Miscibility behavior of polybenzimidazole/sulfonated polysulfone blends for use in fuel cell applications

Polybenzimidazole (PBI) and polysulfone (PSF) compose an immiscible polymer pair; the introduction however of functional groups such as sulfonate groups in the polymeric chain of PSF resulted in the formation of miscible blends with PBI. The miscibility behavior of a series of blends of PBI with sulfonated PSF (SPSF) at various sulfonation levels has been studied by dynamic mechanical analysis (DMA), FT-IR, and FT-Raman spectroscopy. DMA has shown that the sulfonation degree as well as blend composition controls the miscibility behavior of the studied system. In that respect, partially miscible or miscible blends were obtained when the sulfonation level is higher than 10 mol %. Since both polymers exhibit functional groups, which could participate in specific interactions, this possibility has been examined by FT-IR analysis. In absorption FT-IR spectra of PBI-SPSF specimens with high sulfonation degree and high PBI content, band shifts associated with the NH and sulfonate groups are accounted for the induced miscibility in terms of specific interactions. Moreover, a clear red shift of the symmetric vibration of the main chain sulfone group at 1152 cm(-1) has been also observed in the FT-Raman spectra of miscible PBI-SPSF blends. Careful examination of this spectral feature, assisted by curve-fitting analysis, has revealed that, besides the peaks attributed to pure PBI and PSF, a new scattering contribution, which is accounted for a new associated type of sulfone group, appears as the sulfonation degree of SPSF is increasing. This was partially attributed in terms of specific interactions but also to the Raman sensitivity in the medium polarity changes related to the increase of blend homogeneity. Preliminary results have demonstrated the potential usefulness of the blend polymeric membranes as electrolytes for fuel cells.