Multi-sulfonated polyhedral oligosilsesquioxane (POSS) grafted poly(arylene ether sulfone)s for proton conductive membranes

Polymer 123 (2017) 21e29

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Multi-sulfonated polyhedral oligosilsesquioxane (POSS) grafted poly(arylene ether sulfone)s for proton conductive membranes Zhongying Wu a, 1, Yiyao Tang a, 1, Dewen Sun b, Shujiang Zhang a, c, Yixuan Xu a, Hua Wei a, Chenliang Gong a, c, * a b c

State Key Laboratory of Applied Organic Chemistry, College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou 730000, PR China State Key Laboratory of High Performance Civil Engineering, Nanjing, Jiangsu 211103, PR China Key Laboratory of Special Function Materials and Structure Design of Ministry of Education, Lanzhou University, Lanzhou 730000, PR China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 20 December 2016 Received in revised form 23 June 2017 Accepted 1 July 2017 Available online 1 July 2017

Organic-inorganic hybrid polymers based on poly(arylene ether sulfone) (PAES) containing multisulfonated polyhedral oligosilsesquioxane (POSS) pendants (PAES-sPOSS-x) were synthesized to improve the inorganic fillers' distribution in polymer matrix and to enhance the ion conductivity and swelling resistance of aromatic polymer electrolyte membrane. The synthesis of the graft hybrid polymers in this study involves the grafting of mono-functional 3-Iodopropylheptaphenyl POSS with an -OH functionalized polymer main-chain and postsulfonation of the POSS grafted PAES. Notably, nearly all of the benzene rings on the POSS side chains were sulfonated successfully without any sulfonation of main chains. The structure with hydrophobic aromatic backbone and hydrophilic hybrid side chains made an improvement of the nano-phase hydrophilic/hydrophobic separation, thereby facilitating the formation of continuous proton exchange channels. The transmission electron microscopy (TEM) images of the comb-shaped polymer membranes showed homogeneous sulfonated POSS (sPOSS) dispersion, significant phase separation and well-defined interconnected hydrophilic networks. This morphological structure leaded to dramatically enhanced comprehensive properties as proton conductive membranes. © 2017 Elsevier Ltd. All rights reserved.

Keywords: Multi-sulfonated polyhedral oligosilsesquioxane Phase separation Side chain Proton exchange membrane

1. Introduction Proton exchange membrane fuel cells (PEMFCs) are considered as environmentally friendly energy conversion devices for both stationary and portable power applications because of their excellent conversion efficiency, high power density and zero emission of pollutants [1,2]. During the operation of PEMFC, proton exchange membranes (PEMs) play a key role in selective and efficient transport of protons between anode and cathode, isolation of fuel and electrons, and thus considered as the critical component in PEMFCs system [3e5]. Among the various types of ionomers of PEMs, perfluorosulfonic acid membranes such as Nafion®117, are widely investigated and used in this field due to their high conductivity below 90
C, excellent mechanical and chemical stability

* Corresponding author. State Key Laboratory of Applied Organic Chemistry, College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou 730000, PR China. E-mail address: [email protected] (C. Gong). 1 These authors contributed equally to this work. http://dx.doi.org/10.1016/j.polymer.2017.07.001 0032-3861/© 2017 Elsevier Ltd. All rights reserved.

[2]. However, there are several drawbacks such as high fuel permeability, exorbitant price, difficult synthetic procedure, and high methanol permeability, which restrict the potential applications of the membranes in PEMFCs [6,7]. As alternatives to Nafion® membranes, sulfonated aromaticbased polymers such as sulfonated polyimides (SPIs) and sulfonated poly(arylene ether)s (SPAE)s have been paid more and more attentions due to their lower cost and excellent thermal stabilities [8e10]. However, these traditional sulfonated aromatic-based polymers have some disadvantages, such as the excessive water swelling with the high sulfonation degree and the low proton conductivity under low relative humidity [11]. In order to overcome these barriers, the effective formation of well-connected ion clusters structures for rapid proton conduction has been obtained by the polymer architecture including graft or comb-shaped polymers [12e15], highly sulfonated copolymers [16e18], block copolymers [19e22], and organic-inorganic polymers [23e26]. For the graft or comb-shaped copolymers containing ionic groups, wellinterconnected nanostructures or microphase separation will be easily formed via self-assembly of graft or comb-shaped

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copolymers, which contributes to the high proton conductivity [25e28]. Moreover, similar to the structure of the DuPont's Nafion®117 membranes, sulfonated comb-shaped copolymers can separate the hydrophobic domains of copolymer main chains from the hydrophilic bulky SO3H domains of side chains, which strongly inhibits the water swelling of the membrane [12,13,17,25,27]. Guiver et al. have compared the properties of diblock copolymers with those of comb-shaped copolymers having ionic contents [28]. The morphologies of comb-shaped copolymer membranes demonstrated an interconnected network of small sulfonic acid clusters similar to the archetypical Nafion®117, which is the principal reason of attaining high proton conductivity at low ionic contents. In contrast, the morphologies of diblock copolymer membranes exhibited a worms-like microphase separation structure which led to excessive swelling, as well as anisotropic proton conductivity. Based on the above research findings, graft or combshaped copolymers are the most promising approach for proton exchange membranes applications because of their relatively low swelling ratio and high proton conductivities under partially hydrated conditions [26]. Polyhedral oligomeric silsesquioxane (POSS) have been brought into focus as promising oligomerization scaffolds due to their unique cubic structure, monodisperse particles (about 0.5 nm in diameter) and multi-functionality, especially the excellent building blocks for nano-composites and organic-inorganic hybrid materials [29e33]. The elemental and structural features combining reactive functional groups and inorganic segments allowed a “bottom-up” approach to the creation of organic-inorganic hybrid materials. The hybrid organic-inorganic nanostructure of POSS confirms effectively high performance on chemical and thermal stability, and the tethered reactive organic groups are proved to introduce determined molecules to perform special properties such as biocompatibility, self-assembility, extreme rigid environment stability and photooxidative resistance [23,34,35]. Recently, POSS-containing organic-inorganic hybrid materials have been paid more attention as proton exchange membranes (PEMs) in applications of PEMFC [23,36]. For example, Hartmann-Thompson et al. blended sulfonated polyhedral oligomeric silsesquioxane (sPOSS) with sulfonated polyphenylsulfone (S-PPSU) [25]. The sPOSS-S-PPSU composite exhibited higher conductivity in comparison with that of plain S-PPSU membranes. However, the high surface area and surface energy of POSS always lead to its poor dispersion in polymer matrix by direct blending, thereby hindering the further improvement of the proton conductivity and mechanical strength with high content of POSS [37,38]. Additional research demonstrated that cross-linking of multifunctional POSS and polymer can improve the dispersibility of POSS in the membrane [23,24,39e41]. In our previous work, we synthesized octa(aminopheny)silsesquioxane cross-linked sulfonated polyimides [24]. Although the POSS crosslinked membranes exhibited improved proton conductivity, higher strength, superior chemical and thermal stabilities compared with that of the traditional linear sulfonated aromatic polyimide membrane, the high content of POSS led to insolubility and poor processability of the POSS crosslinked polyimide. Additionally, we introduced POSS into the main chain of sulfonated polyimide [23]. The results indicated that the POSS in polymer main chain efficiently improved the dispersibility of POSS in polyimide with high content and increased the proton conductivity due to the increased free volume of ionic clusters around POSS domains. Based on the observations above, the comb-shaped sulfonated polymers are the ideal materials for proton exchange membranes, and introducing of the hybrid POSS in membrane can improve comprehensive properties as proton conductive membranes. Therefore, we proposed to introduce the sulfonated POSS into polymer side chain and expected this organic-inorganic side chain

type membrane could strike a balance between water swelling and proton conductivity. While these POSS composite membranes remain advantages of POSS, such as thermal stability, water retention property and oxidative resistance, the sulfonated POSS can still produce a nanoscale organic-inorganic interface which provides a channel for protons delivering [21]. Moreover, incorporating of POSS into polymer side chain can increase the free volume of hydrophilic units, which facilitates an aggregation of sulfonic acid groups. On the other hand, the hydrophobic aromatic main chain is relatively further from the hydrophilic side chains, which can decrease the free radical attack and improve the membrane stability, in comparison with that of sulfonated aromatic polymer with the acid units dispersed directly along the main chain. This special combination could be expected to make an improvement of the nano-phase hydrophilic and hydrophobic separation of the materials, which may be an effective way to develop high performance PEMs. In this work, a series of multi-sulfonic acid functionized POSS grafted PAES (PAES-sPOSS-x) were prepared for proton conductive membranes. The comb-shaped copolymers with different ionexchange capacity (IEC) of PAES-sPOSS-x were obtained by controlling the -OCH3 molar ratio. The details of the synthesis procedures and the membranes properties such as thermal and oxidative stabilities, mechanical strength and proton conductivity were fully discussed in this article. 2. Experiments 2.1. Materials Phenyltrimethoxysilane (95%), (3-chloropropyl) trimethoxysilane (98%), pentafluoroanisole, borontribromide, potassium carbonate and I2 were purchased from Aladdin (China) used as received. Dimethyl sulphoxide (DMSO), tetrahydrofuran (THF), toluene from Tianjin Guangfu factory were distilled under reduced pressure right before use. Bis(4-hydroxyphenyl) sulfone (99%) and 4-fluorophenylsulfone were obtained from Aldrich and purified by recrystallized from ethanol. Other solvents were used as received. 3-Iodopropylheptaphenyl POSS [(C6H5)7(ICH2CH2CH2)Si8O12] was synthesized by phenyltrimethoxysilane according to the published procedure [42]. The chemical structure was identified by 1H NMR and 29Si NMR (Fig. S1 and Fig. S2). 1H NMR (400 MHz, CDCl3, ppm): 7.81 (7H, m), 7.75e7.36 (14H, m), 3.19 (2H, t), 2.07 (2H, m), 0.98 (2H, t). 29Si NMR (80 MHz, CDCl3): 66.7 ppm, 68.3 ppm. 2.2. Synthesis of copoly(arylene ether sulfone) containing -OCH3 groups (PAES-x-Me) Copoly(arylene ether sulfone) containing -OCH3 groups (PAESx-Me), where x represents the methoxy molar content, was prepared by the condensation reaction between the pentafluoroanisole, bis(4-hydroxyphenyl) sulfone and 4-fluorophenylsulfone monomers. A typical procedure, the synthesis of PAES-11-Me is described below (Scheme S1). 1.500 g (6.00 mmol) of 4,4’-dihydroxydiphenylsulfone, 0.130 g (0.66 mmol) of pentafluoroanisole, 1.335 g (5.34 mmol) of 4-fluorophenylsulfone, 0.994 g (7.20 mmol) of K2CO3 and toluene (10 mL) were mixed and dissolved in 18.50 mL of DMSO. The mixture was refluxed at 140
C for 6 h to remove the toluene/water mixture from the system, and then the reaction continued at 180
C for 10 h. The viscous solution was cooled down and poured into 100 mL deionized water. The precipitate was filtered, washed with water several times and dried at 80
C under vacuum overnight to obtain the PAES-11-OCH3 ionomer (yield: 97.6%). 1H NMR (400 MHz, DMSO-d6, ppm): 8.07e7.81 (22H, m), 7.35e7.08 (22H, m), 3.76 (1H, s).

Z. Wu et al. / Polymer 123 (2017) 21e29

2.3. Synthesis of copoly(arylene ether sulfone) containing -OH groups (PAES-x-OH) (Scheme S1) PAES-11-Me (2.00 g) and CHCl3 (35 mL) were added to a 100 mL three-neck flask equipped with a nitrogen inlet and a magnetic stirrer. Then BBr3 solution (1 mL BBr3 in 3 mL CHCl3) was added dropwise to the PAES-11-Me solution and reacted at room temperature for 24 h. The reaction mixture was then poured into deionized water. The precipitation was filtered, washed with deionized water and dried at 100
C under vacuum overnight to obtain the PAES-11-OH ionomer (yield: 95.0%). 1H NMR (400 MHz, DMSO-d6, ppm): 10.67 (0.26H, s), 8.07e7.81 (22H, m), 7.35e7.08 (22H, m). 2.4. Synthesis of comb copolymers PAES-POSS-x (Fig. 1) The synthesis of comb copolymers PAES-POSS-11 was selected as a typical procedure described below. PAES-11-OH (1.10 g, 0.24 mmol), I-POSS (0.30 g, 0. 27 mmol), K2CO3 (0.04 g, 0.24 mmol), 6 mL of DMSO, and 2 mL of toluene were added into a 25 mL flask equipped with a magnetic stirrer and Dean-Stark trap. The reaction was heated at 130
C for 8 h and then reacted at 180
C for 10 h. After completion of the reaction, the mixture solution was cooled to room temperature and poured into a large excess of deionized water. The obtained comb copolymers PAES-POSS-11 were dried at 80
C in vacuo for 12 h (yield: 93.2%). 1H NMR (400 MHz, DMSO-d6, ppm): 8.17e7.57 (66H, m), 7.55 (2H, d), 7.47e6.95 (103H, m), 3.73 (2H, m), 1.62 (2H, m), 1.35 (2H, t). 2.5. Sulfonation of the comb copolymer PAES-sPOSS-x (Fig. 1)

the synthesis of PAES-sPOSS-x via post-sulfonation of the PAESPOSS-x. The PAES-x-OCH3 was prepared by the polycondensation reaction between the diphenoxyl monomer (4,40 -(hexafluorolsopropylldene)dlphenol) and the difluoro monomers (pentafluoroanisole and 4,40 -difluorobenzophenone) (Scheme S1). The structure of the PAES-x-OCH3 polymer was confirmed by 1H NMR spectroscopy (Fig. S4). For the synthesis of PAES-x-OH, the conversion of the methoxy to reactive phenoxyl using BBr3 was effectively conducted in CHCl3, and confirmed by 1H NMR. The disappearance signals of methoxy group proton at 3.76 ppm, the phenoxyl group proton signals appeared at 10.67 ppm and no peaks around 3.76 ppm confirmed that the methoxy group was completely converted to hydroxyl group. The molecular weights (Mn) and polydispersity indices (PDI, Mw/Mn) of all the PAES-x-OCH3 co-polymers were 31500e42500 and 2.74e4.43, respectively. The broadening of PDI distribution with increasing the content of pentafluoroanisole could be attributed to the steric and electronic effects of the -OCH3 groups reduce the reactive activity of pentafluoroanisole during polymerization. The synthetic scheme of PAES-POSS-x and PAES-sPOSS-x are shown in Fig. 1. The 1H NMR spectrum (Fig. 2a) revealed that the proton of the -OH group at 10.67 ppm disappeared, and the I-POSS protons appeared at 7.48 ppm, 3.51 ppm, 1.79 ppm, and 1.13 ppm, respectively, which provides the evidence for complete grafting of I-POSS to PAES-x-OH side chains. The comb copolymers PAES-POSS-x were then post sulfonated by chlorosulfonic acid in dichloromethane solution. The sulfonated products PAES-sPOSS-x were isolated as white powders by filtering. As shown in Fig. 2b, the PAES-sPOSS-x and the parent copolymer PAES-POSS-x are compared by 1H NMR spectroscopic data. A new

To a mixture of 1.0 g PAES-POSS-11 in 40 mL dry dichloromethane was added dropwise with vigorous stirring a solution of chlorosulfonic acid (0.6 mL, 3 mmol) in 10 mL of CH2Cl2 for 12 h, then the mixture solution was stirred for 30 min. The resulting precipitate was filtered, washed with water several times and dried overnight under vacuum at 80
C to obtain sulfonated combshaped copolymer PAES-sPOSS-11 (yield: 99.2%). 1H NMR (400 MHz, DMSO-d6, ppm): 8.28 (14H, s), 8.17e7.57 (68H, m), 7.46 (14H, s), 7.38e6.89 (68H, m), 3.73 (2H, m), 1.62 (2H, m), 1.35 (2H, t). 2.6. Membrane preparation and acidification To prepare PAES/sPOSS hybrid membrane, the polymer solution in DMSO 5% (w/w) was filtered (0.45 mm filter), and the resulting clear solution was cast onto a flat glass microscope slide in a muffle furnace and dried at 120
C for 10 h. After cooling down, the membranes were immersed in hot water and peeled from the slides. The resulting salt form film was then soaked with 1 M H2SO4 for 24 h, washed with deionized water several times, and drying at 80
C under vacuum overnight. The thickness of the resulting membranes is in the ranged of 35e46 mm. 3. Results and discussion 3.1. The comb-shaped copolymers preparation and molecular structure analysis The synthesis of these PAES-sPOSS-x comb-shaped copolymers, where x represents the molar content of sPOSS, is a four-step procedures including (1) the synthesis of poly(arylene ether sulfone) containing -OCH3 (PAES-x-OCH3); (2) the synthesis of poly(arylene ether sulfone) containing eOH groups (PAES-x-OH) by demethylation; (3) the preparation of the poly(arylene ether sulfone) containing -POSS groups via graft polycondensation; and (4)

23

Fig. 1. Synthetic scheme of PAES-POSS-x and PAES-sPOSS-x.

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Fig. 2. 1H NMR spectra of a) PAES-POSS-11 and b) PAES-sPOSS-11.

signal at 8.21 ppm assigned to the sulfonated pendent phenyl groups of POSS (H13) and the weakened of the H12 signal in comparison with that of Fig. 2a confirm that the phenyl groups of POSS were sulfonated successfully. The areas of the H12 and H13 resonance after sulfonation was used to measure completion rate of sulfonation. The equal area of H12 and H13 in Fig. 2b provides evidence for complete or almost complete sulfonation on the pendent phenyl groups of POSS. Notably, the other aromatic protons in the polymer main chain (H5, H6, H7, H8) were still remained after the sulfonation reaction, which indicates no sulfonation occurred in the main chain of PAES-sPOSS-x (Fig. 2b). Moreover, the IEC value of PAES-sPOSS-x was in the range of 0.71e1.53 mequiv g1 according to the 1H NMR results, which was consistent with the titration values (Table 2). The 29Si NMR spectrum of PAES-sPOSS-11 was also obtained for further study the integrity of POSS after sulfonation reaction (Fig. S3). Compared with that of I-POSS (Fig. S2), the signal at 66.0 ppm corresponding to the Si connected by aliphatic side

chain (Si1) is consistent with the Si1 of I-POSS. The sulfonation reaction contribution to the chemical shifts of Si-2 (from 68.3 to 78.0 ppm) gives the evidence of complete or almost complete sulfonation of I-POSS. Moreover, the presence of only two peaks at 66.0 ppm and 78.0 ppm and no new peak appeared in the spectrum confirm that the cubic structure of POSS remained intact after sulfonation reaction [42]. The FT-IR spectroscopy clearly demonstrates that the combshaped PAES copolymers with pendant sulfonated polyhedral oligosilsesquioxane (POSS) group were synthesized successfully (Fig. S5). The adsorption peaks appeared at 3020 cm1, 2901 cm1 and 1471 cm1 belong to the characteristic adsorptions of -CH2groups of the -POSS structure. The characteristic peaks of sulfonic acid groups were observed at 1088 cm1 (asymmetric S¼O stretching), 1029 cm1 (symmetric S¼O stretching) and 609 cm1 (C-S stretching). The relatively weaker peaks due to POSS were also observed between 1143 cm1 (Si-O-Si asymmetric stretching) and 1231 cm1 (CH2-Si stretching) [23].

Z. Wu et al. / Polymer 123 (2017) 21e29

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Table 1 The Mn, Mn/Mw, tensile strength, elongation at break and Young's modulus of the PAEK-sPOSS-x copolymers. Sample

Mn

Mw/Mn

Tensile strength (MPa)

PAES-sPOSS-5 PAES-sPOSS-8 PAES-sPOSS-11 PAES-sPOSS-14

42500 37200 35800 31500

2.27 3.11 3.37 4.43

18.7 17.2 16.9 14.3

± ± ± ±

Elongation at break (%)

1.1 0.8 0.9 1.0

14.5 19.7 38.3 44.8

± ± ± ±

Young's modulus (GPa)

12 1.4 1.2 1.0

1.7 1.9 2.1 2.6

± ± ± ±

0.2 0.2 0.1 0.1

Table 2 The thickness, IEC, water uptake, l, swelling ratio and oxidative stability of the PAES-sPOSS-x copolymers. sample

Thickness (mm)

IEC (mequiv g1) Cal.

PAES-sPOSS-5 PAES-sPOSS-8 PAES-sPOSS-11 PAES-sPOSS-14 a b c

46 41 32 35

a

0.71 1.03 1.36 1.53

titr.

b

0.65 0.95 1.21 1.44

Water uptake (%)





l (H2O/SO3H)


Swelling ratio (%)





Oxidative stability (wt%)c




20 C

80 C

20 C

80 C

20 C

80 C

9 12 17 21

29 39 47 59

4.1 4.9 5.4 6.7

11 16 19 21

2 5 6 8

13 17 23 28

98.7 98.1 97.7 95.4

Calculated from chemical formula of the polymer repeat unit. Measured by acid-base titration method. The weight remained after treating in Fenton's reagent at 80
C for 1 h.

3.2. Thermal and mechanical properties The thermal stability of the PAES-sPOSS-x comb-shaped copolymers and the effect of the sPOSS fillers on the thermal properties of the hybrid membranes were investigated using TGA under a heating rate of 20
C min1 and nitrogen atmosphere (Fig. S6). The sulfonated comb-shaped copolymers in acid form observed two distinct thermal degradation profiles, which were assigned to the loss of sulfonic acid groups and the degradation of the main chain, respectively. The PAES-sPOSS-x copolymers showed excellent thermal stability, with their 5% weight loss temperatures (Td5%) above 270e370
C, and 10% weight loss temperatures (Td10%) up to 450e520
C, respectively. The first weight loss around 270
C was due to the loss of sulfonic acid groups, while the second degradation step (~420
C) was attributed to decomposition of the copolymers main-chain. Moreover, the degradation temperature of PAES-sPOSS-14 copolymers was lower than that of PAES-sPOSS-5, which is ascribed to the loss of water molecules absorbed by the sPOSS groups. Besides, the catalytic effect of -SO3H leading to the increase of polymer decomposition [43]. Fig. S7 represents the DSC curves of PAES, PAES-POSS-11 and PAES-sPOSS-11. The introduction of POSS into the side chains of PAES displayed slightly higher Tg than that of pure PAES, which can be ascribed to the double role of phenyl functionalized POSS in the polymer matrix. On one hand, the phenyl functionalized POSS present in the form of small crystalline entities is increasing the Tg, because the p-p interaction of phenyl groups constitute a restriction to chain mobility. On the other hand, the POSS acting like a plasticizer increases free volume in the copolymers, which results in the polymer chains being unable to densely pack in the glassy state and causing a decrease of the Tg [44]. Finally, the reinforcement of phenyl interaction exceeds the plasticization effect of POSS, thereby increasing the Tg slightly after POSS grafting. The Tg value of PAES-POSS-11 after sulfonation is reduced from 244
C to 235
C, which could be ascribed to the sulfonation of POSS phenyl group. The hydrophilic sulfonic acid groups on the phenyl groups of POSS disturb the interaction of p-p interaction of phenyl groups and increase the free volume of side chains, which enable a change in the state of polymer from more crystalline to more amorphous resulting in the reduction of the glass transition temperature. In general, all of the results showed that the PAES-sPOSS-x copolymers exhibit high thermal properties, and they can work stably for a long time within 200
C, which satisfies the requirement for a PEM with good thermal performance.

Mechanical strength of the membrane was crucial affects to the durability of PEMFCs. The mechanical strength of the prepared comb-shaped membrane was tested by a tensile tester under fully hydrated state and the mechanical properties of the comb-shaped membranes are summarized in Table 1. The corresponding stressstrain curves are represented in Fig. 3. All obtained comb-shaped membranes had the tensile strengths of 14.3e18.7 MPa, elongation at break of 14.5e44.8% and Young's modulus of 1.7e2.6 GPa. In hydrated state conditions, all of the comb-shaped membranes exhibited high enough mechanical strength to be used in PEMFCs. Moreover, the tensile strength decreased with the increase of sPOSS content, because of the more sulfonic acids promoting more pronounced phase separation that decrease the forces in inter-chain of PAES [45]. Overall, PAES-sPOSS-x membrane performed outstanding mechanical properties in comparison with those of other sulfonated aromatic polymer in hydrated state conditions [46,47]. 3.3. Morphological characterization The hydrophilic-hydrophobic micro-phase separation morphology closely relates to the proton transport pathway and

Fig. 3. Stress-strain curves of the PAES-sPOSS-5, PAES-sPOSS-8, PAES-sPOSS-11, PAESsPOSS-14 membranes in fully hydrated state.

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Z. Wu et al. / Polymer 123 (2017) 21e29

water uptake in the ionomer membranes. The morphologies of comb-shaped membranes were investigated by transmission electron microscopy (TEM) [17,48,49]. The dark domains assigned to POSS exhibited well dispersion in polymer matrix which was proved by energy-dispersive X-ray (EDX) analysis (Fig. 4). The sPOSS aggregates showed smaller and more uniform distribution compared with those of directly blending or cross-linking POSS-polymer composites [49]. Compare the EDX analysis of the bright domains with dark domains, the bright domains also contain Si element, but the content of silicon in the bright parts is less than that in the dark parts, which indicates that the most of sPOSS were aggregated as ionic clusters. For further investigation of the nano-phase separated morphologies, all of the PAES-sPOSS-x membranes were converted into its Pb2þ form by ion exchange of the sulfonic acid groups in 0.5 M lead acetate aqueous solution for the TEM characterization. Although Guiver et al. pointed out that TEM images integrate the morphological structure over 100 nm, which implies that overlapping intensity from randomly distributed 1e10 nm diameter channels through the film thickness will significantly distort the spot sizes in the image [50], the TEM can still give some information for microphase separation, and the comparison between PAESsPOSS-x with different contents of sPOSS also could clarify the influence of sPOSS to the hydrophobic and hydrophilic aggregates. As shown in Fig. 5, the dark domains correspond to the hydrophilic sPOSS graft chains, and the bright regions represented hydrophobic domains (PAES backbone). The distinct phase separation and connected ionic channels of small ionic clusters were observed in TEM images. For PAES-sPOSS-5 membrane with the low IEC values of 0.71 mequiv g1, only small ion clusters could be observed and no significant phase separation was obtained (Fig. 5a). As the content of sPOSS increased, the microphase separation was getting clearer, which demonstrate that the sPOSS could promote the hydrophilic and hydrophobic separation effectively. For PAES- sPOSS-14 combshapes membranes with the high IEC values of 1.53 mequiv g1, the well-defined and interconnected hydrophilic network in the size 4e10 nm (Fig. 5d) were observed. In specific, the morphologies should be caused by two main reasons including (1) the high-

Fig. 5. TEM phase images of lead ion stained comb-shaped copolymer membranes a) PAES-sPOSS-5, b) PAES-sPOSS-8, c) PAES-sPOSS-11 and d) PAES-sPOSS-14.

density sulfonic acid of the POSS in the side chain and the better mobility of the side chains promote the formation of ion clusters with better connectivity; (2) the hydrophobic main chain contains fluorine atoms and the hydrophilic sulfonated POSS in side chain facilitate the hydrophilic and hydrophobic microphase separation in the membranes. The factors above helped to retain water molecules, provided an effective proton transport through hydrophilic ionic channels [26] and then contributed to the proton conductivity [23,27]. Based on these results above, the PAES-sPOSS-x combshaped membranes are expected to show high proton conductivity

Fig. 4. TEM images of membranes of a) PAES-sPOSS-5, b) PAES-sPOSS-8, c) PAES-sPOSS-11, d) PAES-sPOSS-14 and EDX analysis spectra of PAEK-sPOSS-14 membrane.

Z. Wu et al. / Polymer 123 (2017) 21e29

and decrease methanol crossover [26]. The microphase characteristics will be further discussed with swelling ratio and proton conductivity properties later. 3.4. IEC, water uptake and dimensional stability The IEC, swelling ratio, and water uptake of the PAES-sPOSS-x were tested and presented in Table 2. IEC is a constant representing the amount of the exchangeable protons in sulfonation membranes [51,52]. The IEC values of tested comb-shaped membranes were determined by classical acid-base titration method and the measured IEC values of PAES-sPOSS-x were agreed with the theoretical values (0.71e1.53 mequiv g1), which proved that the demethylation and sulfobutylation reaction were complete. The increase of sPOSS content yielded higher IEC values in the PAES-sPOSS-x comb-shaped copolymer membranes and also leaded to the enhancement of proton conductivities of the PAES-sPOSS-x comb-shaped copolymer membranes. Water is the main vehicle by which protons are transported through the membranes according to “vehicle mechanism”, therefore it is an essential requirement for promoting proton conductivity [26]. The water uptake was measured by temperature in the range of 20e80
C with the ratio of the weight after being immersed in water for 24 h. Water uptake can also be expressed as the number of H2O molecules per sulfonic acid group (l ¼ H2O/ SO3H). The water uptake of the comb-shaped membranes increased from 29.8% of PAES-sPOSS-5 to 59.1% of PAES-sPOSS-14 at 80
C with increasing of IEC (Table 2 and Fig. 6a). Moreover, similar to Nafion®117, the water uptake of PAES-sPOSS-x at a given IEC increased slowly with the temperature increasing from 20
C to 80
C. This phenomenon might be attributed to the concentrating sulfonate flexible pendant (sPOSS groups) in PAES-sPOSS-x

27

copolymers change the swelling anisotropy of the membrane [53], and the better hydrophilic performance of multi-sulfonic-base POSS could form hydrophilic regions easily, thereby making the water absorption much easy even at relatively low temperature [26]. As expected, similar values of l (the number of H2O molecules per sulfonic acid group) were calculated for all the prepared membranes (Table 3). Each sulfonic acid group attracts approximately 11e21, which is much higher than that of Nafion®117 (12 water molecules) [54]. The PAES-sPOSS-14 membrane with high IEC values (1.53 mequiv g1) exhibited an obvious increase in water sorption, which may attribute to the hydrophilic ionic clusters forming a continuous morphology, and resulting in rapid increase of water uptake [55]. Fig. 6b compares the swelling ratio of the obtained membranes. The prepared sulfonated POSS comb-shaped membranes (PAES-sPOSS-x) with flexible side chain were expected to minimize or eliminate disadvantage of strong swelling caused by high degree of sulfonation for traditional main-chain-style sPAES, and to enhance nanoscale phase separation of the sulfonated polymer membranes by molecular design of polymer [55]. Experimental results showed that the swelling ratio of PAES-sPOSS-5, -8, -11, and 14 membranes ranged from 13.2% to 28.0% at 80
C, (Fig. 6b). The swelling ratio of the PAES-sPOSS-x comb-shaped membranes increased with increasing IEC, owing to the -SO3H enhance the hydrophilic of the sulfonated polymers. The PAESsPOSS-x comb-shaped copolymers had a much smaller swelling ratio than that of the traditional main-chain-style SPAES copolymers, which could ascribed to the concentrating of the sulfonic acid on the side chain promotes a good nano-phase separation, thereby interrupting swelling increase [26,49]. Besides, the PAESsPOSS-x membranes showed the anisotropic membrane swelling, in which the membranes dimensional change was smaller in plane direction than that in thickness (Fig. 6c). The high rigidity of the

Fig. 6. The water uptake (a), swelling ratio (b), dimensional swelling data in water at 40
C (c) and proton conductivity at 100% RH (d) for PAES-sPOSS-5, PAES-sPOSS-8, PAES-sPOSS11, PAES-sPOSS-14 and Nafion® 117 membranes.

28

Z. Wu et al. / Polymer 123 (2017) 21e29

Table 3 The Ea, conductivity, methanol permeability, selectivity of the PAES-sPOSS-x and Nafion®117 membranes. Sample

PAES-sPOSS-5 PAES-sPOSS-8 PAES-sPOSS-11 PAES-sPOSS-14 Nafion®117 a

Ea (kJ mol1)

12.41 10.37 9.73 7.42 8.94

s (mS cm1) 30
C

80
C

17 26 39 60 78

37 68 98 142 122

Pa (107 cm2 s1)

Selectivity (104 Ss cm3)

1.24 1.72 6.41 7.81 18.6

23.38 32.95 18.20 14.05 4.25

Measured under wet condition at 30
C.

sPOSS groups in main chains prevented the planar orientation of polymer chains, and contributed to the maintaining of dimensional stability [26,55]. Free radicals, such as HO, HO2, typically attack the main chains of the polymer and affect the long-term application of the membranes in PEMFC. The oxidative stability of the prepared membranes was evaluated by measuring the residual weight after the membrane sample immersion in Fenton's reagent (2 ppm FeSO4 in 3% H2O2) at 80
C for 1 h (Table 2). Residual weights of PAES-sPOSSx comb-shaped ionomer were all above 95% after 1 h enduring. The prominent enhanced oxidative stability is mainly due to incorporation of sPOSS groups. The introduction of POSS into polymers can reduce the rate of volatiles' release from the material and shield the weak points from the polymer structures [56]. Moreover, the incorporation of Ph-F groups in the main chain could improve backbone hydrophobicity, which also decreases the attack of HO radicals to polymer backbone in statistically and enhances the oxidative stability of the comb-shaped membrane [26]. In addition, the PAES-sPOSS-x membranes performed a decreasing oxidative property with the increase of the IEC values because the high IEC leads to the high water uptake, but the water uptake of membranes have greatly negative affects to their oxidative stability. The oxidative stability results exhibit that the PAES-sPOSS-x combshaped membranes present higher oxidative stability compared with that of many traditional aromatic sulfonated membranes under the same conditions. 3.5. Proton conductivity, methanol permeability and the relative selectivity Proton conductivity (s) is a key property of the membrane used as PEM. In general, it is necessary for PEMs in fuel cells that the proton conductivity is greater than 102 S cm1 [26,57]. The proton conductivity of PAES-sPOSS-x comb-shaped membranes at fully hydrated was measured in 20e80
C and the results were given in Fig. 6d. As expected, the conductivities increased with increasing IEC values and increasing temperature. As the IEC values increased from 0.71 to 1.53 mequiv g1, the comb-shaped membranes showed high proton conductivity from 0.037 S cm1 to 0.142 S cm1 at 80
C. Remarkably, under fully hydrated conditions at 80
C, the proton conductivity of PAES-sPOSS-14 membrane was 0.142 S cm1, which was higher than that of Nafion®117 (0.122 S cm1). Compared with traditional sulfonated aromatic polymer [11], considering the PAES-sPOSS-x comb-shaped sulfonated membranes possessed similar IEC value and backbone, the excellent conductivity may attribute to two main factors: (1) the highly hydrophobic polymer backbone and the highly sulfonated flexible grafting chain (-sPOSS group) of the polymers could cut down the limitation of backbone rigidity on the mobility of higher concentration sulfonic acid groups, which enhanced the aggregation of ionic clusters and improved the well-defined nano-phase separation [26]. (2) The well dispersed sPOSS in the PAES-sPOSS-x side chain, which displayed in the TEM images (Fig. 4), can alter the

intrinsic micro-structure of comb-shaped copolymer chains and affect the proton connectivity of ionic domains in the hybrid membranes, which may be responsible for its relatively high proton conductivity [26]. The activation energy (Ea) of conductivity is calculated by fitting the Arrhenius equation and the calculated data were shown in Table 3. Apparently, PAES-sPOSS-x comb-shaped membranes had much smaller Ea values even than Nafion®117 membranes except for PAES-sPOSS-5 (due to its insufficient IEC). Compared with traditional SPEEK copolymer (17e20 kJ mol1) [58], PAES-sPOSS-11, PAES-sPOSS-14 exhibited 1.5e2 times smaller Ea value, and the low Ea makes it easier to proton transfer. In summary, the PAES-sPOSS-x comb-shaped membranes exhibited a comparable conductivity. When the methanol was used as fuel in direct methanol fuel cells (DMFCs), the methanol permeability is also a key property for cells. Methanol permeability of polymer membranes was measured in a glass cell which contains the membrane clamped between two compartments. The results are listed in Table 3 and all of the combshaped membranes exhibited excellent methanol permeability, 1.42  107 to 7.81  107 cm2 s1 (30
C), which are much smaller than 18.6  107 cm2 s1 of Nafion®117. Moreover, the methanol permeability of PAES-sPOSS-x comb-shaped membranes increases slowly with the increase of POSS content. The results showed that the introduction of large side chains leaded to an increase of methanol permeation due to the plasticization effect. The selectivity values of the prepared membranes were calculated by the ratio of the proton conductivity and methanol permeability (s/P) at 30
C for evaluation of the combined effect of proton conductivity and methanol permeability. As listed in Table 3, all comb-shaped membranes exhibited higher relative selectivity from 14.05  104 to 23.38  104 S cm3 and 4e6 times higher than Nafion®117 membrane (4.25  104 S cm3). High relative selectivity value revealed that the obtained PAES-sPOSS-x membrane was particularly suitable for DMFCs.

4. Conclusions The hybrid poly(arylene ether sulfone)s contained hydrophobic main chain backbones and hydrophilic highly sulfonated POSS side chains have been successfully synthesized and characterized. The sPOSS was used to provide highly dense sulfonic acid groups without increasing solubility in water, and the main chain containing fluorine atoms formed a hydrophobic domain, which made an improvement of the nano-phase hydrophilic and hydrophobic separation of the materials. This newly poly(arylene ether sulfone) with sPOSS pendant membrane showed high water uptake, good swelling resistance, low methanol permeability and applicable proton conductivity. The obtained membranes also exhibited adequate thermal stability, mechanical strength and chemical durability. The results demonstrated that the obtained combshaped membranes could be potentially applied as polymer electrolytes in PEMFCs.

Z. Wu et al. / Polymer 123 (2017) 21e29

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