Time-evolved growth of semi-aromatic polyamide nanofilms and their structure-performance relationship: Mechanistic insights and implications for nanofiltration membrane synthesis

Abstract

The separation performance of thin film composite nanofiltration (NF) membranes is governed by a semi-aromatic polyamide film. Compared to fully aromatic polyamide films with a well-known self-limiting behavior, the growth kinetics of semi-aromatic polyamide films and the corresponding regulation mechanisms have not been fully understood. This study systematically investigated the time-evolved growth of a piperazine (PIP)-based polyamide film at a free interface over prolonged interfacial polymerization reaction time (up to 60 min). For the first time, we revealed a two-stage growth kinetics, with the film growth rate in the later stage (7.0 nm min⁻¹) one order of magnitude slower than the initial rate (68.2 nm min⁻¹) as a result of reduced availability of PIP monomers. This two-stage growth mechanism led to an asymmetric film structure, with compelling characterization results (i.e., crosslinking degree, film density, and pore size) showing that the newly formed polyamide under the reduced PIP availability was much looser than the incipient film. We demonstrated that such asymmetric structure had profound impact on the separation performance of the resulting NF membrane through mechanisms such as internal concentration polarization and gutter effect. The mechanistic insights into the growth-structure-performance relationship of semi-aromatic polyamide films could guide the synthesis of high-performance NF membranes.