Xunda Feng

Scalable fabrication of polymer membranes with vertically aligned 1 nm pores by magnetic field directed self-Assembly

There is long-standing interest in developing membranes possessing uniform pores with dimensions in the range of 1 nm and physical continuity in the macroscopic transport direction to meet the needs of challenging small molecule and ionic separations. Here we report facile, scalabe fabrication of polymer membranes with vertically (i.e., along the through-plane direction) aligned 1 nm pores by magnetic-field alignment and subsequent cross-linking of a liquid crystalline mesophase. We utilize a wedge-shaped amphiphilic species as the building block of a thermotropic columnar mesophase with 1 nm ionic nanochannels, and leverage the magnetic anisotropy of the amphiphile to control the alignment of these pores with a magnetic field. In situ X-ray scattering and subsequent optical microscopy reveal the formation of highly ordered nanostructured mesophases and cross-linked polymer films with orientational order parameters of ca. 0.95. High-resolution transmission electron microscopy (TEM) imaging provides direct visualization of long-range persistence of vertically aligned, hexagonally packed nanopores in unprecedented detail, demonstrating high-fidelity retention of structure and alignment after photo-cross-linking. Ionic conductivity measurements on the aligned membranes show a remarkable 85-fold enhancement of conductivity over nonaligned samples. These results provide a path to achieving the large area control of morphology and related enhancement of properties required for high-performance membranes and other applications.

Thin Polymer Films with Continuous Vertically Aligned 1 nm Pores Fabricated by Soft Confinement

Membrane separations are critically important in areas ranging from health care and analytical chemistry to bioprocessing and water purification. An ideal nanoporous membrane would consist of a thin film with physically continuous and vertically aligned nanopores and would display a narrow distribution of pore sizes. However, the current state of the art departs considerably from this ideal and is beset by intrinsic trade-offs between permeability and selectivity. We demonstrate an effective and scalable method to fabricate polymer films with ideal membrane morphologies consisting of submicron thickness films with physically continuous and vertically aligned 1 nm pores. The approach is based on soft confinement to control the orientation of a cross-linkable mesophase in which the pores are produced by self-assembly. The scalability, exceptional ease of fabrication, and potential to create a new class of nanofiltration membranes stand out as compelling aspects.

Aligned Nanostructured Polymers by Magnetic-Field-Directed Self-Assembly of a Polymerizable Lyotropic Mesophase

Magnetic-field-directed assembly of lyotropic surfactant mesophases provides a scalable approach for the fabrication of aligned nanoporous polymers by templated polymerization. We develop and characterize a lyotropic liquid crystalline system containing hexagonally packed cylindrical micelles of a polymerizable surfactant in a polymerizable solvent. The system exhibits negative magnetic anisotropy, resulting in the degenerate alignment of cylindrical micelles perpendicular to the magnetic field. Sample rotation during field alignment is used to effectively break this degeneracy and enable the production of uniformly well-aligned mesophases. High-fidelity retentions of the hexagonal structure and alignment were successfully achieved in polymer films produced upon UV exposure of the reactive system. The success of this effort provides a route for the fabrication of aligned nanoporous membranes suitable for highly selective separations, sensing, and templated nanomaterial synthesis.

Highly Selective Vertically Aligned Nanopores in Sustainably Derived Polymer Membranes by Molecular Templating

We describe a combination of molecular templating and directed self-assembly to realize highly selective vertically aligned nanopores in polymer membranes using sustainably derived materials. The approach exploits a structure-directing molecule to template the assembly of plant-derived fatty acids into highly ordered columnar mesophases. Directed self-assembly using physical confinement and magnetic fields provides vertical alignment of the columnar nanostructures in large area (several cm2) thin films. Chemically cross-linking the mesophase with added conventional vinyl comonomers and removing the molecular template results in a mechanically robust polymer film with vertically aligned 1.2-1.5 nm diameter nanopores with a large specific surface area of ∼670 m2/g. The nanoporous polymer films display exceptional size and charge selectivity as demonstrated by adsorption experiments using model penetrant molecules. These materials have significant potential to function as high-performance nanofiltration membranes and as nanoporous thin films for high-density lithographic pattern transfer. The scalability of the fabrication process suggests that practical applications can be reasonably anticipated.

Enhanced antibacterial activity through the controlled alignment of graphene oxide nanosheets

Significance In biomedical and environmental applications, as well as manufacture and disposal, the interaction of graphene-based nanomaterials (GBNs) with living cells is inevitable and sometimes crucial. While the cytotoxic properties of GBNs are well established, the mechanisms behind the cytotoxicity remain controversial. In this study, we first utilize a magnetic field to form films with aligned graphene oxide (GO), showing that the alignment of sharp GO edges plays a crucial role in the antibacterial activity. We then demonstrate using model systems that GO unequivocally induces physical disruption of lipid bilayers and that oxidation stems from a direct electron transfer mechanism. Altogether, our results elucidate the physicochemical, edge-based cytotoxicity of GBNs, while providing guidance for the design of engineered surfaces using GBNs. The cytotoxicity of 2D graphene-based nanomaterials (GBNs) is highly important for engineered applications and environmental health. However, the isotropic orientation of GBNs, most notably graphene oxide (GO), in previous experimental studies obscured the interpretation of cytotoxic contributions of nanosheet edges. Here, we investigate the orientation-dependent interaction of GBNs with bacteria using GO composite films. To produce the films, GO nanosheets are aligned in a magnetic field, immobilized by cross-linking of the surrounding matrix, and exposed on the surface through oxidative etching. Characterization by small-angle X-ray scattering and atomic force microscopy confirms that GO nanosheets align progressively well with increasing magnetic field strength and that the alignment is effectively preserved by cross-linking. When contacted with the model bacterium Escherichia coli, GO nanosheets with vertical orientation exhibit enhanced antibacterial activity compared with random and horizontal orientations. Further characterization is performed to explain the enhanced antibacterial activity of the film with vertically aligned GO. Using phospholipid vesicles as a model system, we observe that GO nanosheets induce physical disruption of the lipid bilayer. Additionally, we find substantial GO-induced oxidation of glutathione, a model intracellular antioxidant, paired with limited generation of reactive oxygen species, suggesting that oxidation occurs through a direct electron-transfer mechanism. These physical and chemical mechanisms both require nanosheet penetration of the cell membrane, suggesting that the enhanced antibacterial activity of the film with vertically aligned GO stems from an increased density of edges with a preferential orientation for membrane disruption. The importance of nanosheet penetration for cytotoxicity has direct implications for the design of engineering surfaces using GBNs.

Controlling orientational order in block copolymers using low-intensity magnetic fields

Significance Magnetic field interactions with condensed matter can produce orientationally ordered states that are important for fundamental research and technological applications. Block copolymer (BCP) mesophases typically exhibit weak field coupling, requiring high-intensity fields generated by superconducting magnets to produce such states. This work advances a strategy for circumventing such field intensity limitations and creates highly aligned mesophases using fields an order of magnitude smaller than typically required and that can be produced by simple permanent magnets. We elucidate the roles of molecular mobility, grain size, and ordering kinetics on the mesophase field response. Low-intensity field-directed BCP ordering has potentially profound implications for processing functional materials and developing complex textures by field shaping. The interaction of fields with condensed matter during phase transitions produces a rich variety of physical phenomena. Self-assembly of liquid crystalline block copolymers (LC BCPs) in the presence of a magnetic field, for example, can result in highly oriented microstructures due to the LC BCP’s anisotropic magnetic susceptibility. We show that such oriented mesophases can be produced using low-intensity fields (<0.5 T) that are accessible using permanent magnets, in contrast to the high fields (>4 T) and superconducting magnets required to date. Low-intensity field alignment is enabled by the addition of labile mesogens that coassemble with the system’s nematic and smectic A mesophases. The alignment saturation field strength and alignment kinetics have pronounced dependences on the free mesogen concentration. Highly aligned states with orientation distribution coefficients close to unity were obtained at fields as small as 0.2 T. This remarkable field response originates in an enhancement of alignment kinetics due to a reduction in viscosity, and increased magnetostatic energy due to increases in grain size, in the presence of labile mesogens. These developments provide routes for controlling structural order in BCPs, including the possibility of producing nontrivial textures and patterns of alignment by locally screening fields using magnetic nanoparticles.

Polymer Nanosheets from Supramolecular Assemblies of Conjugated Linoleic Acid–High Surface Area Adsorbents from Renewable Materials

We present a strategy for robustly cross-linking self-assembled lamellar mesophases made from plant-derived materials to generate polymer nanosheets decorated with a high density of functional groups. We formulate a supramoleclar complex by hydrogen-bonding conjugated linoleic acid moieties to a structure-directing tribasic aromatic core. The resulting constructs self-assemble into a thermotropic lamellar mesophase. Photo-cross-linking the mesophase with the aid of an acrylate cross-linker yields a polymeric material with high-fidelity retention of the lamellar mesophase structure. Transmission electron microscopy images demonstrate the preservation of the large area, highly ordered layered nanostructures in the polymer. Subsequent extraction of the tribasic core and neutralization of the carboxyl groups by NaOH result in exfoliation of polymer nanosheets with a uniform thickness of ∼3 nm. The nanosheets have a large specific area of ∼800 m2/g, are decorated by negatively charged carboxylate groups at a density of 4 nm-2, and exhibit the ability to readily adsorb positively charged colloidal particles. The strategy as presented combines supramolecular self-assembly with the use of renewable or sustainably derived materials in a scalable manner. The resulting nanosheets have potential for use as adsorbents and, with further development, rheology modifiers.

Janus Graft Block Copolymers: Design of a Polymer Architecture for Independently Tuned Nanostructures and Polymer Properties

The graft-through synthesis of Janus graft block copolymers (GBCPs) from branched macromonomers composed of various combinations of homopolymers is presented. Self-assembly of GBCPs resulted in ordered nanostructures with ultra-small domain sizes down to 2.8 nm (half-pitch). The grafted architecture introduces an additional parameter, the backbone length, which enables control over the thermomechanical properties and processability of the GBCPs independently of their self-assembled nanostructures. The simple synthetic route to GBCPs and the possibility of using a variety of polymer combinations contribute to the universality of this technique.

Directed Assembly of Hybrid Nanomaterials and Nanocomposites

Hybrid nanomaterials are molecular or colloidal-level combinations of organic and inorganic materials, or otherwise strongly dissimilar materials. They are often, though not exclusively, anisotropic in shape. A canonical example is an inorganic nanorod or nanosheet sheathed in, or decorated by, a polymeric or other organic material, where both the inorganic and organic components are important for the properties of the system. Hybrid nanomaterials and nanocomposites have generated strong interest for a broad range of applications due to their functional properties. Generating macroscopic assemblies of hybrid nanomaterials and nanomaterials in nanocomposites with controlled orientation and placement by directed assembly is important for realizing such applications. Here, a survey of critical issues and themes in directed assembly of hybrid nanomaterials and nanocomposites is provided, highlighting recent efforts in this field with particular emphasis on scalable methods.