Polyethersulfone containing sulfonimide groups as proton exchange membrane fuel cells

Polyethersulfone containing sulfonimide groups as proton exchange membrane fuel cells

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i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 9 ( 2 0 1 4 ) 2 7 4 0 e2 7 5 0

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Polyethersulfone containing sulfonimide groups as proton exchange membrane fuel cells Luca Assumma a, Cristina Iojoiu a,*, Gulsen Albayrak Ari a,b, Laure Cointeaux a, Jean-Yves Sanchez a a

LEPMI, Grenoble-INP e CNRS e Universite´ Joseph Fourier e Universite´ de Savoie, UMR 5279, Saint Martin d’He`res 38402, France b Istanbul University, Engineering Faculty, Chemical Engineering Department, Avcilar, 34320 Istanbul, Turkey

article info

abstract

Article history:

In this publication a new synthesis approach of polyethersulfone containing sulfonimide

Received 14 February 2013

groups (SI-PES), by chemical modification of sulfonated polyethersulfone (S-PES), was

Received in revised form

developed. The synthesis protocol was optimized in order to turn all the sulfonic function

18 June 2013

into sulfonimide. The effect of replacing arylsulfonic acid function with a more acidic one, i.e.

Accepted 23 July 2013

aryl trifluoromethanesulfonimide, was evaluated through thermal, dynamic mechanical

Available online 8 September 2013

analysis, water uptake and conductivity. For similar ionic exchange capacity (IEC), i.e. 1.8 Hþ/ dm3, at 60  C and 95% relative humidity, the conductivity of SI-PSF is 9.5 mS/cm while that of

Keywords:

S-PSF is only 3.5 mS/cm. However, at 60  C the water uptake is 3 times higher for SI-PSF

Aromatic ionomers

membranes as compare with S-PSF. An important change is observed in the slope of the

Sulfonimide polysulfone

conductivity and water uptake plots of SI-PES membranes, at different temperatures,

PEMFC

depending of IEC. This could be explained by an important change in membrane morphology.

Proton-conducting polymer

Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights

membrane

1.

Introduction

The proton-conducting polymeric membrane (PEM), is one of the key components of PEMFCs. The reference materials are still perfluorosulfonic acid membranes (PFSA) such as Nafion or Aquivion, which provide satisfactory fuel cell performances below 90  C, as long as the degree of humidification is sufficient [1]. However, PFSA ionomers suffer from several drawbacks, including a high production cost, a fairly questionable environmental compatibility, a loss of conductivity and a decrease of the mechanical strength above 80  C. These issues have stimulated the increase of research efforts in order to develop alternative polymer electrolyte membranes, such as polymers based on sulfonated aromatic hydrocarbons.

reserved.

These last ionomers are considered very attractive due to their low cost, easy processability and good oxidation resistance along with high thermal and mechanical stability. Studies on aromatic polymers such as poly(ethereethereketone) (PEEK), poly(aryl-ethereketone) (PEK), poly(aryl-ethersulfone) (PES), poly(benzimidazole) (PBI), polyimide (PI), poly(phenyl quinoxaline) (PPQ), poly(phenylene oxide) (PPO), poly(phenylene sulfide) (PPS) and poly(phosphazenes) (PPZ) have been reported in the literature [2e15]. Some sulfonated aromatic ionomer membranes are claimed to exhibit high conductivities, but at high ionic exchange capacities (IECs) which induces a high water uptake. Furthermore, their conductivities at high temperature and low relative humidity are lower than those of PFSA based

* Corresponding author. Tel.: þ33 476826561. E-mail address: [email protected] (C. Iojoiu). 0360-3199/$ e see front matter Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijhydene.2013.07.090

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membranes. The pKa value of sulfonated polyether ketone, estimated to be approximately 1 as compared to 6 for Nafion, is one of the reasons of the lower conductivity of sulfonated aromatic ionomers when compared to PFSA [16]. According to the comparative study of Brønsted acids in the gas phase, performed by Koppel et al. [17], the Gibbs free energy (DG) for the dissociation of bis(trifluoromethane sulfonyl) imide acid (CF3SO2)2NH, (TFSI) in gas phase is 291.8 kcal mol1, slightly lower than that of trifluoromethanesulfonic acid (DG ¼ 299.5 kcal mol1) but considerably lower than that of methanesulfonic acid (DG ¼ 315.5 kcal mol1). The dissociation depends on the extent of delocalization of the anion negative charge by electron-withdrawing groups through inductive and resonance effects. This is optimized in TFSI anion by the 2 CF3SO2 nitrogen substituents, by both inductive (CF3) and resonance (SO2) effects [18]. Concerning ArylSO2eNeSO2eCF3, the delocalization of the negative charge should be decreased as compared to TFSI anion [19] (i) as the SO2 screens the charge delocalization on the aromatic ring by resonance effect and (ii) as the inductive electron-withdrawing effect of an aryl substituent is much lower than that of a trifluoromethyl [20]. Even if the dissociation of arylsulfonimide acids should be lower than that of TFSI acid, it must be, however, higher than that of arylsulfonic acids. Aromatic ionomers bearing sulfonimide acid, instead of arylsulfonic acids, might therefore allow the improvement of protonconductivities, in particular at low RH. Recently, studies about polymer electrolyte membranes, based on sulfonimide acid groups grafted on polymer main chain, have been carried out. It has been reported that the fuel cell performance of a sulfonimide membrane derived from perfluorosulfonic acid exhibits a similar dependency of conductivity on humidity and temperature as a PFSA membrane, suggesting a similar conduction mechanism for both materials. While at high relative humidity, the conductivities were quite similar for both materials under very low humidity conditions the sulfonimide exhibited a slightly higher conductivity than Nafion 117 [21]. This suggests that the sulfonimide ionomers may possess an internal conducting channel structure that is more favorable for proton conduction at low water availability. It must be emphasized that these perfluorosulfonimide acids, perfectly replicating the TFSI anion, should benefit from the same delocalization of the negative charge. McGrath et al. [22] have synthesized poly(arylenether-sulfone) copolymers by step-growth copolymerization of a comonomer containing two aryl trifluoromethanesulfonimide groups, they reported conductivity close to sulfonated polysulfone. Allcock et al. [23] have obtained similar results for polyphosphazene based on the same acidic functions. However, in both previous papers the reported conductivities were measured in water at temperatures below 60  C, while better performances are expected at higher temperatures and much lower humidity. Recently, Watanabe et al. [24] synthetized styrene/N-phenylmaleimide alternating copolymers with pendant aryl trifluoromethanesulfonimide acid group that showed conductivity higher than their sulfonated homologs and close to the Nafion. This article describes the synthesis of sulfonimide by chemical modification of a commercial polyethersulfone (UDEL P1700). The sulfonimide polyethersulfones were

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obtained by modification of sulfonic functions of sulfonated polyethersulfones. By this approach, both ionomers, i.e. bearing sulfonic and sulfonimide acids, should exhibit the same functionality (substitution degree) and the same distribution of the acidic functions along the ionomer chains allowing an accurate comparison of impact of the acid nature on membrane behavior. McGrath syntheses [22] led to ionomers with randomly distributed ionic functions but, starting from the monomer, the authors faced instability of the sulfonimide groups during the polycondensation reaction. Our method overcomes this synthesis issue. The sulfonimide polyethersulfones were characterized by various methods, their properties being compared to those of sulfonated polyethersulfones. In the first part of this report, the synthesis route is described, and the structural and thermal characterizations of sulfonimide polyethersulfones are compared to the sulfonated polysulfones ones. In the second part, the impact of the acidic function on the membrane water uptakes and conductivities is compared and discussed.

2.

Materials and protocols

2.1.

Materials

Trimethylsilyl chlorosulfonate (TMSClS e purchased from Aldrich), trifluoromethanesulfonic anhydride (from ABCR), phosphorus pentachloride (PCl5 from Fluka) were stored in a glove box and used as received. Benzylamine and N,N-diisopropylethylamine (from Alfa Aesar) were distilled and stored in a glove box. Lithium hydroxide purchased from Alfa Aesar, polyethersulfone (UDEL P1700) from Solvay were used as received.

2.2. Synthesis of polysulfone functionalized with sulfonimide (SI-PSF) The synthesis of polyethersulfone functionalized with sulfonimide was performed by chemical modification of sulfonated polyethersulfone. The transformation of sulfonated polyethersulfone into sulfonimide polyethersulfone involves 4 steps presented in Scheme 1. This method was patented previously by our group on organic arylsulfonic molecules [25,26]. Different ionomers were synthetized; the polymer codes and IEC are presented in Table 1.

2.3.

Synthesis of S-PSF

The sulfonation of polyethersulfone (PSF: UDEL P1700) was performed following the protocol described previously [27]. In a typical procedure, in order to obtain a sulfonated polysulfone with an ionic exchange capacity of 1.6 mol Hþ/kg, 20 g (45.25 mmol of repeating unit) of polymer have been added in 200 ml of dried dichloromethane (DCM) at 45  C. After complete dissolution of the PSF 6.97 ml (45.25 mmol) TMSClS was slowly added. The reaction was kept 24 h under vigorous stirring and argon flow, and then the reaction mixture was poured into a 1 M sodium hydroxide in ethanol, leading to the sodium sulfonate form (S-PSFeNaþ). After filtration the white polymer powder was successively washed with ethanol and

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S

O

S

O

O

O

O

CH

O

CH

O

CH

O

CH

m p

SO Na PCl / DMF

S-PSF Na+

CH S

O

O

S

O

O

O

CH

O

CH

O

S

O

O

O

CH

m p

Cl NH

SCl-PSF

CH S

O

O

S

O

O

O

CH

O

CH

O

S

O

O

O

CH

m

p

NH

DIEA (CF SO ) O

SA-PSF

CH S

O

O

S

O

O

S

O

O

O

O

CH

m p

F

O N

O

CH

O

CH

F

S F

O

EtOH

BSI-PSF

CH S

O O F F

O

S

O

O

O

CH

O

CH

S

O

O

O

O

CH

m p

NH S

O F

SI-PSF

Scheme 1 e Synthesis of SI-PSF from S-PSF.

finally with water up to neutral pH. The polymer was dried under vacuum at 65  C for 48 h. The acidic form (S-PSF) was obtained by treatment of S-PSF Naþ with 1 M solution of HCl in water. After 24 h the S-PSF was filtered and washed up to neutral pH, and then dried at 60  C under vacuum for 48 h.

1

2

CH

2'

4'

1'

R

3'

1

2

CH

2'' 1''

O

O R

3

4

S

O

4'

3'

1

H NMR S-PSF2eNa (DMSO-d6): d 1.66 (CH3CCH3, s, 6H), 6.94 (1 ,40 , d, 3Hxf), 7.03e7.12 (1.4, m, 4Hxf, 8Hx(1-f)), 7.24e7.31 (2,20 , m, 3Hxf, 4Hx(1-f)), 7.72 (200 , s, 1Hxf), 7.81e7.91 (3.30 , m, 4Hxf, 4Hx(1-f)) R ¼ SO3H, f e functionality 0

1

2

CH

2

1

O

3

4

4

3

1

2

CH

2

1

O

3'

4

S

O

4

3

O

O

3'

4

R m p

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Table 1 e Functionality and IEC of S-PSF and SI-PSF calculated by NMR spectroscopy and acidebase titration. Code S-PSF1 S-PSF2 S-PSF3 SI-PSF1 SI-PSF2 SI-PSF3

f* (mol Hþ/mol r.u.) (NMR)

IEC (mol Hþ/kg) (NMR)

IEC (mol Hþ/kg) (Acidebase titration)

Density (g/cm3)

IEC (mol Hþ/dm3) (Acidebase titration)

0.67 0.84 1.05 0.71 0.87 1.08

1.35 1.66 2.0 1.25 1.39 1.61

1.39 1.63 2.04 1.18 1.31 1.58

1.27 1.30 1.34 1.33 1.38 1.40

1.8 2.1 2.7 1.56 1.8 2.2

*Moles sulfonic or sulfonimide function per repeating unit.

2.4.

Synthesis of SCl-PSF

The sulfo-chlorinated polysulfone (SCl-PSF) was synthesized as shown in Scheme 1. In a typical procedure, 10 g of SPSFeNa2þ was dispersed in 440 ml of dichloroethane (DCE) in a round flask. The heterogeneous mixture was heated and a part of DCE was distilled in order to dry the polymer solution by azeotropic removal of water. Then, the temperature of the reaction mixture was decreased down to 50  C under an argon flow and 8.96 g (38.21 mmol) of PCl5 and 8 ml of dimethylformamide (DMF) being added. After a 6 h reaction, the polymers became completely soluble in DCE, proving that the sodium sulfonated form is turned into the sulfonyl chloride function. The reaction mixture was filtered, to remove the NaCl, and poured in diethyl ether. Then, the filtered powder was washed several times with diethyl ether. The polymer was dried under vacuum at room temperature and stored in a glove box to avoid the hydrolysis of sulfonyl chloride groups.

2.5.

Synthesis of SA-PSF

The synthesis of SA-PSF was carried out in anhydrous DCM by addition of large excess of benzylamine at room temperature (Scheme 1). In a typical procedure, 4 g (7.7 mmol of structural units) of SCl-PSF2 (Table 1) was dissolved in 40 ml DCM and 3.36 ml (30.7 mmol) of distilled benzylamine was added. After 24 h of reaction at room temperature, the reaction mixture was poured in methanol. The yellow polymer powder was dried at 60  C under vacuum. R ¼ SO2NHCH2Ar 1 H NMR (DMSO-d6): d 1.63 (CH3CCH3, s, 6H), 4.06 (CH2, d, 2Hxf), 7.02e7.42 (10 ,40 ,1,4,2,20 , AreH from ionic functions, m, 15Hxf, 12Hx(1-f)), 7.63e7.66 (200 , m, 1Hxf), 7.87e7.95 (m, 3 þ 30 , 4Hxf, 4Hx(1-f))

2.6.

Synthesis of BSI-PSF

The SA-PSF was reacted with trifluoromethanesulfonic anhydride, to obtain the sulfonimide groups (eSO2N(CH2eC6H5) SO2CF3) grafted on the polymer backbone (BSI-PSF). In a typical procedure 5 g (8.66 mmol of structural unit) of SA-PSF2 was dissolved in 20 ml of distillated DCE then, 2.18 ml (12.99 mmol) of trifluoromethanesulfonic anhydride and 2.96 ml (17.3 mmol) of Di-Isopropyl-Ethyl-Amine (DIEA) was added. After 30 min at 0  C, the reaction mixture was poured in methanol to obtain the benzylsulfonimide polysulfone.

After 24 h stirring, the polymer was filtered, washed with methanol and dried under vacuum at 60  C. R ¼ SO2N(C6H5)SO2CF3 1 H NMR (DMSO-d6): d 1.66 (CH3CCH3, s, 6H), 5.30 (CH2, s, 2Hxf), 7.03e7.47 (10 ,40 ,1,4,2,20 ,200 , AreH from ionic functions, m, 16Hxf þ 12Hx(1-f)), 7.73e8.01 (3 þ 30 , m, 4Hxf, 4Hx(1-f)). 19 F NMR (DMSO-d6) d: 73.02 (CF3)

2.7.

Synthesis of SI-PSF2

The BSI-PSF was reacted with ethanol at 100  C for a few days to obtain the PSF functionalized with sulfonimide groups (SIPSF) (Scheme 1). In a typical procedure, 5 g (7.32 mmol of structural unit) of BSA-PSF2 (Table 1) was dissolved in 30 ml of ethanol and refluxed. After 3 days, the reaction mixture is poured in diethyl ether to obtain the final polymer in acid form. After 6 h of stirring, the polymer is filtered, washed with diethyl ether and dried under vacuum at 60  C. R ¼ SO2NHSO2CF3 1 H NMR (DMSO-d6): d 1.67 (CH3CCH3, s, 6H), 7.03e7.44 0 0 (1 ,4 ,1,4,2,20 , m, 10Hxf, 12H*(1-f)), 7.78e7.89 (3,30 ,200 , m, 5Hxf, 4Hx(1-f)) 19 F NMR (DMSO-d6): d: 77.77

2.8.

Synthesis of model molecule

In order to calculate the IEC of SI-PSF with the help of NMR a model molecule was synthetized. The synthesis was performed in 3 steps (Scheme 2).

2.9.

Synthesis of SA-T

In a typical procedure, p-toluene sulfonyl chloride 2 g (10.5 mmol) was dissolved in distillated dichloromethane (DCM) (50 ml) and 3.44 ml (31.5 mmol) benzylamine was added. The mixture was stirred for 24 h at room temperature, then, the reaction mixture was washed with an aqueous solution of HCl 3 M. Evaporation of the solvent under reduced pressure gave SA-T as white solid. Yield: 78% (1.13 ge3.9 mmol). 1 H NMR (CDCl3): d 2.43 (1, s, 3H) 4.11 (5, d, 2H) 4.83 (4, s, 1H) 7.18e7.31 (2,6,7,8 m, 7H) 7.75 (3, d, 2H) (cf. Scheme 2)

2.10.

Synthesis of BSI-T

In a typical procedure, 0.261 g (1 mmol) of SA-T (dried under vacuum overnight at 60  C) was dissolved in anhydrous DCM (10 ml) under argon at 0  C and then, 0.274 ml (1.6 mmol) of DIEA

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1

O

4

H3C

S

NH

O H3C

S

Cl

+ NH2

O

O

6

7 8

5

SA-T (CF 3SO2)2O DIEA F

F

2

1

3

H3C

O S O

O

F

S

N

Li

1

O

-

F

F EtOH

+

BSI-T

2

3

H3C

O S

LiOH

O

S

N

O

SI-T

F O

7

4 5

6

Scheme 2 e Synthesis of model molecule (SI-T).

freshly distillated and 0.252 ml (1.5 mmol) of trifluoromethanesulfonic anhydride were added. After 10 min, the mixture was hydrolyzed with aqueous solution of HCl 1 N. The organic phase was further washed with water, then the organic layers were dried over MgSO4 and the solvents evaporated under reduced pressure. The solid was washed with hot pentane and purified by chromatography on silica gel (eluent: CH2Cl2). The product BSI-T was obtained as yellow solid. Yield: 37%. 1 H NMR (in CDCl3): d, 2.35 (1, s, 3H) 5.04 (4, s, 2H) 7.09 (2, d, 2H) 7.28e7.39 (3,5,6,7, m, 7H) (cf. Scheme 2) 19 F NMR d: 73.31 (s)

2.11.

Synthesis of SI-T

In a typical procedure, 0.804 g (2.04 mmol) of BSI-T was dissolved in absolute ethanol (0.2 M) and stirred under reflux for 3 days. After LiOH$H2O 0.088 g (2.04 mmol), was added and the mixture was stirred at room temperature overnight and the solvent was evaporated. The solid obtained was dissolved in diethyl ether and filtered. After evaporation of the solution the solid obtained was washed with pentane and the product SI-T was dried at 60  C under vacuum for 48 h. Yield: 98%. 1 H NMR (CD3COCD3): d 2.36 (1, s, 3H), 7.25 (2, d, 2H) 7.77 (3, d, 2H) (cf. Scheme 2) 19 F NMR d: 79.14 (s)

2.12.

Characterization methods

2.12.1.

1

H NMR and

19

F NMR spectroscopies

The S-PSF and SI-PSF samples were dissolved in DMSO-d6 by warming them to ensure a total dissolution of the samples. A Bruker spectrometer cryospec WM 250 e frequency of 301.2 MHz for the protons and 282.39 for 19F NMR was used.

2.12.2. Ionic exchange capacity (IEC) Generally, the functionality of the polymer is expressed by moles ionic function/mole repeating unit (f). In the case of SPSF is expressed as the sulfonation degree. The relation between f and IEC (mol Hþ/kg) is IEC ¼ (f  1000)/(structural repeating unit molecular weight þ f  Mf), where Mf is the weight of ionic function.

The IECs have been calculated by NMR and acidebase titration in organic solution.

2.12.3. Acidebase titration In a typical procedure, a solution of S-PSF in diethyleneglycolmonomethylether (DGME) (5  103 mol repeating units/L) was titrated with a solution of NaOH in DGME (2.625  102 M) in presence of methyl orange.

2.12.4. Infrared spectroscopy The FT-IR spectra of sulfo-chlorinated polysulfone were recorded from thin films prepared, after total solvent removal, from a dichloromethane solution of SCl-PSF, consisting of 0.05 g of polymer dissolved in 1 ml of dichloromethane.

2.12.5. SEC-MALLS analyses Two columns AGILENT 2PLgel-Mixed-D were used for SECMALLS characterization. The columns were heated at 70  C and a 0.1 M solution of NaNO3 in dimethylformamide (DMF) was used as a solvent with a flow rate of 1 ml/min. Samples’ concentration was 1% (w/w) in DMF; samples were pre-filtered on PTFE 0.45 mm filter. The solvent was filtered and degassed continuously using polypropylene filter 0.2 mm. The pump was WATERS 515 HPLC. A Dawn EOS from Wyatt Technology 690 nm (18 angles) was used. The software ASTRA provided the molecular weights Mi versus elution time (flow rate 1 ml/min).

2.12.6. Water uptake The membrane water uptake was determined at different temperatures ranging between 30  C and 80  C. The polymer membrane previously dried (for 24 h at 60  C under vacuum) was immersed in water during 24 h. The ratio between the amount of water uptake by the membrane and dried membrane was expressed as water uptake percentage.

2.12.7. Proton conductivity of membranes The conductivity measurements were carried out using a HewlettePackard 4192A LF frequency response analyzer at different temperature and 95% relative humidity by using a climatic chamber Vo¨tsch 4018.

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Mechanical analyses were determined using DMA Q 800 spectrometer instrument working in the tensile mode. The strain module was fixed at 0.05%, well below the limit of the linear viscoelastic regime. The measurement of the storage modulus E0 was performed in isochronal condition (1 Hz) in the temperature range from 20  C to 250  C in the case of dried membrane and between 20  C and 80  C with immersion clamps with a temperature ramp of 2  C min1.

1028 cm1 and 1057 cm1 disappear and new bands corresponding to SO2Cl appear at 1178 cm1 and 1357 cm1. Moreover, monitoring the next reaction step, namely the transformation of the eSO2Cl in eSO2eNHeCH2eC6H5, by NMR spectroscopy, allows assessing indirectly, but accurately, the conversion of eSO3Na into eSO2Cl. Indeed, only eSO2Cl can react with the benzylamine to lead to eSO2eNHeCH2eC6H5. Hence, by calculating the integral ratio between the CH2 (from eNHeCH2eC6H5; 4 ppm) and (CH3)2C of PSF backbone (1.6 ppm) we found the same functionality as with sulfonated polysulfone: this allows assuming that both sulfochlorination (synthesis of SCl-PSF) and amination (synthesis of SA-PSF) reactions are quantitative. The synthesis of BSI-PSF was monitored by 1H NMR and 19F NMR. In 1H NMR spectra the peak of CH2 (eNHeCH2eC6H5) shifts from 4 to 5 ppm proved the formation of eSO2eN(SO2eCF3)eCH2eC6H5 intermediate. Additionally, the presence of CF3 at 73 ppm was evidenced by 19F NMR. The acidic form i.e. SI-PSF was obtained by deprotection of N-benzyl trifluoromethanesulfonimide grafted on the polysulfone backbone (BSI-PSF). The 1H NMR spectra of sulfonated polysulfone and sulfonimide polysulfone are presented in Fig. 1. The transformation of sulfonic function into sulfonimide involves a slight chemical shift of the aromatic protons (protons 10 , 20 , 200 and 40 ) to higher ppm values. The stronger acidity of sulfonimide, induced by the larger delocalization of the negative charge in the sulfonimide anion, as compared with the sulfonic one, corroborates these chemical shifts.

2.12.10. Thermal gravimetrical analysis (TGA)

3.1.

The climatic chamber (Vo¨tsch VC 4018) allows the relative humidity to be stabilized in a temperature range from 40  C to 90  C. At constant RH, conductivity is stabilized in 12 h for a step of 10  C in temperature. The equilibrium time is the time necessary to reach a stable value of the resistance at 1%, such 12 h in this case.

2.12.8. Impedance measurements The conductivity measurements were carried out using a HewlettePackard 4192A LF frequency response analyzer. Impedance measurements between the different cells are driven automatically by an Agilent 34970A Switch unit equipped with a 34901A 20 channel card. The spectra were recorded between 13 MHz and 5 Hz. The resistance of the membrane is taken at the high frequency intercept with the real axis in the Nyquist plot, which is usually between 106 and 104 Hz.

2.12.9. Dynamic mechanic analysis (DMA)

The measurements were performed on a NETZSCH STA 409 PC/PG under air flow (mixture of 76% Ar and 25% O2) and a temperature ramp of 5  C min1.

2.12.11. Membrane elaboration The ionomer (SI-PSF or S-PSF) in sodium form was dissolved in dimethylacetamide (10% g/ml). After the complete dissolution of the ionomer, the obtained solution was filtered with 45 mm PTFE filter and then the solution was degassed to remove the air bubbles. The degassed solution was put in a petri dish. The petri dish containing the ionomers solution was kept in an oven at 60  C during 48 h. Then the membrane was dried at 80  C under vacuum for 48 h and then, the membrane was immersed with aqueous HCl solution (2 M) for 48 h in order to acidify the membrane.

3.

Results and discussion

The syntheses of polyethersulfone ionomers based on sulfonimide acidic groups were performed by a multi-step reaction from the commercial UDEL polymer involving the chemical modification of sulfonated polyethersulfone (Scheme 1). The structures and the purity of final and intermediate products were assessed by NMR and IR spectroscopies. The reaction conditions were optimized with the aim of ensuring a total transformation of sulfonic functions into sulfonimide ones. The transformation of eSO3Na into eSO2Cl cannot be monitored by NMR spectroscopy as no significant chemical shift can be observed in the NMR spectra [27]. However, in the FT-IR spectra the bands of SO3  stretching frequency at

Ionic exchange capacity

The functionality and the ionic exchange capacity of SI-PSF were calculated using both 1H and 19F NMR and compared with the starting Ionomers (S-PSF). For S-PSF, both 1H NMR and acidebase titration were used, following the same equations and protocols previously published [27]. Due to the very slight shifts of the aromatic protons close to the sulfonimide function, as compared to those close to the sulfonic function, 1H NMR spectra cannot be used to determine, with great accuracy, the IEC of SI-PSF ionomers and to assess if the whole of the sulfonic functions were turned into sulfonimides. Nevertheless, the presence of CF3 in the sulfonimide allowed, calculation of the IEC using 19F NMR. Hence, in the NMR tubs a known amount of a partially perfluorinated compound, trifluoroethanol (TFEt), was added (Fig. 1) as internal standard. However, different possible relaxation times of fluorine nucleus could exist between the CF3 belonging to the internal standard and the one belonging to the aryl trifluoromethylsulfonimide. In order to avoid any mistake, a model molecule (Scheme 2), mimicking the ionic repeat unit of the ionomer, was synthetized. An NMR tub with a stoichiometric amount of trifluoroethanol and model molecule was prepared. The calculated integral ratio: integral CF3 of TFEt/integral CF3 of model molecule was found to be equal to 1.2. Thus, in order to calculate the functionality and IEC of SIPSF with the help of NMR the following equations were established, the values being gathered in Table 1. f ¼ððintegral CF3 of SI  PSFÞ=ðintegral CF3 of TFEt  1:2ÞÞ=   integral ðCH3 Þ2 C of SI  PSF ðintegral CH2 of TFEt  3Þ

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Fig. 1 e NMR spectra of a) 1H NMR of SI-PSF2 (CF3eCH2eOH internal standard that allowed calculation of SI-PSF IEC) b) 1H NMR of S-PSF2, c) 19F NMR of SI-PSF2 CF3eCH2eOH internal standard that allowed calculation of SI-PSF IEC).

By applying this equation the calculated values of functionality of SI-PSF are very close to those of S-PSF. Thus to calculate the IEC, we considered that all the sulfonic functions were converted in sulfonimides. The gap between the IEC of both ionomers is related to the differences between the molecular weights of ionic moieties. Indeed, the molecular weight, 211 g/mole, of sulfonimide is significantly higher than that of the sulfonic function 80 g/mole. The values obtained by titration were close to those obtained by NMR, which underlines once again the quantitative transformation of sulfonic function into sulfonimide. The modification of sulfonic acid into sulfonimide acid induces a slight increase in polymer density (Table 1). However for close functionalization degrees, f*, the IECs per dm3 of S-PFS are much higher than those of SI-PFS.

3.2.

Molecular weight

The molecular weight of PSF, S-PSF, SI-PSF and some intermediate products were determined by SEC-MALLS and are presented in Table 3. The (dn/dc) values used to determine Mn and Mw were estimated, assuming that the entire polymer injected passed through the column. For S-PSF samples a slight increase of both Mn and Mw, with regard to the starting polymer, was observed. This underlines that chain breaking does not occur during the sulfonation and the slight increase of average molecular weights must be attributed to increase by 80  f g (SO3) of the ionic monomer repeating units [27]. The SEC-MALLS analyses performed on the SI-PSF, led to much higher Mn and Mw than those expected from the higher weight of sulfonimide as compared to sulfonic groups i.e. 211 vs 80.

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Table 2 e Molecular weights and Ip measured by MALSSEC. Code PSF (UDEL 1700) S-PSF1 S-PSF2 S-PSF3 SI-PSF1 SI-PSF2 SI-PSF3

Mn (g/mol) 22 22 23 29 45 32 47

000 500 910 000 780 090 960

Mw (g/mol) 49 46 44 50 92 73 89

000 240 410 430 900 740 180

Ip 2.24 2.05 1.86 1.73 2.3 2.3 1.96

Thus, both Mn and Mw were roughly doubled as compared to their sulfonic homolog S-PSF (Table 2). If the MarkeHouwinkeSakurada coefficients of a polymer are unknown, classical SEC leads, after calibration, to Mw and Mn expressed in equivalent polystyrene. Therefore, changes in the polymer/solvent interactions, upon a chemical modification, can induce a dramatic change of the hydrodynamic volume and, therefore, to a great gap between the molecular weights (in equivalent polystyrene) of the starting and modified polymer. On the contrary, SEC-MALLS allows obtaining the real molecular weights. The high increase observed could be due (i) to the formation of aggregates or/and (ii) to branching occurring during the multi-step process. As there is a good agreement between the refractive index signal and the light scattering, both Mw and Mn increase and no additional peak or even shoulder was detected (Fig. 2), we can discard this the (i) hypothesis. A second possible explanation could be the appearance of ramifications subsequently to branching side reactions e.g. electrophilic substitution of SO2Cl on polymeric chains. Once again, such long branching would mainly increase Mw and the polydispersity index Ip. MALDITOF characterizations of UDEL evidenced oligomeric cycles and linear chains either end-capped by phenol or chlorophenyl [27]. A reaction of phenol ends with PCl5 seems a reasonable assumption and might explain the simultaneous increase of both Mw, Mn. Besides, such a molecular weight increase was previously reported [22]. This assumption is sustained by the slight increase of Ip and the significant increase of Mn and Mw. The well-known reaction between phenol (AreOH) and PCl5, which leads mainly to the phosphoric ester [28], should take place between the phenolic end chains of UDEL.

3.3.

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Thermal and thermo-mechanical characterization

The thermal stability was evaluated by TGA. The onset of weight loss was selected to estimate the stability. TGA was performed, from 20 to 450  C, in a gas mixture i.e. 76% Ar and 24% O2 v/v (Table 2). The acid-form films (S-PSF and SI-PSF) showed a loss of absorbed water of 2e3 wt% at 80e200  C, which indicated their hygroscopic nature (Fig. 3). The sulfonated polysulfone degradation started at 277  C; a slight increase of the degradation temperature being observed when the IEC decreases (Table 2). A slow weight decrease was then observed up to 350  C (Fig. 3a). This behavior, in agreement with a reported one [22], was ascribed to the removal of SO3. The SI-PSF showed different thermal degradation behavior, as compared to S-PSF, and exhibited slightly lower stability (Table 3). The weight losses occured in two steps (Fig. 3b); the weight loss after the second step being close to the weight of SI function. The same results were observed by McGrath et al. [22] who assumed that the sulfonimide is converted to sulfonic acid in the first step, the second step being the loss of the sulfonic functions. A slight increase of the glass transition temperature with IEC was observed for both ionomers (S-PSF and SI-PSF). The Tg of SI-PSF at the same functionality is slightly lower than that of S-PSF. Owing to the longer side groups and to their flexibility, a significant decrease in Tg was expected. However, the increase in free-volume induced by the flexible sulfonimide side chain is partly counterbalanced by its higher dissociation, particularly in the absence of water, which should induce stronger ionic inter-chains interactions and might explain the close Tg of both ionomers. The dynamic mechanical analyses were performed both on a dried membrane and on a membrane immersed in water by using immersion clamp; the membrane being immersed in water during the experiment. The storage moduli of the dried membranes at 100  C are gathered in Table 3. Similar storage modulus values were obtained for both, SI-PES and S-PES, dried membranes. Storage moduli obtained on water-immersed SI-PSF membranes are much lower than for corresponding dried membranes due to the plasticizing effect of water. However, at 30  C, the storage moduli of SI-PSFs are higher than those obtained with S-PSF membranes having the same content in functional groups/repeating unit. This behavior might be

Fig. 2 e SEC-MALLS curves of SI-PSF2.

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Fig. 4 e DMA in water of S-PSF and SI-PSF.

3.4.

Water uptake and conductivity

The behavior of membrane in water DMA performed in water is corroborated by water uptake measurements (Fig. 5). At temperature below 40  C, the water uptake (wt%) of SI-PSF is slightly lower than that of S-PSF (Fig. 5a). However, the number of water molecules per polymer ionic function (l) seems to be slightly higher for SI-PSF as compared to S-PSF (Fig. 5b). Nevertheless, above a certain temperature, and depending on the IEC, the water uptake of SI-PSF increases much more than that of S-PSF despite the higher IEC of the latter. The higher water uptakes of polymers containing sulfonimide have Fig. 3 e TGA and DTG traces of a) S-PSF2 b) SI-PSF2 recorded under 76% Ar and 24% O2.

explained by the lower IEC of SI-PSF as compared to their SPSF counterparts. Indeed, when the temperature increases the storage moduli decreases as water uptake is increased and induces a stronger plasticization. This plasticization was found to be much more significant for SI-PSF. Additionally, a sharper decrease in storage modulus occurred for SI-PSF than for S-PSF (Fig. 4). From this fast decrease in storage modulus a huge membrane swelling must be inferred.

Table 3 e Thermal and thermo-mechanical characterization. Code

S-PSF1 S-PSF2 S-PSF3 SI-PSF1 SI-PSF2 SI-PSF3

Tg (DSC) ( C)

Td (TGA) ( C)

Storage modulus (100  C, air) (MPa)

Storage modulus (30  C, water) (MPa)

196 200 210 194 198 205

296 285 277 282 277 270

2100 2300 e 2050 2220 e

1443 696 638 1770 792 686

Fig. 5 e Water uptake a) % wt b) the corresponding l values of the ionomer membrane as a function of IEC after equilibration in water at different temperatures.

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Conductivity (mS/ cm)

100

S- PSF 1 SI- PSF 1

10

S- PSF 2 SI- PSF 2 S- PSF 3 SI- PSF 3

1

2.7

2.9

3.1

3.3

3.5

1000 /T (K-1)

Fig. 6 e Proton conductivity plots for the ionomers at RH 95% for the ionomers.

already been reported [20e23]. The sulfonimide acid groups might form larger hydration shells than the sulfonic acids groups since the negative charge is highly delocalized. Besides, ab initio calculations on TFSI anion have established that the negative charge is mainly shared between the SO2 surrounding the nitrogen atom, making the interaction with water molecules easier, despite the hydrophobic CF3 [18]. The sharp change in the slope of water uptake of SI-PSF, which occurs roughly at certain temperature depending on IEC, may be ascribed to changes in sulfonimide conformation. The theoretical calculations of TFSI anion showed that the energy barrier for the rotation around the nitrogen is very low and should occur at ambient temperature. Due to the steric hindrance induced by the polyaryl chain of the ionomer, it can be assumed that the energy barrier is much higher in SI-PSF and might occur at temperature above 40  C. Thus, after a given water uptake, the plasticization induced by water favors the conformational changes of side groups, which become more mobile and induce a change in membrane morphology that could accommodate increasing amount of water. The proton-conductivities of membranes, maintained at 95%RH in a climatic chamber, were measured from 20 to 90  C. Fig. 6 shows that the conductivity of the membranes increases with the temperature, while the conductivity curves of S-PSF and SI-PSF membranes are very different. The temperaturedependence conductivities of S-PSF membranes show a VTF behavior. At low temperature the SI-PSF and S-PSF membranes of close functionality, but of markedly different IEC (mol Hþ/dm3), have close conductivities. However, above 40  C, the conductivity of SI-PSF membranes increases more than that of S-PSF ones, which could indicate a gradual change in the membrane morphology. These results corroborate the water uptake behavior that shows, after 40  C, that much more water is accommodated by the SI-PSF membranes. However, the conductivity comparisons between ionomers having close IECs, show that SI-PSF membranes are much more conductive.

4.

Conclusion

Starting from a sulfonated polyethersulfone a series of polysulfone functionalized with sulfonimide acid group were

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successfully synthesized with three different ionic exchange capacities. The resulting ionomers show higher water uptake and higher conductivities than the sulfonated polysulfone. This behavior makes the sulfonimide function most suitable for ionomeric membranes dedicated to PEM Fuel Cells operating above 90  C as, in these conditions, the water vapor pressure is low and the sulfonimide acid should be more dissociated than the sulfonic counterpart. However, taking into account the excessive water uptake of these ionomers, it should be advantageous either to cross-link the membrane or to tailor bloc copolymers, hence nanostructured sulfonimide ionomers.

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