Interplay of Surface Energy and Bulk Thermodynamic Forces in Ordered Block Copolymer Droplets

The wetting state of a simple liquid on a solid substrate, as summarized by Young’s equation, is dictated by the interfacial energies of the different phases that coexist in the system. For simple fluids, rotational symmetry gives rise to symmetric droplets around the axis perpendicular to the substrate. This is not the case for nanostructured fluids, such as block copolymers, where the inherent thermodynamic ordering forces compete with surface tension. This competition is particularly important in nanoscale droplets, where the size of the droplets is a small multiple of the natural periodicity of the block copolymer in the bulk. In the nanoscale regime, droplet shape and internal structure arise from a subtle interplay between interfacial and bulk contributions to the free energy. In this work, we examine the consequences of surface–polymer interaction energies on droplet morphology through a concerted simulation and experimental effort. When the block copolymer is deposited on a neutral substrate, we find noncircular arrangements with perpendicular domains. However, when a preferential substrate is used, the resulting morphology depends on droplet size. In large droplets, we observe bottle-cap-shaped structures with a ring of perpendicular domains along the perimeter, while small droplets exhibit stripes of perpendicular domains

Simulation of Defect Reduction in Block Copolymer Thin Films by Solvent Annealing

Solvent annealing provides an effective means to control the self-assembly of block copolymer (BCP) thin films. Multiple effects, including swelling, shrinkage, and morphological transitions, act in concert to yield ordered or disordered structures. The current understanding of these processes is limited; by relying on a theoretically informed coarse-grained model of block copolymers, a conceptual framework is presented that permits prediction and rationalization of experimentally observed behaviors. Through proper selection of several process conditions, it is shown that a narrow window of solvent pressures exists over which one can direct a BCP material to form well-ordered, defect-free structures

Directly Observing Micelle Fusion and Growth in Solution by Liquid-Cell Transmission Electron Microscopy

Amphiphilic small molecules and polymers form commonplace nanoscale macromolecular compartments and bilayers, and as such are truly essential components in all cells and in many cellular processes. The nature of these architectures, including their formation, phase changes, and stimuli-response behaviors, is necessary for the most basic functions of life, and over the past half-century, these natural micellar structures have inspired a vast diversity of industrial products, from biomedicines to detergents, lubricants, and coatings. The importance of these materials and their ubiquity have made them the subject of intense investigation regarding their nanoscale dynamics with increasing interest in obtaining sufficient temporal and spatial resolution to directly observe nanoscale processes. However, the vast majority of experimental methods involve either bulk-averaging techniques including light, neutron, and X-ray scattering, or are static in nature including even the most advanced cryogenic transmission electron microscopy techniques. Here, we employ in situ liquid-cell transmission electron microscopy (LCTEM) to directly observe the evolution of individual amphiphilic block copolymer micellar nanoparticles in solution, in real time with nanometer spatial resolution. These observations, made on a proof-of-concept bioconjugate polymer amphiphile, revealed growth and evolution occurring by unimer addition processes and by particle–particle collision-and-fusion events. The experimental approach, combining direct LCTEM observation, quantitative analysis of LCTEM data, and correlated in silico simulations, provides a unique view of solvated soft matter nanoassemblies as they morph and evolve in time and space, enabling us to capture these phenomena in solution

Block Copolymer Assembly on Nanoscale Patterns of Polymer Brushes Formed by Electrohydrodynamic Jet Printing

Fundamental understanding of the self-assembly of domains in block copolymers (BCPs) and capabilities in control of these processes are important for their use as nanoscale templates in various applications. This paper focuses on the self-assembly of spin-cast and printed poly(styrene-<i>block</i>-methyl methacrylate) BCPs on patterned surface wetting layers formed by electrohydrodynamic jet printing of random copolymer brushes. Here, end-grafted brushes that present groups of styrene and methyl methacrylate in geometries with nanoscale resolution deterministically define the morphologies of BCP nanostructures. The materials and methods can also be integrated with lithographically defined templates for directed self-assembly of BCPs at multiple length scales. The results provide not only engineering routes to controlled formation of complex patterns but also vehicles for experimental and simulation studies of the effects of chemical transitions on the processes of self-assembly. In particular, we show that the methodology developed here provides the means to explore exotic phenomena displayed by the wetting behavior of BCPs, where 3-D soft confinement, chain elasticity, interfacial energies, and substrate’s surface energy cooperate to yield nonclassical wetting behavior

Chemical Patterns for Directed Self-Assembly of Lamellae-Forming Block Copolymers with Density Multiplication of Features

Lamellae-forming polystyrene-<i>block</i>-poly­(methyl methacrylate) (PS-<i>b</i>-PMMA) films, with bulk period <i>L</i>₀, were directed to assemble on lithographically nanopatterned surfaces. The chemical pattern was comprised of “guiding” stripes of cross-linked polystyrene (X-PS) or poly­(methyl methacrylate) (X-PMMA) mats, with width <i>W</i>, and interspatial “background” regions of a random copolymer brush of styrene and methyl methacrylate (P­(S-<i>r</i>-MMA)). The fraction of styrene (<i>f</i>) in the brush was varied to control the chemistry of the background regions. The period of the pattern was <i>L</i>_s. After assembly, the density of the features (domains) in the block copolymer film was an integer multiple (<i>n</i>) of the density of features of the chemical pattern, where <i>n</i> = <i>L</i>_s/<i>L</i>₀. The quality of the assembled PS-<i>b</i>-PMMA films into patterns of dense lines as a function of <i>n</i>, <i>W</i>/<i>L</i>₀, and <i>f</i> was analyzed with top-down scanning electron microscopy. The most effective background chemistry for directed assembly with density multiplication corresponded to a brush chemistry (<i>f</i>) that minimized the interfacial energy between the background regions and the composition of the film overlying the background regions. The three-dimensional structure of the domains within the film was investigated using cross-sectional SEM and Monte Carlo simulations of a coarse-grained model and was found most closely to resemble perpendicularly oriented lamellae when <i>W</i>/<i>L</i>₀ ∼ 0.5–0.6. Directed self-assembly with density multiplication (<i>n</i> = 4) and <i>W</i>/<i>L</i>₀ = 1 or 1.5 yields pattern of high quality, parallel linear structures on the top surface of the assembled films, but complex, three-dimensional structures within the film

Characterizing the Three-Dimensional Structure of Block Copolymers <i>via</i> Sequential Infiltration Synthesis and Scanning Transmission Electron Tomography

Understanding and controlling the three-dimensional structure of block copolymer (BCP) thin films is critical for utilizing these materials for sub-20 nm nanopatterning in semiconductor devices, as well as in membranes and solar cell applications. Combining an atomic layer deposition (ALD)-based technique for enhancing the contrast of BCPs in transmission electron microscopy (TEM) together with scanning TEM (STEM) tomography reveals and characterizes the three-dimensional structures of poly(styrene-<i>block</i>-methyl methacrylate) (PS-<i>b</i>-PMMA) thin films with great clarity. Sequential infiltration synthesis (SIS), a block-selective technique for growing inorganic materials in BCPs films in an ALD tool and an emerging technique for enhancing the etch contrast of BCPs, was harnessed to significantly enhance the high-angle scattering from the polar domains of BCP films in the TEM. The power of combining SIS and STEM tomography for three-dimensional (3D) characterization of BCP films was demonstrated with the following cases: self-assembled cylindrical, lamellar, and spherical PS-<i>b</i>-PMMA thin films. In all cases, STEM tomography has revealed 3D structures that were hidden underneath the surface, including (1) the 3D structure of defects in cylindrical and lamellar phases, (2) the nonperpendicular 3D surface of grain boundaries in the cylindrical phase, and (3) the 3D arrangement of spheres in body-centered-cubic (BCC) and hexagonal-closed-pack (HCP) morphologies in the spherical phase. The 3D data of the spherical morphologies was compared to coarse-grained simulations and assisted in validating the simulations’ parameters. STEM tomography of SIS-treated BCP films enables the characterization of the exact structure used for pattern transfer and can lead to a better understating of the physics that is utilized in BCP lithography