Perfluoro-p-xylene as a New Unique Monomer for Highly Stable Arylene Main-chain Ionomers Applicable to Low-t and High-t Fuel Cell Membranes

In this study, we present the synthesis and the characterization of novel functionalized arylene main-chain ionomers based on perfluoro-p-xylene (PFX). The polymers were prepared by polycondensation of PFX and 4,4'-dihydroxybiphenyl or bisphenol 2,2-bis(4-hydroxyphenyl)-hexafluoropropane (bisphenol AF). After polymerization, the PFX unit was still able to undergo nucleophilic aromatic substitution reaction, which was used to introduce phosphonic acid groups into the polymer via a reaction with tris(trimethylsilyl)phosphite. Furthermore, electrophilic sulfonation of these polymers was possible in the bisphenol unit when using H2SO4/SO3 as the sulfonation agent. The so-obtained water-soluble PFX-based polyelectrolytes showed excellent chemical stability and were blended with polybenzimidazoles. The blend membranes formed flexible and mechanically robust films with excellent chemical and thermal stabilities.


Introduction
In recent years, the application of polymer electrolyte membrane fuel cells (PEMFCs) and direct methanol fuel cells (DMFCs) as power sources in automotive and portable electronic applications has attracted increasing attention in the scientific community [1].Many different approaches have been investigated to substitute proton-conductive membrane materials based on perfluorinated sulfonic acids (PFSA), such as Nafion ® [1].While the PFSAs show excellent chemical, mechanical and thermal stability coupled with high proton conductivity, these materials unfortunately exhibit a high methanol permeability, which is a severe shortcoming for application in DMFCs.Additionally, a significant drop in proton conductivity occurs at temperatures above 80 °C, because of an increased drying-out of the PFSAs.
Several proton-conducting membrane materials have been investigated with different functionalized polymer families, e.g., sulfonated polysiloxanes [2], polyphosphazenes [3,4], styrene-grafted (partially) fluorinated polyolefins [5] and a significant variety of different aromatic main-chain polymers.When searching for new polymer materials for proton-conductive membranes, it was found that the perfluorinated xylene could serve as a promising building block for partially-fluorinated arylene ionomers.In principle, fluorinated ionomers are expected to have superior stability and increased acid strength, compared to their non-fluorinated analogues [6,7].The focus of this study is the synthesis and characterization of a new partially-fluorinated and, therefore, electron-deficient arylene main-chain polymer prepared by a polycondensation reaction between perfluoro-p-xylene and bisphenols, like 2,2-bis(4-hydroxyphenyl)-hexafluoropropane (bisphenol AF) or 4,4'-dihydroxybiphenyl.Due to the strongly electron-attracting C-F and CF3 groups, the perfluoroxylene building group is capable of undergoing nucleophilic substitution of the F. The reaction between poly(arylene ether perfluoroxylene) and tris(trimethylsilyl)phosphite followed by hydrolysis yielded a phosphonated polyelectrolyte.Moreover, the introduction of benzimidazole groups by nucleophilic substitution reaction was possible with 1-mercaptobenzimidazole (1H-benzo[d]imidazole-2-thiol) via the mercapto group.Additionally, the poly(aryl ether perfluoroxylene) backbone allowed electrophilic substitutions when using SO3/H2SO4 as a sulfonation agent, introducing the SO3H-group into the bisphenol portion of the aromatic polymer.The sulfonated and phosphonated main chain ionomers can subsequently be blended with base group-containing polymers, such as polybenzimidazoles, to obtain acid-or base-excess blend membranes.
Many types of ionically cross-linked acid-excess [8,9] and base-excess acid-base blend membranes and covalently cross-linked (blend) membranes, based on arylene main-chain ionomers, having high mechanical, chemical stability and low methanol permeability have been developed by the authors' group during the last decade.
Since the acid-base blend membranes showed increased thermal and mechanical stability, compared to the pure polymers, which is due to the ionic cross-linking between the acidic and basic functional groups, acid-excess and base-excess blend membranes have been prepared from the novel perfluoroxylene polymers presented in this contribution.

Blend Membrane Preparation
The sulfonated pPFX-DFB in Na + -form was dissolved in DMSO to give a 40% solution.A 4 wt% solution of hexafluoroisopropylidene polybenzimidazole (F6-PBI) in DMSO was prepared and added to the sulfonated pPFX-DFB DMSO solution.The final ratio of sulfonated poly(arylene ether perfluoroxylene)/F6-PBI was adjusted to 30/70 (w/w).The mixed solution was poured onto glass plates and dried at T = 130 °C for 12 h.Subsequently, deionized water was spread on the polymer films to remove the membrane from the glass surface.To induce the formation of the ionic cross-linking in the membrane, the blend membrane was immersed in 10% HCl at 90 °C for 48 h followed by rinsing in water at 90 °C for 24 h.

Instrumentation
NMR spectra were recorded on a Bruker Avance 400 spectrometer at a resonance frequency of 250 MHz for 1 H, 62.9 MHz for 13 C, 235 MHz for 19 F and 250 Hz for 1 H, 13 C-HSQC (heteronuclear single-quantum coherence) at 303 K.Chemical shifts were referenced to external TMS ( 1 H, 13 C).Coupling constants are given as absolute values.FTIR (Fourier transform infrared spectroscopy) spectra were obtained from samples as a KBr pellet on a Nicolet 6700 FTIR instrument (Thermo Fisher Scientific Inc., Waltham, MA, USA).The molecular weight distributions of the polymers and ionomers (Mn, PDI) were determined by gel permeation chromatography (GPC) using an Agilent Technology GPC system (Series 1200) (Agilent Technologies, Inc., Santa Clara, CA, USA) coupled with a viscosity detector (PSS ETA-2010 (Polymer Standards Service, Mainz, Germany)), a refractive index detector (Shodex RI71) (Showa Denko, Tokyo, Japan) and a static light-scattering detector (PSS SLD 7000).A set of three PSS GRAM columns (30, 3000, 3000 A) were used and calibrated with a series of polystyrene standards in N,N-dimethylacetamide containing 5 wt% LiBr.All of the samples were filtered through a Whatman syringe filter over a microporous polytetrafluoroethylene (PTFE) membrane (1.0 μm, Whatman 6878-2510 (Whatman plc, Maidstone, UK)) before injecting into the column system.The thermal stability of the polymers and membranes was determined by TGA (Netzsch GmbH & Co. KG, Selb, Germany), Model STA 449C) with a heating rate of 20 °C/min under an atmosphere enriched with oxygen (65%-70% O2, 35%-30% N2).Non-sulfonated polymers are compared by the temperature at which the sample has lost 5% of its initial weight (T5wt% loss).In the case of the sulfonated polymers the decomposition gases were further examined in a coupled FTIR spectrometer (Nicolet Nexus FTIR spectrometer) in order to identify the splitting-off temperature of the sulfonic acid group (TSO3H onset) for which the asymmetric stretching vibration of the S=O group at 1352-1342 cm −1 was used.Ion-exchange capacities (IECdirect and IECtotal) were determined by titration.Membranes in the H + form were immersed in saturated sodium chloride solution (NaCl) for 24 h to convert them into the Na + form.The exchanged H + ions were then titrated with 0.1 M NaOH to the equivalent point (IECdirect).After that, a defined excess of NaOH was added, and this solution was back-titrated with 0.1 M HCl (IECtotal).For Fenton's Test, a solution of 3 wt% H2O2 containing 4 ppm Fe 2+ was used.Fe 2+ was added as (NH4)2Fe(SO4)2•6H2O in order to accelerate the radical production.Membrane samples were immersed in Fenton's Solution at 68 °C.After a certain period of time, the membrane samples were removed from the oxidizing solution, washed with water, dried at 120 °C for 2 h and weighed.For successive measurements, fresh Fenton's Solutions were prepared and preheated every 24 h, in which the dried membrane samples were immersed continuously.
The H + conductivity in the through plane was determined using the Membrane Test System (MTS 740) from Scribner Associates Inc. (Southern Pines, NC, USA) temperature and humidity sensors in the measuring cell ensure the appropriate temperature and relative humidity in the membrane.A detailed description and technical data are given elsewhere [10].Gas-diffusion electrodes (GDE) (E-TEK ELAT GDE 140-HT) were glued with conductive carbon paint to the platinum electrode sandwiched with the membrane 3 cm × 1 cm in the middle.Porous gas diffusion layers (GDLs) served to facilitate gas-phase diffusion of water vapor to and from the membrane.For sufficient contact between the membrane, GDL and electrodes, the stack was kept under 1.4 MPa.A typical test procedure at relative humidity of 20% and T = 80-150 °C, respectively, with the pre-condition of the sample at 80 °C for 30 min, was followed by increasing the temperature with steps of 10 °C conditioning for 15 min followed by five electrochemical impedance (EIS) measurements.The specific conductivity was obtained by σ = l/(A × R), where l is the membrane thickness, A the overlapping area of the electrodes and R the resistance derived from the high-frequency intercept of the complex impedance with the real axis.   1 H NMR measurements showed a down-field shift in the aromatic region with two doublets at 7.02 and 7.50 ppm for 1 and at 7.12 and 7.35 ppm for 2. Depending on the substituent in the polymer main chain, the 13 C-NMR spectra showed chemical shifts at 115.6, 128.7, 136.5 and 157.6 ppm for 1.Similar chemical shifts were obtained at 63.9, 115.6, 121.8, 128, 132.3 and 157.8 ppm for 2. The related chemical shifts were observed for the perfluoroxylene group independent of the substituent.

Polycondensation and Functionalization
The heat resistance of both polymers 1 and 2 was measured using thermogravimetric analysis (TGA) coupled with an FTIR sensor for analysis of the gaseous decomposition products forming during the TGA experiment (Figure 2).The polymers showed high thermal stability of Tdecomposition = 509 °C, indicated by the appearance of CO bands (between 2241 and 1991 cm −1 ) in the FTIR spectrum of 1 and Tdecomposition = 492 °C for 2. The high thermal stability makes the polymers suitable for high temperature polymer electrolyte membrane application.The weight loss at the TGA profile of 1 in the temperature range of 100-200 °C was attributed to water and residual solvent evaporation.The introduction of a sulfonic acid function of the biphenol and the bisphenol AF units of the pPFX was achieved via sulfonation with oleum at 120 °C for three and five hours respectively (see 3 and 4 in Scheme 2).The sulfonic acid groups served as the proton conductor in the acid-excess acid-base blend membranes or as an ionic cross-linker in the base-excess acid-base blend membranes.Scheme 2. Sulfonation products of poly(arylene ether perfluoroxylene) (pPFX) 3 and 4.
The structure of the sulfonated polymers was determined by 1 H and 13 C NMR spectroscopy.The disappearance of the symmetrical doublets, which are typical for aromatic protons of non-substituted bisphenols at 7.02 and 7.50 ppm and for biphenyl rings at 7.15 and 7.32 ppm, and the appearance of three different peaks in the 1 H NMR spectra confirmed the substitution.Signals with chemical shifts at 8.05, 8.33 and 8.46 ppm for 3 and at 7.15, 7.21 and 7.93 ppm for 4 were observed.In the case of 4, the 1 H-NMR chemical shifts of the protons adjust for the ortho sulfonation in the bisphenol AF unit showing similar values as has been presented in previous work [12].The integration of the peaks of 4 was used to determine the number of protons.The 1 H-NMR signals are too broad to show the typical pattern for the one fold-substituted aromatic ring with two doublets and one singlet peak.Due to the weak signals despite the high concentration, the exact position of the -SO3H groups could not be confirmed by heteronuclear single-quantum correlation measurements.According to the literature, it is expected that the SO3H group is introduced in the ortho position next to the ether-group [12,13].
The sulfonation degree was confirmed by 1 H-NMR spectroscopy (see Figures S1 and S2 in Supplementary Materials).The integral ratio between the signals of the sulfonated and non-sulfonated units at the same position can be used to determine the sulfonation degree [12,13].PFX is incompletely two-fold sulfonated, and all three degrees of substitution of the biphenyl unit can coexist in the polymer backbone (non-sulfonated, one-fold sulfonated and two-fold sulfonated).In the case of bisphenol AF, a sulfonation degree of 80% with a 20% non-sulfonated repeating unit was obtained.In the case of biphenol, the sulfonation degree of 50% was lower than in case of bisphenol AF.The sulfonation of the non-fluorinated aromatic rings was confirmed by 13 C-NMR as the appearance of new signals with chemical shifts at 132.4 and 136.2 ppm for 3 and 4, respectively.Further evidence for sulfonation of pPFX was detected by FTIR measurement.Both of the sulfonated polymers showed absorption bands for the sulfone group found at 1334, 1151 and 1090 cm −1 for 3 and 1334, 1241, 1175 and 1088 cm −1 for 4 (see Figure S3 in Supplementary Materials).The IEC values reflected the partial sulfonation of the poly(arylene ether perfluoroxylene) polymer.The theoretical IEC was not achieved either in the case of 3 or 4 (see Table 1).According to Figure 2, the thermal stability of the sulfonated products 3 and 4 is lower than that of the non-sulfonated polymers.A similar observation has been made for other arylene main chain polymers [12].TGA-FTIR coupling experiments showed clearly that the sulfonated polymers split off -SO3H starting at 317 °C for 3 and 241 °C for 4. The thermal stability of 4 is almost 80 °C lower than that of 3, although 4 has higher fluorine content than 3, as already shown in the literature, that partially fluorinated aromatic polymers are more stable than their non-fluorinated analogues [7].On the other hand, it has been found that the polymer stability can decrease if the aromatic system is too electron-deficient, as in the case for highly fluorinated oxadiazole polymers synthesized recently in the authors' group [12].
The GPC measurements showed a decrease in molecular weights (30% and 46% for 3 and 4, respectively) after the sulfonation of 1 and 2. The degradation is common in view of the harsh reaction conditions required for the substitution reaction.

Phosphonation of Poly(arylene ether perfluoroxylene)
The nucleophilic substitution of a fluorine atom in perfluorinated aromatics is a well-established reaction in the literature [14][15][16][17].Due to the presence of residual C-F groups in the PFX unit, -PO3H2 functions can be introduced via a nucleophilic substitution reaction using tris(trimethylsilyl)phosphite (P(OSiMe3)3, TMSP).This nucleophilic phosphonation reaction has been recently used to prepare poly(2,3,5,6-tetrafluorostyrene phosphonic acid) from pentafluorostyrene in our group [17].To obtain the desired phosphonated polymer, pPFX 1 was reacted with TMSP at 170 °C for 24 h (see Scheme 3).Disappearance of the fluorine signals of the C-F group at −57.7 ppm in the 19 F NMR spectra (Figure 3) and appearance of the phosphorous signal 2.3 ppm in the 31 P NMR spectra (Figure 4) confirmed the formation of the phosphonated polymer.Three resonance signals corresponding to the CF3-groups of one-fold and non-substituted PFX were found in the 19 F NMR spectrum of 5.The non-substituted PFX corresponds to the resonances at −56.2 ppm.The one-fold substituted PFX results in two resonances at −54.7 and −52.9 ppm.From the integral ratio between the non-and the one-fold substituted PFX, a phosphonation degree of about 95% was found.
A condensation reaction between the -PO3H2 groups, which has been shown previously for phosphonated polymers [18], was confirmed in the case of 5 by the 31 P-NMR.
In the 31 P-NMR spectrum of 5, two different phosphorus peaks were found at 2.27 ppm with a coupling constant of 4 JPF = 4.8 Hz and at 6.33 ppm with a coupling constant of 2 JPP = 6.9 Hz, respectively (Figure 4).The reason might be a condensation reaction between the two -PO3H2 groups during the hydrolysis reaction step [18,19].
The integral ratio between the free phosphonic acid functions and their anhydrides was resolved by 31 P-NMR spectroscopy, yielding a condensation degree of the phosphonic acid groups of about 10%.The occurrences of condensed P-O-P groups decrease the attendance of the free -PO3H2 groups, which leads to lower ion exchange capacity.
The IEC value at 100% phosphonation (two protons) was calculated to be 3.36 mmol/g.However, the measured IEC was 0.67 mmol/g for IECdirect and 2.13 mmol/g for IECtotal.This is in agreement with the co-existence of the non-, one-fold-phosphonated and condensed phosphonic acid groups in the polymer.
Additionally, the structure of the phosphonated polymer 5 was confirmed by 1 H-and 13 C-NMR measurements.As expected, the 1 H-NMR spectra showed two doublets for the protons in the aromatic rings of the bisphenol unit.The 13 C-NMR spectra showed clear peaks representing the carbon atoms in the dihydroxy-biphenyl unit.A characteristic peak for the attendance of the C-P bond was delivered by the 13 C-NMR with the chemical shift of 127.21 ppm and the coupling constant of 1 JCP = 52.9Hz.The phosphonation of the product was confirmed by FTIR measurement (see Figure S4 in Supplementary Materials).The -PO3H2 group is represented by several vibration bands, arising in the region 1244-1239, 1008-998, 924-903, 827-823 and 496-430 cm −1 [20].The bands at 1243 and 1239 cm −1 are mainly due to the P=O stretching vibrations (P=O), whereas the 919 and 818 cm −1 bands are assigned to the P-O stretching mode (P-O).
The thermal stability of the phosphonated poly(arylene ether perfluoroxylene) is more than 100 °C lower than the non-phosphonated one.According to the TGA FTIR coupling measurements, the dephosphonation of the substituted polymer was observed at 308 °C.This decomposition temperature was confirmed by the appearance of the stretching vibration of the -PO3 group at 996 cm −1 .

Functionalization of pPFX with Mercaptobenzimidazole
In order to introduce basic functionality, the pPFX was reacted with mercaptobenzimidazole (1H-benzo[d]imidazole-2-thiol).The mercaptobenzimidazole was reacted at T = 130 °C for four hours with potassium carbonate in order to deprotonate the S-H group, followed by the SN reaction of the thiolate with the C-F bond of the perfluorinated aromatic ring (see Scheme 4).
The structure of the thiolated polymer 6 was determined by means of 1 H-, 13 C-NMR and 19 F-NMR.Direct evidence for the substitution could be found by the 19 F-NMR assignments (see Figure 5).The disappearance of the C-F peak at −128.6 ppm in the perfluorinated aromatic unit indicates the completeness of the substitution reaction.In the 19 F-NMR spectra, the typical peaks were found at −53.5 and −54.9 ppm for the one-fold-substituted PFX and a peak for the non-thiolated perfluoroxylene at −56.6 ppm.Integration of the fluorine signals revealed a ratio between one-fold: non-substituted = 1.8:1.1 (62% substitution).
The 1 H-NMR spectra show a complex multiplet spin system in the aromatic region between 6.75 and 7.35 ppm.The multiplet may contain the signals of both the dihydroxy-biphenyl unit and the introduced mercaptobenzimidazole.The doublets for the four protons of the biphenyl group were detected at 6.84 and 7.42 ppm with a coupling constant of 3 JHH = 8.1 Hz.The four protons of the aromatic rings in the substituted mercaptobenzimidazole unit have shown a typical four-spin-system with an AA'BB' pattern at 7.11 ppm.Heteronuclear single-quantum correlation (HSQC) measurements clearly showed four cross-peaks at 6.74 and 7.35 ppm for the coupling between C-H groups in the dihydroxy unit and at 7.01 for the coupling between the carbon and hydrogen atoms in the aromatic ring of the mercaptobenzimidazole side unit (see Figure 6).
Although a high concentration of the measured sample in the deuterated solvent was used, carbon peaks in the 13 C-NMR spectra were only observed for the C-H bonds in the biphenyl unit at 156.63, 131.65, 127.44 and 116.0 ppm.In the FTIR spectra (see Figure S5 in Supplementary Materials), the band at 3239 cm −1 is characteristic for the C-N stretching.The C-S group shows several vibration bands between 824 and 600 cm −1 , while the bands at 740 and 824 cm −1 were attributed to the S group.Supplementary bands were observed at 1628, 1571 and 1493 cm −1 and can be assigned to the aromatic stretching vibration.To obtain an acid-excess acid-base blend membrane with good thermal and mechanical properties, a sulfonated random copolymer having a high C-F ratio in the polymer backbone was synthesized by polycondensation of DHB with both PFX and decafluorobiphenyl (DFBP).By the increase of the fluorine content in the polymer main chain, a better thermal and mechanical stability for the polymer compound was expected.The polycondensation reaction was conducted at 90 °C for 30 min (see Scheme 5).Scheme 5. Reaction scheme of the polycondensation reaction for the preparation of the statistical copolymer poly(aryleneetherperfluoroxylene-co-decafluorobiphenyl) (pPFX-DBF) 7.
Due to the higher chemical reactivity of the DFBP, compared to the PFX, a molar ratio of PFX: DFBP = 60:40 was used in the reaction mixture to obtain a molar ratio of 50:50 in the polymer backbone.The obtained poly(arylene ether perfluoroxylene) (pPFX-DFB) 7 was characterized using 1 H-, 13   The 1 H-NMR measurement showed the typical patterns for the aromatic protons of the dihydroxy-biphenyl ring at 7.33 ppm and 7.71 ppm with a coupling constant of 3 JHH = 7.33 Hz.
The thermal stability measured by TGA revealed the highest value among the prepared pPFX polymers, showing a value of Tdecomposition = 525 °C (see Figure 8).

Sulfonation of pPFX-DFB with Fuming Sulfuric Acid
For the application in the blend membrane as an acidic compound, the pPFX-DFB polymer 7 was sulfonated.The introduction of the -SO3H group into the polymer backbone was realized by using H2SO4 with a SO3 content of 60% at RT for 24 h (see Scheme 6).The -SO3H group acts as proton conductor in acidic excess blend membranes or can serve as an ionic crosslinker in base-excess blend membranes.
The sulfonated pPFX-DFB 8 (spPFX-DFB) was characterized by 1 H-, 13 C-, 19 F-and HSQC NMR.Disappearance of the pattern of the aromatic protons of non-sulfonated bisphenol in the 1 H-NMR at 7.73 and 7.31 ppm confirmed the substitution.Signals with chemical shifts at 7.27, 7.63 and 8.06 ppm were observed.Because of the broad signals in the proton spectra, the coupling constants of the 1 H-atoms in the aromatic ring were difficult to determine.
Additionally, HSQC measurements yielded three cross-peaks for the coupling between the protons and carbon atoms in the dihydroxy unit at ppm 7.28, 7.62 and 8.05 ppm (see Figure 9).
Because of the numerous C-F groups in the polymer backbone and the substituted carbon atoms, the 13 C-NMR spectrum was very complex in the aromatic region between 115 and 155 ppm.The existence of the sulfonated aromatic ring in the polymer backbone was confirmed by the four new peaks for the carbon atoms in the aromatic region.The sulfonation degree was determined from the integral ratio in the 1 H-NMR spectra and yielded a of 1.8 SO3H groups per 4,4'-biphenylene unit in the random pPFX-DFB copolymer.As expected, the TGA of 8 (see Figure 8) revealed a lower thermal stability (Tdecomposition = 373 °C) than that of the non-sulfonated structure.However, this decomposition temperature is still 100 °C higher than that of the sulfonated pPFX 3 and 4, making spPFX-DBF a promising candidate for application in acid-excess blend membranes.

Preparation and Properties of Acid-Excess Acid-Base Blend Membranes MIH418
Due to the high thermal stability and suitable IEC value, the sulfonated pPFX-DFB polymer can serve as the proton-conducting blend component in acid-excess acid-base blend membranes with polybenzimidazole as the basic macromolecular cross-linker.In order to reach a significant excess of the acidic polymer in the membrane for sufficient proton-conductivity in the blend membrane, 80 wt% of the spPFX-DFB and 20 wt% of the F6PBI were mixed in DMSO.The blend membrane was dried at 130 °C, followed by rinsing with 10% HCl for 48 h at 90 °C and deionized water for 48 h at 60 °C.Characteristics of acid excess acid-base blend membrane MIH418 are collected in Table 3.

Conductivity of the Acid Excess Acid-Base Blend Membrane MIH418
Figure 10 shows the H + -conductivity of the acid excess acid-base membrane MIH418 as a function of the temperature (30-130 °C) at constant relative humidity 90% ± 4%.For comparison, the standard PFSA membrane Nafion ® 212 was given.Despite the higher IEC values of membrane MIH418, for most temperatures, lower conductivity was observed than for Nafion ® 212.The reason for the strong deviation of the conductivity versus T curve from the correlation line observed for the MIH418 membrane is still unclear and requires further investigations.

Preparation and Properties of Base-Excess Acid-Base Blend Membranes
Ionically cross-linked base-excess blend membranes for the application in intermediate-T fuel cells has been a research subject in the authors' group for the past 10 years.The research in this field aims at the development of acidic ionomers, which could increase the mechanical stability of the polybenzimidazole membranes and reduce the swelling.Due to its high chemical and thermal stability, PFX-containing polymers were tested as acidic macromolecular cross-linkers for the partially-fluorinated polybenzimidazole F6-PBI.For the preparation of the base-excess blend membranes, the sulfonated poly(arylene ether perfluoroxylene) polymer 4 was blended with F6-PBI.To reach a sufficient molar excess of the basic polymer in the membrane, 70 wt% of the F6-PBI and 30 wt% of the 4 was mixed in DMSO and dried at 130 °C.The base-excess blend membranes were subsequently doped with H3PO4 to make them proton-conductive for application as intermediate-T fuel cell membranes.

Oxidative Stability of the Base-Excess Acid-Base Blend Membranes
Generally, the Fenton test (FT) can serve as a rapid oxidative-degradation test for ionomer membranes.In Fenton's solution, which is composed of an aqueous 3% H2O2 solution with 4 ppm Fe 2+ in the form of (NH4)2Fe(SO4)2•6H2O, high concentrations of OH-and •OOH-radicals are formed, which promote the oxidative radical attack on the membranes, leading to a cumulative decrease of the molecular weight of the membrane polymer(s).The molecular weight decrease leads to a significant loss of mechanical membrane stability.The abovementioned (undoped) base-excess F6PBI blend membrane was subjected to FT, and the mass loss of the membrane was determined.The weight loss vs. FT soaking time dependence of the membrane is presented in Figure 11.It can be concluded that the base excess acid-base-blend membrane shows a high resistance against radical attack, i.e., after 96 h of FT, the membrane still retained more than 86% of its original weight.

Phosphoric Acid Doping of the Base-Excess Blend Membrane
In order to optimize the doping level of the phosphoric acid doped base-excess blend membrane, a series of doped membranes were prepared.The membranes were immersed in concentrated phosphoric acid (85 wt% H3PO4) for given temperatures and times.The best result in terms of enough mechanical integrity at high doping degree was obtained at 240 wt%.This doping degree was reached at 120 °C for 30 min.

Conductivity of the H3PO4-Doped F6-PBI Blend Membranes
The conductivity of the phosphoric acid doped blend membrane was investigated using the Membrane Test Systems from Scribner Associates Inc.The measurement was done at 20% relative humidity in the temperatures range of 80-150 °C.According to Figure 12, the conductivity showed a typical Arrhenius behavior.As expected, the lowest conductivity in the investigated temperature range was observed at 80 °C, being 25.6 mS•cm −1 .The highest conductivity was measured at 150 °C with a value of 91.8 mS•cm −1 .The activation energy Ea of the blend membrane H + conductivity could be derived from the Arrhenius plot, being 21.9 kJ/mol.The conductivity and activation energy values of this membrane are comparable to base-excess blend membranes prepared in our laboratories [11].

Conclusions
Partially-fluorinated poly(arylene ether) polymers with a perfluoro-p-xylene (PFX) building block were successfully synthesized and characterized.It was demonstrated that these polymers can be modified both nucleophilically (in the PFX unit, by nucleophilic exchange of F with other nucleophiles, such as mercaptans and phosphites) and electrophilically (SE sulfonation in the Ar-H of the polymer).Both the non-sulfonated and the sulfonated polymers showed high thermal stabilities up to 450 °C.The successful sulfonation of the pPFX makes it a suitable macromolecular H + -conducting ionomer in acid-excess acid-base blend membranes and a suitable acidic cross-linker in a base-excess blend membrane with the basic engineering polymer F6-PBI.The ionic cross-linking prohibits the dissolution of the membranes during the doping with phosphoric acid.The blend membranes showed higher thermal stability than the pure sulfonated polymers.Due to the doping with phosphoric acid, intermediate T blend membranes were obtained, which show a reasonable conductivity, confirming their suitability for intermediate-T fuel cell application.
In future work, the poly(arylene ether perfluoroxylene)s will be modified with further nucleophiles in the PFX unit to vary the polymer properties in a broader range.The novel sulfonated and phosphonated polymers will also be blended with basic polymers, such as PBIOO and PBI Celazole to yield base-excess blend membranes with a tailored property profile.The new blend membranes will be applied to intermediate T fuel cells, SO2-depolarized electrolysis, direct methanol fuel cells, PEM-electrolysis and redox-flow batteries.
C-and 19 F-NMR.The disappearance of the C-F peaks both for the meta-fluorine atoms at −139.2 ppm for the PFX and at −162.9 ppm for the DFBP revealed the completion of the reaction.The polymer shows chemical shifts of −56.3 and −128.3 ppm for PFX assigned to the CF3 and the CF groups, respectively.The signals at −139.5 and −154.4 were identified as fluorine atoms in the aromatic ring of the DFBP unit (see Figure 7).

Figure 11 .
Figure 11.Fenton's test results for the base-excess blend membrane.

Table 2 .
IEC data of the sulfonated polymer 8.