Introduction

Proton exchange membrane fuel cells (PEMFC) are being explored as the technology of choice for clean and efficient energy conversion systems for automobiles, portable applications and power generation [1–8]. Proton-exchange membrane (PEM), as proton conductive material, is a key component of the PEM fuel cells for transferring protons from the anode to cathode as well as providing a barrier to prevent the fuel cross-leaks between the electrodes. poly(perfluoroalkylsulfonic acid) (Nafion) is the most preferable from the standpoint of chemical and thermal stabilities and physical and proton-conducting properties. [7–17] However, its extremely high cost has limited to a large-scale practical use of Nafion. In this regard, several proton-exchange membranes based on sulfonated aromatic polymers have been investigated for fuel cells [17–24].

Recently, there are several reports on the proton-exchange membranes based on sulfonated poly(aryl ether ketone)s. It was reported that the polymers with acidic groups on the pendant chains were more stable against hydrolysis than those with acidic groups directly on the backbone of the polymers [25, 26].

In this paper, herein, we report the synthesis of fluorene-containing sulfonated poly(ether ether ketone) (SPEEK) with sulfonic acid groups only on the fluorenyl rings. fluorene groups can improve the oxidation resistance of polymers [26]. The ion exchange capacity (IEC), sulfonation degree (SD), equivalent weight (EW), water-uptake, oxidative stabilities, and mechanical properties as well as proton conductivities of the synthesized polymers were fully investigated.

Experimental

Materials

9,9-Bis(4-hydroxyphenyl) fluorene(BPF) (purity > 98 %) was purchased from Tokyo Kasei Kogyo Co. Ltd., Japan (TCI). 2,2-Bis(4-hydroxyphenyl) hexafluoropropane (6 F-BPA) and 4, 4′-difluorobenzophenone (DFBP) were purchased from Aldrich Reagent-grade. Dimethyl sulfoxide (DMSO), methanol, toluene, and anhydrous potassium carbonate were obtained from commercial sources. DMSO was dried over 4A molecule sieves and toluene was dried over sodium wire prior to use. Anhydrous potassium carbonate was vacuum dried at 180 °C for 10 h before use. Other reagents and solvents were used as received.

Synthesis of poly (ether ether ketone)(PEEK)

A typical procedure for the synthesis of the copolymers is as follows [26]0.1.4016 g (4 mmol) of BPF,0.9313 g (4.267 mmol) of DFBP and 0.7189 g (5.2 mmol) of potassium carbonate were added to a three-necked round bottom flask equipped with a magnetic stirrer, a nitrogen inlet, and a Dean-Stark trap. DMSO (18 mL) was introduced to afford a 13 % (w/v) solid concentration. Toluene (18 mL, usually DMSO/toluene = 1/1, v/v) was used as an azeotroping agent. Under the protection of nitrogen atmosphere, the reaction mixture was refluxed at 145 °C for 6 h to dehydrate the system. After the removal of toluene, the temperature was slowly increased to 175 °C and maintained for another 6 h. After cooling to the room temperature, 1.2511 g (5.733 mmol) of DFBP, 2.0178 g (6 mmol) of 6 F-BPA and 1.0793 g (7.8 mmol) of potassium carbonate were added with DMSO (18 mL) and toluene (18 mL). The reaction mixture was refluxed at 145 °C for 6 h to dehydrate the system. The temperature was raised slowly to 175 °C by the controlled removal of toluene. The reaction was allowed to proceed for 6 h. After the solution became very viscous, the reaction mixture was cooled to 80 °C, diluted with the addition of 15 mL DMSO and poured into 300 mL stirred methanol/water solution (1/1, v/v) to precipitate the product. The precipitate was filtered and washed several times with water to remove inorganic salts. Finally, it was vacuum dried at 80 °C for 24 h. An amount of 5.0845 g polymer of Block-6c was obtained in high yield of 97 %.

A series of poly(ether ether ketone) were successfully synthesized using the same procedure as described for synthesis of polymer block-6c. The molecular feed ratio of monomers and the polymerization results were list in Table 1.

Table 1 Polymerization results and properties of block PEEK
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Sulfonation of poly (ether ether ketone)(SPEEK)

The sulfonation of Block-7c was carried out by the reaction of Block-6c with chlorosulfonic acid as shown in Scheme 3. The SD of the Block-7c can be readily controlled by adjusting the molar ratio of the repeated units of BPF to chlorosulfonic acid. The sulfonation procedures are depicted as follows: 75 mL dichloromethane and 0.2582 g (0.2 mmol repeated units of BPF) Block-6c were introduced to a 150 mL round bottom flask. The reaction mixture was stirred vigorously until the polymer was dissolved completely. About 16 mL of 0.1 M solution of chlorosulfonic acid (1.6 mmol) in dichloromethane was then added drop-wise under the vigorously stirring at room temperature for about 3 h, meanwhile the resulting brown product gradually precipitated out from the solution. The reaction mixture was stirred vigorously for another 5 h, then the solvent of the mixture was removed using a rotary evaporator at about 40 °C. The precipitate was washed with hexane for three times (3 × 5 mL) and dissolved in 10 mL DMSO, 10 mL of 3 wt % potassium hydroxide aqueous solution was added to the solution. Reaction for another 5 h, the reaction mixture was acidified with 100 mL of 5 vol % hydrochloric acid for another 5 h. The resulting solution was dialyzed for 3 days to remove all small molecules which molecular weight was less than 8000 Da, the dialyzing water was refreshed everyday. The resulting product Block-7c was recovered by removing the water with rotary evaporator at about 60 °C.

A series of sulfonated block copolymers 7a-c with various SD were synthesized by adjusting the molar ratio of the BPF to chlorosulfonic acid according the above sulfonation procedures. Their relative physical and chemical properties were listed in Table 2.

Table 2 IEC, EW, water-uptake, linear expansion ratio and oxidative stabilities of block SPEEK
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Preparation of polymer membrane

The membranes were prepared by casting from the corresponding 8 % block polymers in DMSO solution onto a glass plate in the dust-free environment, and then dried at 80 °C for 12 h and at 90 °C under vacuum for 48 h. The thickness of all membrane samples was controlled in the range of 100–200 μm.

Measurement

Nuclear magnetic resonance (NMR) spectra was recorded at 400 MHz in a Bruker NMR instrument (model DRX) and listed in parts per million (ppm) downfield from tetramethylsilane(TMS). FT-IR spectra were recorded on a Nicolet520 Fourier transform infrared spectrometer with film samples. Tensile strength was measured at 25°C by a universal test machine (CMT-4014, SANS corporation, Shenzhen, China). The samples were prepared by cutting them into a dumbbell shape (DIN-53504-S3A) and immersing in water at 80°C for 24 h. The cross-head speed was set at a constant speed of 1 mm/min. For each testing reported, at least three measurements were taken and average value was calculated.

Inherent viscosity

Inherent viscosity was determined in 0.5 g dL−1 solution in CHCl3 at 25 °C using a calibrated Ubbelonhde viscometer.

Water uptake and swelling ratio

The water uptake of the membrane was evaluated by measuring the weight change between dried and hydrated state at 80 °C. The membranes were first dried at 150 °C in vacuum to a constant weight. The dried membranes were then immersed in water at 80 °C for 24 h until equilibrium of water absorption. The water uptake was calculated according to the following equation:

$$ \mathrm{Water}\ \mathrm{uptake}\left(\%\right)=\frac{W_{wet}-{W}_{dry}}{W_{dry}}\times 100\% $$

Where Wwet and Wdry are the mass of the wet and dry membranes, respectively. The dimension stability of the membrane was characterized by the swelling ratio, which was calculated from the following equation:

$$ \mathrm{Swelling}\ \mathrm{Ratio}\left(\%\right)=\frac{L_{wet}-{L}_{dry}}{L_{dry}}\times 100\% $$

where Lwet and Ldry are the lengths of dry and wet samples respectively.

Ion exchange capacity (IEC)

The ion-exchange capacity (IEC) was determined by titration. A piece of dried and protonized membranes was weighted, immersed and equilibrated in large excess of 2 M NaCl aqueous solution for 24 h to exchange the protons of sulfonic acid groups with sodium ions. The concentration of HCl released from the membrane sample was determined by titration with 0.01 M NaOH aqueous solution using phenolphthalein as an indicator. Ion-exchange capacity was calculated according to the following equation:

$$ \mathrm{I}\mathrm{E}\mathrm{C}\ \left(\mathrm{mequiv}/\mathrm{g}\right)=\frac{\varDelta {V}_{NaOH}\cdot {C}_{NaOH}}{W_S}\times {10}^3 $$

Where ΔV NaOH is the consumed volume of NaOH solution, C NaOH is the concentration of NaOH solution and Ws is the weight of the membrane sample.

Oxidative and hydrolytic stabilities

Oxidative Stability was tested by immersing a small piece of membrane sample in Fenton’s reagent (3 % H2O2 + 2 ppm FeSO4) in a shaking bath at 80 °C. The oxidative stability was recorded as the expanded time that the membrane began to break into pieces or fracture is observed, the membrane was observed every 10 min to make sure whether or not any change in appearance.

Proton conductivity

Proton conductivity measurement was performed by a Solartron 1255B frequency response analyzer functioning with an oscillating voltage of 10 mV using two probes with the frequency between 1 mHz and 5 kHz. Before the measurement, the membrane samples were full hydrated in de-ionized water for at least 24 h. The proton conductivity of the membrane was measured at 30 °C and 80 °C respectively. The conductivity (σ) of the samples was calculated from the impedance data, using the following equation:

$$ \sigma =d{\mathrm{R}}^{-1}{\mathrm{A}}^{-1} $$

where d and A are the thickness and face area of the sample, respectively, and R was derived from the low intersect of the high-frequency semicircle on a complex impedance plane with the Re (Z)’ axis.

Results and discussion

Polymer synthesis

The block PEEK copolymers were successfully synthesized by a two-stage one-pot method. Firstly, the oligomer terminated with fluorine atoms was first prepared from the monomers of BPF and DFBP, and then the monomers of 6 F-BPA and DFBP were in turn added to the reaction mixture in order to form block copolymers. As illustrated in Scheme 1, fluorine-terminated oligomers were first synthesized by condensation copolymerization of 9, 9-bis (4-hydroxyphenyl) fluorine (BPF) and 4, 4′-difluorobenzophenone (DFBP). Excess of DFBP was added to ensure the formation of fluorine-terminated structure. n (5, 10 or 15) equivalents of BPF were reacted with corresponding (n + 1) (5,10 or 15) equivalents of DFBP to give different segment lengths of the fluorine-terminated oligomers. The desired fluorine-terminated oligomers were confirmed using 1H NMR spectrum (Fig. 1). The characteristic peaks for the aromatic proton can be clearly observed at 6.94, 7.00, 7.15, 7.22, 7.28, 7.31, 7.42, 7.74 and 7.79 ppm. In the second stage, the monomers of 6 F-BPA and DFBP were used to continue the chain growth and condensation copolymerization. The block copolymers were prepared as shown in Scheme 2. In this case, the synthesized block PEEKs have the feed ratio of BPF to 6 F-BPA of 4:6. The block copolymers were confirmed by their 1H NMR spectrums and 13C NMR accordingly. The 1H NMR spectrums of block-6a is show in Fig. 2, with proper assignment of all the resonance peaks. The peaks d 7.85 and 7.08 ppm could be assigned to the chemical shifts of the aromatic proton of the 6 F-BPA. All the other peaks were well assigned according to its chemical structure. Figure 3 shows the 13C NMR spectrums of block-6a, with 30 the resonance peaks, representing 30 different carbon atoms. The peaks at 194.16, 194.10, 194.05 ppm assigned to the carbonyl peak, indicating that there are three types of carbonyl molecules.

Scheme 1
scheme 1

Synthesis of fluorine terminated oligomers

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Fig. 1
figure 1

H-NMR spectrum of oligomer with fluorine end-groups

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Scheme 2
scheme 2

Synthesis of block PEEKs

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Fig. 2
figure 2

H-NMR spectrum of block-6a

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Fig. 3
figure 3

C-NMR spectrum of block-6a

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Sulfonated polymers synthesis

All the polymers were sulfonated using chlorosulfonic acid in methylene chloride. The sulfonation completed at room temperature in a few hours. Because of the insolubility of the resulted products, chlorosulfonic acid must be added as slowly as possible under acutely stirring. Residual sulfonyl chloride was neutralized with aqueous potassium hydroxide in DMSO. Upon treatment with hydrochloric acid followed by dialysis, brownish polymers were obtained after evaporating the water. The sulfonated block copolymers were confirmed by their 1H NMR spectrums accordingly. The 1H NMR spectrums of block-7a is show in Fig. 4, with all the peaks assigned to the molecular structure shown in the figure. FT-IR technique is used to confirm functional groups in the synthesized polymers. As illustration in Fig. 5, sulfonic acid groups can be detected by their corresponding characteristic absorption bands as follows. The absorption bands at 1241, 1020 and 1091 cm−1 can be assigned to symmetric and asymmetric stretching vibrations of O-S-O. The peak at 675 cm−1 is due to the stretching of the S–O. It can be then concluded that the sulfonic acid groups were successfully introduced into the block copolymers as expected (Scheme 3).

Fig. 4
figure 4

H-NMR spectrum of block-7a

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Fig. 5
figure 5

IR spectrum of block-7a

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Scheme 3
scheme 3

Synthesis of block sulfonated SPEEKs

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Oxidative stabilities

The oxidative stability of the samples is listed in Table 2., the maximum anti-oxidative timing was 150 min for 7b-1 and 142 min for 7c-1, respectively. These results indicated that the membranes from 7a-1 to 7c-3 exhibited much better oxidative stability than those previously reported of fluorene-containing sulfonated poy(arylene ether sulfone) [23]. Presumably, this was resulted from that the fluorine groups can enhance the oxidative stability of the polymer. In general, the oxidative stability decrease with increasing sulfonation degree, because higher sulfonation degree can augment the swelling of the membrane, and then increase the attack opportunity of free radicals to the polymer backbone with absorbed water.

Water uptake and proton conductivity measurements

The water content and water state within the membrane are very important factors that directly affect proton transport across the membranes. Water can act as the carrier and medium for proton as it moves through the membranes. However, excess water-uptake can lead to the decrease in mechanical strength and a dimensional mismatch for fuel cell assembly. As can be seen from Table 2, the water uptakes as well as the linear expansion ratio of the membrane increased with increasing IEC, owing to the increase of the hydrophilicity. As the IEC was increased from 0.83 to 1.16, an increase in water uptake was observed for the block polymer. For example, block-7a-1 and block-7a-3 membranes with IEC 0.95 and 1.16 mequiv.g−1 exhibit the value of water uptake of 17.1 and 22.8 %, respectively. Proton conductivity of fully hydrated block membranes with different IEC was determined at 30 and 80 °C respectively. The results are listed in Table 3. As expected, the proton conductivity of the membranes increases with increasing IEC value and temperature. Block-7a-3 membrane with the highest IEC value of 1.16 mequiv.g−1 exhibits the highest conductivity among these 7a-c membranes. This can be explained that more protons participating in conduction are derived from higher content of sulfonic acid groups. For instance, block-7a-3 membrane exhibits conductivity of 4.09 × 10−2 Scm−1 at 30 °C and 5.15 × 10−2 Scm−1 at 80 °C, which is higher than block-7c-3 membrane, 2.97 × 10−2 Scm−1 at 30 °C and 3.75 × 10−2 Scm−1 at 80 °C, respectively.

Table 3 Proton conductivity and mechanical properties of block SPEEK at the relative humidity of 100 %
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Mechanical strength of membranes 1a-6b

The mechanical properties of the membranes were measured at 25 °C and 100 % relative humidity (Table 3). All samples were treated by immersing in deionized water at 25 °C for 24 h prior to test. The membranes samples of block 7a-c showed higher tensile strength (18–32 MPa) and lower strain (28–50 %). These data indicate these membranes are strong and tough enough to be applied as proton exchange membrane in the fuel cell [27].

Conclusions

Fluorene-containing block copolymer can be synthesized by a one-pot two-step methodology. All the polymers were sulfonated using chlorosulfonic acid in methylene chloride. The synthesized membranes exhibit high ion-exchange capacities, excellent mechanical properties in hydrated state, thermal stability, as well as better oxidative stabilities at relatively lower IEC value. Especially the membrane derived from block-7a-3 with IEC value of 1.16 mequiv.g−1 exhibits high conductivities of 4.09 × 10−2S/cm at 30 °C and 5.15 × 10−2 S/cm at 80 °C , respectively. In conclusion, the membranes are suitable for the fuel cell applications as potential proton exchange membrane materials.