Effect of Spin Speed and Solution Concentration on the Directed Assembly of Polymer Blends

Structure evolution during phase separation in spin-coated ethylcellulose/hydroxypropylcellulose films

Porous phase-separated films made of ethylcellulose (EC) and hydroxypropylcellulose (HPC) are commonly used for controlled drug release. The structure of these thin films is controlling the drug transport from the core to the surrounding liquids in the stomach or intestine. However, detailed understanding of the time evolution of these porous structures as they are formed remains elusive. In this work, spin-coating, a widely applied technique for making thin uniform polymer films, was used to mimic the industrial manufacturing process. The focus of this work was on understanding the structure evolution of phase-separated spin-coated EC/HPC films. The structure evolution was determined using confocal laser scanning microscopy (CLSM) and image analysis. In particular, we determined the influence of spin-coating parameters and EC : HPC ratio on the final phase-separated structure and the film thickness. The film thickness was determined by profilometry and it influences the ethanol solvent evaporation rate and thereby the phase separation kinetics. The spin speed was varied between 1000 and 10 000 rpm and the ratio of EC : HPC in the polymer blend was varied between 78 : 22 wt% and 40 : 60 wt%. The obtained CLSM micrographs showed phase separated structures, typical for the spinodal decomposition phase separation mechanism. By using confocal laser scanning microscopy combined with Fourier image analysis, we could extract the characteristic length scale of the phase-separated final structure. Varying spin speed and EC : HPC ratio gave us precise control over the characteristic length scale and the thickness of the film. The results showed that the characteristic length scale increases with decreasing spin speed and with increasing HPC ratio. The thickness of the spin-coated film decreases with increasing spin speed. It was found that the relation between film thickness and spin speed followed the Meyerhofer equation with an exponent close to 0.5. Furthermore, good correlations between thickness and spin speed were found for the compositions 22 wt% HPC, 30 wt% HPC and 45 wt% HPC. These findings give a good basis for understanding the mechanisms responsible for the morphology development and increase the possibilities to tailor thin EC/HPC film structures.

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.

High transparent mid-infrared silicon "window" decorated with amorphous photonic structures fabricated by facile phase separation

High transparency in the infrared (IR) region is desirable for most common IR materials and devices, due to their high interfacial reflectance, resulting from the high refractive indices of constituent substances. Herein, a new strategy, with using phase-separated polystyrene (PS)/polymethylmethacrylate (PMMA) blends as masks, is proposed to fabricate subwavelength structures for Si with significantly enhanced mid-IR transmission. Maximum transmittance approaching to 70% and 90% are achieved with single and double- side structured Si respectively. The fabricated subwavelength structures are short-range ordered amorphous photonic structures (APSs). By using different spin-coating speeds and molar ratios of PS to PMMA and by adjusting the etching duration time, tunable enhanced transmission are also obtained. The good performance of high transmission is confirmed by mid-IR thermal imaging experiments. Furthermore, the enhanced transmission is effective over a wide range of incident angles up to 50° and well maintained at high temperatures up to 600 °C.

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.

Wafer Scale Fabrication of Dense and High Aspect Ratio Sub-50 nm Nanopillars from Phase Separation of Cross-Linkable Polysiloxane/Polystyrene Blend

We demonstrated a simple and effective approach to fabricate dense and high aspect ratio sub-50 nm pillars based on phase separation of a polymer blend composed of a cross-linkable polysiloxane and polystyrene (PS). In order to obtain the phase-separated domains with nanoscale size, a liquid prepolymer of cross-linkable polysiloxane was employed as one moiety for increasing the miscibility of the polymer blend. After phase separation via spin-coating, the dispersed domains of liquid polysiloxane with sub-50 nm size could be solidified by UV exposure. The solidified polysiloxane domains took the role of etching mask for formation of high aspect ratio nanopillars by O₂ reactive ion etching (RIE). The aspect ratio of the nanopillars could be further amplified by introduction of a polymer transfer layer underneath the polymer blend film. The effects of spin speeds, the weight ratio of the polysiloxane/PS blend, and the concentration of polysiloxane/PS blend in toluene on the characters of the nanopillars were investigated. The gold-coated nanopillar arrays exhibited a high Raman scattering enhancement factor in the range of 10⁸-10⁹ with high uniformity across over the wafer scale sample. A superhydrophobic surface could be realized by coating a self-assembled monolayers (SAM) of fluoroalkyltrichlorosilane on the nanopillar arrays. Sub-50 nm silicon nanowires (SiNWs) with high aspect ratio of about 1000 were achieved by using the nanopillars as etching mask through a metal-assisted chemical etching process. They showed an ultralow reflectance of approximately 0.1% for wavelengths ranging from 200 to 800 nm.

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.

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.

Directed assembly of P3HT:PCBM blend films using a chemical template with sub-300 nm features

Surface energy has been demonstrated as a means to direct interfacial-layer composition in polymer:fullerene blends utilized as active layers in organic photovoltaic devices. Combined with recent materials advances in the preparation of nanoscale chemical patterns, surface energy control of nanophase separation presents an opportunity to employ patterned surface energy templates to control the 3D blend morphology of polymer:fullerene blends. This report details the directed assembly of poly(3-hexylthiophene):phenyl-C61-butyric acid methyl ester (P3HT:PCBM) blends atop linear grating patterns with domains of alternating high and low surface energy of 50 to 600 nm in width prepared by nanoscale oxidative lithography of alkyl-terminated self-assembled monolayers on SiO2 and SiH surfaces. Tapping-, contact-, and current-sensing AFM studies demonstrated that chemical patterns were effective at directing the 3D morphology of P3HT:PCBM blends at dimensions of >200 nm. As the dimensionality of domains approached 100 nm, the chemical patterns were no longer able to direct phase segregation, evidence that a directed spinodal decomposition mechanism was responsible for the observed morphology. Surprisingly, the low surface energy component (P3HT) was found to be atop the high surface energy domains of the template, in conflict with current understanding of the role of surface energy directed assembly in polymer blends. These results suggest that the directed spinodal decomposition mechanism applies to conjugated polymer:fullerene blends, but that additional parameters unique to these types of systems will require refinement of the theory to adequately describe and predict the behavior of these scientifically and industrially interesting materials.

Selecting speed-dependent pathways for a programmable nanoscale texture by wet interfaces

The realization of well-defined and ordered structures on the nanoscale is a main issue in nanoscience and nanotechnology, biotechnology and other related fields like plastic or organic electronics. Among the bottom-up approaches, to date, self-assembly (equilibrium aggregates) received a major attention. In spite of this, far from equilibrium conditions allow for the generation of a wider landscape of organized systems depending on the set of control parameters employed. Under an adaptation vision of the structures, here we report some case studies showing how it is possible to programme and control the nanoscale features of ordered super- or supra-aggregates at wet interfaces by modulating the dynamic parameters. In particular, speed is foreseen as a threshold factor for changing the aggregation mechanism along with the shape and degree of order of the structures as well as, within a specific aggregation path, their size and defectivity.

Precise pattern replication of polymer blends into nonuniform geometries via reducing interfacial tension between two polymers

Patterned polymer structures with different functionalities have many potential applications. Directed assembly of polymer blends using chemically functionalized patterns during spin-coating has been used to fabricate the patterned polymer structures. For bridging the gap between laboratorial experiments and manufacturing of nanodevices, the polymer blends structures are required to be precisely patterned into nonuniform geometries in a high-rate process, which still is a challenge. In this Article, we demonstrated for the first time that by decreasing the interfacial tension between two polymers polystyrene and poly(acrylic acid) via adding a compatibilizer (polystyrene-b-poly(acrylic acid) ), a polystyrene/poly(acrylic acid) blend was precisely patterned into nonuniform geometries in a high-rate fashion. The patterned nonuniform geometries included angled lines with angles varied from 30° to 150°, T-junctions, square arrays, circle arrays, and arbitrary letter-shaped geometries. The reduction in the interfacial tension improved the line edge roughness and the patterning efficiency of the patterned polymer blends. In addition, the commensurability between characteristic length and pattern periodicity for well-ordered morphologies was also expanded with decreasing interfacial tension. This approach can be easily extended to other functional polymers in a blend and facilitate the applications of patterned polymer structures in biosensors, organic thin-film electronics, and polymer solar cells.

Polymer blend lithography: A versatile method to fabricate nanopatterned self-assembled monolayers

A rapid and cost-effective lithographic method, polymer blend lithography (PBL), is reported to produce patterned self-assembled monolayers (SAM) on solid substrates featuring two or three different chemical functionalities. For the pattern generation we use the phase separation of two immiscible polymers in a blend solution during a spin-coating process. By controlling the spin-coating parameters and conditions, including the ambient atmosphere (humidity), the molar mass of the polystyrene (PS) and poly(methyl methacrylate) (PMMA), and the mass ratio between the two polymers in the blend solution, the formation of a purely lateral morphology (PS islands standing on the substrate while isolated in the PMMA matrix) can be reproducibly induced. Either of the formed phases (PS or PMMA) can be selectively dissolved afterwards, and the remaining phase can be used as a lift-off mask for the formation of a nanopatterned functional silane monolayer. This "monolayer copy" of the polymer phase morphology has a topographic contrast of about 1.3 nm. A demonstration of tuning of the PS island diameter is given by changing the molar mass of PS. Moreover, polymer blend lithography can provide the possibility of fabricating a surface with three different chemical components: This is demonstrated by inducing breath figures (evaporated condensed entity) at higher humidity during the spin-coating process. Here we demonstrate the formation of a lateral pattern consisting of regions covered with 1H,1H,2H,2H-perfluorodecyltrichlorosilane (FDTS) and (3-aminopropyl)triethoxysilane (APTES), and at the same time featuring regions of bare SiO(x). The patterning process could be applied even on meter-sized substrates with various functional SAM molecules, making this process suitable for the rapid preparation of quasi two-dimensional nanopatterned functional substrates, e.g., for the template-controlled growth of ZnO nanostructures [1].

Random lasing from granular surface of waveguide with blends of PS and PMMA

Lasing from a planar waveguide with the blend of Polystyrene(PS): Poly-methylmethacrylate(PMMA) doped with tris(8-hydroxyquinolinato)aluminum(Alq(3)) and 4-(dicyanomethylene)-2-tert-butyl-6(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-pyran(DCJTB) was investigated. Due to phase separation of the blend of PS:PMMA during the solvent evaporation process, a waveguide with granular surface was obtained, which has 2D island-like nanostructures with diameters ranging between 200 and 400 nm and heights at about 25 nm. Pumped by a YAG laser with wavelength of 355 nm, a significant random lasing was observed. Compared to the amplified spontaneous radiation (ASE) of planar waveguides with only PMMA or PS doped with Alq3:DCJTB prepared under the same conditions, the lasing threshold of the former is decreased by about 5 times, and the full width at half maximum (FWHM) is reduced to 1.7 nm from 12~15 nm. Our experiments show a promising method to achieve lower threshold for organic lasers.

In situ imaging and height reconstruction of phase separation processes in polymer blends during spin coating

Spin coating polymer blend thin films provides a method to produce multiphase functional layers of high uniformity covering large surface areas. Applications for such layers include photovoltaics and light-emitting diodes where performance relies upon the nanoscale phase separation morphology of the spun film. Furthermore, at micrometer scales, phase separation provides a route to produce self-organized structures for templating applications. Understanding the factors that determine the final phase-separated morphology in these systems is consequently an important goal. However, it has to date proved problematic to fully test theoretical models for phase separation during spin coating, due to the high spin speeds, which has limited the spatial resolution of experimental data obtained during the coating process. Without this fundamental understanding, production of optimized micro- and nanoscale structures is hampered. Here, we have employed synchronized stroboscopic illumination together with the high light gathering sensitivity of an electron-multiplying charge-coupled device camera to optically observe structure evolution in such blends during spin coating. Furthermore the use of monochromatic illumination has allowed interference reconstruction of three-dimensional topographies of the spin-coated film as it dries and phase separates with nanometer precision. We have used this new method to directly observe the phase separation process during spinning for a polymer blend (PS-PI) for the first time, providing new insights into the spin-coating process and opening up a route to understand and control phase separation structures.