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Reports: AC7 48373-AC7: Synthesis of Novel Cross-Linked Fluoropolymer Networks and Nanocomposites with High Thermal Stability and Proton Conductivity
We have prepared the copolymer of vinylidene fluoride (VDF) and perfluoro(4-methyl-3,6-dioxane-7-ene) sulfonyl fluoride (PFSVE) containing amino terminal groups by using 4-tert-butoxycarbonylamino benzoyl peroxide as the initiator. The H NMR spectra of the prepared P(VDF-PFSVE)s clearly showed the signals at 1.58, 7.54 and 8.17 ppm corresponding to the protons from tert-butyl and phenyl groups, respectively. Removal of the protecting tert-butoxycarbonyl groups in the copolymers using iodotrimethylsilane yielded the copolymers terminated with amino groups, evidenced by complete disappearance of the peaks from tert-butyl protons at 1.58 ppm and emergence of a singlet at 6.19 ppm attributed to amines in the H NMR spectra. End-group analysis revealed an average degree of functionality of 1.9. By controlling the monomer feed ratio and the reaction condition, P(VDF-PFSVE) copolymers with the PFSVE content ranging from 5.3 to 13.1 mol % were obtained.
The cross-linked network was then formed via thermal condensation between the amine end groups of P(VDF-PFSVE) and a curing agent, 1,3,5-benzene triisocyanate. FTIR confirmed the formation of urea linkages by appearance of the characteristic urea bands at 3348 cm-1 (N-H stretching vibration), 1632 cm-1 (C-N stretching), and 1546 cm-1 (N-H bending) and absence of the absorbance at 2267 cm-1 assigned to isocyanate groups in the cross-linked membrane. Hydrolysis of the sulfonyl fluoride groups to the ammonium salt was achieved in triethylamine methanol/water solution and the sulfonic acid form of the polymer was obtained by ion-exchange with HCl aqueous solution, respectively. The chemical transformation was verified by disappearance of the characteristic bands of -SO2F groups at 1462 cm-1 (S=O stretching vibration) and 816 cm-1 (S-F stretching vibration) and appearance of a broad peak attributed to acid groups at 3000 to 3300 cm-1 in the FTIR spectra of the membranes after hydrolysis. The band at 1632 cm-1 was found to remain in the spectra of the acid form membranes, suggesting that the urea linkages were kept intact during hydrolysis.
It was found that the prepared cross-linked membranes possess excellent thermal, hydrolytic and oxidative stabilities, similar to Nafion 117. Compared with Nafion 117 film that has an ion exchange capacity (IEC) of 0.91 meq/g and a water uptake of 29 wt.%, the cross-linked membranes II and III with similar IEC values of 0.84 and 0.95 meq/g exhibited much lower water uptakes, only 14 and 19 wt.% respectively. In addition, it is worthwhile to note that little change of water uptake was observed in the cross-linked membranes as temperature was increased to 60 oC, implying the cross-linking in the materials stabilized the water domain structures.
The proton conductivity measurements were conducted in water at 30 oC. Methanol permeability measurements were performed through a membrane-separated diffusion cell geometry using 1 M methanol aqueous solution at 30 oC. As expected, in accordance with water uptake and λ, the proton conductivity and methanol permeability increased progressively with IEC. Although the cross-linked membranes generally displayed a lower proton conductivity than Nafion 117, the membranes II, III and IV maintain the proton conductivities at the same order of magnitude as Nafion 117 (i.e. >0.02 S/cm). Remarkably, the membranes II and III with similar IECs as Nafion 117 exhibit methanol permeability of 1.89 x 10-9 and 4.98 x 10-8 cm2/s respectively, which are over two orders of magnitude lower than Nafion 117 with a methanol permeability of 1.92 x 10-6 cm2/s. The selectivities of the membranes II and III are 288 and 22 times, respectively, higher than that of Nafion 117. To the best of our knowledge, this is the highest electrochemical selectivity reported from PEMs with proton conductivities higher than 0.01 S/cm.
The membranes were studied by using cross-sectional transmission electron microscopy (TEM). The dark regions in TEM images are lead acetate-stained ionic domains, and the white areas represent the hydrophobic regions of the polymer nanostructure. It is clear from the TEM micrographs that the sizes of the ionic aggregates in the cross-linked membranes are much smaller than those in Nafion. Apparently, the formation of cross-linked structures restricts the size of hydrophilic domains, and thus limits water absorption to yield smaller conduction channels. It appears that the narrowing of the transport channels and reduced water content affects the diffusion of methanol much more significantly than proton transport in the cross-linked membranes. The selectivity characteristics can also be rationalized based on the state of the absorbed water within the membranes. It has been suggested that the permeation of methanol through ionic membranes is strongly correlated to the unbound or free water. The water binding properties of the membranes were evaluated using low temperature differential scanning calorimetry. The heat of fusion for the water in the membranes was computed using ΔHf = H/MH20, where ΔHf is the heat of fusion for the water contained in the sample, MH2O is the mass of water within the sample, and H is the integrated energy from the melting endotherm. As the water molecules that strongly bound to ionic groups do not contribute to the melting endotherm in DSC measurements, ΔHf indicates the concentration of freezable water (i.e. free and weakly bound water) in the membranes. It was found that the cross-linked membranes exhibited lower ΔHf of the absorbed water than Nafion, consistent with the results on water uptake and λ. The lower ΔHf values suggest a larger fraction of tightly bound water within the cross-linked structures, and thus are correlated to lower methanol permeability.
The influence of the methanol crossover has been demonstrated in the preliminary membrane-electrode assembly (MEA) studies. The open circuit voltages of the membranes II and III under 3 M methanol/air conditions at 50 oC were found in a range of 0.75 to 0.85 V, which are much greater than Nafion 117 (~0.67 V).
In summary, we have demonstrated a new route, using chain-end functionalization of perfluorosulfonic acid polymers, to prepare proton conductive polymer networks showing unprecedented electrochemical selectivities. Considering the facile cross-linking conditions and controlled molecular structures, this strategy is expected to open up new opportunities in the development of high-performance ion-containing polymer membranes for fuel cells.
