Dissecting the effect of substrate on polyamide reverse osmosis membranes via regulating substrate pore size by drying shrinkage

Several studies have proved that the substrate significantly influences the chemical properties of reverse osmosis (RO) membranes, whereas the underlying mechanisms remain elusive. Herein, we adjusted the substrate pore size from 8.6 nm to 4.4 nm by drying shrinkage, ensuring the substrate's chemical properties remained consistent. Interestingly, we observed that an intermediate pore size (7.4 nm) of the substrate resulted in the highest exotherm, with the interfacial temperature rising to 26.8 °C, thus showing that larger pore sizes do not necessarily result in more pronounced exothermic interfacial polymerization (IP) reactions. Through multiscale simulations and various characterization techniques, we found that the IP reaction is governed by two competing effects: (1) the storage capacity of MPD, which provides the reactive monomers; and (2) the water content, which absorbs the heat in the IP reaction. Balancing these two competing factors contributes to enhancing the diffusion of MPD, thereby promoting the progression of the IP reaction. This work revealed a new mechanism through which the substrate affects RO membrane formation.