Piperazine Spacer-Based Co-Diamine-Tailored Pore Structure and Performance of Thin-Film Composite Membranes: Salt and Dye Fractionation

Conventional poly(piperazinamide) [poly(PIP)]-based nanofiltration (NF) membranes possess high rejection of the divalent Na₂SO₄ salt. Consequently, a high operating pressure is needed during the separation of salt and organics by the PIP-based NF membranes, which causes membrane fouling. Herein, a small amount (0.02-0.03% w v^-1) of PIP spacer-containing primary diamine (PDA) is introduced into the aqueous PIP solution to regulate the interfacial polymerization (IP). PDA enlarges the pores and increases the molecular weight cutoff of the membrane. The PDA-modified TFC membranes exhibit characteristic properties of the loose NF (LNF) membrane. PDA slows down the diffusion of PIP toward the interface. The amide bonds formed by the IP between PDA and trimesoyl chloride are heterogeneously distributed in the PA network, generating a limited number of larger pores (3-8%) in the size range of 2-3 nm due to the greater end-to-end amine distance. The PIP spacer (chair conformation) minimizes the pore shrinkage due to its low flexibility. The formed larger pores facilitate the relatively rapid transport of water and salt. The IP between TMC and the mixture of PIP (0.1% w v^-1) and PDA (0.03% w v^-1) gives the LNF-0.03-PDA membrane, which exhibits ∼200% higher water permeation as compared to that of the neat PIP (0.1% w v^-1)-based membrane during fractionation of Direct red 80 + Na₂SO₄. The LNF-0.03-PDA membrane achieves ∼99.9% Congo Red and Direct Red 80 interception in both saline and nonsaline environments. The membrane is stable and has a low fouling propensity during the separation of salt + dye. In contrast to PDA, addition of relatively flexible linear amines into the PIP solution lowers the pore size and permeate flux and enhances the salt rejection of the membranes. Structural features of PDA, such as rigidity-induced pore stability, are crucial to obtain membranes with a tailored pore size and performance.