Long-Range Ordering of Symmetric Block Copolymer Domains by Chaining of Superparamagnetic Nanoparticles in External Magnetic Fields

Electric, magnetic, and shear field-directed assembly of inorganic nanoparticles

Ordered assemblies of inorganic nanoparticles (NPs) have shown tremendous potential for wide applications due to their unique collective properties, which differ from those of individual NPs. Various assembly methods, such as external field-directed assembly, interfacial assembly, template assembly, biomolecular recognition-mediated assembly, confined assembly, and others, have been employed to generate ordered inorganic NP assemblies with hierarchical structures. Among them, the external field-directed assembly method is particularly fascinating, as it can remotely assemble NPs into well-ordered superstructures. Moreover, external fields (e.g., electric, magnetic, and shear fields) can introduce a local and/or global field intensity gradient, resulting in an additional force on NPs to drive their rotation and/or translation. Therefore, the external field-directed assembly of NPs becomes a robust method to fabricate well-defined functional materials with the desired optical, electronic, and magnetic properties, which have various applications in catalysis, sensing, disease diagnosis, energy conversion/storage, photonics, nano-floating-gate memory, and others. In this review, the effects of an electric field, magnetic field, and shear field on the organization of inorganic NPs are highlighted. The methods for controlling the well-ordered organization of inorganic NPs at different scales and their advantages are reviewed. Finally, future challenges and perspectives in this field are discussed.

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.

Kinetic Control of Length and Morphology of Segmented Polymeric Nanofibers in Microfluidic Chips

In this work, we report an approach to prepare segmented polymer nanofibers (SPNFs) composed of rodlike subunits by kinetically controlled self-assembly of polystyrene-b-poly(4-vinylpyridine)-based supramolecules in microfluidic chips. The length and morphology of the SPNFs could be effectively adjusted by changing the total flow rate (V_total) and the molar ratio (x) of 4-vinylpyridine (4VP) unit to a hydrogen-bonding molecule, 3-n-pentadecyphenol. Moreover, the subunits of SPNFs could transform from short rods to spheres when the interfacial tension between PS core and solvent increased. On the contrary, the SPNFs elongated along the major axis when the interfacial tension decreased. This work not only offers mechanism insights into the hierarchical self-assembly of block copolymer-based supramolecules but also provides a versatile and effective method for kinetically controlling the hierarchical structures of assemblies.

Numerical Simulations of Directed Self-Assembly in Diblock Copolymer Films using Zone Annealing and Pattern Templating

Bulk fabrication of surface patterns with sub-20 nm feature sizes is immensely desirable for many existing and emerging technologies. Directed self-assembly (DSA) of block copolymers (BCPs) has been a recently demonstrated approach to achieve such feature resolution over large-scale areas with minimal defect populations. However, much work remains to understand and optimize DSA methods in order to move this field forward. This paper presents large-scale numerical simulations of zone annealing and chemo-epitaxy processing of BCP films to achieve long-range orientational order. The simulations utilize a Time-Dependent Ginzburg-Landau model and parallel processing to elucidate relationships between the magnitude and velocity of a moving thermal gradient and the resulting BCP domain orientations and defect densities. Additional simulations have been conducted to study to what degree orientational order can be further improved by combining zone annealing and chemo-epitaxy techniques. It is found that these two DSA methods do synergistically enhance long-range order with a particular relationship between thermal gradient velocity and chemical template spacing.

Electric-Field-Assisted Assembly of Polymer-Tethered Gold Nanorods in Cylindrical Nanopores

In this report, we demonstrate the confined assembly of polymer-tethered gold nanorods in anodic aluminum oxide (AAO) channels with the assistance of electric field (EF). Various interesting hybrid assemblies, such as single-, double-, triple-, or quadruple-helix, linear, and hexagonally packed structures are obtained by adjusting pore size in AAO channels, ligand length, and EF orientation. Correspondingly, surface plasmonic property of the assemblies can thus be tuned. This strategy, by coupling of external-field and cylindrically confined assembly, is believed to be a promising approach for generating ordered hybrid assemblies with hierarchical structures, which may find potential applications in photoelectric devices, biosensors, and data storage devices.

Phase separation of comb polymer nanocomposite melts

In this work, the spinodal phase demixing of branched comb polymer nanocomposite (PNC) melts is systematically investigated using the polymer reference interaction site model (PRISM) theory. To verify the reliability of the present method in characterizing the phase behavior of comb PNCs, the intermolecular correlation functions of the system for nonzero particle volume fractions are compared with our molecular dynamics simulation data. After verifying the model and discussing the structure of the comb PNCs in the dilute nanoparticle limit, the interference among the side chain number, side chain length, nanoparticle-monomer size ratio and attractive interactions between the comb polymer and nanoparticles in spinodal demixing curves is analyzed and discussed in detail. The results predict two kinds of distinct phase separation behaviors. One is called classic fluid phase boundary, which is mediated by the entropic depletion attraction and contact aggregation of nanoparticles at relatively low nanoparticle-monomer attraction strength. The second demixing transition occurs at relatively high attraction strength and involves the formation of an equilibrium physical network phase with local bridging of nanoparticles. The phase boundaries are found to be sensitive to the side chain number, side chain length, nanoparticle-monomer size ratio and attractive interactions. As the side chain length is fixed, the side chain number has a large effect on the phase behavior of comb PNCs; with increasing side chain number, the miscibility window first widens and then shrinks. When the side chain number is lower than a threshold value, the phase boundaries undergo a process from enlarging the miscibility window to narrowing as side chain length increases. Once the side chain number overtakes this threshold value, the phase boundary shifts towards less miscibility. With increasing nanoparticle-monomer size ratio, a crossover of particle size occurs, above which the phase separation is consistent with that of chain PNCs. The miscibility window for this condition gradually narrows while the other parameters of the PNCs system are held constant. These results indicate that the present PRISM theory can give molecular-level details of the underlying mechanisms of the comb PNCs. It is hoped that the results can be used to provide useful guidance for the future design control of novel, thermodynamically stable comb PNCs.

Magnetic Bead-Magic Bullet

Microfluidics is assumed to be one of the leading and most promising areas of research since the early 1990s. In microfluidic systems, small spherical magnetic particles with superparamagnetic properties, called magnetic beads, play an important role in the design of innovative methods and tools, especially in bioanalysis and medical sciences. The intention of this review paper is to address main aspects from the state-of-the-art in the area of magnetic bead research, while demonstrating the broad variety of applications and the huge potential to solve fundamental biological and medical problems in the fields of diagnostics and therapy. Basic issues and demands related to the fabrication of magnetic particles and physical properties of nanosize magnets are discussed in Section 2. Of main interest are the control and adjustment of the nanoparticles' properties and the availability of adequate approaches for particle detection via their magnetic field. Section 3 presents an overview of magnetic bead applications in nanomedicine. In Section 4, practical aspects of sample manipulation and separation employing magnetic beads are described. Finally, the benefits related to the use of magnetic bead-based microfluidic systems are summarized, illustrating ongoing questions and open tasks to be solved on the way to an approaching microfluidic age.

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.

Kinetics of magnetic field-induced orientational ordering in block copolymer/superparamagnetic nanoparticle composites

The combination of external potential dynamics and Brownian dynamics is introduced to study the kinetics of orientational ordering in block copolymer/superparamagnetic nanoparticle composites where the particles are smaller than the domain spacing and preferentially segregate into one block of the copolymer. This simulation method accounts for both excluded volume interactions and dipolar interactions between particles to quantify alignment kinetics. Two-dimensional simulations reveal that higher dipolar interaction strengths lead to faster alignment of the block copolymer, where the orientation kinetics obeys an exponential rate law. The observed rate of alignment increases with increasing dipolar interaction strength and is dependent on the initial state of the block copolymer. The primary mechanism of orientational ordering is found to be the redistribution of monomer segments leading to bridging and growth of the block copolymer domains around the nanoparticles.

Inkjet printed superparamagnetic polymer composite hemispheres with programmed magnetic anisotropy

We present the fabrication and characterization of large arrays of inkjet-printed superparamagnetic polymer composite (SPMPC) hemispherical microstructures. SPMPCs are appealing for applications in microsystems and nanorobotics due to the added functionality of polymers and the significant magnetic attributes of embedded nanostructures. SPMPC-based microarchitectures can be used to perform different functions wirelessly in various media (e.g. water, solvents) using external magnetic fields: handling and assembling small objects, delivering drugs or biomass, or sensing specific physical or chemical changes. In this work superparamagnetic magnetite nanoparticles are dispersed in SU-8 to form magnetic hemispheres. Magnetically anisotropic hemispheres as well as standard SPMPC hemispheres are fabricated. Magnetic anisotropy is programmed by applying a magnetic field during curing. The distribution of nanoparticles inside the polymer matrix and magnetic characteristics of the SPMPC are investigated. Magnetic manipulation of hemispheres is demonstrated at liquid-liquid interfaces. Different assembly strategies to form lines or geometric shapes from hemispheres as well as their independent dynamic control are demonstrated. Finally, a two-interface assembly strategy is demonstrated to assemble hemispheres into complete spheres for advanced self-assembly tasks.

A generalized method for alignment of block copolymer films: solvent vapor annealing with soft shear

One of the key issues associated with the utilization of block copolymer (BCP) thin films in nanoscience and nanotechnology is control of their alignment and orientation over macroscopic dimensions. We have recently reported a method, solvent vapor annealing with soft shear (SVA-SS), for fabricating unidirectional alignment of cylindrical nanostructures. This method is a simple extension of the common SVA process by adhering a flat, crosslinked poly(dimethylsiloxane) (PDMS) pad to the BCP thin film. The impact of processing parameters, including annealing time, solvent removal rate and the physical properties of the PDMS pad, on the quality of alignment quantified by the Herman's orientational factor (S) is systematically examined for a model system of polystyrene-block-polyisoprene-block-polystyrene (SIS). As annealing time increases, the SIS morphology transitions from isotropic rods to highly aligned cylinders. Decreasing the rate of solvent removal, which impacts the shear rate imposed by the contraction of the PDMS, improves the orientation factor of the cylindrical domains; this suggests the nanostructure alignment is primarily induced by contraction of PDMS during solvent removal. Moreover, the physical properties of the PDMS controlled by the crosslink density impact the orientation factor by tuning its swelling extent during SVA-SS and elastic modulus. Decreasing the PDMS crosslink density increases S; this effect appears to be primarily driven by the changes in the solubility of the SVA-SS solvent in the PDMS. With this understanding of the critical processing parameters, SVA-SS has been successfully applied to align a wide variety of BCPs including polystyrene-block-polybutadiene-block-polystyrene (SBS), polystyrene-block-poly(N,N-dimethyl-n-octadecylammonium p-styrenesulfonate) (PS-b-PSS-DMODA), polystyrene-block-polydimethylsiloxane (PS-b-PDMS) and polystyrene-block-poly(2-vinlypyridine) (PS-b-P2VP). These results suggest that SVA-SS is a generalizable method for the alignment of BCP thin films.

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.

Nano- and microstructures of magnetic field-guided maghemite nanoparticles in diblock copolymer films

The control over the alignment of nanoparticles within a block copolymer matrix was investigated for different external magnetic fields with respect to producing well-aligned, highly oriented metal-oxide-polymer nanopatterns. Hybrid films were prepared by solution casting under a range of external magnetic fields. The nano- and microstructure of maghemite nanoparticles within poly(styrene-b-methyl methacrylate) diblock copolymer films as a function of the nanoparticle concentration was studied using optical microscopy, atomic force microscopy, scanning electron microscopy, and grazing incidence small-angle X-ray scattering. Because of a polystyrene (PS) coating, the nanoparticles are incorporated in the PS domains of the diblock copolymer morphology. At higher nanoparticle concentrations, nanoparticle aggregates perturb the block copolymer structure and accumulate at the films surface into wire-shaped stripes. These wire-shaped nanoparticle aggregates form mainly because of the competition between nanoparticle-polymer friction and magnetic dipolar interaction. The magnetic behavior of the hybrid films was probed at different temperatures for two orthogonal directions (with the line-shaped particle aggregates parallel and perpendicular to the magnetic field). The hybrid film systems show superparamagnetic behavior and remarkable shape anisotropy that render them interesting for magnetic applications.

Theory and simulation studies of effective interactions, phase behavior and morphology in polymer nanocomposites

Polymer nanocomposites are a class of materials that consist of a polymer matrix filled with inorganic/organic nanoscale additives that enhance the inherent macroscopic (mechanical, optical and electronic) properties of the polymer matrix. Over the past few decades such materials have received tremendous attention from experimentalists, theoreticians, and computational scientists. These studies have revealed that the macroscopic properties of polymer nanocomposites depend strongly on the (microscopic) morphology of the constituent nanoscale additives in the polymer matrix. As a consequence, intense research efforts have been directed to understand the relationships between interactions, morphology, and the phase behavior of polymer nanocomposites. Theory and simulations have proven to be useful tools in this regard due to their ability to link molecular level features of the polymer and nanoparticle additives to the resulting morphology within the composite. In this article we review recent theory and simulation studies, presenting briefly the methodological developments underlying PRISM theories, density functional theory, self-consistent field theory approaches, and atomistic and coarse-grained molecular simulations. We first discuss the studies on polymer nanocomposites with bare or un-functionalized nanoparticles as additives, followed by a review of recent work on composites containing polymer grafted or functionalized nanoparticles as additives. We conclude each section with a brief outlook on some potential future directions.

Segregated polymeric nanocomposites with tunable three-dimensional network of nanoparticles by controlling the dispersion and distribution

A facile and robust emulsion approach for fabrication of segregated polymeric nanocomposites with controllable nanoparticle dispersion/distribution is described.

Magnetic Field Induced Morphological Transitions in Block Copolymer/Superparamagnetic Nanoparticle Composites

This two-dimensional computational study investigates the effect of external magnetic fields on thin film nanocomposites comprised of superparamagnetic nanoparticles dispersed within block copolymer melts, which display a variety of morphological transitions based on the field orientation, nanoparticle loading, and selectivity of the nanoparticles for the blocks. In-plane magnetic fields lead to chaining of the nanoparticles; when selective for the minority block in a hexagonal block copolymer nanostructure, this chaining results in the formation of stripe phases oriented parallel to the magnetic field. When selective for the majority block of the hexagonal structure, nanoparticle chains of sufficient persistence length drive the orientation of the hexagonal morphology with the ⟨100⟩ direction oriented parallel to the magnetic field. Out-of-plane magnetic fields induce repulsive dipolar interactions between the nanoparticles that annihilate the defects in the hexagonal morphology of the block copolymer when the nanoparticle is selective for the minority block. When the nanoparticles are selective for the majority block and the field is oriented out of plane, repulsive dipolar interactions lead to the formation of honeycomb lattices. In all cases, the nanoparticle size and volume fraction must be chosen to maximize the commensurability with the block copolymer structure to optimize the ordering of the final composite.

Perpendicular orientation of sub-10 nm channels in polystyrene-b-poly(4-hydroxyl styrene)/PEG oligomer blend thin films

Vertically oriented cylinders via the directional coalescence of the spheres embedded in the blend thin films from polystyrene-b-poly(4-hydroxyl styrene) (PS-b-PHS) and PEG induced by solvent annealing were achieved. Removal of PEG water led to the formation of nanochannels throughout the films. The diameter of these channels could be as small as 9 nm which might enhance the applicability of the nano-porous films as size-selective membranes and controllable drug delivery systems for the objects less than 10 nm.