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Synthesis of Partially Fluorinated Poly(arylene ether sulfone) Multiblock Copolymers Bearing Perfluorosulfonic Functions gis Mercier,3 Sandrine Lyonnard,4 Huu Dat Nguyen,1,2 Luca Assumma,1,2 Cristina Iojoiu,1,2 Re 2,5 Emilie Planes 1
LEPMI, University of Grenoble, F-38000 Grenoble, France LEPMI, CNRS, F-38000 Grenoble, France 3 nierie des Mate riaux Polyme`res, UMR-5223, IMP@LYON1, Universite de Lyon, Universite Lyon 1, 15 Bd. A Latarjet 69622, Inge Villeurbanne CEDEX, France 4 INAC/SPrAM, Groupe Polyme`res Conducteurs Ioniques, UMR-5819, CEA-CNRS-UJF, CEA-Grenoble, 17 Rue de Martyrs 38054 Grenoble, CEDEX 9 France 5 ry, France LEPMI, University of Savoie, F-73000 Chambe Correspondence to: C. Iojoiu (E - mail:
[email protected]) 2
Received 2 January 2015; accepted 19 March 2015; published online 00 Month 2015 DOI: 10.1002/pola.27650
ABSTRACT: Partially fluorinated poly(arylene ether sulfone) multiblock copolymers bearing perfluorosulfonic functions (psPES-FPES), with ionic exchange capacity (IEC) ranging between 0.9 and 1.5 meq H1/g, are synthesized by regioselective bromination of partially fluorinated poly(arylene ether sulfone) multiblock copolymers (PES-FPES), followed by Ullman coupling reaction with lithium 1,1,2,2-tetrafluoro-2-(1,1,2,2-tetrafluoro-2iodoethoxy)ethanesulfonate. The PES-FPES are prepared by aromatic nucleophilic substitution reaction by an original approach, that is, “one pot two reactions synthesis.” The chemical structures of polymers are analyzed by 1H and 19F NMR spectroscopy. The resulted ionomers present two distinct glass transitions and a relaxations revealing phase separation between the hydrophilic and the hydrophobic domains. The phase separation is observed at much lower block lengths of
ps-PES-FPES as compared with the literature. AFM and SANS observations supported the phase separation, the hydrophilic domains are well dispersed but the connectivity to each other depends on the ps-PES block lengths. The thermomechanical behavior, the water up-take, and the conductivity of the psPES-FPES membranes are compared with those of Nafion 117V and randomly functionalized polysulfone (ps-PES). Conductivities close or higher to those of Nafion 117V are obtained. C 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. V Chem. 2015, 00, 000–000
INTRODUCTION Proton Exchange membranes (PEM) in PEM Fuel Cells (PEMFC) play a critical role in transporting hydrated protons. A successful PEM material must meet multiple stringent requirements, such as proton conductivity, good mechanical strength, high oxidative and chemical stability, low fuel and oxidant permeability as well as easy fabrication and excellent water management under low relative humidity (RH) cycling.1,2 Commercially available materials are perfluorosulfonic acid polymer (PFSA) membranes such as NafionV. These membranes consist of an extremely high hydrophobic perfluorinated backbone along with hydrophilic PFSA side chains. Their excellent conduction and overall fuel cell performance originate from two key properties: the highly phase-separated membrane morphology with a welldefined hydrophilic/hydrophobic interface and the acidity of the ionic function.3,4 Although Nafion has been the most
commonly used PEM during recent decades, its properties need to be improved for broader fuel cell application. Considerable efforts have therefore been devoted to the synthesis of sulfonated aromatic polymers such as poly(arylene ethers)s, poly(arylene ether sulfone)s, poly(arylene ether ketone), and polyimide.5–15 However, high proton conductivities are achieved within these types of PEMs only at fully hydrated conditions, and the conductivities drop sharply when RH is decreased. Consequently, a variety of synthesis strategies aiming at increasing the acidity of the ionic function or/and at improving the membrane morphology has been explored to optimize the conductivity of aromatic ionomers. Regarding the search for increased acidity, aromatic ionomers bearing aryl sulfonimide acid16,17 or alkyl perfluoro alkyl sulfonic acid side chains18–26 instead of aryl sulfonic acid were recently proposed. The poly(arylene ether)s
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KEYWORDS: block copolymers; copolymer synthesis; fluoropoly-
mers; ionomers; membrane characterization; membrane; membrane morphology; PEMFC; poly(ether sulfone); proton exchange membranes
C 2015 Wiley Periodicals, Inc. V
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containing pendant PFSA groups showed more phaseseparated morphology and higher proton conductivity than those of the poly(arylene ether)s with main chain grafted sulfonic acid groups. To further improve the proton-conducting properties, block copolymer structures were recently regarded as promising. Accordingly, hydrophilic/hydrophobic multiblock copolymers have been investigated for PEM materials because of their ability to form hydrophilic2hydrophobic phase-separated morphology27–31 with interconnected ionic channels. Mikami et al.26 reported recently on perfluoroalkyle sulfonic acidmodified poly(arylene ether) ionomer membranes and found that the block copolymer structure has an effective impact in improving the proton-conducting behavior without increasing the ion exchange capacity. Usually, the aromatic multiblock copolymers are obtained in 3 steps (two first steps dedicated to the synthesis of the 2 oligomers that correspond to copolymer blocks and a third step concerns the multi-block copolymer synthesis by copolycondensation of two oligomers). However, due to (i) the high viscosity of solution based aromatic polymers, (ii) the high polydispersity of oligomers obtained by polycondensation (close to 2), and (iii) the difficulty to control the stoichiometry (it is not evident to determine the number of reactive end groups per gram for each oligomer) high molecular weight polymers are hardly obtainable by this approach. The use of decafluorobiphenyle as bridging unit between the blocks allows obtaining high enough molecular weight but this adds additional reaction in the multiblock copolymer synthesis.32 Here, we report the synthesis of partially fluorinated multiblock copoly(ether sulfone) bearing superacids, that is, perfluroro alkyl sulfonic acids. These ionomers were synthesized from partially fluorinated multiblock copoly(ether sulfone) followed by the bromination and perfluorosulfonation via Ullmann reaction. The partially fluorinated multiblock copoly(ether sulfone) was synthesized by an original approach “one pot two reactions synthesis.” This method, explored in a pioneering work in the synthesis of polyimide,33 has several decisive advantages, as compared with the synthesis in three steps, of: (i) controlling the stoichiometry of the monomers, (ii) ensuring a shorter duration reaction, and (iii) less solvent consuming as compared with synthesis of multiblocks copolymers in 3 steps. In the first part, we report on the synthesis of partially fluorinated multiblock copoly(ether-sulfone). The chemical structures are discussed with the help of NMR spectroscopic analysis while the characteristic temperatures of resulted copolymers are measured by differential scanning calorimetry (DSC) and thermogravimetrical analysis (TGA). In the second part, the synthesis of ionomers starting from partially fluorinated copoly(ether sulfone) is described. The effect of PFSA groups and block structure on the properties, such as proton conductivity, water affinity, morphology, and mechanical properties are studied and compared with Nafion 117V. R
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Materials Dimethylsulfoxide, copper, purchased from Alfa Aesar, potassium carbonate, bromine, purchased from Acros Organics, were used as received. 4,40 -Difluoro-diphenyl sulfone (DFDPS), 4,40 -Biphenol (BP), and 4,40 -dihydroxydiphenylsulfone (DHDPS), purchased from Alfa Aesar, were recrystallized from isopropanol before use. Decaflurobiphenyl (DFBP), purchased from FluoroChem, was sublimated before use. 1,1,2,2,-Tetrafluoro-2-(1,1,2,2,-tetrafluoro-2-iodoethoxy)ethane sulfonyl fluoride was purchased from Apollo Scientific and used as received. The polysulfone RADEL (the code used in this publication is PESp) was purchased by SOLVAY. The Mn measured by Size Exclusion Chromatography (SEC) is 19,000 g/mol and Mw/ Mn 5 1.9. Synthesis of PES-FPES The block copolymer PES-FPES was synthesized as shown in Scheme 1. In a typical procedure, 4 g (21.5 mmol) of BP, 5 g (19.76 mmol) of 4,40 DFDPS and 9 g (65.12 mmol) of K2CO3 were introduced in a three-necked flask equipped with an argon inlet, mechanical stirrer and Dean-Stark trap. Then DMSO (30 mL) as reaction solvent and toluene (15 mL) as azeotropic agent were added. The reaction mixture was heated at reflux at 160 C for 4 h to dehydrate the system. The temperature was then slowly raised to 180 C to distill off the toluene. After the temperature was cooled down at 120 C, reaction mixture was allowed to proceed at this temperature for 18 h. After this time, the temperature of reaction mixture was decreased to 70 C under argon and a solution of 3.46 g (13.8 mmol) 4,40 -DHDPS in 15 mL DMSO was added. Then 4.5 g (32.6 mmol) of K2CO3 and 5.2 g (15.6 mmol) of DFBP were successively added. Then the reaction mixture was allowed to proceed at this temperature for 1 h 30 min. Finally, the mixture was poured into aqueous solution of HCl (1 M). 1 H-NMR: (CDCl3): d (ppm) 7.97 (d, 4H), 7.89 (d, 4H), 7.58 (d,4H), 7.16-7.06 (m, 12H) 19
F-NMR: (CDCl3): d (ppm) 2136.74 (d, ArAF), 2137.25 (m, ArAF), 2137.77 (m, ArAF), 2149.48 (t, ArAF), 2151.98 (d, ArAF), 2152.22 (m, ArAF), 2152.62 (m, ArAF), 2160.18 (t, ArAF). Synthesis of FPESp Homopolymer
In a typical procedure 10 g (29.9 mmol) of DFBP, 7.5 g (29.9 mmol) of DHDPS an excess of K2CO3 (12.4 g, 89.6 mmol), and 30 mL of DMSO were introduced in a three-necked flask equipped with an argon inlet and a mechanical stirrer. The reaction mixture was heated at 80 C for 8 h and then poured into aqueous solution of HCl (1 M). The white polymer obtained was washed with water and dried under
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SCHEME 1 Synthesis of: (a) copolymer backbone (PES-FPES); (b) Synthesis of ionomers ps-PES-FPES.
vacuum at 60 C. The Mn measured by SEC was 56,000 g/ mol with Mw/Mn 5 2.2.
washed with water and dried under vacuum at 60 C. The Mn measured by SEC was 6000 g/mol with Mw/Mn 5 1.8.
1
1
H-NMR (CDCl3): d (ppm) 7.96 (d, 4H), 7.14 (d, 4H).
19
F-NMR: (CDCl3): d (ppm) 2136.8 (m), 2151.9 (m).
Synthesis of FPES Oligomer
In a typical procedure 5.0 g (15 mmol) of DFBP, 3.3 g (13.3 mmol) of DHDPS, 5.5 g (40 mmol) of K2CO3, and 30 mL of DMSO were introduced in a three-necked flask equipped with an argon inlet and a mechanical stirrer. The reaction mixture was heated at 80 C for 24 h and then poured into aqueous solution of HCl (1 M). The white polymer obtained was
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H-NMR (CDCl3): d (ppm) 7.96 (d, 4H), 7.14 (d, 4H).
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F-NMR (CDCl3): d (ppm) 2136.83 (m), 2137.25 (m, 8H), 2149.59 (t, 2H), 2151.97 (m), 2160.15 (t, 4H). Synthesis of PES-DF Polymer
In a typical procedure, 5.0 g (12.5 mmol of repeating unit) of oligomer PES 1 (2500 g/mol), (1.33 g, 4.0 mmol) of DFBP, an excess of K2CO3 (0.8 g, 6.0 mmol) and 25 mL of dimethylacetamide (DMAc) were introduced in a three-necked flask equipped with an argon inlet and a mechanical stirrer. The reaction
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mixture was heated at 80 C for 8 h and then poured into aqueous solution of HCl (1 M). The white polymer obtained was washed with water and dried under vacuum at 60 C. 1
H-NMR (CDCl3): d (ppm) 7.82 (d, 4H), 7.51 (d, 4H), 7.05– 6.98 (m, 8H). 19
F-NMR: (CDCl3): d (ppm) 2137.78 (m), 2152.82 (m).
Synthesis of Br-PES-FPES The bromination of PES-FPES was carried out in a solution of dichloromethane and acetic acid (10% v/v) by addition of a large excess of bromine (Br2) at room temperature (RT; Scheme 1). In a typical procedure, 10 g (12.2 mmol of BP unit) of PES-FPES 1 was introduced in a three-necked flask equipped with an argon inlet, refrigerant, and mechanical stirrer. Then 200 mL of distillated DCM, 20 mL of distillated acetic acid, and 6.5 mL (121.6 mmol) Br2 were added. After 24 h reaction, the reaction mixture was poured in methanol. The white polymer powder was dried at 60 C under vacuum. 1
H-NMR: (CDCl3): d (ppm) 7.97 (d, 4H), 7.89 (d, 4H), 7.84 (s, 2H), 7.50 (d, 2H), 7.16–7.1 (m, 10H) 19
F-NMR: (CDCl3): d (ppm) 2136.74 (d, ArAF), 2137.25 (m, ArAF), 2137.77 (m, ArAF), 2149.48 (t, ArAF), 2151.98 (d, ArAF), 2152.22 (m, ArAF), 2152.62 (m, ArAF), and 2160.18 (t, ArAF). Synthesis of I-psLi (ICF2CF2OCF2CF2SO3Li) In a typical procedure, 20 g (48 mmol) of 1,1,2,2,-tetrafluoro-2-(1,1,2,2,-tetrafluoro-2-iodoethoxy)ethane sulfonyl fluoride, 4.34 g (103 mmol) of LiOH.H2O and 60 mL of tetrahydrofuran were introduced in a round flask. After 24 h reaction, the reaction mixture was filtered to remove the excess of lithium hydroxide and the filtered solution was evaporated. The solid obtained was solubilized in acetonitrile and filtered to remove lithium fluoride. The filtered was evaporated and dried under vacuum at 60 C. 19
F –NMR (Acetone d6): d (ppm) 269.82 (t, ICF2), 282.87 (t, CF2O), 286.44 (t, OCF2), 2118.54 (m, CF2SO3-Li).
Synthesis of ps-PES-FPES The synthesis of ps-PES-FPES was carried out in anhydrous condition by addition of I-psLi and a large excess of copper powder (Scheme 1). In a typical procedure, 4.0 g (4.85 mmol of BP unit) of Br-PES-FPES 2, 3.09 g (48.6 mmol) of copper and 24 mL of distillated DMSO were introduced in a three necked flask equipped with an argon inlet, refrigerant, and mechanical stirrer. After 2 h reaction at 120 C, a solution of 6.27 g (14.6 mmol) of I-psLi in 20 mL of distillated DMSO (21% w/v) was added drop by drop, and the reaction temperature was slowly increased at 140 C. After 24 h reaction, the reaction mixture was poured into 1 M aqueous solution of HCl. The brown polymer was washed with water and dried under vacuum at 60 C. 1
H-NMR: (Acetone d6): d (ppm) 7.90–8.20 (m), 7.48 (d), 7.34–7.18 (m)
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F-NMR: (Acetone d6): d (ppm) 283.25 (s, ACF2O), 287.34 (s, AOCF2), 2111.60 (s, ArACF2), 2118.80 (s, CF2SO32Li1) 2139.15 (d, ArAF), 2139.40 (m, ArAF), 2139.70 (m, ArAF) 2152.38 (t, ArAF), 2154.70 (d, ArAF), 2155.13 (m, ArAF), 2162.90 (t, ArAF). 19
Synthesis of ps-PES Ionomer
The synthesis of the ps-PES ionomer was carried out in anhydrous condition using distillated DMSO as solvent. In a typical procedure 4.0 g (8.8 mmol of repeating unit) of brominated Radel polysulfone, 5.6 g (88.0 mmol) of copper, and 24 mL of distillated DMSO were introduced in a three necked flask equipped with an argon inlet, refrigerant, and mechanical stirrer. After 2 h reaction at 120 C, a solution of 5.7 g (13.2 mmol) of I-psLi in 20 mL of distilled DMSO (30% w/v) was added drop by drop and the reaction temperature was slowly increased at 140 C. After 24 h reaction, the reaction mixture was poured into 1 M aqueous solution of HCl. The brown polymer was washed with water and dried under vacuum at 60 C. 1
H-NMR: (Acetone d6): d (ppm) 8.24 (d), 8.11–8.02 (m), 7.94 (d), 7.47 (d), 7.34–7.21 (m)
F-NMR: (Acetone d6): d (ppm) 283.25 (s, ACF2O), 287.34 (s, AOCF2), 2111.60 (s, ArACF2), 2118.80 (s, CF2SO32Li1). 19
1
H NMR and 19F NMR Spectroscopies The PES-FPES, FPES, PES-DF, and Br-PES-FPES samples were dissolved in chloroform (CDCl3). The psLi, ps-PES, and psPES-FPES samples were analyzed using acetone (acetone D6) as solvent. A Bruker spectrometer WM 250 –frequency of 30.120 MHz for the protons and 282.39 for 19F- was used. Ionic Exchange Capacity (IEC) The functionality of the polymer is expressed by the mequivalent of proton per g of polymer (meq. H1/g). The IEC have been calculated by NMR and acid-base titration in organic solution. The functionality and the IEC of the random ionomer (ps-PES) were calculated by 19F-NMR using trifluoroethanol as internal standard. Acid Base Titration In a typical procedure, a solution of ps-PES-FPES in DGME (2.63 3 1022 M) was titrated with a solution of NaOH in DGME (2.625 3 1022 M) in presence of methyl orange. Size Exclusion Chromatography SEC analyses were performed at ambient temperature using a Waters 590 GPC equipped with a Waters 410 differential refractometer and a Waters 745 Data Module. The solvents THF and 1 M NaNO3 in DMF in the case of PES-FPES copolymers and ps-PES-FPES ionomers, respectively, were used as eluent with flow rate of 1 mL/min through three
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ultrastyragel columns of 500, 103, and 104 Å. The solutions of 1% of PES-FPES in THF and ps-PES-FPES in 1 M NaNO3 in DMF were filtered through a 0.45 mm Millipore filter based on PP and PTFE, respectively. The calibration was performed using polystyrene standards. The values of Mw and Mn depend on the elution volume and are markedly affected by possible fluctuations, for instance by the presence of bubbles. Therefore, each sample is analyzed twice and the values are calculated if the elution volumes of the two chromatograms are identical. Preparation of PESp, FPESp, and PES-FPES Membranes The polymers were dissolved in DMAc (10% g/mL). The solution was stirred for 24 h at 60 C. After, the solution was centrifuged at 5000 rotations/min during 30 min. The obtained solution was then degased under vacuum for 30 min to remove the air bubbles. The degased solution was put in a petri dish and kept in an oven at 60 C during 48 h. Then the membrane was dried under vacuum at 200 C until the casting solvent was completely removed. The free solvent membrane was verified by 1H-NMR. Preparation of ps-PES and ps-PES-FPES Membranes The ionomers were dissolved in dimethylsulfoxyde (DMSO, 7% g/mL). The solution was stirred for 24 h at 60 C. After the solution was centrifuged for 30 min at 5000 rotations/ min and then was degased under vacuum for 30 min to remove the air bubbles. The degased solution was removed in a petri dish and kept in an oven at 60 C during 48 h. Then the membranes with a thickness ranging between 120 and 150 mm were immersed in aqueous HCl solution (2 M) during 48 h in order to acidify the membrane and completely remove the casting solvent. The free solvent membrane was verified by 1H-NMR. R
Nafion117V Membranes Nafion117V membranes (thickness 176 mm) were kept in 1 M aqueous solution of HNO3 at boiling temperature for 2 h. Then the membranes were washed in deionized water for 48 h to eliminate all HNO3 traces. R
Water Uptake (WU) The membrane WU was determined at different temperatures ranging between 30 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 WU by the membrane and dried membrane was expressed as WU percentage. Proton Conductivity of Membranes The conductivity measurements were carried out by impedance spectroscopy using a Material Mates 7260 frequency response analyzer. The measurements were performed at (i) different temperatures (from 20 to 90 C) and 95% RH (ii) 80 C and different RH (from 30 to 95%) by using a climatic chamber V€ otsch 4018,. The spectra were recorded between 13 MHz and 5 Hz. The resistance of the membrane is taken
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at the high frequency intercept with the real axis in the Nyquist plot, which is usually between 106 and 104Hz. The climatic chamber (V€ otsch VC 4018) allows controlling the RH and the temperature. The RH was kept constant (95%), and the temperature was increased with a step of 10 C. The membrane is stabilized at measurement temperature during 12 h. DMA Mechanical analyses were determined using DMA Q 800 spectrometer instrument working in the tensile mode. The dimensions of samples were 15 3 6 mm2 with a thickness comprised between 120 and 150 mm. The samples were dried 24 h at 60 C under vacuum before characterization. The strain deformation was fixed at 0.05%, well below the limit of the linear viscoelastic regime. The measurement of elastic modulus E0 , dissipative modulus E00 and loss factor tan d were performed in isochronal condition (1 Hz) in the temperature range 20–250 C in the case of dried membrane and between 20 and 80 C with immersion clamps using a temperature ramp of 1 C min21. Thermal Gravimetrical Analysis The measurements were performed on a NETZSCH STA 409 PC/PG devise under air flow (mixture of 80% Ar and 20% O2) and a temperature ramp of 5 C/min from RT to 600 C. Differential Scanning Calorimetry The Modulated-DSC measurements were performed using a TA instruments’ DSC 2920-Modulated DSC. The measurements were carried out under argon atmosphere in two consecutive heating scans from 15 to 300 C with 5 C per minute of heating rate. The glass transition temperatures were measured on the second scan. Density Measurement The density of the membranes was measured by using METTLER TOLEDO’s density kits (using the buoyancy technique) at RT (21 C) with toluene (0.87 g/mL) as the solvent (in fact, a very poor solvent has to be chosen for this measurement). Small Angle Neutron Scattering (SANS) SANS measurements were carried out on the D22 spectrometer at the Institut Laue Langevin, Grenoble, France). The spectrometer was configured to obtain an extended Q-range spanning from 8.5 1023 to 0.6 Å21, Q being the scattering vector defined as q 5 (4p/k)sin(h/2) where k is the wavelength of the incident neutron beam and h is the total scattering angle. The 2D patterns were isotropic and thus radially averaged to extract the 1D scattered intensities I(Q). The SANS I(Q) spectra were corrected for detector efficiency, background and empty cell subtraction. Absolute intensities were obtained by measuring a water-sample for calibration (1 mm thick in Helma cell). The membranes were prepared in quartz Helma cells prior to the SANS experiment, closed quickly and maintained at RT. Atomic Force Microscopy (AFM) The morphology of membranes was further characterized by AFM. After being soaked with a drop of water, the sample
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TABLE 1 Theoretical Mn Values of PES, Mn of FPES Calculated from Units of PES, and Those of FPES Blocks Polymer code
MnPEStha (g/mol)
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F NMR, the Ratio, R, Between the Number of Repeating
MnFPESexpb (g/mol)
Rtha (mol/mol)
Rexp (mol/mol)
PES-FPES 1
2500
5000 (6500)
0.67
0.60 (60.03)
PES-FPES 2
5000
5060 (6300)
1.40
1.31 (60.06)
PES-FPES 3
7500
5100 (6600)
2.13
2.05 (60.10)
a
Theoretical value.
surfaces were immediately imaged at RT and 40–60% RH using the nanoscope 4A Quadrex multimode (Veeco). Experiments were performed in the tapping mode using Si probes with 5 N m21 spring constant and resonance frequencies in the range 60–80 kHz. Phase and height images were recorded simultaneously. The images were acquired in light tapping conditions: the interaction of the tip with the specimen was adjusted by the ratio of the set point (Asp) to the free oscillation amplitude (A0), to obtain a ratio Asp/A0 equal to 0.9. The phase images in these operating conditions are known to be the most to the morphology with this kind of samples. The image processing was conducted in “nanoscope analysis” from Veeco, while “ImageJ” open-source software34 was employed for the image analysis. The ionomers’ surfaces showed morphologies with distinct phases. The ionic domains and hydrophobic backbones were identified following the procedure developed by James et al.35 Post treatment of the phase images permitted to characterize the different domains. After fixing the phase scale between 220 and 20 , a tailored procedure, which automatically iterates a binarization, a watershed segmentation, and a particle analysis, furnished the proportion of surface of ionic domains. Five images for each polymer were analysed. As the polymer surface response looks like modulated at the submicrometer scale whatever the lateral scan direction, this modulation was studied more quantitatively by applying the azimuthal averaged on the square 2D-Fourier transform of the phase image. For this purpose, a mathematical Power Spectral Density function from Nanoscope Analysis software was used and from the graph power versus inverse of wavelength, it was possible to extract a correlation length, characteristic of the average distance between hydrophobic domains. RESULTS AND DISCUSSION
The ionomers synthesis is performed in 3 steps (Scheme 1), that is, (i) synthesis of multiblock copolysulfone (PES-FPES), (ii) selective bromination of PES-FPES copolymer, and (iii) grafting of alkyl sulfonic acid groups by substitution of the Br atoms (ps-PES-FPES). The structure of PES-FPES block copolymer is carefully tailored in order to ensure a selective functionalization of blocks. The presence of high electron withdrawing groups such as F and SO2 in the FPES blocks deactivates the reactivity of aromatic cycles through bromination reaction and
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b Determinate by NMR; Rexp determinate by NMR (integral of 200 /integral of 600 ).
hence the alkyl perfluorinated acid can be grafted only on PES blocks.
Multiblock Copolymer Synthesis (PES-FPES) The backbones of multiblock copolymers are prepared in one pot two reactions synthesis. In a first reaction the PES oligomers ended by hydroxyl biphenyl units were performed. The lengths of PES blocks are modulated by controlling the molar ratio between BP and DHDPS monomers; hence, oligomers with target molecular weight of 2500, 5000, and 7500 g/mol are prepared (Table 1). Further, the copolymer PES-FPES is synthesized by adding in one pot, without PES oligomers purification, the monomers composing the FPES blocks, that is, DHDPS and DF, respectively. The target of molecular weight of FPES block is 5000 g/mol and the molar ratio between the blocks PES, FPES is calculated to be equal to 1. The DF monomer is highly reactive toward the nucleophilic aromatic substitution and high aromatic polymers form at relatively low temperature and short period time, therefore for the second reaction the temperature is decreased from 120 to 70 C. Indeed, during the second reaction (synthesis of FPES blocks and PES-FPES copolymer), two nucleophiles are present in the reaction media, that is, the phenolats derived from (i) monomer, DHDPS, and (ii) hydroxyl biphenyl ended PES oligomers. Thus, the two F from para position of DF monomer can be substituted (i) either by two PES oligomers (PES-DF -see experimental part) (ii) or by two DHDPS monomers (FPES oligomers) or (iii) by both nucleophiles (PES-FPES). However, the concentration, the reactivity, the solubility in DMSO and the mobility of phenolats derived from DHSPS monomer are higher; hence it might be expected that first the FPES oligomers are formed with a certain polydispersity and then the FPES oligomers are linked to PES oligomers. The chemical structure of copolymer is analyzed by 1H and 19 F NMR. In Figure 1 the 1H NMR of copolymer PES-FPES 2 is compared with those of FPES and PES oligomers. The presence in the 1H NMR spectrum of PES-FPES 2 of the peaks characteristics to both oligomers, that is, PES and FPES, indicates, at least qualitatively, the formation, during the second synthesis reaction, of FPES sequence. The diphenylsulfone appears in the repeating units of both blocks, the ratio, R, between the number of repeating units of PES and those of FPES was calculated by using the ratio between the integration of biphenylsulfone from both blocks. The
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FIGURE 1 1H NMR spectra of: (a) oligomer PES 2 (5000 g/mol); (b) oligomer FPES; (c) PES-FPES 2 (Solvent CDCl3).
obtained values appear to be very close to the expected ones (Table 1). The copolymer structure is analyzed by 19F NMR as well, the spectrum of PES-FPES 2 being represented in Figure 2(c) and compared with those of FPES oligomer [Fig. 2(a)] and PES-DF polymer [Fig. 2(b)]. The 19F NMR spectra of FPES oligomer shows two intense peaks at 136.8 and 151.9 ppm corresponding to aromatic fluorine on the chain, and 3 small peaks at 160.1, 149.5, and 137.2 ppm assigned to the aromatic fluorine on the end of chain. The 19F NMR spectrum of PES-DF shows two aromatic fluorine signals at 137.7 and 152.8 ppm. As shown on the spectrum of PES-FPES 2, the peaks corresponding to FPES oligomers are observed, the peaks assigned to the aromatic fluorine on the chain being very intense while those corresponding to the fluorinated biphenyl located at the chain ends are very weak. Additionally to these, new peaks are depicted at 152.2, 152.6, and 137.8 ppm. These peaks are attributed to the fluorines of octafluorobiphenyls connecting the PES, FPES blocks, or two PES (Fig. 2). However, the chemical shift of peaks assigned to the aromatic fluorine in ether ortho position (F100 and
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F400 ) are slightly shifted as compared with those of PES-DF (152.2 and 152.6 instead of 152.8) indicating that the electronically environments of fluorine situated on 100 , 400 , and 10 positions are different. Hence, it can be assumed that the majority of DF that joints the blocks is bonded on a site to a PES block and on the other site to DHDPS (FPES blocks). The peaks assigned to F on 10 position (PES-DF) are not detectable in 19F NMR spectra of PES-FPES and accordingly it might be assumed that there is no formation of PES-DF or it forms in very low quantity. The two kinds of fluorine atoms on the chain (600 ) and at the end of FPES blocks (100 ) of PES-FPES have a molar ration of 4n/2, where n is the number of repeating units. Using this equation and 19F NMR integration a n close to 7.5 is calculated. The n multiplied by the molecular weight of repeat unit and with the addition of molecular weight of end groups, a Mn close to 5060 g/mol for FPES blocks from PES-FPES 2 is obtained, in good agreement with the theoretical value. The reaction of FPES segments with preformed PES oligomers proceeded rapidly, as evidenced by sharp increase in viscosity of reaction mixture in the first hour and by the high
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FIGURE 2
19
F NMR spectra of (a) FPES; (b) PES-DF; (c) PES-FPES 2. (Solvent CDCl3).
molecular weight (Mw > 130 kDa; Table 2) measured by SEC in THF solvent. Moreover, the presence in SEC curve of only one peak with polydispersity lower than 2.9 excludes the formation of oligomers and sustains the formation of multiblock copolymers. Highly white fibrous materials were obtained by precipitation and tough films were resulted by solution casting. Thermal Properties The glass transition temperature (Tg) and degradation temperature (Td) of polymers and copolymers were, respectively, determined by DSC and TGA. The results of DSC analyses are presented in Table 3. The Tg of PESp is 10 C higher than that of FPESp. Interestingly, the DSC thermogram of an intimate mixture of PESp and FPESp, obtained after the evaporation of solvent from a solution of 10% of 50/50 w/w PESp/FPESp in chloroform, shows two glass transitions, one corresponding to PES and the other one to FPES (Table 3).
This result proves that the two polysulfones, despite similar structures, are not miscible and during the solvent evaporation, PES–FPES phase separation takes place. Regarding the DSC thermograms, a single glass transition is observed for the three copolymers. The Tg being higher than that of FPESp and increases as the PES lengths increase. The presence of only one Tg can be explained by the fact that the lengths of two blocks of copolymers PES-FPES are not enough large to allow a phase separation to induce a more or less ordered nanostructure.36–38 As for the degradation temperature of copolymers, it seems to be independent of the block length. The values obtained are close to FPESp (Table 2). Ionomer Synthesis (ps-PES-FPES) The bromination reaction was performed under inert atmosphere using bromine as reactive in the presence of acetic acid (10% v/v). The bromination degree, determined from 1H
TABLE 2 Molecular Weights of PES-FPES Block copolymer
PES-FPES 1a
PES-FPES 2a
PES-FPES 3a
ps-PES-FPES 1b
ps-PES-FPES 2b
ps-PES-FPES 4b
Mn (kDa)
90
70
130
143
116
263
Mw (kDa)
260
160
310
479
278
626
Mw/Mn (kDa)
2.9
2.4
2.4
3.3
2.3
2.4
a
8
SEC performed in THF, solution filtered with filter based on PP.
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b SEC performed in 1M NaNO3 in DMF, solution filtered with filter based on PTFE.
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TABLE 3 Glass Transition Temperature (Tg) and Degradation Temperature (Td) of the Various Polymers Tg ( C) (DSC)
Polymer code b
Td ( C)a (ATG)
PESp
220 6 2
530 6 2
FPESpc
210 6 2
590 6 2
FPESp 1 PESp
212 6 2; 219 6 2
–
19
F NMR, the IECs are calculated with a good accuracy (Table 4), using the eqs 1 and 2. IECRMN 5
sdRMN XnPES 1000 ðMnps-PES 1 MnF-PES Þ
(1)
2 I1 ½ðI6 Þ Rexp
(2)
sdRMN 5
PES-FPES 1
215 6 2
532 6 2
PES-FPES 2
220 6 2
531 6 2
sdRMN number of ionic function per PES repeat unit
PES-FPES 3
222 6 2
529 6 2
I10 5 integral peak of F1, (2111.6 ppm).
a b c
The weight loss of 5%. Commercial PES (RADEL). FPES synthesized polymer with molecular weight 40,000 g/mol.
I60 5 integral peak of fluorine atoms of the FPES block [F60 (2139.1 ppm) or F50 (2154.7 ppm)] 1 integral peak of fluorine atoms at the end of the FPES block.
NMR analysis and elementary analysis indicates bromination yield higher than 90%, each repeating unit has at least 1.8 bromine atoms. The grafting of perfluoalkyl sulfonic groups was performed through a copper mediated coupling reaction involving bromine and the iodoperfluoroalkylsulfonate compound.18,20,23,26 No gelation was observed that means the coupling reaction between two PES bromines did not take place as expected. The molecular weights increase as compared with corresponding PES-FPES in coherence with the grafting of alkyl perfuorosulfonic function (Table 2). During the coupling reaction, the peak at 269.82 ppm assessed to I-CF22 decreases in intensity, and a new peak at 2111.6 emerged and increase in intensity, the last being assessed to the fluorine group of ArACF2 (Ar-aromatic ring from PES; Fig. 3).
The values of IEC determined by NMR are similar to those obtained by titration. The random ionomer (ps-PES) was obtained by the grafting of perfluoalkyl sulfonic groups on the commercial polymer (RADEL), PESp. The IEC was determined by using trifluoroethanol (CF3CH2OH) as internal standard.
The grafting degree of perfuoroalkyl sulfonate groups expressed as the IEC of ps-PES-FPES was determined by 19F NMR spectroscopy and by acid base titration. Based on the
To compare the conductivity of membranes derived from different ionomers, which have different densities, the values of IECv expressed in meq. H1/cm3 were preferred to the IEC
FIGURE 3
XnPES: average repeating unit number in PES blocks. MnFPES: molar mass of FPES block. : molar mass of ps-PES blockMnps-PES 5 (MPES 1 sd*Mps)*XnPES (where MPES: molar mass of structural unit of PES and Mps: molar mass of side chain).
19
F NMR spectra of (a) iodo perfluoroalkyl sulfonic acid; (b) ps-PES-FPES 2. (Solvent: Acetone D6).
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R
TABLE 4 IEC of ps-PES-FPES, ps-PES, and Nafion 117V Determined from NMR Spectra and by Titration Method IECexp (meq H1/g)
Polymer
IECtha (meq. H1/g)
ps-PES-FPES 1 ps-PES-FPES 2
IECaverage (meq. H1/g)
u (g/cm3)
IECv (meq.H1/cm3)
0.90 6 0.05
0.9 6 0.1
1.60
1.5 6 0.1
1.20 6 0.05
1.2 6 0.1
1.61
1.9 6 0.1
1.5 6 0.2
1.6 6 0.1
1.6 6 0.2
1.61
2.5 6 0.2
1.2
1.2b 6 0.1
1.30 6 0.04
1.3 6 0.1
1.50
1.95 6 0.1
0.90
0.92
0.9
0.9
2.04
1.84
NMR
Titr.
1.1
1.0 6 0.1
1.4
1.2 6 0.1
ps-PES-FPES 3
1.6
ps-PES Nafion
a It was calculated considering two ionic function/repeat unit of PES block.
b
generally expressed in meq. H1/g. The IECv of PES-FPES 2, Nafion 117, and ps-PES are close (Table 4).
of E0 and tand versus temperature are reported for the statistic ionomer, ps-PES, ps-PES-FPES 1 PESp, and FPESp.
Thermal and Thermomechanical Analysis of ps-PES-FPES The grafting of the ionic function on the aromatic block chains induces a significant decrease on the thermal stability (Table 5) as compared with PES-FPES copolymers. A weight loss of 5% is observed for ionomer ps-PES-FPES 1 at 250 C. The Td decreases slightly with the increase of IEC (Table 5). A slight weight loss is observed between 100 and 200 C ( 50% the conductivities of ps-PESFPES 3 are higher than those of Nafion 117.
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The authors are indebted to the Institut Laue Langevin for beam time allocation. One of us (RS) acknowledges the Minister for Higher Education and Research for PhD grant. This work was performed within the framework of the Centre of Excellence of Multifunctional Architectured Materials “CEMAM” n AN-10-LABX-44-01.
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