Block Copolymer Layer Research Articles

Liquid Metal Covered with Thermoplastic Conductive Composites for High Electrical Stability and Negligible Electromechanical Coupling at Large Strains.

Stretchable electrode is an essential part of soft electronic devices. Practical stretchable electrodes must meet the following requirements: metallic conductivity and no resistance change in various situations such as repeated large deformation, toxic environment, and large temperature change. This study suggests a simple electrode design that meets all of these requirements simultaneously. The electrode consists of a liquid metal (LM) mesh pattern that is sandwiched between a thermoplastic block copolymer (BCP) film and a BCP/Ag flake composite film with a microfibril network structure on its surface. The electrode has a high conductivity (1.2 × 104 S/cm) and is stretchable up to 600% uniaxial strain (ε). Its resistance remains unchanged during repeated stretching cycles at ε = 300% (ΔR < 0.04 Ω) as well as under simultaneous situation of large deformation (ε = 400%) and large temperature change (20-70 °C). The electrode is anticorrosive in an acidic solution owing to the hydrophobic BCP layer that protects the LM from being etched. This study shows the connection of two separate electrodes and complete healing of scratched electrodes by finger pressing. In addition, it demonstrates the fabrication of superstretchable electroluminescence display as an example of potential uses of the electrode.

Irreversible Physisorption of PS-b-PMMA Copolymers on Substrates for Balanced Interfacial Interactions as a Versatile Surface Modification.

We present a direct approach to fabricating the perfect neutral layer for block copolymer (BCP) self-assembled thin films. An irreversible physisorption of polystyrene-b-poly(methyl methacrylate) (PS-b-PMMA) itself onto the bottom substrates, though extremely thin, offers such a compositional randomness for the substrates, directing the balanced interfacial interactions toward both blocks of the top-layer PS-b-PMMA. Owing to the neutral property from the skin layers, the chemically identical BCP self-assembles into perpendicular microdomains on the adsorbed layer composed of itself, as identified in symmetric PS-b-PMMA films. Intriguingly, the compositional randomness turns out to be valid when the correlation length (ξ) of the BCP layer adsorbed on the substrates is shorter than equilibrium lamellar spacing (L0) of the BCP. Our strategy provides its versatility applicable to various substrates without any necessity of specific random copolymer brushes or mats, enabling the design of a neutral platform for PS-b-PMMA films.

Monolayer Arrays of Nanoparticles on Block Copolymer Brush Films.

Two-dimensional arrays of nanoparticles (NPs) have widespread applications in optical coatings, plasmonic sensors, and nanocomposites. Current bottom-up approaches that use homogeneous NP templates, such as silane self-assembled monolayers or homopolymers, are typically plagued by NP aggregation, whereas patterned block copolymer (BCP) films require specific compositions for specific NP distributions. Here, we show, using polystyrene- b-poly(4-vinylpyridine) (PS- b-P4VP) and gold NPs (AuNPs) of various sizes, that a nanothin PS- b-P4VP brushlike coating (comprised of a P4VP wetting layer and a PS overlayer), which is adsorbed onto flat substrates during their immersion in very dilute PS- b-P4VP tetrahydrofuran solutions, provides an excellent template for obtaining dense and well-dispersed AuNPs with little aggregation. These non-close-packed arrays have similar characteristics regardless of immersion time in solution (about 10-120 s studied), solution concentration below a critical value (0.1 and 0.05 mg/mL studied), and AuNP diameter (10-90 nm studied). Very dilute BCP solutions are necessary to avoid deposition, during substrate withdrawal, of additional material onto the adsorbed BCP layer, which typically leads to patterned surfaces. The PS brush coverage depends on immersion time (adsorption kinetics), but full coverage does not inhibit AuNP adsorption, which is attributed to PS molecular rearrangement during exposure to the aqueous AuNP colloidal solution. The simplicity, versatility and robustness of the method will enable applications in materials science requiring dense, unaggregated NP arrays.

Effect of the Micelle Opening in Self-assembled Amphiphilic Block Co-polymer Films on the Infiltration of Inorganic Precursors.

Infiltration of the polymer templates with inorganic precursors using the selective vapor-phase infiltration approach, or sequential infiltration synthesis (SIS), allows the design of materials with advanced properties. Swelling of the block co-polymer (BCP) templates enables the additional control of the structure, porosity, and thickness of the composite or inorganic materials. Here, we use the highly precise quartz crystal microbalance (QCM) technique to investigate quantitatively the effect of the micelle opening by swelling and inorganic precursor infiltrating on the evolution of porosity in amphiphilic BCPs. We show that swelling of the polystyrene- block-poly-4-vinyl pyridine (PS- b-P4VP) BCP in ethanol at 75 °C occurs rapidly and results in a stable polymer structure in 30 min. By using an alumina model system, we found that swelling enables access to all available polar domains of the PS- b-P4VP film leading to an increase in the SIS-infiltrated alumina mass as compared to the nonswelled BCP layer. Our results demonstrate that swelling of the 110 nm thick BCP template results in the formation of 192 nm thick alumina films with 2 times larger alumina mass and 4 times larger effective pore volume than in case of the nonswelled sample. In the case of the thicker polymer template, the difference due to swelling becomes even more substantial because the fraction of accessible polymer is increased much more than in thin films. Our findings provide important insights into the mechanism of the infiltration of the inorganic precursors into swelled and nonswelled, spin-coated BCP templates enabling the design of highly porousthick ceramic films by SIS.

Highly permeable membranes enabled by film formation of block copolymers on water surface

Separation membranes derived from block copolymers (BCPs) have attracted significant interest due to their well-defined pores and functional surfaces. However, it still remains a challenge to prepare ultrathin BCP composite membranes in an efficient way. Here we report a facile approach for the fabrication of ultrathin BCP composite membranes by forming thin films on water surface. The BCP layer as thin as ~ 17 nm can be prepared by spreading polystyrene-block-poly(2-vinyl pyridine) (PS-b-P2VP) solutions on water surface followed by controlled evaporation. The thin layer can be readily collected and composited with macroporous supports. A thickness of ~ 260 nm of the BCP layer is necessary to guarantee the integrity. Interconnected nanoporous structures are then obtained in the BCP layer by the process of selective swelling induced pore generation. On account of the ultrathin separation layer, the composite membranes exhibit excellent permselectivity, better than many other ultrafiltration membranes. Moreover, the separation performances can be facilely regulated by tuning the swelling durations. This method is featured as cost-effective and convenient, and is expected to provide a higher chance for the production of BCP membranes at large scale.

Formation of Thin, Isoporous Block Copolymer Membranes by an Upscalable Profile Roller Coating Process-A Promising Way to Save Block Copolymer.

In this work we present a method to manufacture flat sheet membranes with a thin isoporous block copolymer (BCP) layer (thickness <3 µm) by profile roller coating (breadth: 30 cm) on top of a porous support membrane. Highly diluted BCP-solutions were used for this coating process. While we cast membranes with dimensions of 30 cm × 50 cm in this work, the procedure can easily be extended to endless dimensions in this roll to roll (R2R) process. The method offers the possibility to save >95% of BCP raw material compared to common doctor blade casting, by strongly decreasing the layer thickness to below 3 µm in combination with a highly open substructure. Additionally, we report a straightforward method to investigate the influence of the solvent evaporation time between coating and precipitation (phase inversion) on the membrane morphology using one sample only, which also ensures that all other influencing parameters remain constant.

Highly permeable nanoporous block copolymer membranes by machine-casting on nonwoven supports: An upscalable route

Block copolymer (BCP) membranes are distinguished for their well-defined porosities, tunable pore geometries, and functionable pore walls. However, it remains challenging to produce robust BCP membranes by affordable, convenient methods. Herein, we demonstrate a facile and easily upscalable approach to produce highly permeable BCP membranes in large areas. The membranes possess a bi-layered composite structure with nanoporous polystyrene-block-poly(2-vinylpyrdine) BCP layers directly supported on macroporous nonwoven substrates. The BCP layers are machine-cast on the water-prefilled nonwoven, and interconnected nanoporosities are created in the BCP layers by ethanol swelling. The nanoporous BCP layers exhibit a thickness of ~10µm and are tightly adhered to the nonwoven. Changes in the swelling temperatures and durations modulate both pore sizes and surface hydrophilicity of the BCP layers, and consequently the permselectivity of the membranes. By increasing swelling duration from 15min to 12h, the permeability of the membrane swollen at 65°C can be increased from ~100 to ~850Lm−2h−1bar−1 with the retention to 15-nm gold nanoparticles reduced from ~93% to ~54%. Moreover, we demonstrate that the composite membrane can efficiently fractionate nanoparticles and narrow down their size distribution from ~3–20nm to ~3–10nm.

Imprint strategy for directed self-assembly of block copolymers

The directed self-assembly (DSA) of block copolymers (BCP) has attracted high interest for the definition of nanostructures in an almost self-forming way when adequate boundary conditions are given. At present, grapho- and chemo-epitaxy are the workhorses but they require precisely patterned substrates to serve as the guiding pattern. Nanoimprint may replace this laborious pre-patterning of each substrate by employing an adequate stamp that can be used multiple times, inducing the guided DSA from the top of the film. Here, the DSA of BCPs is revisited in view of the specific nanoimprint situation. As a consequence, the BCP layer is imprinted in a partial cavity-filling mode, using a stamp of sufficient height provided with a conventional anti-sticking layer; substrate pre-treatment is minimized or rather avoided. Even with a highly preferential substrate it is possible to obtain vertical lamellae that are largely oriented in parallel to the stamp edges in PS-b-PMMA (polystyrene-block-polymethyl methacrylate) already after 3min of imprint. The vertical lamellae are at least 70nm high, freestanding on the substrate. Though optimization is required the results indicate the high potential of nanoimprint to simplify the DSA of BCPs for technical applications, also beyond Si technology.

Determining drug release rates of hydrophobic compounds from nanocarriers.

Obtaining meaningful drug release profiles for drug formulations is essential prior to in vivo testing and for ensuring consistent quality. The release kinetics of hydrophobic drugs from nanocarriers (NCs) are not well understood because the standard protocols for maintaining sink conditions and sampling are not valid owing to mass transfer and solubility limitations. In this work, a new in vitroassay protocol based on 'lipid sinks' and magnetic separation produces release conditions that mimic the concentrations of lipid membranes and lipoproteins in vivo, facilitates separation, and thus allows determination of intrinsic release rates of drugs from NCs. The assay protocol is validated by (i) determining the magnetic separation efficiency, (ii) demonstrating that sink condition requirements are met, and (iii) accounting for drug by completing a mass balance. NCs of itraconazole and cyclosporine A (CsA) were prepared and the drug release profiles were determined. This release protocol has been used to compare the drug release from a polymer stabilized NC of CsA to a solid drug NP of CsA alone. These data have led to the finding that stabilizing block copolymer layers have a retarding effect on drug release from NCs, reducing the rate of CsA release fourfold compared with the nanoparticle without a polymer coating.This article is part of the themed issue 'Soft interfacial materials: from fundamentals to formulation'.

Directed Self-Assembly of Diblock Copolymer Thin Films on Prepatterned Metal Nanoarrays.

The sequential layer by layer self-assembly of block copolymer (BCP) nanopatterns is an effective approach to construct 3D nanostructures. Here large-scale highly ordered metal nano-arrays prepared from solvent annealed thin films of polystyrene-block-poly(2-vinylpyridine) (PS-b-P2VP) diblock copolymer are used to direct the assembly of the same BCP. The influence of initial loading concentration of metal precursor, the type of metal nanoparticle (gold, platinum, and silver), and the nanoparticle-substrate interaction on the directed assembly behavior of the upper BCP layer have been focused. It is found that the upper BCP film can be completely directed by the gold nanoarray with P2VP domain exclusively located between two adjacent gold nanowires or nanodots, which behaves the same way as on the platinum nanoarray. While the silver nanoarray can be destroyed during the upper BCP self-assembly with the silver nanoparticles assembled into the P2VP domain. Based on the discussions of the surface energy of nanoparticles and the interplay between nanoparticle-substrate interaction and nanoparticle-polymer interaction, it is concluded that the effect of immobilization of nanoparticles on the substrate, together with entropy effect to minimize the energetically unfavorable chain stretching contributes to the most effective alignment between each layer.

Ionic Liquid-Triggered Redox Molecule Placement in Block Copolymer Nanotemplates toward an Organic Resistive Memory.

The integration of functional components such as metal nanoparticles, metal salts, or ionic liquids with well-defined block copolymer (BCP) nanotemplates via noncovalent bond interactions has afforded hybrid functional materials. Here, we designed an ionic liquid (IL)-functionalized redox-active TEMPO (2,2,6,6-tetramethylpiperidine-N-oxy) radical (guest), investigated phase-selective incorporation/placement into host BCP nanostructured matrices, and established a rational approach to functionalize BCP templates. On-demand domain functionalization of poly(styrene-b-ethylene oxide) (PS-b-PEO) was triggered by ion-ionophore interaction, as verified by the suppression of PEO melting transition in DSC, and the swelling behavior of the PEO spherical domain in AFM, TEM, and X-ray scattering characterizations. The obtained BCP layer containing the redox-active TEMPO and IL was utilized as an active layer in the diode-structured memory device, which exhibited on/off resistive switching (on/off ratio >103). Systematic placement of TEMPO and IL in the BCP spherical domain allowed for tuning of the switching characteristics and revealed that the formation of a discontinuous redox-active domain was critical for rewritable resistive switching.

Tunable Encapsulation Structure of Block Copolymer Coated Single-Walled Carbon Nanotubes in Aqueous Solution

Nanosized and shape-tunable molecular building blocks can provide great opportunities for the fabrication of precisely controlled nanostructures. In this work, we have fabricated a molecular building block of single-walled carbon nanotubes (SWNTs) coated by PPO–PEO–PPO block copolymers whose encapsulation structure can be controlled via temperature or addition of small molecules. The structure and optical properties of SWNT block copolymers have been investigated by small-angle neutron scattering (SANS), ultraviolet–visible (UV–vis) spectroscopy, atomic force microscopy (AFM), and molecular dynamics (MD) simulation. The structure of the hydrated block copolymer layer surrounding SWNT can be controlled reversibly by varying temperature as well as by irreversibly adding 5-methylsalicylic acid (5MS). Increasing hydrophobicity of the polymers with temperature and strong tendency of 5MS to interact with both block copolymers and π orbitals of the SWNTs are likely to be responsible for the significant structural change of the block copolymer encapsulation layer, from loose corona shell to tightly encapsulating compact shell. Our result shows an efficient and simple way to fabricate and manipulate carbon-based nano building blocks in aqueous systems with tunable structure.

Mono- and Di-valent Salts as Modifiers of PEO-PPO-PEO Block Copolymer Interactions with Silica Nanoparticles in Aqueous Dispersions

Colloidal stabilization of nanoparticle dispersions is central to applications including coatings, mineral extraction, and dispersion of oil spills in oceanic environments, which often involves oil-mineral-aggregates (OMAs). We have an ongoing interest in the modulation of amphiphile micellization and adsorption behavior in aqueous colloidal dispersions in the presence of various additives. Here we evaluate the effect of added salts CaCl2, MgCl2, and NaCl on the micellization and adsorption behavior of the poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) triblock copolymer Pluronic P105 (EO37PO56EO37). In 0.10 wt% silica nanoparticle (10.6 nm average diameter) dispersion, adsorbed block copolymer layer formation begins at a critical surface micelle concentration (csmc) of 0.02 wt%, well below the critical micellization concentration of Pluronic P105 in water. Dye solubilization experiments demonstrate an increase in the csmc upon addition of each salt. Each added salt reaches a level of maximum effectiveness in its ability to disfavor Pluronic P105 adsorption at the silica surface. These peak levels occur at concentrations of 0.005, 0.03, and 0.05 M for CaCl2, MgCl2, and NaCl, respectively, in the presence of 0.10 wt% silica nanoparticles. We explain these results in the context of an electrostatic displacer mechanism and discuss possible connections to OMA-dispersant formation.

Filtration-based synthesis of micelle-derived composite membranes for high-flux ultrafiltration.

Ideal membrane configurations for efficient separation at high flux rates consist of thin size-selective layers connected to macroporous supports for mechanical stabilization. We show that micelle-derived (MD) composite membranes combine efficient separation of similarly sized proteins and water flux 5-10 times higher than that of commercial membranes with similar retentions. MD composite membranes were obtained by filtration of solutions of amphiphilic block copolymer (BCP) micelles through commercially available macroporous supports covered by sacrificial nanostrand fabrics followed by annealing and removal of the nanostrand fabrics. Swelling-induced pore generation in the BCP films thus covering the macroporous supports yielded ∼210 nm thin nanoporous size-selective BCP layers with porosities in the 40% range tightly connected to the macroporous supports. Permselectivity and flux rates of the size-selective BCP layers were adjusted by the BCP mass deposited per membrane area and by proper selection of swelling times. The preparation methodology described here may pave the way for a modular assembly system allowing the design of tailored separation membranes.

Isoporous membranes with gradient porosity by selective swelling of UV-crosslinked block copolymers

Selective swelling of amphiphilic block copolymers (BCPs) is an effective and nondestructive pore-making strategy. Here we coupled swelling-induced pore generation with UV crosslinking to fabricate BCP isoporous membranes with gradient porosity. Polystyrene-block-poly(2-vinylpyridine) (PS-b-P2VP) solutions were coated onto macroporous supporting membranes to achieve composite films, which were then annealed in solvent vapor for the perpendicular alignment of the P2VP phases near the surface of the coating BCP layer. After swelling of BCP in hot ethanol and drying, isopores of ~8nm formed at the surface of BCP layer following the selective swelling-induced pore-formation mechanism. Then UV exposure and subsequent secondary swelling at stronger condition of the membranes were conducted to enlarge the inner pores while maintaining the surface structures. With balanced UV crosslinking and secondary swelling, the finally obtained membranes showed ordered perpendicular pores at the outmost layer and gradient porosity with enlarged interconnected pores inside the BCP layer. Due to the gradient structures, the membranes exhibited much higher flux while the surface structures and retention remained essentially unchanged. Moreover, compared to the membranes without UV treatment, the membranes showed better performances in discriminating polyethylene glycol molecules with different molecular weights and still kept a sensitive pH-responsive property.

Composite membranes with hydrophilic nanopores derived from the self-assembly of block copolymer supramolecular complexes

Selective composite membranes with high and controlled porosity have been fabricated using patterned silicon microsieves as support and nanoporous polystyrene-block-poly(ethylene oxide) (PS-b-PEO) membranes as selective layer. The desired morphology of the block copolymer layer is obtained by a supramolecular approach based on the hydrogen bonding of the PEO block with resorcinol. The formation of PS-b-PEO/resorcinol complexes allows to easily control the self-assembly of the block copolymer and to obtain normally oriented PEO/resorcinol nanocylinders by spin coating PS-b-PEO/resorcinol solutions. In a further step resorcinol works as a porogen and by its selective dissolution densely packed, normally oriented nanochannels coated by PEO chains have been obtained. These hydrophilic nanochannels have a diameter of approximately 20nm and cross the membranes from top to bottom. The nanoporous polymer layer is crack-free and well adheres to the support microsieve. The composite membrane is permeable to methylene blue while blocks larger molecules, such as bovine serum albumin.

Large-area nanosquare arrays from shear-aligned block copolymer thin films.

While block copolymer lithography has been broadly applied as a bottom-up patterning technique, only a few nanopattern symmetries, such as hexagonally packed dots or parallel stripes, can be produced by spontaneous self-assembly of simple diblock copolymers; even a simple square packing has heretofore required more intricate macromolecular architectures or nanoscale substrate prepatterning. In this study, we demonstrate that square, rectangular, and rhombic arrays can be created via shear-alignment of distinct layers of cylinder-forming block copolymers, coupled with cross-linking of the layers using ultraviolet light. Furthermore, these block copolymer arrays can in turn be used as templates to fabricate dense, substrate-supported arrays of nanostructures comprising a wide variety of elements: deep (>50 nm) nanowells, nanoposts, and thin metal nanodots (3 nm thick, 35 nm pitch) are all demonstrated.

Janus Nanodiscs: Janus Nanodisc of Diblock Copolymers (Adv. Mater. 26/2014)

Z. Z. Yang, J. Zhu, F. X. Liang, and co-workers develop on page 4469 a facile approach to synthesize regular and uniform polymeric Janus nanodiscs with alternative layers of block copolymers by partial disintegration of ellipsoidal nano particles. Further selective growth of materials onto the desired sides of the discs extends the composition and tunes the microstructure for achieving functional nanodiscs. Many other Janus nanomaterials with tunable shapes are expected using similar methods.

28 nm pitch of line/space pattern transfer into silicon substrates with chemo-epitaxy Directed Self-Assembly (DSA) process flow

Directed Self-Assembly (DSA) using block-copolymer (BCP) materials is considered as an alternative to Extreme Ultra-Violet (EUV) lithography for sub-14nm half pitch technology. A BCP with natural periodicity of 28nm was assembled on 84nm pitch pre-pattern. 3X feature multiplication can be achieved with the chemo-epitaxy DSA flow. The BCP thickness should be thin in order to reduce the self-assembly defectivity. As a consequence, plasma etch process to transfer such thin polymer material to subsequent underlayer becomes a challenging task. The main purpose of this paper is to study the DSA pattern transfer capability of 20–25nm BCP layer aiming at silicon fin patterning through the Si3N4 hardmask. Alternative Ar/O2 and Ar plasmas were developed to increase the selectivity of PMMA to PS, and as well as reducing hydrogen passivation plasma for neutral brush layer etch. A SiCl4 encapsulation process was introduced to increase the selectivity towards Polystyrene during the Si3N4 hardmask patterning. Finally, DSA full pattern transfer to create 14nm half-pitch line trenches with the silicon depth of 67nm was achieved. Line width roughness (LWR) and line edge roughness (LER) of 2.56nm and 2.64nm respectively were achieved after PMMA removal. This DSA-based 14nm half pitch line and space enable the building of fin structures meeting future 7nm-technology node (N7) design rules.

Defect Precursor and Metal Reduction from DSAL Resist using Filtration and Ion Exchange

Gel particle is a primal defect in block copolymer (BCP) layer in directed self assembly lithography(DSAL). We studied gel removal performance for various lithography process filters and found that nylon 6,6 membrane outperformed other membrane materials exerting adsorption performance similarly to the conventional chemical amplified resist (CAR) filtration. Contrary to the CAR, BCP is synthesized using metal polymerization initiator and the metal in the resist layer should be removed to prevent potential impact to the device integrity failure or the yield loss. We also studied metal reduction from the BCP solution using ion exchange filter and particle removal filters. As a result, ion exchange filter effectively reduced metals from BCP solution as expected. In addition, unexpectedly, nylon 6,6 membrane best reduced some metal species in the BCP solution.