Ultrathin
polyamide nanofilms are desirable as the separation layers for the
highly permeable thin-film composite (TFC) membranes, and recently,
their lowest thickness limits have attracted a lot of attention from
researchers. Due to the interference of the underlying substrate,
preparing a defect-free, ultrathin polyamide nanofilm directly on
top of a membrane substrate remains a great challenge. Herein, we
report a novel fabrication technique of TFC membranes, named in situ free interfacial polymerization (IFIP), where the
IP reaction occurs at the uniform, free oil–water interface
dozens of microns above the substrate, and then the resulting nanofilm
spontaneously assembles into the TFC structure without extra manual
transfer. This IFIP method not only overcomes the limitations of conventional
IP, succeeding in preparing ultrathin–nanofilm composite membranes
for nanofiltration and reverse osmosis application, but also enables
scale membrane manufacturing that is not feasible via previously reported
free-standing IP. Based on the IFIP method, the thickness of the polyamide
nanofilm was successfully reduced to ca. 3–4 nm, which we believe
is close to the ultrathin limit of the polyamide nanofilm for separation
application. Meanwhile, the structure–performance relationship
revealed that the strategy of increasing TFC membrane permeance by
reducing polyamide layer thickness also had a limit. Besides, the
IP mechanisms in regard to the formation of surface morphology and
film growth were explored by combining experimental and molecular
simulation methods. Overall, this work is expected to push forward
the fundamental study and practical application of the ultrathin–film
composite membrane.
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