Side-chain effects on the properties of highly branched imidazolium-functionalized copolymer anion exchange membranes

Applied Surface Science 493 (2019) 1306–1316

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Side-chain effects on the properties of highly branched imidazoliumfunctionalized copolymer anion exchange membranes

T

Mingliang Fanga, Dong Liua, Sivasubramaniyan Neelakandana, Muzi Xua, Danqing Liua,b, , ⁎ Lei Wanga,b, ⁎

a b

Shenzhen Key Laboratory of Polymer Science and Technology, College of Materials Science and Engineering, Shenzhen University, Shenzhen 518060, China Guangdong Research Centre for Interfacial Engineering of Functional Materials, Shenzhen University, Shenzhen 518060, China

ARTICLE INFO

ABSTRACT

Keywords: Anion exchange membrane Branched poly(arylene ether sulfone)s Imidazolium groups Flexible alkyl side chains

A set of highly branched imidazolium-functionalized poly(arylene ether sulfone)s copolymer bearing with flexible alkyl side chains of different lengths are designed and synthesized. The ion exchange capacity (IEC), ionic conductivity, water uptake, thermal stability, mechanical property and alkaline resistance of the anion exchange membranes (AEMs) were evaluated in detail. Atomic force microscopy and small-angle X-ray scattering are used to study morphology which reveals that the branched membrane with an alkyl side chain (6 carbons) achieves the highest conductivity (up to 115.8 mS cm−1 at 80 °C) due to the well-developed hydrophilic/hydrophobic phase separation. In addition, the branched co-polymer AEM with a longer alkyl side chain (12 carbons) exhibit the best robust alkaline stability, it decreases only 27% of ionic conductivity after 550 h in 1 M KOH. Therefore, this study provides a comprehensive insight into the tuneable membrane properties of highly branched copolymers as a change in the length of flexible alkyl side chains.

1. Introduction Anion exchange membrane fuel cells (AEMFCs) are attracting widespread attention as one of the most promising clean energy-conversion technologies because of their rapid oxygen reduction reaction kinetics and the ability to use nonprecious metals as catalysts (such as Co and Ni), thereby significantly reducing the cost of fuel cell devices [1–4]. Although AEMFCs have many advantages, the practical application of anion exchange membranes (AEMs) in AEMFCs still faces major obstacles such as poor alkaline stability and low conductivity of the AEM materials [5–7]. Therefore, polymer backbones such as poly(arylene ether)s [4,8–10], poly(phenylene oxide)s [6,11,12] and polybenzimidazoles [13] have been used as AEMs. Among these polymer backbones, poly (arylene ether sulfone)s have been extensively studied because of their good solubility, excellent thermal properties, good mechanical properties and versatile chemical modification [4,9,14–18]. In addition, various functionalized cations [19], such as quaternary ammonium (QA) [4,20–22], imidazolium [23–29], guanidinium [30,31], cycloaminium [17], pyrrolidinium [32] and metal-based cations [33] have all been employed in AEMs to increase the stability and conductivity of the

corresponding membranes. Among these cationic groups, imidazoliumfunctionalized AEMs have recently attracted wide interest, because they demonstrated robust alkaline stability. The main reason is the presence of steric hindrance and π-conjugated structures. Unfortunately, the alkaline stability and ionic conductivity of these cations, including imidazolium groups, still do not meet the requirements of AEM materials. [5,10,23,24,26,27] Recently, some researchers have developed comb-shaped AEMs grafted with flexible alkyl side chains between the polymer backbone and cationic groups because they exhibit higher alkaline stability and ionic conductivity than AEM materials with benzyl-substituted cations [11,34]. The flexible side chains promote the local mobility of the cationic groups and are conducive to the development of the hydrophilic/ hydrophobic phase-separated morphology, thereby increasing the conductivity [11,13,26,35,36]. Additionally, the electron-withdrawing effect of the cationic groups is weakened since the cationic groups are away from the polymer backbone, which could protect the functionalized-cation groups from ion attack [36,37]. Jannasch et al. systematically developed a set of polymers attached to QA groups on backbone via the flexible side chain and found that the membranes containing alkyl side chains with 5–7 carbon atoms achieved good conductivity

⁎ Corresponding authors at: Shenzhen Key Laboratory of Polymer Science and Technology, College of Materials Science and Engineering, Shenzhen University, Shenzhen 518060, China. E-mail addresses: [email protected] (D. Liu), [email protected] (L. Wang).

https://doi.org/10.1016/j.apsusc.2019.07.059 Received 2 February 2019; Received in revised form 26 June 2019; Accepted 8 July 2019 Available online 09 July 2019 0169-4332/ © 2019 Elsevier B.V. All rights reserved.

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[38]. Liu and co-workers reported a poly(ether sulfone) AEM grafted with hexyleneoxy QA side chains that displayed optimum ionic conductivity [36]. Kim et al. reported that a polymer backbone grafted with an imidazolium group containing alkyl spacers demonstrated higher stability and conductivity than conventional benzylic counterparts [27]. In order to further improve the alkaline stability and ionic conductivity of comb-shaped polymer AEMs, we introduced a bulky and rigid three-prong branched structure into a poly(arylene ether sulfone)s copolymer bearing pendent flexible alkyl functionalized-imidazolium side chains (6C), which exhibited excellent stability and conductivity, especially for highly branched membrane [7]. The branched structure can enhance the free volume of membranes, which can promote the water absorption and facilitate the construction of efficient ion transport channels [39,40]. Therefore, there is an urgent need to develop strategies for the preparation of a novel branched comb-shaped copolymers as AEMs. However, the structure-property relationship, especially the effect of the length of the flexible side chain on the properties of the branched membrane materials, is significantly important for the preparation of AEMs with enhanced alkaline stability and ionic conductivity, and it is not yet clear. In this work, we designed and synthesized a set of novel branched AEMs containing flexible alkyl side chains of different lengths attaching the imidazolium groups on the poly(arylene ether sulfone)s backbone. In addition, the relationships between the lengths of the alkyl functionalized-imidazolium side chain and the properties of the corresponding branched AEMs, including water uptake, mechanical properties, ionic conductivity and alkaline stability, were systematically studied. The change in the length of the flexible side chain significantly affected the properties of the branched copolymer AEM materials.

acetone in the filtrate was removed by rotary evaporation. After several extractions with ethyl acetate and diethyl ether, a viscous pale-yellow oil was obtained after. The product was dried under vacuum at 50 °C for 24 h (yield: 68%). 2.3. Synthesis of highly branched poly(arylene ether sulfone)s with methoxy groups (BPES-OCH3) As shown in Scheme 2, the detailed synthesis of BPES-OCH3 is described as follows. FPS (1.1568 g, 4.55 mmol), DMHF (0.9406 g, 2 mmol), B3 (0.2343 g, 0.3 mmol), K2CO3 (1.382 g, 10 mmol), 6F-BPA (1.0084 g, 3 mmol), 18 ml of DMAc and 10 mL of toluene were stirred in a 100 mL three-necked flask placed with a magnetic stirrer, a condenser, a Dean-Stark trap and nitrogen inlet/outlet. The reaction mixture was heated to 140 °C for 4 h and then heated to 170 °C for another 4 h. After the reaction was completed, the reaction mixture was diluted with 5 mL of DMAc, then cooled to room temperature, and the reaction was slowly poured into a 200 mL mixture of methanol and deionized water with 2 mL of HCl. The white fibrous product was filtered and washed several times with deionized water. Finally, the product was dried under vacuum at 80 °C for 24 h (yield: 95%). 2.4. Synthesis of highly branched poly(arylene ether sulfone)s with hydroxyl groups (BPES-OH) As previously reported, BPES-OH was synthesized from BPES-OCH3 through a demethylation reaction. [7] As shown in Schemes 2, 3 g of BPES-OCH3 was dissolved in 120 mL of CH2Cl2 in a 250 mL roundbottom flask with a magnetic stirrer under nitrogen protection. The flask was placed in an ice-water bath to react for 6 h, and a mixture of 3 mL of BBr3 and 50 mL CH2Cl2 was dropped into the container dropwise during the reaction. The resulting precipitate was poured slowly into boiling water and filtered. The solid product was dried under vacuum at 80 °C for 24 h (yield: 92%).

2. Experimental 2.1. Materials 1,4-dibromobutane, 1,12-dibromododecane, potassium hydroxide (KOH), boron tribromide (BBr3), 1,6-dibromohexane, 2,6-dimethoxyphenol, 1,4-butanesultone, 4,4-difluorodiphenyl sulfone (FPS), potassium iodide (KI), 1-methylimidazole, dimethyl sulfoxide (DMSO), 4,4-(hexafluoroisopropylidene)diphenol (6F-BPA), dichloromethane (CH2Cl2), dimethylacetamide (DMAc), dichloromethane, 1,8-dibromooctane and potassium carbonate (K2CO3) were purchased from Energy Chemical (Shanghai, China) and used as received. Two monomers (9,9-bis(3,5-dimethoxy-4-hydroxyphenyl) fluorene (DMHF) and 1,3,5-tris[4-(4-fluorophenylsulfonyl)phenoxy]-benzene (B3)) were synthesized [18,41]. All other chemicals were obtained from commercial sources and used without further purification.

2.5. Synthesis of highly branched poly(arylene ether sulfone)s grafted with Br-n-Im groups (BPES-n-Im) The procedure for the preparation of BPES-n-Im (n = 4, 6, 8 and 12) is shown in Scheme 2. Taking BPES-6-Im as an example, 1.0 g of BPESOH was dissolved in 20 mL of DMSO at room temperature under nitrogen protection in a 100 mL three-necked flask equipped with a condenser and a magnetic stirrer. Then, 1.6 g of Br-6-Im, 0.94 g of K2CO3 and 0.05 g of KI were added to the flask. At 100 °C, the reaction continued for 12 h. After cooling to room temperature, the reactant was slowly poured into 500 mL of acetone and stirred to obtain a brown precipitate. The final product was washed several times with deionized water, then filtered and dried under vacuum at 60 °C for 24 h (yield: 88%).

2.2. Synthesis of (ω-bromoalkyl) imidazolium bromide (Br-n-Im)

2.6. Preparation of membranes

As shown in Scheme 1, Br-n-Im were prepared according to the previous method with some modifications [26], in which n (4, 6, 8 and 12) is the number of carbon atoms of alkyl chains between the imidazolium groups and the eBr groups. Taking Br-6-Im as an example, 1.6dibromohenane (61.0 g, 250 mmol) was added into a three-necked round bottom flask. 1-Methylimidazole (4.1 g, 50 mmol) and acetone (50 mL) were added dropwise into the bottom flask. The reaction was stirred via a magnetic stirrer at 40 °C for 15 h. The product was filtered and the precipitate was washed several times with acetone. Then, the

1 g of BPES-n-Im was dissolved in DMSO (10 mL) to give a 10 wt% solution. The resulting solution was filtered with cotton and cast on clean, flat glass plate and dried at 60 °C under vacuum for 24 h to prepare the BPES-n-Im membranes. The as-prepared membranes in bromide form (Br−) were dipped in a 1.0 M NaOH solution at room temperature for 48 h to obtain the hydroxide form (OH−) of the materials. Prior to performance testing, these membranes were washed Scheme 1. The synthetic route of Br-n-Im monomers.

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Scheme 2. The synthetic of branched poly(arylene ether sulfone) copolymers BPES-OCH3, BPES-OH, BPES-n-Im. 1308

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Scheme 3. Degradation pathways of imidazolium cationic groups under alkaline conditions.

several times with deionized water and soaked in deionized water for at least 48 h to remove free hydroxide ions.

2.7.3. Swelling ratio (SR) The swelling ratio was calculated by the area expansion ratio, which was depended on the dimensional change between the wet and dry samples (2 cm × 2 cm) after soaking in deionized water at a specific temperature (30 °C, 60 °C, 80 °C) for 24 h. The SR was determined by the following equations:

2.7. Characterization and measurements The nuclear magnetic resonance spectroscopy (1H NMR) was obtained on a Varian 400 MHz spectrometer using DMSO‑d6 or CDCl3 as solvents and the deuterium signal as an internal reference. The thermal stability of membrane materials was recorded in the range of 50–700 °C via a Q50 TGA instrument at a heating rate of 10 °C min−1 with a 20 mL min−1 of nitrogen flow. The mechanical properties were tested at room temperature on an Instron 1185 analyser using a 2 mm min−1 of crosshead speed. Each membrane sample was measured three times and the average was taken as the reported value. The pendant-drop method was used to investigate water contact angles of membrane materials at room temperature. Morphologies of the membranes were observed by scanning electron microscopy using a Zeiss Supra 550 SEM. The phase images of membrane materials were achieved through atomic force microscopy (AFM, Bruker Multimode instrument) with tapping mode. Small angle X-ray scattering (SAXS) profiles of the branched AEMs in Br− form were recorded using a SAXSess-MC2 X-ray diffractometer (Anton Paar, Austria) at RT under vacuum.

SR (%) =

n1

n2 m

,

( ms cm 1) =

m dry

m dry

WU (%) =

m dry

m dry

,

l R×A

( ms cm 1) =

l , R×A

(4)

2.7.5. Alkaline stability The alkaline stability of the membranes was investigated by soaking the samples in hydroxide form into a 1 M KOH solution at 60 °C for up to 550 h to record the difference in hydroxide conductivity and the 1H NMR spectra. The conductivity of each sample was evaluated in triplicate in deionized water at 60 °C.

(1)

m wet

(3)

in which l is the distance between the two electrodes and A is the crosssectional area of the samples. Three AEM specimen were measured for each sample and the average values were adopted.

3. Results and discussion

2.7.2. Water uptake (WU) The WU of the samples was investigated by measuring the weight of the dry and wet membranes. The membrane samples were soaked in deionized water at a specific temperature (30 °C, 60 °C, 80 °C) for 24 h, and the mass of the wet samples (mwet) was quickly measured. Then, the dry weight of the membranes (wdry) was obtained after drying at 80 °C under vacuum for 24 h. The calculation was based on the following equation:

m wet

,

2.7.4. Hydroxide conductivity (σ) The resistance of the membranes (R) was obtained in the frequency range from 1 Hz to 105 Hz using an electrochemical workstation (Zahner IM6ex, Germany) by a four-point probe AC impedance method. Conductivity of the membranes (4 cm × 1 cm) were measured in deionized water at different temperatures in the range from 30 to 80 °C. The hydroxide conductivity (σ) was calculated by the following equations:

in which m(g) is the weight of the dried membrane, and n1 and n2 are the amount of H+ before and after titration in the 60 mL of HCl solution, respectively.

WU (%) =

l dry wdry

l dry wdry

in which lwet and wwet are the length and the weight of the wet membranes, and ldry and wdry are the width and the weight of the dry membranes, respectively. For WU and SW, three specimens were measured for each sample.

2.7.1. Ion exchange capacity (IEC) The IEC of the AEM samples was investigated by the traditional back titration method. 0.05 g of the BPES-n-Im membranes were immersed in 0.01 M HCl standard solution (60 mL) for 48 h and then titrated with 0.01 M NaOH standard solution using phenolphthalein as an indicator. The experimental IEC value was given by the following equation:

IEC (meq g 1) =

l wet wwet

3.1. Synthesis of Br-n-Im monomers As shown in Scheme 1, the Br-n-Im monomers were synthesized through a nucleophilic substitution reaction according to a reported study [26]. The 1H NMR spectrum confirmed the structure of Br-n-Im and used DMSO‑d6 as a solvent. As shown in Fig. 1, taking Br-6-Im as an example, the peaks at approximately 9.30, 7.84 and 7.76 ppm were assigned to the protons on the imidazolium ring, and the signal at approximately 3.87 ppm was assigned to the methyl of the imidazolium cationic group. Additionally, the signals at 4.18, 3.50, 1.37, and 1.23 ppm were assigned to the chemical shifts of the other –CH2– groups in the alkyl chain. The area integration ratio of Hd to H1 was close to 1.50 (Fig. S1), which is consistent with the theoretical value of 1.50, indicating that the successful

(2)

in which mwet and mdry are the weight of the wet and dry membranes, respectively. 1309

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Fig. 1. 1H NMR spectra of the Br-n-Im monomers.

synthesis of Br-6-Im without forming di-imidazolium-functionalized byproducts. Fig. 1 also shown the 1H NMR spectra of the other Br-n-Im monomers and all proton peaks were well assigned to the proton of monomers, indicating that these monomers were successfully prepared.

copolymers by grafting Br-n-Im onto the poly(arylene ether sulfone) backbone via Williamson reaction (Scheme 2). The 1H NMR spectra of the BPES-n-Im copolymers are shown in Fig. S2 (SI). Taking BPES-6-Im as an example, the proton resonance arising from the –OH groups at approximately 9.80 ppm disappeared, whereas the characteristic peaks of imidazolium groups and alkyl chains were appeared at approximately 7.71–7.73, 9.13, 3.85 ppm and 1.05–1.61, 3.89–4.08 ppm, respectively. These results suggested the successful grafting of Br-6-Im to the poly(arylene ether sulfone)s backbone. In addition, the degree of the alkyl chain of various membranes was calculated by 1H NMR spectrum (Fig. S2, ESI) in the range of 95% to 99%.

3.2. Synthesis of branched poly(arylene ether sulfone) grafting with flexible alkyl functionalized-imidazolium side chains BPES-OCH3 was synthesized by a nucleophilic polycondensation reaction (Scheme 2). According to previous reports, the demethylation reaction of BPES-OCH3 was carried out in chloroform and using BBr3 as the demethylation reagent. [18,42] The polymer structure was confirmed by 1H NMR spectroscopy analysis. Fig. S2 (SI) shown that the 6.6–8.0 ppm peaks in the spectrum were assigned to protons on the benzene rings of the polymer backbone. Furthermore, the signal peaks at about 8.08 and 8.10 ppm were attributed to the protons of the B3 structure. The peak at approximately 3.55 ppm was attributed to proton signals from –OCH3 groups and disappeared after the demethylation reaction. However, a new peak appeared at approximately 9.80 ppm assigned to the –OH group, which means that the BPES-OH was successfully synthesized. Synthesis of branched comb-type BPES-n-Im

3.3. Morphologies of membranes Taking BPES-6-Im as an example, the flexible and transparent membranes were obtained by dissolving a branched copolymer in an organic solvent and casting them on glass plates. SEM images showed the morphology of the membranes (Fig. 2), illustrating the preparation of a smooth, compact and homogeneous membrane without defects. The thickness of membrane materials was controlled in the range of 40 and 60 μm. To confirm the structure-morphology relationship of the as-

Fig. 2. (a) Photographs, (b) surface and (c) cross-sectional SEM images of BEPS-6-Im membrane. 1310

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Fig. 3. AFM phase images of (a) BPES-4-Im, (b) BPES-6-Im, (c) BPES-8-Im, and (d) BPES-12-Im.

calculated according to the SAXS data [28]. As shown in Fig. 4, obvious scattering peaks can be observed in the SAXS spectra for all the BPES-nIm membranes due to the existence of highly branched comb-shaped structure and hydrophilic/hydrophobic for side-chain type polymers, and the scattering vectors (qmax) is related with the ionic clusters. The ionomer peaks of BPES-4-Im, BPES-6-Im, BPES-8-Im and BPES-12-Im are located at qmax = 0.91, 0.84, 1.06 and 1.25 nm−1, respectively. Meanwhile, the corresponding inter-domain spacing (d) of BPES-4-Im, BPES-6-Im, BPES-8-Im and BPES-12-Im were calculated by the Bragg's equation (d = 2π/qmax) is 6.90, 7.48, 5.93, 5.03 nm, respectively. BPES-6-Im had the maximum size of self-assemble ionic clusters, which lead to form more developed inter-connected ion transport channels as a result of the spacing length with 6 carbons between polymer backbones and imidazolium groups. It suggested that BPES-6-Im exhibited the most instinct micro-phase separated morphology. The result is in accordance with related as-mentioned AFM investigation.

synthesized membranes, AFM tapping mode was applied to investigate the microphase separation architecture. Fig. 3 shows the AFM phase images of the BPES-n-Im membranes grafted to different lengths of flexible alkyl functionalized-imidazolium side chains, in which the brighter regions represent hydrophobic domains and the darker regions represent hydrophilic domains [43]. All four membranes demonstrated a distinct hydrophilic/hydrophobic phase separation. The flexible alkyl side chains between imidazolium cationic groups and the copolymer mainchain are conducive to self-assembly and the fabrication of hydrophilic nano pathways for branched comb-shaped membranes, which is beneficial for hydroxide transportation [44]. The BPES-6-Im with a 6‑carbon atom side chain exhibited better hydrophilic/hydrophobic phase separated structures than that of membranes containing shorter or longer side chains. This observation indicated that there was an optimal alkyl side chain length to obtain the largest ion cluster domains. This may be due to the enhanced local mobility of cationic groups favouring the promotion of microphase separation (x ≤ 6), while the hydrophobic alkyl side chain between the backbone and imidazolium groups becomes too long, which decreases the hydrophilicity of the polymer. These results indicated that the suitable lengths of the flexible alkyl side chains tend to form ionic clusters and the optimum alkyl chain length was 6 carbon atoms. AFM analysis demonstrated that phase separation favoured ion cluster formation, which can be facilitated by the introduction of the flexible alkyl side chain. As demonstrated below, self-assembled morphology resulting from the comb-shaped structure of the branched BPES-n-Im membranes containing alkyl side chains of various lengths had a strong influence on the ionic conductivity. SAXS was also used for deep studying of the phase segregation and microstructure of the branched AEMs, because ionic clusters size can be

3.4. IEC, water uptake and dimensional stability In general, high IEC values of membrane materials will obtain high water uptake and ionic conductivity [45]. The IEC value for all the BPES-n-Im membrane materials was evaluated by the back-titration method and the values between 1.51 and 1.80 meq. g−1, as summarized in Table 1. The theoretical IEC value is calculated from the imidazolium-functionalized groups and found to be in good agreement with the experimental IEC. As the length of the flexible alkyl imidazolium side chain increased, the IEC value decreased because of the increased hydrophobicity of the alkyl side chains; the lowest IEC value was obtained for BPES-12-Im (1.51 meq g−1). Water uptake plays a significant role in conductivity because water 1311

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Fig. 4. SAXS profiles of the (a) BPES-4-Im, (b) BPES-6-Im, (c) BPES-8-Im, and (d) BPES-12-Im.

absorption by the AEM facilitates ion cluster formation and ion transportation [19,46]. However, an excess of water uptake caused excessive swelling, which is not beneficial for dimensional and mechanical stability [47]. Therefore, it is important to balance the water uptake and ionic conductivity of membrane materials. Fig. 5 and Table 1 shown the water uptake of the BPES-n-Im membranes measured at a temperature between 30 °C and 80 °C. The water uptake of all four membrane materials increased with the increase of temperature, whereas the water uptake of BPES-n-Im membranes with different alkyl chain lengths did not change significantly. Maximum value at a certain temperature was observed for BPES-6-Im. The well-developed hydrophobic domains could contribute to the suppression of excessive water uptake. The presence of an optimal alkyl side chain (BPES-6-Im) between the imidazolium groups and backbone promoted more ionic aggregation and thus the formation of distinct water filled domains. Therefore, the BPES-6-Im membrane obtained more large water uptake than BPES-4Im membrane. In addition, the introduction of more methylene groups

into alkyl side chains could enhance the hydrophobicity of the membranes, resulting in the restriction of the water uptake of BPES-8-Im and BPES-12-Im [38]. The number of absorbed water molecules on each alkyl imidazolium group was calculated for all the membranes, denoted by λ. The trend of λ was in accordance with the water uptake and the value ranged from 9.3 to 10.8, as listed in Table 1. Additionally, the static water contact angle of all four membrane materials was observed at room temperature, which was directly related to the hydrophilicity. As shown in Fig. 5, the BPES-6-Im membrane showed the minimum water contact angle (60.5°), which was lower than other membranes with relatively flexible alkyl side chains [BPES-4-Im (66.0°), BPES-8-Im (67.5°), and BPES-12-Im (77.4°)]. The tendency of the water contact angle for all membranes coincided well with the water uptake. These results indicated that an appropriate alkyl side chain length in the membranes is beneficial for facilitating water absorption. Moreover, the change of dimensional swelling behaviours of the membrane materials were obtained by comparing the swelling ratio of

Table 1 Ion exchange capacity (IEC), Water uptake (WU), Swelling ratio (SW), and Ionic conductivity (σ) of BPES-n-Im membranes. Membrane

BPES-4-Im BPES-6-Im BPES-8-Im BPES-12-Im a b c d

Length of side chains

4 6 8 12

IEC (meq g−1) The.a

Exp.b

1.88 1.78 1.71 1.56

1.80 1.75 1.64 1.51

Water uptakec (%)

Swelling ratioc (%)

Ionic conductivity

30.0 33.9 28.5 26.4

19.4 20.9 17.5 16.4

104.8 115.8 98.4 77.6

Theoretical value, calculated from the polymer composition and the degree of function. Experimental value, determined by back-titration. Determined at 30 °C. Determined at 80 °C. 1312

d

(mS cm−1)

Conductivity remained (%)

λ

60.1 67.0 70.0 73.0

9.3 10.8 9.6 9.7

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Fig. 5. (a) Water uptake, (b) swelling ratio and (c) static water contact angle of the BPES-n-Im membranes.

the BPES-n-Im membrane at various temperatures (Fig. 4 and Table 1). The swelling ratio tends to be similar to that of water absorption. As the temperature increases, the swelling ratio of all the membranes increased. When n ≤ 6, the swelling ratio increased with the length of the flexible side chains, whereas the membrane materials grafted with an excessively long flexible side chains (BPES-8-Im and BPES-12-Im) displayed a lower swelling than that of BPES-6-Im. The introduction of more methylene groups into the flexible alkyl side chains could increase the hydrophobicity of the membranes, which reduced water uptake and thus restricted the swelling. All branched copolymer membranes grafted to the various length of alkyl imidazolium side chains exhibited acceptable swelling ratio and water uptake. These branched comb-shaped membranes are promising for application as AEM materials.

trend of ionic conductivity was similar to that of the water uptake, which indicated that ion transport in the membrane materials was heavily dependent on the water molecules inside the membrane. As the temperature raised from 30 °C to 80 °C, the ionic conductivity gradually increased. This was because the rate of migration of water molecules in the membrane material increased with increasing temperature. Although the highest IEC value was obtained for BPES-4-Im, the hydroxide ions were dispersed over the membrane, resulting in inefficient ion conduction. By increasing the flexible alkyl side chain length to 6C, the BPES-6-Im membranes showed the highest conductivity at all temperatures measured, which was consistent with the clearest phase separation and maximum water uptake of the membrane, leading to ionic clusters formation. However, the ionic conductivity of the membrane materials was decreased as the length of the alkyl side chain (n = 8 and 12) further increased, which results from over self-aggregations of the membranes, as confirmed by AFM phase, SAXS and water uptake. This means that the over self-aggregation reduced the absorption of water molecules and weakened the hydrophilic/hydrophobic phase-separated morphology. However, the water molecules in the membrane can facilitate the transport of ions, and the well-developed microphase separation can build an effective ion channel for ion transport. As shown in Fig. 6 (b), the ion transport activation energy of the AEM materials with flexible alkyl imidazolium side chains was in the range of 14.13–17.45 KJ mol−1, which was similar to the reported AEM materials [49]. The BPES-6-Im membrane displayed the lowest activation energy owing to the best well-developed phase separation. These results suggested that ion transportation in the membrane required

3.5. Ionic conductivity The ionic conductivity of the AEM materials plays an important role in fuel cell performance and is related to many factors, including test temperature, IEC and WU [35,48]. The hydroxide conductivities (OH−) of the BPES-n-Im membrane materials were measured in the range of 30–80 °C at 100% RH (Fig. 6(a) and Table 1). These BPES-n-Im membranes with relatively flexible alkyl side chains exhibited good conductivities of over 77.6 mS cm−1 at 80 °C. The distinct hydrophilic/ hydrophobic phase separation formed by comb-shaped and branched structure could create an efficient channel for ion transportation and contribute to the high conductivity of the membranes. The changing 1313

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Table 2 Mechanical properties of BPES-n-Im membranes. Membrane

Tensile strength (MPa)

Elongation at break (%)

Young's modulus (MPa)

BPES-4-Im BPES-6-Im BPES-8-Im BPES-12-Im

24.7 23.9 25.0 27.1

8.4 9.3 8.7 7.6

572.9 546.6 583.8 621.3

± ± ± ±

1.3 1.5 1.1 1.7

± ± ± ±

0.5 1.1 0.3 0.8

± ± ± ±

21 16 25 25

Fig. 7. TGA curves of the BPES-n-Im membranes.

protection. Typical TGA curves are shown in Fig. 7. Three-stage weight loss of BPES-n-Im membrane materials was observed. The first stage weight loss from 260 to 340 °C could be due to the degradation of the imidazolium cationic groups. The second stage weight loss was observed at 340 °C because of the degradation of the flexible alkyl functionalized-imidazolium side chains of the membrane materials. The third step above 410 °C originated from the degradation of the copolymer backbone [10,27]. The trend of these TGA curves was consistent with the degradation of the structure of the branched comb-shaped copolymers grafted with various flexible alkyl side chains. The degradation temperature of all four membranes reached 260 °C and was far above the practical operating temperature of AEMFCs in the approximate range of 30 to 80 °C. Therefore, these membrane materials have excellent thermal stability.

Fig. 6. (a) Temperature dependence of the conductivity of the BPES-n-Im membranes and (b) Arrhenius-type temperature plot.

lower activation energy. The optimum hydroxide conductivity of the branched membranes with various alkyl side chains was obtained when the length of alkyl side chain was 6 carbon atoms (n = 6). This was mainly because the membrane material achieved the highest water uptake and the best hydrophilic/hydrophobic phase separation morphology.

3.7. Alkaline stability

3.6. Mechanical properties and thermal stability

In addition to excellent mechanical properties, dimensional stability, thermal stability and, the long-term stability of AEM is also important because it works under alkaline conditions for AEMFCs [6]. To evaluate the alkaline stability, the branched AEMs with various flexible alkyl imidazolium side chains was tested by comparing the ionic conductivity before, during and after soaking the membrane materials in 1 M KOH at 60 °C for accelerated aging. The change in the ionic conductivity was studied at 60 °C. Although those membranes with relatively flexible alkyl side chains maintained their flexibility and original appearance even after 550 h, some small floes were observed in the KOH aqueous solution. The results suggested that the branched combshaped AEMs partially degraded during testing. Fig. 8 showed that all the membranes remained above 61% of the initial conductivity after 550 h. As the alkyl side chain length increased, the change in the ionic conductivity of the branched comb membrane decreased. For example, the BPES-12-Im membrane material containing the longest length of alkyl imidazolium-functionalized side chains displayed the greatest alkaline stability, for which the conductivity decreased by 73.0%. This was mainly attributable to the increase in the electron density around

For membrane materials used in AEMFCs, their mechanical properties were indispensable parameters. [22] Because the membrane materials are used under fully hydrated conditions, the membrane materials were soaked in deionized water for > 24 h before investigation. The mechanical properties of the BPES-n-Im membranes in hydroxide form were tested at room temperature. All the branched combshaped membranes exhibited tensile strengths in the range of 23.9–27.1 MPa, Young's moduli of 546.6–621.3 MPa, and elongations at break of 7.6–9.3%, as summarized in Table 2. The BPES-6-Im membrane displayed the lowest tensile strength because it had the largest water uptake, whereby the water molecules can act as plasticizers and thus affect the mechanical properties of the membrane materials. These results suggested that the BPES-n-Im membranes were very tough and strong enough to be used as potential AEM materials. The thermal stability of the AEM materials is an important parameter because these membranes are applied in AEMFCs at a wide range of temperatures. The thermal stability of the BPES-n-Im was investigated at a heating rate of 10 °C min−1 by TGA under nitrogen 1314

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the C2 position by hydroxide ions, followed by a ring-opening reaction. 4. Conclusions We successfully synthesized highly branched poly(arylene ether sulfone)s with different lengths (4C to 12C) of flexible alkyl imidazolium-functionalized side chain imidazolium cationic groups as novel anion exchange membrane materials. The relationships between the lengths of the alkyl functionalized-imidazolium side chains on the branched copolymers and their properties were systematically investigated. The optimum ionic conductivity (OH−) was observed for the BPES-6-Im membrane. Increasing the length of the flexible alkyl side chains increases the hydrophobicity of the membrane, which was not beneficial for the construction of efficient ion transportation pathways owing to the poor water absorption. The degradation of the imidazolium groups was inhibited by increasing the length of the flexible side chains owing to the suppression of the electron-withdrawing effect; thus, the BPES-12-Im membrane displayed the highest alkaline stability. The incorporation of the comb-shaped structure of alkyl imidazolium side chains and a branched structure in BPES-n-Im membranes is a promising strategy for the preparation of AEM materials for AEMFCs. Declaration of Competing Interest There are no conflicts to declare. Acknowledgments The authors are very grateful for the financial support from the National Basic Research Program of China (2014CB643600), the Natural Science Foundation of Guangdong Province (2015A030313546), and the National Natural Science Foundation of China (51773318), the Shenzhen Sci & Tech Research Grant (JCYJ201303291 05010137, ZDSYS201507141105130, and JCYJ 20150331142303052). Appendix A. Supplementary data

Fig. 8. (a) Hydroxide conductivities and (b) hydroxide conductivity of the BPES-n-Im membranes in a 1 M KOH solution at 60 °C for 550 h.

Supplementary data to this article can be found online at https:// doi.org/10.1016/j.apsusc.2019.07.059.

the β‑hydrogen as the length of the alkyl imidazolium-functionalized side chains, which suppressed the electron-withdrawing effect and reduced the susceptibility of the imidazolium groups to attack by hydroxide ions. Therefore, the stability of the cationic groups was significantly enhanced by increasing the length of the flexible alkyl imidazolium-functionalized side chains between the copolymer mainchain and imidazolium groups [36]. The alkaline stability of these membrane materials was indeed very high compared with those of typical AEM materials, which are generally unstable at elevated temperature in a concentrated alkaline solution [35,50–53]. In summary, the as-prepared branched membranes with various alkyl imidazolium side chains demonstrated better alkaline stability than those of typical imidazolium-functionalized membrane materials composed of aliphatic polymers and aromatic polymers. The conjugation bonds present in the imidazolium groups and the phase separation promoted by the branched structure and the comb structure contributed to the improvement of the alkali stability of these imidazolium-functionalized branched membranes containing a flexible alkyl side chain [35,54,55]. Additionally, the ionic conductivity of prepared branched combshaped membranes was gradually reduced after the accelerated aging test (1 M KOH at 60 °C for 550 h). This is mainly because the imidazolium groups were degraded under alkaline conditions at a certain temperature. The degradation mechanism of the imidazolium cation was described in Scheme 3. The degradation of the imidazolium cationic groups under alkaline conditions mainly involved in the attack of

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