Faculty Profiles - TAKEOKA, Yukikazu

Highly Transparent and Tough Filler Composite Elastomer Inspired by the Cornea Reviewed International journal

Kenta Watanabe, Eiji Miwa, Fumio Asai, Takahiro Seki, Kenji Urayama, Tomotaka Nakatani, So Fujinami, Taiki Hoshino, Masaki Takata, Chang Liu, Koichi Mayumi, Kohzo Ito, Yukikazu Takeoka

ACS MATERIALS LETTERS Vol. 2 ( 4 ) page： 325 - 330 2020.4

Language：English Publishing type：Research paper (scientific journal) Publisher：AMER CHEMICAL SOC

We propose a strategy to develop a colorless, transparent, and tough composite elastomer inspired by the cornea, which is the transparent front portion of the eyeball. The composite elastomer, in which 34 vol % hard silica particles with a uniform particle size are dispersed as a filler in a low-crosslinking polymer network exhibits a fracture energy that is similar to 13.5 times higher than that of a system without the silica particles. This strategy also makes the elastomer optically transparent, because the light scattered by each silica particle that forms an ordered structure in the polymer network is cancelled by interference. This research may pave the way for the development of optically transparent and durable materials for applications such as advanced medical devices and soft robots.

DOI： 10.1021/acsmaterialslett.9b00520

Improvement of mechanical properties of elastic materials by chemical methods International journal

Yukikazu Takeoka, Sizhe Liu, Fumio Asai

SCIENCE AND TECHNOLOGY OF ADVANCED MATERIALS Vol. 21 ( 1 ) page： 817 - 833 2020.2

Language：English Publishing type：Research paper (scientific journal) Publisher：TAYLOR & FRANCIS LTD

Elastomers such as gels and rubbers play various roles in our lives. Elastomers, which guarantee the safety of airplanes and automobiles and the stability of buildings, are materials that have made the lives of people in the twentieth century extremely convenient. The existence of macromolecules, that is, giant molecules, has been clarified; the development of synthetic macromolecules has progressed; and understanding of elastomers has progressed. By introducing new ideas, it has become possible to obtain tough and hard elastomers, which was difficult under conventional ideas. In this paper, we will explain the development from the classical theory of elastomers to current efforts.

DOI： 10.1080/14686996.2020.1849931

Precise Synthesis of a Homogeneous Thermoresponsive Polymer Network Composed of Four-Branched Star Polymers with a Narrow Molecular Weight Distribution Reviewed International journal

Yuumi Okaya, Yuto Jochi, Takahiro Seki, Kotaro Satoh, Masami Kamigaito, Taiki Hoshino, Tomotaka Nakatani, So Fujinami, Masaki Takata, Yukikazu Takeoka

Macromolecules Vol. 53 ( 1 ) page： 374 - 386 2020.1

Language：English Publishing type：Research paper (scientific journal) Publisher：American Chemical Society ({ACS})

In this study, the synthesis of a star-shaped polymer with a narrow molecular weight distribution and subsequent formation of a homogeneous polymer network composed of the star-shaped polymer were realized by combining single-electron transfer living radical polymerization, click reaction, and amide bond formation with a condensing agent, which are toolized reactions. First, a 4-armed star polymer consisting of N-isopropylacrylamide was synthesized by living radical polymerization using N,N'-ethylenebis (2,2-dichloroacetamide) as a 4-branched initiator. By this polymerization method, a terminal Cl 4-branched star poly(N-isopropylacrylamide) (PNIPA) with a narrow molecular weight distribution could be obtained, but it was found that the reaction activity of the polymer terminal was lost in the process of purification and isolation. Therefore, after obtaining the terminal Cl 4-branched star PNIPA by living radical polymerization, an azide reaction was carried out in one pot without purification and isolation of the star-shaped polymer. As a result, the azide group was successfully introduced to all ends of the star-shaped polymer. By introducing a carboxyl group or an amino group into the 4-branched star polymer with azide groups using a click reaction, two types of 4-branched star polymers with different end groups were obtained. Equal amounts of both 4-branched star polymers were mixed at a polymer concentration equal to or higher than the overlapping concentration, and as a result of forming an amide bond with a condensing agent, a polymer gel was obtained. The swelling behaviors of the polymer gel indicate that almost no unreacted carboxyl group or amino group was present in the obtained polymer gel. That is, it was found that both 4-branched star polymers reacted efficiently to form a polymer network. In addition, structural observation of the polymer network by the small-angle X-ray scattering method showed that a polymer gel consisting of a network of uniform size was obtained. As mentioned above, we succeeded in constructing a polymer gel consisting of a homogeneous network structure using a temperature responsive 4-branched star polymer as the building block. The living radical polymerization method, the click reaction, and the amide formation by condensation reaction used in this study can be applied not only to the NIPA used here but also to various other monomers. If the construction of a precise network structure is realized by many polymers and the relation with the functional expression derived from the structure is clarified, it will be possible to design the network structure in accordance with the usage of the polymer gel.

DOI： 10.1021/acs.macromol.9b01616

Characterization of Colloidal Amorphous Arrays Prepared by Uniaxial Pressure Application Reviewed International journal

Yui Naoi, Takahiro Seki, Ryosuke Ohnuki, Shinya Yoshioka, Yukikazu Takeoka

LANGMUIR Vol. 35 ( 43 ) page： 13983 - 13990 2019.10

Language：English Publishing type：Research paper (scientific journal) Publisher：AMER CHEMICAL SOC

We prepared a colloidal amorphous array by applying uniaxial pressure to a powder of monodispersed colloidal silica particles. Pellet-shaped samples were obtained that exhibit different structural colors depending on the diameter of the particles. We characterized the optical properties of the arrays by measuring the angle-dependent scattering spectrum wherein several spectral peaks were observed. The peak at the longest wavelength was caused by the short-range order of the particle arrangement. Interestingly, this peak exhibited a smaller shift in wavelength than that observed in similar samples prepared by several different methods. The other spectral peaks were thought to originate from Mie scattering, which produces a color when the diameter of the colloidal particles is appropriately chosen. Our results showed that uniaxial pressure application can be a suitable method to prepare structurally colored pigments with low angle dependence.

DOI： 10.1021/acs.langmuir.9b02622

Colorful Photonic Pigments Prepared by Using Safe Black and White Materials Reviewed International journal

Miki Sakai, Takahiro Seki, Yukikazu Takeoka

ACS SUSTAINABLE CHEMISTRY & ENGINEERING Vol. 7 ( 17 ) page： 14933 - 14940 2019.9

Language：English Publishing type：Research paper (scientific journal) Publisher：AMER CHEMICAL SOC

The methods used to prepare vivid color materials that have supported our rich lifestyle are due for review, with the goal of using chemical substances that minimize the significant adverse effects on human health and the environment. In this research, we found that pigments of various colors can be obtained by utilizing submicron-sized amorphous silica fine particles and a complex of iron and tannic acid (Fe-TA complex), which are used for foods, cosmetics, etc. Silica fine particles with uniform submicron particle size appear as a white powder. When silica fine particles form spherical colloidal crystals with short-range order, long-distance order, and periodicity or form colloidal amorphous arrays with only short-range order, the fine particle arrays strongly scatter light with a specific wavelength in all directions. If the wavelength is in the visible light region, these arrays can be used as color materials displaying colors with low angle dependence. However, such arrays formed only from silica fine particles can appear whitish when observed with the naked eye because of noncoherent multiple scattering of light from the interior. Introducing a black Fe-TA complex inside the arrays reduces the effect of the noncoherent multiple scattering of light, and the arrays exhibit bright colors. Safe and inexpensive pigments exhibiting various hues and saturations can be obtained by controlling the size of the silica fine particles, the form of the fine particle arrays, and the incorporated state of the Fe-TA complex. By including TA, the mechanical stability of fine particle arrays and their adhesion to glass substrates are improved, and therefore, the fine particle arrays can be used as coating films.

DOI： 10.1021/acssuschemeng.9b03165

Development of angle-independent color material using colloidal amorphous array Reviewed International journal

Yui Naoi, Takahiro Seki, Shinya Yoshioka, Yukikazu Takeoka

MOLECULAR CRYSTALS AND LIQUID CRYSTALS Vol. 688 ( 1 ) page： 105 - 113 2019.7

Language：English Publishing type：Research paper (scientific journal) Publisher：TAYLOR & FRANCIS LTD

Color materials utilizing physical phenomena such as scattering, diffraction, and interference of light have drawn attention as alternatives to organic dyes that fade by light and some inorganic pigments that are highly toxic. For example, a color material exhibiting structural color caused by a fine structure with a size that is on the order of the wavelength can be prepared using only human body or environmentally friendly materials. If the fine structure is kept stable, the structural colored material can be a nonfading color material. We know that a conventional structural colored material exhibits a property that the hue greatly changes depending on the light irradiation direction and the viewing direction. Recent studies have shown that colloidal amorphous arrays obtained with spherical colloidal particle aggregates with short-range order have almost no angle dependence of the hue under natural light where light is irradiated from all directions. However, the structural color generated from the colloidal amorphous array exhibits angle dependence on the hue under the condition that light is irradiated only from one direction. In this study, we found a light scattering peak, in the scattering spectrum observed from the colloidal amorphous array, showing no angle dependence at all on the shorter wavelength side than the light scattering peak caused by the existence of the short-range order. By exploiting this scattering phenomenon, we will explain the possibility of producing a color material with no angle dependence on the hue even under the condition that light is irradiated only from one direction.

DOI： 10.1080/15421406.2019.1651075

Grating Diffraction or Bragg Diffraction? Coloration Mechanisms of the Photonic Ball Reviewed International journal

Ryosuke Ohnuki, Shinji Isoda, Miki Sakai, Yukikazu Takeoka, Shinya Yoshioka

ADVANCED OPTICAL MATERIALS Vol. 7 ( 13 ) 2019.7

Language：English Publishing type：Research paper (scientific journal) Publisher：WILEY-V C H VERLAG GMBH

The coloration mechanism of the photonic ball is investigated, which is a spherical aggregation of submicrometer-sized colloidal particles. An interesting optical property is reported for photonic balls with colloidal particles larger than 400 nm: a ring-like iridescent reflection appears on the peripheral part of the ball when observed under an optical microscope. Previous studies considered grating diffraction or Bragg diffraction separately to explain this iridescence. Here, it is shown from detailed structural and optical investigations that both diffraction mechanisms are important. In particular, Bragg diffraction explains the presence of a specific color for the photonic ball when observed with the naked eye in contrast to the iridescence seen under the microscope. The essential idea of the optical analysis is to combine the Bragg diffraction that occurs in the 3D periodic structure with the grating diffraction that originates from the surface with a 2D periodicity. The macroscopic color is almost independent of the angle of observation because of this unique reflection mechanism; hence, the photonic ball can be a suitable candidate for the structurally colored pigment.

DOI： 10.1002/adom.201900227

Optically transparent, high-toughness elastomer using a polyrotaxane cross-linker as a molecular pulley Reviewed International journal

Hiroaki Gotoh, Chang Liu, Abu Bin Imran, Mitsuo Hara, Takahiro Seki, Koichi Mayumi, Kohzo Ito, Yukikazu Takeoka

SCIENCE ADVANCES Vol. 4 ( 10 ) page： eaat7629 2018.10

Language：English Publishing type：Research paper (scientific journal) Publisher：AMER ASSOC ADVANCEMENT SCIENCE

An elastomer is a three-dimensional network with a cross-linked polymer chain that undergoes large deformation with a small external force and returns to its original state when the external force is removed. Because of this hyperelasticity, elastomers are regarded as one of the best candidates for the matrix material of soft robots. However, the comprehensive performance required of matrix materials is a special challenge because improvement of some matrix properties often causes the deterioration of others. For example, an improvement in toughness can be realized by adding a large amount of filler to an elastomer, but to the impairment of optical transparency. Therefore, to produce an elastomer exhibiting optimum properties suitable for the desired purpose, very elaborate, complicated materials are often devised. Here, we have succeeded in creating an optically transparent, easily fabricated elastomer with good extensibility and high toughness by using a polyrotaxane (PR) composed of cyclic molecules and a linear polymer as a cross-linking agent. In general, elastomers having conventional cross-linked structures are susceptible to breakage as a result of loss of extensibility at high cross-linking density. We found that the toughness of the transparent elastomer prepared using the PR cross-linking agent is enhanced along with its Young's modulus as cross-linking density is increased.

DOI： 10.1126/sciadv.aat7629

Spontaneous synthesis of a homogeneous thermoresponsive polymer network composed of polymers with a narrow molecular weight distribution

Jochi Yuto, Seki Takahiro, Soejima Takamasa, Satoh Kotaro, Kamigaito Masami, Takeoka Yukikazu

NPG ASIA MATERIALS Vol. 10 page： 840-848 2018.8

Bioinspired Color Materials Combining Structural, Dye, and Background Colors

Sakai Miki, Seki Takahiro, Takeoka Yukikazu

SMALL Vol. 14 ( 30 ) 2018.7

Spontaneous Preparation of Monodispersed, Structural Colored, Spherical Particles by Rotational Stirring

Teshima Midori, Suzuki Motoki, Seki Takahiro, Takeoka Yukikazu

CHEMNANOMAT Vol. 4 ( 7 ) page： 621-625 2018.7

Environment and human friendly colored materials prepared using black and white components

Takeoka Yukikazu

CHEMICAL COMMUNICATIONS Vol. 54 ( 39 ) page： 4905-4914 2018.5

Structural Coloration of a Colloidal Amorphous Array is Intensified by Carbon Nanolayers

Takeoka Yukikazu, Iwata Masanori, Seki Takahiro, Nueangnoraj Khanin, Nishihara Hirotomo, Yoshioka Shinya

LANGMUIR Vol. 34 ( 14 ) page： 4282-4288 2018.4

Angle-independent structural colored materials by black and white components

Takeoka Yukikazu

ABSTRACTS OF PAPERS OF THE AMERICAN CHEMICAL SOCIETY Vol. 255 page： . 2018.3

Preparation of angle-independent color materials by applying Mie resonances

Naoi Yui, Takeoka Yukikazu, Seki Takahiro

ABSTRACTS OF PAPERS OF THE AMERICAN CHEMICAL SOCIETY Vol. 255 page： . 2018.3

Simple preparation of magnetic field-responsive structural colored Janus particles

Teshima Midori, Seki Takahiro, Takeoka Yukikazu

CHEMICAL COMMUNICATIONS Vol. 54 ( 21 ) page： 2607-2610 2018.3

Angle-independent structural colored materials by black and white components Reviewed

Yukikazu Takeoka

ABSTRACTS OF PAPERS OF THE AMERICAN CHEMICAL SOCIETY Vol. 255 page： . 2018.3

Preparation of angle-independent color materials by applying Mie resonances Reviewed

Yui Naoi, Yukikazu Takeoka, Takahiro Seki

ABSTRACTS OF PAPERS OF THE AMERICAN CHEMICAL SOCIETY Vol. 255 page： . 2018.3

Structurally colored coating films with tunable iridescence fabricated via cathodic electrophoretic deposition of silica particles Reviewed International journal

Kiyofumi Katagiri, Kensuke Uemura, Ryo Uesugi, Kei Inumaru, Takahiro Seki, Yukikazu Takeoka

RSC ADVANCES Vol. 8 ( 20 ) page： 10776 - 10784 2018

Language：English Publishing type：Research paper (scientific journal) Publisher：ROYAL SOC CHEMISTRY

In recent years, colloidal arrays of submicrometer-sized monodisperse particles used as structurally colored coatings have drawn great attention due to their non-bleaching properties and low impact on human health and the environment. In this paper, structurally colored coating films were fabricated using monodisperse SiO2 particles via the cathodic electrophoretic deposition (EPD) technique. The addition of a strong polycation, poly(diallyldimethylammonium chloride) (PDDA), enables the cathodic EPD of SiO2 particles and carbon black (CB) additives. Optimizing the quantities of PDDA and CB results in the appearance of vivid structural color from the coating films. The arrangement of the particle array is controllable by varying the pH of the water added to the coating sols for EPD. Structurally colored coating films with and without iridescence, i.e., angular dependence, can be fabricated on demand by a simple operation of the EPD process. In addition, the coating film prepared by cathodic EPD displayed high abrasion resistance because PDDA acts not only as a charge control agent but also as a binder.

DOI： 10.1039/c8ra01215f