Enhancing the Separation Performance of Aqueous Phase Separation-Based Membranes through Polyelectrolyte Multilayer Coatings and Interfacial Polymerization

Abstract

The aqueous phase separation (APS) technique allows membrane fabrication without use of unsustainable organic solvents, while at the same time, it provides extensive control over membrane pore size and morphology. Herein, we investigate if polyelectrolyte complexation-induced APS ultrafiltration membranes can be the basis for different types of nanofiltration membranes. We demonstrate that APS membranes can be used as support membranes for functional surface coatings like thin polyelectrolyte multilayer (PEMs) and interfacial polymerization (IP) coatings. Three different PEMs were fabricated on poly(sodium 4-styrene sulfonate) (PSS) poly(allylamine hydrochloride) (PAH) APS ultrafiltration membranes, and only 4.5 bilayers were needed to create nanofiltration membranes with molecular weight cut-off (MWCO) values of 210-390 Da while maintaining a roughly constant water permeability (∼1.7 L·m^-2·h^-1·bar^-1). The PEM-coated membranes showed excellent MgCl₂ (∼98%), NaCl (∼70%), and organic micropollutant retention values (>90%). Similarly, fabricating thin polyamide layers on the ultrafiltration PSS-PAH APS membranes by IP resulted in nanofiltration membranes with MWCO values of ∼200 Da. This work shows for the first time that APS membranes can indeed be utilized as excellent support membranes for the application of functional coatings without requiring any form of pretreatment.

Figure 1

Chemical structures of the polyelectrolytes used for performing PEM coatings on the APS membrane supports. (a) Poly(sodium 4-styrene sulfonate) (PSS), (b) poly(allylamine hydrochloride) (PAH), (c) poly(diallyldimethylammonium chloride) (PDADMAC), and (d) branched polyethyleneimine (PEI).

Figure 2

Top surface and cross-section SEM images of (a, b) APS support membrane and the PEM membranes coated with 4.5 bilayers of (c, d) PSS-PAH, (e, f) PSS-PDADMAC, and (g, h) PSS-PEI.

Figure 3

(a) Pure water permeability and molecular weight cut-off. (b) Salt retentions of PSS-PAH_(4.5), PSS-PDADMAC_(4.5), and PSS-PEI_(4.5) membranes. The support membrane showed negligible salt retentions as compared to the enhanced salt retentions by the PEM-coated membranes. The retention tests were conducted at a feed pressure of 4 bar.

Figure 4

FTIR spectra of the ultrafiltration type PSS-PAH support and the IP coated thin-film composite (TFC) membrane. The appearance of absorbance bands at 1545, 1611, and 1660 cm^–1 confirms the presence of polyamide in TFC membranes.

Figure 5

Top surface and cross-section SEM images of the (a and c) APS support membrane and (b and d) IP coated membrane.

Figure 6

Salt retentions of the TFC membranes. Retention tests were conducted using 5 mM salt solution at a feed pressure of 4 bar.