A facile approach to construct hierarchical dense membranes via polydopamine for enhanced propylene/nitrogen separation

Bio-inspired polydopamine has been established as a facile and versatile surface modification agent in recent years, many applications have been found such as tailoring the wettability of membrane surfaces, constructing antifouling or antimicrobial surface and serving as a platform for further modifications. To our knowledge, these applications were almost all aimed at liquid systems. For gas separations, however, polydopamine (PDA) was rarely investigated and exploited in literature. In this work, we report a facile method to fabricate ultrathin reverse-selective gas separation membranes (bigger molecules permeate more) via self-polymerization and adhesion of polydopamine onto a thin PDMS interlayer (spin-coated on a porous substrate). The as-prepared hierarchical structured membranes (PDA@PDMS) were used to selectively collect hydrocarbons from nitrogen (propylene/nitrogen as a model gas pair) with considerably enhanced separation selectivity and high gas permeance. Evidence supported that in two ways polydopamine skin layer favorably promoted the overall separation performance. Firstly, polydopamine exhibits much better affinity with propylene than that with nitrogen as indicated by gas sorption test. Secondly, owing to the excellent adhesion property and the dense structure of polydopamine, the in-situ polymerized polydopamine contributes to reducing the overall membrane thickness as well as minimizing the possible microstructural defects. Furthermore, polydopamine provides abundant functional groups for further design of membrane surface chemistry, which we believe would have good potential in other possible gas applications (e.g., carbon capture, olefin/paraffin).