Unraveling the Morphology–Function Relationships of Polyamide Membranes Using Quantitative Electron Tomography

Abstract

An understanding of how complex nanoscale morphologies emerge from synthesis would offer powerful strategies to construct soft materials with designed structures and functions. However, these kinds of morphologies have proven difficult to characterize, and therefore manipulate, because they are three-dimensional (3D), nanoscopic, and often highly irregular. Here, we studied polyamide (PA) membranes used in wastewater reclamation as a prime example of this challenge. Using electron tomography and quantitative morphometry, we reconstructed the nanoscale morphology of 3D crumples and voids in PA membranes for the first time. Various parameters governing film transport properties, such as surface-to-volume ratio and mass-per-area, were measured directly from the reconstructed membrane structure. In addition, we extracted information inaccessible by other means. For example, 3D reconstruction shows that membrane nanostructures are formed from PA layers 15–20 nm thick folding into 3D crumples which envelope up to 30% void by volume. Mapping local curvature and thickness in 3D quantitatively groups these crumples into three classes, “domes”, “dimples”, and “clusters”, each being a distinct type of microenvironment. Elemental mapping of metal ion adsorption across the film demonstrates that these previously missed parameters are relevant to membrane performance. This imaging–morphometry platform can be applicable to other nanoscale soft materials and potentially suggests engineering strategies based directly on synthesis–morphology–function relationships.

Mechanism and performance relevance of nanomorphogenesis in polyamide films revealed by quantitative 3D imaging and machine learning

“…We used low-dose rate (4 to 7 e − Å −2 s −1 ) electron tomography with no staining ( 11 ) to minimize beam-induced alteration and reconstruct the 3D nanostructure (with a final voxel size of 6.8 Å) of polyamide membranes synthesized with a range of monomer concentrations and a fixed reaction time (movie S1). Their synthesis by interfacial polymerization involves an aqueous phase containing an amine monomer [ m -phenylenediamine (MPD) here] and an organic phase containing an acyl chloride monomer [here, trimesoyl chloride (TMC) in hexane] ( Fig.…”

2D Material Based Thin‐Film Nanocomposite Membranes for Water Treatment

“…According to previous works on the mechanism of IP, the commonalities are that the faster the diffusion rate of the aqueous monomer, the thicker the PA film and the rougher surface. [69][70][71][72][73] Generally, after IP consumes the amide monomer at the interface, other amide monomers in the solution will diffuse to the interface and participate in the reaction. This maintains the localized concentration of the monomer at the reaction interface.…”

Analysis of membrane fouling by Brunauer-Emmett-Teller nitrogen adsorption/desorption technique

“…below pore size of 2 nm. Porosities can be determined also by scanning or transmission electron microscopy imaging, but due to the resolution and contrast limitations pores with size of less than few nanometers cannot be usually detected 26,27 .…”