Cite this: DOI: 10.1039/c3ra41508b

Impact on the ionic channels of sulfonated poly(etherether ketone) due to the incorporation ofpolyphosphazene: a case study in direct methanol fuelcells

Received 27th March 2013,Accepted 21st May 2013

DOI: 10.1039/c3ra41508b

www.rsc.org/advances

S. Gouse Peera, S. Meenakshi, K. Hari Gopi, Santoshkumar D. Bhat,* P. Sridharand S. Pitchumani

Blend membranes are fabricated from sulfonated poly(ether ether ketone) (SPEEK) and poly[bi-

s(phenoxy)phosphazene] (POP). The effect of POP content on the distribution of ionic channels is

investigated by atomic force microscopy (AFM). The water uptake and methanol permeability for the

blend membranes are also investigated. The blend membranes are characterized in terms of their thermal

and mechanical properties in conjunction with their ionic conductivity. The proton conductivity of the

blend membranes slightly decreased with increasing POP content in comparison with the pristine SPEEK

membrane. The hydrophobic nature of POP blocks the ionic channels in the SPEEK matrix, subsequently

decreasing its water uptake and methanol permeability. The blend membranes showed higher power

density compared to a pristine SPEEK membrane in direct methanol fuel cells (DMFCs).

Introduction

Direct methanol fuel cells are promising power sources forportable and stationary applications due to their advantages,including high efficiency, simple design and low emissions tothe environment. The polymer electrolyte membrane (PEM) isan important component of DMFCs, it transfers protons fromthe anode to the cathode and also acts as a barrier to avoid thecrossover of methanol.1 Nafion, a perfluorosulfonic acidpolymer is the state-of-the-art membrane material for DMFCsand shows high mechanical and chemical stability.2 Thispolymeric membrane also has excellent ionic conductivity,long term durability and unique hydrophilic percolation.However, materials with permeable morphologies suffer fromhigh methanol permeability and swelling in the presence ofmethanol during DMFC operation. Hence, for enhancedDMFC performance, membranes with low permeability tomethanol, that can resist swelling are highly desirable.3,4

Among the various sulfonated polymers studied in theliterature, SPEEK has attracted considerable attention becauseof its good mechanical strength, high chemical stability, lowcost and ease of preparation.5 SPEEK is also resistant tomethanol permeability, in comparison with Nafion in DMFCs,due to its narrow and more branched hydrophilic channels.6

The proton conductivity, methanol permeability, water uptake

and swelling ratio of SPEEK membranes are all dependent onthe degree of sulfonation (DS). Although the SPEEK membranewith a low DS shows good performance in reducing perme-ability to methanol, there is an apparent reduction in ionicconductivity. It also becomes extremely difficult to processSPEEK with low DS due to its poor solubility in organicsolvents. In contrast, a high DS results in excessive wateruptake, increasing the swelling ratio and limiting the use ofthe SPEEK membrane.7,8 The above factors may lead to a lossin the mechanical stability of the membrane making itunsuitable for long term operation in DMFCs. In order toaddress the above issues different routes for modification ofthe SPEEK polymer have been explored. One of the mostimportant routes is to fabricate a blend of SPEEK withdifferent hydrophobic polymers to control water uptake andmethanol permeability.9–14

Polyphosphazenes are polymers which possess a backboneof alternating phosphorus and nitrogen atoms. The propertiesof polyphosphazenes are mainly controlled by the choice ofside groups.15 Polyphosphazenes with various properties canbe synthesized by careful choice of side groups.Polyphosphazenes have several advantages over establishedhydrocarbon based polymers. One of their prime attributes isthe thermal and chemical stability of the polymer backbone,both phosphorus and nitrogen are in their highest oxidationstates resulting in a high degree of thermo-oxidative stability.16

The polar nature of the bonding along the polymeric backboneinhibits chemical attack by free radicals.17

CSIR-Central Electrochemical Research Institute-Madras Unit, CSIR Madras

Complex, Taramani, Chennai, 600 113, India. E-mail: sdbhatcecri@gmail.com;

Fax: +91-44-22542456; Tel: +91-44-22542068

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The blend membrane approach is one of the favoured waysto improve the properties of the PEM in DMFCs, as therequired properties of the two components can be combinedin one blend. Hence, the present study is an attempt toestablish a blend of SPEEK and poly[bis(phenoxy)phospha-zene] (POP), taking into consideration the unique features ofthese two polymers. POP is chosen as the inert (no charge/hydrophobic) component of the blend membrane. Blendmembranes containing varying contents of POP with SPEEKare prepared and characterized in detail before subjectingthese blends to DMFC performance evaluation.

Experimental

Membrane and electrode materials

Sulfonated poly(ether ether ketone) (SPEEK, Mw = 50 000 gmol21, Mn = 14 000, with IEC of 1.4 meq g21 and degree ofsulfonation of 54%) was purchased from FuMA-Tech GmbH,Germany. Poly[bis(phenoxy)phosphazene] and dimethylaceta-mide (DMAc) were obtained from Aldrich, USA. Toray TGP-H-120 carbon paper was procured from Nikunj Exim Pvt. Ltd.,India. Vulcan XC-72R carbon, Pt–Ru (60 wt% in 1 : 1 atomicratio)/C and Pt/C (40 wt% Pt on Vulcan XC-72R carbon) wereobtained from Alfa Aesar (Johnson Matthey, India) Chemicals.All chemicals were used as received. De-ionized (DI) water(conductivity, 18.4 MV cm) from a Millipore system was usedduring the study.

Membrane fabrication

5 wt% SPEEK was dissolved in DMAc at 70 uC to form ahomogeneous viscous solution. The required wt% of POP (1, 2or 3 wt%) in relation to SPEEK was added to form a polymerblend. The resulting solution was cast on a Plexiglass plate andthe solvent was evaporated at 70 uC under vacuum. Themembranes were peeled off the plate after drying andimmersed in 1 M H2SO4 solution for 4 h at 30 uC for furtherdoping. The membranes were washed with DI water to removeresidual H2SO4. The thickness of every membrane was y160mm. Pristine SPEEK membrane was also prepared using asimilar procedure, without the addition of POP, for compara-tive study.

Water and water–methanol mixture sorption measurements

Sorption measurements were conducted for pristine recastNafion, pristine SPEEK, SPEEK-POP (1 wt%), SPEEK-POP (2wt%) and SPEEK-POP (3 wt%). Before subjecting the mem-branes to sorption, the membrane samples were dried at 100uC to remove moisture. Dry membrane samples were weighed(Wo) and equilibrated in DI water for 24 h at 30 uC. Themembranes were surface blotted and membranes aftersorption were weighed (W‘). Percentage sorption for theaforesaid membranes was calculated using eqn (1).

Sorption %ð Þ~ W?{Wo

Wo

� �|100 (1)

For the water–methanol mixture sorption measurement at30 uC, a feed composition of 2 M aq. methanol was selected.Pre-weighed dry membranes (Wo) were dipped in feed solutionfor 24 h to attain equilibrium. The equilibrated membraneswere surface blotted, final weights (W‘) were recorded and thesorption values were calculated from eqn (1).

Physicochemical characterization

Mechanical properties of the membranes were studied with aUniversal testing machine (UTM) (Model AGS-J, Shimadzu,Japan) with an operating head-load of 10 kN following aprocedure reported in the literature.18 The tensile strength andelongation-at-break measurements were conducted for pris-tine SPEEK, SPEEK-POP (1 wt%), SPEEK-POP (2 wt%) andSPEEK-POP (3 wt%) blend membranes.

Cross-sectional morphologies for pristine SPEEK andSPEEK-POP (2 wt%) membranes were obtained using a JEOLJSM 35CF Scanning Electron Microscope (SEM). Thermo-gravimetric analysis (TGA) for pristine SPEEK, SPEEK-POP (1wt%), SPEEK-POP (2 wt%) and SPEEK-POP (3 wt%), mem-branes were conducted using a NETZSCH STA 449F3 TGA-DSCinstrument over a temperature range between 30 uC and 800uC at a heating rate of 5 uC min21 under a nitrogenatmosphere. FTIR spectra for pristine SPEEK, SPEEK-POP (1wt%), SPEEK-POP (2 wt%) and SPEEK-POP (3 wt%) blendmembranes were recorded using a Nicolet IR 860 Spectrometer(Thermo Nicolet Nexus-670). Topological and phase images forpristine SPEEK and its blend membranes were determined bytapping mode atomic force microscopy (AFM, PicoSPM–Picoscan 2100, Molecular Imaging, USA).

Proton-conductivity measurements

Proton-conductivity measurements were performed for all themembranes using a four-probe DC method described else-where.19,20 In brief, four probes were placed on the surface ofthe membrane and a current was passed through two outerelectrodes while the potential was measured across the innerpair. Conductivity data for the membranes were collectedunder fully-humidified conditions (100%). Conductivity mea-surements were carried out at 30 uC and 60 uC.

Methanol permeability studies

A DMFC single cell assembly was used for the measurement ofmethanol permeability at OCV conditions, as reported in theliterature.19 In brief, 2 M aq. methanol was initially supplied tothe DMFC and the cell was allowed to equilibrate for 1 h. Afterattaining steady state, the difference in the amount ofmethanol supplied to the cell and the amount of methanolcollected at the anode outlet, for a particular time (t), wasmeasured at open circuit voltage (OCV) keeping the celltemperature at 60 uC under ambient pressure. The concentra-tion of methanol transported through the membranes wasdetermined by sampling a small amount of the solution fromthe outlet of the cell by gas chromatography (ThermoFisherScientific, Model Trace GC 700 with Capillary Column and FIDdetector). The methanol permeability of the membrane wasdetermined by eqn (2).21

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P~Cout|T|Vout

t{t0ð ÞA|Cin

(2)

Where P is the membrane permeability, Vout is the volume ofthe outlet solution, t is the time, Cin and Cout are theconcentration of methanol in the inlet and outlet respectively,T is the thickness, and A is the area of the test membrane.

Single-cell assembly and performance evaluation in DMFCs

The performance of the aforesaid membranes was evaluated inDMFCs comprised of membrane electrode assemblies (MEAs)prepared in a similar manner to those reported in our earlierstudies.19 The active area for the DMFCs was 4 cm2. MEAscomprising pristine recast Nafion, pristine SPEEK, SPEEK-POP(1 wt%), SPEEK-POP (2 wt%) and SPEEK-POP (3 wt%)membranes were obtained by sandwiching the membranebetween the anode and the cathode followed by hot-pressing at100 uC for 3 min at a pressure of 20 kg cm22. MEAs wereevaluated using a conventional fuel cell fixture and the cellswere tested at 60 uC with 2 M aq. methanol, with a flow rate of 2mL min21 at the anode side, and oxygen at the cathode sidewith a flow rate of 300 mL min21, at atmospheric pressure.Measurements of cell potential as a function of current densitywere conducted galvanostatically using Bitrode Instruments(US) (Model-LCN4-25-24/LCN 50-24). After the polarization test,the stabilities of the pristine recast Nafion and SPEEK-POP (2wt%) blend membranes were evaluated by following the changein OCV of DMFCs comprised of these membranes for 50 h.

Results and discussion

FT-IR spectral analysis

FT-IR analyses of the blend membranes were carried out toinvestigate the presence of POP in SPEEK polymer.22 The

spectra shown in Fig. 1 and the corresponding functionalgroups tabulated in Table 1 confirm that POP forms acompatible blend with the SPEEK matrix. The characteristicpeaks related to P–N, P–N–P, P–OAr and PLN stretchingindicate the presence of POP in the SPEEK matrix. Theabsorption band at 1081 cm21 is assigned to the asymmetricOLSLO stretching vibrations of sulfonated groups in theSPEEK matrix. However, the related vibrations for thesulfonated groups in the blend membranes are shown at1076 cm21, as seen in the inset to Fig. 1. The absorption bandof the sulfonated groups in the SPEEK-POP blend membranesshowed a slight blue shift in comparison with pristine SPEEK.The blue shift may be induced by the presence of POP in theSPEEK matrix.

Thermal stability of the membranes

TGA analyses were performed to evaluate the thermal proper-ties of the blend membranes. Fig. 2 shows the TGA curves forpristine SPEEK and SPEEK-POP blend membranes under anitrogen atmosphere. Three distinct weight loss steps areobserved, wherein the first weight loss in the region 100 to 120uC is due to thermal dehydration, by the removal ofphysisorbed water (residual humidity). The second weightloss in the region of 300 to 400 uC is mainly associated with thethermal degradation of sulfonic acid groups in SPEEK. Thethird weight loss region at approximately 450 to 800 uC is

Fig. 1 Typical FT-IR spectra for the pristine and blend membranes of SPEEK.

Table 1 Functional groups of SPEEK and SPEEK-POP blend membranes withtheir corresponding FT-IR bands.

Wave number (cm21) Functional groups

857 P–N stretching919 P–N–P stretching1048 P–OAr stretching1152 PLN stretching1594 Aromatic –CLC– stretching of SPEEK1641 CLO stretching of SPEEK1020 and 1081 Symmetric and asymmetric stretching

vibrations of OLSLO of SPEEK

Fig. 2 TGA analysis for pristine and blend membranes of SPEEK.

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attributed to the thermal decomposition of the main chain ofthe copolymers.10 These results indicate that all the mem-branes are thermally stable up to 300 uC, which is sufficient foruse as proton conducting materials. Although all the mem-branes show similar TGA profiles corresponding to the onsetof thermal degradation, the slope of mass loss is different.Compared to SPEEK membrane, the blend membranes haverelatively high onset temperatures. In addition, the weight lossof SPEEK is about 20 and 37 wt% at 300 uC and 500 uC,respectively whereas the weight loss of the blend membranesis only 15 and 33 wt% at the same temperatures. Incorporationof the POP polymer into the SPEEK matrix contributes to thehigher thermal stability of the blend membranes.

From the results of the differential thermogravimetric (DTG)analysis of pristine SPEEK and blend membranes (Fig. 3),three exothermic peaks can be clearly observed. In the case ofthe SPEEK membrane, the first peak around 90 uC correspondsto the removal of adsorbed water molecules. The second peakat 350 uC is ascribed to sulfonic acid group’s degradation. Thethird peak at 530 uC corresponds to the decomposition of themain chain of SPEEK. For blend membranes, the thirdendothermic peak is observed at a higher temperature (556uC) in comparison to the pristine SPEEK membrane (530 uC) asseen in Fig. 3. This also further confirms the enhancedthermal stability of the blend membrane in relation to theSPEEK membrane.

Mechanical stability of the membranes

The tensile strength and percentage elongation at the breakfor pristine SPEEK and blend membranes are shown in Fig. 4.From the figure it is clear that the tensile strength of the blendmembranes is higher than that of pristine SPEEK. This can beattributed to the addition of mechanically stable POP in theSPEEK matrix. In contrast, the percentage elongation at breakdecreases with increasing POP content which may be due torestricted SPEEK chain mobility as a result of the addition ofPOP.

Fig. 3 DTG curves for pristine and blend membranes of SPEEK.

Fig. 4 Tensile strength and elongation at break for pristine and blendmembranes of SPEEK.

Fig. 5 Cross-sectional morphology of (a) pristine SPEEK, (b) SPEEK-POP (2 wt%)membranes.

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Morphology of the membranes

Blend membranes were subjected to scanning electronmicroscopy in order to study their surface morphology(Fig. 5). From the figure, it is clear that agglomerates of POPmolecules are well distributed in the SPEEK matrix. Theseagglomerates are expected to implant in the SPEEK matrix andreduce the domain size of the ionic channels. To furtherconfirm the effect of POP on the microstructure of themembranes, AFM analysis was performed. The microstructurecan have significant impact on the properties of themembranes, particularly, the spatial distribution of the ionicsites. The aggregation of ionic polymers has been widelystudied by SAXS, AFM and TEM.23–25 Eisenberg has studied theaggregation of sulfonic acid groups in ionic transport path-ways or channels.26 The distribution and connectivity of theconductive pathways can have a major influence on the protonconductivity of membranes.27,28 Fig. 6 and 7 show topologicaland corresponding phase images of the pristine SPEEK andSPEEK-POP blend membranes, wherein the light regions areassigned to the hydrophilic sulfonic acid groups and the darkregions are assigned to the hydrophobic polymer back-bone.28,12 All the images show even dispersions of POP,however, aggregations of the particles can still be observed. Itis worth noting from the AFM that the connectivity anddomain size of SPEEK varies depending on the content of POP,affecting its ionic channels. The hydrophilic and hydrophobicdomains of the blend membranes decreased in size and werepacked more densely. The microstructures of SPEEK varysignificantly with the addition of POP, unlike the hydrophilicfillers, which may be due to the hydrophobic nature of POP.The decreasing hydrophilic domains may impact the change

in water uptake, proton conductivity and methanol perme-ability properties of the membranes. Similar results werereported for polyaniline, poly(amide imide) and TiO2 incorpo-rated SPEEK matrices.9,12,29,30

Water and water–methanol mixture sorption

The water content has a great influence on the properties of aPEM. High water content can facilitate the transport ofprotons, but too much water absorption results in loss of themechanical stability of the membrane.31 For a membraneintended for DMFC applications, the mechanical stability isvery important due to the membrane’s direct contact with theliquid methanol solution, which can enhance its swellingbehaviour. Reduced swelling of the PEM represents anessential advantage in fuel cell applications especially inDMFCs because of four important factors i.e. (a) reducedelectro-osmotic drag,32 (b) reduced methanol permeation, (c)improved membrane–electrode contact and (d) restricted lossin mechanical stability of the membrane. It is interesting tonote that the presence of POP has an impact on the wateruptake and consequently swelling to some extent. As can beseen in Fig. 8 the water and methanol uptake of the blendmembranes decreased upon the addition of POP which ishydrophobic in nature (no charge). When the content ofhydrophobic POP is increased, the content of the hydrophilicsulfonic acid functional groups that are mainly responsible forthe water uptake decreased. Incorporation of POP makes themembranes more compact and partially sulfonated groupsmay not participate in the proton exchange process. Therefore,water uptake in the membrane decreased.

Fig. 6 AFM topological images for (a) pristine SPEEK, (b) SPEEK-POP (1 wt%), (c) SPEEK-POP (2 wt%) and (d) SPEEK-POP (3 wt%) blend membranes.

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Proton conductivity and methanol permeability

Compared with pristine SPEEK, proton conductivities mea-sured at 30 uC and 60 uC for blend membranes decreased withthe increase in POP content (Table 2). Proton transport inmembranes requires well connected proton conductingchannels formed from ionic clusters of hydrophilic sulfonated

functional groups. The content and the diameter of theconnected channels have a significant impact on the protontransport rate in membranes.33,34 When the density of sulfonicgroups is low, the hydrophilic sulfonic acid groups can formisolated ionic clusters in the continuous hydrophobic phase.Whereas, when the density of sulfonic acid groups increases,up to a certain limit, the isolated ionic clusters form cross-linked channels with good connectivity to allow faciletransport of protons through these channels.35,36 The intro-duction of hydrophobic polymer POP reduces the density ofsulfonic acid groups of SPEEK in the membrane therebyslightly reducing the proton transport which is evident fromthe ionic conductivity data for the blend membranes as seenin Table 2.

Fig. 8 Percentage water and water–methanol mixture sorption for pristine andblend membranes of SPEEK.

Table 2 Ionic conductivity data for the membranes

Membrane type

Proton conductivity (mS cm21)

30 uC 60 uC

Pristine recast Nafion 51 86Pristine SPEEK 38 50SPEEK-POP (1 wt%) 35 44SPEEK-POP (2 wt%) 32 41SPEEK-POP (3 wt%) 26 30

Fig. 7 AFM phase images for (a1) pristine SPEEK, (b1) SPEEK-POP (1 wt%), (c1) SPEEK-POP (2 wt%) (d1) SPEEK-POP (3 wt%) blend membranes.

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The transport of methanol through the membrane alsorequires ionic channels with good connectivity. As discussedabove, the content of well-connected channels in SPEEKdecreased for blend membranes upon the introduction ofPOP. As shown in Fig. 9, the methanol permeabilities of theSPEEK-POP blend membranes decreased with increasing POPcontent. Usually transport through the membrane is con-trolled by kinetic and equilibrium factors.11 It is important tonote that for porous membranes, methanol and/or watertransport properties may be affected by the bulk water in poresbut SPEEK membranes, which are non-porous, have differenttransport properties due to the compact and smooth surface.In contrast, SPEEK-POP membranes have a dense entangledstructure which may lead to low methanol permeability incomparison with the pristine SPEEK. The electrochemicalselectivity, which is defined as the ratio between proton

conductivity and methanol permeability, is often used toevaluate the possibility of using membranes in DMFCs. Amembrane with a higher selectivity is desirable for DMFCs.Fig. 10 shows the selectivities of pristine recast Nafion andSPEEK-POP blend membranes, based on the conductivitiesand methanol permeabilities measured at 60 uC. Of all themembranes investigated, the blend membranes with 3 wt% ofPOP possessed the highest selectivity.

DMFC performance and stability study

Performance curves for DMFCs comprising pristine recastNafion, pristine SPEEK and blend membranes of SPEEK-POPat 60 uC are presented in Fig. 11. It is noteworthy that a peakpower density of 47 mW cm22 is obtained for the DMFCs withthe pristine recast Nafion membrane. Although the protonconductivity of the blend membranes is lower than that ofpristine SPEEK, MEAs fabricated with blend membranes (1

Fig. 12 Stability test for DMFCs with pristine recast Nafion and SPEEK-POP (2wt%) membranes.

Fig. 11 Performance curves for DMFCs with pristine recast Nafion, pristineSPEEK and blend membranes of SPEEK at 60 uC.

Fig. 10 Electrochemical selectivity values for the pristine and blend membranesof SPEEK.

Fig. 9 Methanol permeability of pristine recast Nafion, pristine SPEEK and blendmembranes of SPEEK.

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wt% POP and 2 wt% POP) showed higher performance, whichis attributed to the lower methanol cross-over and betterelectrochemical selectivity. Presence of excess POP (3 wt%) inthe blend membrane hinders the proton conduction path asevidenced from the conductivity data, which results indecreased DMFC performance. The stability study on pristinerecast Nafion membrane and SPEEK-POP (2 wt%) blendmembrane was performed under OCV conditions for 50 h ata cell temperature of 60 uC and the data are presented inFig. 12. It was found that the blend membrane showed higherstability in comparison with the pristine recast Nafionmembrane under identical operating conditions, presumablydue to the lower methanol permeability. The ionic conductiv-ity, methanol permeability and DMFC performance of theSPEEK-POP membranes are compared with the data availablefrom the literature on ionic channel affected SPEEK mem-branes in Table 3. It is noteworthy that the SPEEK-POP blendmembranes with a lower degree of sulfonation showed lowermethanol permeability and higher DMFC performance, onaccount of effective blockage of microchannels of SPEEK withthe addition of POP, when compared with the other SPEEK-based membranes that had a higher degree of sulfonation andmethanol permeability.

Conclusions

The effect of incorporation of the hydrophobic polymer POPon ionic channels in SPEEK is studied. AFM topological andphase images proved that ionic channels of SPEEK get blockedby the addition of POP agglomerates restricting the methanolpermeability in DMFCs. Overall, an increase in electrochemi-cal selectivity and concomitant improved performance inDMFCs were achieved using the blend membranes of SPEEK-POP in comparison with pristine recast Nafion.

Acknowledgements

The authors thank DST for providing financial supportthrough DST-GAP-06/12 project. The authors also thank MrRavishanker and Mrs Bhagyalakshmi from CSIR-CECRI-Karaikudi, for helping us with the SEM and AFM measure-ments.

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Table 3 Comparative literature data

Base matrix Hydrophobic phaseProton conductivity(mS cm21) 30 uC

Methanol permeability(1027 cm2 s21)

Fuel cell performance(mW cm22) Ref.

SPEEK (DSb = 0.78) Poly(vinylidene fluoride) 40 7.5 NRa 9SPEEK (DS = 1.31) Poly(amide imide) 66 8.4 NR 10SPEEK (DS NR) Polypyrrole 51 5.3 8 11SPEEK (DS = 1.21) Polyaniline 51 3.8 3 12SPEEK (DS = 0.64) Poly(amide imide) 10 2.1 NR 13SPEEK (DS = 0.69) Poly(vinyl pyrrolidone) 22 5.3 13 14SPEEK (DS = 0.54) POP (present study) 32 2.0 96 —

a NR = Not reported. b DS = Degree of sulfonation.

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