Block Copolymer Films Research Articles

Dewetting-Induced Alignment and Ordering of Cylindrical Mesophases in Thin Block Copolymer Films

The fabrication of ordered self-assembled nanopatterns in block copolymer films, especially aligned nanostructures, has gained considerable attention. Here, we present a simple approach based on de...

Degradation of Block Copolymer Films Confined in Elastic Media: Molecular Dynamics Simulations

We performed coarse-grained molecular dynamics simulations of swelling and degradation of copolymer films confined within polymer networks with different values of shear modulus. The microphase sep...

Self-Directed Self-Assembly of 3D Tailored Block Copolymer Nanostructures.

Directed self-assembly (DSA) of block copolymers (BCPs) provides a powerful tool to fabricate various 2D nanostructures. However, it still remains a challenge to extend DSA to make uniform and complex 3D nanostructures through BCP self-assembly. In this paper, we introduce a method to fabricate various nanostructures in 3D and test it using simulations. In particular, we employ dissipative particle dynamics (DPD) simulation to demonstrate that uniform multilayer nanostructures can be achieved by alternating the stacking of two "orthogonal" BCPs films, AB copolymer film and AC copolymer film, without the need to cross-link or etch any of the components. The assembly of a new layer occurs on top of the previous bottom layer, and thus the structural information from the substrate is propagated upward in the film, a process we refer to as self-directed self-assembly (SDSA). If this process is repeated many times, one can have tailored multilayer nanostructures. Furthermore, the natural (bulk) phases of the block copolymers in each layer do not need to be the same, so one can achieve complex 3D assemblies that are not possible with a single-phase 3D system. This method in conjunction with grapho (or chemo) epitaxy is able to evolve a surface pattern into a 3D nanostructure. Here we show several examples of nanostructures fabricated by this process, which include aligned cylinders, spheres on top of cylinders, and orthogonal nanomeshes. Our work should be useful for creating complex 3D nanostructures using self-assembly.

Synthesis of Block Copolymer Brush by RAFT and Click Chemistry and Its Self-Assembly as a Thin Film.

A well-defined block copolymer brush poly(glycidyl methacrylate)-graft-(poly(methyl methacrylate)-block- poly(oligo(ethylene glycol) methyl ether methacrylate)) (PGMA-g-(PMMA-b-POEGMA)) is synthesized via grafting from an approach based on a combination of click chemistry and reversible addition-fragmentation chain transfer (RAFT) polymerization. The resulting block copolymer brushes were characterized by 1H-NMR and size exclusion chromatography (SEC). The self-assembly of the block copolymer brush was then investigated under selective solvent conditions in three systems: THF/water, THF/CH3OH, and DMSO/CHCl3. PGMA-g-(PMMA-b-POEGMA) was found to self-assemble into spherical micelle structures as analyzed by transmission electron microscopy (TEM) and dynamic light scattering (DLS). The average size of the particles was much smaller in THF/CH3OH and DMSO/CHCl3 as compared with the THF/water system. Thin film of block copolymer brushes with tunable surface properties was then prepared by the spin-coating technique. The thickness of the thin film was confirmed by scanning electron microscopy (SEM). Atom force microscopy (AFM) analysis revealed a spherical morphology when the block copolymer brush was treated with poor solvents for the backbone and hydrophobic side chains. The contact angle measurements were used to confirm the surface rearrangements of the block copolymer brushes.

Centimeter-scale gas-sieving nanoporous single-layer graphene membrane

High-permeance, molecular-sieving, nanoporous single-layer graphene (NSLG) membranes are highly promising for gas separation. However, the formation of cracks during the transfer of NSLG to a low-cost porous support is difficult to avoid. These cracks are detrimental to gas selectivity, and therefore, make the scale-up of the gas-sieving NSLG membranes challenging. To mitigate the crack formation on low-cost macroporous supports, herein, we demonstrate mechanical reinforcement of the graphene film with a two-layer composite carbon film. The bottom layer of the composite film is a 100-nm-thick block-copolymer film derived nanoporous carbon (NPC) film with a pore size of 20–30 nm. This layer makes an intimate contact with NSLG and prevents generation of crack. However, the NPC film by itself is not robust enough to cover the rough surface of low-cost macroporous supports and tends to generate occasional cracks. This is prevented by spin-coating a 500-nm-thick multi-walled carbon nanotube (MWNT) film, hosting pore size of 200–300 nm, on top of the NPC film. This imparts enough mechanical strength to NSLG/NPC film to be successfully suspended on a low-cost, macroporous, nonwoven metal wire mesh on a centimeter-scale while completely avoiding cracks. As a result, H2/CH4 and H2/CO2 selectivities of 11–23 and 5–8, respectively, higher than the corresponding Knudsen selectivities of 2.8 and 4.7, respectively, are obtained from the centimeter-scale NSLG membranes. The reinforced membranes are mechanically robust and can successfully withstand transmembrane pressure difference of 4 bar. When the MWNT film is directly coated on NSLG without using the intermediate NPC layer, the gas sieving behavior is not observed, likely due to the development of nanoscale cracks. This underlines the crucial role of the hierarchical pore structure in the composite carbon film in realizing the gas-sieving graphene membranes.

Self-template–assisted micro-phase segregation in blended liquid-crystalline block copolymers films toward three-dimensional structures

In-plane mesopatterns derived from block-copolymer (BCP) micro-phase segregation in thin films have attracted much interest in practical applications as well as fundamental research programs. However, phase segregation along the film-normal direction has been less studied. Here, we describe a strategy to concurrently, yet independently, control in-plane micro-phase and out-of-plane macro-phase segregation in multiblended films composed of liquid-crystalline BCPs (LCBCPs), affording spontaneously layered three-dimensional (3D) mesostructures. This strategy relies on sequential liquid crystallization during the cooling process in thermal annealing as follows. The constituent LCBCP with the highest isotropic-transition temperature (Tiso) first liquid-crystallizes and segregates from the other LCBCP mixture remaining in isotropic states to form a noncontaminated layer at the top surface. This preformed LCBCP layer preserves its inherent in-plane pattern and acts as a template guiding the subsequent micro-phase segregations of the other low-Tiso LCBCPs underneath. This self-template-assisted micro-phase segregation (STAMPS) readily provides 3D mesostructures, the potential toward rational material design of which is also demonstrated in water-separation applications.

Self-assembly of block copolymers under non-isothermal annealing conditions as revealed by grazing-incidence small-angle X-ray scattering

An accurate knowledge of the parameters governing the kinetics of block copolymer self-assembly is crucial to model the time- and temperature-dependent evolution of pattern formation during annealing as well as to predict the most efficient conditions for the formation of defect-free patterns. Here, the self-assembly kinetics of a lamellar PS-b-PMMA block copolymer under both isothermal and non-isothermal annealing conditions are investigated by combining grazing-incidence small-angle X-ray scattering (GISAXS) experiments with a novel modelling methodology that accounts for the annealing history of the block copolymer film before it reaches the isothermal regime. Such a model allows conventional studies in isothermal annealing conditions to be extended to the more realistic case of non-isothermal annealing and prediction of the accuracy in the determination of the relevant parameters, namely the correlation length and the growth exponent, which define the kinetics of the self-assembly.

Mechanical and chemical robustness of the aluminum oxide-infiltrated block copolymer films and the resulting aluminum oxide coatings

Block copolymer (BCP) templates have been widely used for the design of functional organic-ceramic composites and ceramic structures. In this study, we explore the mechanical characteristics of such structures designed using sequential infiltration synthesis (SIS). We demonstrate that SIS significantly reinforces the BCP template and decreases their sensitivity to temperature and chemical environment. We also probe tribological characteristics of nanoporous alumina films obtained from the organic-ceramic composites after the BCP template removal. We determined with atomic force microscopy (AFM) that the lateral forces between the tip and the substrate are highly affected by the surface reactivity, surface structure, area of contact, environment, and porosity accessibility and interconnectivity. As confirmed by the quartz crystal microbalance (QCM) analysis, the high accessibility of the pores in the alumina film to the water penetration leads to a decrease in the friction for the ~4 μm in size colloidal tip sliding against the nanoporous alumina in contrast to the bulk alumina. The effect diminishes once the size of the AFM probe becomes comparable to the size of the pores, which is attributed to the limited water flow under the tip to support the applied during sliding stresses. Understanding the surface structure and its effect on the mechanical and frictional characteristics of materials is important for the predictive design of coatings that can sustain thermomechanical stresses and can be used to enhance the lubrication of fluids in various mechanical systems.

Kinetic Monitoring of Block Copolymer Self-Assembly Using In Situ Spectroscopic Ellipsometry.

Understanding the kinetic pathways of self-assembly in block copolymers (BCPs) has been a long-standing challenge, mostly due to limitations of in situ monitoring techniques. Here, we demonstrate an approach that uses optical birefringence, determined by spectroscopic ellipsometry (SE), as a measure of domain formation in cylinder- and lamellae-forming BCP films. The rapid experimental acquisition time in SE (ca. 1 sec) enables monitoring of the assembly/disassembly kinetics of BCP films during solvent-vapor annealing (SVA). We demonstrate that upon SVA, BCP films form ordered domains that are stable in the swollen state, but disorder upon rapid drying. Surprisingly, the disassembly during drying strongly depends on the duration of solvent exposure in the swollen state, explaining previous observations of loss of order in SVA processes. SE thus allows for decoupling of BCP self-assembly and disordering that occurs during solvent annealing and solvent evaporation, which is difficult to probe using other, slower techniques.

Nanostructured Films of Oppositely Charged Domains from Self-Assembled Block Copolymers

The exploration of poly(tert-butyl methacrylate)-block-poly(4-vinylpyridine), PtBMA-b-P4VP, as a precursor material for the creation of nanostructured films with oppositely charged domains is reported. Thin films of hexagonally packed P4VP cylinders were self-assembled perpendicular to the surface and subsequently treated with bromoethane vapor at various times to quaternize pyridinyl nitrogens. The PtBMA matrix was then partially hydrolyzed to poly(methacrylic acid), PMAA, through HCl vapor treatment followed by neutralization by brief submersion in KOH solution. Under optimal treatment conditions, atomic force microscopy and contact angle measurements confirm that the film morphologies remain intact and become more hydrophilic. Time-of-flight secondary ion mass spectrometry confirms the presence and location of specific anions and cations within each domain throughout the block copolymer film and corroborates successful ionization during each treatment step. These results bolster the viability of PtBMA-b-P4VP as a suitable material for creating self-assembled nanostructures bearing oppositely charged domains under relatively facile conditions and open the door to future investigations for their potential application in charged mosaic membranes.

Effect of Molecular Weight and Layer Thickness on the Dielectric Breakdown Strength of Neat and Homopolymer Swollen Lamellar Block Copolymer Films.

Designing next-generation lightweight pulsed power devices hinges on understanding the factors influencing the energy storage performance of dielectric materials. Polymer dielectric films have a quadratic dependence of energy storage on the voltage breakdown strength, and strategies to enhance the breakdown strength are expected to yield a path toward high energy storage densities. Highly stratified lamellar block copolymer (L-BCP) films of model polystyrene-b-polymethylmethacrylate (PS-b-PMMA) exhibited as much as ~50% enhancement in breakdown voltage ( ) (225% increase in stored energy density, ) compared to unordered as-cast L-BCP films. Such an energy density using amorphous polymer is on par with industry-standard semicrystalline biaxially oriented polypropylene (BOPP) and as such a notable development in the field. This work develops a deeper understanding of the molecular mechanisms of enhancement in L-BCP films, relating directly to molecular weight ( ), with interpretation to effects of chain-end density and distribution, interface formation, layer thickness, and their relative contributions. As-cast disordered L-BCP films show decreasing with decreasing similar to homopolymer studies because of the increase of homogeneously distributed chain ends in the film. increases significantly in parallel ordered L-BCP films because of the combination of interface formation and spatial isolation of the chain ends into segregated zones. We further confirm the role of chain ends in the breakdown process blending a low L-BCP with matched homopolymers to attain the same layer spacing as neat L-BCP of higher . shows a significant decrease at low homopolymer fractions because of increased net chain-end density within swollen ordered L-BCP domains in wet-brush regime, followed by increased because of layer thickness increase via segregated "interphase layer" formation by excess homopolymers. Notably, of homopolymer swollen L-BCPs is always lower than that of neat L-BCPs of the same domain spacing because of overall adverse chain-end contribution from homopolymers. These findings provide important selection rules for L-BCPs for designing next-generation flexible electronics with high energy density solid-state BCP film capacitors.

Reversible polystyrene-block-poly(methyl methacrylate) copolymer films with perpendicular orientation by ultra-thin polystyrene substrates

Detecting the inner structures in the perpendicularly orientated block copolymer (BCP) films is beneficial to understand the in-depth arrangement of nano-domains. This study presents a method to reverse the perpendicularly orientated polystyrene-block-poly(methyl methacrylate) (PS-b-PMMA) BCP films for observing their bottom morphologies. Only PS brush or mat was used as neutral substrates, because strongly polar groups inside the PS chains exposed to balance the interfacial energies of PS-OH chemical substrates. The perpendicular window for fingerprint morphology of PS-b-PMMA BCP was among 1.16∼1.44 nm of PS brush on silicon wafer with silanol group. For crosslinked PS mat on water soluble substrate of sodium polyacrylate, however, perpendicular window was shortened to be around 1.37 nm, which was confirmed by both interfacial energy and images of scanning electron microscope (SEM). The bottom views of PS-b-PMMA block copolymer films with perpendicular and mixed orientation were observed freshly. There was no obvious difference between the fingerprints of top and bottom, indicating that perpendicular orientation in the block copolymer was through the film up to down. Therefore, the challenges of degrees of perfection from directed assembly of various morphologies are deduced to be originated from match of the morphology with the e-beamed prepattern or defects in the prepattern.

Universal Interfacial Control through Polymeric Nanomosaic Coating for Block Copolymer Nanopatterning.

The employment of self-assembly of soft materials has been accepted as an inexpensive, robust, and reliable patterning method. As their self-assembly relies on the delicate molecular interactions near the substrate, a precise prediction/control of the interface structure and dynamics is critical to achieve desired nanostructures. Herein, a polymeric nanomosaic (PNM) pattern is created from the air/water interfacial self-assembly of a block copolymer (BCP) and introduced as an effective interfacial energy control for substrates. As a demonstration, the PNM coating is employed to control the BCP film structures. The perpendicular orientation of BCP self-assembly, which requires neutral wetting conditions for both blocks, is difficult to achieve but can readily be obtained with the PNM coating upon a fine resolution of the pattern quality. The universal applicability of the PNM coating as an interfacial control has been confirmed on curved, flexible, and three-dimensional substrates. In addition, the PNM is introduced as an etching-free and reusable topcoat imparting free surface neutralization even for the high-χ BCP nanopatterning.

Room-temperature swelling of block copolymers for nanoporous membranes with well-defined porosities

Selective swelling of block copolymers (BCPs) has been an efficient way to produce nanoporous materials with well-defined porosities. Currently, the cavitation of bulk BCP films by selective swelling needs to be performed at elevated temperatures (typically 50–70 °C), which is not only tedious in operation but also energy extensive. Herein, we develop the room-temperature (RT) swelling strategy to enable pore formation in BCP films with no need of heating. Simply introducing a solvent selective to the majority blocks of BCPs into the swelling bath allows the initiation of selective swelling-induced pore generation at room temperature because of plasticizing effect on BCP matrix. The swelling degrees and porosities of BCP films can be flexibly tuned by changing the compositions of the swelling bath and the swelling durations. The universality of this RT swelling strategy is also demonstrated by its applicability to both polystyrene (PS)-based BCPs and polysulfone-based BCPs with high glass transition temperatures. We prepare composite membranes with nanoporous polystyrene-block-poly (2-vinyl pyridine) (PS-b-P2VP) selective layers on top of macroporous substrates by using this RT swelling approach. Interestingly, thus-produced membranes exhibit more uniform pores with narrow pore size distributions, resulting in better selectivity. RT swelling is featured as convenient and energy-saving, and is desired in large-scale manufacturing of nanoporous BCP materials for various applications.

Block copolymer directed self-assembly defect modes induced by localized errors in chemoepitaxial guiding underlayers: A molecular simulation study

Block copolymer (BCP) directed self-assembly (DSA) has been presented as a potential economically attractive enhancement to extend the capabilities of optical lithography for semiconductor manufacturing. One concern in DSA is the level of defectivity that can be achieved in such a process. Although entropic effects will always lead to some degree of defectivity, highly ordered structures with a low theoretical equilibrium defect density can be produced by guiding the ordering and placement of the BCP domains using a patterned underlayer. Recent experimental studies have shown that while DSA processes can significantly reduce the observed defect density, defectivity levels are generally still higher than allowable for high-volume manufacturing and higher than what would be anticipated from free energy estimates of the observed defect modes. In particular, bridge defects are one of the most commonly observed defect modes in experimental DSA studies. A number of hypotheses have been proposed to explain the origins of these defects. One hypothesis is that so-called affinity defects present in the underlayer can spawn bridge defects in the overlying BCP film. The goal of the work reported here was to investigate the extent to which bridge defects can be generated or further reinforced in lamellae-forming block copolymer films due to affinity defects in the underlayer pattern. Coarse-grained molecular dynamics simulations were used to simulate the chemoepitaxial DSA of monodisperse block copolymer films atop underlayers with varying affinity defect sizes. Affinity defects were simulated by creating circular regions of a single polymer block type (which is the opposite block type of that used to pattern the underlayer guiding stripes) in the nominally neutral background region of the underlayer. These affinity defects were positioned in regions of the underlayer where they were the incorrect type to match the overlying block copolymer pattern. It was observed that the presence of an affinity defect in the neutral region of the underlayer caused the energetically preferential polymer block to wet the affinity defect, thus creating the nucleus of what could potentially become a bridge defect—even when the affinity defects were very small. As the radius of the underlayer affinity defect (RoD) increased, the amount of block copolymer of incorrect type (with respect to a perfectly assembled copolymer pattern) that assembled above the affinity defect increased; but, in general, the thickness of the wetting layer in contact with the affinity defect was only roughly one polymer chain thick. These data suggest that an affinity defect in the underlayer alone is unlikely to be noticeably enhanced by significant bridge defect formation in a monodisperse block copolymer film.

Microdomain Orientation of Star-Shaped Block Copolymer Thin Film Depending on Molecular Weight

The vertical orientation of microdomains of a block copolymer (BCP) on a substrate is essential to applying nanolithography. Thin films of star-shaped block copolymers have shown vertical orientati...

Self‐assembled nanostructures of diblock copolymer films under homopolymer topcoats

AbstractTo mitigate the interface energies of block copolymers with high interaction parameters, a topcoat strategy has been experimentally proposed to produce perpendicular oriented domains on a length scale of sub‐10 nm. However, the origin of perpendicular oriented domains and the effect of topcoats on the self‐assembled nanostructures remain to be uncovered. Herein, we use the dynamic self‐consistent field theory to explore the self‐assembly behaviors of symmetric block copolymer films under homopolymer topcoats. It is clearly demonstrated that the introduction of homopolymer topcoats enables a wide formation range of perpendicular oriented lamellae, originating from the fluctuating diblock copolymer/homopolymer interfaces in the process of in‐plane microphase separation of block copolymer films. Our simulation results also demonstrate that the formation range of perpendicular oriented lamellae can be tuned by changing the wetting properties of homopolymer topcoats and the thickness of block copolymer films, but is weakly dependent upon the chain length of homopolymers. Our theoretical findings have wide implications for understanding the formation of perpendicular oriented domains of block copolymer films, which are important for the rational design of self‐assembled nanostructures with new horizons for block copolymer lithography. © 2020 Society of Chemical Industry

Porous Ultra-Thin Films from Photocleavable Block Copolymers: In-Situ Degradation Kinetics Study of Pore Material.

On the basis of the major application for block copolymers to use them as separation membranes, lithographic mask, and as templates, the preparation of highly oriented nanoporous thin films requires the selective removal of the minor phase from the pores. In the scope of this study, thin film of polystyrene-block-poly(ethylene oxide) block copolymer with a photocleavable junction groups based on ortho-nitrobenzylester (ONB) (PS-hν-PEO) was papered via the spin coating technique followed by solvent annealing to obtain highly-ordered cylindrical domains. The polymer blocks are cleaved by means of a mild UV exposure and then the pore material is washed out of the polymer film by ultra-pure water resulting in arrays of nanoporous thin films to remove one block. The removal of the PEO materials from the pores was proven using the grazing-incidence small-angle X-ray scattering (GISAXS) technique. The treatment of the polymer film during the washing process was observed in real time after two different UV exposure time (1 and 4 h) in order to draw conclusions regarding the dynamics of the removal process. In-situ X-ray reflectivity measurements provide statistically significant information about the change in the layer thickness as well as the roughness and electron density of the polymer film during pore formation. 4 H UV exposure was found to be more efficient for PEO cleavage. By in-situ SFM measurements, the structure of the ultra-thin block copolymer films was also analysed and, thus, the kinetics of the washing process was elaborated. The results from both measurements confirmed that the washing procedure induces irreversible change in morphology to the surface of the thin film.

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.

Enhancing Optoelectronic Properties of Conjugated Block Copolymers through Crystallization of Both Blocks

Microphase-separated block copolymers composed of electron donor and acceptor blocks may provide morphology control to address many challenges in organic electronics. Crucial to controlling the self-assembly of fully conjugated block copolymers is tuning the interplay between crystallization of the individual blocks and microphase separation between the donor and the acceptor. Thus, we have examined the kinetics of the morphological evolution in P3HT-b-PFTBT block copolymer films during two processes: solution casting and thermal annealing. We use in situ wide-angle and small-angle grazing incidence X-ray scattering to monitor the crystallization of P3HT and microphase separation between the two blocks. We find that during film drying, initial P3HT crystallization happens quickly, before phase separation of the two blocks. However, crystallization is significantly suppressed with respect to neat materials, enabling microphase separation to proceed at time scales after some initial crystallization of the donor block takes place. This enables a mesoscale structure to develop during processes such as thermal annealing because self-assembly of the lamellar structure takes place before the crystallization of the donor block is complete. We also find that significant crystallization of PFTBT blocks after P3HT crystallization is possible at elevated temperatures. Crystallization of both blocks is important to maximize the performance of solar cells and transistors with block copolymer active layers. As a consequence, we exceed 3% average power conversion efficiencies in P3HT-b-PFTBT photovoltaic devices.