Jörg G. Werner

Tuning Structure and Properties of Graded Triblock Terpolymer-Based Mesoporous and Hybrid Films

Despite considerable efforts toward fabricating ordered, water-permeable, mesoporous films from block copolymers, fine control over pore dimensions, structural characteristics, and mechanical behavior of graded structures remains a major challenge. To this end, we describe the fabrication and performance characteristics of graded mesoporous and hybrid films derived from the newly synthesized triblock terpolymer, poly(isoprene-b-styrene-b-4-vinylpyridine). A unique morphology, unachievable in diblock copolymer systems, with enhanced mechanical integrity is evidenced. The film structure comprises a thin selective layer containing vertically aligned and nearly monodisperse mesopores at a density of more than 10(14) per m(2) above a graded macroporous layer. Hybridization via homopolymer blending enables tuning of pore size within the range of 16 to 30 nm. Solvent flow and solute separation experiments demonstrate that the terpolymer films have permeabilities comparable to commercial membranes, are stimuli-responsive, and contain pores with a nearly monodisperse diameter. These results suggest that moving to multiblock polymers and their hybrids may open new paths to produce high-performance graded membranes for filtration, separations, nanofluidics, catalysis, and drug delivery.

A silica sol–gel design strategy for nanostructured metallic materials

Batteries, fuel cells and solar cells, among many other high-current-density devices, could benefit from the precise meso- to macroscopic structure control afforded by the silica sol-gel process. The porous materials made by silica sol-gel chemistry are typically insulators, however, which has restricted their application. Here we present a simple, yet highly versatile silica sol-gel process built around a multifunctional sol-gel precursor that is derived from the following: amino acids, hydroxy acids or peptides; a silicon alkoxide; and a metal acetate. This approach allows a wide range of biological functionalities and metals--including noble metals--to be combined into a library of sol-gel materials with a high degree of control over composition and structure. We demonstrate that the sol-gel process based on these precursors is compatible with block-copolymer self-assembly, colloidal crystal templating and the Stöber process. As a result of the exceptionally high metal content, these materials can be thermally processed to make porous nanocomposites with metallic percolation networks that have an electrical conductivity of over 1,000 S cm(-1). This improves the electrical conductivity of porous silica sol-gel nanocomposites by three orders of magnitude over existing approaches, opening applications to high-current-density devices.

Transient laser heating induced hierarchical porous structures from block copolymer–directed self-assembly

Laser patterning polymer membranes Porous materials are useful for membranes, filters, energy conversion, and catalysis. Their utility often depends on the ability to finely control both the pore sizes and their connectivity. Tan et al. prepared porous thin films of block copolymers mixed with phenol-formaldehyde resins (resols) on silicon substrates using a simple laser process. On exposure to ultraviolet light, rapid heating of the substrate causes polymerization of the resols and decomposition of the block copolymer. This method allows direct patterning of the films on a local scale, with tunable pore sizes and size distributions. Science, this issue p. 54 Laser heating drives block copolymers to self-assemble into patterned hierarchical porous structures. Development of rapid processes combining hierarchical self-assembly with mesoscopic shape control has remained a challenge. This is particularly true for high-surface-area porous materials essential for applications including separation and detection, catalysis, and energy conversion and storage. We introduce a simple and rapid laser writing method compatible with semiconductor processing technology to control three-dimensionally continuous hierarchically porous polymer network structures and shapes. Combining self-assembly of mixtures of block copolymers and resols with spatially localized transient laser heating enables pore size and pore size distribution control in all-organic and highly conducting inorganic carbon films with variable thickness. The method provides all-laser-controlled pathways to complex high-surface-area structures, including fabrication of microfluidic devices with high-surface-area channels and complex porous crystalline semiconductor nanostructures.

Synthesis and Characterization of Gyroidal Mesoporous Carbons and Carbon Monoliths with Tunable Ultralarge Pore Size

Ordered mesoporous carbons with high pore accessibility are of great interest as electrodes in energy conversion and storage applications due to their high electric and thermal conductivity, chemical inertness, and low density. The metal- and halogen-free synthesis of gyroidal bicontinuous mesoporous carbon materials with uniform and tunable pore sizes through bottom-up self-assembly of block copolymers thus poses an interesting challenge. Four double gyroidal mesoporous carbons with pore sizes of 12, 15, 20, and 39 nm were synthesized using poly(isoprene)-block-poly(styrene)-block-poly(ethylene oxide) (ISO) as structure-directing triblock terpolymer and phenol-formaldehyde resols as carbon precursors. The highly ordered materials were thermally stable to at least 1600 °C with pore volumes of up to 1.56 cm(3) g(-1). Treatment at this temperature induced a high degree of sp(2)-hybridization and low microporosity. Increasing the resols/ISO ratio led to hexagonally packed cylinders with lower porosity. A single gyroid carbon network with high porosity of 80 vol % was obtained using a similar synthesis strategy. Furthermore, we present a method to fabricate monolithic materials of the gyroidal carbons with macroscopic shape and thickness control that exhibit an open and structured surface with gyroidal features. The gyroidal materials are ideally suited as electrode materials in fuel cells, batteries, and supercapacitors as their high, three-dimensionally connected porosity is expected to allow for good fuel or electrolyte accessibility and to prevent total pore blockage.

Block copolymer self-assembly–directed synthesis of mesoporous gyroidal superconductors

Block copolymer self-assembly is used to synthesize three-dimensionally continuous gyroidal mesoporous superconductors of niobium nitride. Superconductors with periodically ordered mesoporous structures are expected to have properties very different from those of their bulk counterparts. Systematic studies of such phenomena to date are sparse, however, because of a lack of versatile synthetic approaches to such materials. We demonstrate the formation of three-dimensionally continuous gyroidal mesoporous niobium nitride (NbN) superconductors from chiral ABC triblock terpolymer self-assembly–directed sol-gel–derived niobium oxide with subsequent thermal processing in air and ammonia gas. Superconducting materials exhibit a critical temperature (Tc) of about 7 to 8 K, a flux exclusion of about 5% compared to a dense NbN solid, and an estimated critical current density (Jc) of 440 A cm−2 at 100 Oe and 2.5 K. We expect block copolymer self-assembly–directed mesoporous superconductors to provide interesting subjects for mesostructure-superconductivity correlation studies.

Characterization of Sulfur and Nanostructured Sulfur Battery Cathodes in Electron Microscopy Without Sublimation Artifacts

Lithium sulfur (Li-S) batteries have the potential to provide higher energy storage density at lower cost than conventional lithium ion batteries. A key challenge for Li-S batteries is the loss of sulfur to the electrolyte during cycling. This loss can be mitigated by sequestering the sulfur in nanostructured carbon-sulfur composites. The nanoscale characterization of the sulfur distribution within these complex nanostructured electrodes is normally performed by electron microscopy, but sulfur sublimates and redistributes in the high-vacuum conditions of conventional electron microscopes. The resulting sublimation artifacts render characterization of sulfur in conventional electron microscopes problematic and unreliable. Here, we demonstrate two techniques, cryogenic transmission electron microscopy (cryo-TEM) and scanning electron microscopy in air (airSEM), that enable the reliable characterization of sulfur across multiple length scales by suppressing sulfur sublimation. We use cryo-TEM and airSEM to examine carbon-sulfur composites synthesized for use as Li-S battery cathodes, noting several cases where the commonly employed sulfur melt infusion method is highly inefficient at infiltrating sulfur into porous carbon hosts.

Characterizing Sulfur in TEM and STEM, with Applications to Lithium Sulfur Batteries

The lithium sulfur (Li-S) battery is a promising technology with the potential to provide greater energy density at lower cost than current lithium ion batteries. One of the main challenges to improving the performance of Li-S batteries is the dissolution and loss of sulfur to the electrolyte as the battery is cycled [1]. Recently much effort has focused on nanostructured electrodes that could sequester the sulfur and prevent its loss during battery operation. Analyzing the distribution of sulfur in these electrodes is critical for creating durable Li-S batteries with high energy density.

Ordered mesoporous crystalline aluminas from self-assembly of ABC triblock terpolymer–butanol–alumina sols

A one-pot synthesis approach is described to generate ordered mesoporous crystalline γ-alumina–carbon composites and ordered mesoporous crystalline γ-alumina materials via the combination of soft and hard-templating chemistries using block copolymers as soft structure-directing agents. Periodically ordered alumina hybrid mesostructures were generated by self-assembly of a poly(isoprene)-block-poly(styrene)-block-poly(ethylene oxide) terpolymer, n-butanol and aluminum tri-sec-butoxide derived sols in organic solvents. The triblock terpolymer was converted into a rigid carbon framework during thermal annealing under nitrogen to support and preserve the ordered mesoporous crystalline γ-alumina–carbon composite structures up to 1200 °C. The carbon matrix was subsequently removed in a second heat treatment in air to obtain ordered mesoporous crystalline γ-alumina structures. Such thermally stable, highly crystalline, and periodically ordered mesoporous ceramic and ceramic–carbon composite materials may be promising candidates for various high temperature catalysis, separation, and energy-related applications.

Pathways to Mesoporous Resin/Carbon Thin Films with Alternating Gyroid Morphology

Three-dimensional (3D) mesoporous thin films with sub-100 nm periodic lattices are of increasing interest as templates for a number of nanotechnology applications, yet are hard to achieve with conventional top-down fabrication methods. Block copolymer self-assembly derived mesoscale structures provide a toolbox for such 3D template formation. In this work, single (alternating) gyroidal and double gyroidal mesoporous thin-film structures are achieved via solvent vapor annealing assisted co-assembly of poly(isoprene-block-styrene-block-ethylene oxide) (PI-b-PS-b-PEO, ISO) and resorcinol/phenol formaldehyde resols. In particular, the alternating gyroid thin-film morphology is highly desirable for potential template backfilling processes as a result of the large pore volume fraction. In situ grazing-incidence small-angle X-ray scattering during solvent annealing is employed as a tool to elucidate and navigate the pathway complexity of the structure formation processes. The resulting network structures are resistant to high temperatures provided an inert atmosphere. The thin films have tunable hydrophilicity from pyrolysis at different temperatures, while pore sizes can be tailored by varying ISO molar mass. A transfer technique between substrates is demonstrated for alternating gyroidal mesoporous thin films, circumventing the need to re-optimize film formation protocols for different substrates. Increased conductivity after pyrolysis at high temperatures demonstrates that these gyroidal mesoporous resin/carbon thin films have potential as functional 3D templates for a number of nanomaterials applications.

Block copolymer derived 3-D interpenetrating multifunctional gyroidal nanohybrids for electrical energy storage

Electrical energy storage systems such as batteries would benefit enormously from integrating all device components in three-dimensional (3-D) architectures on the nanoscale to improve their power capability without negatively impacting the device-scale energy density. However, the lack of large scale synthesis methods of 3-D architectures with precise spatial control of multiple, functional energy materials at the nanoscale remains a key issue holding back the development of such intricate device designs. To achieve fully integrated, multi-material nano-3-D architectures, next-generation nanofabrication requires departure from the traditional top-down patterning methods. Here, we present an approach to such systems based on the bottom-up synthesis of co-continuous nanohybrids with all necessary functional battery components rationally integrated in a triblock terpolymer derived core–shell double gyroid architecture. In our design three-dimensional periodically ordered, functional anode and cathode nanonetworks are separated by an ultrathin electrolyte phase within a single 3-D nanostructure. All materials are less than 20 nm in their layer dimensions, co-continuous and interpenetrating in 3-D, and extended throughout a macroscopic monolith. The electrochemical analysis of our solid-state nano-3-D Li-ion/sulfur system demonstrated battery-like characteristics with stable open circuit voltage, reversible discharge voltage and capacity, and orders of magnitude decreases in footprint area compared to two-dimensional thin layer designs.