Surface-induced structure formation of polymer blends on patterned substrates

Tip-Induced Nanopatterning of Ultrathin Polymer Brushes

Patterned, ultra-thin surface layers can serve as templates for positioning nanoparticlesor targeted self-assembly of molecular structures, for example, block-copolymers. This work investigates the high-resolution, atomic force microscopebased patterning of 2 nm thick vinyl-terminated polystyrene brush layers and evaluates the line broadening due to tip degradation. This work compares the patterning properties with those of a silane-based fluorinated self-assembled monolayer (SAM), using molecular heteropatterns generated by modified polymer blend lithography (brush/SAM-PBL). Stable line widths of 20 nm (FWHM) over lengths of over 20000 µm indicate greatly reduced tip wear, compared to expectations on uncoated SiO_x surfaces. The polymer brush acts as a molecularly thin lubricating layer, thus enabling a 5000 fold increase in tip lifetime, and the brush is bonded weakly enough that it can be removed with surgical accuracy. On traditionally used SAMs, either the tip wear is very high or the molecules are not completely removed. Polymer Phase Amplified Brush Editing is presented, which uses directed self-assembly to amplify the aspect ratio of the molecular structures by a factor of 4. The structures thus amplified allow transfer into silicon/metal heterostructures, fabricating 30 nm deep, all-silicon diffraction gratings that could withstand focused high-power 405 nm laser irradiation.

Anisotropic Wettability on One-Dimensional Nanopatterned Surfaces: The Effects of Intrinsic Surface Wettability and Morphology

Large-scale molecular dynamic simulations were conducted to study anisotropic wettability on one-dimensional (1D) nanopatterned surfaces. Hexadecane (C₁₆H₃₄) and decane (C₁₀H₂₂) nanodroplets were used as wetting liquids. Initially, surfaces with various intrinsic wettability (oleophobic and oleophilic) were produced using surface lattice size as a control parameter. These surfaces were subsequently patterned with 1D grooves of different sizes, and their anisotropic wettability was examined. The results show that anisotropic wettability strongly depends on intrinsic surface wettability and surface morphology. The results also demonstrate that the anisotropy in the contact angle is negligible for oleophobic surfaces. However, the anisotropy becomes more evident for oleophilic surfaces and increases with the degree of oleophilicity. Results suggest that anisotropy also depends on the surface morphology, including the patterns' width and height. Monitoring the droplet shape showed that more significant droplet distortion was associated with higher anisotropy. A clear association was lacking between the roughness ratio, r, and the degree of anisotropy. The observed average contact angle for 1D patterned oleophilic surfaces disagreed with the predicted values from the Wenzel theory. However, the theory could correctly predict the state of the droplet being Cassie-Baxter or Wenzel.

Simulation on Pore Formation from Polymer Solution at Surface in Contact with Solid Substrate via Thermally Induced Phase Separation

The formation of porous structures from polymer solutions at the surface in contact with various solid surfaces via a thermally-induced phase separation (TIPS) process is investigated. The pore formation process at the bulk and the surface of the poly(methyl methacrylate)/cyclohexanol solution is simulated with a model based on the phase field method. When the compatibilities between the polymer-rich phase formed by the phase separation and the solid surface are high or low, surface porosity decreases. In contrast, for the solid surface having similar compatibilities with the polymer and solvent, high surface porosity is achieved. This indicates that the compatibility between the solid surface and polymer solution is important, and that optimal compatibility results in high surface porosity. The knowledge obtained in this work is useful to design the coagulation bath component in the membrane preparation process by TIPS for achieving high surface porosity.

Impact of the process conditions on polymer pattern morphology during spin coating over topological surfaces

Micro and nanofabrication techniques depend on the technology of polymer film casting. Spin coating is a relatively robust method to develop uniform polymer films over the substrate surface. However, polymer casting over a topographically prepatterned surface using the spin coating technique is challenging because of the complex transport phenomena involved in the process. Apart from the substrate wettability and the polymer composition, the geometry of the substrate prepatterns affects the polymer phase separation characteristics and thus the morphology of the polymer pattern. In this work the phase separation dynamics during the spinodal decomposition of a polymer-solvent system in a spin coating process is mathematically investigated. The effect of the prepattern topography, substrate wettability, spin-coating rotational speed, and polymer composition on the phase separation dynamics is investigated. The results reveal that the periodicity and phase difference of the polymer peaks with the topography are dependent on the geometric parameters and substrate wettability. The impact of the rotational motion, on the polymer film, is restricted by the surface roughness (due to the topological prepatterns). On reducing the polymer fraction in the solution, the transition from a uniform coating to film defects to isolated patches (wetting to dewetting) occurs. The surface wettability plays a crucial role in topology directed dewetting, which is not observed in flat substrates.

Nanoparticle Induced Morphology Modulation in Spin Coated PS/PMMA Blend Thin Films

The influence of adding nanoparticles on the ascast morphology of spin coated immiscible polystyrene/poly(methyl methacrylate) (PS/PMMA) thin films of different thickness (h_E) and composition (R_B, volume ratio of PS to PMMA) has been explored in this article. To understand the precise effect of nanoparticle addition, the morphology of PS/PMMA thin blend films spin cast from toluene on a native oxide covered silicon wafer substrate was first investigated. It is seen that in particle free films, the generic morphology of the films remains nearly unaltered with increase in h_E, for R_B = 3:1 and 1:3. In contrast, strong h_E dependent morphology transformation is observed in films with R_B = 1:1. Subsequently, thiol-capped gold nanoparticles (AuNP) containing films with different particle concentrations (C_NP) were cast from the same solvent along with the polymer mixture. We observe that addition of AuNPs barely alters the generic morphology of the films with R_B = 3:1. In contrast, the presence of the particles significantly influences the morphology of the films with R_B = 1:1 and 1:3, particularly at higher C_NP (≈10.0%). X-ray photoelectron spectroscopy and X-ray reflectivity of some samples reveal that the AuNPs tend to migrate to the free surface through the PS phase, thereby stabilizing this layer partially or fully (depending on C_NP) against dewetting over a surface of adsorbed PMMA layer and influencing the ascast morphology as a function of C_NP. The work is fundamentally important in understanding largely overlooked implications of nanoparticle addition on the morphology of PS/PMMA blend thin films which forms the fundamental basis for future interesting studies involving dynamics of nanoparticles within the blend thin films.

UV-Blocking, Transparent, and Antioxidant Polycyanoacrylate Films

Applications of cyanoacrylate monomers are generally limited to adhesives/glues (instant or superglues) and forensic sciences. They tend to polymerize rapidly into rigid structures when exposed to trace amounts of moisture. Transforming cyanoacrylate monomers into transparent polymeric films or coatings can open up several new applications, as they are biocompatible, biodegradable and have surgical uses. Like other acrylics, cyanoacrylate polymers are glassy and rigid. To circumvent this, we prepared transparent cyanoacrylate films by solvent casting from a readily biodegrade solvent, cyclopentanone. To improve the ductility of the films, poly(propylene carbonate) (PPC) biopolymer was used as an additive (maximum 5 wt.%) while maintaining transparency. Additionally, ductile films were functionalized with caffeic acid (maximum 2 wt.%), with no loss of transparency while establishing highly effective double functionality, i.e., antioxidant effect and effective UV-absorbing capability. Less than 25 mg antioxidant caffeic acid release per gram film was achieved within a 24-h period, conforming to food safety regulations. Within 2 h, films achieved 100% radical inhibition levels. Films displayed zero UVC (100–280 nm) and UVB (280–315 nm), and ~15% UVA (315–400 nm) radiation transmittance comparable to advanced sunscreen materials containing ZnO nanoparticles or quantum dots. Transparent films also exhibited promising water vapor and oxygen barrier properties, outperforming low-density polyethylene (LPDE) films. Several potential applications can be envisioned such as films for fatty food preservation, biofilms for sun screening, and biomedical films for free-radical inhibition.

Surface-directed spinodal decomposition on chemically patterned substrates

Surface-directed spinodal decomposition (SDSD) is the kinetic interplay of phase separation and wetting at a surface. This process is of great scientific and technological importance. In this paper, we report results from a numerical study of SDSD on a chemically patterned substrate. We consider simple surface patterns for our simulations, but most of the results apply for arbitrary patterns. In layers near the surface, we observe a dynamical crossover from a surface-registry regime to a phase-separation regime. We study this crossover using layerwise correlation functions and structure factors and domain length scales.

Impact of particle arrays on phase separation composition patterns

We examine the symmetry-breaking effect of fixed constellations of particles on the surface-directed spinodal decomposition of binary blends in the presence of particles whose surfaces have a preferential affinity for one of the components. Our phase-field simulations indicate that the phase separation morphology in the presence of particle arrays can be tuned to have a continuous, droplet, lamellar, or hybrid morphology depending on the interparticle spacing, blend composition, and time. In particular, when the interparticle spacing is large compared to the spinodal wavelength, a transient target pattern composed of alternate rings of preferred and non-preferred phases emerges at early times, tending to adopt the symmetry of the particle configuration. We reveal that such target patterns stabilize for certain characteristic length, time, and composition scales characteristic of the pure phase-separating mixture. To illustrate the general range of phenomena exhibited by mixture-particle systems, we simulate the effects of single-particle, multi-particle, and cluster-particle systems having multiple geometrical configurations of the particle characteristic of pattern substrates on phase separation. Our simulations show that tailoring the particle configuration, or substrate pattern configuration, a relative fluid-particle composition should allow the desirable control of the phase separation morphology as in block copolymer materials, but where the scales accessible to this approach of organizing phase-separated fluids usually are significantly larger. Limited experiments confirm the trends observed in our simulations, which should provide some guidance in engineering patterned blend and other mixtures of technological interest.

Lamellar Orientation of a Block Copolymer via an Electron-Beam Induced Polarity Switch in a Nitrophenyl Self-Assembled Monolayer or Si Etching Treatments

Directed self-assembly (DSA) was investigated on self-assembled monolayers (SAMs) chemically modified by electron beam (EB) irradiation, which is composed of 6-(4-nitrophenoxy) hexane-1-thiol (NPHT). Irradiating a NPHT by EB could successfully induce the orientation and selective patterning of block copolymer domains. We clarified that spatially-selective lamellar orientations of polystyrene-block-poly(methyl methacrylate) (PS-b-PMMA) could be achieved by a change of an underlying SAM. The change of an underlying SAM is composed of the transition of an NO2 group to an NH2 group, which is induced by EB. The modification in the polarity of different regions of the SAM with EB lithography controlled the lamellar orientation of PS-b-PMMA. The reduction of the NPHT SAM plays an important role in the orientation of block copolymer. This method might significantly simplify block copolymer DSA processes when it is compared to the conventional DSA process. By investigating the lamellae orientation with EB, it is clarified that only suitable annealing temperatures and irradiation doses lead to the vertical orientation. We also fabricated pre-patterned Si substrates by EB lithographic patterning and reactive ion etching (RIE). DSA onto such pre-patterned Si substrates was proven to be successful for subdivision of the lithographic patterns into line and space patterns.

Spatially resolved polymer classification using laser induced breakdown spectroscopy (LIBS) and multivariate statistics

Synthetic polymers and plastics have become one of the most important materials in our modern world and everyday life with all kinds of applications mainly due to their wide range of excellent and tuneable properties. Several novel materials consisting of multiple different synthetic polymers or composite materials like natural-fiber-reinforced polymer composites have already been reported in literature. Additionally, materials consisting of multiple synthetic polymers already found their way in our daily lives (e.g. double-sided adhesive tape). With emerging materials consisting of different structured synthetic polymers, the need for analytical methods characterizing these kinds of sample also arises. Conventionally, analytical techniques such as FT-IR or Raman spectroscopy are used for polymer classification. Although, these techniques offer laterally resolved investigations they lack the possibility of analyzing depth profiles. In this work, we present laser induced breakdown spectroscopy (LIBS) as a novel and powerful analytical method for spatially resolved polymer classification. As a feasibility study, two exemplary structured synthetic polymer samples (2D structured and multilayer system) have been analyzed using LIBS and the spatial distribution of 5 different synthetic polymers, namely acrylonitrile butadiene styrene (ABS), polylactic acid (PLA), polyethylene (PE), polyacrylate (PAK) and polyvinylchloride (PVC) have been successfully classified using multivariate statistical approaches (principal component analysis (PCA) and k-means clustering). Spatially resolved classification results were validated by comparing the obtained distribution of the 2D structured sample to the elemental distribution of a contamination present in one of the synthetic polymers. Classification of the polymeric multilayer system was validated by comparing the obtained results to a microscopic cross-section. It was shown that LIBS cannot only be used to investigate 2D structured polymer samples but also for direct analysis of depth profiles. Besides synthetic polymer classification, LIBS provides simultaneous analysis of the elemental composition of the sample, which increases the total amount of information accessible with only one measurement.

Structural hierarchy in blends of amphiphilic block copolymers self-assembled at the air-water interface

We present a concurrent self-assembly strategy for patterning hierarchical polymeric surface features by depositing variable-composition blends of polystyrene-block-poly(ethylene oxide) (PS-b-PEO) and polybutadiene-block-poly(ethylene oxide) (PB-b-PEO) block copolymers at the air-water interface. Hierarchical strand networks of hydrophobic PS/PB blocks anchored via PEO blocks to the water surface, with an internal phase-separation structure consisting of periodic domains of PS blocks surrounded and connected by a matrix of PB blocks, are generated by the interplay of interfacial amphiphilic block copolymer aggregation and polymer/polymer phase separation. In contrast to the cylinder-in-strand structures previously formed by our group in which interfacial microphase separation between PS and PB blocks was constrained by chemical connectivity between the blocks, in the current system phase separation between PS and PB is not constrained by chemical connectivity and yet is confined laterally within surface features at the air-water interface. Investigations of multi-component polymer systems with different connectivities constraining repulsive and attractive interactions provides routes to new hierarchical surface patterns for a variety of applications, including photolithography masks, display technology, surface-guided cell growth and tissue engineering.

Formation of High-Density Brush of Liquid Crystalline Polymer Block Associated with Dewetting Process on Amorphous Polymer Film

The understanding of polymer dewetting on solid surfaces is significant in both fundamental polymer physics and practical film technologies. When liquid crystalline (LC) polymers are dewetted, LC ordering is involved in the dewetting process. Here, we report on the characteristic dewetting processes of a diblock copolymer composed of a cyanobiphenyl side chain liquid crystalline polymer (SCLCP) block connected with polystyrene (PS) taking place on a PS base film. Thin films of the block copolymer were prepared by the water-floating method onto the PS film, and the dewetting process is observed in a softened state above the glass transition temperature of the PS. At the smectic A phase temperature of the SCLCP block, the dewetted surface layer generated a flat unique fingering pattern leading to a monolayered (two-dimensional) high-density LC polymer brush through the LC ordering. The important role of the anchoring PS block on the base PS film surface is suggested for the formation of highly stretched LC polymer brush. Above the isotropization temperature, in contrast, ordinary three-dimensional droplet morphologies with smooth round edges were observed. By photo-cross-linking the base PS film, the lateral diffusion rate was significantly reduced. This can be applied to an entropy-driven morphology patterning via dewetting. The polymer brush formation and its spatial controls are expected to provide new opportunities for the modification strategies of polymer surfaces.

Rapid and Selective Deposition of Patterned Thin Films on Heterogeneous Substrates via Spin Coating

The selective deposition of polymer thin films can be achieved via spin coating by manipulating interfacial interactions. While this "spin dewetting" approach sometimes generates spatial localization on topographic and chemical patterns, the connection between material selection, process parameters, and resulting film characteristics remains poorly understood. Here, we demonstrate that accurate control over these parameters allows incomplete trichlorosilane self-assembled monolayers (SAMs) to induce spin dewetting on both homogeneous (SiO₂) and heterogeneous (Cu/SiO₂ or TiN/SiO₂) surfaces. Glassy polymers undergo a sharp transition from uniform wetting to complete dewetting depending on spin speed, solution concentration, polymer molecular weight, and SAM chemistry. Under optimal conditions, spin dewetting on line-space patterns results in the selective deposition of polymer over regions not functionalized with SAM. The insights described herein clarify the importance of different variables involved in spin dewetting and provide access to a versatile strategy for patterning polymeric thin films.

Layer-by-Layer Assembly of Three-Dimensional Optical Functional Nanostructures

Nanotransfer printing (nTP) technology can generate highly functional three-dimensional (3D) nanostructures in a low-cost and high-throughput fashion. Nevertheless, the fabrication yield and quality of the transferred nanostructures are often limited by the merging of the surface patterns of replica stamps during transfer printing. Here, an nTP technology was developed to fabricate large-area and crack-free 3D multilayer nanostructures. Instead of directly depositing materials on the patterned flexible stamp in conventional nTPs, we transferred the nanostructures straightforwardly onto an attached polydimethylsiloxane slab by removing a sacrificial water-soluble poly(acrylic acid) film, which can avoid the cracking of metal film and the failures of printing nanostructures onto target substrates. Based on this approach, subwavelength-thick polarization rotators working at infrared wavelengths were fabricated. Excellent performance of linear polarization rotation over a broadband was realized. This nTP approach could complement existing fabrication techniques and benefit the development of various functional nanostructures with complex multilayer hierarchies.

Substrate micropatterns produced by polymer demixing regulate focal adhesions, actin anisotropy, and lineage differentiation of stem cells

Stem cells are adherent cells whose multipotency and differentiation can be regulated by numerous microenvironmental signals including soluble growth factors and surface topography. This study describes a simple method for creating distinct micropatterns via microphase separation resulting from polymer demixing of poly(desaminotyrosyl-tyrosine carbonate) (PDTEC) and polystyrene (PS). Substrates with co-continuous (ribbons) or discontinuous (islands and pits) PDTEC regions were obtained by varying the ratio of PDTEC and sacrificial PS. Human mesenchymal stem cells (MSCs) cultured on co-continuous PDTEC substrates for 3 days in bipotential adipogenic/osteogenic (AD/OS) induction medium showed no change in cell morphology but exhibited increased anisotropic cytoskeletal organization and larger focal adhesions when compared to MSCs cultured on discontinuous micropatterns. After 14 days in bipotential AD/OS induction medium, MSCs cultured on co-continuous micropatterns exhibited increased expression of osteogenic markers, whereas MSCs on discontinuous PDTEC substrates showed a low expression of adipogenic and osteogenic differentiation markers. Substrates with graded micropatterns were able to reproduce the influence of local underlying topography on MSC differentiation, thus demonstrating their potential for high throughput analysis. This work presents polymer demixing as a simple, non-lithographic technique to produce a wide range of micropatterns on surfaces with complex geometries to influence cellular and tissue regenerative responses.

STATEMENT OF SIGNIFICANCE

A better understanding of how engineered microenvironments influence stem cell differentiation is integral to increasing the use of stem cells and materials in a wide range of tissue engineering applications. In this study, we show the range of topography obtained by polymer demixing is sufficient for investigating how surface topography affects stem cell morphology and differentiation. Our findings show that co-continuous topographies favor early (3-day) cytoskeletal anisotropy and focal adhesion maturation as well as long-term (14-day) expression of osteogenic differentiation markers. Taken together, this study presents a simple approach to pattern topographies that induce divergent responses in stem cell morphology and differentiation.

Percolation of functionalized colloids on patterned substrates

We study the percolation properties for a system of functionalized colloids on patterned substrates via Monte Carlo simulations. The colloidal particles are modeled as hard disks with three equally-distributed attractive patches on their perimeter. We describe the patterns on the substrate as circular potential wells of radius Rp arranged in a regular square or hexagonal lattice. We find a nonmonotonic behavior of the percolation threshold (packing fraction) as a function of Rp. For attractive wells, the percolation threshold is higher than the one for clean (non-patterned) substrates if the circular wells are non-overlapping and can only be lower if the wells overlap. For repulsive wells we find the opposite behavior. In addition, at high packing fractions the formation of both structural and bond defects suppress percolation. As a result, the percolation diagram is reentrant with the non-percolated state occurring at very low and intermediate densities.

Making Nonsticky Surfaces of Sticky Materials: Self-Organized Microtexturing of Viscoelastic Elastomeric Layers by Tearing

Fabrication of large area, multiscale microtextured surfaces engineered for antiadhesion properties remains a challenge. Compared to an elastic surface, viscoelastic solids show much higher surface stickiness, tack, and adhesion owing to the increased contact area and energy dissipation. Here, we show a simple, low cost, large-area and high throughput method with roll-to-roll compatibility to fabricate multiscale, rough microstructures resistant to adhesion in a viscoelastic layer by controlled tearing of viscous film. Even a high adhesive strength viscoelastic solid layer, such as partially cured PDMS, is made nonsticky simply by its controlled tearing. The torn surface shows a fracture induced, self-organized leaflike micropattern resistant to sticking. The topography and adhesion strength of these structures are readily tuned by changing the tearing speed and the film thickness. The microtexture displays a springlike recovery, low adhesive strength, and easy release properties even under the high applied loads.

Capillary Force Lithography Pattern-Directed Self-Assembly (CFL-PDSA) of Phase-Separating Polymer Blend Thin Films

We report capillary force lithography pattern-directed self-assembly (CFL-PDSA), a facile technique for patterning immiscible polymer blend films of polystyrene (PS)/poly(methyl methacrylate) (PMMA), resulting in a highly ordered phase-separated morphology. The pattern replication is achieved by capillary force lithography (CFL), by annealing the film beyond the glass transition temperature of both the constituent polymers, while confining it between a patterned cross-linked poly(dimethyl siloxane) (PDMS) stamp and the silicon substrate. As the pattern replication takes place because of rise of the polymer meniscus along the confining stamp walls, higher affinity of PMMA toward the oxide-coated silicon substrate and of PS toward cross-linked PDMS leads to well-controlled vertically patterned phase separation of the two constituent polymers during thermal annealing. Although a perfect negative replica of the stamp pattern is obtained in all cases, the phase-separated morphology of the films under pattern confinement is strongly influenced by the blend composition and annealing time. The phase-separated domains coarsen with time because of migration of the two components into specific areas, PS into an elevated mesa region and PMMA toward the substrate, because of preferential wetting. We show that a well-controlled, phase-separated morphology is achieved when the blend ratio matches the volume ratio of the elevated region to the base region in the patterned films. The proposed top-down imprint patterning of blends can be easily made roll-to-roll-compatible for industrial adoption.

Columnar-Structured Low-Concentration Donor Molecules in Bulk Heterojunction Organic Solar Cells

We investigate the arrangement of donor molecules in vacuum-deposited bulk heterojunction (BHJ) 1,1-bis-(4-bis(4-methyl-phenyl)-amino-phenyl)-cyclohexane (TAPC):C₇₀-based organic solar cells (OSCs). Even a low dose of donors (∼10%) forms columnar structures that provide pathways for efficient hole transport in the BHJ layer; however, these structures disappear at donor concentrations below 10%, generating disconnected and isolated hole pathways. The formation of columnar donor structures is confirmed by the contrast of the contact potential difference, measured by Kelvin probe force microscopy, and by the trap-assisted charge injection at low donor concentrations. The mobility of electrons and holes is well balanced in OSCs owing to the preservation of the hole mobility at such low donor concentrations, consequently maximizing the internal quantum efficiency of the OSCs. A high power conversion efficiency of 6.24% was achieved in inverted TAPC:C₇₀ (1:9) OSCs.

Gradated Mixed Hole Transport Layer in a Perovskite Solar Cell: Improving Moisture Stability and Efficiency

We demonstrate a simple and facile way to improve the efficiency and moisture stability of perovskite solar cells using commercially available hole transport materials, 2,2',7,7'-tetrakis-(N,N-di-4-methoxyphenylamino)-9,9'-spirobifluorene (spiro-OMeTAD) and poly(3-hexylthiophene) (P3HT). The hole transport layer (HTL) composed of mixed spiro-OMeTAD and P3HT exhibited favorable vertical phase separation. The hydrophobic P3HT was more distributed near the surface (the air atmosphere), whereas the hydrophilic spiro-OMeTAD was more distributed near the perovskite layer. This vertical separation resulted in improved moisture stability by effectively blocking moisture in air. In addition, the optimized composition of spiro-OMeTAD and P3HT improved the efficiency of the solar cells by enabling fast intramolecular charge transport. In addition, a suitable energy level alignment facilitated charge transfer. A device fabricated using the mixed HTL exhibited enhanced performance, demonstrating 18.9% power conversion efficiency and improved moisture stability.

Directed ordering of phase separated domains and dewetting of thin polymer blend films on a topographically patterned substrate

Substrate pattern guided self-organization of ultrathin and confined polymeric films on a topographically patterned substrate is a useful approach for obtaining ordered meso and nano structures over large areas, particularly if the ordering is achieved during film preparation itself, eliminating any post-processing such as thermal or solvent vapor annealing. By casting a dilute solution of two immiscible polymers, polystyrene (PS) and polymethylmethacrylate (PMMA), from a common solvent (toluene) on a topographically patterned substrate with a grating geometry, we show the formation of self-organized meso patterns with various degrees of ordering. The morphology depends on both the concentration of the dispensed solution (C_n) and the blend composition (R_B). Depending on the extent of dewetting during spin coating, the final morphologies can be classified into three distinct categories. At a very low C_n the solution dewets fully, resulting in isolated polymer droplets aligned along substrate grooves (Type 1). Type 2 structures comprising isolated threads with aligned phase separated domains along each substrate groove are observed at intermediate C_n. A continuous film (Type 3) is obtained above a critical concentration (C_n*) that depends on R_B. While the extent of ordering of the domains gradually diminishes with an increase in film thickness for Type 3 patterns, the size of the domains remains much smaller than that on a flat substrate, resulting in significant downsizing of the features due to the lateral confinement imposed on the phase separation process by the topographic patterns. Finally, we show that some of these structures exhibit excellent broadband anti-reflection (AR) properties.

Deposition and Characterization of Roughened Surfaces

Phase separation occurs whenever a solvent leaves a solution of strongly incompatible polymers. This can happen in bulk and in films. Films can be tailored as substrates for multiple applications such as solar cells, surface catalysis, and antireflection coatings. In this study, polystyrene (PS) was dissolved with polyvinyl acetate (PVAc) in different ratios using chlorobenzene as the solvent. Thin films of different ratios of PS and PVAc were deposited on glass via spin coating. The deposited films were investigated for their morphology, strain, surface area, and Raman scattering. The incompatibility between the two polymers leads to the growth of roughened PVAc islands supported by the PS matrix. A down shift in the Raman PVAc signal was observed in the combined film as compared with a 100% PVAc film, which was attributed to the high strain of PVAc that grew as tips. As the PVAc concentration in the polymer blend increases, the porous regions in the film expand and the amount and height of PVAc tips increase as well, up to the point where the pores merge to create a uniform surface. The optimal ratio for the deposition of a uniformly roughened surface is 75% PVAc and 25% PS. For demonstrating a possible application, we applied the partially roughened surface as a substrate for surface-enhanced Raman scattering and demonstrated at least 500% increase in the signal intensity measured in roughened areas. This is explained by the rod effect from the PVAc tips.

Impact of surface roughness on liquid-liquid transition

Liquid-liquid transition (LLT) in single-component liquids is one of the most mysterious phenomena in condensed matter. So far, this problem has attracted attention mainly from the fundamental viewpoint. We report the first experimental study on an impact of surface nanostructuring on LLT by using a surface treatment called rubbing, which is the key technology for the production of liquid crystal displays. We find that this rubbing treatment has a significant impact on the kinetics of LLT of an isotropic molecular liquid, triphenyl phosphite. For a liquid confined between rubbed surfaces, surface-induced barrierless formation of the liquid II phase is observed even in a metastable state, where there should be a barrier for nucleation of the liquid II phase in bulk. Thus, surface rubbing of substrates not only changes the ordering behavior but also significantly accelerates the kinetics. This spatiotemporal pattern modulation of LLT can be explained by a wedge-filling transition and the resulting drastic reduction of the nucleation barrier. However, this effect completely disappears in the unstable (spinodal) regime, indicating the absence of the activation barrier even for bulk LLT. This confirms the presence of nucleation-growth- and spinodal decomposition-type LLT, supporting the conclusion that LLT is truly a first-order transition with criticality. Our finding also opens up a new way to control the kinetics of LLT of a liquid confined in a solid cell by structuring its surface on a mesoscopic length scale, which may contribute to making LLT useful for microfluidics and other industrial applications.

Ambipolar Small-Molecule:Polymer Blend Semiconductors for Solution-Processable Organic Field-Effect Transistors

We report on the fabrication of an organic thin-film semiconductor formed using a blend solution of soluble ambipolar small molecules and an insulating polymer binder that exhibits vertical phase separation and uniform film formation. The semiconductor thin films are produced in a single step from a mixture containing a small molecular semiconductor, namely, quinoidal biselenophene (QBS), and a binder polymer, namely, poly(2-vinylnaphthalene) (PVN). Organic field-effect transistors (OFETs) based on QBS/PVN blend semiconductor are then assembled using top-gate/bottom-contact device configuration, which achieve almost four times higher mobility than the neat QBS semiconductor. Depth profile via secondary ion mass spectrometry and atomic force microscopy images indicate that the QBS domains in the films made from the blend are evenly distributed with a smooth morphology at the bottom of the PVN layer. Bias stress test and variable-temperature measurements on QBS-based OFETs reveal that the QBS/PVN blend semiconductor remarkably reduces the number of trap sites at the gate dielectric/semiconductor interface and the activation energy in the transistor channel. This work provides a one-step solution processing technique, which makes use of soluble ambipolar small molecules to form a thin-film semiconductor for application in high-performance OFETs.

Reversible Diels-Alder Reaction To Control Wrinkle Patterns: From Dynamic Chemistry to Dynamic Patterns

A facile and robust strategy to produce a reversible wrinkle pattern is presented by controlling a dynamic D-A reaction between furan and maleimide. The smart surface with highly reversible morphology and tunable adhesion, wettability, self-healing, and transparency is realized by the thermoreversible generation and erasure of the wrinkle pattern, which might find broad applications in functional intelligent materials with properties that can be tuned on-demand without altering the material's intrinsic properties.

Rhodamine-doped nanoporous polymer films as high-performance anti-reflection coatings and optical filters

We demonstrate a simple and robust procedure for the fabrication of rhodamine-doped nanoporous poly(methyl methacrylate) (PMMA) films, whose optical properties, such as anti-reflection, fluorescence and absorption can be tailored to specific applications. By exploiting phase separation of a binary polymer blend (PMMA and polystyrene), we fabricated foam-like nanoporous films that could be easily and cost-effectively integrated into the fabrication process of optical components. We link film morphology, studied by multifrequency atomic force microscopy (AFM), to the effective refractive index of the films for use as anti-reflection coatings. The film's morphology leads to superior broadband anti-reflection performance compared with homogeneous films. For applications involving optical filters and spectral conversion layers (e.g., for photovoltaic applications), we doped the films with the fluorescent molecule rhodamine, whereby simple variations in the fabrication process enabled us to prepare rhodamine-doped nanoporous PMMA with tunable fluorescence and absorption, without losing the anti-reflective properties. The above combination of optical properties makes the films attractive for a wide range of applications.

Controlling Marangoni-induced instabilities in spin-cast polymer films: How to prepare uniform films

In both research and industrial settings spincoating is extensively used to prepare highly uniform thin polymer films. However, under certain conditions, spincoating results in films with non-uniform surface morphologies. Although the spincoating process has been extensively studied, the origin of these morphologies is not fully understood and the formation of non-uniform spin-cast films remains a practical problem. Here we report on experiments demonstrating that the formation of surface instabilities during spincoating is dependent on temperature. Our results suggest that non-uniform spin-cast films form as a result of the Marangoni effect, which describes flow due to surface tension gradients. We find that both the wavelength and amplitude of the pattern increase with temperature. Finally, and most important from a practical viewpoint, the non-uniformities in the film thickness can be entirely avoided simply by lowering the spin coating temperature.

Phase Separation on Bicontinuous Cubic Membranes: Symmetry Breaking, Reentrant, and Domain Faceting

We study the phase separation of binary lipid mixtures that form bicontinuous cubic phases. The competition between the nonuniform Gaussian membrane curvature and line tension leads to a very rich phase diagram, where we observe symmetry breaking of the membrane morphologies and reentrant phenomena due to the formation of bridges between segregated domains. Upon increasing the line tension contribution, we also find faceting of lipid domains that we explain using a simple argument based on the symmetry of the underlying surface and topology.

Control of Polymer Phase Separation by Roughness Transfer Printing for 2D Microlens Arrays

Great efforts have been devoted to the control of phase separation between blended polymers in terms of the advantages for engineering functional topologies. A simple and straightforward pathway through roughness transfer printing (RTP) is proposed to realize the control of polymer phase separation. The additional roughness difference, which is introduced by trace agarose transferred from a hydrogel stamp, offers a great effect on the rate of nucleation and coalescence orientation of polymethylmethacrylate (PMMA) protrusions grown from a polydimethylsiloxane (PDMS) network. Using a particular topography of agarose stamp and a proper growth time in toluene atmosphere, a 2D microlens array with high uniformity is obtained that shows great potential for optical applications. Moreover, the control of polymer phase separation was successfully extended to the collection and identification of fingerprints with a high degree of replication.

Electrochemical decompatibilisation leads to morphology rearrangements in host-guest polymer blend films

Controlled phase separation in a polymer film, with subsequent morphology rearrangement on the micro-scale, provides novel perspectives in smart materials. Based on our experience on supramolecularly compatibilised polymer blends consisting of polystyrene and poly(butyl methacrylate), we demonstrate here physical segregation of the blend in the solid state by the application of an electrochemical stimulus. The thereby occurring changes in film morphology, namely the appearance of voids and grains, have been characterised by atomic force microscopy in spin coated and in Langmuir-Schaefer deposited films.

Bio-inspired strategies for designing antifouling biomaterials

Contamination of biomedical devices in a biological medium, biofouling, is a major cause of infection and is entirely avoidable. This mini-review will coherently present the broad range of antifouling strategies, germicidal, preventive and cleaning using one or more of biological, chemical and physical techniques. These techniques will be discussed from the point of view of their ability to inhibit protein adsorption, usually the first step that eventually leads to fouling. Many of these approaches draw their inspiration from nature, such as emulating the nitric oxide production in endothelium, use of peptoids that mimic protein repellant peptides, zwitterionic functionalities found in membrane structures, and catechol functionalities used by mussel to immobilize poly(ethylene glycol) (PEG). More intriguing are the physical modifications, creation of micropatterns on the surface to control the hydration layer, making them either superhydrophobic or superhydrophilic. This has led to technologies that emulate the texture of shark skin, and the superhyprophobicity of self-cleaning textures found in lotus leaves. The mechanism of antifouling in each of these methods is described, and implementation of these ideas is illustrated with examples in a way that could be adapted to prevent infection in medical devices.

Significantly Increasing the Ductility of High Performance Polymer Semiconductors through Polymer Blending

Polymer semiconductors based on donor-acceptor monomers have recently resulted in significant gains in field effect mobility in organic thin film transistors (OTFTs). These polymers incorporate fused aromatic rings and have been designed to have stiff planar backbones, resulting in strong intermolecular interactions, which subsequently result in stiff and brittle films. The complex synthesis typically required for these materials may also result in increased production costs. Thus, the development of methods to improve mechanical plasticity while lowering material consumption during fabrication will significantly improve opportunities for adoption in flexible and stretchable electronics. To achieve these goals, we consider blending a brittle donor-acceptor polymer, poly[4-(4,4-dihexadecyl-4H-cyclopenta[1,2-b:5,4-b']dithiophen-2-yl)-alt-[1,2,5]thiadiazolo[3,4-c]pyridine] (PCDTPT), with ductile poly(3-hexylthiophene). We found that the ductility of the blend films is significantly improved compared to that of neat PCDTPT films, and when the blend film is employed in an OTFT, the performance is largely maintained. The ability to maintain charge transport character is due to vertical segregation within the blend, while the improved ductility is due to intermixing of the polymers throughout the film thickness. Importantly, the application of large strains to the ductile films is shown to orient both polymers, which further increases charge carrier mobility. These results highlight a processing approach to achieve high performance polymer OTFTs that are electrically and mechanically optimized.

Phase Separation of Silicon-Containing Polymer/Polystyrene Blends in Spin-Coated Films

In this Article, two readily available polymers that contain silicon and have different surface tensions, polydimethylsiloxane (PDMS) and polyphenylsilsequioxane (PPSQ), were used to produce polymer blends with polystyrene (PS). Spin-coated thin films of the polymer blends were treated by O2 reactive-ion etching (RIE). The PS constituent was selectively removed by O2 RIE, whereas the silicon-containing phase remained because of the high etching resistance of silicon. This selective removal of PS substantially enhanced the contrast of the phase separation morphologies for better scanning electron microscope (SEM) and atomic force microscope (AFM) measurements. We investigated the effects of the silicon-containing constituents, polymer blend composition, concentration of the polymer blend solution, surface tension of the substrate, and the spin-coating speed on the ultimate morphologies of phase separation. The average domain size, ranging from 100 nm to 10 μm, was tuned through an interplay of these factors. In addition, the polymer blend film was formed on a pure organic layer, through which the aspect ratio of the phase separation morphologies was further amplified by a selective etching process. The formed nanostructures are compatible with existing nanofabrication techniques for pattern transfer onto substrates.

Three-dimensional inverse design of nanopatterns with block copolymers and homopolymers

We propose a facile inverse design strategy to generate three-dimensional (3D) nanopatterns by using either block copolymers or a binary homopolymer blend via dissipative particle dynamics simulations. We find that the composition window of block copolymers to form a specific 3D morphology can be expanded when the self-assembly of block copolymers is directed by templates. We also find that a binary homopolymer blend can serve as a better candidate in the inverse templating design, since they have similar performances on recovering the target pattern, with much lower cost. This strategy is proved efficient for fabricating templates with desired topographical configuration, and the inverse design idea sheds lights on better control and design of materials with complex nanopatterns.

Simultaneously enhanced device efficiency, stabilized chromaticity of organic light emitting diodes with lambertian emission characteristic by random convex lenses

An optical functional film applicable to various lighting devices is demonstrated in this study. The phase separation of two immiscible polymers in a common solvent was used to fabricate the film. In this paper, a self-organized lens-like structure is realized in this manner with optical OLED functional film. For an OLED, there are a few optical drawbacks, including light confinement or viewing angle distortion. By applying the optical film to an OLED, the angular spectra distortion resulting from the designed organic stack which produced the highest efficiency was successfully stabilized, simultaneously enhancing the efficiency of the OLED. We prove the effect of the film on the efficiency of OLEDs through an optical simulation. With the capability to overcome the main drawbacks of OLEDs, we contend that the proposed film can be applied to various lighting devices.

Stability and break-up of thin liquid films on patterned and structured surfaces

Solid surfaces with chemical patterning or topographical structure have attracted attention due to many potential applications such as manufacture of flexible electronics, microfluidic devices, microscale cooling systems, as well as development of self-cleaning, antifogging, and antimicrobial surfaces. In many configurations involving patterned or structured surfaces, liquid films are in contact with such solid surfaces and the issue of film stability becomes important. Studies of stability in this context have been largely focused on specific applications and often not connected to each other. The purpose of the present review is to provide a unified view of the topic of stability and rupture of liquid films on patterned and structured surfaces, with particular focus on common mathematical methods, such as lubrication approximation for the liquid flow, bifurcation analysis, and Floquet theory, which can be used for a wide variety of problems. The physical mechanisms of the instability discussed include disjoining pressure, thermocapillarity, and classical hydrodynamic instability of gravity-driven flows. Motion of a contact line formed after the film rupture is also discussed, with emphasis on how the receding contact angle is expected to depend on the small-scale properties of the substrate.

Assessing Mixing Quality of a Copovidone-TPGS Hot Melt Extrusion Process with Atomic Force Microscopy and Differential Scanning Calorimetry

Atomic force microscopy (AFM) and modulated differential scanning calorimetry (mDSC) were used to evaluate the extent of mixing of a hot melt extrusion process for producing solid dispersions of copovidone and D-α-tocopherol polyethylene glycol 1000 succinate (TPGS 1000). In addition to composition, extrusion process parameters of screw speed and thermal quench rate were varied. The data indicated that for 10% TPGS and 300 rpm screw speed, the mixing was insufficient to yield a single-phase amorphous material. AFM images of the extrudate cross section for air-cooled material indicate round domains 200 to 700 nm in diameter without any observed alignment resulting from the extrusion whereas domains in extrudate subjected to chilled rolls were elliptical in shape with uniform orientation. Thermal analysis indicated that the domains were predominantly semi-crystalline TPGS. For 10% TPGS and 600 rpm screw speed, AFM and mDSC data were consistent with that of a single-phase amorphous material for both thermal quench rates examined. When the TPGS concentration was reduced to 5%, a single-phase amorphous material was achieved for all conditions even the slowest screw speed studied (150 rpm).

Pattern formation in thin films of polymer solutions: Theory and simulations

Self-assembly has been recognized as an efficient tool for generating a wide range of functional, chemically, or physically textured surfaces for applications in small scale devices. In this work, we investigate the stability of thin films of polymer solutions. For low concentrations of polymer in the solution, long length scale dewetting patterns are obtained with wavelength approximately few microns. Whereas, for concentrations above a critical value, bimodal dispersion curves are obtained with the dominant wavelength being up to two orders smaller than the usual dewetting length scale. We further show that the short wavelength corresponds to the phase separation in the film resulting in uniformly distributed high and low concentration regions. Interestingly, due to the solvent entropy, at very high concentration values of polymer, a re-entrant behaviour is observed with the dominant length scale now again corresponding to the dewetting wavelength. Thus, we show that the binary films of polymer solutions provide additional control parameters that can be utilized for generating functional textured surfaces for various applications.