6FBPAQSH-40 showed a very slow increase in water uptake (WUW), that is, 2− 8% at 30 °C and 4−13% at 80 °C. This lower water uptake gives an indication of isolated hydrophilic domains distributed in a predominantly hydrophobic matrix. Meanwhile, substantial higher water uptake of 6FBPAQSH-50 (IECW = 1.35 mequiv/ g; WUW = 13% at 30 °C and WUW = 23% at 80 °C) and 6FBPAQSH-60 (IECW = 1.63 mequiv/g; WUW = 23% at 30 °C

Figure 7. Water uptake dependence of (a) IECw and (b) IECv values of copolymer membranes.

and WUW = 38% at 80 °C) membranes with good mechanical strength indicated (validation of the microstructure evidence from TEM analysis) hydrophilic domains distributed in hydrophobic domains. When IECV (wet) was plotted with volumetric water uptake (WUV) (Figure 8), a large gap was exhibited between curves of highly fluorinated 6FBPAQSH-XX series and partially fluorinated BPAQSH-XX series due to the difference in the density values. The inflection in the figure appears much more sharp and obvious with higher temperature.

Figure 8. Water uptake dependence of IECv (wet) of copolymer membranes in the wet state. 2779



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The inflection point indicated that a percolation threshold for both 6FBPAQSH-XX and BPAQSH-XX is reached at 40 mol percentage of SDCDPS comonomer. After the inflection point, for a very small increase of IECV (wet), both highly fluorinated 6FBPAQSH-XX (XX = 50, 60) and partially fluorinated BPAQSH-XX (XX = 50, 60) membranes showed a substantial increase in WUV. Although highly fluorinated 6FBPAQSH-XX showed a relatively lower water uptake as compared to BPAQSH-XX at any given IECV (wet) value, the increase in the sulfonic acid concentration of 6FBPAQSH-XX with IECW was retained at different measured temperatures after equilibration with water. For 80 °C, it is interesting to note that at higher sulfonation content, 6FBPAQSH-XX membranes exhibited very close IECV (wet) value to their counterpart in BPAQSH-XX series membranes, because the difference in mass normalized IEC (IECW) is counterbalanced by the difference of their density values. 6FBPAQSH-XX membranes showed similar features of water uptake figure for λ value with increasing IECV (wet) value in Figure 9. At similar degrees of

Figure 10. Complex impedance spectra of 6FBPAQSH-XX membranes.

the decreasing size of the arc. For increasing DS, the conductivities of 6FBPAQSH-XX membranes were 4, 5, 19, 38, and 54 mS/cm at 30 °C, and 8,11, 38, 68, 69, and 98 mS/ cm at 80 °C, which are considered as high enough for PEM for fuel cell application. The values are lower than the reported value for Nafion 117 (80 mS/cm at 30 °C, and 100 mS/cm at 80 °C).43 The reason may be due to the more rigid backbone structure of the 6FBPAQSH-XX copolymers and the direct attachment of the sulfonic acid groups to the polymer backbone.11,20 The proton conductivity values were also lower as compared to the values reported for BPAQSH-XX membranes when considering the DS up to 40. The reason may be due to the relatively low IECW and low water uptake. However, it may be noted that despite the relatively lower IECW and low water uptake, the proton conductivities of 6FBPAQSH-50 and -60 membranes were higher than the corresponding values in the BPAQSH-XX membranes. This may be attributed to the better phase separation and connectivity among the hydrophilic domains in 6FBPAQSHXX membranes. When comparing the ratio of conductivity per WUV (%), 6FBPAQSH-XX shows higher values as compared to the BPAQSH-XX series, indicating better management of water in the facilitation of proton conductivity. For example, at 30 °C, the ratio of conductivity per WUV (%) for 6FBPAQSH-60 is 1.86 (σ = 54 mS/cm and WUV (%) = 29), whereas it is 0.85 (σ = 40 mS/cm and WUV (%) = 47) for BPAQSH-60. It is also noted that the proton conductivities of 6FBPAQSH-XX membranes showed a marked increment with the increase of temperature than that of Nafion117 membrane. The probable reason may be due to the fact that aromatic sulfonic acid groups of 6FBPAQSH-XX (particularly XX = 50 and 60) are more weakly acidic in nature in comparison to the perfluorosulfonic acid groups of the Nafion, and dissociation constants of weak acids generally vary with temperature. In comparison with 6FBPAQSH-XX membranes, the Nafion 117 membrane is almost fully ionized at room temperature, so an increase in temperature will have no effect on the ion density and hence the conductivity.24 The activation energy (Ea) for proton conductivity of the copolymers was calculated from Figure 11 according to the Arrhenius equation:

Figure 9. Number of water molecules associated with sulfonic acid group (λ) as a function of IECV (wet).

sulfonation (or IECW values), water uptake of the 6FBPAQSHXX membranes was lower than the reported value (20%) for Nafion 117.14 We attribute it to the higher rigidity of the aromatic chain of 6FBPAQSH-XX and contributing hydrophobic character of −CF3 groups. Moreover, the strong ionic interaction among sulfonic acid groups increases the rigidity of network structure. This results in the restriction of free volume for water absorption and decreases the water uptake and swelling of 6FBPAQSH-XX copolymers.26 3.5. Proton Conductivity. Proton conductivity is the property of prime importance for fuel cell application. Before the proton conductivity measurement, all of the acid form membranes were immersed in deionized water (18 MΩ) for at least 72 h at room temperature to hydrate. In-plane proton conductivity (σ) of the 6FBPAQSH-XX series membranes was measured in deionized water as a function of DS and temperature by AC impedance spectroscopy. Figure 10 shows the complex impedance spectra of the 6FBPAQSH-30 membrane at different temperatures, and the inset spectra of 6FBPAQSH-XX (XX = 20−60) membranes at 90 °C. The membrane resistance was derived from the low intersect of the high-frequency semicircle with the Re(Z) axis. The conductivity of the membranes increased with increasing temperatures and sulfonated comonomer mole percentage, as was obvious from

σ = A e−Ea / RT 2780



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However, when DS is further increased to DS = 50 and 60, IECV values of 6FBPAQSH-XX membranes exceeded those of BPAQSH-XX membranes, a fact that was also reflected in their relative higher proton conductivity.

4. CONCLUSION Several new fluorinated sulfonated poly(arylene ether) copolymers were prepared by aromatic nucleophilic polycondensation of 4,4′-bis(4′-fluoro-3′-trifluoromethyl benzyl) biphenyl (QBF), 3,3′-disodiumsulfonyl-4,4′-dichlorodiphenylsulfone (SDCDPS), and 4,4′-hexafluoroisopropylidene diphenol (6FBPA). All of the 6FBPAQSH-XX copolymers were converted into flexible membranes from their solution in DMAc. The structure was determined using 1H, 13C, 19F NMR, and FTIR spectroscopy. The use of QBF having rigid aromatic moiety in the main chain was manifested in realizing good thermal stability and high mechanical strength. The 6FBPAQSH-XX copolymer membranes showed good thermal stability with 5% decomposition temperature in the range from 407 to 300 °C and Tg’s higher than 266 °C. The tensile strength for 6FBPAQSH-XX copolymer membranes was obtained in the range 35−52 MPa, which was several times higher than the reported value of Nafion (10 MPa) and comparable to our previous BPAQSH-XX series (27−57 MPa). 6FBPAQSH-XX copolymer membranes proved to be a very promising candidate in terms of lower water uptake, lower swelling, and relatively high oxidative stability, which could be attributed to the stabilizing effects of the −CF3 substituents and rigid backbone structures. The presence of QBF having trifluoromethyl group increased the hydrophobicity of nonsulfonated segments, which contributed to the hot water stability of the membranes and isolated hydrophilic domains distributed in a predominantly hydrophobic matrix. TEM observations suggested that relatively high fluorine content in the copolymer showed improved nanophase separation and was distinct with higher sulfonation degree, favorable for the effective proton transportation through the membrane. The 6FBPAQSH-XX copolymer membranes showed good proton conductivities in the range of 8−98 mS/cm at 80 °C even at relatively lower water uptake and lower IECW value, which indicated better management of water in 6FBPAQSH-XX membranes as compared to BPAQSH-XX series membranes. The combination of high thermal stability, good mechanical properties, low swelling ratio, and high proton conductivity renders 6FBPAQSH-XX polymers highly interesting for potential usage as promising PEM materials.

Figure 11. Arrhenius temperature dependence of proton conductivity (σ) of 6FBPAQSH-XX membranes.

where σ is the proton conductivity (mS/cm), R is the universal gas constant (8.314 J/mol K), A is the pre-exponential factor, and T is the absolute temperature (K). The activation energies for 6FBPAQSH-XX membranes (XX = 20, 30, 40, 50, 60) were in the range 15.55−10.71 kJ/mol, which was closer to the reported value of Nafion 117 (9.56 kJ/mol).44 Thus, similar to Nafion, 6FBPAQSH-XX membranes might have involved proton conduction mechanism, involving hydronium ion. Figure 12 shows the increase in the proton conductivity value

Figure 12. IECv dependence of proton conductivity of the 6FBPAQSH-XX membranes.

of the 6FBPAQSH-XX membranes with increase in IECV value. Beginning with a low IECV value, proton conductivity increased very slightly to the inflection point. After the inflection (percolation threshold), a rapid increase in proton conductivity was noticed. So, at low IECV, hydrophilic sulfonic acid groups are distributed as isolated cluster (narrow channels with dead end) in the hydrophobic domain. Yet as IECV increased, the hydrophilic domains get closer, forming a bigger ionic cluster for increased proton conductivity. With increasing temperature, a significant sharp transition in the proton conductivity is noticed at the inflection point, because proton conductivity is generally a thermally stimulated process. Similar to the water uptake figure, proton conductivities of 6FBPAQSH-XX membranes showed predictable change with increasing sulfonated comonomer mole percentage. As shown in Table 1, the IECV values of 6FBPAQSH-XX membranes were lower as compared to BPAQSH-XX membranes for DS = 20−40.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +91-3222-283972. Fax: +91-3222-255303. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS A.K.M. thanks the Indian Institute of Technology, Kharagpur, India, for financial support in the form of a Senior Research Fellowship (SRF) to carry out this work. We thank Mrs. L. Häuβler (IPF Dresden) for thermal analyses of the samples. 2781



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