Papers - Oaki, Yuya

Amorphous conjugated polymer networks as an emerging class of polymer nanostructures

Oaki Y., Sato K.

Rsc Applied Polymers 3 ( 1 ) 78 - 91 2024.11

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The organizational states of functional monomer molecules have a significant impact on the properties of polymer materials. This Perspective summarizes amorphous conjugated polymer networks (CPNs) as a new family of polymerized structures. CPNs have been studied since the 1990s. The number of papers about CPNs increased in the 2010s after the earlier work was summarized in a review in 2005. However, the amorphous types had not attracted much attention in previous articles. Amorphous CPNs have potential structural advantages compared with conventional crystalline polymers and framework materials. Diverse combinations of monomers and linkers are used to synthesize amorphous CPNs. Conjugated monomers as functional units are not densely aggregated but are homogeneously dispersed in the network. The amorphous network contributes to structural flexibility for molecular motion related to dynamic properties. The low-crystalline nature affords control over the nanoscale morphology. This Perspective starts with a brief summary of polymerized structures. Our recent work is introduced to show the structures and properties of amorphous CPNs. Simultaneous and random copolymerization of multiple conjugated monomers provides amorphous CPNs. Enhanced electrochemical performance in energy-related applications was extracted from the resultant amorphous CPNs containing redox-active moieties, thanks to their structural characteristics. These results imply that a variety of advanced functional materials can be developed based on the concept of amorphous CPNs.

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Machine-learning-assisted prediction of the size of microgels prepared by aqueous precipitation polymerization

Suzuki D., Minato H., Sato Y., Namioka R., Igarashi Y., Shibata R., Oaki Y.

Chemical Communications 60 ( 93 ) 13678 - 13681 2024.10

ISSN  13597345

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The size of soft colloids (microgels) is essential; however, control over their size has typically been established empirically. Herein, we report a linear-regression model that can predict microgel size using a machine learning method, sparse modeling for small data, which enables the determination of the synthesis conditions for target-sized microgels.

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Assembly-promoted repeatable enhancement of photoluminescence from cesium lead tribromide nanocubes under light illumination

Hashimoto S., Watanabe H., Iso Y., Oaki Y., Isobe T., Imai H.

Nanoscale Advances 6 ( 22 ) 5563 - 5567 2024.10

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A repeatable enhancement of the photoluminescence (PL) from CsPbBr<inf>3</inf> nanocubes (NCs) is promoted by the assembling of NCs. The PL quantum yield (QY) of ordered NC arrays increases with photoirradiation and decreases in the dark. The repeatable enhancement of the PLQY cannot be observed from isolated NCs. The nanospaces between NCs in the ordered arrays allow a reversible change in thermally stimulated desorption and photo-induced adsorption of surface ligands that affect the PLQY.

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Phase-separated structures of tunable thermoresponsive and matrix polymers for large-scale temperature monitoring coatings

Shibata R., Matsuda S., Kawakubo H., Imai H., Oaki Y.

Journal of Materials Chemistry B 12 ( 42 ) 10886 - 10892 2024.10

ISSN  2050750X

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Temperature monitoring is significant and fundamental in the fields of healthcare and medicine. In surgery, ultrasonic cutting devices are used for cutting, coagulation, and hemostasis. However, surgeons are concerned about the collateral thermal damages. For example, due to the limited space during endoscopic surgery, thermal damage is caused by touching the surrounding tissues of the targeted organ with the heated shaft of the ultrasonic cutting device. The present work exhibits thermoresponsive and reversible color-change coatings for temperature-distribution imaging. The phase-separated structures of the layered conjugated and matrix polymers enable both the tuned thermoresponsivity and large-scale coating. Layered polydiacetylene (PDA) with intercalated guests exhibits reversible and thermoresponsive gradual color changes from blue to red. A matrix polymer facilitates formation of the phase-separated layered PDA and large-scale coating. Spraying the precursor solution containing the diacetylene monomer, guest molecule, and matrix polymer provides the self-organized large-scale coatings on substrates. The temperature distribution on the shaft of an ultrasonic cutting device is monitored using the coatings. The phase-separated structure of thermoresponsive and matrix polymers can be applied to tunable temperature monitoring in a variety of fields.

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Motion and Orientation of Janus Spherical Particles at a Liquid-Air Interface Governed by the Moses Effect

Kaushal A., Gendelman O., Shoval S., Kuroiwa K., Oaki Y., Fujii S., Bormashenko E.

Langmuir 40 ( 33 ) 17835 - 17842 2024.08

ISSN  07437463

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We investigated the motion of spherical polystyrene/polypyrrole-coated polystyrene Janus particles placed at an air/saline interface and driven by a permanent magnetic field of ca. 0.5 T. For the sake of comparison, the motion of pure floating polystyrene particles was studied. Both kinds of the studied particles moved toward the magnet and stopped at the boundary of the near-surface well produced by the magnetic field. The Moses effect-driven motion of floating Janus particles was analyzed and investigated under different strengths of the magnetic field and salt concentrations. The study of the Janus particle displacement led to the development of a unified theoretical framework explaining the mechanism of the motion. This framework predicts that the motion of particles placed at an air-salt solution interface is not only dictated by magnetic energy but also intricately influenced by the interplay of factors, including the curvature of the interface caused by the static magnetic field, gravitational potential, and capillary forces. The orientation of the particles was observed. A qualitative explanation of the observed phenomena is suggested. The investigated process has potential for the self-assembly of particles placed at the liquid/air interface.

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Changes in crystal morphology induced by lanthanide doping into diacetylene lamellar crystals

Kohri M., Isomura S., Tachibana K., Hikichi R., Oaki Y., Wadati H., Kanoh H., Kishikawa K.

Polymer Journal 56 ( 8 ) 765 - 775 2024.08

ISSN  00323896

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In this study, we show that doping lanthanides into lamellar crystals reorganizes the lamellar structure and dramatically changes the crystal morphology. Azo-DA, a compound with azobenzene derivatives and carboxylic acids at both ends of the diacetylene moiety, formed plate-like lamellar crystals. The doping of holmium (Ho), a lanthanide, into the film obtained by stacking Azo-DA lamellar crystals, promoted a dramatic change in crystal morphology, resulting in the formation of an Azo-DA/Ho film with a radial lamellar crystal structure. A detailed investigation of the crystal growth process revealed that Azo-DA/Ho, which is slightly formed in the solution phase during Ho doping, acts as a pseudonucleating agent and dramatically changes the morphology of the lamellar crystals. Additionally, the morphological changes in the lamellar crystal films significantly changed the surface properties of the films, such as their appearance and water repellency. Similar morphological changes in lamellar crystals were induced when other lanthanide elements were used instead of Ho, and the type of lanthanide dopant can affect the magnetic properties of the films.

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Cascading responses of stimuli-responsive materials

Oaki Y., Fujii S.

Chemical Communications 60 ( 69 ) 9163 - 9176 2024.07

ISSN  13597345

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Responsiveness to stimuli is important in daily life: natural biological activity is governed by continuous stimulus responsiveness. The design of stimuli-responsive materials is required for the development of advanced sensing systems. Although fully controlled stimuli-responsive systems have been constructed in nature, artificial systems remain a challenge. Conventional stimuli-responsive materials show direct responsiveness to an applied stimulus (Stimulus 1), with structural changes in their molecules and organized states. This feature article focuses on cascading responses as a new concept for integrating stimuli-responsive material design. In cascading responses, an original stimulus (Stimulus 1) is converted into other stimuli (Stimulus 2, 3, …, N) through successive conversions. Stimulus N provides the eventual output response. Integration of multiple stimuli-responsive materials is required to achieve cascading responses. Although cascade, domino, and tandem chemical reactions have been reported at the molecular level, they are not used for materials with higher organized structures. In this article, we introduce functional carriers and sensors based on cascading responses as model cases. The concept of cascading responses enables the achievement of transscale responsivity and sensitivity, which are not directly induced by the original stimulus or its responsive material, for the development of advanced dynamic functional materials.

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High-Sensitive Spatiotemporal Distribution Imaging of Compression Stresses Based on Time-Evolutional Responsiveness

Ono N., Seishima R., Shigeta K., Okabayashi K., Imai H., Fujii S., Oaki Y.

Small 20 ( 24 )  2024.06

ISSN  16136810

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Mechanoresponsive materials have been studied to visualize and measure stresses in various fields. However, the high-sensitive and spatiotemporal imaging remain a challenging issue. In particular, the time evolutional responsiveness is not easily integrated in mechanoresponsive materials. In the present study, high-sensitive spatiotemporal imaging of weak compression stresses is achieved by time-evolutional controlled diffusion processes using conjugated polymer, capsule, and sponge. Stimuli-responsive polydiacetylene (PDA) is coated inside a sponge. A mechanoresponsive capsule is set on the top face of the sponge. When compression stresses in the range of 6.67–533 kPa are applied to the device, the blue color of PDA is changed to red by the diffusion of the interior liquid containing a guest polymer flowed out of the disrupted capsule. The applied strength (F/N), time (t/s), and impulse (F·t/N s) are visualized and quantified by the red-color intensity. When a guest metal ion is intercalated in the layered structure of PDA to tune the responsivity, the device visualizes the elapsed time (τ/min) after unloading the stresses. PDA, capsule, and sponge play the important roles to achieve the time evolutional responsiveness for the high-sensitive spatiotemporal distribution imaging through the controlled diffusion processes.

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Coral-mimetic production of aragonite films from CO2 captured by biogenic polyamines

Takashina K., Watanabe H., Oaki Y., Ohno Y., Yasumoto K., Imai H.

Crystengcomm 26 ( 15 ) 2065 - 2071 2024.03

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We designed CaCO<inf>3</inf> films comprised of aragonite nanorods by mimicking the microstructure and the formation process of the calcareous skeleton of a stony coral in the sea. Hierarchically structured calcareous frameworks are produced from the center of calcification (COC) under alkaline conditions in the organism. In the first step, a seed layer consisting of crystalline nanoparticles was produced as an artificial COC on a polyvinyl alcohol sheet in a solution containing Ca²⁺ ions, polyacrylic acid, and a biogenic polyamine, such as putrescine, through the gradual absorption of atmospheric CO<inf>2</inf>. In the second step, aragonite nanorods grew on the artificial COC in the mother liquid obtained by mixing a putrescine solution saturated with CO<inf>2</inf> and a CaCl<inf>2</inf> solution containing Mg²⁺ and alginate. Finally, we obtained a dense, homogeneous film consisting of aragonite nanorods similar to the coral skeleton through the fixation of CO<inf>2</inf>

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The size-dependent valence and conduction band-edge energies of Cu quantum dots

Matsui T., Watanabe H., Somekawa S., Yanagida S., Oaki Y., Imai H.

Chemical Communications 60 ( 33 ) 4419 - 4422 2024.03

ISSN  13597345

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Ultra-small metal particles having band gaps are regarded as a new class of functional materials. We investigated the size dependencies of the band-edge energies on Cu quantum-dots in the size range of 0.7-2.1 nm. The extremely high conduction band-edge energies owing to the strong quantum-size effects were observed for sizes below 1 nm.

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Efficient design and synthesis of an amorphous conjugated polymer network for a metal-free electrocatalyst of hydrogen evolution reaction

Hamada W., Hishida M., Sugiura R., Tobita H., Imai H., Igarashi Y., Oaki Y.

Journal of Materials Chemistry A 12 ( 6 ) 3294 - 3303 2024.01

ISSN  20507488

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Performances of functional polymers are enhanced by designing the structures in different hierarchies, such as monomers, polymers, secondary structures, and nanoscale morphologies. In the present work, an amorphous conjugated polymer network was designed and synthesized to obtain a metal-free electrocatalyst for hydrogen evolution reaction (HER) in an efficient manner. A prediction model for the catalytic performance, i.e. overpotential for HER, was constructed using machine learning on small data based on the literature. The straightforward linear prediction model assisted in the design of the network polymer containing quinone and heteroaromatic moieties. The simultaneous multiple reactions of benzoquinone (BQ) and benzoxazole (BO) formed the amorphous conjugated polymer network at 200 °C. After the morphology control, the BQ-BO polymer showed an overpotential of 230 mV for the electrochemical HER at 10 mA cm⁻², which is one of the highest performances in the metal-free electrocatalysts synthesized at low temperatures. Moreover, we found that the amorphous conjugated polymer networks showed specific hydration behavior in aqueous media. The results indicate that designing an amorphous conjugated polymer network coupled with machine learning is a potential approach for the development of functional materials.

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Morphology and size control of an amorphous conjugated polymer network containing quinone and pyrrole moieties via precipitation polymerization

Sugiura R., Imai H., Oaki Y.

Nanoscale Advances 6 ( 4 ) 1084 - 1090 2023.12

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Morphology and size control of insoluble and infusible conjugated polymers are significant for their applications. Development of a precipitation polymerization route without using a surface stabilizer is preferred to control the reaction, morphology, and size. In the present work, precipitation polymerization for an amorphous conjugated polymer network, a new type of polymerized structure containing functional units, was studied for the size and morphology control in the solution phase at low temperature. The random copolymerization of benzoquinone (BQ) and pyrrole (Py) monomers formed microspheres of the BQ-Py network polymers as the precipitates in the solution phase. The particle diameter was controlled in the range of 70 nm and 1 μm by changing the pH of the solution and concentration of the monomers. The resultant nanoparticles were applied to a metal-free electrocatalyst for the hydrogen evolution reaction (HER). The catalytic activity of the BQ-Py nanoparticles was higher than that of the bulk micrometer-sized particles. The results imply that the morphology and size of amorphous conjugated polymer networks can be controlled by precipitation polymerization.

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Imaging of Accumulated Mechanical Stresses Using Self-Assembled Layered Conjugated Polymer

Mochizuki Y., Imai H., Oaki Y.

ACS Applied Materials and Interfaces (ACS Applied Materials and Interfaces)  15 ( 41 ) 48725 - 48735 2023.10

ISSN  19448244

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When mechanical stresses, such as tensile, compressive, and frictional stresses, are applied to objects by various motions, they are accumulated in materials. Conventional mechanoresponsive materials and sensors detect one-time applied stress. However, the accumulated stresses are not visualized or measured in previous works. The present study demonstrated imaging and sensing of not only one-time but also accumulated tensile, compressive, and frictional stresses. Polyurethane (PU) film was combined with 2D layered polydiacetylene (PDA), a stimuli-responsive color-changing polymer. PDA generally exhibits no color changes with the application of tensile and compression stresses because the molecular motion leading to the color change is not induced by such mechanical stresses. Here the versatile mechanoresponsiveness was achieved using a block copolymer guest partially intercalated in the layered PDA. As the interlayer and outerlayer segments interact with PDA and PU, respectively, the applied stresses to the film are transferred from PU to PDA via the block copolymer guest. The color changes of the film imaged and quantified the accumulated work depending on the number and strength of the applied multiple stresses such as tensile, compressive, and frictional stresses. The design strategy of materials and methodology of sensing can be applied to the development of new sensors for accumulated mechanical stresses in a wide range of length and strength scales.

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Fluorine-Free Substrate-Independent Superhydrophobic Coatings by Nanoarchitectonics of Polydispersed 2D Materials

Hikichi R., Tokura Y., Igarashi Y., Imai H., Oaki Y.

Bulletin of the Chemical Society of Japan (Bulletin of the Chemical Society of Japan)  96 ( 8 ) 766 - 774 2023.08

ISSN  00092673

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Fluorine-free superhydrophobic surfaces have been required to meet environmental regulations in recent years. A new nanoarchitectonics is required to achieve superhydrophobicity without using perfluoroalkyl compounds. In the present work, surface-modified exfoliated nanosheets with polydispersed sizes were utilized to prepare coatings with high surface roughness and low surface energy. Fluorine-free superhydrophobic surfaces were obtained by spray coating of surface-modified exfoliated nanosheets based on transition-metal oxides. The water contact angle (ªw) 166.0 « 0.5° is the highest compared with that of the fluorine-free substrate-independent coatings in previous reports. The significant factors related to ªw were studied using machine learning combined with our chemical insight in the small data obtained in the present work. The resultant superhydrophobic coatings exhibited unique properties, such as sliding of water droplets, superoleophilic behavior, recyclability, and durability. The present design strategy can be applied to development of various functional coatings using exfoliated 2D materials with polydispersity.

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Departmental characterization of structural and mechanical properties of crossed lamellae of a gastropod Murex pecten

Nishio N., Sugiyama S., Yoshimura T., Terasaki R., Takasaki M., Oaki Y., Sasaki T., Imai H.

CrystEngComm (CrystEngComm)  25 ( 29 ) 4213 - 4218 2023.07

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We characterized departmental variations of the hierarchical lamellar structures of whorls, a siphonal canal, and spines of shells of a gastropod Murex pecten (Venus comb) and compared their mechanical properties. The crossed lamellae are basically composed of bundled aragonite nanorods ∼20 nm wide that are elongated along the (110) plane. The main body, including whorls and a siphonal canal, consists of four- and three-layer crossed lamellae, and thin spines comprise double- and single-layered structures including strained and distorted lamellae. The macroscopic bending strength and nanoscopic hardness of the spines were found to be greater than those of the main body of the shell. The specific strained and distorted lamellar structures are deduced to contribute to the enhanced mechanical properties of the long and thin parts.

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Capacity-prediction models for organic anode-active materials of lithium-ion batteries: advances in predictors using small data

Tobita H., Namiuchi Y., Komura T., Imai H., Obinata K., Okada M., Igarashi Y., Oaki Y.

Energy Advances (Energy Advances)  2 ( 7 ) 1014 - 1021 2023.05

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Organic energy storage has attracted a lot of interest in enhancing performance and reducing the consumption of resources. If performance predictors are prepared, the exploration of new compounds can be accelerated without consumption of time, energy, and effort. In the present work, a new straightforward capacity predictor is constructed for the exploration of organic anode-active materials. Sparse modeling for small data (SpM-S) combining machine learning (ML) and our chemical insights was used to construct linear regression models of specific capacity. In our previous work, two predictors (models G1 and G2) were prepared using small datasets. However, the descriptors and prediction accuracy of these models were not validated. In the present work, a new improved model (model G3) has been constructed with the addition of new data. These three models were studied in terms of data science: namely, prediction accuracy, validity of the descriptors, amount of training data used, and effect of ML algorithms. The straightforward, generalizable, and interpretable model G3 can be applied to explore new organic anode-active materials. Moreover, these data-scientific approaches to model construction and validation can be used to explore new energy-related materials even with small data.

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Development of prediction model for cloud point of thermo-responsive polymers by experiment-oriented materials informatics

Hayakawa M., Sakano K., Kumada R., Tobita H., Igarashi Y., Citterio D., Oaki Y., Hiruta Y.

Polymer Chemistry (Polymer Chemistry)  14 ( 19 ) 2383 - 2389 2023.04

ISSN  17599954

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Thermo-responsive polymers having a lower critical solution temperature (LCST) have attracted attention for biological applications such as drug delivery, diagnosis, and coating materials. In recent years, research on predicting LCST by utilizing machine learning has been conducted. However, since these methods targeted only copolymers combining specific monomer structures, they are not versatile, and multiple trials are still required to obtain new thermo-responsive polymers with the desired LCST. In this study, a prediction model for cloud point temperature (TCP) was built by a combination of materials informatics and chemical insight, named sparse modeling for small data (SpM-S) using a small dataset of polymers collected from the literature as training data. This approach created a model that is interpretable, easy to calculate, and versatile. The prediction accuracy was validated using data from different literature sources and experimental test data. The model was able to predict the TCP of polymers containing monomers not included in the dataset as well as polymers containing monomers included in the dataset. The predictive model has the potential to guide the design of new thermo-responsive polymers, and to contribute to efficient development of thermo-responsive polymers.

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Enhanced and stabilized photoluminescence of perovskite cesium lead bromide nanocubes through ordered assemblies

Sasaki M., Hashimoto S., Iso Y., Oaki Y., Isobe T., Imai H.

Nanoscale Advances (Nanoscale Advances)  5 ( 9 ) 2553 - 2557 2023.04

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This work clarified the effects of self-assembly of perovskite cesium lead bromide (CsPbBr3) nanocubes (NCs) covered with didodecyldimethyl ammonium bromide (DDAB) on photoluminescence (PL) properties. Although the PL intensity of isolated NCs was weakened in the solid state even under inert conditions, the quantum yield of PL (PLQY) and the photostability of DDAB-covered NCs were drastically improved by the formation of two-dimensional (2D) ordered arrays on a substrate. The PLQY of the 2D arrays increased to ca. 60% by initial excitation illumination at 468 nm and was maintained for over 4000 h. The improved PL properties are attributable to the fixation of the surface ligand around the NCs in the specific ordered arrays.

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Construction of Millimeter-Wide Monolayers of Ordered Nanocubes as a Stain of “Wineglass Tears” Driven by the Marangoni Flow

Takatsuji Y., Matsumoto R., Sazaki G., Oaki Y., Imai H.

Langmuir (Langmuir)  39 ( 11 ) 4091 - 4099 2023.03

ISSN  07437463

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We constructed millimeter-wide monolayers consisting of tetragonally ordered BaTiO3 (BT) nanocubes through the liquid film formation caused by the Marangoni flow in a toluene-hexane binary liquid containing oleic acid. A thin liquid film containing BT nanocubes was overspread on a standing silicon substrate through the condensation of toluene at the advancing front after the preferential evaporation of hexane. Then, the oscillatory droplet formation like “wineglass tears” occurred on the substrate. Finally, two-dimensionally ordered BT nanocubes were observed as a stain of “wineglass tears” on the substrate after the liquid film receded through evaporation. The presence of a thin liquid film in the binary system is essential for the production of millimeter-wide monolayers on the substrate because multilayer deposition occurs without the formation of a thin liquid film in monocomponent systems. We improved the regularity of the ordered arrays of nanocubes by adjusting the liquid component and evaporation conditions.

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Stimuli-Responsive Sponge for Imaging and Measuring Weak Compression Stresses

Ono N., Seishima R., Okabayashi K., Imai H., Fujii S., Oaki Y.

Advanced Science (Advanced Science)  10 ( 3 )  2023.01

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Imaging and measuring compression stresses secure a safe and healthy life. Compression stresses in kPa range are not easily detected by conventional mechanoresponsive materials because microscopic molecular motion of the chromophores is not induced by such weak stresses. Moreover, imaging of the stress distribution is not achieved so far. The present study shows a sponge device combining two stimuli-responsive materials, a capsule releasing interior liquid and color-changing polymer in responses to compression stress and chemical stimulus, respectively. The stimuli-responsive capsule is dispersed on a melamine sponge comprised of the fibers with coating the layered polydiacetylene (PDA). The application of weak compression stresses induces collapse of the capsules, outflow of the interior liquid, and subsequent irreversible color change of PDA. The cascading response in the sponge device colorimetrically enables imaging of the distribution and measuring the strength of the compression stresses in kPa range. Furthermore, the device demonstrates imaging and measuring unknown weak compression stresses applied by the irregular-shaped objects. A couple of clinical issues in surgical operation of intestine are studied using the stress-imaging sponge device. The device and its design strategy can be applied to stress imaging in a variety of fields.

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