Chemically recyclable nanofiltration membranes fabricated from two circular polymer classes of the same monomer origin

Nanofiltration is widely used in various industries to separate solutes from solvents. To foster a circular plastic economy, establishing a closed-loop lifecycle for the membrane materials is highly important. In this study, we fabricated recyclable nanofiltration membranes from chemically recyclable polymers —polyester P(BiL=)ROP and poly(cyclic olefin) P(BiL=)ROMP— using γ-butyrolactone as a green solvent. These two distinct polymers were obtained from a single monomer, which could be recycled back to the parent monomer, exhibiting the unique "one monomer–two polymers–one monomer" closed-loop chemical circularity. The effect of physical treatment, such as annealing, hot-pressing, and air exposure on the morphological characteristics and performance of the nanofiltration membranes was investigated. We revealed the interplay between membrane pore size, thickness, density and the molecular sieving performance of the nanofiltration membranes. Solute rejections were mainly governed by the membrane pore size. However, solvent flux was mainly governed by the membrane density that determines the free volume interconnectivity. The membranes exhibited a tunable molecular weight cutoff between 553 and 777 g mol−1 and methanol permeance between 5.9 and 9.8 L m−2 h−1 bar−1. The membranes exhibited excellent long-term nanofiltration stability over 1 week. The combination of the green solvent used for membrane fabrication and the circular life cycle of the polymer membrane brings one step closer to closing the sustainability loop of membrane technology.