Improvement of Properties of Poly(ether ketone) Ionomer Membranes

Abstract. Sulfonated poly(ether ketone)s are suitable alternative membrane materials to perfluorinated ionomers for application in electromembrane pro...
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Ind. Eng. Chem. Res. 2004, 43, 4571-4579

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Improvement of Properties of Poly(ether ketone) Ionomer Membranes by Blending and Cross-Linking Jochen Kerres,* Chy-Ming Tang, and Christian Graf Institut fur Chemische Verfahrenstechnik, Universitaet Stuttgart, Boeblinger Strasse 72, D-70199 Stuttgart, Germany

Sulfonated poly(ether ketone)s are suitable alternative membrane materials to perfluorinated ionomers for application in electromembrane processes and in membrane fuel cells. However, they show too high water uptake/swelling when having sufficiently low specific H+ resistance for the applications mentioned above, especially at elevated temperatures of >80 °C. To overcome the swelling problems, sulfonated poly(ether ketone) ionomers were blended with polybenzimidazole. By blending, ionical cross-links form by proton transfer from the sulfonic acid group to the basic nitrogen atom of the imidazole. This led to a reduction in swelling but, however, also to an increase in specific H+ resistance because the overall number of hydrated protons associated with the SO3- groups was reduced by the formation of the ionic cross-links. To again decrease the specific H+ resistance, the two heteropolyacids, molybdophosphoric acid (MPA) H3PMo12O40‚xH2O and tungstophosphoric acid (TPA) H3PW12O40‚xH2O, were added. Although both materials are water-soluble, the TPA interestingly did only partially bleed out in water at T ) 90 °C. The reason for the stabilization of the TPA in the membrane is probably the formation of strong interactions between TPA and the polymeric blend membrane, preferentially as hydrogen bridges. poly(ether ketone)-PBI-heteropolyacid blend membranes are presented and discussed in this paper.

Introduction In the past decade a lot of different ionomer membranes have been developed for application in membrane fuel cells as an alternative to perfluorinated ionomer membranes. Among these materials are sulfonated arylene main-chain ionomers,1-8 e.g., sulfonated poly(ether ketone)s.3 A drawback of these materials is their large water uptake (swelling) at proton conductivities sufficient for the fuel cell application. To reduce this effect, cross-linking techniques were applied for the sulfonated ionomer membranes, among them ionical cross-linking by mixing with polymeric bases9 and covalent cross-linking by applying sulfinate groupcontaining polymers.10 To decrease the specific H+ resistance of these membranes particularly at temperatures higher than 100 °C, the organopolymers were blended with an inorganic component.12 Herein the inorganic phase serves as water storage so that the H+ conduction can be maintained even at T > 100 °C. Other interesting inorganic materials are heteropolyacids such as molybdophosphoric acid MPA H3PMo12O40‚xH2O and tungstophosphoric acid TPA H3PW12O40‚xH2O.14 Due to the proton delocalization within their structure they are strong Bronsted acids15,16 and hence provide a high proton conductivity. A drawback of these complexes is their high water solubility. However, recent studies suggest that the heteropolyacids are stabilized in the ionomer membrane matrix by interaction with the ionexchange groups.17 In this study sulfonated poly(ether ketone) membranes are presented whose properties where systematically improved by blending9,20 with (i) polybenzimidazole and (ii) two different heteropolyacids, MPA and TPA. The characterization results of the sulfonated poly(ether ketone)s, the binary sulfonated poly(ether ketone)PBI blend membranes, and the ternary sulfonated

Experimental Section 1. Sulfonation of Polymers. Two different poly(ether ketone)s have been sulfonated: poly(ether ether ketone) PEEK Victrex (Victrex Co.) and poly(ether ketone ether ketone ketone) PEKEKK (BASF). The sulfonation of PEEK was performed in the following way: Concentrated H2SO4 (Aldrich) was placed in a glass reactor equipped with a stirring motor, a cooler, and a drying tube. Then powdered PEEK was introduced into the glass vessel in spoon portions (1 g of polymer/20 g of H2SO4). The vessel was closed, and the dissolution and reaction was allowed to proceed for the desired time at the preset reaction temperature. The PEKEKK sulfonation was accomplished in a very similar way, except that oleum was added after PEKEKK had been dissolved in the concentrated H2SO4. The reaction was allowed to proceed for the desired time at the preset reaction temperature. An amount of 25 g of polymer (PEKEKK)/200 mL of acid was applied. After sulfonation, the polymer was precipitated in an ice/water mixture. Afterward the polymer was filtered out. Then, in case of a water-insoluble polymer, the polymer was stirred in water, again filtered out, and washed until a neutral reaction of the washing water was obtained. Finally the polymer was dried in the drying oven at T ) 80-100 °C and finely milled. In case of a water-soluble polymer, the raw polymer from the first filtering was dissolved in water at elevated temperature (T ) 60-80 °C). The solution, still containing a lot of sulfuric acid, was dialyzed until neutral reaction of the water surrounding the dialysis tubes was achieved. Then the water was evaporated from the aqueous polymer solution in a drying oven at T ) 90 °C until weight constancy of the polymer was reached.

10.1021/ie030762d CCC: $27.50 © 2004 American Chemical Society Published on Web 04/20/2004

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Figure 1. Sulfonation of PEEK and PEKEKK. All possible sulfonation sites are shown.

2. Membrane Preparation. The polymers (sPEEK, sPEKEKK, and polybenzimidazole PBI Celazol) were dissolved in NMP to 10 wt % solutions. The solutions of sPEEK and sPEKEKK were neutralized with npropylamine to avoid precipitation of polyelectrolyte complexes during mixing with PBI. The proportion of the particular materials used for the preparation of the membranes is given in Table 3. In the case of ternary blend membranes, the desired amount of heteropolyacid was added to the solution as a solid (Table 4). It had to be taken care that enough n-propylamine was introduced in excess in the solution to neutralize the heteropolyacid too. After homogenization the mixture was spread onto a glass plate by means of a doctor knife (depth of the doctor knife 1 mm) and placed in a vacuum oven at 130 °C at an initial pressure of 100 mbar for 90 min to evaporate the solvent. After cooling and removal from the glass, the membranes were subjected to the following posttreatment: (i) conditioning in 10% HCl at 90 °C for 24 h; (ii) conditioning in water at 90 °C for 24 h to wash out residual acid. 3. Polymer and Membrane Characterization.9,21 3.1. Titration. The ion-exchange capacity (IEC) of the polymers and membranes was determined by titration with 0.1 N NaOH. 3.2. FTIR Spectroscopy. The FTIR spectra of the polymers and membranes were recorded with a BioRad FTS 155 machine. Therefore, thin membrane pieces were directly mounted in the specimen holder. 3.3. Thermogravimetry. For TGA measurements a Netzsch STA 449C instrument was used. The TGA experiments were conducted under 65% O2 atmosphere (rest N2). The applied heating/cooling program was as follows: isothermal at 30 °C for 21 min; dynamic heating during 30-600 °C at 20 K/min; isothermal at 600 °C for 5 min; dynamic cooling during 600-30 °C at -20 K/min. 3.4. Specific H+ Resistance. The specific H+ resistance was determined via impedance spectroscopy (EIS) with a Zahner Elektrik IM6 machine both in 0.5 N HCl and water at room temperature. The measurement conditions are described in ref 9. 3.5. Swelling (Water Uptake). The water uptake of the membranes was determined via immersion in liquid water at temperatures 25, 40, 60, and 90 °C for 48 h. First experiments with different sulfonated membranes showed that within this interval the sorption equilibrium of water is adjusted for all membranes. After immersion they were quickly removed from the water bath, immediately dry-wiped, and weighed. Then they were dried at 90 °C to weight constancy. The

Table 1. Sulfonation Results SO3 IEC RspH+ sulfonation temp content time (min) (°C) (wt %) (mequiv/g) DSa (Ω cm)

polymer SPEEK1 SPEEK2 SPEEK3 SPEEK4 SPEKEKK1 SPEKEKK2 SPEKEKK3 SPEKEKK4 SPEKEKK5 SPEKEKK6 SPEKEKK7 SPEKEKK8 SPEKEKK9 SPEKEKK10

60 120 180 1440 180 240 360 60 360 60 1440 360 60 360

60 60 60 60 70 70 70 70 70 60 70 70 40 70

0 0 0 0 15 15 15 15 30 15 30 15 15 15

0.93 1.53 1.58 2.12 2.9 2.87 3.32 2.52 4.13 2.31 4.2 3.38 0.79 3.08

0.29 0.5 0.52 0.74 1.87 1.85 2.24 1.57 3.06 1.41 3.14 2.3 0.42 2.03

46.7 14.77 14.31 sb s s s s s s s s 67.73 s

a DS (degree of substitution) can be calculated from IEC using the following equation: DS ) (MWu × IEC)/[1000 - (MWSO3 × IEC)[ with MWu ) molecular weight of the repeating unit of the unsulfonated poly(ether ketone). b s: polymer is water soluble.

swelling degree (SW) was calculated via the following formula:

SW ) (mwet - mdry)/mdry × 100% Results and Discussion 1. Synthesis and Characterization of Sulfonated PEEK and PEKEKK. Sulfonations of PEEK and PEKEKK both proceed via electrophilic sulfonation. For PEEK sulfonation concentrated sulfuric acid (95-98 wt %) is sufficient as a sulfonation agent, due to the electron-rich phenylene between two ether bridges, while for PEKEKK oleum is required, due to the electron-deficient phenylene units between one ether and one carbonyl bridge. The sulfonation procedure and the sulfonation sites are shown schematically in Figure 1. In Table 1 the sulfonation results of PEEK and PEKEKK under different conditions are presented. Our focus was to study the sulfonation of PEKEKK, because the sulfonation of PEEK, PEEKK, and PEK has been studied in detail by many groups.27,28 The latter served only as reference for the sulfonation of PEKEKK from which we expected a lower swelling than sPEEK at the same specific H+ resistance. From the IEC measurements it can be concluded that the sulfonation was successful. FTIR spectroscopy confirmed these results (Figure 2). The assignment of the IR bands was made according to ref 22. The band at 1025 cm-1 can be assigned to the symmetrical SdO stretching vibration, while the vibration band at 1255 cm-1 can be identified as a asymmetric OdSdO stretching vibration. Moreover, the band

Ind. Eng. Chem. Res., Vol. 43, No. 16, 2004 4573 Table 2. Weight Loss Percentages of sPEKEKK5 and -6 in the Temperature Range 280-380 °C polymer

IEC (mequiv/g)

calcd wt loss of SO3Ha [(%)

found wt lossb (%)

SPEKEKK6 SPEKEKK5

2.3 4.1

18.4 32.8

21.2 28.8

a The weight loss was calculated as follows: wt loss ) 100 100(MWu/MWs), with MWu ) molecular weight of the repeating unit of the unsulfonated polymer and MWs ) molecular weight of the repeating unit of the sulfonated polymer. b Determined via TGA.

Figure 2. FTIR spectra of sPEKEKK. The assignment of the IR bands is made according to ref 22.

at 1488 cm-1 can be identified as an aromatic C-C vibration which is splitting up into two bands at 1472 and 1491 cm-1 when sulfonation takes place. It is indicative for a 3-fold substituted aromatic system. The thermal stability of the sulfonated polymers was determined via thermogravimetry (TGA). In Figure 3, the TGA traces of different sPEEK and sPEKEKK are presented. The weight losses can be assigned as follows. The first step at the TGA between RT (room temperature) and 300 °C is a water loss, while the second weight loss step at the TGA between 300 and 400 °C signalizes the splitting-off of sulfonic acid groups, which can be proven by FTIR analysis of the TGA combustion gases by TGA-FTIR coupling.23 Interestingly, at sPEKEKK5, which has a very high IEC of 4.13 mequiv/g, the splitting-off from SO3H groups starts at a lower temperature than at sPEKEKK6, which has an IEC of 2.3 mequiv/gsthe higher the IEC, the lower the thermal stability. Generally, all ionomers are stable under typical operating conditions of PEM fuel cells. An analysis of the sPEEK trace between 260 and 390 °C shows a weight loss of 11.3%, while the calculated weight loss at the splitting-off of SO3H is 8.1%. For sPEEK2 the measured weight loss in the SO3H-splitting-off temperature range is 14.3%, while the respective calculated value is 14%. The calculated and measured SO3H splitting-off weight losses of these polymers are in good correspondence. When we look at sPEKEKK5 and -6 in the temperature range from 280 to 380 °C, the TGA results obtained are listed in Table 2.

For both sulfonated PEKEKKs the calculated and determined weight loss values are in good agreement. The sulfonation kinetics of both PEEK and PEKEKK was qualitatively monitored. In Figure 4, the sulfonation degree in dependence of the reaction time is shown. Sulfonation proceeds very fast in the beginning of the reaction, and with increasing time the reaction is slowing down. This effect can be explained by progressing deactivation of the polyaromatic chains because of the electron-withdrawing effect of the SO3H group.27 The observed sulfonation kinetics is in good agreement with sulfonation kinetics reported by other groups.27,28 For PEKEKK the time dependence of the sulfonation reaction was studied using 15% and 30% oleum, respectively. The results are also given in Figure 4. Under both conditions the reaction proceeds very fast and tends to an upper limit because of an increasing deactivation of the aromatic system.27,28 For PEKEKK the temperature dependence of DS at fixed reaction time (60 min) was also investigated (Figure 5). A strong dependence of DS from temperature is observed, especially when it is increased from 40 to 60 °C. The graph has the typical shape of an activated process. Using an Arrhenius approach the activation energy is calculated to 0.59 eV, in agreement to the work of Ulrich.27 Here the evaluation of the PEEK sulfonation yields an activation energy of 0.31 eV according to the favored substitution. The sulfonated polymers have furthermore been characterized by swelling and specific H+ resistance measurements. It was observed that all sulfonated poly(ether ketone)s with an ion-exchange capacity (IEC) of higher than ≈2.1 mequiv of SO3H/g of polymer are water-soluble. For a series of three sPEEKs the specific proton resistance was determined. The resistance values of the three sPEEKs sPEEK1, sPEEK2, and sPEEK3

Figure 3. TGA of sPEEK and sPEKEKK. As a reference, the unsulfonated trace of PEKEKK is shown.

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Figure 4. Sulfonation of PEEK and PEKEKK in dependence of reaction time with 15% oleum and 30% oleum at 60 °C.

Figure 5. Dependence of DS from T at sulfonation of PEKEKK (time 60 min).

Figure 6. Swelling of sPEEK1 and sPEKEKK9 in dependence of T.

are given in Table. 1. The strong decrease of sPEEK resistance with increasing IEC is obvious. sPEEK and sPEKEKK with similar IECs have been compared to decide which polymer type is more suitable for the preparation of ion-exchange membranes. The data for the two polymers are listed in Table 1. The dependence of water uptake of the two polymers from temperature is given in Figure 6. For comparable IECs, sPEEK1 exhibits a higher water uptake than sPEKEKK9. Furthermore sPEEK1

shows an extremely high and therefore nonmeasurable swelling at 90 °C, while the swelling of sPEKEKK1 at 90 °C is only moderate. On the other hand, sPEKEKK9 shows a markedly higher proton resistance than sPEEK1 despite both polymers have a similar IEC. These findings can be explained with the chemical composition of both polymers: PEKEKK has a more rigid polymer backbone, due to its higher amount of carbonyl bridges, compared to PEEK (in PEEK the relation between ketone and ether bridges is 1:1, whereas in PEKEKK this relation is 3:2). Ketone bridges increase the stiffness of a macromolecule, while ether bridges lead to softening of a polymer.24 This leads to the increased resistance and the reduced swelling of sPEKEKK. From the aforementioned it can be concluded that sPEKEKK is more suitable for the preparation of ionomer membranes than sPEEK, due to its lower swelling and therefore better mechanical stability. The increased resistance of sPEKEKK, compared to sPEEK, can be compensated by increase of IEC or addition of ion-conducting species such as heteropolyacids (see below). 2. Preparation and Characterization of Binary and Ternary Sulfonated Poly(ether ketone) Blend Membranes. 2.1. Binary Blend Membranes [Poly(ether ketone) and Polybenzimidazole]. In the following, the results of the characterization of binary blend membranes of sPEEK and sPEKEKK with the highly chemically and thermally stable25 polybenzimidazole PBI Celazol (producer: Celanese) are presented and discussed. Table 3 lists the composition and the IEC of binary blend membranes from sulfonated poly(ether ketone)s and PBI. The theoretical IEC was calculated from the molar relation between sulfonic acid and basic nitrogen groups. Due to the fact that PBI is a relatively strong polymeric base (pKs of the conjugated acid ≈ 5.7), all basic N sites of PBI should be protonated by the SO3H groups of the sulfonated poly(ether ketone) component, and therefore, the calculated and experimentally determined IEC should be similar. As can be seen from Table 3, a good agreement exists between theoretical and experimental IEC for most of the blend membranes. The impact of ionical cross-linking can be determined from the comparison of water uptake versus temperature curves of the ionically cross-linked membranes and pure sulfonated poly(ether ketone)s. This is shown in Figure 7 for sPEEK1 (IEC ) 0.93 mequiv/g), the membrane G007 (13.87% PBI, IEC ) 1.18 mequiv/g), and the membrane G009 (10.75% PBI, IEC ) 1.36 mequiv/g) as examples of the ionically cross-linked species. As expected sPEEK1 and G007, having similar IECs, show similar swelling behavior up to T ) 60 °C. While the swelling of the ionically cross-linked membrane G007 shows a value of 47% at 90 °C, the swelling of sPEEK1 is so high that it cannot be measured. Interestingly, also the swelling of membrane G009 is much lower at 90 °C than the swelling of sPEEK1, although the IEC of G009 is markedly higher than the IEC of pure sPEEK1. Obviously, the swelling is strongly reduced by the ionical cross-links in the acid-base blend membrane. In a comparison of the specific H+ resistance of the different membranes, G007 and G009 show lower specific resistance than sPEEK1 (Figure 8), although sPEEK1 and G007 have comparable IECs. The specific

Ind. Eng. Chem. Res., Vol. 43, No. 16, 2004 4575 Table 3. Composition and IEC of Binary Acid-Base Blend Membranes membr

acidic composn

basic composn

macidic (g)

mbasic (g)

acida (wt %)

basea (wt %)

IECdirect (expt) (mequiv/g]

IECdirect (calcd) (mequiv/g)

G007 G009 G011 G012 G013 G014 G017 G018 G019 G020 G033 G074

sPEEK4 sPEEK4 sPEKEKK3 sPEKEKK3 sPEKEKK3 sPEKEKK3 sPEKEKK5 sPEKEKK3 sPEKEKK1 sPEKEKK4 sPEKEKK10 sPEKEKK10

PBI PBI PBI PBI PBI PBI PBI PBI PBI PBI PBI PBI

2.0 2.0 2.0 2.0 2.0 2.0 3.0 3.0 3.0 3.0 2.0 2.0

0.322 0.241 0.384 0.254 0.653 0.484 1.052 0.746 0.593 0.440 0.5 0.5

86.13 89.25 83.89 88.73 75.39 80.52 74.04 80.09 83.5 87.21 80.0 80.0

13.87 10.75 16.11 11.27 24.61 19.48 25.96 19.91 16.5 12.79 20.0 20.0

1.18 1.36 1.18 1.48 0.7 1.2 1.29 1.33 1.45 1.5 1.34

0.93 1.2 1.74 2.21 0.9 1.41 1.35 1.35 1.35 1.35 1.33 1.33

a

Weight percent of the polymeric acid and the polymeric base in the blend membrane.

Figure 7. Swelling versus temperature curves of a pure sPEEK (IEC ) 0.93 mequiv/g) and the ionically cross-linked membranes G007 (IEC ) 1.18 meq/g) and G009 (IEC ) 1.36 meqiuv/g).

Figure 8. RspH+ of sPEEK1 (IEC ) 0.93 mequiv/g) and of the binary membranes G007 (IEC ) 1.18 mequiv/g) and G009 (IEC ) 1.36 mequiv/g) in 0.1 N HCl and at temperature 20 °C.

resistance of G009 is even lower than the specific resistance of Nafion 1135 membranes. According to Leung and Bailly26 the morphology of ionomers (such as sulfonated poly(ether ketone)s) is changing with increasing ion concentration in the following way: the higher the IEC, the higher the number of ion aggregates whose size is reduced with increasing IEC. In this case there are more pathways for proton transport and the resistance is reduced. In the case of the membrane G007, the overall sulfonate group content is 1.82 mequiv of SO3H/g, because it is made from sPEEK4 having an IEC of 2.12 mequiv/g. This markedly higher overall sulfonate group content of G007, compared to sPEEK1, may lead to

Figure 9. Swelling (water uptake) of the membranes G017-G020 in dependence of temperature. All membranes have the same IEC.

smaller sized clusters in high concentration which finally yields a lower specific H+ resistance of the G007 membrane, compared to sPEEK1 (Figure 8). As a check of this hypothesis, the determination of the ion-aggregate size of the sulfonated poly(ether ketone)s and of the acid-base blend membranes is required which can be accomplished by SAXS or possibly by highresolution SEM. Figure 9 shows how the ionical cross-linking density of the binary blend membranes influences the swelling degree of the membranes. For this purpose, the ionically cross-linked binary blend membranes G017, G018, G019, and G020 have been prepared, all having the same calculated IEC of 1.35 mequiv of SO3H/g, starting from sulfonated PEKEKK polymers with different sulfonation degrees. As expected, the swelling of the membranes at 90 °C is lower as the higher is the crosslinking density of the membranes (e.g., the higher the PBI content). Interestingly, the water uptake of the membrane G017 at room temperature is higher than the water uptake of the membranes G019 and G018 under the same conditions. This might be due to the high hydrophilicity of G017sthe ionical cross-links (acid-base pairs) are also hydrophilic, not only the free SO3H groups. In Figure 10, the specific proton resistance values of the membranes G017-G020 are presented. Although the DS of the deployed sPEKEKKs rises from G020 to G017, the specific proton resistance of the membranes is increasing with the PBI content and therefore with the ionical cross-linking density. The presence of ionical cross-links obviously hinders the proton transport within

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Figure 10. Specific H+ resistance values of the membranes G017-G020 in 0.1 N HCl and at temperature 20 °C. All membranes have the same IEC.

Figure 11. Water uptake of the membranes G011-G014 in dependence of temperature. All membranes have the same sPEKEKK.

the ion-aggregate structure. A hypothesis for explanation of this finding is that the proton transport paths within the ion-aggregate structure of the membrane are partially interrupted by the ionical cross-links. In a second membrane series (membranes G011, G012, G013, G014), the IEC of the binary blend membranes was varied by combining one of the sulfonated PEKEKKs (sPEKEKK3) with an increasing amount of PBI (Table 3). In Figure 11, the temperature dependence of water uptake of these membranes is shown. As expected, the water uptake is strongly increasing with decreasing cross-linking density. The membranes G013 and G014, having the highest PBI content, show only little increase in swelling from room temperature to 90 °C, while the other two membranes show unacceptable high water uptake. Moreover, the influence of ionical cross-linking density onto specific H+ resistance is shown in Figure 12. As can be seen, the resistance of the membranes is rising strongly from G012 to G013, which supports the hypothesis of impeding ionical cross-links integrated into the ion aggregates. The stability of the cast membranes was proven via TGA measurements. In Figure 13, the resulting traces of the three membranes G012, G013, and G014 and their main component, sPEKEKK3, are shown. From Figure 13 it follows that the thermal stability of the blend membranes is obviously not affected by the ionical cross-linking density of the membranes. 2.3. Ternary Blend Membranes [Poly(ether ketone), Polybenzimidazole, and Heteropolyacid].

Figure 12. Specific H+ resistance of the membranes G011-G014, measured in 0.1 N HCl and at temperature 20 °C. All membranes have the same sPEKEKK.

Table 4 presents the composition and the IEC of (i) pure sPEKEKK10-PBI membrane (G033) and (ii) sPEKEKK10-PBI membranes doped with 10, 20, and 40 wt % TPA. As can be seen in Table 4, the IEC is decreasing with increasing TPA content. This is due to an IEC of the TPA of 1.04 mequiv of H+/g, which is in accordance with 3 free protons/TPA molecule, leading to a “dilution” of the IEC. The influence of the TPA on the membrane resistance is shown in Figure 14. A marked reduction of the resistance of the membranes with increasing TPA content is observed, although the IEC of the membranes is decreasing with increasing TPA content. Similar results were obtained by Kim17 and Zaidi.14 These authors propose a homogeneous distribution of the proton-conductive TPA molecules over the whole cluster/ channel structure of the membranes leading to a decrease of specific H+ resistance of the composite membranes with increasing TPA content. The corresponding swelling values of the membranes G033-G036 are shown in Figure 15 as a function of temperature. As expected, the swelling values are increasing with increasing TPA content, due to the water solubility of TPA. In comparison with the results for the pure ionically cross-linked membranes (Figures 11 and 12), the swelling appears to be lower for the same reduction in proton resistance. The mentioned water solubility of the heteropolyacids requires a check of the stability of the ternary blend membranes against leaching-out of the heteropolyacid. In case of MPA membranes, it was observed that the blue-colored MPA was washed out during aqueous membrane posttreatment. In contrast to this, TPA is colorless and therefore its possible out-diffusion behavior from the membrane matrix was determined via thermogravimetry. In a very recent paper the leachingout of TPA, MPA, and other heteropolyacids from sulfonated PEK was determined, yielding the result that MPA is much stronger leaching out of sulfonated PEEK membranes during aqueous posttreatment than TPA.29 The respective weight loss/time TGA diagram is shown in Figure 16. It is assumed that at 600 °C weight constancy of the membrane and TPA residuals is reached. From the weight loss of TPA from 30 to 600 °C, which is mainly due to dehydratation, the expected TPA residuals of the composite membranes are calculated. The experimental and calculated TPA and membrane residuals and weight loss values from 30 to 600 °C are shown in Table 5.

Ind. Eng. Chem. Res., Vol. 43, No. 16, 2004 4577

Figure 13. TGA traces of G012, G013, G014, and sPEKEKK3. Table 4. Composition and Properties of Ternary Membranes sPEKEKK10-PBI-TPA membr

macidic (g)

mbasic (g)

mTPA (g)

acida (wt %)

basea (wt %)

TPAa (wt %)

IECdirect(expt) (mequiv/g)

IECdirect(calcd) (mequiv/g)

G033 G034 G035 G036

2.0 2.0 2.0 2.0

0.5 0.5 0.5 0.5

0.278 0.625 1.667

80 72 64 48

20 18 16 12

10 20 40

1.34 1.37 1.35 1.15

1.33 1.30 1.27 1.21

a

Weight percent of the polymeric acid, the polymeric base, and TPA in the blend membrane.

Figure 14. Specific resistance of the membranes G033 (0% TPA) to G036 (40% TPA) in 0.1 N HCl and at temperature 20 °C. All membranes have same PBI content and nearly the same IEC.

From Table 5 and Figure 16 it is obvious that the membranes lose TPA during membrane posttreatment to an intolerable extent. Consequently it is required to stabilize the heteropolyacid molecules in the blend membrane matrix, which can be possibly accomplished by, as suggested in the relevant literature for ionomer membrane systems other than ours,29,30 the formation of covalent or ionical links between the heteropolyacid molecules and the polymer or by immobilization of the heteropolyacid molecules in the pores of another compound, such as porous silica, added to the membranes. Conclusions Different series of alternative binary and ternary proton conducting membranes based on sPEKEKK have been synthesized. If one started from simple sulfonated poly(ether ketone)s, the properties of the membranes were stepwise improved by blending with PBI and insertion of hetereopolyacids (MPA or TPA) into the membrane matrix. The membranes were investigated in terms of specific H+ resistance, water uptake, and

Figure 15. Swelling values of membranes G033 (0% TPA) to G036 (40% TPA) in dependence of temperature. All membranes have the same PBI content.

thermal stability and compared with each other. Sulfonated PEKEKK exhibits a lower water uptake than sulfonated PEEK at the same specific H+ resistance. This result can be explained by the higher rigidity of PEKEKK, compared to PEEK. Both sulfonated PEEK and PEKEKK showed good thermal stabilities up to 250 °C. In a second step the sulfonated poly(aryl ether ketone) membranes were blended with PBI. The blend membranes had a lower water uptake because of ionic cross-links between the sulfonated polymer and the basic PBI, compared to the pure sulfonated polymers. Because of the decrease in free proton concentration, the specific H+ resistance rises with increasing PBI content. By addition of tungstophosphoric acid (TPA) or molybdophosphoric acid (MPA) to the membranes, it was possible to decrease the resistance of the membranes markedly. Interestingly, the water uptake of the membranes remained nearly constant even at increas-

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Figure 16. Weight loss/time TGA traces of TPA and the membranes G033-G036. Table 5. Experimental and Calculated TPA and Membrane Residuals and Weight Loss Values from 30 to 600 °C (Data from TGA) TPA residuals calcd residuals TPA content wt loss to at 600 °C (dehydratn of loss component (%) 600 °C (%) (%) TPA) (%) (%) TPA G033 G034 G035 G036

100 0 10 20 40

14.4 98.5 92.4 86.8 75.4

85.6 1.5 7.6 13.2 24.6

85.6 0 8.56 17.12 34.24

0 11.2 22.9 28.2

ing TPA quantities, which can be explained by the TPA location within the membrane pores. The ternary blend membranes exhibited a moderate swelling over a wide temperature range and simultaneously had a specific resistance comparable with Nafion 1135. However, the membranes lacked compositional stability, which needs to be improved. Therefore, our effort in future investigations will lie in the stabilization of the TPA in the membrane, which can be accomplished by (i) generation of chemical bonds between heteropolyacid molecules and the organomembrane matrix, (ii) introduction of functional groups into the heteropolyacid molecules which are capable of generation of strong interactions between the organomembrane and the heteropolyacid molecules or which reduce the water solubility of the heteropolyacid, and (iii) lock-up of the heteropolyacid molecules in the membrane matrix by, e.g., generation of heteropolyacid-inpermeable membrane surface layers or by immobilization of the heteropolyacid in a porous filler added to the membrane, e.g., porous silica. In our further development work, we will concentrate on the achievement of a better retention of the heteropolyacid molecules in the membrane by the abovementioned measures. Acknowledgment It was a privilege for me to work nearly 13 years in the Institute for Chemical Process Engineering of the University of Stuttgart, which is headed by Gerhart Eigenberger. I warmly thank Gerhart Eigenberger for the continuous support and encouragement with which he provided me, my group, and the work of my group and me at his institute. Financial support by the DFG (DFG-Schwerpunkt 1060 “Schichtstrukturen”) is gratefully acknowledged. We thank BASF AG for supplying the PEKEKK.

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Received for review October 10, 2003 Revised manuscript received February 13, 2004 Accepted February 16, 2004 IE030762D