Sulfonated poly(arylene ether sulfone) functionalized polysilsesquioxane hybrid membranes with enhanced proton conductivity

Abstract Sulfopropylated polysilsesquioxane and –COOH containing fluorinated sulfonated poly(arylene ether sulfone) composite membranes (SPAES-SS-X) have been prepared via an in situ sol–gel reaction through the solution casting technique. The composite membranes showed excellent thermal and chemical stability, compared to the pristine SPAES membrane. The uniform dispersion of the sulfonated SiOPS nanoparticles on the polymer matrix was observed from the scanning electron microscope images. Atomic force microscopy and transmission electron microscopy images indicated significantly better phase-separated morphology and connectivity of the ionic domains of the composite membranes than the pristine SPAES membrane. The composite membranes showed considerable improvement in proton conductivity and oxidative stability than the pristine copolymer membrane under similar test conditions. Graphical abstract

To improve some of the inadequate properties of PEMs, an efficient and convenient approach has been considered by forming hybrid materials such as organicinorganic nanocomposites. Usually, sulfonated polymers/copolymers filled with inorganic nanofillers show lower ion exchange capacity (IEC) values and subsequently reduce the proton conductivity of the PEMs (29)(30)(31). The hydrophilic fillers, such as SiO 2 , TiO 2 , and ZrO 2 , are generally added to improve the water retention ability and enhance the proton conductivity of the PEMs (32)(33)(34). It envisioned that the introduction of these hygroscopic inorganic nanoparticles into the polymer matrix improves the hydration of the membrane and reduces the evaporation of water. Marani et al. synthesized titania nanosheets (TNS) and investigated the effects of TNS as an additive in sulfonated (poly ether ether ketone) (SPEEK) nanocomposites. They reported that the composite membrane with 1.67 wt% TNS showed a maximum conductivity value at 100% relative humidity (RH), which is much higher than that of a pure SPEEK membrane under the same conditions. In contrast, the degree of volume swelling was reduced by 80% to that of the reference SPEEK membrane (35).
In recent times, sulfonated nanofillers are being used as an additive to compensate for the reduction in IEC W values with the addition of inorganic fillers to PEMs. Rhee et al. prepared sulfonated montmorillonite (SO 3 H-MMT) and developed a composite electrolyte membrane with Nafion ® 115. The proton conductivity of the composite membrane was slightly lower than that of pristine Nafion ® 115 (36). Bebin et al. synthesized Nafion ® 115/sulfonic acid groups containing Laponite nanocomposite membranes and reported higher proton conductivity and water retention ability than pristine Nafion ® 115 (37). Sua et al. reported a similar result toward proton conductivity when sulfonated silica nanoparticle (s-SNP) was added to the poly(phthalazinone ether ketone) (SPPEK) membrane (38). A 3.6-fold increase in proton conductivity than pristine SPPEK membrane was reported when 7.5 phr of silica-SO 3 H was added. Nevertheless, the conductivity value of the composite membrane was still lower than that of Nafion ® 117. Later on, Xu et al. synthesized a series of new Nafion-based composite membranes via an in situ sol-gel reaction of 3-(trihydroxysilyl)propane-1-sulfonic acid (THSPSA) through the solution casting technique. More significantly, the composite membranes exhibit an increased water uptake and two times higher proton conductivity, compared to unmodified Nafion at high temperatures and low humidity (39). All these studies indicated that the composite approach might be an alternate way to enhance the PEM properties (40,41).
The main objective of the present research work was to exploit the effect of incorporated inorganic compounds in the presence of -COOH functionalized sulfonated polymers and fabricated highly proton-conducting hybrid membranes. Accordingly, a -COOH functionalized fluorinated sulfonated copolymer was prepared. Different amounts of THSPSA were added to this -COOH functionalized copolymer to make hybrid membranes by in situ sol-gel reactions of THSPSA during the film casting step (shown in Figure 1). The oxide precursor THSPSA was chosen due to its multifunctional structure. In contrast, the -COOH functionalized fluorinated sulfonated copolymers were selected for better interaction between the inorganic phases (-OH groups) and the ionic domains of the polymer matrix through -COOH groups. Besides, the sulfonic acid groups of the sulfonated poly(arylene ether sulfone) chains and the resulting sulfopropylated polysilsesquioxane (SiOPS) interacted with each other and formed a network structure, which was responsible for efficient proton-conducting channels. Thus, the utility of the newly prepared composite membranes (regarded as SPAES-SS-X, where X represents the theoretical amount in wt% of sulfonated silica in the hybrid membrane) has been demonstrated by investigating the morphology and selected PEM properties such as proton conductivity, water uptake, mechanical strength, swelling ratio, and thermal and chemical stability.

Synthesis of the -COOH functionalized
fluorinated sulfonated poly(arylene ether) copolymer The -COOH functionalized fluorinated sulfonated poly (arylene ether) copolymer (TPQSH-COOH-50) was synthesized via standard K 2 CO 3 -mediated polycondensation of a bisphenol monomer (TP-COOH) with two bishalide monomers (QBF and SDCDPS) in the ratio of 50:50 where the NMP and toluene were used as solvent and cosolvent, respectively. The detailed synthetic procedure with characterization of TPQSH-COOH-50 has been demonstrated in an earlier publication (44). The copolymer structure used in this investigation is shown in Figure 2.

Composite membrane preparation
The composite membrane is prepared using the following procedure. A measured amount of TPQSH-COOH-50 copolymer was dissolved in 15 mL of DMAc and filtered to obtain a transparent polymer solution. THSPSA (2.5-10 wt%) was added dropwise under vigorous stirring to this solution. The resulting mixture was sonicated for 30 min using a water bath sonicator. The homogeneous mixture was transferred into a roundbottomed flask for partial esterification reaction (between the -COOH group of the polymer and the -OH group of the silica filler), which was carried out at 80°C for 12 h in DMAc with 5 mg of p-toluenesulfonic acid per gram of polymer for catalysis. The resulting solution with different amounts of THSPSA was cast onto a glass petri dish and heated at   80°C for 12 h to complete the esterification process as well as to form a Si-O-Si network. Then the temperature was slowly increased to 100°C, 120°C, 140°C, and 160°C, and the membranes were kept for 2 h in each temperature to remove any residual solvent. After cooling down to room temperature, the composite membranes were peeled off from the glass plate and acidified by immersing in the 1.5 M H 2 SO 4 solution at room temperature for 24 h, followed by washing with deionized (DI) water several times until pH 7. All the hybrid membranes were dried under vacuum overnight at 120°C before use. The typical thickness of the membranes was in the range of 45-50 µm.

Characterization
Infrared spectra of the composite membranes were recorded on a NEXUS 870 FTIR (Thermo Nicolet) spectrophotometer under a humidity-free atmosphere at room temperature. The DSC measurements were performed under nitrogen atmosphere using TA Instruments DSC Q20 at a heating rate of 20 K/min and the glass transition temperatures (T g ) determined by taking the middle point of the step transition in the second heating run. Low-temperature dynamic scanning calorimetry (DSC) measurements were performed to determine the state of water (the frozen water content and the nonfrozen water content) within the polymer membranes using a NETZSCH DSC 200PC instrument at a heating rate of 5 K/min under nitrogen atmosphere. Initially, 5-10 mg swollen membranes were sealed hermetically in a sample pan and cooled to −40°C. Then after reaching the equilibrium, the sample was heated to 40°C. The single melting enthalpy peak (enthalpy of fusion) that arose near 0°C was considered in order to determine the amount of free water present in the membrane during the course of the experiment (an average value was taken for calculation). Thermogravimetric analysis (TGA) was performed using a TA Instruments TGAQ50 thermal analyzer under synthetic air (N 2 :O 2 = 80:20) at a heating rate of 10 K/min. The mechanical properties of the acid form of the hybrid membranes (10 mm × 25 mm) were measured using a TINIUS OLSEN tensile testing machine (Model No. H5KS). The measurements were performed at a crosshead speed of 5 mm/min of the specimen length. All the membranes were predried at 80°C for 4 h before testing and the measurements were performed at 30°C and 60% RH. The IEC W , water uptake, swelling ratio, and oxidative stability of the membranes were determined, according to the reported protocol (11,14). The surface images of the composite membranes were taken using a field emission scanning electron microscope manufactured by Carl Zeiss, Model Supra 40. The instrument was operated at an accelerated voltage of 5-20 kV equipped with an energy-dispersive spectrometer. The atomic force microscopy (AFM) analysis was carried out using AFM 5500 (Agilent Technology) in tapping mode. Transmission electron microscopy (TEM) images of the ultra-microtome membranes were taken by using an FEI-TECNAI G2 20 S-TWIN TEM instrument at an operating voltage of 80 kV. The acid form of the membranes was stained with Pb 2+ ions using 0.5 M lead acetate solution for overnight to enhance the contrast between phases. The membranes were then washed with DI water several times to remove the surface salt solution and dried at room temperature for 24 h. The stained samples were sectioned by ultra-microtome under cryogenic condition and placed on carbon-coated copper grids before TEM viewing. In plane, proton conductivity (σ, mS/cm) of all the composite membranes was measured at different temperatures by four-probe AC impedance spectroscopy using an electrode system connected to a Gamry instrument reference 3000 potentiostat/galvanostat/ZAR over the frequency range of 100 Hz-1 MHz under fully hydrated condition (11,12). The reproducibility in the measurements of proton conductivity was within 2-3%, and the maximum value obtained is reported.

Results and discussion
The formation of sulfopropylated polysilsesquioxane in all the composite membranes was confirmed by the FT-IR spectra, as shown in Figure 3. The characteristic symmetric stretching and asymmetric stretching bands at 809 and 1,018 cm −1 , respectively, suggest the formation of the Si-O-Si bond in the polymer composite (45). Additionally, the formation of polysilsesquioxane was also confirmed by the disappearance of Si-OH bands around 930-940 cm −1 . The aromatic C═C stretching band appeared at 1,584 cm −1 , whereas the C-F stretching band was found at 1,240-1,127 cm −1 (46). The characteristic symmetric stretching band at 1,051 cm −1 corresponds to the ether linkage of the polymers. The symmetric stretching of the aromatic S-O bond of the -SO 3 H group overlaps with the asymmetric stretching of the Si-O-Si bond, but a strong absorption band was found at 1,095 cm −1 which corresponds to the asymmetric stretches of the aromatic S-O bond of the -SO 3 H group. Moreover, the absorption band of S-O stretching from the -SO 3 H group of sulfopropylated polysilsesquioxane at 1,125 cm −1 was superimposed with the C-F stretching band of the pristine copolymer. All these results suggest that the sulfopropylated polysilsesquioxane was successfully developed into the polymer matrix during the sol-gel process. The hybrid membranes do not show any noticeable signature for glass transitions in the DSC analysis (temperature range of 100-350°C) attributed to the locking of the segmental motion caused by the ionic interactions of the hydrophilic sulfonic groups of both SiOPS and the sulfonated poly(arylene ether sulfone) chains (12,44,46). All hybrid membranes displayed twostep degradation profiles in TGA (Figure 4) under air. The first weight loss (280-320°C) was assigned to the degradation of sulfonic acid groups. The 10% weight loss temperature of hybrid membranes was around 320°C and was lower than that of the pristine membrane (44). However, no significant change in 10% weight loss temperature of the hybrid membranes with the loading percentage of sulfonated SiOPS nanoparticles was observed. The weight loss range from 495 to 508°C was attributed to the decomposition of the polymer backbone ( Table 1).
Mechanical stability is critical of the electrolyte membranes for effective utilization in a fuel cell as PEMs. Table 1 summarizes the mechanical properties of the composite membranes and the pristine membrane and Nafion ® 117. Figure 5 shows that all the composite membranes exhibit a lower tensile strength value than the -COOH functionalized pristine copolymer membrane. The existence of additional -SO 3 H groups in the polymer matrix and the higher structural rigidity of the inorganic nanofillers make them mechanically weak (33). In general, all these hybrid membranes showed higher tensile strength and Young's modulus with a lower elongation at break than Nafion ® 117. In the dry state, the tensile strength values of SPAES-SS-X hybrid membranes decreased steadily with the increase in filler loading, but no absolute trend was observed in Young's modulus and elongation at break values. The membranes with 7.5 wt% (SPAES-SS-7.5) filler loading showed an optimal set of mechanical properties with elongation at break as high and sufficient for fabricating a membrane electrode assembly (47,48).
Oxidative stability of the hybrid membranes was checked by immersing the samples in Fenton's reagent (3 wt% H 2 O 2 , 2 ppm FeSO 4 ) at 80°C ( Table 1). The hybrid membrane with 2.5 wt% filler exhibited the highest oxidative stability, with τ 1 = 22.5 h, whereas the least oxidative stability was observed for 10% filler loaded membrane (τ 1 = 7.5 h). Generally, compared to the pristine membrane, the oxidative stability of the hybrid membranes increases with increasing filler loading. Here, the presence of additional -SO 3 H groups in the nanofiller reduces the oxidative stability of the membranes. Hydrophilic domains are prone to an oxidative attack by the free radical species (HO˙and HOO˙). Thus, increasing the percentage of s-SNPs (THSPSA) increases the number of hydrophilic domains, and consequently, the oxidative stability of the hybrid membranes with higher filler loading decreases. However, due to the formation of the crosslinked and interpenetrating network structure, the hybrid membrane, even with 10 wt% filler, showed excellent oxidative stability, compared to many other pristine membrane materials (24,44,46). On the other hand, the -CF 3 groups in QBF and the wholly aromatic nature of the copolymers are mainly responsible for enhancing the oxidative stability and making them resistant toward attack by the oxidative species (49).
The hydrolytic stability of the hybrid membranes was investigated at 80°C, keeping them for 24 h in water. The membranes were substantially stable in wet conditions, as no apparent changes were observed in their appearance and weight even after 24 h. Although the SiOPS alone dissolves immediately in hot water, the hybrid membranes exhibit a high hydrolytic stability. This stability arises from a strong interaction (possibly hydrogen bonding) between the sulfonic acid groups of SiOPS and the sulfonated poly(arylene ether sulfone) chains (50).
IEC W usually signifies the number of exchangeable protons present in the polymer electrolytes and was measured by the classical titrimetric analysis with 0.1 M NaOH solution. The IEC W values of SPAES-SS-X copolymer membranes and Nafion ® 117 are given in Table 2. The titrimetric IEC W value of the bare TPQSH-COOH-50 was found to be 1.32 mequiv./g, and with an increase of THSPSA loading, the IEC W was found to increase rapidly up to 1.48 mequiv./g, but a further rise in THSPSA content from 7.5 to 10 wt% did not lead to a noticeable increase in IEC W . It is attributed to the trapping of H + ions (present in the inner sulfonic acid groups) by large SiOPS nanoclusters at high precursor concentrations (39). This result corroborates with the water uptake of the hybrid membranes. The water uptake values of the hybrid membranes were significantly higher than that of the pristine TPQSH-COOH-50 membrane, which is attributed to the hygroscopic nature of the SiOPS nanofiller (39). With increase in THSPSA loading up to 7.5 wt%, the water uptake values gradually increased from 47% to 61% (at 80°C). Beyond 7.5% loading of THSPSA, the rise in water uptake of the SPAES-SS-10 hybrid membrane increased marginally (64%), as shown in Figure 6a.
A low-temperature DSC experiment was performed to investigate the state of water inside the hydrated membranes by measuring the latent heat of fusion (ΔH f ) for water (44). Generally, a higher ΔH f value indicated a high concentration of free and weakly bound waters that are responsible for the melting enthalpy peaks near 0°C in the DSC curves. It is consistent with the literature reports claiming that the loosely bounded interfacial water molecules are mainly responsible for the interconnected channels between ionic domains and play a crucial role in proton conduction of Nafion ® (2,51). In contrast with Nafion ® 117 (39), the SPAES-SS-2.5 membrane showed a sharp melting peak near 0°C, which is attributed to a more significant number of free and weekly bound water molecules in it (Figure 6b). However, the same behavior was not observed with the hybrid membranes when the filler loading increased to 5 wt% or higher. It could be observed that due to the Table 1: Thermal, mechanical, and oxidative stability of SPAES-SS-X membranes as well as selected reference polymers Oxidative stability τ f (h) Ref.  presence of highly dense sulfonic acid groups, the hybrid membranes with more filler loading contain a higher number of strongly bonded water molecules (probably through hydrogen bonding) and do not contribute to the melting endotherm in the DSC curves. The hydration number (λ, the average number of water molecules per sulfonic acid group) of the hybrid membranes was calculated to probe the state of water inside the hydrated membranes. From Table 2, it is seen that at 80°C, with an increase in THSPSA, the hydration number (λ) sharply increased from 19 to 25 for SPAES-SS-7.5 and then slowly to 26 for SPAES-SS-10. Analogous to the water uptake, the swelling ratios (in-plane) of the SPAES-SS-X hybrid membranes gradually enhanced with increasing sulfonated silica nanofiller loading and temperatures. However, compared to the pristine copolymer membrane, the hybrid membranes showed a much lower-dimensional swelling attributed to the formation of a cage-like structure of the silica network. They suppressed the free movement of water molecules through the bulk of the polymer (52). At 80°C, all the hybrid membranes possess a much better dimensional stability than Nafion ® 117, resulting from the bulky TP-COOH groups and hydrophobic -CF 3 groups along with a rigid quadriphenyl moiety (18).
Scanning electron microscope (SEM) techniques have extensively studied the surface morphology of the hybrid membranes to examine the homogeneity of the silica sulfonic acid nanoparticles (THSPSA) in the polymer matrix. The SEM image (Figure 7) reveals that the s-SNPs were uniformly dispersed on the polymer surface, indicating homogeneity. The compatibility between the Table 2: IEC W , hydration number, water uptake, and proton conductivity of pristine copolymer, SPAES-SS-X composite membranes, and Nafion ® 117  organic and inorganic components improved, probably due to the existence of two types of covalent bonds (the sulfonated silica particles formed the covalent bonds with a -COOH functionalized polymer and the bond was formed after the formation of SiOPS) present in the hybrid membranes. The density of the -SO 3 H groups on the membrane surface gradually increases with a gradual increase in silica loading (2.5-10 wt%). In addition to that, the surface morphology of the composite membranes was studied by AFM in tapping mode (Figure 8).
There are two regions in the AFM image where the bright part corresponds to the hard hydrophobic domains and the dark area corresponds to the soft hydrophilic domains. The interconnectivity of the ionic domains was vividly observed in the composite membrane, with 7.5 wt% of silica loading. The bulk morphology of the membranes played an important role in elucidating the proton conductivity and in being investigated by the TEM. The Pb 2+ -stained sample membranes showed ( Figure 9) an excellent micro-phase separated morphology wherein the dark spherical regions correspond to the hydrophilic ionic domains. The brighter areas represent the hydrophobic nonionic domains of the polymer matrix. However, the connectivity of the ionic domains of the composite membranes was significantly better than that of the pristine SPAES membrane (44). The hybrid membranes with 2.5 wt% SiOPS consist of both small hydrophilic clusters (particle size ∼20-25 nm) and some interconnected medium-sized ionic clusters (particle size ∼40-45 nm). The connectivity of the ionic domains was more pronounced on increasing the percentage of hydrophilic silica nanoparticles. It is believed that the ionic domains resulted from the aggregation of sulfonic acid groups from the polymer electrolyte and THSPSA/SiOPS, and the maximum interconnectivity was found in the SPAES-SS-10 composite membrane. The result suggests that the SiOPS aggregates distributed relatively more uniformly to form a welldispersed nanocomposite membrane.
Proton conductivity is the prime property for a PEM material for its effective utilization in fuel cells. In plane, proton conductivity of all the composite membranes was measured using AC impedance spectroscopy within the temperature ranging from 30 to 80°C under fully hydrated condition. In comparison with the pristine TPQSH-COOH-50, the hybrid membranes exhibited an improved proton conductivity due to the presence of additional sulfonic acid groups in nanoparticles. Table 2 summarizes the proton conductivity of the membranes, and Figure 10a represents the change in proton conductivity of the composite membranes as a function of temperatures along with Nafion ® 117.
The proton conductivity increases progressively with the loading of the THSPSA content, as shown in Figure 10b and, in particular, the conductivity of the hybrid membranes (mainly XX = 7.5 and 10) at low temperature was significantly improved, compared to the pristine membrane (TPQSH-COOH-50). Depending on the amount of filler loading, the proton conductivity of the hybrid membranes was exhibited in the ranges of 38-54 mS/cm and 93-138 mS/cm, respectively, at 30°C and 80°C ( Table 2). The 10 wt% SiOPS loaded membrane showed more than two times higher proton conductivity (138 mS/cm) than the pristine TPQSH-COOH-50 membrane (55 mS/cm) at 80°C under the same test conditions (44).
It is noted that the proton conductivity of this 10 wt% filler loaded membrane did not increase that sharply, compared to that of the 7.5 wt% SiOPS charged membrane. This feature was attributed to the trapping of inner -SO 3 H groups by forming a high precursor concentration of the large SiOPS nanoclusters in 10 wt% filler loaded membrane and making them inert for the interaction with water (50). The proton conductivity values are in close agreement with the water uptake values of the hybrid membranes. The proton transportation of hybrid membranes occurred by both surface and bulk transport paths. The protons hopped on the surface of the pore channels in the form of H 3 O + in the surface transport pathway, whereas, in bulk transport mechanisms, the protons jumped in the bulk of the pore channels in the form of H 5 O 2 + /H 9 O 4 + (53-55). The density of the functional groups and the distance between two neighboring functional groups control the rate of surface transport pathway, whereas the rate of bulk transport pathway was controlled by the RH of the membrane surroundings and the cross-sectional area of the pore channels. The incorporation of functionalized silica (THSPSA) into the polymer matrix reduced the distance between two neighboring proton-conducting moieties, compared to the pristine TPQSH-COOH-50 membrane. It facilitated the surface transportation effectively through the  hybrid membranes. For a better understanding, the temperature-dependent proton conductivity of the membranes is plotted in Figure 11. The activation energies of composite membranes were calculated from the slope of different lines presented in Figure 11 using the Arrhenius equation and the values were found to be in the range of 15.6-18.8 kJ/mol, close to that of Nafion ® 117. The activation energies of the composite membranes are higher than that of the pristine TPQSH-COOH-50. The hydrogen bond present in the crosslinked composite membranes needs to be broken at the high temperature to facilitate proton transport, resulting in relatively high activation energies (12). These results were in concurrence with the morphological data. They suggested that the incorporation of SiOPS evokes a significant improvement in the polymer electrolyte membranes' water transport behavior by forming an interconnected ionic network that causes high proton conductivity.

Conclusions
A series of fluorinated sulfonated hybrid membranes were fabricated using a practical approach to improve the proton conductivity by a simple incorporation of an acid-functionalized THSPSA nanofiller. The prepared hybrid membranes showed excellent mechanical stability. The SEM studies suggest that the sulfonated SiOPS nanoparticles were dispersed uniformly on the polymer matrix in all the hybrid membranes with high homogeneity, probably due to the existence of two types of covalent bonds. However, the TEM micrographs of the composite membranes showed a clear phase-separated morphology with a significantly better connectivity of the ionic domains than the pristine SPAES membrane. The pendant sulfonic acids on the polysilsesquioxane fillers help to increase the interconnectivity of the proton transport channels by reducing the distance between two neighboring proton-conducting moieties, compared to the pristine TPQSH-COOH-50 membrane and facilitated the proton transportation effectively through the composite membranes. The SPAES-SS-10 membrane showed a maximum proton conductivity of 138 mS/cm at 80°C. This work demonstrates that the prepared nanocomposite conductive membrane materials are expected to find their applications in PEM fuel cells.