Studies on Nanostructure Formation via Self-assembly and Applications to Antireflective Coatings and Cell Culture Substrates

概要

1.1 Background
Surface roughness is one of the most important factors in material development because it determines various properties such as adhesive strength,1 wettability,2 and light scattering.3 In nature, excellent functions are expressed by utilizing the surface structure. For example, lotus leaves form microscopic protrusions with a hierarchical structure on their surfaces, and universal materials exhibit superhydrophobicity with a contact angle to water of 150° or more.4 This performance cannot be achieved by the conventional approach of controlling the interfacial energy by introducing fluorine into the material, as typified by Teflon. Thus, there are many attempts to realize excellent functions by mimicking the surface structure of living organisms, which is called biomimetics. One of the oldest materials developed by biomimetics is Velcro that imitates the hook structure of the plant, Cordyceps sinensis. With the development of observation and processing technologies for biological surfaces, many bio-inspired functional materials have been developed.5 For instance, anti-reflection films mimicking moth eyes,6–9 superhydrophobic materials mimicking lotus leaves,10 and adhesive tapes mimicking gecko legs11 have been reported. Many other interesting studies have been reported, such as a container in which mayonnaise slides down that mimics an insectivorous plant.12

Nanoimprinting and bottom-up approaches are typical methods for forming these nanostructures. In the nanoimprinting method, nanostructures are produced using molds, 13 and this method has issues of productivity when the size of structures moves from the submicron level to the nanoscale. Conversely, the bottom-up approaches have the potential for forming nanostructures with high productivity. For examples of the bottom-up approaches, vertical structural growths of nanowires, nanotubes, nanosails, and nanobelts have been reported.14–16 The problem of these methods is that they require long time to fabricate the nanostructures.

Above mentioned problem can be solved using nanoparticle array or polymer phase separation. For instance, close-packed particle arrays are produced using the capillary force generated during solvent volatilization.17 In general, capillary forces between nanoparticles result in the formation of 2D colloidal crystals when they are coated on a smooth surface (Figure 1-1a). In the colloidal crystals, particles were arranged in a close- packed hexagonal lattice, hence, the standard deviation of the inter-particle distance is small (Figure 1-1b). However, this method cannot fabricate non-close-packed arrays because the capillary force attracts the particles to each other. When the number of particles for the coating is reduced, non-uniform spacing between the particles appear ( Figure 1- 1c). In this case, two different regions are present: one is the region where particles are close-packed and the other is the region where no particles exist. Several optical applications such as anti-reflection, photonic crystal, and surface-enhanced Raman scattering require homogenous, non-close-packed arrays.

The fabrication methods of non-close-packed particle arrays are classified based on two approaches: by application of an external force or by direct self-assembly. The methods that use external force include reactive and plasma etchings which are used to create spacings between particles (Figure 1-2). Examples of this approach include the etching of close-packed particle arrays to reduce particle size,18–21 the plasma etching of nanoparticle coatings embedded in acrylic resin to expose the colloids.22–24 As an alternative to etching for uncovering particles, high-speed spin coating of monolayer particle arrays has been reported.25,26 Soft lithography methods have also been reported (Figure 1-3).27,28 However, etching, high-speed spin coating, and soft lithography processes are less productive, and the use of direct self-assembly methods is ideal for increasing productivity.

Another method to form nanostructures by direct self-assembly is to use phase separation of block copolymers, where dissimilar blocks are conjugated by covalent bonds. Thin films of block copolymers can be deposited on a variety of substrates using solution processing. The type of monomer used and the polymer chain length control the morphologies of block copolymers such as lamellar, cylinder, and co-continuous structures.29 Degree of long-range order and orientation of these domains can be also controlled by the interaction of the block copolymer molecules with the substrate, the film thickness, and the post-deposition annealing procedures.30,31 In a typical block copolymer, the dimension of the domains range from 10 to 100 nm. To control the pattern of the block domains, application of chemical contrast between SiO2 and polystyrene is reported.32 Nanostructure formation by combining the block polymers with top-down methods such as reactive etching has also been reported.33 It has been reported that temperature-responsive segments are added to block polymers in order to change the structure formed by the block polymers.34,35 Although the application in encapsulation for drug delivery applications is expected, there are few examples of its application in thin films. There have been attempts to apply the wrinkled structure formed by the phase separation of block copolymers to the light out-coupling of OLEDs,36 but there are few practical examples due to the technical hurdles in controlling the structure of block copolymers.

1.2 Summary of this thesis
In this thesis, control of nanostructures by self-assembly via coating and drying processes is presented. Designing of the surface properties of particles and materials to be added to the coating solution is described. This thesis consists of following seven chapters.

In Chapter 2, three types of novel self-assembly approaches for synthesizing homogenous 2D colloid arrays using sublimation, polymer solidification, and thermal fusion are presented. In the first approach, the sublimation method was applied by coating silica colloids and sublimable compounds on a substrate, followed by exposing them via sublimation. In the second one, polymer solidification method was developed by using the colloids, polymers, and fluorinated solvents with high boiling points, followed by evaporation of the solvents. Final approach involved forming 2D arrays of silica-polymer core-shell nanoparticles using layer-by-layer techniques and the subsequent thermal fusion of polymer shells on the particles. Homogenous nano-protrusions were obtained using these three methods.

In Chapter 3, a novel self-assembly method for forming vertically aligned arrays of anisotropic-shaped particles is described. The particles could be vertically aligned with controlled spacing on substrates using a layer-by-layer method and thermal fusion. This method was applied to particles of various shapes; e.g., dumbbell-shaped particles with an aspect ratio in the range of 1.2–1.9 and bullet-shaped particles with an aspect ratio of two or more were aligned on substrates. From the results of the simulation of particle arrangement, the particles were oriented in the layer-by-layer process if they have anisotropy in terms of the shape, specific gravity, and surface charge of the particles. Furthermore, to make the particles stand upright, it was necessary to heat the particles at a temperature equal to or higher than the glass transition temperature of the polymer that covers the particles.

In Chapter 4, self-assembly approaches for synthesizing colloidal crystals with square lattice protrusions are presented. In the first approach, spin-coating and subsequent heating of colloidal silica and acrylic monomer resulted in a body-centered cubic (bcc)- like arrangement; the number of stacked colloid layers and the colloid density of these compounds were important in the formation of this structure. Moreover, prolonged heating exposed colloids from acrylic monomer, with square lattice nano-protrusions formed. In the second approach, the protrusions were fabricated in a shorter time by replacing some of the acrylic monomer with volatile media. Only certain volatile media, such as a solvent with a structure similar to diethylene glycol, exhibited the formation of square lattice protrusions.

In Chapter 5, results of fabrication of antireflective films, as an application of the nanostructure formation technology described in the Chapter 2 and 3, are described. The surface of a moth's eye has projections with a height of 100 to 200 nm in a regular periodic pattern, and antireflective materials that mimic this structure are known as moth-eye films.7,37–40 Moth-eye film has both excellent antireflective properties and high transparency. Any disorder in the distance between the nano-protrusions caused light scattering, resulted in film bleaching. It was confirmed that the spherical nanoparticles arranged two-dimensionally at uniform spacing exhibited high transparency. The wavelength of light that can be antireflected depended on the height of the protrusions. Formation of high-aspect-ratio protrusions by standing anisotropic-shaped nanoparticles upright on a substrate enabled antireflection of light up to a higher wavelength range.

In Chapter 6, applications of nanostructures for the culture of induced pluripotent stem cells (iPSCs) are presented. iPSCs can differentiate into all the tissues and organs that constitute the human body41,42 and are widely studied in regenerative medicine,43,44 pathology modeling,45–47 and drug screening.45,48 Mass culture of iPSCs in a stable manner is essential for these applications. It was found that using colloidal arrays described in the Chapter 2, cultured iPSCs were easily detached from the substrate without manual cell scraping. In addition to planar culture, spheroids of iPSCs attached to the substrate were formed by combining hydrophilic surface patterning on the colloidal array.

In Chapter 6, co-culture of iPSCs and differentiated cells on the thermoresponsive polymer poly[(butyl methacrylate)-block-(N-isopropylacrylamide)] (PBMA-PNIPAAm) is also described. Interesting surface morphology changes due to temperature variation was observed, which affected protein adsorption. Quantitative analysis of the selectivity of detachment revealed that iPSCs with a purity of more than 98% could be recovered from a cell population containing differentiated cells. By using thermoresponse, it was found that long-term maintenance culture of iPSCs was possible without manual removal of differentiated cells.

Chapter 7 summarizes the results obtained and outlines future prospects.