es9b05892_si_001.pdf (1.22 MB)
Intrinsic Nanoscale Structure of Thin Film Composite Polyamide Membranes: Connectivity, Defects, and Structure–Property Correlation
journal contribution
posted on 2020-03-05, 18:14 authored by Xiaoxiao Song, Bowen Gan, Saren Qi, Hao Guo, Chuyang Y. Tang, Yong Zhou, Congjie GaoTransport of water, solutes, and
contaminants through a thin film
composite (TFC) membrane is governed by the intrinsic structure of
its polyamide separation layer. In this work, we systematically characterized
the nanoscale polyamide structure of four commercial TFC membranes
to reveal the underlying structure–property relationship. For
all the membranes, their polyamide layers have an intrinsic thickness
in the range of 10–20 nm, which is an order of magnitude smaller
than the more frequently reported apparent thickness of the roughness
protuberances due to the ubiquitous presence of nanovoids within the
rejection layers. Tracer filtration tests confirmed that these nanovoids
are well connected to the pores in the substrates via the honeycomb-like
opening of the backside of the polyamide layers such that the actual
separation takes place at the frontside of the polyamide layer. Compared
to SW30HR and BW30, loose membranes XLE and NF90 have thinner intrinsic
thickness and greater effective filtration area (e.g., by the creation
of secondary roughness features) for their polyamide layers, which
correlates well to their significantly higher water permeability and
lower salt rejection. With the aid of scanning electron microscopy,
transmission electron microscopy, and tracer tests, the current study
reveals the presence of nanosized defects in a polyamide film, which
is possibly promoted by excessive interfacial degassing. The presence
of such defects not only impairs the salt rejection but also has major
implications for the removal of pathogens and micropollutants.