Rapid Fabrication by Lyotropic Self-Assembly of Thin Nanofiltration Membranes with Uniform 1 Nanometer Pores

Abstract

Nanostructured materials with precisely defined and water-bicontinuous 1-nm-scale pores are highly sought after as advanced materials for next-generation nanofiltration membranes. While several self-assembled systems appear to satisfy this need, straightforward fabrication of such materials as submicron films with high-fidelity retention of their ordered nanostructure represents a nontrivial challenge. We report the development of a lyotropic liquid crystal mesophase that addresses the aforementioned issue. Films as thin as ∼200 nm are prepared on conventional support membranes using solution-based methods. Within these films, the system is composed of a hexagonally ordered array of ∼3 nm diameter cylinders of cross-linked polymer, embedded in an aqueous medium. The cylinders are uniformly oriented in the plane of the film, providing a transport-limiting dimension of ∼1 nm, associated with the space between the outer surfaces of nearest-neighbor cylinders. These membranes exhibit molecular weight cutoffs of ∼300 Da for organic solutes and are effective in rejecting dissolved salts, and in particular, divalent species, while exhibiting water permeabilities that rival or exceed current state-of-the-art commercial nanofiltration membranes. These materials have the ability to address a broad range of nanofiltration applications, while structure–property considerations suggest several avenues for potential performance improvements.

Tunable organic solvent nanofiltration in self-assembled membranes at the sub–1 nm scale

“…Hence, the rejection data with PEG provide a realistic representation of the minimum dimension of the slit-like pores in between cylinders. This situation contrasts with that of highly anisotropic solutes that could, in principle, transit through the slit-like pores despite having a molar mass above the nominal cutoff ( 39 ). The measured hydraulic permeance is in good agreement with theoretical estimates made for ordered planar oriented cylinders ( 45 ) and a hypothetical mixture of perpendicular and planar orientations ( 46 ), as shown in fig.…”

“…A recently developed strategy addresses these challenges however. The internally cross-linked cylinders from a direct lyotropic hexagonal (H I ) mesophase present an attractive medium for nanofiltration, because of the bicontinuous nature of the solvent transport and the well-defined ~1-nm slit-like pores provided by the space between the surfaces of nearest-neighbor cylinders uniformly oriented in the plane of the film ( 38 , 39 ).…”

Sub-3-Nanometer Domain Spacings of Ultrahigh-χ Multiblock Copolymers with Pendant Ionic Groups

“…Utilization of self-assembled materials is an essential technology for producing nanostructured materials such as nanowires, , nanodots, , and nanoporous films. − Nanostructured materials made by using self-assembled templates of various shapes and length scales exhibit a wide variety of material properties that enable new applications for semiconductors, , metamaterials, , and 2D materials. − Among the various templating materials, block copolymers have emerged as exceptionally versatile materials, as they can self-assemble into ordered nanostructures with sub-10-nm feature sizes . More recently, many studies have contributed to developing the block copolymers with smaller feature sizes particularly focused on transferring nanopatterns for microelectronic devices. , Synthetically, the underlying mechanism of controlling the domain spacing ( d ) of block copolymers is a well-known scaling to the Flory–Huggins interaction parameter (χ) and degree of polymerization ( N ) .…”

Flexible Nanoporous Materials by Matrix Removal from Cylinder-Forming Diblock Copolymers

“…The above results conceptually resemble recent reports from Osuji and co-workers, in which mechanically stable porous ultrafiltration membranes were prepared by cross-linking shear-aligned, cylindrical surfactant micelles in a continuous aqueous matrix. 60,61 The mechanical stability of their micellar network stems from a water-soluble cross-linker that links the fibrils to resist structural change. Our findings regarding the intrinsic continuity of the cylinders through grain boundaries suggest new design principles for other mechanically stable porous materials derived from this simple and scalable preparation process.…”