Hierarchical nanostructure control in rod-coil block copolymers with magnetic fields

Hierarchical Nanobiosensors at the End of the SARS-CoV-2 Pandemic

Nanostructures have played a key role in the development of different techniques to attack severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). Some applications include masks, vaccines, and biosensors. The latter are of great interest for detecting diseases since some of their features allowed us to find specific markers in secretion samples such as saliva, blood, and even tears. Herein, we highlight how hierarchical nanoparticles integrated into two or more low-dimensional materials present outstanding advantages that are attractive for photonic biosensing using their nanoscale functions. The potential of nanohybrids with their superlative mechanical characteristics together with their optical and optoelectronic properties is discussed. The progress in the scientific research focused on using nanoparticles for biosensing a variety of viruses has become a medical milestone in recent years, and has laid the groundwork for future disease treatments. This perspective analyzes the crucial information about the use of hierarchical nanostructures in biosensing for the prevention, treatment, and mitigation of SARS-CoV-2 effects.

Alignment of organic conjugated molecules for high-performance device applications

High-performance organic semiconductor materials as the electroactive components of optoelectronic devices have attracted much attention and made them ideal candidates for solution-processable, large-area, and low-cost flexible electronics. Especially, organic field-effect transistors (OFETs) based on conjugated semiconductor materials have experienced stunning progress in device performance. To make these materials economically viable, comprehensive knowledge of charge transport mechanisms is required. The alignment of organic conjugated molecules in the active layer is vital to charge transport properties of devices. The present review highlights the recent progress of processing-structure-transport correlations that allow the precise and uniform alignment of organic conjugated molecules over large areas for multiple electronic applications, including OFETs, organic thermoelectric devices (OTEs), and organic phototransistors (OPTs). Different strategies for regulating crystallinity and macroscopic orientation of conjugated molecules are introduced to correlate the molecular packing, the device performance and charge transport anisotropy in multiple organic electronic devices. This article is protected by copyright. All rights reserved.

100th Anniversary of Macromolecular Science Viewpoint: Opportunities for Liquid Crystal Polymers in Nanopatterning and Beyond

Liquid-crystal polymers (LCPs) integrate at a molecular level the characteristics of two important material classes, i.e., liquid crystals (LCs) and polymers. As a result, they exhibit a wide variety of intriguing physical phenomena and have useful properties in various settings. In the nearly 50 years since the discovery of the first melt-processable LCPs, there has been a remarkable expansion in the field encompassing the development of new chain architectures, the incorporation of new classes of mesogens, and the exploration of new properties and applications. As engineering materials, LCPs are historically best known in the context of high strength fibers. In a more contemporary study, the pairing of LC mesophase assembly with block copolymer (BCP) self-assembly in LC BCPs has resulted in a fascinating interplay of ordering phenomena and rich phase behavior, while lightly cross-linked networks, LC elastomers, are extensively investigated as shape memory materials based on their thermomechanical actuation. As this Viewpoint describes, these and other examples are active areas of research in which new, compelling opportunities for LCPs are emerging. We highlight a few selected areas that we view as being potentially significant in the near future, with a particular emphasis on nanopatterning. Here, the ability to readily access small feature sizes, the fluidity of the LC mesophase, and LC-based handles for achieving orientation control present a compelling combination. Opportunities for LCPs are also presented under the broad rubric of "beyond nanopatterning", and we discuss relevant challenges and potential new directions in the field.

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.

Large-Grained Cylindrical Block Copolymer Morphologies by One-Step Room-Temperature Casting

We report a facile method of ordering block copolymer (BCP) morphologies in which the conventional two-step casting and annealing steps are replaced by a single-step process where microphase separation and grain coarsening are seamlessly integrated within the casting protocol. This is achieved by slowing down solvent evaporation during casting by introducing a nonvolatile solvent into the BCP casting solution that effectively prolongs the duration of the grain-growth phase. We demonstrate the utility of this solvent evaporation annealing (SEA) method by producing well-ordered large-molecular-weight BCP thin films in a total processing time shorter than 3 min without resorting to any extra laboratory equipment other than a basic casting device, i.e., spin- or blade-coater. By analyzing the morphologies of the quenched samples, we identify a relatively narrow range of polymer concentration in the wet film, just above the order-disorder concentration, to be critical for obtaining large-grained morphologies. This finding is corroborated by the analysis of the grain-growth kinetics of horizontally oriented cylindrical domains where relatively large growth exponents (1/2) are observed, indicative of a more rapid defect-annihilation mechanism in the concentrated BCP solution than in thermally annealed BCP melts. Furthermore, the analysis of temperature-resolved kinetics data allows us to calculate the Arrhenius activation energy of the grain coarsening in this one-step BCP ordering process.

Solvent Vapor-Assisted Magnetic Manipulation of Molecular Orientation and Carrier Transport of Semiconducting Polymers

Effective control of the molecular orientation and the degree of ordering in organic semiconductors is important to achieve high-performance organic electronics. Herein, we have successfully achieved highly oriented films in centimeter scale for a naphthalenedicarboximide-based semiconducting polymer (P(NDI2OD-T2)) by solvent vapor annealing (SVA) of precast films under a high magnetic field (HMF). As revealed by the microstructural studies, the SVA-HMF films exhibit a remarkably higher degree of chain alignment and high morphological uniformity compared to the HMF-guided drop-cast films. Based on the structural evolution of the films with the SVA time, a mechanism is proposed to elucidate the alignment process, which emphasizes that the chain aggregates re-formed in the swollen films trigger magnetic alignment and determine the film order. Compared with the unaligned films, field-effect transistors of the magnetic aligned P(NDI2OD-T2) films have exhibited a 19-fold enhancement of electron mobility and an extraordinarily large mobility anisotropy of 125. Furthermore, a significantly reduced energetic barrier for activated transport is observed on the aligned devices from temperature-variable measurements. The improved performance achieved by the HMF-SVA process has indicated its potential for high-performance organic electronic applications.

Adsorption and ordering of amphiphilic rod-coil block copolymers on a substrate: conditions for well-aligned stripe nanopatterns

Controlling the ordering of self-assembled nanostructures is vital in block copolymer nanotechnology but still presents a challenge. Here we demonstrated that the adsorption and ordering of amphiphilic rod-coil block copolymers on a substrate can generate well-aligned stripe nanopatterns by dissipative particle dynamics simulations. The effects of the copolymer concentration and the surface affinity on the formation of stripe nanopatterns were examined. The simulation results revealed that the well-aligned stripe nanopatterns with controllable thickness and stripe width can be obtained in the systems with higher copolymer concentration and surface affinity. An immersion coating experiment was designed to verify the simulation results, and an agreement is shown. The present work provides a strategy for constructing well-aligned stripe nanopatterns in a controllable way.

Tunable structural color of bottlebrush block copolymers through direct-write 3D printing from solution

Additive manufacturing of functional materials is limited by control of microstructure and assembly at the nanoscale. In this work, we integrate nonequilibrium self-assembly with direct-write three-dimensional (3D) printing to prepare bottlebrush block copolymer (BBCP) photonic crystals (PCs) with tunable structure color. After varying deposition conditions during printing of a single ink solution, peak reflected wavelength for BBCP PCs span a range of 403 to 626 nm (blue to red), corresponding to an estimated change in d-spacing of >70 nm (Bragg- Snell equation). Physical characterization confirms that these vivid optical effects are underpinned by tuning of lamellar domain spacing, which we attribute to modulation of polymer conformation. Using in situ optical microscopy and solvent-vapor annealing, we identify kinetic trapping of metastable microstructures during printing as the mechanism for domain size control. More generally, we present a robust processing scheme with potential for on-the-fly property tuning of a variety of functional materials.

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.

Self-assembly of maltoheptaose-b-PMMA block copolymer systems: 10nm Resolution in thin film and bulk states

This paper describes the self-assembly of oligosaccharide-based hybrid block copolymers (BCPs) consisting of maltoheptaose (MH) and poly(methyl methacrylate) (PMMA) into 10nm scale lamellar and cylindrical phases depending on the volume fractions of MH (ϕ_MH) and the annealing process. Time resolved SAXS study of the BCP bulk samples during thermal annealing indicated that the BCPs phase separate into 10nm scale periodical structures. The solvent vapor annealing induced self-organizations of the BCP into different phases depending on ϕ_MH and the weight fraction of THF/H₂O. BCPs with relatively higher Ø_MH, MH-b-PMMA_3k (ϕ_MH=0.27) and MH-b-PMMA_5k (ϕ_MH=0.16) self-organized into lamellar phases while the BCP sample with relatively lower ϕ_MH, MH-b-PMMA_9k (ϕ_MH=0.10), self-organized into cylindrical phase by using THF/H₂O=1/4 (w/w). On the other hand, the solvent vapor annealing with larger fraction of THF, i.e. THF/H₂O=2/3 (w/w), induced cylindrical phases for MH-b-PMMA_3k and MH-b-PMMA_5k.

Vapour-Induced Liquid Crystallinity and Self-Recovery Mechanochromism of Helical Block Copolymer

New molecular design of conjugated polymer that possess high sensitivity to vapour and self-recovering property against pressure is proposed. We synthesised a rod-rod diblock copolymer, poly(3-((3S)-3,7-dimethyl-octyl)-thiophene)-block-poly(4-octyl phenylisocyanide) (PTh- b -PPI), composed of a π-conjugated polymer and a rod-type helical coiled polymer. Introduction of PPI block in the block copolymer architecture enabled PTh- b -PPI film to exhibit solid-to-liquid crystal phase transition by exposure to chloroform vapour, accompanied with colour change (purple-to-yellow), which is the first report on a new phenomenon of "vapour-induced liquid crystallinity". In addition, PTh- b -PPI film showed colour change (purple-to-vermillion) during mechanical shearing, and spontaneously recovered under ambient conditions. We concluded that rod-type helical coiled polymer PPI block performs crucial roles as intrinsically vapour-induced liquid crystallinity and self-reassembling property in the architecture of PTh- b -PPI.

Molecular Considerations for Mesophase Interaction and Alignment of Lyotropic Liquid Crystalline Semiconducting Polymers

Intermolecular interactions in conjugated polymers influence crystallinity, self-assembly, and packing motif, factors which in turn crucially impact charge transport properties such as carrier mobility in organic electronic devices. Correlated alignment of molecular and crystalline morphologies provides direct pathways for charge carriers to follow; however, the role of intermolecular interactions in achieving this is unexplored. Herein, we synthesize a series of lyotropic liquid crystalline conjugated polymers with variable side-chain structure to lend distinct steric repulsion and van der Waals attractive forces to each mesophase. We use this to investigate the role of intermolecular interactions on mesophase alignment. The strength of intermolecular interaction for each mesophase is compared by measuring melting temperature, π-stacking distance, and the Maier-Saupe interaction parameter. In general we find that side-chain structure can impact interaction strength by varying steric repulsion and backbone attractions and that the Maier-Saupe interaction parameters correlate with higher degrees of alignment after shearing, achieving a dichroic absorbance ratio of up to 2. This observation is used to develop equilibrium processing methods for fabricating macroscopically aligned polymer substrates used in transistors, improving mobility by a factor of 3 compared to spin-coated devices.

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

Recent experiments have highlighted the intrinsic magnetic anisotropy in coil-coil diblock copolymers, specifically in poly(styrene-block-4-vinylpyridine) (PS-b-P4VP), that enables magnetic field alignment at field strengths of a few tesla. We consider here the alignment response of two low molecular weight (MW) lamallae-forming PS-b-P4VP systems. Cooling across the disorder-order transition temperature (T_odt) results in strong alignment for the higher MW sample (5.5K), whereas little alignment is discernible for the lower MW system (3.6K). This disparity under otherwise identical conditions of field strength and cooling rate suggests that different average grain sizes are produced during slow cooling of these materials, with larger grains formed in the higher MW material. Blending the block copolymers results in homogeneous samples which display T_odt, d-spacings, and grain sizes that are intermediate between the two neat diblocks. Similarly, the alignment quality displays a smooth variation with the concentration of the higher MW diblock in the blends, and the size of grains likewise interpolates between limits set by the neat diblocks, with a factor of 3.5× difference in the grain size observed in high vs low MW neat diblocks. These results highlight the importance of grain growth kinetics in dictating the field response in block copolymers and suggests an unconventional route for the manipulation of such kinetics.

Strong Orientational Coupling of Block Copolymer Microdomains to Smectic Layering Revealed by Magnetic Field Alignment

We elucidate the roles of the isotropic-nematic (I-N) and nematic-smectic A (N-SmA) transitions in the magnetic field directed self-assembly of a liquid crystalline block copolymer (BCP), using in situ X-ray scattering. Cooling into the nematic from the disordered melt yields poorly ordered and weakly aligned BCP microdomains. Continued cooling into the SmA, however, results in an abrupt increase in BCP orientational order with microdomain alignment tightly coupled to the translational order parameter of the smectic layers. These results underscore the significance of the N-SmA transition in generating highly aligned states under magnetic fields in these hierarchically ordered materials.

Theoretical simulations of nanostructures self-assembled from copolymer systems

This article provides an overview of recent simulation investigations of the nanostructures and structure–property relationships in copolymer systems.

Magnetic Alignment of Block Copolymer Microdomains by Intrinsic Chain Anisotropy

We examine the role of intrinsic chain susceptibility anisotropy in magnetic field directed self-assembly of a block copolymer using in situ x-ray scattering. Alignment of a lamellar mesophase is observed on cooling across the disorder-order transition with the resulting orientational order inversely proportional to the cooling rate. We discuss the origin of the susceptibility anisotropy, Δχ, that drives alignment and calculate its magnitude using coarse-grained molecular dynamics to sample conformations of surface-tethered chains, finding Δχ≈2×10^{-8}. From field-dependent scattering data, we estimate that grains of ≈1.2  μm are present during alignment. These results demonstrate that intrinsic anisotropy is sufficient to support strong field-induced mesophase alignment and suggest a versatile strategy for field control of orientational order in block copolymers.

Synthesis and assembly of nanomaterials under magnetic fields

Traditionally, magnetic field has long been regarded as an important means for studying the magnetic properties of materials. With the development of synthesis and assembly methods, magnetic field, similar to conventional reaction conditions such as temperature, pressure, and surfactant, has been developed as a new parameter for synthesizing and assembling special structures. To date, magnetic fields have been widely employed for materials synthesis and assembly of one-dimensional (1D), two-dimensional (2D) or three-dimensional (3D) aggregates. In this review, we aim to provide a summary on the applications of magnetic fields in this area. Overall, the objectives of this review are: (1) to theoretically discuss several factors that refer to magnetic field effects (MFEs); (2) to review the magnetic-field-induced synthesis of nanomaterials; the 1D structure of various nanomaterials, such as metal oxides/sulfide, metals, alloys, and carbon, will be described in detail. Moreover, the MFEs on spin states of ions, magnetic domain and product phase distribution will be also involved; (3) to review the alignment of carbon nanotubes, assembly of magnetic nanomaterials and photonic crystals with the help of magnetic fields; and (4) to sketch the future opportunities that magnetic fields can face in the area of materials synthesis and assembly.

Directed self-assembly of block copolymers: a tutorial review of strategies for enabling nanotechnology with soft matter

Self-assembly of soft materials is broadly considered an attractive means of generating nanoscale structures and patterns over large areas. However, the spontaneous formation of equilibrium nanostructures in response to temperature and concentration changes, for example, must be guided to yield the long-range order and orientation required for utility in a given scenario. In this review we examine directed self-assembly (DSA) of block copolymers (BCPs) as canonical examples of nanostructured soft matter systems which are additionally compelling for creating functional materials and devices. We survey well established and newly emerging DSA methods from a tutorial perspective. Special emphasis is given to exploring underlying physical phenomena, identifying prototypical BCPs that are compatible with different DSA techniques, describing experimental methods and highlighting the attractive functional properties of block copolymers overall. Finally we offer a brief perspective on some unresolved issues and future opportunities in this field.

Microphase separation in comblike liquid-crystalline diblock copolymers

The interplay between liquid crystallinity and microphase separation in comblike liquid-crystalline diblock copolymers is examined via a Brazovskii-type phenomenological model using both analytical and numerical calculations. For symmetric diblock copolymers we determine a critical electric field that is required to tilt the orientation of the constituent liquid crystals of the polymer side chains in the microphase-separated lamellar state. Such electrically induced reorientation of the liquid-crystal molecules can lead to substantially large changes of lamellar periodicity. Our numerical results show that highly aligned polymer lamellar domains can self-assemble when the liquid-crystal ordering precedes microphase separation, and that weak electric fields can be used to direct the self-assembly process due to the dielectric anisotropy of the liquid-crystal side chains. We also find that phase separation of asymmetric diblock copolymers can coexist with a network of liquid-crystal nematic orientations, with domain morphology depending on the details of copolymer and liquid-crystal coupling.

Intelligent supramolecular assembly of aromatic block molecules in aqueous solution

The construction of supramolecular nanoscopic architectures has been intensively pursued because of their unique features for applications in nanoscience and biomimetic chemistry. Molecular self-assemblies of aromatic rod-coil amphiphiles consisting of rigid rod segments and hydrophilic flexible chains in aqueous solution provide a facile avenue into this area. This feature article highlights the recent progress regarding the construction of aqueous assemblies that result from the sophisticated design of aromatic rod-coils, with the aim to develop stimuli-responsive systems and bioactive materials. Important factors affecting the self-assembly morphologies are discussed and summarized. Dynamic structural changes triggered by temperature and guest molecules are demonstrated. Finally, the perspective of bioactive nanostructures originated from self-assembly of aromatic block amphiphiles is also introduced.

Self-assembly and chemical processing of block copolymers: a roadmap towards a diverse array of block copolymer nanostructures

Block copolymers can yield a diverse array of nanostructures. Their assembly structures are influenced by their inherent structures, and the wide variety of structures that can be prepared especially becomes apparent when one considers the number of routes available to prepare block copolymer assemblies. Some examples include self-assembly, directed assembly, coupling, as well as hierarchical assembly, which can yield assemblies having even higher structural order. These assembly routes can also be complemented by processing techniques such as selective crosslinking and etching, the former technique leading to permanent structures, the latter towards sculpted and the combination of the two towards permanent sculpted structures. The combination of these pathways provides extremely versatile routes towards an exciting variety of architectures. This review will attempt to highlight destinations reached by LIU Guojun and coworkers following these pathways.

Electric field induced selective disordering in lamellar block copolymers

External electric fields align nanostructured block copolymers by either rotation of grains or nucleation and growth depending on how strongly the chemically distinct block copolymer components are segregated. In close vicinity to the order-disorder transition, theory and simulations suggest a third mechanism: selective disordering. We present a time-resolved small-angle X-ray scattering study that demonstrates how an electric field can indeed selectively disintegrate ill-aligned lamellae in a lyotropic block copolymer solution, while lamellae with interfaces oriented parallel to the applied field prevail. The present study adds an additional mechanism to the experimentally corroborated suite of mechanistic pathways, by which nanostructured block copolymers can align with an electric field. Our results further unveil the benefit of electric field assisted annealing for mitigating orientational disorder and topological defects in block copolymer mesophases, both in close vicinity to the order-disorder transition and well below it.

Inducement by directional fields of rotational and translational phase ordering in polymer liquid-crystals

The effects of aligning fields on models of polymer liquid crystals were simulated using the dissipative particle dynamics method. Exposing a liquid crystal of rod-like particles to a directional field causes a stabilization of the phases with orientational order, shifts the isotropic-nematic and nematic-smectic-A phase transitions to higher temperatures, makes the transitions continuous beyond a critical field strength, and induces weak para-nematic alignment in the zero-field isotropic phase. The interplay of liquid-crystalline ordering, microphase separation, and an alignment field endows the diblock and triblock copolymers studied here with rich phase behavior. The simulations suggest that field-induced orientational ordering can give rise to positional ordering. Reversely, positional ordering resulting from rod-coil demixing may be accompanied by orientational ordering, which is enhanced by external fields. For highly asymmetric rod-coil copolymers, the microphase separation pattern formed by the rigid segments can be altered by an aligning field.

Order-disorder transition and alignment dynamics of a block copolymer under high magnetic fields by in situ x-ray scattering

We examine the influence of magnetic fields on the order-disorder transition (ODT) in a liquid crystalline block copolymer. This is motivated by a desire to understand the dynamics of microstructure alignment during field annealing as potentially dictated by selective destabilization of nonaligned material. Temperature resolved scattering across the ODT and time-resolved measurements during isothermal field annealing at sub-ODT temperatures were performed in situ. Strongly textured mesophases resulted in each case but no measurable field-induced shift in T(ODT) was observed. This suggests that selective melting does not play a discernable role in the system's field response. Our data indicate instead that alignment occurs by slow grain rotation within the mesophase. We identify an optimum subcooling that maximizes alignment during isothermal field annealing. This is corroborated by a simple model incorporating the competing effects of an exponentially decreasing mobility and divergent, increasing magnetic anisotropy on cooling below T(ODT). The absence of measurable field effects on T(ODT) is consistent with estimates based on the relative magnitudes of the field interaction energy and the enthalpy associated with the ODT.

Block Copolymer Nanocomposites in Electric Fields: Kinetics of Alignment

We investigate the kinetics of block copolymer/nanoparticle composite alignment in an electric field using in situ transmission small-angle X-ray scattering. As a model system, we employ a lamellae forming polystyrene-block-poly(2-vinyl pyridine) block copolymer with different contents of gold nanoparticles in thick films under solvent vapor annealing. While the alignment improves with increasing nanoparticle fraction, the kinetics slows down. This is explained by changes in the degree of phase separation and viscosity. Our findings provide extended insights into the basics of nanocomposite alignment.