Gradient nanoporous phenolics as substrates for high-flux nanofiltration membranes by layer-by-layer assembly of polyelectrolytes

Thin film composite (TFC) membranes represent a highly promising platform for efficient nanofiltration (NF) processes. However, the improvement in permeance is impeded by the substrates with low permeances. Herein, highly permeable gradient phenolic membranes with tight selectivity are used as substrates to prepare TFC membranes with high permeances by the layer-by-layer assembly method. The negatively charged phenolic substrates are alternately assembled with polycation polyethylenimine (PEI) and polyanion poly(acrylic acid) (PAA) as a result of electrostatic interactions, forming thin and compact PEI/PAA layers tightly attached to the substrate surface. Benefiting from the high permeances and tight surface pores of the gradient nanoporous structures of the substrates, the produced PEI/PAA membranes exhibit a permeance up to 506 L ∙ m⁻² ∙ h⁻¹ ∙ MPa⁻¹, which is ~ 2–10 times higher than that of other membranes with similar rejections. The PEI/PAA membranes are capable of retaining > 96.1% of negatively charged dyes following the mechanism of electrostatic repulsion. We demonstrate that the membranes can also separate positively and neutrally charged dyes from water via other mechanisms. This work opens a new avenue for the design and preparation of high-flux NF membranes, which is also applicable to enhance the permeance of other TFC membranes.