Sulfonated fluorinated-aromatic polymers as proton exchange membranes

2.1 Sulfonated fluorinated random copoly(arylene ether)s

Sulfonated poly(aryl ether ether ketone ketone)s (SPEEKK-xx, where xx=40, 50, 60, 70, 80, and 100 with DS=0.8, 1.0, 1.2, 1.4, 1.6, and 2.0) were prepared by Liu et al. (34). The structures of the polymer containing the 6F or 6FP groups are shown in Scheme 1 (1-1). The aim of the study was to make a comparative study of the effect of the length of hydrophobic segments and to investigate the effect of various molecular structures on the properties related to proton conductivity. The resulting SPEEKK-6FP polymer series in acid form showed higher T_g values in the range of 212–271°C due to the bulky nature of the 6FP groups in comparison to the SPEEKK-6F polymer series, in acid form, which showed T_g values in the range of 196–236°C and moderate thermal stability in the range of 255–358°C. In general, the mechanical properties as evaluated by Young’s moduli and elongation at break showed values in the range of 0.61–1.61 GPa and 79–141%, respectively, indicating the film-forming ability of the polymers. The proton conductivities of the membranes in sulfonated acid form (–SO₃ H) were investigated as a function of the temperature. The conductivities of all the samples increased with increasing temperature. At the same temperature, high DS content samples showed higher proton conductivities, >100 mS/cm at room temperature, which is the minimum value regarded for a membrane to serve as a PEM in fuel cells. The SPEEKK with 6FP moieties exhibited higher water uptake and swelling ratios than those of the SPEEKK with 6F moieties, which showed a more regular structure of molecular chains. This was because the bulky pendant 6FP groups led to the large free volume between polymer chains, in which water molecules could be confined. The swelling ratios also showed an increase, considering the irregular structure of the 6FP moieties in the polymer architecture in comparison to the more regular 6F moieties in the polymer structure. For instance, SPEEKK-6FP-60 and SPEEKK-6F-60 at room temperature (23°C) showed a water uptake of 15% and 9%, respectively, and swelling ratios of 8% and 5%, respectively. This finding was also in accordance with the previous finding by Miyatake et al. (35, 36). SPEEKK-6FP-60 at 80°C showed a proton conductivity value of 160 mS/cm, and, at the same temperature, SPEEKK-6FP-50 showed a proton conductivity value of 100 mS/cm, which was higher than that of Nafion 112 (proton conductivity value at 80°C was 56 mS/cm), and the polymer SPEEKK-6FP-60 showed a higher value than that of Nafion 117 (proton conductivity value at 80°C was 96 mS/cm). SPEEKK-6FP-60 and SPEEKK-6F-60 showed an oxidative stability of 1.5 and 2 h, respectively, as tested in Fenton’s reagent (30 ppm FeSO₄ in 30% H₂ O₂). The polymers SPEEKK-6FP-70 and SPEEKK-6F-70 at the same DS of 1.6 showed IEC values as high as 1.95 and 1.92 mequiv/g, respectively, but, unfortunately, the oxidative stability of these polymers was poor as they dissolved in Fenton’s reagent after 30 min.

Scheme 1

Structures of sulfonated fluorinated poly(aryl ether ketone)s containing bulky groups in the main chain or as grafted chain.

Pan et al. (37) synthesized –CF₃ substituted sulfonated poly(phthalazinone ether ketone)s (SPPFEKK) by the direct copolymerization method. The polymer structure of SPPFEKK is shown in Scheme 2. SPPFEKK showed high thermal stability as required for PEMs. All the membranes exhibited a tensile strength ranging from 57.2 to 69.1 MPa and an elongation at break ranging from 8.0% to 14.7%. The tensile strength of the SPPFEKK membranes was higher than those of Nafion 112 (26.6 MPa) (38) and Nafion 117, which indicated their mechanical suitability for PEM. All the membranes exhibited proton conductivity in the range of 2.2–100 mS/cm. SPPFEKK-120 (DS=1.14) with an IEC value of 1.63 mmol/g showed a proton conductivity value of 100 mS/cm at 95°C, which was comparable to that of Nafion 117 and was higher than that of the non-fluorinated analogue with a similar DS, which showed a proton conductivity value of 45 mS/cm. The authors believed that the higher proton conductivity of the sulfonated fluorinated polymer SPPFEKK-120 was due to the greater amount of water confined in comparison to the non-fluorinated analogue. All the SPPFEKK membranes exhibited a methanol permeability <2.76×10^-7 cm²/s, which was much lower than that of Nafion 117 (15.5×10^-7 cm²/s at room temperature), indicating that SPPFEKK membranes were much better methanol barriers compared to Nafion 117. SPPFEKK membranes containing the –CF₃ groups and with DS in the range of 0.38–1.14 showed higher hydrolytic stability (>8000 h) in comparison with the non-fluorinated analogous membranes, which showed a lower hydrolytic stability of 864 and 552 h, respectively (39, 40). The SPPFEKK-100 membrane with a DS value of 0.98 exhibited a better oxidative stability (19 h) in comparison to the related non-fluorinated analogues with the same DS, which showed an oxidative stability of only 7 h (39, 40); this increase in oxidative stability was attributed to the presence of the –CF₃ groups introduced by 4,4-hexafluoroisopropylidene-diphenol (6F-BPA).

Scheme 2

Structure of the SPP-co-PAEK copolymer in proton form. [Adapted from Ref. (44).]

Jeong et al. (41) utilized cross-linking to improve the thermal stability of membranes. Sulfonated cross-linked membranes were obtained by thermal curing of the copolymers end-functionalized with alkyne moieties (Scheme 1, 1-3). The T_g of the sulfonated fluorinated cross-linked membranes was found to be higher than that of the non-cross-linked membrane and was in the range of 208–258°C. The 5% weight loss temperature (T_d,5) in nitrogen ranged from 309°C to 325°C in the acid forms of the membranes. Here the sulfonated polymers containing the 6F-BPA units showed a higher T_d,5 in comparison to that of the non-fluorinated analogue containing biphenyl units in the polymer main chain. The high thermal stability of the fluorinated polymers was due to the high bond strength of the C-F linkage compared to the C-H bond strength. As expected, the fluorinated cross-linked membranes with DS values of 0.59 and 0.72 showed a proton conductivity in the range of 45–72 mS/cm at room temperature, with an increasing trend of proton conductivity with an increase in the IEC value from 1.23 to 1.64 mequiv/g. Again, at a similar IEC value (∼1.23–1.28 mequiv/g), the sulfonated fluorinated cross-linked membrane showed a lower water uptake (14%) in comparison to the water uptake (23%) of the non-cross-linked analogue, which might be due to the cross-linked network, which imbibed less water. The sulfonated fluorinated cross-linked membrane also showed a lower water uptake in comparison to Nafion 117 (water uptake 17%). The proton conductivity of the sulfonated fluorinated cross-linked membrane (72 mS/cm at room temperature) at an IEC value of 1.64 mequiv/g was lower in comparison to that of Nafion 117 (conductivity being 85 mS/cm at room temperature), which the authors attributed to the difference in the morphology reported in previous studies (7, 42). The cross-linked membranes showed a smaller hydrophilic and hydrophobic phase separation, leading to narrow ionic pathways, and hence a lower proton conductivity was realized.

Jeong et al. (43) synthesized a sulfonated fluorinated poly(aryl ether ketone)s with pendent phenyl rings as shown in Scheme 1 (1-4). Both proton conductivities and the water uptake increased with an increase in temperature. For example, the proton conductivity of polymer membranes with a DS value of 0.96 was 83 mS/cm at 25°C and was 98 mS/cm at 80°C, which was comparable with that of Nafion 117 (proton conductivity of 45 mS/cm at 25°C and 96 mS/cm at 80°C). The pendent phenyl ring containing the sulfonic acid group improved the water uptake from 45% to 63% for the temperature range of 25–80°C, respectively, for the polymer with the same DS value, indicating its applicability as a PEM material for fuel cell application.

Zhang et al. (44) used Ni(0)-catalyzed coupling copolymerization for the synthesis of a series of sulfonated fluorinated poly(p-phenylene-co-aryl ether ketone)s (SPP-co-PAEKs). Copolymerization with the sulfonated monomer 2,5-dichloro-3′-sulfobenzophenone (DCSB) and 2,2′-bis[4-(4-chlorobenzoyl)]phenoxyl perfluoropropane (BCPPF) was carried out by varying the molar ratio of DCSB/BCPPF. Here aryl chloride with the electron-withdrawing group was chosen, which the authors attributed to the higher reactivity of such monomer reported previously (45, 46). The structure of SPP-co-PAEKs is shown in Scheme 2.

The incorporation of ether linkage and –CF₃ groups in the polymer backbone also led to enhanced solubility required for the fabrication of membranes for evaluating the properties for PEM application and for comparing the same with Nafion 112. In this study, fuel cell performance was evaluated using a SPP-co-PAEK (DCSB/BCPPF ratio 3:1) membrane because at this particular ratio the SPP-co-PAEK membrane showed good mechanical properties (Young’s modulus 1.8 GPa, elongation at break 63%, and tensile strength 51 MPa). The stress-strain curves of SPP-co-PAEKs and Nafion 112 are shown in Figure 1A, indicating the superior mechanical property of SPP-co-PAEK (3/1) in comparison to the other ratios. Figure 1B shows the proton conductivity of SPP-co-PAEK membranes as a function of relative humidity (RH). The membrane of SPP-co-PAEK (2/1) with an IEC value of 1.65 mequiv/g showed an in-plane proton conductivity of 93 mS/cm in water at 60°C. The SPP-co-PAEK (5:1) membrane exhibited an in-plane proton conductivity value as low as 1.7 mS/cm at 30% RH, in comparison to that of Nafion 112; however, it increased to 92 and 223 mS/cm on increasing the RH to 80% and on full hydrated condition, at the same temperature with an IEC value of 2.51 mequiv/g, respectively.

Figure 1

(A) Stress-strain curves of SPP-co-PAEKs and Nafion 112. (B) Proton conductivity of SPP-co-PAEK membranes as a function of RH. [Reproduced with permission from Ref. (44).]

In water at 60°C, the SPP-co-PAEK (3:1) membrane with an IEC value of >2.03 mequiv/g exhibited a higher in-plane proton conductivity value (164 mS/cm) compared to that of Nafion 112 (in-plane proton conductivity value of 143 mS/cm). The SPP-co-PAEK (3:1) membrane with an IEC value of 2.03 mequiv/g as shown in Figure 2 showed a proton exchange fuel cell performance of open circuit voltage (OCV) of 0.94 V, cell voltage at current density of 1.0 A/cm² (V_1.0 of 0.61 V), and output at 1.7 A/cm² of 0.85 W/cm² at 90°C, and 82:68% RH condition (H₂/air), which was comparable to the fuel cell performance of Nafion 112 under a similar condition (OCV of 0.93 V, V_1.0 of 0.61 V, and output at 1.7 A/cm² of 0.86 W/cm²), indicating the SPP-co-PAEK membrane as a suitable material for PEM.

Figure 2

Fuel cell performance of SPP-co-PAEK (3:1) with an open circuit voltage (OCV) of 0.94 V, cell voltage at current density of 1.0 A/cm² (V_1.0 of 0.61 V), and output at 1.7 A/cm² of 0.85 W/cm² under 90°C and 82/68% RH condition (H₂/air). [Reproduced with permission from ref. (44).]

Densely sulfonated fluorinated poly(arylene ether ketone) copolymers (SPAEK-xx, where xx=15, 20, 25, 30, and 35) were prepared by Pang et al. (47) and were evaluated for proton conductivity. The T_g of the copolymers was in the range of 193–231°C, and T_d,5 was in the range of 364–390°C, with a decreasing trend with an increase in the DS value. The structure of the tetra-sulfonated fluorinated poly(arylene ether ketone) copolymer is shown in Scheme 3 (1-5).

Scheme 3

Structures of densely sulfonated fluorinated poly(arylene ether)s.

The proton conductivities of the sulfonated membranes increased with an increase in the DS value from 0.6 to 1.4, and IEC also increased in the range from 0.93 to 1.81 mequiv/g. The SPAEK-35 membrane with high IEC (1.81 mequiv/g) exhibited a relatively low water uptake (23.5%) and was hydrolytically stable since the swelling ratio (in-plane direction) was only 5.1% at 80°C, which the authors attributed to the greater length of the fluorinated hydrophobic segment. SPAEK-xx showed good oxidative stability with almost no weight loss after 1 h, and mechanical properties were retained even at 80% RH (tensile strength of 44–53 MPa and elongation at break of 38–110%, with the trend decreasing with an increase in DS). The SPAEK-35 membrane (DS 1.4) showed a proton conductivity value of 84 mS/cm at 100°C, which was comparable to the proton conductivity of Nafion 117 (proton conductivity 100 mS/cm at 100°C). The SPAEK-35 membrane showed a good phase separation with sulfonic acid containing a segment in the hydrophilic phase and –CF₃ containing a hydrophobic segment, as observed through a transmission electron micrograph with ionic clusters of diameter ∼20 nm, which was much higher than that of Nafion 117 (∼8 nm). As the IEC and water uptake increased, the methanol permeability of SPAEK-xx membranes increased from 4.38×10^-9 to 5.08×10^-8 cm²/s at room temperature, which was significantly lower than that of the Nafion 117 membrane (15.5×10^-7 cm²/s at room temperature).

Nakabayashi et al. (48) incorporated a chain extender, namely, decafluorobiphenyl, and prepared fluorinated random multiblock copolymers (Scheme 3, 1-6) with IEC values in the range of 1.71–2.08 mequiv/g (as per ¹H NMR spectra). At an IEC value of 2.08 mequiv/g, the ionic cluster sizes were in the range of 50–60 nm and the membranes maintained a relatively high proton conductivity of 60 mS/cm under 50% RH at 80°C.

Matsumoto et al. (49) synthesized densely sulfonated fluorinated poly(arylene ether)s containing phenylene ether units in the main chain and decorated the periphery of the phenylene ether units with sulfonic acid groups by post-sulfonation using chlorosulfonic acid, with the aim to improve the proton conductivity. The phenylene ether moiety contained eight sulfonic acid groups per repeat unit. The RH dependence on the proton conductivity showed a decreasing trend with increasing IEC value, indicating the formation of well-connected proton pathways. However, the sulfonated membrane with an IEC value of 2.4 mequiv/g was soluble in water, restricting its utility as a PEM. Matsumoto et al. (50) further modified the phenylene ether moiety by increasing the rigidity of the polymer backbone with 10 sulfonic acid groups per repeat unit at the periphery, and the sulfonated membrane with an IEC value of 2.4 mequiv/g showed a proton conductivity of 40 mS/cm, which was comparable to that of Nafion 117 even at 30% RH, according to the manufacturer’s test protocol.

Li et al. (51) designed ABA-type densely fluorinated triblock copolymers (Scheme 3, 1-8), where sulfonated poly(2,6-diphenyl-1,4-phenylene oxide)s were the A blocks and poly(arylene ether sulfone)s were the B blocks, and a well-separated nanophase morphology was observed by them as reported by the atomic force microscopy (AFM) images. The sulfonated polymer at an IEC value of 1.83 mequiv/g showed a proton conductivity value of 190 mS/cm at 20°C in water, which was much higher in comparison to that of Nafion 112 (proton conductivity 90 mS/cm) at the same temperature.

Kim et al. (52) synthesized a comb-shaped densely sulfonated fluorinated poly(arylene ether)s with two and four sulfonic acid groups as grafted side chains. The sulfonated copolymers with four sulfonic acid groups obtained proton conductivities in the range of 63–125 mS/cm at room temperature, showing an increasing trend with an increase in water uptake from 27% to 53%. As the side chain grafting of sulfonic acid groups showed interesting results, Li et al. (53) prepared another series of comb-shaped densely sulfonated fluorinated poly(arylene ether)s in which the grafted side chain contained two sulfonic acid groups per unit. The comb-shaped polymers self-assembled to worm-like morphology, and the proton conductivity attained was 100 mS/cm at 30% RH at an IEC value as low as 0.92 mequiv/g. The structures of comb-shaped sulfonated fluorinated poly(arylene ether)s are shown in Scheme 4.

Scheme 4

Comb-shaped sulfonated fluorinated poly(arylene ether)s. [Adapted from Refs. (52) and (53).]

Wang et al. (54), who synthesized two series of sulfonated poly(arylene ether sulfone) copolymers (SPAEs) grafted with two or four sulfobutoxyphenyl side chains (2-SPAEs-xx, where xx=50, 60, 70, and 80, and 4-SPAEs-xx, where xx=25 and 35) of reasonable weight, with average molecular weights (M_w) in the range of 136,300–209,300 g/mol. The structure of the sulfonated copolymers with two pendent sulfobutoxyphenyl side chains is shown in Scheme 5 (1-9). The sulfonated copolymers showed a low water uptake of 12.8–32.3% and 10.4–17.5% at 80°C with two or four sulfobutoxyphenyl groups by the membranes, respectively. Again, at the same IEC value (1.24 mequiv/g), the 4-SPAEs-25 membrane showed a lower water uptake (10.4%) compared to the 2-SPAEs-50 membrane (water uptake 12.8%) at 80°C, which Wang et al. (54). attributed to the higher local concentrations of hydrophilic sulfonic acid groups in the 4-SPAEs-xx membranes. The plots of the dimensional swelling of 2-SPAEs-xx, 4-SPAEs-xx, and Nafion 117 in the through-plane and in-plane direction are presented in Figure 3A. The sulfonated copolymers showed a lower dimensional swelling due to the lower water uptake compared to that of Nafion 117 at 80°C and also in comparison with other polymers where the pendent sulfonic acid group was in the main chain at a similar IEC value (55). The authors attributed this to the good phase separation of the hydrophilic and hydrophobic domains, as evaluated by transmission electron microscopy (TEM), by the formation of small hydrophilic clusters of 1–3 nm, which provided ionic channels for proton transfer. Hence, the water molecules were confined well in the hydrophilic domains, preventing water swelling. The sulfonated copolymers with two or four pendent sulfobutoxyphenyl groups showed proton conductivities in the range of 108–258 mS/cm for 2-SPAEs-xx and 135–194 mS/cm for 4-SPAEs-xx at 80°C, respectively. The proton conductivity of 4-SPAEs-25 was slightly higher (135 mS/cm at 80°C) than that of copolymer 2-SPAEs-50 (108 mS/cm at 80°C) with the same IEC value (1.24 mequiv/g), which the authors attributed to the high density of the sulfoalkyl segments in the 4-SPAES-xx series, which promoted a higher proton conduction.

Scheme 5

Structures of sulfonated fluorinated poly(arylene ether)s with side chain grafting.

Figure 3

(A) Dimensional swelling of 2-SPAES-xx, 4-SPAES-xx, and Nafion 117 at 80°C. (B) Trade-off plot of proton conductivity as a function of methanol permeability, relative to Nafion 117. [Reproduced with permission from Ref. (54).]

This finding was in accordance with the previous findings by Li et al. (56), where two or four pendent phenyl sulfonic acid groups were introduced as side chains. However, the proton conductivity values of the sulfonated copolymers obtained by Li et al. (56) were somewhat higher, lying in the range between 82 and 298 mS/cm for 2-SPAEs-xx and between 106 and 311 mS/cm for 2-SPAEs-xx at 80°C. This might be attributed to the formation of wider hydrophilic clusters of 2–5 nm, as evaluated by TEM in their work, in comparison to the 1–3 nm obtained by Wang et al. (54). For comparison, the structure of the sulfonated copolymer with two pendent phenyl sulfonic acid groups is presented in Scheme 5 (1-10).

The performance of the membranes was evaluated by generating a trade-off plot containing both relative methanol permeability and relative proton conductivity (57, 58).

As shown in Figure 3B, all the data points are situated in the upper left-hand corner of the plot, which indicated high conductivity and low methanol permeability in the range of 1.59×10^-7–4.69×10^-7 cm²/s, which was much lower than the value for Nafion 117 (15.5×10^-7 cm²/s).

Our group prepared sulfonated fluorinated poly(arylene ether) copolymers containing pendent –CF₃ groups (BPAQSH-XX, xx=20, 30, 40, 50, and 60) with varying DS values (0.39, 0.57, 0.8, 0.97, and 1.12, calculated from ¹H NMR spectra), utilizing aromatic nucleophilic polycondensation (59). The fluorinated sulfonated poly(arylene ether) copolymers were prepared by direct copolymerization of 4,4′-bis(4′-fluoro-3′-trifluoromethyl benzyl) biphenyl (QBF), 3,3′-disodiumsulfonyl-4,4′-dichlorodiphenylsulfone (SDCDPS), and bisphenol A (BPA), and inherent viscosities were obtained in the range of 0.72–2.11 dl/g, indicating the formation of high molar masses. Scheme 6 shows the synthesis of ArQSH-XX copolymers by varying the n values, with different DS values, and their properties are presented in Table 1.

Scheme 6

Synthesis of sulfonated fluorinated poly(arylene ether)s utilizing different bisphenol units in the polymer main chain.

The sulfonated polymers were soluble in polar aprotic solvents such as N-methylpyrrolidinone, dimethyl sulfoxide, dimethylformamide, and dimethylacetamide, which was required for fabrication into membranes and for evaluation of the properties required for PEM. The T_g values of BPAQSH-XX (where XX=20 and 30) polymers measured by differential scanning calorimetry showed an increasing trend (245–257°C) with the increase in DS. This increase in T_g with increasing DS was attributed to the increase in intermolecular interactions as well as to the presence of bulky –SO₃ H groups, which hindered the internal rotation. For the remaining polymers, BPAQSH-XX (where XX=40, 50, and 60), no T_g could be detected up to 300°C, which might be in the temperature range when the decomposition of the polymers already started. As the DS increased, T_d,5 values were in the range of 283–365°C with a decreasing tendency with an increase in DS. Compared to the acid form of copolymers (BPAQSH-XX), the copolymers in sodium-salt form (BPAQS-XX) showed markedly high thermal stabilities with T_d above 277°C and T_d,5 above 302°C. The tensile strength for BPAQSH-XX membranes was in the range of 27–57 MPa, and Young’s modulus was in the range of 0.91–1.55 GPa. The tensile strength and Young’s modulus values were much higher than those of Nafion 117 membrane (tensile strength 25.65 MPa and Young’s modulus 0.26 GPa) (60). The elongation at break (5–59%) of these membranes was much lower than that of Nafion 117 membrane (>200%) because of rigid quadriphenyl moiety coming from QBF, in the BPAQSH-XX polymers. The mechanical properties of the membranes showed no significant effect on acidification.

In the series, BPAQSH-20 showed the highest oxidative stability (t=29.6 h), whereas BPAQSH-60 showed the lowest oxidative stability (t=3.8 h). The sulfonic acid groups in an electron-deficient aromatic moiety and the –CF₃ groups in aromatic ring ortho to the ether linkage greatly minimized the probability of hydrolysis of the polymer main chain. The water uptake and swelling ratio of the films increased with increasing temperature and IEC value. This was because the sulfonic acid groups were hydrophilic in nature and hence the membranes with higher DS absorbed more water due to the increase in the hydrophilicity. At room temperature, all the BPAQSH-XX membranes showed water uptake in the range of 4–40% and swelling ratios of <10%. At elevated temperatures, the polymer chain mobility and the free volume increased; hence the water uptake and swelling were enhanced. At room temperature and at 80°C, membranes showed gradual increase in water uptake up to a certain IEC value and then increased sharply due to the formation of large and continuous ion network in the sulfonated polymers. All the BPAQSH-XX membranes maintained their dimensional shapes and mechanical strengths in the temperature range under investigation. It was noteworthy that BPAQSH-XX with the same DS (or IEC values) showed a lower water uptake than the value of 38% for Nafion 117 reported by Zhang et al. (60). This was attributed to the higher rigidity of the aromatic chain of BPAQSH-XX and to the lower acidity of the sulfonic acid groups, together with the contributing hydrophobic character of the –CF₃ groups, which decreased the absorption of water compared to that of Nafion 117. Moreover, the strong ionic interaction among sulfonic acid groups increased the rigidity of the network structure. These effects resulted in the restriction of free volume for water absorption and decreased the water uptake and swelling of BPAQSH-XX copolymers (61). The proton conductivities of BPAQSH-XX membranes were obtained in the range of 9–40 mS/cm at 30°C and 25–80 mS/cm at 80°C. However, the values were lower than the reported values (80 mS/cm at 30°C and 100 mS/cm at 80°C) for Nafion 117 (62).

We also prepared sulfonated fluorinated poly(aryl ether sulfone) (PAQSH-XX) (where XX=30, 40, 50, and 60) copolymers (DS=0.29, 0.39, 0.47, and 0.57, calculated from ¹H NMR spectra) containing bulky phthalimidine moiety in the main chain by direct copolymerization of QBF, SDCDPS, and N-phenyl-3,3-bis(4-hydroxyphenyl)phthalimidine (PA), and the properties were evaluated as required for PEMs (63). The number average molecular weights (M_n) for the PAQSH-XX were in the range of 50,700–57,200 g/mol, with PDI values of ∼2.22–2.63, indicating reasonable molar mass products. Scheme 6 shows the structure of sulfonated fluorinated poly(arylene ether sulfone) containing PA moiety. These sulfonated copolymers with phthalimidine moiety showed high T_g and T_d,5 compared to the polymers containing isopropylidene units in the main chain (59), which could be attributed to the bulky nature of the phthalimidine moiety, which restricted free rotation, and hence a rise in T_g and T_d,5 was observed. The mechanical properties measured at room temperature and at 65% RH showed high tensile strength (43.7–68.8 MPa), much higher than those of Nafion 117 membrane (25.65 MPa) (60) and analogous copolymers containing isopropylidene units in the main chain (59). This was attributed to the incorporation of cardo-type PA group and rigid QBF moiety in the main chain. As shown in Figure 4A, all the PAQSH-XX membranes exhibited an Arrhenius-type, temperature-dependent proton conductivity behavior.

Figure 4

(A) The Arrhenius-type, temperature-dependant proton conductivity behavior of PAQSH-XX membranes. (B) Proton conductivity of PAQSH-XX membranes as a function of IEC. [Reproduced with permission from Ref. (63).]

All the sulfonated membranes in this study showed a slightly higher activation energy value (14.3–10.1 kJ/mol) than the reported value of Nafion 117 (9.56 kJ/mol) (64). The PAQSH-60 copolymer membranes, however, showed an activation energy value close to that of Nafion 117, which was probably due to an analogous proton conduction mechanism, involving hydronium ion (vehicular mechanism), suggesting PAQSH-XX copolymers as potential candidates for PEM materials. Furthermore, the dependence of the proton conductivity of PAQSH-XX membrane on the IEC value as shown in Figure 4B showed a gradual increase up to a certain IEC value (1.25 mequiv/g). After that, it showed a marked increment with an increase in IEC. At low IEC values, hydrated sulfonic acid groups formed mainly isolated dispersed clusters and less interconnected channels, which resulted in low proton conductivity. With increasing IEC, the isolated clusters came closer and formed bigger size ionic clusters, which resulted in increasing trend of proton conductivity values, together with low swelling, leading to a good dimensional stability of PAQSH membranes. For example, PAQSH-60 membrane exhibited a swelling ratio of 13% at 80°C, which was much lower than that of Nafion 117 (18, 19), proving that the PAQSH-60 membrane exhibited better dimensional stability than Nafion 117, resulting from the bulky phthalimidine groups and hydrophobic –CF₃ groups, along with rigid quadriphenyl moiety. The proton conductivity of PAQSH-XX membranes was in the range of 5–28 mS/cm at 30°C and 11–50 mS/cm at 80°C. The low value of the proton conductivity was probably due to the presence of more rigid phthalimidine and QBF moieties, which restricted the proton transport (28, 32). However, the phase-separated morphology observed in PAQSH-XX membranes was better than that observed in many other reported sulfonated fluorinated poly(arylene ether sulfone)s (59, 65). The presence of hydrophobic –CF₃ groups in the polymer backbone led to a well-defined nanophase separated structure.

Recently, our group prepared sulfonated fluorinated poly(arylene ether sulfone) (IBQSH-XX) copolymers (where XX=20, 30, 40, 50, and 60, and DS=0.16, 0.26, 0.39, 0.47, and 0.56, respectively) based on imidoaryl biphenol, namely, 3,8-bis(4-hydroxyphenyl)-N-phenyl-1,2-naphthalimide (IB), by direct copolymerization with the SDCDPS as the sulfonated monomer and QBF as the fluorinated monomer (66). Sulfonated copolymers were obtained with M_n values in the range of 41,200–60,200 g/mol, with PDI values in the range of 0.96–1.89, the trend increasing with the increase in DS (0.16–0.56 from ¹H NMR). Scheme 6 shows the representative structure of sulfonated fluorinated poly(arylene ether sulfone) containing IB moiety. In the IBQSH-XX series, none of the sulfonated copolymers showed any T_g up to 350°C, which was attributed to the presence of highly rigid quadriphenyl and bulky IB moieties, along with some strong intermolecular interactions, which hindered the movement of molecular segments in the sulfonated copolymers. The sulfonated copolymers containing IB moiety showed very high thermal stability and mechanical and tensile strength. The oxidative stability of the sulfonated copolymers measured at 80°C was in the range of 4.4–30.1 h, which decreased with the increase in DS, which could be attributed to the stabilizing effects of the hydrophobic –CF₃ groups and to the rigidity of QBF and IB moieties, and was higher in comparison to the PAQSH-XX copolymers containing phthalimidine moiety (oxidative stability measured at 80°C ∼2.2–25.3 h). The polar IB moieties in the polymer backbone helped in the exclusion of water and low swelling, and incorporation of fluorine resulted in well-defined nanophase separation as obtained by TEM. Figure 5 shows the temperature dependence of water uptake and the number of water molecules associated with the sulfonic acid group, hydration number λ (H₂ O/SO₃^-) value as a function IEC.

Figure 5

Variation in the water uptake (%) and number of water molecules associated with the sulfonic acid group (λ) of IBQSH-XX with the IEC. [Reproduced with permission from Ref. (66).]

At room temperature, all the IBQSH-XX membranes showed, in general, very low water uptake (below 20%) and low swelling ratios (<6%) in comparison to the polymers containing isopropylidene units reported earlier by us (59) and to the various other PEMs containing sulfone (56), keto (28), and naphthalene moieties (61). In addition to the rigid and bulky IB moieties present in the IBQSH-XX, the strong interaction with the sulfonic acid groups increased the rigidity of the network structure, resulting in lower water uptake (3–20% at room temperature) and lower swelling (1.2–5.9%) in comparison to those of Nafion 117 (water uptake of 19% at room temperature and swelling of 11.2%), and which were lower than those of the analogous PAQSH-XX (water uptake of 13–38% at room temperature and swelling of 4–11%) as reported by us (63). TEM showed a good phase-separated morphology, with ionic cluster size in the range of 3–60 nm for the copolymer membranes. The IBQSH-XX copolymer membranes showed proton conductivities in the range of 3–49 mS/cm at 80°C but were lower in comparison to those of Nafion 117.

In order to obtain proton conductivity comparable to that of Nafion 117, our group further explored a different monomer combination utilizing 4,4′-hexafluoroisopropylidene diphenol (6F-BPA) and evaluated the effect of increased fluorine content on IEC and related PEM properties, such as water uptake, and on the swelling properties of the resulting membranes (67, 68). Accordingly, sulfonated fluorinated poly(aryl ether) copolymers were prepared by direct copolymerization of 6F-BPA, QBF, and SDCDPS (Scheme 6). 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 (where XX=20, 30, 40, 50, and 60, and DS=0.16, 0.29, 0.35, 0.44, and 0.53, calculated from ¹H NMR spectra) showed good thermal stability with T_d,5 of ∼407–300°C and T_g values >266°C. The tensile strength for 6FBPAQSH-XX copolymer membranes was obtained in the range of 35–52 MPa, which was higher than the reported value for Nafion (25.65 MPa) (54) and comparable to those of our previous BPAQSH-XX series (27–57 MPa) (53).

When the BPA in our previous BPAQSH-XX series was replaced by a heavier 6F-BPA comonomer to obtain the 6FBPAQSH-XX series, elemental fluorine content increased from 7.2–13.9% to 21.6–26.9% and the IEC_w value decreased from 1.91–0.61 to 1.63–0.53 mequiv/g (a 13–15% decrease). Figure 6 shows the effect of IEC_w and volumetric IEC (IEC_v) (dry) on the water uptake of 6FBPAQSH-XX and BPAQSH-XX copolymers as a function of temperature (30°C and 80°C). The figure shows a deflection at an IEC_w value of 1.07 mequiv/g [IEC_v (dry)=1.45 mequiv/ml] for the 6FBPAQSH-XX series and an IEC_w value of 1.25 mequiv/g [IEC_v (dry)=1.53 mequiv/ml] for the BPAQSH-XX series. 6FBPAQSH-XX showed a very slow increase in water uptake (WU), i.e., 2–8% at 30°C and 4–13% at 80°C.

Figure 6

Water uptake dependence of (A) IEC_w and (B) IEC_v values of copolymer membranes. [Reproduced with permission from Ref. (67, 68).]

This lower water uptake was attributed to the isolated hydrophilic domains distributed in a predominantly hydrophobic matrix. Meanwhile, a substantially higher water uptake of 6FBPAQSH-50 (IEC_w 1.35 mequiv/g, WU_w 13% at 30°C, and WU_w 23% at 80°C) and 6FBPAQSH-60 (IEC_w 1.63 mequiv/g, WU_w 23% at 30°C, and WU_w 38% at 80°C) membranes did not affect the mechanical strength, and the dimensional stability of the membranes was well maintained as evaluated by TEM, indicating a well-defined phase-separated morphology, as shown in Figure 7. The presence of QBF having –CF₃ groups increased the hydrophobicity of non-sulfonated segments, which contributed to the hot water stability of the membranes and to isolated hydrophilic domains distributed in a predominantly hydrophobic matrix. The transmission electron micrograph of 6FBPAQSH-40 exhibited a clear microphase-separated structure with somewhat larger ionic clusters (12–20 nm). 6FBPAQSH-50 and 6FBPAQSH-60 exhibited an excellent phase-separated morphology with a large number of medium-sized ionic clusters (15–26 nm), along with a certain number of bigger ionic clusters (30–65 nm). Thus, the TEM images of membranes containing higher sulfonic acid suggested that the sulfonic acid groups might have aggregated to form bigger hydrophilic clusters where water was well confined and to provide much better ionic pathways for proton transport. In contrast, the analogous BPAQSH-XX membranes showed a phase-segregated co-continuous morphology of the softer region (black) and harder region (bright or gray), and fibril-like cylindrical structures along the thickness of the membrane, according to our previous finding (59).

Figure 7

Transmission electron micrograph of lead-stained 6FBPAQS-XX copolymers: (A) XX=20, (B) XX=30, (C) XX=40, (D) XX=50, and (E) XX=60. [Reproduced with permission from Ref. (67, 68).]

The 6FBPAQSH-XX copolymer membranes showed a proton conductivity value of 108 mS/cm at 80°C, which was comparable to that of Nafion 117 (110 mS/cm at 90°C), even at relatively lower water uptake and lower IEC_w value than those of BPAQSH-XX copolymer membranes.

Previously, Schönberger et al. (69) prepared sulfonated fluorinated poly(aryl ether)s by post-sulfonation using chlorosulfonic acid and achieved a DS value of 1.26, and a specific conductivity value as high as 167 mS/cm was reached at room temperature at an IEC value of 1.67 mmol/g, which was higher than that of Nafion 117 (specific conductivity value of 133 mS/cm at room temperature). The sulfonated polymer showed very good oxidative stability at 24 h, which the authors attributed to the stabilizing effect of the –CF₃ groups present in the polymer backbone. However, the swelling of such a sulfonated polymer was quite higher in comparison to that of Nafion^® 117, which the authors attributed that the swelling could be restricted by either lowering the DS or by exploring a suitable cross-linking mechanism.

2.2 Sulfonated fluorinated block copoly(arylene ether)s

Chu et al. (70) prepared sulfonated and partially fluorinated block copolymers of high thermal stability containing 6F-BPA and perfluorobiphenylene units. Block copolymers were synthesized from the hydrophilic (29,000 g/mol) and hydrophobic parts (11,000 g/mol) with decafluorobiphenyl, the structure of which is shown in Scheme 7 (0:1:1 ratio for Block 0, 1:9:10 ratio for Block 10, 3:7:10 ratio for Block 30, and 5:5:10 ratio for Block 50). The M_n values of the copolymers were in the range of 15,000–90,000 g/mol, with PDI values of ∼1.3–4.4.

Scheme 7

Representative structure of sulfonated poly(biphenylsulfone ketone) block copolymers. [Adapted from Ref. (70).]

The T_g of the block copolymers in the acidified form increased from 137°C to 157°C, with an increase in DS as expected (71, 72), and the T_d,5 values of the sulfonated polymers were in the range of 175–505°C, and the T_g values as well as the T_d,5 values were lower in comparison to the sulfonated random polymers containing PA moiety (63) and IB moiety (66), as reported by us. Interestingly, Block 30 with a DS value of 31 and an IEC value of 0.79 mequiv/g exhibited the highest proton conductivity of 75 mS/cm at 90°C and at 100% RH in comparison to Block 50 with a DS value of 47 and a higher IEC value of 1.02 mequiv/g, which showed a lower proton conductivity of 62 mS/cm at the same temperature and RH. The authors attributed the difference in proton conductivity to the difference in morphology of the block copolymers. The morphology of the block copolymers was studied by AFM, which the authors attributed to the fact that, although the DS value of Block 50 was high, the ionic pathways for the proton were not well defined in comparison to Block 30, which formed better proton channels.

Previously, Ghassemi et al. (73) prepared multiblock sulfonated fluorinated poly(aryl ether)s and tailored the IEC value to 2.29 mequiv/g, which attained a proton conductivity at room temperature of 320 mS/cm by varying the sulfonated block length; however, the water uptake was very high (470%). Furthermore, validation of these multiblock polymers for practical use in terms of oxidative stability needs to be studied as well.

2.3 Sulfonated fluorinated branched poly(arylene ether)s

Guo et al. (74) studied the proton conductivity and methanol permeability of branched polymer membranes for DMFC application. Branched sulfonated poly(aryl ether ketone) copolymers (Br-SPAEKs) were synthesized by nucleophilic aromatic substitution reaction based on 6F-BPA (A₂ monomer), 4,4′-difluorobenzophenone (DFBP, unsulfonated B₂ monomer), 3,3′-disodiumsulfonyl-4,4′-difluorobenzophenone (SDFBP, sulfonated B₂ monomer), and 2,4′,6-trifluoro-benzophenone (TFBP, branched BB₂′ monomer). 6F-BPA, DFBP, and sulfonated monomer SDFBP with or without the branched monomer TFBP were copolymerized to form linear (L-SPAEK) and Br-SPAEK copolymers, respectively. The synthetic scheme is presented in Scheme 8. To study the effect of branching, a series of Br-xx-SPAEK-30 (sulfonated monomer of 30 mol% and xx=10, 20, and 30) were prepared. In order to elucidate the influence of the increase in sulfonated monomer on the properties of the branched polymer, a series of Br-20-SPAEK-xx (branched monomer of 20 mol% and sulfonated monomer of 40–60 mol%, xx=40, 50, and 60) were also prepared.

Scheme 8

Synthesis of copolymers: (A) L-SPAEK, (B) Br-SPAEKs. Br-SPAEKs included Br-xx-SPAEK-30 (xx=10, 20, and 30; xx represents the content of the branched monomer) and Br-20-SPAEK-xx (xx=40, 50, and 60; xx represents the content of the sulfonated monomer). Br-xx-SPAEK-30 (xx=10, 20, and 30) was obtained with m=0.1, 0.2, and 0.3, and n=0.3. Br-20-SPAEK-xx (xx=40, 50, and 60) was obtained with m=0.2 and n=0.4, 0.5, and 0.6. [Reproduced with permission from Ref. (74).]

The proton conductivity of Br-20-SPAEK-40 was reported to be 121 mS/cm at 80°C, which was comparable to that of Nafion 117 (139 mS/cm) under the same laboratory test condition. In the case of Br-20-SPAEK-50 and Br-20-SPAEK-60, the proton conductivity further increased to 176 and 187 mS/cm, respectively, which was higher than that of the linear analogue (164 mS/cm), which the authors attributed to the increase in free volume, leading to enhanced water uptake by introducing branched units in the polymer structure.

The methanol permeability of Br-xx-SPAEK-30 was lower (1.25×10^-7–1.55×10^-7 cm²/s) than that of L-SPAEK (1.78×10^-7 cm²/s) at similar IEC value (1.06 mequiv/g) and slightly decreased with an increase in the amount of branched monomer, which the authors attributed to the branched network that suppressed the swelling of the membranes, which led to a more compact membrane structure restricting methanol penetration. The methanol permeability of Br-20-SPAEK-xx, however, increased by increasing the amount of sulfonated monomer as expected, due to the enhancement of the water swelling. At a glance, the methanol permeability of all the membranes containing branching units at room temperature was in the range of 1.25×10^-7 to 1.35×10^-6 cm²/s, which was much lower than that of Nafion 117 (15.5×10^-7 cm²/s). The authors calculated the selectivity (defined as the ratio of proton conductivity to the methanol permeability of membranes) for evaluating membrane performances, considering both proton conductivity and methanol permeability. As shown in Figure 8, compared with the selectivity of L-SPAEK with similar IEC, Br-20-SPAEK-30 showed the highest selectivity of 1.69×10⁵ Ss/cm³, which was almost three times higher than that of L-SPAEK (0.59×10⁵ Ss/cm³).

Figure 8

Selectivity of membranes at 20°C: (A) L-SPAEK and Br-xx-SPAEK-30 (xx=10, 20, and 30). (B) Nafion 117 and Br-20-SPAEK-xx (xx=40, 50, and 60). [Reproduced with permission from Ref. (74).]

Li et al. (75) reported on branched sulfonated poly(ether ether ketone) (Br-SPEEK) and evaluated the proton conductivity and methanol permeability, and compared the properties with the linear analogue and also with Nafion 117. The structure of the branched sulfonated poly(ether ether ketone) is shown in Scheme 9.

Scheme 9

Structure of branched sulfonated poly(ether ether ketone) (Br-SPEEK). [Adapted from Ref. (75).]

The amount of branching monomer was kept to 3, 5, and 10 mol%, respectively. Beyond 10 mol%, the copolymers were cross-linked. The proton conductivity value of linear sulfonated poly(ether ether ketone) (L-SPEEK) was higher (152 mS/cm at 80°C) than those of Br-SPEEK copolymers (146–126 mS/cm at 80°C). The value of proton conductivity for Br-SPEEKs also showed a decreasing trend with an increase in the branching unit from 3 to 10 mol%. The authors attributed these findings to the increase in branching agent, which restricted the ionic pathways for proton conduction. However, both L-PEEK and Br-SPEEK-10 showed comparable values to that of Nafion 117 at 80°C. This finding in terms of proton conductivity is in contrast to that reported by Guo et al. (74), where branched polymers showed a higher proton conductivity compared to the linear analogues. The methanol permeability of Br-SPEEK at room temperature decreased compared to the L-SPEEK membrane (8×10^-7 cm²/s), and the values were lower than those of Nafion 117 (15.5×10^-7 cm²/s). With the content of branching agent increasing from 3 to 10 mol%, the methanol permeability decreased from 7.1×10^-7 to 6.3×10^-7 cm²/s, as the branching restricted the swelling, leading to less water confined in the hydrophilic domains. The methanol permeability values of branched polymers reported by Guo et al. (74) were lower, indicating better performance in terms of DMFCs compared to the methanol permeability values of branched polymers, as reported by Li et al. (75).

2.4 Sulfonated fluorinated poly(arylene ether) based composites

Li et al. (76) prepared composite membranes utilizing cross-linking between the sulfonic acid and the amine groups based on sulfonated fluorinated poly(aryl ether ketone)s SPEEK-6F and poly(amic acid) (PAA) with oligoaniline in the main chain. The blend membranes composed of 2, 5, 10, and 15 wt% of PAA in SPEEK-6F were prepared by casting the polymer in DMF solutions. The combination of sulfonated poly(aryl ether ketone) with polyamic acid(PAA) containing oligoaniline is shown in Scheme 10 (1-11).

Scheme 10

(A) Combination of sulfonated poly(aryl ether ketone) with polyamic acid (PAA) containing oligoaniline. (B) Structure of the sulfonated poly(arylene biphenylsulfone ketone) block copolymer.

Due to the specific interaction between SPEEK-6F and PAA, a dense cross-linking structure was formed, which resulted in the decrease in methanol diffusion. The SPEEK-6F/PAA-15 showed a methanol permeability of 0.9×10^-7 cm²/s, and the proton conductivity of SPEEK-6F/PAA-15 (90 mS/cm at 80°C) was comparable to that of Nafion 117. The proton conductivity was improved to a value of 128 mS/cm when phosphoric acid-doped membranes (SPEEK-6F/PAA-15) were prepared (77). However, the methanol permeability was increased to a value of 4×10^-7 cm²/s as a result of doping, even though the value was still lower in comparison to the methanol permeability of Nafion 117.

Densely sulfonated poly(arylene biphenylsulfone ketone) block copolymer and its composite membranes containing 10%, 20%, or 30% of phosphotungstic acid (PWA) were investigated by Lee et al. (78). The structure of the sulfonated poly(arylene biphenylsulfone ketone) block copolymer is shown in Scheme 10 (1-12). The T_g of the block copolymer/PWA-30 membrane showed a lower value (163°C) compared to the T_g of the block copolymer (174°C), which the authors attributed to the possible interactions between sulfonic acid and PWA. The thermal stability of the composite membranes was also lower, lying in the range of 370–416°C, showing a decreasing trend with an increase in the content of PWA compared to the block copolymer, which showed a T_d,10 value of 470°C. All the composite membranes showed an oxidative stability >24 h. The water uptake of the composite membranes with a PWA content of 10–30% showed a decreasing trend in water uptake from 18% to 10%. Interestingly, the composite membrane with 30% PWA showed a water uptake value of 10%, which was the same as that of the block copolymer. The authors attributed that lower water uptake value to the possible interactions between the sulfonic acid groups and PWA, which reduced the available water absorption sites. However, the IEC value of the composite membranes was found to increase in the range of 1.46–1.58 mequiv/g, with an increase in PWA content from 105 to 30%. The proton conductivities of the composite membranes also showed an increasing trend of 32–40 mS/cm at room temperature and 65–96 mS/cm at 90°C, measured at 100% RH, which was higher than that of block copolymer (6 mS/cm at room temperature and 21 mS/cm at 90°C) and comparable to that of Nafion 117 (45 mS/cm at room temperature and 110 mS/cm at 90°C), which the authors attributed to the PWA content helping to form ionic channels for proton conduction in the composite membranes. The fabricated composite membrane exhibited a maximum fuel cell power density of 150 mW/cm², which was lower in comparison to the power density (250 mW/cm²) of Nafion 117.

Guo et al. (79) prepared sulfonated-fluorinated poly(aryl ether ether ketone)/epoxy-based composite membranes by a solution casting method. They used epoxy cross-linking reaction to improve the dimensional stability and to prevent excessive water swelling. They attributed that increase in the epoxy content to the feed composition, which increased the probability of cross-linking, preventing the diffusion of water in the composite membranes. The composite membranes were cured by following two different thermal curing profiles. In one procedure, the composite membranes were treated at 200°C for 0.5 h for enhancing the cross-linking reaction and were further heated at 120°C for 24 h. In another procedure, the composite membranes were heated only at 120°C for 24 h. After curing at 200°C, the composite membranes showed a decrease in water uptake more evidently from 11.6% to 5.6% at room temperature and from 43.6% to 29% at 100°C, respectively. The water uptake of composite membranes cured at 120°C decreased from 20.2% to 10.1% at room temperature and from 69.3% to 33.7% at 100°C, respectively. The composite membranes cured at 200°C showed lower methanol permeability values compared to the composite membranes cured at 120°C (0.33×10^-6 cm²/s cured at 200°C and 1.28×10^-6 cm²/s cured at 120°C) and compared to that of Nafion 117. The proton conductivities of the samples after curing at 200°C were 12.6 mS/cm at room temperature and 86.7 mS/cm at 100°C; however, the values were much lower than those of Nafion 117. The proton conductivities of membranes cured at 120°C ranged from 22.2 mS/cm at room temperature to 94.2 mS/cm at 100°C, which were comparable to those of Nafion 117.

3.1 Sulfonated fluorinated block copolyimides

Nakano et al. (91) prepared sulfonated fluorinated block copolyimides (see the structure in Scheme 12, 1-17) by chemical imidization using a two-pot procedure with 1,4,5,8-naphthalene tetracarboxylic dianhydride (NTDA), 4,4′-diamino-biphenyl 2,2′-disulfonic acid (BDSA), and 2,2′-bis(4-aminophenoxy)hexafluoropropane (6FAP) by varying the diamine compositions and block chain lengths (m/n=112/48, 70/30, and 49/21), and then investigated the proton conductivity. They also prepared sulfonated random copolymers for comparison. The proton conductivity of the membranes strongly depended on the block chain lengths and increased with the increase in the block chain lengths. The proton conductivity of the block copolyimide membranes was in the range of 190–350 mS/cm at 80°C and increased with the increase in the block chain lengths. The random analogue showed a proton conductivity value of 150 mS/cm, which was lower than that of the sulfonated block copolyimide (proton conductivity value of 250 S/cm) at the same diamine composition and theoretical IEC value of 2.44 (91). The proton conductivity value was also higher for the sulfonated block copolyimides in comparison to the proton conductivity value of 150 mS/cm for Nafion 117. The hydration number (λ) of block copolyimide membranes was found to be larger (λ=15) than those of the random copolyimide membranes (λ=11) and Nafion 117 (λ=12) and was similar to that of Nafion 115 (λ=15), indicating that water in the block membranes was significantly located in the hydrophilic domains; however, morphological studies are further required to validate their findings. The increase in proton conductivity with the increase in block chain length was somewhat contrary to the results reported by Genies et al. (92), who observed a decreasing trend in proton conductivity with the increase in block lengths. These authors were of the opinion that, at higher block lengths, the length of the inter-domain connections increased, leading to a decrease in proton conductivity.

Scheme 12

Structures of fluorinated block SPIs.

Recently, Chen et al. (93) prepared multiblock SPIs (Scheme 12, 1-18) and observed that the increase in block lengths (hydrophilic/hydrophobic ratio of 20:10 and 20:20) showed higher in-plane and through-plane proton conductivity even at a lower IEC value of 1.35 mequiv/g. The through-plane proton conductivity for the longer block length was recorded to be 29 mS/cm compared to a value of only 11 mS/cm for the smaller block length at 59°C and 27% RH, which the authors attributed to the more effective back diffusion for water at longer block length.

3.2 Sulfonated fluorinated branched polyimide

A sulfonated anhydride-terminated polyimide, NTDA-BDSA, was synthesized by Suda et al. (94), and three kinds of NTDA-BDSA polyimides with different molecular weights (M_w=59,000, 200,000, and 300,000 g/mol) were obtained. Furthermore, sulfonated fluorinated hyperbranched polyimides [SHB-PI(1), M_w=320,000 g/mol; SHB-PI(2), M_w=500,000 g/mol; and SHB-PI(3), M_w=830,000 g/mol] with core-shell structures were synthesized from the mixture of sulfonated anhydride-terminated polyimide and amine-terminated polyimide 6FDA-TAPA (M_w=44,000 g/mol). The scheme for the preparation of star-hyperbranched polyimide (SHB-PI) is shown in Scheme 13.

Scheme 13

Synthetic scheme of star-hyperbranched polyimide (SHB-PI). [Reproduced with permission from Ref. (94).]

The proton conductivities of the SHB-PI membranes were similar to or higher than those of Nafion 117 from 70% to 98% RH. The conductivity of the star-hyperbranched polyimide membranes was higher than that of Nafion 117 and was in the range of 340–510 mS/cm at 80°C and at 98% RH; the highest value of 510 mS/cm corresponded to the polyimide with an IEC value of 2.8 mequiv/g.

Interestingly, the proton conductivity value for SHB-PI(3) was 510 mS/cm, which was slightly higher than the 460 mS/cm achieved for SHB-PI(2), although the IEC value was the same (2.8 mequiv/g) for both polymers. The authors attributed a slightly higher conductivity for SHB-PI(3), which was due to more chain entanglements in SHB-PI(3) prepared from higher molecular weight of NTDA-BDSA, which formed better ionic channels for proton conduction in comparison to SHB-PI(2). However, the proton conductivity of SHB-PIs prepared in this study was lower in comparison to that of Nafion 117 at low humidity. Unfortunately, all the SHB-PI membranes broke within 1 h and their oxidative stabilities were not sufficient for any practical use.

3.3 Sulfonated fluorinated polyimide-based composites

Pan et al. (95) prepared cross-linked fluorinated polyimide-based composites with Nafion 212 and showed improvement in thermal properties, mechanical properties, and dimensional stability in comparison to Nafion 212. The in situ cross-linking was executed by adding an azide-based cross-linker to the fluorinated polyimide end-functionalized with alkynyl moieties in the presence of Nafion 212. The structure of fluorinated polyimide end-functionalized with alkynyl moieties is shown in Scheme 14. The degree of cross-linking depended on the alkynyl content present in the fluorinated polyimide. The water uptake values were reduced in the range of 8.5–11.3% at room temperature and 14.7–22.9% at 80°C due to the formation of semi-interpenetrating network via cross-linking in comparison to the water uptake of Nafion 212 (19.6% at room temperature and 28.8% at 80°C). The water uptake increased with the increase in temperature due to the high mobility of the polymer chain and to the larger free volume available to the polymer membrane at a higher temperature. The proton conductivity of the composite membranes (96.9 mS/cm at 100°C) was close to that Nafion 212 (100 mS/cm at 100°C).

Scheme 14

Structure of cross-linkable fluorinated polyimide. [Adapted from Ref. (95).]

Previously, Pan et al. (96) also synthesized fluorinated polyimide with vinyl groups as cross-linking sites and prepared composite membranes with Nafion 212 and observed a similar improvement in high temperature properties and mechanical properties, with reduction in water uptake. The proton conductivity of the composite membranes decreased with increasing degree of cross-linking and increased with increasing temperature (proton conductivity: from 20 mS/cm at room temperature to 89 mS/cm at 100°C). Pan et al. (97) prepared fluorinated polyimide with side chain carboxylic acid groups as cross-linking sites and prepared composites with Nafion 212. The composite membranes exhibited proton conductivity in the range of 23–91 mS/cm, showing an increasing trend with the increase in temperature.

4.1 Sulfonated fluorinated polybenzimidazole-based blends

Polybenzimidazoles (PBI), show excellent chemical, thermal, and mechanical stabilities in the temperature range of 100–200°C. The benzimidazole units of PBI are the amphoteric groups and can form specific interactions with acidic polymers or basic polymers, facilitating the formation of miscible blends (102). However, PBI membranes have a limitation when doped with phosphoric acid: if the doping level is too high, the membrane dissolves. An option for the stabilization of PBI against dissolution during the H₃ PO₄ doping process and for improvement of the thermal stability of PBI is to covalently or ionically cross-link PBI. Seyb and Kerres (103) developed base-excess acid-base blend membranes from PBI Celazol (Professional Plastics Inc., Fullerton, CA, USA) and a sulfonated fluorinated poly(arylene ether), and evaluated their thermal stability (blend: T_onset=426°C, M_w =10,100 g/mol) and oxidative stability [10% weight loss detected >144 h for poly(aryl ether)/PBI-Celazol]. A representative structure of a sulfonated fluorinated poly(arylene ether) used for preparing blend membrane with PBI is shown in Scheme 17 (1-21). The oxidative stability of the blend membranes was higher than that of pure PBI. The poly(arylene ether)s blended with PBI-Celazol (103) were further doped with H₃ PO₄ to allow ionic cross-linking. Poly(arylene ether)/PBI-Celazol exhibited a proton conductivity of 60 mS/cm at 180°C, and by ionic cross-linking the proton conductivity increased four times compared to the pure and doped PBI (104).

Scheme 17

Representative structures of a sulfonated fluorinated poly(arylene ether) used for preparing blend membrane with PBI.

Schönberger et al. (105) controlled the water uptake of the PBI blend membranes by using the ratio of two different bisphenol monomers, namely, 6F-bisphenol A and 2,2-bis(4-hydroxyphenyl)sulfone (6F-BPA/BHPS). The structure of sulfonated fluorinated poly(arylene ether) used for preparing the blend membrane with PBI is shown in Scheme 17 (1-22). Depending on BHPS content, the sulfonated polymers were highly swellable or even soluble in water due to the increase in water uptake and possibly due to the interaction between water molecules with polar and hydrophilic sulfone bridging groups. A series of acid-base blend membranes prepared from sulfonated polymers and PBI with an IEC value of 1.35 mequiv/g were investigated for specific proton resistance, water uptake, and onset of sulfonic acid group splitting-off temperature. The specific proton resistance decreased to as low as 3.82 Ωcm, and the water uptake increased to ∼105% with increasing content of BHPS, and the onset of the splitting-off temperature of the sulfonic acid group was ∼271°C. This lower specific resistance was caused by the strong interaction of the sulfone bridging group (–SO₂–) in meta-position to the sulfonic acid group, which resulted in higher acidity, thus leading to a lower specific proton resistance value. Hence, by choosing a suitable building block like BHPS, it is possible to obtain high proton conductivities.

Liu et al. (106) prepared blend membranes composed of sulfonated fluorinated poly(arylene thioether phosphine oxide)/sulfonated PBI (sPATPO/sPBI) in two steps. Firstly, highly soluble sulfonated fluorinated PBIs were synthesized by polycondensation of 2,2′-bis(4-carboxyphenyl)hexafluoropropane and bis(3-sulfonate-4-carboxyphenyl) sulfone with 3,3′-diaminobenzidine followed by blending with sPATPO. Several miscible blends were formed by blending sPATPO with sPBI by varying the sPBI content from 2 wt% to 6.5 wt%. The blend membranes showed no T_g and showed onset decomposition temperature at around 390°C. The blend membranes exhibited a tensile strength in the range of 31.4–42.2 MPa and Young’s modulus of 0.50–0.81 MPa with elongation at break of >20%. The oxidative stability was as high as >288 h for the blend membrane containing 6.5 wt% of sPBI. The sPBIs were employed as a blend component, which led to ionic cross-linking between sPATPO and sPBI, which increased with increasing sPBI content, leading to a reduction in swelling of blend membranes. The blend membranes showed a proton conductivity value in the range of 74–84 mS/cm at 80°C, which was higher than that of synthesized sPBIs but was still lower than that of Nafion 117 at 80°C. The proton conductivity decreased with the increase in sPBI content due to the interaction of the sulfonic acid groups and the H atom of the imidazole groups, making the sulfonic acid groups partly unavailable for proton conduction. This finding was proven by morphological studies through AFM images.