The interaction of H3PO4 and steam with PBI and TPS polymeric membranes. A TGA and Raman study

Abstract

Thermogravimetric analysis (TGA) and Raman spectroscopic experiments were carried out in order to study the interaction of H₃PO₄ and steam with the chemical structure of polybenzimidazole (PBI) and pyridine bearing aromatic polyethers (TPS). The interaction of the polymers with H₃PO₄ causes a blue Raman shift on the imidazole and the pyridine bands, which is larger for the imidazole ring. It has been found that one H₃PO₄ molecule is needed to neutralize one imidazole group, while ca. five H₃PO₄ molecules are necessary for full protonation of the pyridine group. This observation is reflected on the hydration procedure of the imbibed membranes at high temperatures (150–170 °C) as this was studied by TGA. At low acid doping levels (130–150 wt%) the hydrolysis of pyrophosphoric acid into ortho-phosphoric acid is not reversible in the case of PBI, thus indicating that the chemical structure influences the chemical equilibrium of the hydrolysis reaction. The evaporation rate of H₃PO₄ increases with increasing steam partial pressure and is lower in the case of PBI than in TPS. Nevertheless in all cases the evaporation rate is by a factor of two lower than that of the pure phosphoric acid under the same steam partial pressure.