Mechanistic insights of nanofillers tuning polymer microstructures for enhanced separation performance of nanofiltration membrane

Nanomaterials, added in small quantities as “nanofillers,” have been shown to improve the performance of NF membranes in terms of mechanical properties, anti-fouling characteristics, selectivity, and transport. The goal of this work is to explore the mechanisms of nanofiller improvements on selectivity and transport of NF membranes. Carboxylated cellulose nanocrystals, multi-walled carbon nanotubes, and graphene oxide were selected as nanofillers to be added in the fabrication of free-standing polyamide (PA) films for the characterization of the internal structure of the polymer materials. Investigations of polymer characteristics revealed that the PA film with nanofillers was a semi-crystalline cross-linked polymer that exhibited an increase in chain length and free volume, and a reduction of cross-linkage and crystallite size. Molecular dynamics simulation results showed that the growth of free volume caused by polymeric chain stacking allowed water molecules to readily penetrate the spaces between polymer chains, which was primarily responsible for the enhanced permeability of PA films with nanofillers. The changes in the crystallite structures influenced the dielectric properties of the membrane matrix and the confined water in the pores, leading to the enhancement of dielectric exclusion, which can be harnessed to improve the separation performance. Taken together, our findings elucidated the structural changes of PA polymers induced by nanomaterials governing the properties and performance of NF membranes, which should be considered in the design of high-performance NF membranes.