Michinari Kohri

Biomimetic non-iridescent structural color materials from polydopamine black particles that mimic melanin granules

A novel approach for creating non-iridescent bright structural color materials from polydopamine (PDA) black particles is presented. Two biomimetic features, melanin granules and the amorphous structures found in nature, were incorporated into these materials, consisting of PDA black particles to develop a bright and non-iridescent structural color without using any additives.

A colorless functional polydopamine thin layer as a basis for polymer capsules

Herein, we describe a facile method to prepare a colorless functional polydopamine (PDA) thin layer by the in situ oxidative copolymerization of dopamine (DA) and ATRP initiator-bearing DA (DA-BiBB) onto polystyrene (PSt) core particles: PSt@PDA/BiBB2. Surface-initiated ATRP of 2-hydroxyethyl methacrylate (HEMA) was performed on PSt@PDA/BiBB2 particles, followed by the removal of the template particles, which generated PHEMA capsules that were based on a colorless PDA thin layer and have tailored hollow core sizes and capsule wall thicknesses. The PDA thin layer is used as a basis for polymer brushes. The present method does not require cross-linking the polymer brushes. Furthermore, subsequent preparation of functional PHEMA capsules by post-functionalization of hydroxy groups of PHEMA chains was successful.

Facile synthesis of free-standing polymer brush films based on a colorless polydopamine thin layer.

A free-standing polymer brush film with tailored thicknesses based on a colorless polydopamine (PDA) thin layer is prepared and characterized. The surface-initiated atom transfer radical polymerization (ATRP) of 2-hydroxyethyl methacrylate (HEMA) is performed on a PDA layer with thickness of ca. 6 nm, which generated an optically transparent and colorless free-standing PHEMA brush film (1.5 cm × 1.5 cm). Because the cross-linked PDA layer is used as the base for the polymer brushes, the reported method does not require cross-linking the polymer brushes. The free-standing film thicknesses of ≈16-75 nm are controlled by simply changing the ATRP reaction time. The results show that the free-standing PHEMA brush film transferred onto a plate exhibits a relatively smooth surface and is stable in any solvent.

Full-Color Biomimetic Photonic Materials with Iridescent and Non-Iridescent Structural Colors

The beautiful structural colors in bird feathers are some of the brightest colors in nature, and some of these colors are created by arrays of melanin granules that act as both structural colors and scattering absorbers. Inspired by the color of bird feathers, high-visibility structural colors have been created by altering four variables: size, blackness, refractive index, and arrangement of the nano-elements. To control these four variables, we developed a facile method for the preparation of biomimetic core-shell particles with melanin-like polydopamine (PDA) shell layers. The size of the core-shell particles was controlled by adjusting the core polystyrene (PSt) particles’ diameter and the PDA shell thicknesses. The blackness and refractive index of the colloidal particles could be adjusted by controlling the thickness of the PDA shell. The arrangement of the particles was controlled by adjusting the surface roughness of the core-shell particles. This method enabled the production of both iridescent and non-iridescent structural colors from only one component. This simple and novel process of using core-shell particles containing PDA shell layers can be used in basic research on structural colors in nature and their practical applications.

Preparation of organic/inorganic composites by deposition of silica onto shell layers of polystyrene (core)/poly[2-(N,N-dimethylamino)ethyl methacrylate] (shell) particles

Organic/inorganic composites were prepared by catalytic hydrolysis and subsequent condensation of tetraethoxysilane (TEOS) in a shell layer of core-shell polymer particles. First, core-shell particles were prepared by emulsifier-free emulsion polymerization of styrene (St) with 2-chloropropionyloxyethyl methacrylate (CPEM) using potassium persulfate as an initiator, followed by surface-initiated activator generated electron transfer-atom transfer radical polymerization (AGET-ATRP) of 2-(N,N-dimethylamino)ethyl methacrylate (DMAEMA). Dynamic light scattering revealed that hydrodynamic diameter of the particle increased from 482 to 931 nm after AGET-ATRP of DMAEMA. The amount of grafted DMAEMA was determined to be ca. 10 mol% with respect to (wrt) St by (1)H NMR. Second, the composite particles were prepared by adding TEOS into a water/methanol dispersion of the P(St-CPEM)-g-P(DMAEMA). The P(St-CPEM)-g-P(DMAEMA)-SiO(2) composite particles containing ca. 50 wt.% of silica wrt the total weight were obtained. Hollow silica shell particles were also obtained by extraction of polymer components from the composites with tetrahydrofuran.

Enzymatic miniemulsion polymerization of styrene with a polymerizable surfactant

Enzymatic polymerization is a powerful tool for synthesizing useful polymeric materials. However, the preparation of polymer particles by enzymatic polymerization has been reported only rarely. We demonstrated herein the preparation of polystyrene particles by enzyme-mediated miniemulsion polymerization with a polymerizable surfactant, i.e., N,N-dimethyl-N-n-dodecyl-N-2-methacryloyloxyethylammonium bromide (C12-DMAEMA), using a horseradish peroxidase (HRP)/H2O2/β-diketone system in water. Effects of various amounts of HRP, H2O2, and β-diketone on styrene conversion and particle size were investigated. The obtained polystyrene particles were characterized using DLS, GPC, and SEM measurements, and the polymerization kinetics was examined. Comparison of five β-diketones was also conducted: acetylacetone (ACAC), ethylacetoacetate (EAA), 1,3-cyclopentanedione (CP), tetronic acid (TA), and dibenzoylmethane (DM), for enzymatic miniemulsion polymerization.

Surface-initiated enzymatic vinyl polymerization: synthesis of polymer-grafted silica particles using horseradish peroxidase as catalyst

A novel method of surface-initiated enzymatic vinyl polymerization was used to synthesize organic–inorganic hybrid core–shell particles comprising a silica core and a polyacrylamide shell covalently attached to the core surface.

Simple and efficient chiral dopants to induce blue phases and their optical purity effects on the physical properties of blue phases.

Blue phases (BPs) have received considerable attention as light shutters in the next generation of liquid crystal (LC) displays. However, no simple and efficient chiral dopant for induction of BPs of commercially available rodlike LC compounds has been reported. In this study, both (R) and (S) forms of novel chiral dopants were synthesized, showed extremely high helical twisting power values in nematic LC compounds, and induced stable BPs with a small amount of our chiral dopants (3-5 mol %). In enantiomeric excess controlled experiments, we found novel phenomena in their physical properties, such as generation of a metastable chiral nematic phase between an isotropic state and a BP.

Preparation of organic/inorganic hybrid and hollow particles by catalytic deposition of silica onto core/shell heterocoagulates modified with poly[2-(N,N-dimethylamino)ethyl methacrylate]

The organic/inorganic hybrid particles PSt/P(St-CPEM)(θ)-g-PDMAEMA/SiO(2) were prepared by catalytic hydrolysis and subsequent polycondensation of tetraethoxysilane in the poly[2-(N,N-dimethylamino)ethyl methacrylate] (PDMAEMA) layers grafted on the PSt/P(St-CPEM)(θ) core/shell heterocoagulates. The micron-sized PSt core and the submicron-sized P(St-CPEM) shell particles bearing ATRP initiating groups were synthesized by dispersion polymerization of styrene (St) and emulsifier-free emulsion polymerization of St with 2-chloropropionyloxyethyl methacrylate (CPEM), respectively. The raspberry-shaped PSt/P(St-CPEM)(θ) heterocoagulates with a controlled surface coverage (θ=0.51, 0.81) were prepared by hydrophobic coagulation between the core and the shell particles in an aqueous NaCl solution near the T(g) of P(St-CPEM). Surface modification of heterocoagulates was carried out by ATRP of DMAEMA from the shell particles adsorbed on the core particles. Silica deposition was performed by simply adding tetraethoxysilane to a water/methanol dispersion of PSt/P(St-CPEM)(θ)-g-PDMAEMA. The SEM and TGA revealed that the resulting PSt/P(St-CPEM)(θ)-g-PDMAEMA/SiO(2) composites maintain a raspberry-like morphology after deposition of silica onto the PDMAEMA layer grafted on heterocoagulates. The micron-sized, raspberry-shaped or the submicron-sized, hole-structured silica hollow particles were obtained selectively by thermal decomposition of the PSt/P(St-CPEM)(θ)-g-PDMAEMA/SiO(2). The oriented particle array was fabricated by dropping anisotropically perforated silica particles onto a glass substrate settled at the bottom of a bottle filled with chloroform.

Structural Color Tuning: Mixing Melanin-Like Particles with Different Diameters to Create Neutral Colors

We present the ability to tune structural colors by mixing colloidal particles. To produce high-visibility structural colors, melanin-like core-shell particles composed of a polystyrene (PSt) core and a polydopamine (PDA) shell, were used as components. The results indicated that neutral structural colors could be successfully obtained by simply mixing two differently sized melanin-like PSt@PDA core-shell particles. In addition, the arrangements of the particles, which were important factors when forming structural colors, were investigated by mathematical processing using a 2D Fourier transform technique and Voronoi diagrams. These findings provide new insights for the development of structural color-based ink applications.