Orientation Of Microdomains Research Articles

Control of the Microdomain Orientation in Block Copolymer Thin Films with Homopolymers for Lithographic Application

Block copolymer lithography is a promising method for fabricating periodical nanopatterns of less than 20 nm by self-assembly and can be applicable for fabricating patterned magnetic media with a recording density over 1 Tb/in.2. We found a simple technique to control the orientation of cylindrical microdomains in thin films. Simply by mixing polystyrene-block-poly(methyl methacrylate) (PS-b-PMMA) diblock copolymers with the homopolymer (PS or PMMA) of the major component, we could align the cylindrical microdomains perpendicular to the film surface. The added homopolymer induces conformational entropic relaxation of the block chains in microdomain space and stabilizes the perpendicular orientation of hexagonally packed cylindrical microdomains. Thus formed perpendicular cylinders can be readily aligned in a regular array with a grating substrate.

Orientationally Controlled Nanoporous Cylindrical Domains in Polystyrene-b-poly(ferrocenylethylmethylsilane) Block Copolymer Films

The self-assembly of thin films of organometallic cylinder-forming amorphous polystyrene-block-polyferrocenylsilane diblock copolymers is described. By varying film thickness and/or the conditions of toluene during evaporation annealing, well-ordered arrays of hexagonally packed iron-rich cylindrical microdomains oriented either parallel to or normal to the substrate were produced. In the latter case, when the film thickness was very small (12−15 nm), well-defined nanoporous cylindrical domains were found. This unique morphology persists in UV ozone etched films where inorganic, ring-like cylindrical domains were produced. By varying the rate of solvent evaporation from solvent-swollen films, control over the orientation, order, and size of the cylindrical microdomains was achieved and variation of the fundamental morphology was observed.

Effect of Chain Architecture and Surface Energies on the Ordering Behavior of Lamellar and Cylinder Forming Block Copolymers

We investigate the effect of surface energy and chain architecture on the orientation of microdomains in relatively thick films (600−800 nm) of lamellar and cylindrical block copolymers of poly(cyclohexylethylene) (C) and poly(ethylene) (E). The E block has 26 ethyl branches per 1000 backbone carbon atoms. Melt surface energies of the C and E blocks are 22.3 and 20.9 mJ/m2, respectively. Grazing-incidence small-angle X-ray scattering (GISAXS), scanning force microscopy (SFM), and cross-sectional transmission electron microscopy (TEM) show that cylindrical and lamellar CEC triblock copolymers orient their microdomains normal to the surface throughout the film thickness. However, a lamellar CE diblock copolymer prefers a parallel orientation of the lamellae relative to the surface with an E surface layer. Moreover, a cylindrical CEBC triblock copolymer where the EB block has 125 ethyl branches per 1000 backbone carbon atoms leads to EB cylinder domains that always orient parallel to the surface. In this cas...

Grain Rotation in Ion-Complexed Symmetric Diblock Copolymer Thin Films under an Electric Field

In symmetric polystyrene-block-poly(methyl methacrylate) (PS-b-PMMA) diblock copolymer thin films, lithium−PMMA complexes were formed with the addition of lithium chloride (LiCl), significantly increasing both χ and dielectric constant. These led to a transition in the kinetic pathway of the orientation of lamellar microdomains under an applied electric field from a disruption and re-formation of the microdomains to a grain rotation mediated by movement of defects. By controlling the number of lithium−PMMA complexes, the microdomain alignment is possibly regulated in PS-b-PMMA copolymer thin films.

Salt Complexation in Block Copolymer Thin Films

Ion complexation within cylinder-forming block copolymer thin films was found to affect the ordering process of the copolymer films during solvent annealing, significantly enhancing the long-range positional order. Small amounts of alkali halide or metal salts were added to PS-b-PEO, on the order of a few ions per chain, where the salt complexed with the PEO block. The orientation of the cylindrical microdomains strongly depended on the salt concentration and the ability of the ions to complex with PEO. The process shows large flexibility in the choice of salt used, including gold or cobalt salts, whereby well-organized patterns of nanoparticles can be generated inside the copolymer microdomains. By further increasing the amount of added salts, the copolymer remained highly ordered at large degrees of swelling and demonstrated long-range positional correlations of the microdomains in the swollen state, which holds promise as a route to addressable media.

Side-Chain-Grafted Random Copolymer Brushes as Neutral Surfaces for Controlling the Orientation of Block Copolymer Microdomains in Thin Films

Random copolymers of P(S-r-MMA-r-HEMA)s with a distribution of surface reactive hydroxyl groups were synthesized to formulate neutral surface layers on a SiO2 substrate. The layers were designed to drive vertical orientation of lamellar microdomains in a top P(S-b-MMA) thin film. Copolymers with a styrene weight fraction (f(St)) of 0.58 and a HEMA fraction (f(HEMA)) ranging from 0.01 to 0.03, with a corresponding MMA fraction (f(MMA)) ranging from 0.41 to 0.39, in the P(S-r-MMA-r-HEMA) copolymer showed neutral surface characteristics. The morphology of block copolymer thin films was studied by scanning electron microscopy (SEM). P(S-r-MMA-r-HEMA) copolymers prepared by both living and classical free-radical polymerizations were equally effective in demonstrating the neutrality of the surface. These side-chain-grafted random copolymer brushes showed faster grafting kinetics than the end-chain-grafted P(S-r-MMA) because of multipoint attachment to the surface. The modified surfaces had a very thin layer of random copolymer brush (5-7 nm), which is desirable for effective pattern transfer. Furthermore, neutral surfaces could be obtained even when the grafting time was reduced to 3 h. These results indicate that the composition of the random copolymer brush, rather than its PDI or molecular weights, is the most important factor in controlling the neutrality of the surface. These results also demonstrate the feasibility of using a third comonomer (C) in the random copolymer brush P(A-r-B-r-C) to alter the interfacial and surface energies of a diblock copolymer (A-b-B).

Deformation and Fracture of Lamellar and Cylindrical Block Copolymers with Unentangled Glassy Matrices: Effect of Chain Architecture and Microdomain Orientation

We investigate the influence of chain architecture and microdomain orientation on the deformation and fracture properties of relatively thick films (600−800 nm) of poly(cyclohexylethylene)−poly(eth...

Grazing-incidence small-angle X-ray scattering from thin polymer films with lamellar structures – the scattering cross section in the distorted-wave Born approximation

Grazing-incidence small-angle X-ray or neutron scattering of thin polymer films reveals information about the ordering and preferential orientations of the phase-separated microdomains within the films. The grazing-incidence geometry enhances the surface sensitivity; however, the scattering has to be treated within the framework of the distorted-wave Born approximation. In this work, the case of thin films with lamellar mesostructure is studied, where the orientation of the lamellae is either perpendicular or parallel to the film interfaces. For perpendicular lamellae, Bragg rods are found, which are extended along the film normal, whereas for parallel lamellae, peaks along the film normal appear. The positions of the maxima present in the latter case are explained by accounting for refraction at the film surface and reflection at the film–substrate interface. The results are relevant for thin films of lamellar diblock copolymers.

Tailoring Microdomain Orientation in Block Copolymer Thin Films for Lithographic Application

ABSTRACTBlock copolymer lithography is a promising method for fabricating periodical nano patterns less than 20nm by self-assembly and can be applicable for fabricating patterned magnetic media with recording density over 1Tbit/inch2. We found a simple technique to control the orientation of cylindrical microdomains in thin films. Only by mixing polystyrene-block-poly(methyl methacrylate) (PS-b-PMMA) diblock copolymers with the constituent homopolymer (PS or PMMA), we could align the cylindrical microdomains perpendicular to the film surface. The added homopolymer induces conformational entropic relaxation of the block chains in microdomain space and stabilizes perpendicular orientation of hexagonally packed cylindrical microdomains. Thus formed perpendicular cylinders can be readily aligned in a regular array with a grating substrate.

Directed Self-Assembly of Thin Block Copolymer Films under Controlled Atmosphere

ABSTRACTWe have developed a simple and fast methodology to control the orientation of cylindrical microdomains in thin block copolymer films under controlled atmosphere conditions. Asymmetric block copolymers of polystyrene and poly(ethylene oxide) (PS-b-PEO) were dissolved in toluene and spin coated with less than one minute exposure to different solvent vapors and controlled humidity. The specific choice of solvents was based on the polymer solubility parameters and the solvent vapor pressure. By controlling the spin coating environment we were able to produce cylinders with orientations ranging from parallel to perpendicular with respect to the substrate. Orientation was apparently controlled by the preferential affinity of the vapor atmosphere. A combinatorial gradient technique was employed to investigate the mechanism of solvent-induced microdomain orientation.

Electric Field Alignment of Asymmetric Diblock Copolymer Thin Films

The electric field alignment of cylindrical microdomains in diblock copolymer thin films was studied using small-angle neutron scattering and transmission electron microscopy. The alignment process was followed with the block copolymer films in different initial states. Starting from a poorly ordered state, the cylindrical microdomain orientation was biased by the surface field that initially drove the cylindrical microdomains to be oriented parallel to the film surface. With further annealing, the cylinders were disrupted locally and formed ellipsoid-shaped microdomains that, with time, connected into cylindrical microdomains oriented in the field direction. Starting from an ordered state with cylinders parallel to the surface, the applied electric field enhanced fluctuations at the interfaces of the microdomains. The growth of the fluctuations continued until the cylindrical microdomains broke up into spherical microdomains, similar to that seen in the thermoreversible cylinder-to-sphere order−order tra...

Solvent annealing thin films of poly(isoprene- b-lactide)

The use of solvent annealing to control the microdomain orientation and long-range ordering in poly(isoprene- b- d, l-lactide) thin films was investigated using scanning force microscopy and grazing incidence small angle X-ray scattering (GISAXS). Benzene and chloroform were used as annealing solvents. Both were found to improve the long-range order in the films. Additionally, at high concentrations of chloroform in the film, a perpendicular orientation of cylinders was observed where the solvent was able to mediate interfacial interactions sufficiently to prevent a preferential segregation of one of the blocks to the surface. In situ GISAXS measurements made during solvent swelling and evaporation allowed an examination of the film morphology over a wide range of solvent concentrations providing a efficient route to optimize conditions for morphology control by solvent annealing.

Influence of Initial Order on the Microscopic Mechanism of Electric Field Induced Alignment of Block Copolymer Microdomains

We investigate the mechanism of microdomain orientation in concentrated block copolymer solutions exposed to a dc electric field by in situ synchrotron small-angle X-ray scattering (SAXS). As a model system, we use concentrated solutions of a lamellar polystyrene-b-polyisoprene block copolymer in toluene. We find that both the microscopic mechanism of reorientation and the kinetics of the process strongly depend on the initial degree of order in the system. In a highly ordered lamellar system with the lamellae being aligned perpendicular to the electric field vector, only nucleation and growth of domains is possible as a pathway to reorientation and the process proceeds rather slowly. In less ordered samples, grain rotation becomes possible as an alternative pathway, and the process proceeds considerably faster. The interpretation of our finding is strongly corroborated by dynamic self-consistent field simulations.

Orthogonal fields: A path to long-range three-dimensional order in block copolymers

Large-scale computer simulations show that two orthogonal external fields can control the orientation of lamellar microdomains in diblock copolymers in three dimensions and lead to an enhanced long-range ordering.

Interfacial Interaction Dependence of Microdomain Orientation in Diblock Copolymer Thin Films

The dependence of the orientation of lamellar microdomains in thin films of symmetric diblock copolymers of polystyrene-b-polymethyl methacrylate was quantitatively investigated by small angle neutron scattering, transmission electron microscopy (TEM), and reflectance optical microscopy. A mixed orientation of the lamellar microdomains was found when the difference in the interfacial energies between each block and the substrate was not strong enough to force the orientation of the lamellae parallel to the substrate throughout the film. A limiting film thickness was found, below which a parallel alignment of the lamellar microdomains was seen throughout the film. This film thickness was found to depend on the strength of the interfacial interactions of each block with the substrate. TEM images indicate that the surface field suppresses the fluctuations in the microdomain, reduces defects, and consequently, propagates the orientation of the domains parallel to the surface into the film.

Symmetric Diblock Copolymer Thin Films on Rough Substrates. Kinetics and Structure Formation in Pure Block Copolymer Thin Films

The effect of substrate roughness on the orientation of lamellar microdomains of symmetric poly(styrene)-block-poly(methyl methacrylate) [PS-b-PMMA] was investigated. Thin films of three molecular weights of PS-b-PMMA were prepared on organic polyimide and inorganic indium tin oxide substrates whose surfaces were characterized for roughness and surface energy. It was shown, through cross-section transmission electron microscopy (TEM) and dynamic secondary ion mass spectroscopy (dSIMS), that above a critical substrate roughness all three molecular weights of PS-b-PMMA produced a perpendicular lamellar orientation. Using atomic force microscopy (AFM) and PS-b-PMMA thin films on an array of polyimide substrates of varied substrate roughness, a critical substrate roughness was identified, below which a parallel orientation was observed. This behavior was modeled simply and showed that the critical roughness determined by AFM represents an underestimate of the true critical roughness of the substrate. Finally,...

Enhancement in the Orientation of the Microdomain in Block Copolymer Thin Films upon the Addition of Homopolymer

Cylindrical microdomains in thin films of mixtures of polystyrene‐block‐ poly(methyl methacrylate) and poly(methyl methacrylate) (PMMA) homopolymer are oriented normal to the film surface (see Figure). Confining the PMMA homopolymer to the microdomains increases the persistence of microdomain orientation over large distances; thus, aspect ratios up to ∼10 are achievable without the use of an external field.

Order-Enhancing Processing in Self-Assembly

Self-assembly of building blocks from solutions into ordered arrays is a promising approach toward functional materials. However, self-assembly alone forms locally ordered structures. The processing to enhance ordering is necessary for real world applications. This article presents an overview of current research activities in order-enhancing processing. The first part discusses the processing needed for controlling microdomain orientations and macroscale-ordering in block copolymer systems. The second part is concerned with the processing needed for creating two and three dimensional ordered arrays from colloidal suspensions. Understanding of the underlying physical-nature in the order-enhancing processing will provide a scientific platform in the chemical engineering field.

Electric Field Induced Alignment of Concentrated Block Copolymer Solutions

We investigate the microdomain orientation kinetics of concentrated block copolymer solutions exposed to a dc electric field by time-resolved synchrotron small-angle X-ray scattering. As a model system, we use a lamellar polystyrene-b-polyisoprene block copolymer dissolved in toluene. Our results indicate two different microscopic mechanisms, i.e., nucleation and growth of domains and grain rotation. The former dominates close to the order−disorder transition, while the latter prevails under more strongly segregated conditions. This conclusion is corroborated by computer simulations based on dynamic density functional theory. The orientation kinetics follows a single-exponential behavior with characteristic time constants varying from a few seconds to some minutes depending on polymer concentration, temperature, and electric field strength. From the experimental results we deduce optimum conditions for the preparation of highly anisotropic bulk polymer samples via solvent casting in the presence of an electric field.

Sequential, Orthogonal Fields: A Path to Long-Range, 3-D Order in Block Copolymer Thin Films

The use of two orthogonal, external fields is shown to control the orientation of lamellar microdomains in three dimensions in diblock copolymer thin films. An elongational flow field was applied to obtain an in-plane orientation of the microdomains of the copolymer melt, and an electric field, applied normal to the surface, was then used to further align the microdomains. Thin films of symmetric diblock copolymers of poly(styrene-b-methyl methacrylate) with long-range order and orientation of the lamellar microdomains were obtained as evidenced by small-angle X-ray scattering and transmission electron microscopy.