Plasticization mechanism in polybenzimidazole membranes for organic solvent nanofiltration: Molecular insights from in situ FTIR spectroscopy

Despite substantial research efforts being devoted to design polymer membranes for organic solvent nanofiltration (OSN) exhibiting enhanced plasticization resistance, detailed studies of membrane structural stability in chemically challenging environments are rare. This study sets forth a multiscale method, which combines in situ FTIR measurements in the transmission mode with sorption and transport measurements, to investigate the molecular mechanism of polymer plasticization upon exposure to organic species. We recently reported that polybenzimidazole (PBI), a polymer that has been considered for OSN application, experiences severe plasticization upon exposure to methanol. FTIR measurements suggest that the mechanism of PBI plasticization relies on competitive hydrogen bonding. According to this mechanism, methanol molecules disrupt the hydrogen bonded network characteristic of dry PBI, by forming mutual polymer/penetrant hydrogen bonds, which enhances polymer chain mobility and favors polymer plasticization. The analysis of the isosteric heat of sorption supports the picture sketched above. The method developed in this study can be readily extended to many other systems of practical interest in membrane science, to shed fundamental light on the phenomenon of plasticization, whose molecular aspects are still largely unknown. Equally important, this study provides some useful guidelines to design OSN membrane materials.