Deformation in Layer-by-Layer Hollow Fiber Nanofiltration Membranes: Compaction–Expansion Model

High operating pressure causes membrane structural deformation, thereby affecting the separation performance, long-term stability, and operational efficiency. This paper reported for the first time the impact of hydraulic pressure on hollow fiber (HF) layer-by-layer (LBL) nanofiltration (NF) membranes. The evolution of microdeformation and consequent performance degradation of LBL HF membranes was analyzed by focusing on the Young's modulus of the substrate. Irreversible radial compaction and circumferential expansion under high pressure in a poly(ether sulfone) (PES) substrate were observed in the membrane morphology and performance changes. The lower the substrate Young's modulus, the more pronounced the deformation at the same operating pressure and the lower the critical pressure at which performance begins to deteriorate. In responding to the deformation of the substrate, we propose a compaction-expansion model where the LBL skin layer imbedded into the substrate pore exhibits a three-stage response: (1) compaction, (2) limited stretching, and (3) significant stretching. Beyond stage (3), when stress was released, the decreased separation performance was irreversible. The experimental observations and modeling provided a physical origin of the performance degradation of the LBL NF membranes at high pressure. This work offered valuable practical and theoretical insights for the development of highly mechanically stable LBL HF membranes.