Highly durable superhydrophobic bilayer nanofibrous composite membrane with intermediate interlocked network inspired by mortise and tenon connections for membrane distillation

Abstract

The membranes for membrane distillation (MD) posed challenges for long-term stable operation due to poor mechanical strength, low flux and susceptibility to wetting. In this study, inspired by conventional mortise and tenon (MT) structure, we constructed a novel robust and porous bilayer composite membrane consisting of superhydrophobic microsphere layer and nanofibrous substrate along with interfacial interlocked networks via a facile integrated casting-recrystallization (ICR) method for highly efficient direct contact membrane distillation (DCMD). During one-step ICR process, amorphous polypropylene (aPP) and isotactic polypropylene (iPP) (a-iPP) with certain mass ratio were completely dissolved in xylene and then cast on the surface of highly porous poly(vinylidene fluoride) (PVDF) nanofibrous substrate at high temperature, in which a crystallization process of the mixed solution occurred in a single step to form PP microsphere layer with a flower-like structure for guarantee of the superhydrophobicity and permeability of the composite membrane. Meanwhile, PP solution infiltrated into the PVDF nanofibrous substrate and then solidified along the fibers and fiber junctions at the initial pouring to create intermediate interlocking connections based on MT construction between the nanofibrous substrate and the microsphere layer, which resulted in the composite membrane with extremely high structural integrity and mechanical properties. The optimal a-iPP/PVDF composite membranes exhibited outstanding mechanical properties (36.6 MPa in tensile strength and 118.0% in strain), significantly superior to PVDF electrospun nanofibrous membrane and commercial PVDF membrane. This unique a-iPP composite membrane with unrelenting superhydrophobicity and high permeability demonstrated a complete barrier to salts with a considerable permeation flux of 54 kg m⁻² h⁻¹ in a 70-h DCMD test (ΔT = 40 °C).