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Synthesis and properties of highly branched star-shaped sulfonated block polymers with sulfoalkyl pendant groups for use as proton exchange membranes
Journal of Membrane Science 497 (2016) 55–66
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Synthesis and properties of highly branched star-shaped sulfonated block polymers with sulfoalkyl pendant groups for use as proton exchange membranes Huixiong Xie a, Dan Tao a, Jiangpeng Ni a, Xiongzhi Xiang a, Chunmei Gao b,n, Lei Wang a,n a b
Shenzhen Key Laboratory of Special Functional Materials, College of Materials Science and Engineering, Shenzhen University, Shenzhen 518060, PR China The Ministry-Province Jointly Constructed Base for State Key Lab-Shenzhen Key Laboratory of Chemical Biology, Shenzhen 518055, PR China
art ic l e i nf o
a b s t r a c t
Article history: Received 23 June 2015 Received in revised form 23 August 2015 Accepted 15 September 2015
Branched sulfonated copoly(ether ketone)s exhibit excellent properties for use as proton exchange membranes (PEMs). However, to date, the use of highly branched star-shaped sulfonated block polymers with flexible side chains as PEMs has not been reported. In this study, two novel types of star-shaped block poly(arylene ether sulfone)s containing sulfoalkyl pendant groups with 6% branching agent were synthesized for the first time. The properties of the polymer with hydrophilic segments surrounded by hydrophobic segments (SPAES-6s) and of the polymer with hydrophobic segments surrounded by hydrophilic segments (SPAES-6f) were investigated, with a focus on their potential use as PEMs. The block polymers SPAES-6f and SPAES-6s exhibited higher proton conductivities (0.42 S cm 1 and 0.30 S cm 1 at 80 °C) than those of the random polymer SPAES-6r and Nafion 117, and SPAES-6s exhibited better oxidative stability (435 min) and dimensional stability than SPAES-6f. The results indicate that the highly branched sulfonated block poly(arylene ether sulfone)s with sulfoalkyl pendant groups are good candidate PEM materials for fuel cell applications. & 2015 Elsevier B.V. All rights reserved.
Keywords: Branched polymer Comb-shaped Poly(arylene ether)s Oxidative stability Proton exchange membrane
1. Introduction Proton exchange membrane fuel cells (PEMFCs) have received global attention because of their high energy conversion efficiency, near zero harmful emissions and convenient operation [1,2]. As one of the key components of a PEMFC, the PEM is closely related to the performance and durability of the entire fuel cell system [3,4]. Generally, among all possible candidates, perfluorosulfonic acid polymer membranes, such as Nafion membranes, are currently considered the most suitable PEM materials because of their excellent chemical and oxidative stability [5]. However, the many disadvantages of these materials, such as their high cost, high methanol permeability and loss of conductivity at high temperatures ( 480 °C) [6], have limited their commercial applications. In recent years, many aromatic hydrocarbon polymers have been extensively explored because of their satisfactory thermal properties and electrochemical stability [7–9]. Among aromatic hydrocarbon polymeric PEMs, sulfonated poly(arylene ether)s (SPAEs) are considered one of the most promising candidates for n
Corresponding authors. E-mail addresses: [email protected] (C. Gao), [email protected] (L. Wang). http://dx.doi.org/10.1016/j.memsci.2015.09.035 0376-7388/& 2015 Elsevier B.V. All rights reserved.
PEMFC applications by virtue of their stable thermal properties and high solubility in organic solvents [2,10–12]. However, most of them have failed to be used as PEMs because of their short lifetimes due to a combination of hydrolysis and oxidative degradation. Therefore, the improvement of the durability of inexpensive membranes has become an active area of PEM research. It is widely accepted that cross-linking is an effective method of improving the oxidative stability of a membrane [13–16]. However, cross-linked membranes are generally insoluble in common organic solvents and are difficult to reprocess, which has restricted their development as commercial membranes. Recently, studies have shown that branching is also an excellent method for this purpose [17–23]. Compared with cross-linked membranes, branched membranes not only exhibit improved oxidative stability and conductivity but also demonstrate good solubility in common organic solvents [17,19,20]. However, the mechanical properties of highly branched membranes notably decrease with an increasing degree of branching (DB) and cannot satisfy the requirements for PEMs. This could be because of a decrease in chain entanglement with increasing branching caused by the hard arms and short chains between branching points [17]. A viable strategy for overcoming the disadvantages of branched PEMs is to introduce flexible side chains into the branched
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polymer, which could help to increase the entanglement of the polymer chain, thus improving its mechanical properties. Recently, a series of branched polymers with pendant acid groups on the side chains was synthesized by our group [24]. Investigation of these polymers revealed that the mechanical strength of the branched polymer with 6% branching agent was 24.3 MPa, higher than that of the corresponding branched polymer with 6% branching agent without pendant groups (18.0 MPa). In addition, the oxidative stability (420 min) of the branched polymers with pendant acid groups on the side chains was also improved compared with that of the corresponding branched polymer without pendant groups (281 min). This is attributed to the fact that in polymers [9,25–28] with pendant acid groups on their side chains, the main polymer chain can be maintained in a hydrophobic environment. Thus, the chains initially separate into nanophases, which can reduce the probability of attack by water and peroxy radicals on the main chain, leading to excellent oxidative stability. In addition, Guiver and Lee's group report that poly(arylene ether sulfone) membranes [29] containing clustered flexible pendant sulfonic acids exhibited better dimensional stability and higher proton conductivity than Nafion 117, even at elevated temperatures (80 °C) and 450% relative humidity. From the results, we can infer that branched polymers with pendant acid groups on their side chains show promise as PEMs, and it would be worthwhile to further improve their relevant properties, such as proton conductivity at low relative humidity, oxidative stability and mechanical properties. It is widely believed that the preparation of block copolymers can significantly increase phase separation [23,30–34]. Because of their well-separated microstructure, such membranes exhibit high proton conductivity and superior hydrolytic stability, especially at high temperatures and low relative humidity. To enhance the properties of branched membranes with pendant acid groups on their side chains, in this study, two types of highly branched starshaped block poly(arylene ether sulfone)s with sulfoalkyl pendant groups were successfully synthesized. The properties of membranes of these highly branched star-shaped sulfonated block copolymers with flexible side chains, including their proton conductivity, hydrolytic and oxidative stability, mechanical properties, ion-exchange capacity (IEC), water uptake, thermal stability and swelling ratio, were investigated and compared with those of their random polymer counterparts.
2. Experimental 2.1. Materials 1,3,5-Triphenylbenzene, 4-fluorobenzenesulfonyl chloride, mercaptopropionic acid (MPA), 2,6-dimethoxyphenol, 9-fluorenone, 4,4′-(hexafluoroisopropylidene)diphenol (6F BPA), 4,4′-difluorodiphenyl sulfone (DFDS), boron tribromide (BBr3), 1,4-butanesultone, dimethylacetamide (DMAc), dimethyl sulfoxide (DMSO), and dichloromethane were purchased from commercial sources and used as received. Toluene was dried using sodium wire, and the DMAc, DMSO, and dichloromethane were dried using 4 Å molecular sieves prior to use. Anhydrous potassium carbonate was dried at 300 °C for 24 h in a furnace prior to use. All other solvents and reagents were of reagent grade and were used as received. 2.2. Measurements 1 H NMR spectra, reported in ppm, were recorded using a Varian 400 Hz NMR instrument with tetramethylsilane (TMS) as the internal standard. Gel permeation chromatography (GPC) analysis
was performed using a Waters HPLC 2695 Separations Module (with tetrahydrofuran as the eluent and polystyrene as the standard). The thermal stability of the polymers was investigated over a temperature range from 50 °C to 600 °C at a heating rate of 10 °C/min using a Q50 thermogravimetric analysis (TGA) under a nitrogen flow of 50 mL/min. Tapping-mode atomic force microscopy (AFM) images were acquired using a Dimension Icon scanning probe microscope. 2.3. Synthesis of 9,9-bis(3,5-dimethoxy-4-hydroxyphenyl)fluorene (DMHF) DMHF was synthesized from 2,6-dimethoxyphenol and 9-fluorenone using a method reported previously in the literature [29]. Pure white powder was obtained with an overall yield of 63% after purification via recrystallization from toluene (Mp: 186– 187 °C). 2.4. Synthesis of 1,3,5-tris[4-(4-fluorophenylsulfonyl)phenoxy] benzene (B3) B3 was synthesized from 1,3,5-triphenylbenzene and 4-fluorobenzenesulfonyl chloride using a method reported previously in the literature [22]. A white powder was obtained with an overall yield of 84.8% after purification via recrystallization from acetic acid (Mp: 143–144 °C). 2.5. Synthesis of the oligomer b6f 6F BPA (0.900 g, 2.68 mmol), 4,4′-difluorodiphenyl sulfone (0.589 g, 2.32 mmol), B3 (0.187 g, 0.24 mmol), potassium carbonate (0.552 g, 4.00 mmol), DMAc (7 mL) and toluene (8 mL) were carefully introduced into a 50 mL three-neck round-bottom flask equipped with a Dean–Stark trap and a condenser under nitrogen protection. The reaction mixture was stirred at 140 °C for 4 h. After the removal of the toluene, the reaction temperature was increased to 170 °C, and the reaction was continued for 2 h. After cooling to room temperature, the mixture was poured slowly into 150 mL of water that contained 4 mL of concentrated HCl to precipitate the formed polymer. The precipitates were filtered and washed with water three times to remove any inorganic salts. The fibrous polymer was collected and dried at 110 °C under vacuum for 24 h. 1 H NMR (300 MHz, CDCl3; ppm): 8.02 (d, 0.33H), 7.90 (d, 0.86H), 7.77 (d, 0.26H), 7.39 (d, 0.86H), 7.08 (d, 1H), 7.01(d, 1H). 2.6. Synthesis of the highly branched block polymer MPAES-6f containing methoxy groups 6F-BPA (0.900 g, 2.68 mmol), 4,4′-difluorodiphenyl sulfone (0.589 g, 2.32 mmol), B3 (0.187 g, 0.24 mmol), and potassium carbonate (0.552 g, 4.00 mmol) were added to a 50 mL three-neck round-bottom flask equipped with a Dean–Stark trap. Then, DMAc (7 mL) and toluene (8 mL) were added to the flask under nitrogen. The reaction mixture was stirred at 140 °C for 4.0 h. After the water was removed, the reaction temperature was increased to 170 °C, and the reaction was continued for 2 h. After the mixture had cooled to 120 °C, 4,4′-difluorodiphenyl sulfone (0.335 g, 1.32 mmol), DMHF (0.620 g, 1.32 mmol), potassium carbonate (0.276 g, 2.00 mmol), DMAc (6 mL) and toluene (8 mL) were carefully introduced into the flask. The reaction mixture was again heated to 140 °C for 4.0 h for water removal, and the temperature was then increased to 170 °C. The reaction mixture was maintained at this temperature for 2.5 h. After cooling to room temperature, the mixture was slowly poured into 200 mL of water that contained 5 mL of concentrated HCl to precipitate the formed
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polymer. The precipitates were filtered and washed with water three times to remove any inorganic salts. The fibrous polymer was collected and dried at 110 °C under vacuum for 24 h. 1 H NMR (300 MHz, CDCl3; ppm): 7.98 (d, 0.33H), 7.90 (d, 0.86H), 7.83–7.72 (m, 0.76H), 7.34–7.50 (m, 1.86H), 7.01–7.10 (m, 2H), 6.82–6.90 (m, 0.5H), 6.47 (s, 0.5H), 3.54 (s, 1.48H, –OCH3). 2.7. Synthesis of the oligomer b6s DMHF (0.620 g, 1.32 mmol), 4,4′-difluorodiphenyl sulfone (2.440 g, 0.96 mmol), B3 (0.187 g, 0.24 mmol), potassium carbonate (0.273 g, 1.98 mmol), DMAc (6 mL) and toluene (7 mL) were carefully introduced into a 50 mL three-neck round-bottom flask equipped with a Dean–Stark trap and a condenser under nitrogen protection. The reaction mixture was stirred at 140 °C for 4.0 h. After the removal of the toluene, the reaction temperature was increased to 170 °C, and the reaction was continued for 2.5 h. After cooling to room temperature, the mixture was poured slowly into 150 mL of water that contained 4 mL of concentrated HCl to precipitate the formed polymer. The precipitates were filtered and washed with water three times to remove any inorganic salts. The fibrous polymer was collected and dried at 110 °C under vacuum for 24 h. 1 H NMR (300 MHz, CDCl3; ppm): 7.96–7.99 (d, 0.81H), 7.80 (d, 1.99H), 7.48 (d, 1H), 7.33–7.43 (m, 2.45H), 6.82–6.90 (m, 2H), 6.47 (s, 2H), 3.54 (s, 6H, –OCH3). 2.8. Synthesis of the highly branched block polymer MPAES-6s containing methoxy groups DMHF (0.620 g, 1.32 mmol), 4,4′-difluorodiphenyl sulfone (2.440 g, 0.96 mmol), B3 (0.187 g, 0.24 mmol), and potassium carbonate (0.273 g, 1.98 mmol) were introduced into a 50 mL threeneck round-bottom flask equipped with a Dean–Stark trap. Then, DMAc (6 mL) and toluene (7 mL) were added to the flask under nitrogen. The reaction mixture was stirred at 140 °C for 4.0 h. After the water was removed, the reaction temperature was increased to 170 °C, and the reaction was continued for 2.5 h. After the mixture was cooled to 120 °C, 4,4′-difluorodiphenyl sulfone (0.680 g, 2.68 mmol), 6F BPA (0.900 g, 2.68 mmol), potassium carbonate (0.555 g, 4.02 mmol), DMAc (7 mL) and toluene (8 mL) were carefully introduced into the flask. The reaction mixture was again heated at 140 °C for 4.0 h for water removal, and the temperature was then increased to 170 °C. The reaction mixture was maintained at this temperature for 3 h. After cooling to room temperature, the mixture was poured slowly into 200 mL of water that contained 5 mL of concentrated HCl to precipitate the formed polymer. The precipitates were filtered and washed with water three times to remove any inorganic salts. The fibrous polymer was collected and dried at 110 °C under vacuum for 24 h. 1 H NMR (300 MHz, CDCl3; ppm): 8.02–8.05 (d, 0.40H), 7.92 (d, 2.03H), 7.74–7.85 (m, 0.99H), 7.36–7.52 (m, 3.75H), 7.03–7.12 (m, 4H), 6.84–6.90 (m, 1H), 6.49 (s, 1H), 3.56 (s, 3H, –OCH3). 2.9. Synthesis of the highly branched random polymer MPAES-6r containing methoxy groups For comparison with the branched block poly(arylene ether sulfone)s, the branched random polymer MPAES-6r was synthesized. 6F BPA (0.900 g, 2.68 mmol), DMHF (0.620 g, 1.32 mmol), 4,4′-difluorodiphenyl sulfone (0.924 g, 3.64 mmol), B3 (0.187 g, 0.24 mmol), and potassium carbonate (0.828 g, 6 mmol) were introduced into a 50 mL three-neck round-bottom flask equipped with a Dean–Stark trap. Then, DMAc (10 mL) and toluene (10 mL) were added to the flask under nitrogen. The reaction mixture was stirred at 140 °C for 4.0 h. After the water was removed, the
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reaction temperature was increased to 170 °C, and the reaction was continued for 3.5 h. After cooling to room temperature, the mixture was poured slowly into 200 mL of water that contained 5 mL of concentrated HCl to precipitate the formed polymer. The precipitates were filtered and washed with water three times to remove any inorganic salts. The fibrous polymer was collected and dried at 110 °C under vacuum for 24 h. 1 H NMR (300 MHz, CDCl3; ppm): 7.95–8.03 (d, 0.40H), 7.71– 7.91 (m, 3.02H), 7.47 (d, 0.5H), 7.38 (m, 2.75H), 7.33 (d, 0.5H), 6.98– 7.09 (m, 4.06H), 7.81–6.88 (m, 1H), 6.48 (s, 1H), 3.55 (s, 3H, –OCH3). 2.10. Conversion of methoxy groups (MPAES-6x) into hydroxyl groups Polymers containing hydroxyl groups were synthesized via a demethylation reaction; the reaction procedure for the MPAES-6x (6x ¼6f, 6s and 6r) polymers is described below. A typical experimental procedure (MPAES-6f) for MPAES was as follows: a sample of 2.000 g of MPAES-6f was dissolved in 100 mL of anhydrous dichloromethane in a 500 mL three-neck flask with a nitrogen inlet. BBr3 (3 mL) was mixed with dichloromethane (30 mL), and the mixture solution was added dropwise to the MPAES-6f solution at 0 °C (ice bath). After 6 h, the resulting copolymer was filtered, washed with methanol and deionized water, recovered, and dried under vacuum at 100 °C for 24 h to finally obtain the product HPAES-6f (yield: 93%). 1 H NMR (300 MHz, DMSO-d6; ppm): 9.44 (s, 0.5H, –OH), 7.94– 8.06 (m, 1.09H), 7.76–7.83 (m, 0.76H), 7.38–7.44 (m, 1.86H), 7.20 (d, 2H), 6.68–6.86 (m, 0.5H), 6.27 (s, 0.5H). 2.11. Preparation of sulfonated copolymers (SPAES-6x) The synthesis of the SPAES-6f polymer, a typical example of the synthesis of SPAES-6x, is described below. HPAES-6f (1.000 g) and 0.300 g of NaOH were introduced into a 100 mL three-neck roundbottom flask. Then, DMSO (30 mL) was added to the flask under nitrogen. The reaction mixture was stirred at room temperature for 2 h. Then, 2 mL of 1,4-butanesultone was added, and the reaction was heated to 100 °C for another 12 h. The resulting polymer was obtained by carefully pouring the solution into 300 mL of isopropanol, filtering, washing thoroughly with water, and drying under vacuum at 80 °C for 10 h (yield: 92%). 1 H NMR (300 MHz, DMSO-d6; ppm): 7.94–8.01 (t, 1.09H), 7.76– 7.80 (m, 0.64H), 7.19–7.37 (d, 2.88H), 6.93 (d, 0.5H), 6.45 (d, 0.5H), 3.73 (s, 1H), 2.65 (s, 1H), 1.71(s, 1H), 1.43 (d, 1H). 2.12. Film casting and membrane acidification The dried sulfonated copolymer was dissolved in DMAc (10 mL) under stirring at room temperature for 12 h. Then, the solution was filtered using a funnel and cast onto flat glass slides. After careful drying at 60 °C for 24 h under vacuum, tough, smooth membranes were obtained. The resulting membranes were acidified with a 1 M H2SO4 solution for 24 h to exchange Na þ with H þ . Finally, the membranes were immersed in deionized water overnight to eliminate excess H2SO4 and were stored in deionized water for testing purposes. 2.13. Water uptake and dimensional stability Prior to absorption, the membranes were first dried under vacuum at 80 °C for 24 h to determine their masses and were then immersed in deionized water at different temperatures for 24 h to determine their masses after absorption. The water uptake of each membrane was calculated in weight percent using Eq. (1):
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WU = ( Ws − Wd )/Wd × 100%
(1)
where Wd and Ws are the masses of the membrane before and after water absorption, respectively. The swelling ratio was calculated in terms of the linear expansion rate of the wet membrane using Eq. (2):
Swelling ratio (%) = ( L s − L d )/L d × 100%
(2)
where Ld and Ls are the thicknesses of the membrane before and after water absorption, respectively. 2.14. Proton conductivity and ion-exchange capacity The proton conductivity of each hydrated film was measured using an impedance analyzer (Solartron 1260A) with an oscillating voltage of 10 mV and a frequency range of 10 MHz to 500 Hz. The proton conductivity measurement of each fully hydrated membrane was performed with the cell immersed in liquid water. The temperature range tested in the impedance measurements was from 30 °C to 90 °C. The proton conductivity was calculated using Eq. (3):
σ = d/RS
(3) 1
where s is the proton conductivity (S cm ), d is the distance between the two electrodes (d¼ 1.88 cm), R is the resistance of the membrane, and S is the cross-sectional area of the membrane (cm2). The IEC values of the membranes were determined via titration. Each membrane was immersed in a 2 M NaCl solution at room temperature for 24 h for the exchange of H þ ions with Na þ ions. The exchanged protons within the solution were titrated with a 0.01 M NaOH solution using phenolphthalein as an indicator. The IEC was calculated from the titration data using Eq. (4):
(
)
IEC meq g −1 = (MNaOH × VNaOH )/Wd
(4)
where MNaOH is the molarity of NaOH, VNaOH is the consumed volume of NaOH, and Wd is the weight of the dry membrane. 2.15. Oxidative and hydrolytic stability A small piece of each membrane sample was soaked in Fenton's reagent (3% H2O2 containing 2 ppm FeSO4) at 80 °C. The oxidative stability was evaluated by recording the time at which the membrane began to disintegrate. The hydrolytic stability was also investigated by placing a membrane sample in boiling water. 2.16. Hydration number and membrane density The hydration number (λ) refers to the number of water molecules absorbed per sulfonic acid group and was calculated by combining the water-uptake and IEC data. Using the dry (Wd) and wet (Ws) membrane weights, the λ values were calculated using Eq. (5):
λ = ⎡⎣ ( Ws − Wd )/18⎤⎦ × 1000/( Wd × IEC) = (WU × 1000)/(18 × IEC)
(5)
The membrane density was calculated based on measurements of the membrane dimensions and weight after drying at 110 °C for 24 h in accordance with a method previously reported in the literature [35].
3. Results and discussion 3.1. Synthesis and characterization of the oligomers and block copolymers To achieve suitable film performance, the sulfonation of the polymers was set to 66%, and the DB of the polymers was chosen to be 6% because of the excellent properties of highly branched polymers for use as PEMs [24,29]. Highly branched block poly (arylene ether sulfone)s containing methoxy groups (MPAES-6f and MPAES-6s) were synthesized via two-step direct polycondensation using different feed sequences, resulting in copolymers with two different block structures, as shown in Schemes 1 and 2. As illustrated in Scheme 1, the oligomer b6f was synthesized via a nucleophilic substitution reaction to form the core of the branched structure in the first step. Controlling the reaction time is very important for successful core synthesis. When the reaction time was longer than 2 h at 170 °C, the oligomer b6f was easily cross-linked because of the relatively high B3 content. Therefore, the reaction time for b6f in the first reaction step was set to 2 h. For the same reason, the reaction time for b6s was set to 2.5 h. High-molecular-weight polymers b6f and b6s (Mn4 10,000 g/ mol) with good solubility were obtained, as shown in Table 1. Because of the high reaction activity of the monomers, including DMHF and 4,4′-difluorodiphenyl sulfone, the block polymers were synthesized by allowing the reactions in the second step to progress for only 3.5 h at 170 °C. When the reaction time of the second step exceeded 4 h, the branched block polymers MPAES-6f became cross-linked during polymerization and were not soluble in polar organic solvents. The reaction time of the MPAES-6s block polymer was also determined using the same method. In addition, the random polymer MPAES-6r was synthesized via a one-step direct polycondensation reaction for comparison. The chemical structures and compositions of the synthesized copolymers were characterized via 1H NMR spectroscopy with DMSO-d6 and CDCl3 as the solvent. The 1H NMR spectra of the polymer b6f and the block polymer MPAES-6f are shown in Fig. 1. The peaks at 8.04 and 7.98 ppm suggest that the B3 moiety was introduced into the branched polymers b6f. New peaks at approximately 3.54, 6.47, 6.82–6.89, 7.34, 7.50 and 7.83 ppm, which were absent in the b6f spectrum, were observed in the spectrum of polymer MPAES-6f, with the signal at 3.54 ppm corresponding to –OCH3. These peaks were attributed to the introduction of DMHF. The spectra of polymer b6s and block polymer MPAES-6s are shown in Fig. 2. New peaks at approximately 7.03–7.12 and 7.92 ppm, which were not present in the b6s spectrum, were observed in the spectrum of polymer MPAES-6f, suggesting the formation of a block structure. The conversion of the –OCH3 into reactive –OH was realized by using BBr3 to graft flexible pendant sulfoalkyl groups. Although the copolymers MPAES-6x were completely soluble in dichloromethane, the resulting copolymers containing –OH were insoluble because of the polar nature of the hydroxyl groups. The precipitate was collected, and the yield was 96%. In the last step of the reactions, the parent polymers containing hydroxyl groups in the ortho-position of the aromatic ring were sulfopropylated through a nucleophilic ring-opening reaction with 1,4-butanesultone using NaOH to obtain the final polymers SPAES6x, as shown in Schemes 1 and 2. The chemical structures of the SPAES-6x polymers were confirmed by their 1H NMR spectra (Figs. 3 and 4). As expected, the signals of the four sulfobutyl methylene groups appeared at lower frequencies. Interestingly, the proton of H8 in the spectrum of polymer SPAES-6f and that of H4 in the spectrum of polymer SPAES-6s appeared as broad multiplets because the steric hindrance of the bulky sulfobutyl groups
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Scheme 1. Synthesis of highly branched sulfonated block poly(arylene ether sulfone)s with the 6f structure.
prevented rotational freedom of the benzene ring, providing further evidence supporting the formation of SPAES-6x containing sulfobutyl groups. High-molecular-weight polymers (Mn 4500,000 g/mol) were prepared, as shown in Table 1, and the molecular weight distributions (PDIs) ranged from 1.04 to 1.17, consistent with the results of the polycondensation reaction. As shown in Table 2, the sulfonated polymers were readily soluble in polar organic solvents, such as DMAc and DMSO, and produced flexible and transparent films upon polymer solution casting. Diagrams of SPAES-6f and SPAES-6s are shown in Fig. 5. In SPAES-6f, the pendant sulfoalkyl groups generally avoid the core of the branching agent to form a block structure; by contrast, the pendant sulfoalkyl groups of SPAES-6s are located near the branching points. These unique structures affect the performance of the polymer films. Generally, block copolymers exhibit significantly increased phase separation and thus demonstrate improved properties as PEMs [23,30–32,36]. Compared with the random polymer SPAES-6r, the branched sulfonated block polymers may exhibit clearer phase segregation between the ionic groups and the polymer backbone because of their uniquely designed structure, thus endowing them with excellent properties. 3.2. Membrane microstructure and density It is widely believed that the properties of membranes are closely related to their microstructures. To gain a comprehensive
understanding of the membranes, the microstructures of the membranes was examined via AFM (Fig. 6). The bright and dark regions in the images were assigned to the hard structures corresponding to hydrophobic domains and the soft structures corresponding to hydrophilic domains with sulfonic acid and water, respectively [37,38]. Generally, sulfonated block copolymers are known to exhibit well-defined phase-separated morphologies of incompatible domains that form continuous channels, thus enabling the rapid transport of protons [30]. Compared with the block polymers SPAES-6f and SPAES-6s, the random membrane SPAES-6r exhibited less pronounced phase separation, and its proton-conducting channels were relatively narrow (3–5 nm), leading to a relatively low proton conductivity. As expected, a well-defined phase separation between the hydrophilic and hydrophobic domains was observed in the block membranes. Wide proton-conducting channels formed by hydrophilic domains are beneficial for the movement of protons and improve the proton conductivity of PEMs. Compared with the block membrane SPAES6s, which possessed moderate block sizes (7–9 nm), the block membrane SPAES-6f exhibited excessively large hydrophilic ionic domains with sizes in the range of 10–14 nm. We can infer that the block membrane SPAES-6f should demonstrate higher proton conductivity than the SPAES-6s membrane. Table 3 compares the densities of the copolymer membranes as measured using a previously reported method [35]. The density of the block membrane SPAES-6f was the lowest among all branched membranes because of the unique microstructure of SPAES-6f. The
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Scheme 2. Synthesis of highly branched sulfonated block poly(arylene ether sulfone)s with the 6s structure.
Table 1 Properties of the polymers and oligomers.
SPAES-6r SPAES-6f SPAES-6s b6f b6s a
Yield (%)
Mna ( 104)
Mwa ( 104)
PDI
Td5% (°C)
92 90 91 95 96
64.0 62.2 76.7 1.7 1.2
74.9 65.0 85.4 2.2 1.5
1.17 1.04 1.11 1.29 1.25
257 269 255 – –
Measured at 40 °C using THF as a solvent.
branched cores connect the hydrophobic segments with long, hard arms, leading to an increase in the free volume of the branched membranes; moreover, the hydrophilic segments are located on the exterior of the branched block polymer. The sulfonic groups of the hydrophilic segments present considerable steric hindrance and force the polymer chain to close, further increasing the free volume of the membrane. This increased free volume of SPAES-6f may help to trap water, leading to high water uptake and proton conductivity. 3.3. Thermal and mechanical properties The thermal properties of the block polymer and random copolymer membranes were measured via TGA at a heating rate of 10 °C/min under a nitrogen atmosphere. All samples were
preheated to 150 °C for 20 min in the TGA furnace to remove any bound water. Two weight loss transitions of the sulfonated copolymer membranes were evident from the TGA curves (Fig. 7). The first weight loss observed for the polymers at approximately 230– 280 °C was attributed to the degradation of the sulfonic acid group of the side chains. The polymer residues in the second weight loss region (at temperatures of 4370 °C) were further degraded, corresponding to the decomposition of the main chains of the polymers. The 5% weight loss temperatures of the sulfonated polymers are listed in Table 1, and the results show that the branched sulfonated polymers exhibited excellent thermal stability under conditions applicable to medium-temperature fuel cells. Table 3 lists the mechanical properties of the membranes measured at room temperature. The polymer membranes had tensile strengths in the range of 24.17–27.57 MPa, and the tensile strengths of the block membranes were greater than those of the random membranes, which were also higher than that of Nafion 117 (25.7 MPa). The three membranes exhibited elongations at break of 10.32%, 10.79% and 9.73%, respectively, and tensile moduli in the range of 0.45–0.58 GPa. Although branched structures in polymer membranes should impair their mechanical properties, the synthesized branched polymers nevertheless exhibited better mechanical properties. This is because the introduction of flexible side chains increased the entanglement of the polymer chains, thus improving the mechanical properties of the branched membranes. The results indicate that branched block polymers with flexible side chains can satisfy the mechanical criteria for
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Fig. 1. 1H NMR spectra of the b6f polymer b6f and polymer MPAES-6f.
application as PEM materials. 3.4. Ion-exchange capacity, water uptake and dimensional stability The IEC of a membrane plays an important role in determining its water uptake, swelling ratio and proton conductivity, which represents the amount of exchangeable protons in the PEM. The IECs of the tested membranes were determined via classical acid– base titration (Table 4). The experimental IEC values agreed with the theoretical values (1.60–1.67 meq/g), representing the quantitative reaction yields for methoxy conversion and sulfobutylation. These results also confirmed that the DMHF in the obtained MPAES-6x was consistent with the feed ratios used in the copolymerization, indicating that no side reactions occurred. The properties of water uptake and water transport through membranes are of paramount importance in PEM applications. Water uptake was evaluated by measuring the weight ratio between each membrane after water absorption upon immersion in water and the dry membrane. Fig. 8 shows the water uptake of the
block and random polymers at various temperatures. The IEC is one of the most important factors influencing the water uptake value, and the values for the three synthesized polymers were similar. However, the water uptake values of the resulting membranes were very different. The block polymer membrane SPAES6f exhibited the highest water uptake among the three SPAES-6x membranes, whereas the water uptake of block membrane SPAES6s was slightly less than that of the SPAES-6r membrane. This difference in water uptake is attributed to the unique microstructure of the branched membranes. As shown in Fig. 5, the hydrophobic cores of block polymer SPAES-6f are surrounded by hydrophilic segments. Sulfonic groups in the hydrophilic segments of SPAES-6f can easily absorb water molecules, whereas the inner hydrophobic core contains branched structures with long, rigid arms, which effectively increase the free volume of the three-dimensional structure, leading to greater capacity for water uptake. These two factors may be responsible for the high water uptake and the larger hydrophilic ionic domains of SPAES-6f. Although the SPAES-6s polymer contains more highly aggregated hydrophilic
Fig. 2. 1H NMR spectra of polymer b6s and polymer MPAES-6s.
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Fig. 3. 1H NMR spectrum of polymer SPAES-6f.
segments, the hydrophilic cores of the SPAES-6s block polymer are surrounded by hydrophobic segments. It is difficult for these hydrophobic segments to absorb water molecules, and the free volume available to trap water molecules in the hydrophilic cores is relatively small, leading to low water uptake and relatively small hydrophilic domains. This result suggests that the microstructures of the polymers play an important role in influencing water uptake in the branched membranes. The dimensional stability of the branched membranes was evaluated by comparing the length or thickness of the hydrated and dry-state membranes. Fig. 9 shows that the swelling ratios of the membranes increased with increasing temperature. All copolymer membranes exhibited a lower in-plane swelling ratio compared with the through-plane swelling ratio, and a similar tendency was observed in the water uptake at different temperatures, possibly because the branched structures suppressed the movement of the polymer chains, preventing membrane swelling. The block membranes exhibited strongly anisotropic swelling behavior because of the high content of hydrophilic and hydrophobic domains in the phase-separated regions, with larger dimensional changes in the through-plane direction than in the in-plane direction (Fig. 10). The anisotropic swelling behavior of the block membranes may suggest the formation of ordered hydrophilic domains within the copolymers, and membrane swelling occurred predominantly in the thickness direction, suggesting the presence of a nanophase-separated block structure [39,40]. Moreover, because of the excessively large hydrophilic ionic domains of SPAES6f, the regularity of the SPAES-6s microstructure was superior to
Table 2 Solubilities of the polymers and oligomers.
SPAES-6r SPAES-6f SPAES-6s b6f b6s a b
DMAc
DMSO
DMF
NMP
H2O
CH3OH
CHCl3
þa þ þ þ þ
þ þ þ þ þ
þ þ þ þ þ
þ þ þ þ þ
b
þ þ
Soluble. Insoluble.
that of SPAES-6f, probably leading to more strongly anisotropic behavior than that in SPAES-6f, and the swelling ratio in the through-plane direction was nearly two times that in the in-plane direction. The random polymer SPAES-6r exhibited mildly anisotropic membrane swelling due to the sulfonic groups on the side chains, giving rise to a slight phase separation. Furthermore, the block polymer SPAES-6s exhibited better dimensional stability, perhaps because of the unique microstructure of SPAES-6s. In this structure, the hydrophilic cores are surrounded by hydrophobic segments, thereby limiting the swelling of the membrane and leading to better dimensional stability. 3.5. Proton conductivity Generally, a conductivity higher than 10 2 S cm 1 at room temperature is required for PEM materials; this is the lowest value
Fig. 4. 1H NMR spectrum of polymer SPAES-6s.
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Fig. 5. Diagrams of the SPAES-6s and SPAES-6f polymers.
Fig. 6. AFM phase images of the block and random polymer membranes.
Table 3 Mechanical properties, densities and oxidation stabilities of the block and random polymers. Tensile strengtha (MPa) SPAES-6r 24.17 SPAES-6f 26.32 SPAES-6s 27.57 a b c
Young's modulusa (MPa)
Elongation at break a (%)
Densityb (g cm 3)
Oxidation stabilityc (min)
458 526 583
10.32 10.79 9.73
1.34 1.19 1.29
420 402 435
Measured at room temperature. Based on the dry state. Measured in Fenton's reagent at 80 °C.
of practical interest for materials to be used as PEMs in fuel cells [4,41]. From Fig. 11, it can be seen that all obtained membranes exhibited conductivities higher than 10 2 S cm 1 at 30 °C and 100% relative humidity, surpassing that of Nafion 117 (Table 4). Sulfonic acid groups on the flexible aliphatic side chains of these polymers could reduce the influence of the main-chain rigidity on the mobility of sulfonic acid groups, thereby enabling the formation of an ionic network and consequently enhancing proton conductivity. The proton conductivity of a membrane is strongly related to its IEC value, water uptake and microstructure. Considering that the target membranes possessed similar IEC values, the differences in their conductivities are believed to have resulted from the differences in water uptake among the polymers and the polymer microstructures. The obtained SPAES-6f with the high water absorption and phase separation, also demonstrated the highest proton conductivity (0.45 S cm 1 at 90 °C). Although the block membrane SPAES-6s demonstrated slightly lower water uptake than the random SPAES-6r membrane, the conductivity of
Fig. 7. TGA curves of the block and random polymers.
SPAES-6s was higher than that of SPAES-6r. Because the block polymer structure is beneficial for the aggregation of ionic clusters, which in turn leads to more obvious hydrophilic/hydrophobic separation and the formation of continuous channels, both block membranes exhibited superior proton conductivity to that of SPAES-6r. Moreover, compared with the proton conductivity of the corresponding linear copolymer (0.25 S cm 1 at 80 °C, IEC¼1.70 meq g 1) [29], the branched polymers exhibited higher proton conductivity in the range of 0.26–0.42 S cm 1, as shown in Table 4. All three membranes exhibited a similar dependence of the conductivity on the relative humidity at 80 °C, as shown in Fig. 12. Additionally, the proton conductivity of the membranes
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Table 4 Properties of the polymer membranes.
SPAES-6r SPAES-6f SPAES-6s Nafion 117
Water uptakea (wt%)
IECb (meq g 1)
IECc (meq g 1)
λ
Conductivityd (S cm 1)
52.5 59.5 46.5 30.6
1.69 1.69 1.69 –
1.66 1.60 1.67 0.91
17.5 20.6 15.5 18.6
0.26 0.42 0.30 0.11
a
Measured at 80 °C. Theoretical IEC. c Experimental IEC. d Measured for membrane samples at 80 °C and 100% relative humidity. b
Fig. 10. Dimensional swelling of the block and random polymer membranes at 30 °C.
Fig. 8. Water uptake of the block and random polymer membranes.
Fig. 11. Proton conductivities of the block and random polymer membranes.
Fig. 9. In-plane and through-plane swelling ratios of the block and random polymer membranes.
increased with increasing relative humidity. The corresponding reduction in proton transport capability with decreasing relative humidity is associated with the evaporation of proton carrier water molecules [13]. From Fig. 12, we can observe that the proton conductivities of the block membranes were greater than that of the random SPAES-6r membrane at different relative humidities because of the obvious phase separation of the block structure. These results may be explained by the improved water retention capability facilitated by the higher acidity of the hydrophilic domains and the existence of well-connected ionic channels [30]. This finding illustrates the positive effect of obvious phase
Fig. 12. Relative humidity dependences of the proton conductivities of the block and random polymer membranes and Nafion 117 at 80 °C.
separation on the proton conductivity under partially humidified conditions. However, the values were lower than those of the reference sample of Nafion 117 at low relative humidity, likely because of the relatively large hydrophilic domains, which possessed a weak capacity for water retention compared with the narrower
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thereby improving the oxidative stability of the polymer. By contrast, SPAES-6f, with its hydrophilic segments on the periphery, was more easily oxidized and exhibited poor oxidative stability. The hydrolytic stability was also investigated by immersing the membranes in boiling water. All membranes remained whole and unchanged in shape or appearance, even after 8 days, indicating that no hydrolysis occurred during this treatment.
4. Conclusions
Fig. 13. The Arrhenius-type temperature-dependent proton conductivity (s) behavior of the block and random polymer membranes.
hydrophilic domains. The hydration number (λ), which is the number of water molecules per sulfonic acid group, is presented in Table 4. The λ and water uptake values of the membranes exhibited similar behaviors because of the similar IEC values, which also reflected the proton conductivity of the branched membranes. Furthermore, although the λ of Nafion 117 is higher than those of SPAES-6r and SPAES-6s, the conductivities of the resulting SPAES6x membranes were superior because of the branched structure and flexible side chains of the branched polymers. Fig. 13 also displays the Arrhenius plots of the conductivities of the analyzed membranes in the temperature range of 30–90 °C. The activation energy (Ea) of conductivity was calculated by linearly fitting the Arrhenius equation, s¼ Aexp( Ea/RT). The activation energy for proton conduction was found to be in the range of 10.64–13.80 kJ mol 1. These high activation energy values suggest that proton conductivity increases significantly with increasing temperature. The activation energy of the block polymer SPAES-6f was 13.80 kJ mol 1, which is higher than that of SPAES-6s or SPAES-6r. This may be attributed to the enhancement of water absorption with increasing temperature induced by the hydrophilic segments of SPAES-6f.
Two novel types of highly branched star-shaped block copolymers were successfully synthesized at high yield via a facile condensation reaction. Sulfonated poly(arylene ether sulfone)s with four sulfoalkyl pendant groups were obtained through methoxy conversion and sulfobutylation. These polymers possessed high molecular weights and could form tough and flexible membranes via solvent casting. The sulfonated block polymers SPAES-6s and SPAES-6f exhibited excellent proton conductivities and tensile strengths, which were comparable to or even higher than those of Nafion 117. The block polymer SPAES-6s, with hydrophilic cores surrounded by hydrophobic segments, displayed excellent oxidative and dimensional stability, whereas block polymer SPAES-6f, with hydrophobic cores surrounded by hydrophilic segments, exhibited excellent proton conductivity and high water uptake. The combination of the facile synthetic routes for these polymers and their good proton transport, antioxidant capacity and reasonably good mechanical properties make these membranes attractive for further investigation for use as PEM materials in fuel cell applications.
Acknowledgments The authors gratefully acknowledge the National Natural Science Foundation of China (Nos. 51003060, 51171117 and 51101103), Shenzhen Sci & Tech research grant (JC20110, 42100070A and ZYC201105170225A) and the Special Fund of the Central Finance for the Development of Local Universities (No. 000022070140) for financial support.
Appendix A. Supplementary material 3.6. Oxidative and hydrolytic stability Free radicals, such as oxygen, hydroxide and peroxide, typically attack the hydrophilic domains of polymer membranes, resulting in degradation of the polymer chain. To evaluate whether the SPAES-6x membranes are capable of withstanding strong oxidizing environments during fuel cell operation, the oxidative stability of the branched membranes was evaluated by measuring the elapsed time before a membrane sample began to disintegrate after immersion in Fenton's reagent (2 ppm FeSO4 in 3% H2O2) at 80 °C. As shown in Table 3, the branched SPAES-6x membranes with pendant sulfoalkyl groups exhibited better oxidative stability (4 400 min) than the corresponding linear polymer (210 min) or the branched membranes without sulfoalkyl side chains (280 min) [17,29]. This improved stability is attributed to the branched structure and the flexible side chains, which effectively separate the sulfonic acid groups from the polymer main chain to form highly concentrated hydrophobic domains that prevent or reduce the probability of radical attack, leading to better oxidative stability. From Table 3, we also observe that the block membrane SPAES-6s exhibited the best oxidative stability (435 min). This is because the hydrophilic cores of SPAES-6s are surrounded by hydrophobic segments, which can protect the hydrophilic segments,
Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.memsci.2015.09. 035.
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Contents lists available at ScienceDirect
Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci
Synthesis and properties of highly branched star-shaped sulfonated block polymers with sulfoalkyl pendant groups for use as proton exchange membranes Huixiong Xie a, Dan Tao a, Jiangpeng Ni a, Xiongzhi Xiang a, Chunmei Gao b,n, Lei Wang a,n a b
Shenzhen Key Laboratory of Special Functional Materials, College of Materials Science and Engineering, Shenzhen University, Shenzhen 518060, PR China The Ministry-Province Jointly Constructed Base for State Key Lab-Shenzhen Key Laboratory of Chemical Biology, Shenzhen 518055, PR China
art ic l e i nf o
a b s t r a c t
Article history: Received 23 June 2015 Received in revised form 23 August 2015 Accepted 15 September 2015
Branched sulfonated copoly(ether ketone)s exhibit excellent properties for use as proton exchange membranes (PEMs). However, to date, the use of highly branched star-shaped sulfonated block polymers with flexible side chains as PEMs has not been reported. In this study, two novel types of star-shaped block poly(arylene ether sulfone)s containing sulfoalkyl pendant groups with 6% branching agent were synthesized for the first time. The properties of the polymer with hydrophilic segments surrounded by hydrophobic segments (SPAES-6s) and of the polymer with hydrophobic segments surrounded by hydrophilic segments (SPAES-6f) were investigated, with a focus on their potential use as PEMs. The block polymers SPAES-6f and SPAES-6s exhibited higher proton conductivities (0.42 S cm 1 and 0.30 S cm 1 at 80 °C) than those of the random polymer SPAES-6r and Nafion 117, and SPAES-6s exhibited better oxidative stability (435 min) and dimensional stability than SPAES-6f. The results indicate that the highly branched sulfonated block poly(arylene ether sulfone)s with sulfoalkyl pendant groups are good candidate PEM materials for fuel cell applications. & 2015 Elsevier B.V. All rights reserved.
Keywords: Branched polymer Comb-shaped Poly(arylene ether)s Oxidative stability Proton exchange membrane
1. Introduction Proton exchange membrane fuel cells (PEMFCs) have received global attention because of their high energy conversion efficiency, near zero harmful emissions and convenient operation [1,2]. As one of the key components of a PEMFC, the PEM is closely related to the performance and durability of the entire fuel cell system [3,4]. Generally, among all possible candidates, perfluorosulfonic acid polymer membranes, such as Nafion membranes, are currently considered the most suitable PEM materials because of their excellent chemical and oxidative stability [5]. However, the many disadvantages of these materials, such as their high cost, high methanol permeability and loss of conductivity at high temperatures ( 480 °C) [6], have limited their commercial applications. In recent years, many aromatic hydrocarbon polymers have been extensively explored because of their satisfactory thermal properties and electrochemical stability [7–9]. Among aromatic hydrocarbon polymeric PEMs, sulfonated poly(arylene ether)s (SPAEs) are considered one of the most promising candidates for n
Corresponding authors. E-mail addresses: [email protected] (C. Gao), [email protected] (L. Wang). http://dx.doi.org/10.1016/j.memsci.2015.09.035 0376-7388/& 2015 Elsevier B.V. All rights reserved.
PEMFC applications by virtue of their stable thermal properties and high solubility in organic solvents [2,10–12]. However, most of them have failed to be used as PEMs because of their short lifetimes due to a combination of hydrolysis and oxidative degradation. Therefore, the improvement of the durability of inexpensive membranes has become an active area of PEM research. It is widely accepted that cross-linking is an effective method of improving the oxidative stability of a membrane [13–16]. However, cross-linked membranes are generally insoluble in common organic solvents and are difficult to reprocess, which has restricted their development as commercial membranes. Recently, studies have shown that branching is also an excellent method for this purpose [17–23]. Compared with cross-linked membranes, branched membranes not only exhibit improved oxidative stability and conductivity but also demonstrate good solubility in common organic solvents [17,19,20]. However, the mechanical properties of highly branched membranes notably decrease with an increasing degree of branching (DB) and cannot satisfy the requirements for PEMs. This could be because of a decrease in chain entanglement with increasing branching caused by the hard arms and short chains between branching points [17]. A viable strategy for overcoming the disadvantages of branched PEMs is to introduce flexible side chains into the branched
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polymer, which could help to increase the entanglement of the polymer chain, thus improving its mechanical properties. Recently, a series of branched polymers with pendant acid groups on the side chains was synthesized by our group [24]. Investigation of these polymers revealed that the mechanical strength of the branched polymer with 6% branching agent was 24.3 MPa, higher than that of the corresponding branched polymer with 6% branching agent without pendant groups (18.0 MPa). In addition, the oxidative stability (420 min) of the branched polymers with pendant acid groups on the side chains was also improved compared with that of the corresponding branched polymer without pendant groups (281 min). This is attributed to the fact that in polymers [9,25–28] with pendant acid groups on their side chains, the main polymer chain can be maintained in a hydrophobic environment. Thus, the chains initially separate into nanophases, which can reduce the probability of attack by water and peroxy radicals on the main chain, leading to excellent oxidative stability. In addition, Guiver and Lee's group report that poly(arylene ether sulfone) membranes [29] containing clustered flexible pendant sulfonic acids exhibited better dimensional stability and higher proton conductivity than Nafion 117, even at elevated temperatures (80 °C) and 450% relative humidity. From the results, we can infer that branched polymers with pendant acid groups on their side chains show promise as PEMs, and it would be worthwhile to further improve their relevant properties, such as proton conductivity at low relative humidity, oxidative stability and mechanical properties. It is widely believed that the preparation of block copolymers can significantly increase phase separation [23,30–34]. Because of their well-separated microstructure, such membranes exhibit high proton conductivity and superior hydrolytic stability, especially at high temperatures and low relative humidity. To enhance the properties of branched membranes with pendant acid groups on their side chains, in this study, two types of highly branched starshaped block poly(arylene ether sulfone)s with sulfoalkyl pendant groups were successfully synthesized. The properties of membranes of these highly branched star-shaped sulfonated block copolymers with flexible side chains, including their proton conductivity, hydrolytic and oxidative stability, mechanical properties, ion-exchange capacity (IEC), water uptake, thermal stability and swelling ratio, were investigated and compared with those of their random polymer counterparts.
2. Experimental 2.1. Materials 1,3,5-Triphenylbenzene, 4-fluorobenzenesulfonyl chloride, mercaptopropionic acid (MPA), 2,6-dimethoxyphenol, 9-fluorenone, 4,4′-(hexafluoroisopropylidene)diphenol (6F BPA), 4,4′-difluorodiphenyl sulfone (DFDS), boron tribromide (BBr3), 1,4-butanesultone, dimethylacetamide (DMAc), dimethyl sulfoxide (DMSO), and dichloromethane were purchased from commercial sources and used as received. Toluene was dried using sodium wire, and the DMAc, DMSO, and dichloromethane were dried using 4 Å molecular sieves prior to use. Anhydrous potassium carbonate was dried at 300 °C for 24 h in a furnace prior to use. All other solvents and reagents were of reagent grade and were used as received. 2.2. Measurements 1 H NMR spectra, reported in ppm, were recorded using a Varian 400 Hz NMR instrument with tetramethylsilane (TMS) as the internal standard. Gel permeation chromatography (GPC) analysis
was performed using a Waters HPLC 2695 Separations Module (with tetrahydrofuran as the eluent and polystyrene as the standard). The thermal stability of the polymers was investigated over a temperature range from 50 °C to 600 °C at a heating rate of 10 °C/min using a Q50 thermogravimetric analysis (TGA) under a nitrogen flow of 50 mL/min. Tapping-mode atomic force microscopy (AFM) images were acquired using a Dimension Icon scanning probe microscope. 2.3. Synthesis of 9,9-bis(3,5-dimethoxy-4-hydroxyphenyl)fluorene (DMHF) DMHF was synthesized from 2,6-dimethoxyphenol and 9-fluorenone using a method reported previously in the literature [29]. Pure white powder was obtained with an overall yield of 63% after purification via recrystallization from toluene (Mp: 186– 187 °C). 2.4. Synthesis of 1,3,5-tris[4-(4-fluorophenylsulfonyl)phenoxy] benzene (B3) B3 was synthesized from 1,3,5-triphenylbenzene and 4-fluorobenzenesulfonyl chloride using a method reported previously in the literature [22]. A white powder was obtained with an overall yield of 84.8% after purification via recrystallization from acetic acid (Mp: 143–144 °C). 2.5. Synthesis of the oligomer b6f 6F BPA (0.900 g, 2.68 mmol), 4,4′-difluorodiphenyl sulfone (0.589 g, 2.32 mmol), B3 (0.187 g, 0.24 mmol), potassium carbonate (0.552 g, 4.00 mmol), DMAc (7 mL) and toluene (8 mL) were carefully introduced into a 50 mL three-neck round-bottom flask equipped with a Dean–Stark trap and a condenser under nitrogen protection. The reaction mixture was stirred at 140 °C for 4 h. After the removal of the toluene, the reaction temperature was increased to 170 °C, and the reaction was continued for 2 h. After cooling to room temperature, the mixture was poured slowly into 150 mL of water that contained 4 mL of concentrated HCl to precipitate the formed polymer. The precipitates were filtered and washed with water three times to remove any inorganic salts. The fibrous polymer was collected and dried at 110 °C under vacuum for 24 h. 1 H NMR (300 MHz, CDCl3; ppm): 8.02 (d, 0.33H), 7.90 (d, 0.86H), 7.77 (d, 0.26H), 7.39 (d, 0.86H), 7.08 (d, 1H), 7.01(d, 1H). 2.6. Synthesis of the highly branched block polymer MPAES-6f containing methoxy groups 6F-BPA (0.900 g, 2.68 mmol), 4,4′-difluorodiphenyl sulfone (0.589 g, 2.32 mmol), B3 (0.187 g, 0.24 mmol), and potassium carbonate (0.552 g, 4.00 mmol) were added to a 50 mL three-neck round-bottom flask equipped with a Dean–Stark trap. Then, DMAc (7 mL) and toluene (8 mL) were added to the flask under nitrogen. The reaction mixture was stirred at 140 °C for 4.0 h. After the water was removed, the reaction temperature was increased to 170 °C, and the reaction was continued for 2 h. After the mixture had cooled to 120 °C, 4,4′-difluorodiphenyl sulfone (0.335 g, 1.32 mmol), DMHF (0.620 g, 1.32 mmol), potassium carbonate (0.276 g, 2.00 mmol), DMAc (6 mL) and toluene (8 mL) were carefully introduced into the flask. The reaction mixture was again heated to 140 °C for 4.0 h for water removal, and the temperature was then increased to 170 °C. The reaction mixture was maintained at this temperature for 2.5 h. After cooling to room temperature, the mixture was slowly poured into 200 mL of water that contained 5 mL of concentrated HCl to precipitate the formed
H. Xie et al. / Journal of Membrane Science 497 (2016) 55–66
polymer. The precipitates were filtered and washed with water three times to remove any inorganic salts. The fibrous polymer was collected and dried at 110 °C under vacuum for 24 h. 1 H NMR (300 MHz, CDCl3; ppm): 7.98 (d, 0.33H), 7.90 (d, 0.86H), 7.83–7.72 (m, 0.76H), 7.34–7.50 (m, 1.86H), 7.01–7.10 (m, 2H), 6.82–6.90 (m, 0.5H), 6.47 (s, 0.5H), 3.54 (s, 1.48H, –OCH3). 2.7. Synthesis of the oligomer b6s DMHF (0.620 g, 1.32 mmol), 4,4′-difluorodiphenyl sulfone (2.440 g, 0.96 mmol), B3 (0.187 g, 0.24 mmol), potassium carbonate (0.273 g, 1.98 mmol), DMAc (6 mL) and toluene (7 mL) were carefully introduced into a 50 mL three-neck round-bottom flask equipped with a Dean–Stark trap and a condenser under nitrogen protection. The reaction mixture was stirred at 140 °C for 4.0 h. After the removal of the toluene, the reaction temperature was increased to 170 °C, and the reaction was continued for 2.5 h. After cooling to room temperature, the mixture was poured slowly into 150 mL of water that contained 4 mL of concentrated HCl to precipitate the formed polymer. The precipitates were filtered and washed with water three times to remove any inorganic salts. The fibrous polymer was collected and dried at 110 °C under vacuum for 24 h. 1 H NMR (300 MHz, CDCl3; ppm): 7.96–7.99 (d, 0.81H), 7.80 (d, 1.99H), 7.48 (d, 1H), 7.33–7.43 (m, 2.45H), 6.82–6.90 (m, 2H), 6.47 (s, 2H), 3.54 (s, 6H, –OCH3). 2.8. Synthesis of the highly branched block polymer MPAES-6s containing methoxy groups DMHF (0.620 g, 1.32 mmol), 4,4′-difluorodiphenyl sulfone (2.440 g, 0.96 mmol), B3 (0.187 g, 0.24 mmol), and potassium carbonate (0.273 g, 1.98 mmol) were introduced into a 50 mL threeneck round-bottom flask equipped with a Dean–Stark trap. Then, DMAc (6 mL) and toluene (7 mL) were added to the flask under nitrogen. The reaction mixture was stirred at 140 °C for 4.0 h. After the water was removed, the reaction temperature was increased to 170 °C, and the reaction was continued for 2.5 h. After the mixture was cooled to 120 °C, 4,4′-difluorodiphenyl sulfone (0.680 g, 2.68 mmol), 6F BPA (0.900 g, 2.68 mmol), potassium carbonate (0.555 g, 4.02 mmol), DMAc (7 mL) and toluene (8 mL) were carefully introduced into the flask. The reaction mixture was again heated at 140 °C for 4.0 h for water removal, and the temperature was then increased to 170 °C. The reaction mixture was maintained at this temperature for 3 h. After cooling to room temperature, the mixture was poured slowly into 200 mL of water that contained 5 mL of concentrated HCl to precipitate the formed polymer. The precipitates were filtered and washed with water three times to remove any inorganic salts. The fibrous polymer was collected and dried at 110 °C under vacuum for 24 h. 1 H NMR (300 MHz, CDCl3; ppm): 8.02–8.05 (d, 0.40H), 7.92 (d, 2.03H), 7.74–7.85 (m, 0.99H), 7.36–7.52 (m, 3.75H), 7.03–7.12 (m, 4H), 6.84–6.90 (m, 1H), 6.49 (s, 1H), 3.56 (s, 3H, –OCH3). 2.9. Synthesis of the highly branched random polymer MPAES-6r containing methoxy groups For comparison with the branched block poly(arylene ether sulfone)s, the branched random polymer MPAES-6r was synthesized. 6F BPA (0.900 g, 2.68 mmol), DMHF (0.620 g, 1.32 mmol), 4,4′-difluorodiphenyl sulfone (0.924 g, 3.64 mmol), B3 (0.187 g, 0.24 mmol), and potassium carbonate (0.828 g, 6 mmol) were introduced into a 50 mL three-neck round-bottom flask equipped with a Dean–Stark trap. Then, DMAc (10 mL) and toluene (10 mL) were added to the flask under nitrogen. The reaction mixture was stirred at 140 °C for 4.0 h. After the water was removed, the
57
reaction temperature was increased to 170 °C, and the reaction was continued for 3.5 h. After cooling to room temperature, the mixture was poured slowly into 200 mL of water that contained 5 mL of concentrated HCl to precipitate the formed polymer. The precipitates were filtered and washed with water three times to remove any inorganic salts. The fibrous polymer was collected and dried at 110 °C under vacuum for 24 h. 1 H NMR (300 MHz, CDCl3; ppm): 7.95–8.03 (d, 0.40H), 7.71– 7.91 (m, 3.02H), 7.47 (d, 0.5H), 7.38 (m, 2.75H), 7.33 (d, 0.5H), 6.98– 7.09 (m, 4.06H), 7.81–6.88 (m, 1H), 6.48 (s, 1H), 3.55 (s, 3H, –OCH3). 2.10. Conversion of methoxy groups (MPAES-6x) into hydroxyl groups Polymers containing hydroxyl groups were synthesized via a demethylation reaction; the reaction procedure for the MPAES-6x (6x ¼6f, 6s and 6r) polymers is described below. A typical experimental procedure (MPAES-6f) for MPAES was as follows: a sample of 2.000 g of MPAES-6f was dissolved in 100 mL of anhydrous dichloromethane in a 500 mL three-neck flask with a nitrogen inlet. BBr3 (3 mL) was mixed with dichloromethane (30 mL), and the mixture solution was added dropwise to the MPAES-6f solution at 0 °C (ice bath). After 6 h, the resulting copolymer was filtered, washed with methanol and deionized water, recovered, and dried under vacuum at 100 °C for 24 h to finally obtain the product HPAES-6f (yield: 93%). 1 H NMR (300 MHz, DMSO-d6; ppm): 9.44 (s, 0.5H, –OH), 7.94– 8.06 (m, 1.09H), 7.76–7.83 (m, 0.76H), 7.38–7.44 (m, 1.86H), 7.20 (d, 2H), 6.68–6.86 (m, 0.5H), 6.27 (s, 0.5H). 2.11. Preparation of sulfonated copolymers (SPAES-6x) The synthesis of the SPAES-6f polymer, a typical example of the synthesis of SPAES-6x, is described below. HPAES-6f (1.000 g) and 0.300 g of NaOH were introduced into a 100 mL three-neck roundbottom flask. Then, DMSO (30 mL) was added to the flask under nitrogen. The reaction mixture was stirred at room temperature for 2 h. Then, 2 mL of 1,4-butanesultone was added, and the reaction was heated to 100 °C for another 12 h. The resulting polymer was obtained by carefully pouring the solution into 300 mL of isopropanol, filtering, washing thoroughly with water, and drying under vacuum at 80 °C for 10 h (yield: 92%). 1 H NMR (300 MHz, DMSO-d6; ppm): 7.94–8.01 (t, 1.09H), 7.76– 7.80 (m, 0.64H), 7.19–7.37 (d, 2.88H), 6.93 (d, 0.5H), 6.45 (d, 0.5H), 3.73 (s, 1H), 2.65 (s, 1H), 1.71(s, 1H), 1.43 (d, 1H). 2.12. Film casting and membrane acidification The dried sulfonated copolymer was dissolved in DMAc (10 mL) under stirring at room temperature for 12 h. Then, the solution was filtered using a funnel and cast onto flat glass slides. After careful drying at 60 °C for 24 h under vacuum, tough, smooth membranes were obtained. The resulting membranes were acidified with a 1 M H2SO4 solution for 24 h to exchange Na þ with H þ . Finally, the membranes were immersed in deionized water overnight to eliminate excess H2SO4 and were stored in deionized water for testing purposes. 2.13. Water uptake and dimensional stability Prior to absorption, the membranes were first dried under vacuum at 80 °C for 24 h to determine their masses and were then immersed in deionized water at different temperatures for 24 h to determine their masses after absorption. The water uptake of each membrane was calculated in weight percent using Eq. (1):
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WU = ( Ws − Wd )/Wd × 100%
(1)
where Wd and Ws are the masses of the membrane before and after water absorption, respectively. The swelling ratio was calculated in terms of the linear expansion rate of the wet membrane using Eq. (2):
Swelling ratio (%) = ( L s − L d )/L d × 100%
(2)
where Ld and Ls are the thicknesses of the membrane before and after water absorption, respectively. 2.14. Proton conductivity and ion-exchange capacity The proton conductivity of each hydrated film was measured using an impedance analyzer (Solartron 1260A) with an oscillating voltage of 10 mV and a frequency range of 10 MHz to 500 Hz. The proton conductivity measurement of each fully hydrated membrane was performed with the cell immersed in liquid water. The temperature range tested in the impedance measurements was from 30 °C to 90 °C. The proton conductivity was calculated using Eq. (3):
σ = d/RS
(3) 1
where s is the proton conductivity (S cm ), d is the distance between the two electrodes (d¼ 1.88 cm), R is the resistance of the membrane, and S is the cross-sectional area of the membrane (cm2). The IEC values of the membranes were determined via titration. Each membrane was immersed in a 2 M NaCl solution at room temperature for 24 h for the exchange of H þ ions with Na þ ions. The exchanged protons within the solution were titrated with a 0.01 M NaOH solution using phenolphthalein as an indicator. The IEC was calculated from the titration data using Eq. (4):
(
)
IEC meq g −1 = (MNaOH × VNaOH )/Wd
(4)
where MNaOH is the molarity of NaOH, VNaOH is the consumed volume of NaOH, and Wd is the weight of the dry membrane. 2.15. Oxidative and hydrolytic stability A small piece of each membrane sample was soaked in Fenton's reagent (3% H2O2 containing 2 ppm FeSO4) at 80 °C. The oxidative stability was evaluated by recording the time at which the membrane began to disintegrate. The hydrolytic stability was also investigated by placing a membrane sample in boiling water. 2.16. Hydration number and membrane density The hydration number (λ) refers to the number of water molecules absorbed per sulfonic acid group and was calculated by combining the water-uptake and IEC data. Using the dry (Wd) and wet (Ws) membrane weights, the λ values were calculated using Eq. (5):
λ = ⎡⎣ ( Ws − Wd )/18⎤⎦ × 1000/( Wd × IEC) = (WU × 1000)/(18 × IEC)
(5)
The membrane density was calculated based on measurements of the membrane dimensions and weight after drying at 110 °C for 24 h in accordance with a method previously reported in the literature [35].
3. Results and discussion 3.1. Synthesis and characterization of the oligomers and block copolymers To achieve suitable film performance, the sulfonation of the polymers was set to 66%, and the DB of the polymers was chosen to be 6% because of the excellent properties of highly branched polymers for use as PEMs [24,29]. Highly branched block poly (arylene ether sulfone)s containing methoxy groups (MPAES-6f and MPAES-6s) were synthesized via two-step direct polycondensation using different feed sequences, resulting in copolymers with two different block structures, as shown in Schemes 1 and 2. As illustrated in Scheme 1, the oligomer b6f was synthesized via a nucleophilic substitution reaction to form the core of the branched structure in the first step. Controlling the reaction time is very important for successful core synthesis. When the reaction time was longer than 2 h at 170 °C, the oligomer b6f was easily cross-linked because of the relatively high B3 content. Therefore, the reaction time for b6f in the first reaction step was set to 2 h. For the same reason, the reaction time for b6s was set to 2.5 h. High-molecular-weight polymers b6f and b6s (Mn4 10,000 g/ mol) with good solubility were obtained, as shown in Table 1. Because of the high reaction activity of the monomers, including DMHF and 4,4′-difluorodiphenyl sulfone, the block polymers were synthesized by allowing the reactions in the second step to progress for only 3.5 h at 170 °C. When the reaction time of the second step exceeded 4 h, the branched block polymers MPAES-6f became cross-linked during polymerization and were not soluble in polar organic solvents. The reaction time of the MPAES-6s block polymer was also determined using the same method. In addition, the random polymer MPAES-6r was synthesized via a one-step direct polycondensation reaction for comparison. The chemical structures and compositions of the synthesized copolymers were characterized via 1H NMR spectroscopy with DMSO-d6 and CDCl3 as the solvent. The 1H NMR spectra of the polymer b6f and the block polymer MPAES-6f are shown in Fig. 1. The peaks at 8.04 and 7.98 ppm suggest that the B3 moiety was introduced into the branched polymers b6f. New peaks at approximately 3.54, 6.47, 6.82–6.89, 7.34, 7.50 and 7.83 ppm, which were absent in the b6f spectrum, were observed in the spectrum of polymer MPAES-6f, with the signal at 3.54 ppm corresponding to –OCH3. These peaks were attributed to the introduction of DMHF. The spectra of polymer b6s and block polymer MPAES-6s are shown in Fig. 2. New peaks at approximately 7.03–7.12 and 7.92 ppm, which were not present in the b6s spectrum, were observed in the spectrum of polymer MPAES-6f, suggesting the formation of a block structure. The conversion of the –OCH3 into reactive –OH was realized by using BBr3 to graft flexible pendant sulfoalkyl groups. Although the copolymers MPAES-6x were completely soluble in dichloromethane, the resulting copolymers containing –OH were insoluble because of the polar nature of the hydroxyl groups. The precipitate was collected, and the yield was 96%. In the last step of the reactions, the parent polymers containing hydroxyl groups in the ortho-position of the aromatic ring were sulfopropylated through a nucleophilic ring-opening reaction with 1,4-butanesultone using NaOH to obtain the final polymers SPAES6x, as shown in Schemes 1 and 2. The chemical structures of the SPAES-6x polymers were confirmed by their 1H NMR spectra (Figs. 3 and 4). As expected, the signals of the four sulfobutyl methylene groups appeared at lower frequencies. Interestingly, the proton of H8 in the spectrum of polymer SPAES-6f and that of H4 in the spectrum of polymer SPAES-6s appeared as broad multiplets because the steric hindrance of the bulky sulfobutyl groups
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59
Scheme 1. Synthesis of highly branched sulfonated block poly(arylene ether sulfone)s with the 6f structure.
prevented rotational freedom of the benzene ring, providing further evidence supporting the formation of SPAES-6x containing sulfobutyl groups. High-molecular-weight polymers (Mn 4500,000 g/mol) were prepared, as shown in Table 1, and the molecular weight distributions (PDIs) ranged from 1.04 to 1.17, consistent with the results of the polycondensation reaction. As shown in Table 2, the sulfonated polymers were readily soluble in polar organic solvents, such as DMAc and DMSO, and produced flexible and transparent films upon polymer solution casting. Diagrams of SPAES-6f and SPAES-6s are shown in Fig. 5. In SPAES-6f, the pendant sulfoalkyl groups generally avoid the core of the branching agent to form a block structure; by contrast, the pendant sulfoalkyl groups of SPAES-6s are located near the branching points. These unique structures affect the performance of the polymer films. Generally, block copolymers exhibit significantly increased phase separation and thus demonstrate improved properties as PEMs [23,30–32,36]. Compared with the random polymer SPAES-6r, the branched sulfonated block polymers may exhibit clearer phase segregation between the ionic groups and the polymer backbone because of their uniquely designed structure, thus endowing them with excellent properties. 3.2. Membrane microstructure and density It is widely believed that the properties of membranes are closely related to their microstructures. To gain a comprehensive
understanding of the membranes, the microstructures of the membranes was examined via AFM (Fig. 6). The bright and dark regions in the images were assigned to the hard structures corresponding to hydrophobic domains and the soft structures corresponding to hydrophilic domains with sulfonic acid and water, respectively [37,38]. Generally, sulfonated block copolymers are known to exhibit well-defined phase-separated morphologies of incompatible domains that form continuous channels, thus enabling the rapid transport of protons [30]. Compared with the block polymers SPAES-6f and SPAES-6s, the random membrane SPAES-6r exhibited less pronounced phase separation, and its proton-conducting channels were relatively narrow (3–5 nm), leading to a relatively low proton conductivity. As expected, a well-defined phase separation between the hydrophilic and hydrophobic domains was observed in the block membranes. Wide proton-conducting channels formed by hydrophilic domains are beneficial for the movement of protons and improve the proton conductivity of PEMs. Compared with the block membrane SPAES6s, which possessed moderate block sizes (7–9 nm), the block membrane SPAES-6f exhibited excessively large hydrophilic ionic domains with sizes in the range of 10–14 nm. We can infer that the block membrane SPAES-6f should demonstrate higher proton conductivity than the SPAES-6s membrane. Table 3 compares the densities of the copolymer membranes as measured using a previously reported method [35]. The density of the block membrane SPAES-6f was the lowest among all branched membranes because of the unique microstructure of SPAES-6f. The
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Scheme 2. Synthesis of highly branched sulfonated block poly(arylene ether sulfone)s with the 6s structure.
Table 1 Properties of the polymers and oligomers.
SPAES-6r SPAES-6f SPAES-6s b6f b6s a
Yield (%)
Mna ( 104)
Mwa ( 104)
PDI
Td5% (°C)
92 90 91 95 96
64.0 62.2 76.7 1.7 1.2
74.9 65.0 85.4 2.2 1.5
1.17 1.04 1.11 1.29 1.25
257 269 255 – –
Measured at 40 °C using THF as a solvent.
branched cores connect the hydrophobic segments with long, hard arms, leading to an increase in the free volume of the branched membranes; moreover, the hydrophilic segments are located on the exterior of the branched block polymer. The sulfonic groups of the hydrophilic segments present considerable steric hindrance and force the polymer chain to close, further increasing the free volume of the membrane. This increased free volume of SPAES-6f may help to trap water, leading to high water uptake and proton conductivity. 3.3. Thermal and mechanical properties The thermal properties of the block polymer and random copolymer membranes were measured via TGA at a heating rate of 10 °C/min under a nitrogen atmosphere. All samples were
preheated to 150 °C for 20 min in the TGA furnace to remove any bound water. Two weight loss transitions of the sulfonated copolymer membranes were evident from the TGA curves (Fig. 7). The first weight loss observed for the polymers at approximately 230– 280 °C was attributed to the degradation of the sulfonic acid group of the side chains. The polymer residues in the second weight loss region (at temperatures of 4370 °C) were further degraded, corresponding to the decomposition of the main chains of the polymers. The 5% weight loss temperatures of the sulfonated polymers are listed in Table 1, and the results show that the branched sulfonated polymers exhibited excellent thermal stability under conditions applicable to medium-temperature fuel cells. Table 3 lists the mechanical properties of the membranes measured at room temperature. The polymer membranes had tensile strengths in the range of 24.17–27.57 MPa, and the tensile strengths of the block membranes were greater than those of the random membranes, which were also higher than that of Nafion 117 (25.7 MPa). The three membranes exhibited elongations at break of 10.32%, 10.79% and 9.73%, respectively, and tensile moduli in the range of 0.45–0.58 GPa. Although branched structures in polymer membranes should impair their mechanical properties, the synthesized branched polymers nevertheless exhibited better mechanical properties. This is because the introduction of flexible side chains increased the entanglement of the polymer chains, thus improving the mechanical properties of the branched membranes. The results indicate that branched block polymers with flexible side chains can satisfy the mechanical criteria for
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Fig. 1. 1H NMR spectra of the b6f polymer b6f and polymer MPAES-6f.
application as PEM materials. 3.4. Ion-exchange capacity, water uptake and dimensional stability The IEC of a membrane plays an important role in determining its water uptake, swelling ratio and proton conductivity, which represents the amount of exchangeable protons in the PEM. The IECs of the tested membranes were determined via classical acid– base titration (Table 4). The experimental IEC values agreed with the theoretical values (1.60–1.67 meq/g), representing the quantitative reaction yields for methoxy conversion and sulfobutylation. These results also confirmed that the DMHF in the obtained MPAES-6x was consistent with the feed ratios used in the copolymerization, indicating that no side reactions occurred. The properties of water uptake and water transport through membranes are of paramount importance in PEM applications. Water uptake was evaluated by measuring the weight ratio between each membrane after water absorption upon immersion in water and the dry membrane. Fig. 8 shows the water uptake of the
block and random polymers at various temperatures. The IEC is one of the most important factors influencing the water uptake value, and the values for the three synthesized polymers were similar. However, the water uptake values of the resulting membranes were very different. The block polymer membrane SPAES6f exhibited the highest water uptake among the three SPAES-6x membranes, whereas the water uptake of block membrane SPAES6s was slightly less than that of the SPAES-6r membrane. This difference in water uptake is attributed to the unique microstructure of the branched membranes. As shown in Fig. 5, the hydrophobic cores of block polymer SPAES-6f are surrounded by hydrophilic segments. Sulfonic groups in the hydrophilic segments of SPAES-6f can easily absorb water molecules, whereas the inner hydrophobic core contains branched structures with long, rigid arms, which effectively increase the free volume of the three-dimensional structure, leading to greater capacity for water uptake. These two factors may be responsible for the high water uptake and the larger hydrophilic ionic domains of SPAES-6f. Although the SPAES-6s polymer contains more highly aggregated hydrophilic
Fig. 2. 1H NMR spectra of polymer b6s and polymer MPAES-6s.
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Fig. 3. 1H NMR spectrum of polymer SPAES-6f.
segments, the hydrophilic cores of the SPAES-6s block polymer are surrounded by hydrophobic segments. It is difficult for these hydrophobic segments to absorb water molecules, and the free volume available to trap water molecules in the hydrophilic cores is relatively small, leading to low water uptake and relatively small hydrophilic domains. This result suggests that the microstructures of the polymers play an important role in influencing water uptake in the branched membranes. The dimensional stability of the branched membranes was evaluated by comparing the length or thickness of the hydrated and dry-state membranes. Fig. 9 shows that the swelling ratios of the membranes increased with increasing temperature. All copolymer membranes exhibited a lower in-plane swelling ratio compared with the through-plane swelling ratio, and a similar tendency was observed in the water uptake at different temperatures, possibly because the branched structures suppressed the movement of the polymer chains, preventing membrane swelling. The block membranes exhibited strongly anisotropic swelling behavior because of the high content of hydrophilic and hydrophobic domains in the phase-separated regions, with larger dimensional changes in the through-plane direction than in the in-plane direction (Fig. 10). The anisotropic swelling behavior of the block membranes may suggest the formation of ordered hydrophilic domains within the copolymers, and membrane swelling occurred predominantly in the thickness direction, suggesting the presence of a nanophase-separated block structure [39,40]. Moreover, because of the excessively large hydrophilic ionic domains of SPAES6f, the regularity of the SPAES-6s microstructure was superior to
Table 2 Solubilities of the polymers and oligomers.
SPAES-6r SPAES-6f SPAES-6s b6f b6s a b
DMAc
DMSO
DMF
NMP
H2O
CH3OH
CHCl3
þa þ þ þ þ
þ þ þ þ þ
þ þ þ þ þ
þ þ þ þ þ
b
þ þ
Soluble. Insoluble.
that of SPAES-6f, probably leading to more strongly anisotropic behavior than that in SPAES-6f, and the swelling ratio in the through-plane direction was nearly two times that in the in-plane direction. The random polymer SPAES-6r exhibited mildly anisotropic membrane swelling due to the sulfonic groups on the side chains, giving rise to a slight phase separation. Furthermore, the block polymer SPAES-6s exhibited better dimensional stability, perhaps because of the unique microstructure of SPAES-6s. In this structure, the hydrophilic cores are surrounded by hydrophobic segments, thereby limiting the swelling of the membrane and leading to better dimensional stability. 3.5. Proton conductivity Generally, a conductivity higher than 10 2 S cm 1 at room temperature is required for PEM materials; this is the lowest value
Fig. 4. 1H NMR spectrum of polymer SPAES-6s.
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Fig. 5. Diagrams of the SPAES-6s and SPAES-6f polymers.
Fig. 6. AFM phase images of the block and random polymer membranes.
Table 3 Mechanical properties, densities and oxidation stabilities of the block and random polymers. Tensile strengtha (MPa) SPAES-6r 24.17 SPAES-6f 26.32 SPAES-6s 27.57 a b c
Young's modulusa (MPa)
Elongation at break a (%)
Densityb (g cm 3)
Oxidation stabilityc (min)
458 526 583
10.32 10.79 9.73
1.34 1.19 1.29
420 402 435
Measured at room temperature. Based on the dry state. Measured in Fenton's reagent at 80 °C.
of practical interest for materials to be used as PEMs in fuel cells [4,41]. From Fig. 11, it can be seen that all obtained membranes exhibited conductivities higher than 10 2 S cm 1 at 30 °C and 100% relative humidity, surpassing that of Nafion 117 (Table 4). Sulfonic acid groups on the flexible aliphatic side chains of these polymers could reduce the influence of the main-chain rigidity on the mobility of sulfonic acid groups, thereby enabling the formation of an ionic network and consequently enhancing proton conductivity. The proton conductivity of a membrane is strongly related to its IEC value, water uptake and microstructure. Considering that the target membranes possessed similar IEC values, the differences in their conductivities are believed to have resulted from the differences in water uptake among the polymers and the polymer microstructures. The obtained SPAES-6f with the high water absorption and phase separation, also demonstrated the highest proton conductivity (0.45 S cm 1 at 90 °C). Although the block membrane SPAES-6s demonstrated slightly lower water uptake than the random SPAES-6r membrane, the conductivity of
Fig. 7. TGA curves of the block and random polymers.
SPAES-6s was higher than that of SPAES-6r. Because the block polymer structure is beneficial for the aggregation of ionic clusters, which in turn leads to more obvious hydrophilic/hydrophobic separation and the formation of continuous channels, both block membranes exhibited superior proton conductivity to that of SPAES-6r. Moreover, compared with the proton conductivity of the corresponding linear copolymer (0.25 S cm 1 at 80 °C, IEC¼1.70 meq g 1) [29], the branched polymers exhibited higher proton conductivity in the range of 0.26–0.42 S cm 1, as shown in Table 4. All three membranes exhibited a similar dependence of the conductivity on the relative humidity at 80 °C, as shown in Fig. 12. Additionally, the proton conductivity of the membranes
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Table 4 Properties of the polymer membranes.
SPAES-6r SPAES-6f SPAES-6s Nafion 117
Water uptakea (wt%)
IECb (meq g 1)
IECc (meq g 1)
λ
Conductivityd (S cm 1)
52.5 59.5 46.5 30.6
1.69 1.69 1.69 –
1.66 1.60 1.67 0.91
17.5 20.6 15.5 18.6
0.26 0.42 0.30 0.11
a
Measured at 80 °C. Theoretical IEC. c Experimental IEC. d Measured for membrane samples at 80 °C and 100% relative humidity. b
Fig. 10. Dimensional swelling of the block and random polymer membranes at 30 °C.
Fig. 8. Water uptake of the block and random polymer membranes.
Fig. 11. Proton conductivities of the block and random polymer membranes.
Fig. 9. In-plane and through-plane swelling ratios of the block and random polymer membranes.
increased with increasing relative humidity. The corresponding reduction in proton transport capability with decreasing relative humidity is associated with the evaporation of proton carrier water molecules [13]. From Fig. 12, we can observe that the proton conductivities of the block membranes were greater than that of the random SPAES-6r membrane at different relative humidities because of the obvious phase separation of the block structure. These results may be explained by the improved water retention capability facilitated by the higher acidity of the hydrophilic domains and the existence of well-connected ionic channels [30]. This finding illustrates the positive effect of obvious phase
Fig. 12. Relative humidity dependences of the proton conductivities of the block and random polymer membranes and Nafion 117 at 80 °C.
separation on the proton conductivity under partially humidified conditions. However, the values were lower than those of the reference sample of Nafion 117 at low relative humidity, likely because of the relatively large hydrophilic domains, which possessed a weak capacity for water retention compared with the narrower
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thereby improving the oxidative stability of the polymer. By contrast, SPAES-6f, with its hydrophilic segments on the periphery, was more easily oxidized and exhibited poor oxidative stability. The hydrolytic stability was also investigated by immersing the membranes in boiling water. All membranes remained whole and unchanged in shape or appearance, even after 8 days, indicating that no hydrolysis occurred during this treatment.
4. Conclusions
Fig. 13. The Arrhenius-type temperature-dependent proton conductivity (s) behavior of the block and random polymer membranes.
hydrophilic domains. The hydration number (λ), which is the number of water molecules per sulfonic acid group, is presented in Table 4. The λ and water uptake values of the membranes exhibited similar behaviors because of the similar IEC values, which also reflected the proton conductivity of the branched membranes. Furthermore, although the λ of Nafion 117 is higher than those of SPAES-6r and SPAES-6s, the conductivities of the resulting SPAES6x membranes were superior because of the branched structure and flexible side chains of the branched polymers. Fig. 13 also displays the Arrhenius plots of the conductivities of the analyzed membranes in the temperature range of 30–90 °C. The activation energy (Ea) of conductivity was calculated by linearly fitting the Arrhenius equation, s¼ Aexp( Ea/RT). The activation energy for proton conduction was found to be in the range of 10.64–13.80 kJ mol 1. These high activation energy values suggest that proton conductivity increases significantly with increasing temperature. The activation energy of the block polymer SPAES-6f was 13.80 kJ mol 1, which is higher than that of SPAES-6s or SPAES-6r. This may be attributed to the enhancement of water absorption with increasing temperature induced by the hydrophilic segments of SPAES-6f.
Two novel types of highly branched star-shaped block copolymers were successfully synthesized at high yield via a facile condensation reaction. Sulfonated poly(arylene ether sulfone)s with four sulfoalkyl pendant groups were obtained through methoxy conversion and sulfobutylation. These polymers possessed high molecular weights and could form tough and flexible membranes via solvent casting. The sulfonated block polymers SPAES-6s and SPAES-6f exhibited excellent proton conductivities and tensile strengths, which were comparable to or even higher than those of Nafion 117. The block polymer SPAES-6s, with hydrophilic cores surrounded by hydrophobic segments, displayed excellent oxidative and dimensional stability, whereas block polymer SPAES-6f, with hydrophobic cores surrounded by hydrophilic segments, exhibited excellent proton conductivity and high water uptake. The combination of the facile synthetic routes for these polymers and their good proton transport, antioxidant capacity and reasonably good mechanical properties make these membranes attractive for further investigation for use as PEM materials in fuel cell applications.
Acknowledgments The authors gratefully acknowledge the National Natural Science Foundation of China (Nos. 51003060, 51171117 and 51101103), Shenzhen Sci & Tech research grant (JC20110, 42100070A and ZYC201105170225A) and the Special Fund of the Central Finance for the Development of Local Universities (No. 000022070140) for financial support.
Appendix A. Supplementary material 3.6. Oxidative and hydrolytic stability Free radicals, such as oxygen, hydroxide and peroxide, typically attack the hydrophilic domains of polymer membranes, resulting in degradation of the polymer chain. To evaluate whether the SPAES-6x membranes are capable of withstanding strong oxidizing environments during fuel cell operation, the oxidative stability of the branched membranes was evaluated by measuring the elapsed time before a membrane sample began to disintegrate after immersion in Fenton's reagent (2 ppm FeSO4 in 3% H2O2) at 80 °C. As shown in Table 3, the branched SPAES-6x membranes with pendant sulfoalkyl groups exhibited better oxidative stability (4 400 min) than the corresponding linear polymer (210 min) or the branched membranes without sulfoalkyl side chains (280 min) [17,29]. This improved stability is attributed to the branched structure and the flexible side chains, which effectively separate the sulfonic acid groups from the polymer main chain to form highly concentrated hydrophobic domains that prevent or reduce the probability of radical attack, leading to better oxidative stability. From Table 3, we also observe that the block membrane SPAES-6s exhibited the best oxidative stability (435 min). This is because the hydrophilic cores of SPAES-6s are surrounded by hydrophobic segments, which can protect the hydrophilic segments,
Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.memsci.2015.09. 035.
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