Diblock Copolymer Films Research Articles

Memory Effect of Self‐Assembled PS‐b‐PEO Block Copolymer Films with Selectively Embedded Functionalized TiO2 Nanoparticles

The memory effect in organic thin film transistors is described with a dielectric hybrid material composed of selectively localized charge trapping titania nanoparticles in a polystyrene‐b‐polyethyleneoxide (PS‐b‐PEO) diblock copolymer film. Upon solvent annealing in toluene atmosphere, the films readily form nanophase‐separated features of PEO cylinders in a matrix of PS. X‐ray reflectivity (XRR) investigations reveal that the TiO2 particles are selectively distributed in the top half of the hybrid layers. Applied in organic thin film transistor devices with 2‐tridecyl‐[1]benzothieno[3,2‐b][1]benzothiophen (mono‐C13‐BTBT) semiconductor, the hybrid layers induce clockwise drain current hysteresis with promising memory ratio, long‐term stability, and endurance. The occurrence of nonvolatile charge trapping directly relates to the hierarchically assembled nanostructure of the nanoparticles in the block copolymer since reference devices with PS homopolymers blended with TiO2 are found to be hysteresis free. Morphological characterization of the hybrid dielectric layers by atomic force microscopy and XRR experiments is presented to shed light on the internal electron density distribution. The layers are subsequently tested in capacitor stacks and transistors.

Photoalignment of Vertically Oriented Microphase Separated Lamellae in LC-LC Diblock Copolymer Thin Film.

The microphase separation and photoalignment behavior of a diblock copolymer consisting of two kinds of side chain liquid crystalline (LC) polymers are investigated. Here, the mesogens of photoresponsive azobenzene and non-photoresponsive cyanobiphenyl are introduced in the side chains. The azobenzene and cyanobiphenyl mesogens orient homeotropically and planarly, respectively, in a thin film state. The combination of these contrasting orienting properties leads to a formation of vertically oriented lamella in the microphase separation structure. Irradiation with linearly polarized light onto this polymer film induces a highly oriented microphase separation structure having regularly elongated stripe morphology with 30 nm periodicity. The nearly defect-free morphology of this LC-LC diblock copolymer after the photoalignment is a distinct feature when compared with those of coil (amorphous)-LC diblock copolymer films possessing the same azobenzene side chain.

Nanopatterned polystyrene-b-poly(acrylic acid) surfaces to modulate cell-material interaction

In this work we explore the effect of surface nanoarchitecture of polystyrene (PS) and polystyrene-b-poly(acrylic acid) (PS-b-PAA) diblock copolymer films on cell viability. PS and PS-b-PAA have been nanopatterned at temperatures of 110, 120 and 140°C using nanoporous aluminium oxide membranes (AAO) as a template. Surface architecture strongly depends on the infiltration temperature and the nature of the infiltrated polymer. High patterning temperatures yield hollow fibre shape architecture at the nanoscale level, which substantially modifies the surface hydrophobicity of the resulting materials. Up to date very scarce reports could be found in the literature dealing with the interaction of microstructured/nanostructured polymeric surfaces with cancer cells. Therefore, MCF-7 breast cancer cells have been selected as a model to conduct cell viability assays. The findings reveal that the fine-tuning of the surface nanoarchitecture contributes to the modification of its biocompatibility. Overall, this study highlights the potential of AAO membranes to obtain well-defined tailored morphologies at nanoscale level and its importance to develop novel soft functional surfaces to be used in the biomedical field.

Effects of nanoparticles on phase morphology in thin films of phase-separated diblock copolymers

This study investigates the morphology changes in thin diblock copolymer (DiBCP) films occurring in the interaction with modified nanoparticles (NPs). Magnetite (Fe3O4) and silica (SiOx) were prepared and used. Poly(pentyl methacrylate-b-methyl methacrylate) (PPMA-b-PMMA) (70/30 mol mol−1, hcp cylinders of the PMMA phase) DiBCP were employed to prepare thin films having thicknesses to realize standing cylinders in pure DiBCP films. The investigations aimed at two topics: (1) morphology after controlled incorporation of organo-modified NP (gold, silver, Fe3O4, SiOx) and (2) additional solvent vapour annealing (SVA) with tetrahydrofuran (and chloroform for comparison). The laterally ordered morphology in thin films was examined by GISAXS and atomic force microscopy. Keeping the same type of morphology in nanocomposites, the dimensions of the periodic nanostructure altered depending on type and amount of incorporated NP. It was found that SiOx clusters enlarge the lateral distance of the PMMA cylinders, whereas metallic NPs reduce this parameter. Applying SVA improves the phase separation slightly, whereas lateral distances were kept constant or were reduced a little. Switching of domain orientation upon SVA could not be detected in the presence of NPs located at the polymer/substrate interface.

Controlled solvent vapor annealing of a high χ block copolymer thin film.

Molecular self-assembling block copolymers (BCPs) have shown promise as a next generation bottom-up lithography technology. However, a critical step in advancing this approach is the elimination of polymer dewetting due to bulk solvent nucleation and thermodynamically driven film rupture that can occur during the solvent vapor annealing process. We report on the pattern formation via phase segregation of spin coated diblock copolymer films through the investigation of annealing parameters in the limit of high solvent vapor saturation conditions that results in wafer-scale patterning without observing polymer dewetting defects. Specifically, the work addresses polymer dewetting in diblock copolymer nanodot templates through the use of a "neutral" functionalization layer and the development of a custom-built solvent vapor annealing chamber to precisely control saturation conditions. Furthermore, the long anneal times (4 h) using a standard static solvent vapor annealing procedure were reduced to ∼15-30 minutes with our dynamic solvent vapor annealing system for the high χ, cylindrical forming poly(styrene)-block-poly(4-vinyl-pyridine) [PS-b-P4VP] diblock copolymer system. We discuss the kinetic mechanism governing the phase segregation process that highlights the small processing window bounded by long phase segregation timescales (≳1 min) on one side and the initiation of polymer film dewetting on the other. These results demonstrate a key step towards realizing a high fidelity, low cost BCP patterning technique for large-scale "bottom-up" feature definition at nanometer length scales.

Defect-Free Perpendicular Diblock Copolymer Films: The Synergy Effect of Surface Topography and Chemistry

We propose a direct self-assembly mechanism towards obtaining defect-free perpendicular lamellar phases of diblock copolymer (BCP) thin films. In our numerical study, a thin BCP film having a flat top surface is casted on a uni-directional corrugated solid substrate. The substrate is treated chemically and has a weak preference toward one of the two BCP components. Employing self-consistent field theory (SCFT), we find that there is an enhanced synergy between two substrate characteristics: its topography (geometrical roughness) combined with a weak surface preference. This synergy produces the desired perpendicular lamellar phase with perfect inplane ordering. Defect-free BCP lamellar phases are reproducible for several random initial states, and are obtained for a range of substrate roughness and chemical characteristics, even for a uni-directional multi-mode substrate roughness. Our theoretical study suggests possible experiments that will explore the interplay between uni-directional substrate corrugation and chemical surface treatment. It may lead to viable and economical ways of obtaining BCP films with defect-free lateral alignment.

Thermodynamics and Kinetics of Defect Motion and Annihilation in the Self-Assembly of Lamellar Diblock Copolymers

The thermodynamics of dislocations in thin films of lamella-forming diblock copolymers and their climb and glide motions are investigated using single-chain-in-mean-field (SCMF) simulations and self-consistent field theory (SCFT) in conjunction with the string method. The glide motion of a defect perpendicular to the stripe pattern is characterized by large free energy barriers. The barriers not only stem from altering the domain topology; an additional barrier arises from a small-amplitude but long-range domain displacement. In contrast, the climb motion along the stripes does not involve a free energy barrier in accord with the continuous translational invariance along the stripe. Thus, the perpendicular distance (“impact parameter”) between a pair of defects is approximately conserved. Dislocation pairs with opposite Burgers vectors attract each other and move toward each other (“collide”) via climb motion. We find that the forces between apposing defects significantly depend on system size, and the Pe...

Hierarchical Directed Self‐Assembly of Diblock Copolymers for Modified Pattern Symmetry

Block copolymer lithography exploiting diblock copolymer thin films is promising for scalable manufacture of device‐oriented nanostructures. Nonetheless, its intrinsic limitation in the degree of freedom for pattern symmetry within hexagonal dot or parallel line array greatly diminishes the potential application fields. Here, we report multi‐level hierarchical self‐assembled nanopatterning of diblock copolymers for modified pattern symmetry. Sequential hierarchical integration of two layers of diblock copolymer films with judiciously chosen molecular weights and chemical composition creates nanopatterned morphology with modified pattern symmetry, including sparse linear cylinder or lamellar arrays. Internal structure of the hierarchically patterned morphology is characterized by grazing‐incidence small‐angle X‐ray scattering throughout the film thickness. Pattern transfer of the modified nanopattern generates linear metal nanodot array with uniform size and regular spacing as a typical example of functional nanopatterned structures.

Two‐dimensionally ordered AuNP array formation via microcontact printing on lamellar diblock copolymer films

The construction of nano‐sized, two‐dimensionally ordered nanoparticle (NP) superstructures is important for various advanced applications such as photonics, sensing, catalysis, or nano‐circuitry. Currently, such structures are fabricated using the templated organization approach, in which the templates are mainly created by photo‐lithography or laser‐lithography and other invasive top‐down etching procedures. In this work, we present an alternative bottom‐up preparation method for the controlled deposition of NPs into hierarchical structures. Lamellar polystyrene‐block‐poly(2‐vinylpyridinium) thin films featuring alternating stripes of neutral PS and positively charged P2VP domains serve as templates, allowing for the selective adsorption of negatively charged gold NPs. Dense NP assembly is achieved by a simple immersion process, whereas two‐dimensionally ordered arrays of NPs are realized by microcontact printing (μCP), utilizing periodic polydimethylsiloxane wrinkle grooves loaded with gold NPs. This approach enables the facile construction of hierarchical NP arrays with variable geometries. Copyright © 2016 John Wiley & Sons, Ltd.

Assembly structure and rod orientation of rod–coil diblock copolymer films

The self-assembly and rod orientation of rod–coil (RC) diblock copolymer (DBC) films confined between two identical impenetrable and rod-selective walls was investigated by performing dissipative particle dynamics simulations. Various structures, such as a wetting layer near the surfaces, perpendicular cylinders, parallel cylinders and island-like structures, were observed. Thus a morphological phase diagram as a function of the film thickness and rod length was presented. A long-range rod–rod orientational order was observed in the assembly structures for a rod block above a critical rod length. The critical rod length for the disorder–order transition of the rod orientation was found to be dependent on the assembly structure and film thickness. Our results indicate that the rod length and film thickness influence both the assembly structure and the rod orientation of RC DBCs. The assembly structure and the critical rod length for the disorder–order orientational transition in rod–coil diblock copolymer films depend on the property of the wall surface and the film thickness.

Polymer Surface Transport Is a Combination of in-Plane Diffusion and Desorption-Mediated Flights.

Previous studies of polymer motion at solid/liquid interfaces described the transport in the context of a continuous time random walk (CTRW) process, in which diffusion switches between desorption-mediated "flights" (i.e., hopping) and surface-adsorbed waiting-time intervals. However, it has been unclear whether the waiting times represented periods of complete immobility or times during which molecules engaged in a different (e.g., slower or confined) mode of interfacial transport. Here we designed high-throughput, single-molecule tracking measurements to address this question. Specifically, we studied polymer dynamics on either chemically homogeneous or nanopatterned surfaces (hexagonal diblock copolymer films) with chemically distinct domains, where polymers were essentially excluded from the low-affinity domains, eliminating the possibility of significant continuous diffusion in the absence of desorption-mediated flights. Indeed, the step-size distributions on homogeneous surfaces exhibited an additional diffusive mode that was missing on the chemically heterogeneous nanopatterned surfaces, confirming the presence of a slow continuous mode due to 2D in-plane diffusion. Kinetic Monte Carlo simulations were performed to test this model and, with the theoretical in-plane diffusion coefficient of D2D = 0.20 μm2/s, we found a good agreement between simulations and experimental data on both chemically homogeneous and nanopatterned surfaces.

Confinement effects on the miscibility of block copolymer blends.

Thin films of long and short symmetric AB diblock copolymers are examined using self-consistent field theory (SCFT). We focus on hard confining walls with a preference for the A component, such that the lamellar domains orient parallel to the film with an even number ν of monolayers. For neat melts, confinement causes the lamellar period, D, to deviate from its bulk value, Db, in order to be commensurate with the film thickness, i.e., L = νD/2. For blends, however, the melt also has the option of macrophase separating into ν(l) large and ν((s)) small monolayers so as to provide a better fit, where L = ν(l)D(l)/2 + ν(s)D((s))/2. In addition to performing full SCFT calculations of the entire film, we develop a semi-analytical calculation for the coexistence of thick and thin monolayers that helps explain the complicated interplay between miscibility and commensurability.

Combined Mesoscale/Experimental Study of Selective Placement of Magnetic Nanoparticles in Diblock Copolymer Films via Solvent Vapor Annealing

We present a combined theoretical/experimental study to investigate the effect of selective solvent vapor annealing treatment on the obtainment of highly ordered morphologies of symmetric poly(styrene-b-methyl methacrylate) diblock copolymer (PS-b-PMMA DBC) films loaded with compatibilized magnetic (Fe3O4) nanoparticles (NPs). Different amounts of NPs were considered (1, 2, and 5 wt %) to study the effect of the inorganic content on the final properties of Fe3O4/PS-b-PMMA films. A precise control of the DBC nanostructure could be obtained by very simple and cost-effective fabrication steps, compatible with current industrial processes. Moreover, the modified NPs could be selectively placed into the PMMA domains of the DBC up to NP concentration of 5 wt % while preserving the corresponding DBC lamellar morphology.

Multi-tube area detector developed for reactor small-angle neutron scattering spectrometer SANS-J-II

A newly developed multi-tube area detector for a small-angle neutron scattering (SANS) spectrometer (SANS-J-II) at the research reactor JRR-3 in Tokai, Japan, has been implementedviathe use of one-dimensional position-sensitive3He detectors (tubes). Ninety-six active tubes of 8 mm in diameter and 650 and 580 mm in length were filled with 15 atm (1.52 MPa) of3He and aligned vertically parallel in order to cover a sufficiently large area for small-angle scattering measurement. These tubes are enclosed in an air chamber together with neutron encode and GATENET modules (VME boards), which compose a standard data acquisition system for the spallation neutron source of the Japan Proton Accelerator Research Complex. This system facilitates the acquisition of time-of-flight neutron event data. The multi-tube detector is mounted on a truck moving in a vacuum chamber of the SANS spectrometer. After discriminating noise originating from γ-rays, and calibrating the positions and sensitivities of individual tubes, the resolution was determined (i.e.channel widths along parallel and vertical directions along a tube). The counting rate of one tube was determined to be 1.4 × 103counts per second with a counting loss of 1%. This implies that the new detector, composed of 96 tubes, can detect more than 105neutrons per second with a counting loss of 1%. To demonstrate its use, small-angle scattering originating from a diblock copolymer film with a highly oriented lamellar microdomain was observed. The data acquisition in event mode has a great advantage in time-resolved measurements that are synchronized with external stimuli imposed on a sample.

Domain configurations in Co/Pd and L10-FePt nanowire arrays with perpendicular magnetic anisotropy.

Perpendicular magnetic anisotropy [Co/Pd]15 and L10-FePt nanowire arrays of period 63 nm with linewidths 38 nm and 27 nm and film thickness 27 nm and 20 nm respectively were fabricated using a self-assembled PS-b-PDMS diblock copolymer film as a lithographic mask. The wires are predicted to support Néel walls in the Co/Pd and Bloch walls in the FePt. Magnetostatic interactions from nearest neighbor nanowires promote a ground state configuration consisting of alternating up and down magnetization in adjacent wires. This was observed over ∼75% of the Co/Pd wires after ac-demagnetization but was less prevalent in the FePt because the ratio of interaction field to switching field was much smaller. Interactions also led to correlations in the domain wall positions in adjacent Co/Pd nanowires. The reversal process was characterized by nucleation of reverse domains, followed at higher fields by propagation of the domains along the nanowires. These narrow wires provide model system for exploring domain wall structure and dynamics in perpendicular anisotropy systems.

Order-disorder transition in thin films of horizontally-oriented cylinder-forming block copolymers: thermal fluctuations vs. preferential wetting.

We present experimental and theoretical investigations of the order-disorder transition (ODT) in thin films of cylinder-forming diblock copolymers with asymmetric wetting conditions. Grazing incidence small-angle X-ray scattering (GISAXS) was implemented to determine the ODT temperatures (TODT) for poly(styrene-b-2-vinyl pyridine) (PS-P2VP) block copolymer thin films on a P2VP-preferential silicon substrate. Specifically, films consisting of multilayers of horizontally-oriented cylindrical structures (from 1- to 9-layers) were tested. We find that films with more than 2 cylindrical layers have a TODT comparable to the bulk case. However, TODT decreases as the film becomes thinner and the monolayer system has an ODT 30 °C below the bulk. Using self-consistent field theory (SCFT), we studied the ordering in corresponding thin films with asymmetric (top and bottom surface) wetting conditions. Surprisingly, SCFT is found to predict an opposite trend in TODT with film thickness than observed experimentally. Field-theoretic simulations with complex Langevin sampling were employed to resolve this discrepancy and demonstrate that thermal fluctuations in the PS-P2VP thin-film system dominate its ordering behavior in monolayer and bilayer films.

Vertical Domain Orientation in Cylinder-Forming Diblock Copolymer Films upon Solvent Vapor Annealing

Dissipative particle dynamics was applied to simulate solvent vapor annealing in films of cylinder-forming diblock copolymers. Our simulations reveal a wide range of conditions, under which the vertical domain orientation is promoted by the fast solvent evaporation. First, the order-to-disorder transition must be induced upon swelling of the film. This condition ensures the independence of the structure formation during drying on the history, i.e., the preparation conditions. Second, the weak segregation regime has to be achievable during the evaporation process. This condition provides more “pathways” for structural transformations which are not accompanied by a strong penalty in the interfacial energy. The third condition for vertical orientation of the cylindrical nanodomains is a weak selectivity of the solvent. In the swollen state, a stronger swelling of the majority domains results in the formation of spherical micelles (rather than cylindrical). Upon drying of the film, these spherical micelles jo...

Structural details and digital memory performances of difluorene-containing diblock copolymers in nanoscale thin films

Amphiphilic poly(4-di(9,9-dihexylfluoren-2-yl)styrene)-b-poly(2-vinylpyridine)s (PStFl2m-b-P2VPn) in two different compositions and their homopolymers were synthesized: PStFl211-b-P2VP89 (50/50, volume ratio), PStFl212-b-P2VP33 (75/25), PStFl2, and P2VP. They were thermally stable up to around 350°C. In nanoscale thin films, the diblock copolymers exhibited various phase-separated nanostructures depending on the composition and film process condition: random two phases, horizontal hexagonal P2VP cylinders, and hexagonally-close packed (HCP) P2VP spheres. Surprisingly, the hexagonal cylinder and HCP sphere structures are quite different from those of common diblock copolymers with similar compositions. The structural details of these thin film morphologies were investigated by synchrotron grazing incidence X-ray scattering. The thin film morphologies were found to make influences on the electrical memory performances of the polymers. In particular, the switching-ON voltage was influenced by the nanostructures and the film layer thickness as well as by the composition. Overall, the diblock copolymer films demonstrated excellent p-type permanent digital memory behaviors with unipolarity, long retention time, high ON/OFF current ratio and low power consumption. These memory behaviors were governed by a trap-limited space charge limited conduction mechanism combined with ohmic conduction and a hopping process.

Electron Propagation within Redox-Active Microdomains in Thin Films of Ferrocene-Containing Diblock Copolymers.

This paper reports the electrochemical behavior of redox-active microdomains in thin films of ferrocene-containing diblock copolymers, polystyrene-block-poly(2-(acryloyloxy)ethyl ferrocenecarboxylate) (PS-b-PAEFc). PS-b-PAEFc with different PAEFc volume fractions (PS154-b-PAEFc51, PS154-b-PAEFc26, and PS154-b-PAEFc12, where the subscripts represent the polymerization degree of each block; f(PAEFc) = 0.47, 0.30, and 0.17, respectively) was synthesized by sequential atom transfer radical polymerization. PS-b-PAEFc films of controlled thicknesses (20-160 nm) were prepared on gold substrates via spin-coating and characterized by ellipsometry. Microdomains were observed via atomic force microscopy on the surfaces of PS154-b-PAEFc51 and PS154-b-PAEFc26 thin films but not on the surfaces of PS154-b-PAEFc12 thin films. Electrochemical behavior of films was assessed by cyclic voltammetry and chronocoulometry in acetonitrile solution. The redox potential of ferrocene moieties was similar (ca. + 0.29 V vs Fc(+)/Fc) regardless of fPAEFc and film thickness. For PS154-b-PAEFc51 and PS154-b-PAEFc26, thicker films afforded larger faradaic peak currents and exhibited diffusion-controlled voltammograms at faster sweep rates. PS154-b-PAEFc26 produced voltammograms less influenced by solvent-induced swelling than PS154-b-PAEFc51, reflecting the improved morphological stability of PAEFc microdomains by redox-inert PS frameworks. In contrast, PS154-b-PAEFc12 films yielded similar faradaic peak currents regardless of film thickness and exhibited voltammograms indicative of surface-confined species. These observations suggest that PS154-b-PAEFc51 and PS154-b-PAEFc26 films contain continuous PAEFc microdomains extending from the electrode to the surface, in contrast to the PS154-b-PAEFc12 films which contain isolated PAEFc microdomains buried within the PS matrix. Electron propagation took place only through PAEFc microdomains that could electrically communicate with the underlying electrode. Apparent diffusion coefficients within PAEFc microdomains were similar (≈ 2 × 10(-11) cm(2)/s) for PS154-b-PAEFc51 and PS154-b-PAEFc26. The relatively low efficiency in electron propagation was attributable to ineffective electron self-exchange reaction within the PAEFc microdomains and/or limited counterion migration through the acetonitrile-swollen microdomains. These results provide guidance in design of redox-active metalloblock copolymers for various applications, which include electrocatalysis, electrochemical mediation in enzyme sensors, and redox-controlled molecular deposition.

Adaptable Ultraviolet Reflecting Polymeric Multilayer Coatings of High Refractive Index Contrast

A synthetic route is demonstrated to build purely polymeric nanostructured multilayer coatings, adaptable to arbitrary surfaces, and capable of efficiently blocking by reflection a targeted and tunable ultraviolet (UV) range. Reflection properties are determined by optical interference between UV light beams reflected at the interfaces between polystyrene layers of different porosity and hence refractive index. As no dopant absorber intervenes in the shielding effect, polymer degradation effects are prevented. Alternated porosity results from the modulation of photochemical effects at the few tens of nanometers length scale, combined with the collective osmotic shock induced during the processing of the precursor diblock copolymer film. Experimental evidence of the application of this method to coat rough surfaces with smooth and conformal UV protecting films is provided.