Asymmetric polyamide nanofilms with highly ordered nanovoids for water purification

Abstract

Tailor-made structure and morphology are critical to the highly permeable and selective polyamide membranes used for water purification. Here we report an asymmetric polyamide nanofilm having a two-layer structure, in which the lower is a spherical polyamide dendrimer porous layer, and the upper is a polyamide dense layer with highly ordered nanovoids structure. The dendrimer porous layer was covalently assembled in situ on the surface of the polysulfone (PSF) support by a diazotization-coupling reaction, and then the asymmetric polyamide nanofilm with highly ordered hollow nanostrips structure was formed by interfacial polymerization (IP) thereon. Tuning the number of the spherical dendrimer porous layers and IP time enabled control of the nanostrips morphology in the polyamide nanofilm. The asymmetric polyamide membrane exhibits a water flux of 3.7−4.3 times that of the traditional monolayer polyamide membrane, showing an improved divalent salt rejection rate (more than 99%), which thus surpasses the upper bound line of the permeability−selectivity performance of the existing various structural polyamide membranes. We estimate that this work might inspire the preparation of highly permeable and selective reverse osmosis (RO), organic solvent nanofiltration (OSNF) and pervaporation (PV) membranes.

High‐Precision and High‐Flux Separation by Rationally Designing the Nanochannels and Surface Nanostructure of Polyamide Nanofiltration Membranes

“…Meanwhile, the more negatively charged membrane surfaces of the TFN membrane in the basic region (pH > 7) are ascribed to the larger number of carboxylic acid groups. [ 45 ] Note that the anionic CDs are embedded below the polyamide surface layer as revealed by the XPS measurement and hence do not contribute to the negative surface charges of the TFN membranes. With the largest number of the unreacted amine and carboxylic acid groups, the TFN‐C 8 ‐CDs membrane exhibits highest hydrophilicity, followed by the TFN‐C 4 ‐CDs, TFN‐C 1 ‐CDs, and TFC membranes, as indicated by the water contact angle analysis (Figure 2g).…”

Proteoliposome-Incorporated Seawater Reverse Osmosis Polyamide Membrane: Is the Aquaporin Water Channel Effect in Improving Membrane Performance Overestimated?

“…By re-examining the microscopic images of the AQP-based TFC PA membranes in prior studies, ,− we found that many proteoliposome/liposome-incorporated PA layers had enlarged protuberances. Such changes in the PA morphology were usually ignored for AQP-based TFC PA membranes although they could be one of the important factors for enhancing membrane separation performance. − Hence, the real role of AQPs in AQP-based PA membranes demands more comprehensive investigation. In addition, the existing study mainly focused on the development of AQP-based PA brackish water RO membranes for brackish water desalination .…”

Zwitterionic Copolymer-Regulated Interfacial Polymerization for Highly Permselective Nanofiltration Membrane

“…Generally, PA thin film composite (TFC) nanofiltration membranes are mass-produced on porous substrates with interfacial polymerization (IP), where an irreversible step-growth polymerization between the piperazine (PIP) aqueous phase and trimesoyl chloride (TMC) organic phase occurs at the water/organic interface using the Schotten–Baumann reaction. , However, such uncontrolled diffusion and fast polymerization processes tend to facilitate the formation of thick PA layers (∼200 nm) with highly cross-linked structures (Figure a), which severely restrict the rapid transport of water molecules and degenerates the precise molecular sieving capability of the membrane. , Progressive efforts have been dedicated to understanding the IP mechanism, to simultaneously improve the membrane permeance and selectivity toward target solutes . This improvement can be achieved by precisely controlling the kinetics of the IP by manipulating the stoichiometric equilibrium at the interface .…”

“…This result agrees well with the trend of the Freger equation (Figure S10), wherein the thickness of the PA layer is proportional to one-third of the diffusivity of the amine-based PIP monomers toward the organic phase (as shown in eq 1 in the Supporting Information). Additionally, the formation of a ridge-shaped morphology after P­[MPC- co -AEMA] incorporation can be ascribed to the synergistic effects of (1) the differences in the aqueous template formed on the substrate surfaces, since more hydrophilic poly­[MPC] moieties tend to absorb more aqueous microphase that takes shapes on the substrate, which may directly influence the transition of the membrane surface morphology from a relatively smooth 2D-like nodular structure (Figure a) to more crumpled 3D-like striped (Figure d) surface microstructures. − (2) The diffusion-driven instability caused by the hindrance effect at the water/organic interface, where the self-assembled P­[MPC- co -AEMA] network may alter the diffusion of the amine-dispersed aqueous phase (PIP molecules) toward the organic phase (as demonstrated in Figure c–e), thus forming a ridge-shaped structure consisting of closely packed hexagonal arrays or interconnected labyrinthine networks. , …”

“…28−30 (2) The diffusion-driven instability caused by the hindrance effect at the water/organic interface, where the self-assembled P[MPC-co-AEMA] network may alter the diffusion of the amine-dispersed aqueous phase (PIP molecules) toward the organic phase (as demonstrated in Figure 3c−3e), thus forming a ridge-shaped structure consisting of closely packed hexagonal arrays or interconnected labyrinthine networks. 8,15 By considering the balance between the rejection ratio and water permeance of the TFC membranes, the optimum concentrations of PIP (1 wt %) and TMC (0.15 wt %%), as well as the IP duration (60 s), were selected for the preparation of all the TFC membranes (Figures S11−S13). According to the results in Figure 4a and 4b, the Co@PA TFC membranes exhibited an increase in the water permeance from 4.1 for pristine PA to 15.7 L•m −2 •h −1 •bar −1 for Co_7:3@PA, after embedding P[MPC-co-AEMA] with additional hydrophilic poly[MPC] segments.…”