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About the Interactions Controlling Nafion’s Viscoelastic Properties and Morphology Jan-Patrick Melchior,† Thomas Braü niger,*,† Andreas Wohlfarth,† Giuseppe Portale,‡ and Klaus-Dieter Kreuer*,† †

Max Planck Institute for Solid State Research, Heisenbergstr. 1, 70569 Stuttgart, Germany DUBBLE, BM26 at ESRF, 6 rue Jules Horowitz, BP220, F-38043 Grenoble, France

‡

S Supporting Information *

ABSTRACT: Interactions controlling the viscoelastic properties of Nafion are identified by investigating morphological changes induced through stretching at a wide range of controlled temperature and relative humidity. 2H-goniometer NMR exploiting the pseudonematic effect in D2O-containing membranes provides information on induced anisotropy on the 70 nm scale while small-angle X-ray scattering (SAXS) is used to reveal local structural correlations on the low nanometer scale. Under highly humidified conditions, stress is suggested to be mainly transmitted through the robust polymeric domain with shearing mainly occurring within the soft aqueous domain. Since the latter deteriorates the uniformity of the flat aqueous domain thickness, also the structural nanocorrelation decays. With decreasing relative humidity, where the weakly hydrated ionic domain is thought to form an increasingly stable Coulomb structure, and with increasing temperature reducing the elastic modulus of the polymeric domain, shearing is suggested to preferentially occur within the mechanically weaker polymer aggregates, leaving the nanoscale structural correlation intact. Structural anisotropy on the 70 nm scale, as evidenced by a residual 2H quadrupolar splitting, is only observed after stretching. Unstretched Nafion is virtually isotropic on this scale; i.e., any genuine anisotropy of Nafion’s morphology must be on a significantly smaller scale.

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INTRODUCTION Nafion as produced by the DuPont Company is still the benchmark ion-conducting membrane material for numerous electrochemical applications such as chlor-alkali electrolysis, low-temperature fuel cells, and redox-flow batteries. It stands for the family of perfluorosulfonic acids (PFSA) comprising polytetrafluoroethylene (PTFE) backbones with perfluorinated side chains of different lengths attached to the backbone through ether linkages and terminated by sulfonic (−SO3M) cation (M +) exchanging groups. PFSAs are ionomers combining in one macromolecule the hydrophobicity of their backbones with the hydrophilicity of their superacidic sulfonic acid groups. The unique membrane properties of PFSAs, i.e., the combination of good morphological stability and high ionic conductivity, have much to do with their characteristic nanomorphology, which is the result of a spontaneous hydrophilic/hydrophobic separation especially in the presence of water. Despite their technological importance, details of the nanomorphology including its crystallinity and the controlling interaction are still a matter of debate.1−20 In this debate on Nafion’s structure, the “parallel cylinder model” became popular during the past few years,16 whereas our own recent work provides further evidence for locally flat and narrow water structures.19 Such water “films” of quite uniform thickness are proposed to be positively charged keeping together negatively © 2015 American Chemical Society

charged polymer aggregates through ionic cross-linking. From dynamical mechanical analysis (DMA), ionic interactions were suggested to affect even the macroscopic viscoelastic behavior of Nafion for moderate levels of hydration where ionic charges are hardly screened. Under very wet conditions, hydrophobic interactions between polymer backbones are thought to control the viscoelastic properties.21,22 While the structure on the low nanometer scale ( 10% do not show the same single Lorentzian peak shapes as reported in the literature for stretching at high temperature and low humidity.37,38 Instead, we frequently encountered spectra which possessed shoulders, such as those displayed in Figure 6b. These shoulders were typically present for orientations in the vicinity of the maximum residual splitting, whereas for orientations with smaller splittings, they tended to merge with the main doublet peaks. Only for the samples stretched at very low relative humidity, the shoulders were reliably absent in the experiments. We note that such shoulders are visible in 2H NMR spectra reported in some of the earlier papers.32−34 At the moment, the origins of this effect are not understood, with surface effects in the uptake of water21,67 being one likely candidate. Another feature which needs to be inspected more closely is the severe asymmetry η of the Q-tensor induced through stretching. This corresponds to the biaxiality of the tensor which appears to differ significantly for stretching under wet and dry conditions (Table 2), indicating that this is reflecting some material properties. However, this large difference is only

Figure 6. (a) The parameter η (eq 3) for samples stretched to twice their initial length (membrane width 11.5 cm; T = 70 °C) as a function of relative humidity. (b) Typical spectra as a function of orientations (mounting c). 8540

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existing anisotropic objects. Anisotropy can also develop through the stretching process itself. In fact, this is the case for most polymers investigated with this technique29−31,53−55 which is the immediate consequence of the one-dimensional nature of any polymer. This principal behavior is also relevant for the cases discussed in the following. Stretched Nafion. With above considerations in mind, it is quite expected that some splitting Δν emerges after stretching under any condition as a consequence of induced anisotropy of Nafion’s morphology. Important information is then obtained from the degree of anisotropy and how this develops with temperature T and relative humidity RH, the only two parameters which were varied for the different stretching experiments (stretching rate and degree (strain) were kept constant, and characterization after stretching was carried out at room temperature and a hydration number of λ ∼ 6 for all samples). Since the viscoelastic properties change significantly with T and RH (see Figure 3), stress occurring in the membranes during stretching varied accordingly. The relevant questions we address here are how far stress transmits in the different parts of Nafion’s structure and what the resulting irreversible morphological changes are. We may recall that stretching as described in the Experimental Section goes far beyond the yield point where most of the induced changes (e.g., anisotropy) are quasi-irreversible and preserved under the characterization conditions. The variation of these changes with T and RH then provides qualitative information on the relative T/RH dependence of those interactions which control Nafions viscoelastic properties and morphology. In this context the additional information from small-angle X-ray scattering (SAXS) proves to be very useful. For this discussion we have to keep in mind that the aqueous ionic domain and the polymeric domain are connected through the common −SO3− groups; i.e., these groups are covalently bound to the molecular structure of this ionomer, and they are interacting with the aqueous ionic domain through hydration forces and residual ionic interactions (with other hydrated sulfonic groups and protonic charge carriers). Therefore, any deformation in one domain must lead to some structural changes in the other domain as well. The other aspect we include into the discussion of the effects of deformation is the origin of Nafion’s flat morphology which had been found to be a common feature of various strongly acidic ionomers and polyelectrolytes.19 We had therefore suggested that the formation of this morphology is essentially driven by residual ionic interactions within the aqueous domain (including the −SO3− group which is also part of the polymeric structure). That is to say structure and interactions in the ionic part are more similar for different ionomers than structure and interactions within the polymeric part. We may also recall that SAXS patterns of chemically very different ionomers such as PFSAs (perfluorosulfonic acids) and sulfonated poly(ether ketone)s are quite similar7 and that surfactants structurally organize into well-ordered lamellar domains provided the water content is not too high.69 This point of view leads one to expect the existence of film-like aqueous structures of quite uniform thickness separated by polymeric domains of less regularity (Figure 8a). Then the appearance of structural correlation essentially originates from the uniformity of the aqueous film thickness and does not depend on the existence of flat polymeric primary objects such as Langmuir−Blodgett-like bilayers. (The latter are observed as part of the very regular morphologies of surfactant/water

Figure 7. Effect of the membrane width w (stretching at T = 70 °C submerged in D2O) on the parameter η (eq 3) for Nafion stretched to twice its original length (for orientation-dependent 2H NMR spectra see Figure S2 in the Supporting Information).

Figure 8. Schematic two-dimensional illustrations of proposed Nafion morphologies viewed along the locally flat aqueous ionic domains separating irregular polymeric domains (note that this corresponds to a distribution of view angles compensating for out of plane tortuosities): (a) unstretched, (b) after stretching under wet conditions, and (c) after stretching at low levels of hydration (dry) (see text). Arrows illustrate where forces are mainly transmitted during stretching.

already be visible for unstreched membranes. For a random (isotropic) distribution of orientations of those objects on larger scales, a Pake spectrum is expected which does not change with the sample’s orientation in the magnetic field B0. Any preferred orientation of the objects, e.g., as a result of stretching, then would lead to an increasing dependence of the spectrum’s shape on sample orientation.68 For uniform orientation of anisotropic objects within the entire sample the spectrum simply consists of a single sharp doublet. However, the appearance of a doublet after stretching must not necessarily be the consequence of the orientation of pre8541

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Macromolecules mixtures only;69 for entangled polymeric structures, however, structural organization is most likely constrained to small (nm) scales leaving some structural disorder as evidenced by relatively broad ionomer peaks in corresponding SAXS patterns.) Nevertheless, the thickness of the polymeric domains is limited (to an average of about 2.7 nm in the case of Nafion)19 as dictated by the requirement that all −SO3− groups aggregate at the interface of polymeric and aqueous domain (see above). With these preconsiderations the induced anisotropies as a function of T and RH and the changes of structural correlations on the nanometer scale find quite natural explanations. From T and RH dependent DMA (dynamical mechanical analysis) measurements there is already evidence that at least two types of local interactions govern the macroscopic viscoelastic properties.21,40 It is therefore not surprising that for an identical macroscopic storage modulus E′ the local structural changes induced by stretching may differ significantly. This is clearly the case for the two conditions documented in Table 2. Stretching under very wet condition (T = 80 °C) leads to significantly higher anisotropy (indicated by χpseudo) and asymmetry (biaxiality) η compared to stretching under dry and hot (T = 117 °C) conditions. These two extreme cases are embedded into two major trends; i.e., under relatively dry conditions (RH = 5−10%) the induced anisotropy severely decreases with temperature (Figure 9) while stretching under

ionic cross-linking between polymeric domains is therefore weaker than the hydrophobic forces stabilizing the irregularly shaped polymeric objects. The stress building up during the stretching process is therefore mainly transmitted through the polymeric domain deforming on large scales while local shearing most likely takes place along the soft aqueous films leaving the local (nm) tortuosity of the polymeric structure intact (Figure 8). Because of the irregularity of the polymeric objects, this shearing process destroys the uniformity of the aqueous film thickness. This is clearly confirmed by the loss of structural correlation as indicated by the severe attenuation of the so-called ionomer peak in SAXS patterns recorded after stretching under wet conditions (Figure 10). The resulting

Figure 10. SAXS patterns of Nafion 117 membranes after stretching to twice their initial length at T = 70 °C and various levels of hydration. The patterns have been obtained through integration of intensity data recorded by a 2-D detector (see Experimental Section and Figure 12)

morphology, schematically illustrated in Figure 8b, is probably highly metastable, local structural relaxation through electrostatic interactions being suppressed by the robustness of the polymeric domain at not too high temperature. The alignment of the polymeric structure on scales larger than a few nanometers then still leads to the relatively large anisotropy on the probing length of the pseudonematic effect (∼70 nm). With decreasing level of hydration, the aqueous ionic domain is progressively getting stronger passing through a region where aqueous and polymeric domain appear to exhibit comparable mechanical strength. This may lead to plastic instability which is clearly indicated by the appearance of a local maximum in the transition from elastic to plastic behavior in the corresponding stress/strain curves. This feature, which is frequently observed for phase separated block copolymers and for Nafion at certain water contents,41−44,70,71 emerges for RH < 20% at T = 70 °C (Figure 11). At higher T, stress/strain curves are uniform again (Figure 11). Under these conditions (dry, increased T) the polymeric domain softens and the aqueous ionic domain is the strongest part of Nafion’s structure. Then, relatively stable flat ionic films exist within a softer polymeric matrix. This situation is kind of complementary to that under wet and cold conditions (see above). Therefore, external stress is primarily transmitted through the flat ionic domain keeping its characteristic flat morphology on the nanometer scale (as the polymeric domain keeps its local irregular shape under cold and wet conditions,

Figure 9. Anisotropy χpseudo after stretching at various temperature T under wet (membrane submerged in water) and dry (RH = 5−10%) conditions. All data points have been recorded at the same D2O content (λ ∼ 6). The stretching conditions for which all tensor elements have been determined (see Table 2) are highlighted by open circles. For the corresponding storage moduli E′ please refer to Figure 3.

wet conditions leads to a high T independent anisotropy corresponding to χpseudo ∼ 1500 Hz (Figure 9). Again, it is interesting to note that the evolution of the anisotropy does not correlate with the evolution of the macroscopic storage modulus and the related yield stress building up during the stretching process (Figure 3). For understanding the induced anisotropy, one has to keep in mind that this is probed through the restricted diffusion within the aqueous domain. Under very wet conditions, the ions within this domain are hydrated to an extent (RH > 20%) that residual ionic interactions are weak. Especially at low T, 8542

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up to at least T = 80 °C (Figure 9), where the rather wide aqueous film (>2 nm)5,19 is still much softer than the polymeric structure, the induced anisotropy in relatively dry environment decreases with temperature (Figure 9). This is actually the regime in which the polymeric structure severely softens with increasing temperature (see above). Especially at very high T and very low RH, where Li et al. had performed their stretching experiments,38 also the ionic domain loses its strength,21 leading to a macroscopically soft and highly plastic state. Therefore, only very small stresses build up during stretching, and any residual stress may relax across very short distances. This situation differs fundamentally from that under wet conditions and low T, under which stresses may be transmitted over larger distances within the polymeric domain as indicated by the severe asymmetry (biaxiality) observed after stretching. Under hot and very dry conditions, however, stretching leads to less deviation from equilibrium corresponding to the much smaller anisotropy induced under these conditions (χpseudo ∼ 500 Hz, Figure 9).

Figure 11. Tensile curves with constant strain rate of 1 mm min−1 for different temperatures and relative humidity.

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see above) albeit aligning in the direction of external strain on larger scales, as illustrated in Figure 8c. The polymeric domain then adjusts to these changes on all scales within the degrees of freedom described at the beginning of this section. Since the uniformity of the aqueous film thickness is preserved the structural nanoscale correlation is preserved as well (see SAXS pattern recorded after stretching, Figure 10). The 2-dimensional SAXS data (Figure 12) indicate that the correlation

SUMMARY AND CONCLUSION

The combined use of orientation-dependent 2H NMR spectroscopy, small-angle X-ray scattering (SAXS), and tensile tests on as-received Nafion and Nafion membranes stretched to twice there initial length as a function of temperature T and relative humidity RH, including wet conditions, provide insight into those interactions which control the viscoelastic properties and the morphology of PFSA ionomers. The absence of any significant quadrupolar splitting of the 2 H NMR spectrum of as-received (unstretched) Nafion containing D2O clearly signifies that Nafion is virtually isotropic on the ∼70 nm scale, i.e., any genuine anisotropy of Nafion’s morphology must be on a significantly smaller scale. The wellknown structural correlation on the nanometer scale is suggested to be the consequence of residual ionic interactions within the aqueous domain (favoring the formation of locally flat aqueous structures of uniform thickness, Figure 8a) rather than the presence of flat primary polymeric objects (such as well-ordered bilayers). This locally flat morphology may be considered to be a feature of Nafion’s genuine morphology, i.e., the local “equilibrium” state of an otherwise irregular frustrated morphology characteristic for any polymeric material. Long-range anisotropy is only induced through uniaxial stretching. From the orientation-dependent doublet splitting present in the 2H NMR spectra after stretching, there is clear evidence that the tensor’s principal axis and the stretching direction coincide. The anisotropy caused by stretching (quantified by χpseudo) depends on hydration level and temperature during the stretching process, which demonstrates that the probe molecule (D2O) has an effect on the structure it is probing (such effects can usually be neglected in the case of nonionic elastomers for which this technique has been developed). Under very wet and not too hot (T < 80 °C) conditions (see Figure S3, green regime), stress is suggested to be mainly transmitted through the robust polymeric domain with local shearing mainly occurring within the soft aqueous domain. Since the latter deteriorates the uniformity of the aqueous film thickness, also the structural nanocorrelation decays while longrange anisotropy is induced through the stretching process (Figure 8b) which elongates the polymeric domain on larger scales only.

Figure 12. The 2-dimensional SAXS pattern of Nafion 117 after stretching under dry condition (T = 70%, RH = 20%). Stretching direction in real space is indicated (see text).

length is dependent on direction with a preferred orientation of its reciprocal vector perpendicular to the stretching direction in direct space. In other words, the flat aqueous ionic domains preferentially orient with the stretching direction being inplane. Interestingly, the correlation length between domains in preferred orientation is slightly reduced while it increases with increasing deviation from this orientation. This is clearly indicated by the deformation of the corresponding Debye ring of the diffraction pattern (Figure 12) as already observed by Li et al.37,38 While the observed quadrupolar splitting after stretching under wet conditions appears to be independent of temperature 8543

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(3) Litt, M. H. Polym. Prep. 1997, 38, 80−81. (4) Elliott, J. A.; Hanna, S.; Elliott, A. M. S.; Cooley, G. E. Macromolecules 2000, 33, 4161−4171. (5) Gebel, G. Polymer 2000, 41, 5829−5838. (6) Haubold, H.-G.; Vad, T.; Jungbluth, H.; Hiller, P. Electrochim. Acta 2001, 46, 1559−1563. (7) Kreuer, K.-D. J. Membr. Sci. 2001, 185, 29−39. (8) Rubatat, L.; Rollet, A. L.; Gebel, G.; Diat, O. Macromolecules 2002, 35, 4050−4055. (9) Rubatat, L.; Gebel, G.; Diat, O. Macromolecules 2004, 37, 7772− 7783. (10) van der Heijden, P. C.; Rubatat, L.; Diat, O. Macromolecules 2004, 37, 5327−5336. (11) Ghielmi, A.; Vaccarono, P.; Troglia, C.; Arcella, V. J. Power Sources 2005, 145, 108−115. (12) Hickner, M. A.; Pivovar, B. S. Fuel Cells 2005, 5, 213−229. (13) Page, K. A.; Landis, F. A.; Phillips, A. K.; Moore, R. B. Macromolecules 2006, 39, 3939−3946. (14) Liu, D.; Kyriakides, S.; Case, S. W.; Lesko, J. J.; Li, Y.; McGrath, J. E. J. Polym. Sci., Part B: Polym. Phys. 2006, 44, 1453−1465. (15) Alberti, G.; Narducci, R.; Sganappa, M. J. Power Sources 2008, 178, 575−583. (16) Schmidt-Rohr, K.; Chen, Q. Nat. Mater. 2008, 7, 75−83. (17) Knox, C. K.; Voth, G. A. J. Phys. Chem. B 2010, 114, 3205− 3218. (18) Kusoglu, A.; Karlsson, A. M.; Santare, M. H. Polymer 2010, 51, 1457−1464. (19) Kreuer, K.-D.; Portale, G. Adv. Funct. Mater. 2013, 23, 5390− 5397. (20) Allen, I. A.; Comolli, L. R.; Kusoglo, A.; Modestino, M. A.; Minor, A. M.; Weber, A. Z. ACS Macro Lett. 2015, 4, 1−5. (21) Kreuer, K.-D. Solid State Ionics 2013, 252, 93−101. (22) Kreuer, K.-D. Chem. Mater. 2014, 26, 361−380. (23) Gebel, G.; Diat, O. Fuel Cells 2005, 5, 261−276. (24) Bass, M.; Berman, A.; Singh, A.; Konovalov, O.; Freger, V. Macromolecules 2011, 44, 2893−2899. (25) DeCaluwe, S. C.; Kienzle, P. A.; Bhargava, P.; Baker, A. M.; Dura, J. A. Soft Matter 2014, 10, 5763−5776. (26) Elliott, J. A.; Wu, D.; Paddison, S. J.; Moore, R. B. Soft Matter 2011, 7, 6820−6827. (27) Kreuer, K.-D.; Paddison, S. J.; Spohr, E.; Schuster, M. Chem. Rev. 2004, 104, 4637−4678. (28) Mantsch, H. H.; Saito, H.; Smith, I. C. P. Prog. Nucl. Magn. Reson. Spectrosc. 1977, 11, 211−271. (29) Deloche, B.; Samulski, E. T. Macromolecules 1981, 14, 575−581. (30) Samulski, E. T. Polymer 1985, 26, 177−189. (31) Gottlieb, H. E.; Luz, Z. Macromolecules 1984, 17, 1959−1964. (32) Xu, G.; Pak, Y. S. Solid State Ionics 1992, 50, 339−343. (33) Chen, R. S.; Jayakody, J. P.; Greenbaum, S. G.; Pak, Y. S.; Xu, G.; McLin, M. G.; Fontanella, J. J. J. Electrochem. Soc. 1993, 140, 889− 895. (34) Rankothge, M.; Haryadi; Moran, G.; Hook, J.; Van Gorkom, L. Solid State Ionics 1994, 67, 241−248. (35) Li, J.; Wilmsmeyer, K. G.; Madsen, L. A. Macromolecules 2008, 41, 4555−4557. (36) Li, J.; Wilmsmeyer, K. G.; Madsen, L. A. Macromolecules 2009, 42, 255−262. (37) Park, J. K.; Li, J.; Divoux, G. M.; Madsen, L. A.; Moore, R. B. Macromolecules 2011, 44, 5701−5710. (38) Li, J.; Park, J. K.; Moore, R. B.; Madsen, L. A. Nat. Mater. 2011, 10, 507. (39) Kyu, T.; Eisenberg, A. In Mechanical Relaxations in Perfluorosulfonate Ionomer Membranes; Eisenberg, A., Yeager, H. L., Eds.; American Chemical Society: Washington, DC, 1982; Vol. 180, Chapter 7, pp 79−110. (40) Bauer, F.; Denneler, S.; Willert-Porada, M. J. Polym. Sci., Part B: Polym. Phys. 2005, 43, 786−795. (41) Tang, Y.; Karlsson, A. M.; Santare, M. H.; Gilbert, M.; Cleghorn, S.; Johnson, W. B. Mater. Sci. Eng., A 2006, 425, 297−304.

With decreasing hydration level, increasing ionic interaction within the aqueous ionic domain stabilizes the uniformity of the flat domain thickness which is essentially preserved during stretching under dry conditions (see Figure S3, red regime). Then, stress is transmitted through the ionic domain deforming on larger scales, which leads to macroscopic anisotropy while the softer polymeric phase tends to adapt to these structural changes (Figure 8c). This is especially true for higher temperature where the polymeric domain severely softens. Where the transition between regimes with prevailing ionic interaction and prevailing hydrophobic interaction takes place (red and green regimes in Figure S3) is afflicted with some uncertainty. In any case, our present and earlier results21 suggest the presence of at least two relevant interactions which is in contrast to other author’s ideas explaining Nafions properties with a single controlling interaction within the entire T/RH regime under consideration (e.g., clustering of hydrophilic domains45). At very high temperature and low relative humidity (see Figure S3, yellow regime), also the ionic domain is suggested to soften. Therefore, only very small stress builds up during the stretching process, and any morphological change tends to relax on a local scale. As a consequence, the nanoscale morphology is virtually unaffected by stretching, and the induced anisotropy is relatively small.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.5b01559. Figures S1−S3 (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (T.B.). *E-mail: [email protected] (K.D.K.). Present Address

T.B.: LMU, Department of Chemistry, Butenandtstrasse 5-13, 81377 München, Germany. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Ewald Schmitt, Udo Klock, Sabine Seiffert, and Uwe Traub for help constructing, maintaining, and computational support of the stretching apparatus, Annette Fuchs and Michael Marino for sample preparation and helpful discussions, and Sina Stobinski for artwork. We also acknowledge Igor Moudrakovski for reading the proofs. Partial funding of the spectrometer hardware by the German Science Foundation (DFG grant BR 3370/5-1) is gratefully acknowledged. A.W. and K.D.K. kindly acknowledge the Bundesministerium für Bildung und Forschung and Energie BadenWürttemberg EnBW for financial support (project PSUMEA 03ET2004A).

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