Rigid twisted structured PA membranes for organic solvent nanofiltration via co-solvent assisted interfacial polymerization

Interfacial polymerization has always been a research hotspot for the preparation of polyamide (PA) membranes for molecular sieving. In previous reports, water-soluble amines (such as piperazine (PIP), m-phenylenediamine (MPD), etc.) were mostly used as monomers containing amino functional groups in the aqueous phase to conduct interfacial polymerization with 1,3,5-mesotribenzoyl chloride (TMC) in hexane, resulting in a dense PA film. However, this approach limits the diversity of polymer structures in PA films. To introduce water-insoluble 9,9-Bis(4-aminophenyl)fluorene (BAF) with a rigid twisted structure into interfacial polymerization, this study reports the interfacial polymerization of BAF in DMF/water solution with TMC in n-hexane. BAF-PA films with a larger free volume compared with MPD-PA were successfully prepared. The anomalous interfacial diffusion phenomenon was explained by the interfacial properties and molecular diffusion mechanism of the two-phase solvent and was confirmed by observing the microscopic morphology of the film. Since the structure of BAF-PA has a larger free volume, the BAF-PA film prepared by co-solvent interfacial polymerization has higher permeance to methanol (23.88 L m−2 h−1 bar−1). Meanwhile, it has more than 90% rejection of dyes with molecular weights of 585 Da and above. The large free volume PA film prepared with DMF/water mixed solvent exhibited excellent performance in organic solvent nanofiltration. This method also offers the possibility of introducing other water-insoluble amines into the interfacial polymerization to prepare PA films.