Implications of Grain Size Variation in Magnetic Field Alignment of Block Copolymer Blends

Block Copolymer-Templated, Single-Step Synthesis of Transition Metal Oxide Nanostructures for Sensing Applications

The synthesis of transition metal oxide nanostructures, thanks to their high surface-to-volume ratio and the resulting large fraction of surface atoms with high catalytic activity, is of prime importance for the development of new sensors and catalytic materials. Here, we report an economical, time-efficient, and easily scalable method of fabricating nanowires composed of vanadium, chromium, manganese, iron, and cobalt oxides by employing simultaneous block copolymer (BCP) self-assembly and selective sequestration of metal-organic acetylacetonate complexes within one of the BCP blocks. We discuss the mechanism and the primary factors that are responsible for the sequestration and conformal replication of the BCP template by the inorganic material that is obtained after the polymer template is removed. X-ray photoelectron spectroscopy (XPS) and powder X-ray diffraction (PXRD) studies indicate that the metal oxidation state in the nanowires produced by plasma ashing the BCP template closely matches that of the precursor complex and that their structure is amorphous, thus requiring high-temperature annealing in order to sinter them into a crystalline form. Finally, we demonstrate how the developed nanowire array fabrication scheme can be used to rapidly pattern a multilayered iron oxide nanomesh, which we then used to construct a prototype volatile organic compound sensor.

Responsive blends of block copolymers stabilize the hexagonally perforated lamellae morphology

Blends of block copolymers can form phases and exhibit features distinct from the constituent materials. We study thin film blends of cylinder-forming and lamellar-forming block copolymers across a range of substrate surface energies. Blend materials are responsive to interfacial energy, allowing selection of pure or coexisting phases based on surface chemistry. Blending stabilizes certain motifs that are typically metastable, and can be used to generate pure hexagonally perforated lamellar thin films across a range of film thicknesses and surface energies. This tolerant behavior is ascribed to the ability of blend materials to redistribute chains to stabilize otherwise high-energy defect structures. The blend responsiveness allows the morphology to be spatially defined through multi-tone chemical surface patterns.

Computational Study of Mechanochemical Activation in Nanostructured Triblock Copolymers

Force-driven chemical reactions have emerged as an attractive platform for diverse applications in polymeric materials. However, the microscopic chain conformations and topologies necessary for efficiently transducing macroscopic forces to the molecular scale are not well-understood. In this work, we use a coarse-grained model to investigate the impact of network-like topologies on mechanochemical activation in self-assembled triblock copolymers. We find that mechanochemical activation during tensile deformation depends strongly on both the polymer composition and chain conformation in these materials. Activation primarily occurs in the tie chains connecting different glassy domains and in loop chains that are hooked onto each other by physical entanglements. Activation also requires a higher stress in materials having a higher glassy block content. Overall, the lamellar samples show the highest percent activation at high stress. In contrast, at low stress, the spherical morphology, which has the lowest glassy fraction, shows the highest activation. Additionally, we observe a spatial pattern of activation, which appears to be tied to distortion of the self-assembled morphology. Higher activation is observed in the tips of the chevrons formed during deformation of lamellar samples as well as in the centers between the cylinders in the cylindrical morphology. Our work shows that changes in the network-like topology in different morphologies significantly impact mechanochemical activation efficiencies in these materials, suggesting that this area will be a fruitful avenue for further experimental research.

Developing Anisotropy in Self-Assembled Block Copolymers: Methods, Properties, and Applications

Block copolymers (BCPs) self-assembly has continually attracted interest as a means to provide bottom-up control over nanostructures. While various methods have been demonstrated for efficiently ordering BCP nanodomains, most of them do not generically afford control of nanostructural orientation. For many applications of BCPs, such as energy storage, microelectronics, and separation membranes, alignment of nanodomains is a key requirement for enabling their practical use or enhancing materials performance. This review focuses on summarizing research progress on the development of anisotropy in BCP systems, covering a variety of topics from established aligning techniques, resultant material properties, and the associated applications. Specifically, the significance of aligning nanostructures and the anisotropic properties of BCPs is discussed and highlighted by demonstrating a few promising applications. Finally, the challenges and outlook are presented to further implement aligned BCPs into practical nanotechnological applications, where exciting opportunities exist.

Towards Macroscopically Anisotropic Functionality: Oriented Metallo-supramolecular Polymeric Materials Induced by Magnetic Fields

Based on the predesigned self-selective complexation, metallo-supramolecular P3HT-b-PEO diblock copolymers with varying block ratios were synthesized, and their oriented polymer films generated during solvent evaporation in a 9 T magnetic field were investigated. An anisotropic, ordered layer structure was achieved using [P3HT₂₀ -Zn-PEO₁₀₇ ] and carefully characterized by polarized optical microscopy (POM), AFM, polarized UV/Vis spectroscopy, and GI-SAXS/WAXS. The PEO-removed [P3HT₂₀ -Zn-PEO₁₀₇ ] film was obtained after decomplexation with TEA-EDTA under mild conditions, and the selective removal of PEO domains was evidenced by UV/Vis and ATR-FTIR spectroscopy. Anisotropic photoconductivity of the magnetically aligned film was evaluated by flash-photolysis time-resolved microwave conductivity (FP-TRMC) measurements. The results indicated that the presence of insulating crystalline PEO segments diminished the photoconductivity along the P3HT backbone direction.

Macroscopic Alignment of Block Copolymers on Silicon Substrates by Laser Annealing

Laser annealing is a competitive alternative to conventional oven annealing of block copolymer (BCP) thin films enabling rapid acceleration and precise spatial control of the self-assembly process. Localized heating by a moving laser beam (zone annealing), taking advantage of steep temperature gradients, can additionally yield aligned morphologies. In its original implementation it was limited to specialized germanium-coated glass substrates, which absorb visible light and exhibit low-enough thermal conductivity to facilitate heating at relatively low irradiation power density. Here, we demonstrate a recent advance in laser zone annealing, which utilizes a powerful fiber-coupled near-IR laser source allowing rapid BCP annealing over a large area on conventional silicon wafers. The annealing coupled with photothermal shearing yields macroscopically aligned BCP films, which are used as templates for patterning metallic nanowires. We also report a facile method of transferring laser-annealed BCP films onto arbitrary surfaces. The transfer process allows patterning substrates with a highly corrugated surface and single-step rapid fabrication of multilayered nanomaterials with complex morphologies.

Creating Aligned Nanopores by Magnetic Field Processing of Block Copolymer/Homopolymer Blends

We describe the phase behavior of a cylinder-forming block copolymer (BCP)/homopolymer blend and the generation of aligned nanopores by a combination of magnetic field alignment and selective removal of the minority-block-miscible homopolymer. Alignment is achieved by cooling through the order-disorder transition temperature (T_odt) in a 6 T field. The system is a blend of poly(styrene-block-4-vinylpyridine) (PS-b-P4VP) and poly(ethylene glycol) (PEG). PEG is miscible with P4VP and partitions preferentially into the cylindrical microdomains. Calorimetry and X-ray scattering show that T_odt decreases linearly with PEG concentration until the onset of macrophase separation, inferred by PEG crystallization. Beyond this point, T_odt is invariant with PEG content. Increasing PEG molar mass decreases the concentration at which macrophase separation is observed. Nanopore formation is confirmed by dye uptake experiments that show a clear dependence of dye uptake on PEG content before removal. We anticipate that this strategy can be extended to other BCP/homopolymer blends to produce nanoporous materials with reliable control of pore alignment and effective pore dimensions.

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.

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

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.