Stable Sequestration of Single-Walled Carbon Nanotubes in Self-Assembled Aqueous Nanopores

We demonstrate the ability to stably sequester individual single-walled carbon nanotubes (SWNTs) within self-contained nanometer-scale aqueous volumes arrayed in an organic continuum. Large areal densities of 4 × 10⁹ cm^–2 are readily achieved. SWNTs are incorporated into a surfactant mesophase which forms 2.3 nm diameter water channels by lyotropic self-assembly. Near-infrared fluorescence spectroscopy demonstrates that the SWNTs exist as well-dispersed tubes that are stable over several months and through multiple cycles of heating and cooling. Absence of physical distortion of the mesophase suggests that the SWNTs are stabilized by adsorbed surfactants that do not extend considerably from the surface. Our findings have important implications for templated assembly of carbon nanotubes using soft mesophases and the development of functional nanocomposites

Morphology Development in Thin Films of a Lamellar Block Copolymer Deposited by Electrospray

Electrospray has been recently advanced as a novel approach for the continuous deposition of self-assembled block copolymer thin films. It represents an analogue of physical vapor deposition in which the development of well-ordered microstructures is predicated on relatively rapid relaxation of the polymer compared to its rate of deposition. Here we describe the morphology development of a lamellae-forming poly­(styrene-<i>b</i>-4-vinylpyridine) deposited by electrospray. Morphology was considered in the context of relative changes of the deposition and relaxation rates, with the latter significantly affected in some cases by the presence of residual solvent. We observe that the presence of residual solvent in deposited material accelerates the equilibration kinetics such that well-ordered alternating lamellar morphologies could be produced at deposition rates as high as 55 nm/min under “wet” spray conditions, whereas hexagonally packed micelles were produced when the polymer was deposited free of solvent, denoted as the “dry” spray limit. Molecular weight (MW) plays an important role in equilibration kinetics in the “dry” limit with a transition from poorly ordered to well-ordered lamellae produced by reducing MW. Film morphology was largely insensitive to temperature and flow rate over a broad range from 150 to 210 °C and from 3 to 18 μL/min respectively, although the orientation of the lamellae switched from parallel to perpendicular at elevated flow rates, potentially due to the influence of rapid solvent evaporation

Dual-Functionality Fullerene and Silver Nanoparticle Antimicrobial Composites via Block Copolymer Templates

We present the facile prepartion of C₇₀ and Ag nanoparticle (NP) loaded block copolymer (BCP) thin films, with C₇₀ and Ag NPs working in tandem to provide virucidal and bactericidal activities, respectively. Polystyrene-<i>block</i>-poly-4-vinylpyridine (PS-P4VP) was used as a template, allowing C₇₀ integration into PS domains and in situ formation of Ag NPs in P4VP domains, while providing control of the nanoscale spatial distribution of functionality as a function of BCP molecular weight (MW). C₇₀ loaded PS-P4VP films were found to generate significant amounts of ¹O₂ under visible light illumination with no apparent dependence on BCP MW. An analogous C₇₀ loaded PS homopolymer film produced notably less ¹O₂, highlighting a possible critical role of morphology on C₇₀ photoactivity. The antimicrobial activity of Ag NP and C₇₀ loaded composites against the model PR772 bacteriophage and Escherichia coli was assessed, finding synergistic inactivation afforded by the dual functionality. BCPs were demonstrated as versatile platforms for the preparation of multifunctional antimicrobial coatings toward combating diverse microbial communities

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

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

Sub-10 nm Self-Assembly of Mesogen-Containing Grafted Macromonomers and Their Bottlebrush Polymers

We explore the morphology and phase behavior of branched diblock macromonomers and their polymers. A series of macromonomers was synthesized based on a disubstituted norbornene. The first branch consists of polydimethylsiloxane (PDMS) while the second branch is a quasi-mesogenic structure incorporating one or more cyanobiphenyl (CB) moieties. Bottlebrush polymers with varying degrees of polymerization were prepared by “graft-through” ring-opening metathesis of the macromonomers. The molecules in the resulting library of macromonomers and bottlebrush polymers self-assemble to form classically observed microphase-separated structures, including spheres, hexagonally packed cylinders, bicontinuous gyroid, and lamellae. The systematic variation of molecular structure, molecular weight of each branch, and degree of polymerization of the polymers results in a diverse set of structures and properties. We report the observation of well-ordered lamellae and cylinders with <i>d</i>-spacings as low as 6.1 and 8.0 nm, respectively. The system displays an asymmetric phase diagram, with large deviations from the canonical phase behavior of linear coil–coil diblocks. Hexagonally packed cylinders and lamellae are observed at remarkably small mass fractions of the mesogen-containing block of 0.07 and 0.21, respectively. The samples are highly birefringent, and polarized optical microscopy revealed the formation of well-developed textures in microphase-separated states formed by cooling samples through the order–disorder transition. The textures are reminiscent of the classic fan-like or focal-conic textures observed in small molecule liquid crystal mesophases, highlighting the formation of unusually large and well-ordered grains of the microphase-separated PDMS and CB microdomains. Apparent crystallization of the CB units in systems with two or three CB moieties per monomer results in distortion of the microphase-separated structure. The small <i>d</i>-spacings and large grain sizes observed here highlight the versatility and potential utility of this molecular architecture for designing and engineering new functional materials

Selectivity and Mass Transfer Limitations in Pressure-Retarded Osmosis at High Concentrations and Increased Operating Pressures

Pressure-retarded osmosis (PRO) is a promising source of renewable energy when hypersaline brines and other high concentration solutions are used. However, membrane performance under conditions suitable for these solutions is poorly understood. In this work, we use a new method to characterize membranes under a variety of pressures and concentrations, including hydraulic pressures up to 48.3 bar and concentrations of up to 3 M NaCl. We find membrane selectivity decreases as the draw solution concentration is increased, with the salt permeability coefficient increasing by a factor of 2 when the draw concentration is changed from 0.6 to 3 M NaCl, even when the applied hydraulic pressure is maintained constant. Additionally, we find that significant pumping energy is required to overcome frictional pressure losses in the spacer-filled feed channel and achieve suitable mass transfer on the feed side of the membrane, especially at high operating pressures. For a meter-long module operating at 41 bar, we estimate feedwater will have to be pumped in at a pressure of at least 3 bar. Both the reduced selectivity and increased pumping energy requirements we observe in PRO will significantly diminish the obtainable net energy, highlighting important new challenges for development of systems utilizing hypersaline draw solutions

Magnetic Field Alignment of a Diblock Copolymer Using a Supramolecular Route

Large-area uniform magnetic alignment of a self-assembled diblock copolymer has been achieved by the selective sequestration of rigid moieties with anisotropic diamagnetic susceptibility within one block of the system. The species is based on a biphenyl core and is confined in the acrylic acid domains of a poly­(styrene-<i>b</i>-acrylic acid) block copolymer by hydrogen bonding between an imidazole headgroup and the acrylic acid units. Microphase separation produces hierarchically ordered systems of smectic layers within lamellae and smectic layers in the matrix surrounding hexagonally packed poly­(styrene) cylinders, as a function of imidazole/acrylic acid stoichiometry. The magnetic field aligns the smectic layers as well as the block copolymer superstructure in a manner dependent on the anchoring condition of the biphenyl species at the block copolymer interface. Surprisingly, this is found to depend on the composition of the system. This approach is synergistic with recent efforts to engineer functional supramolecular block copolymer assemblies based on rigid chromophores. It offers a facile route to large area control of microstructure as required for full exploitation of functional properties in these systems

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

Omniphobic Membrane for Robust Membrane Distillation

In this work, we fabricate an omniphobic microporous membrane for membrane distillation (MD) by modifying a hydrophilic glass fiber membrane with silica nanoparticles followed by surface fluorination and polymer coating. The modified glass fiber membrane exhibits an anti-wetting property not only against water but also against low surface tension organic solvents that easily wet a hydrophobic polytetrafluoroethylene (PTFE) membrane that is commonly used in MD applications. By comparing the performance of the PTFE and omniphobic membranes in direct contact MD experiments in the presence of a surfactant (sodium dodecyl sulfate, SDS), we show that SDS wets the hydrophobic PTFE membrane but not the omniphobic membrane. Our results suggest that omniphobic membranes are critical for MD applications with feed waters containing surface active species, such as oil and gas produced water, to prevent membrane pore wetting