Kenji Ishida

We experimentally investigated the instability of periodic roll structures in electroconvection of a planar-aligned nematic liquid crystal, with particular focus on the system size along the roll axis. We systematically varied the system size and compared the transitions in the convection pattern as the applied voltage was increased. In larger systems, the pattern transitioned to defect turbulence, a form of weak turbulence. In this state, defects are generated irregularly within the periodic structure as the convection rolls become inclined. In smaller systems, we observed angle-deflective oscillation, a time-periodic oscillation of the roll inclination angle. To describe the instability of these periodic convective structures, we propose an activator-inhibitor model for the coupled dynamics of the nematic director and the roll orientation, and show that the type of Hopf-unstable mode is selected by the system size along the roll axis.

To realize electrically pumped organic lasing, single‐layer devices without organic heterointerfaces have been employed to suppress efficiency rolloff. In such architectures, on the other hand, extrinsic trap states result in trap‐assisted nonradiative recombination, thereby decreasing electroluminescence efficiency. Here, we elucidate the dominant factors governing extrinsic trap formation by combining current density ( J )–voltage ( V ) characteristics of single‐layer hole‐only‐devices (HODs) with molecular dynamics (MD) simulations of 6,6′‐bibenzo[ d ]oxazole (BZ) derivatives exhibiting high photostability and low amplified spontaneous emission (ASE) thresholds. Drift‐diffusion analysis of the J – V characteristics reveals that 2,2′‐bis(4‐(9‐phenyl‐9 H ‐carbazol‐2‐yl)phenyl)‐6,6′‐bibenzo[ d ]oxazole (BZCz2) exhibits a higher density of extrinsic trap states than the other BZ derivatives studied. MD simulations further show that amorphous BZCz2 layers contain a high population of large voids arising from bulky substituents, potentially facilitating the incorporation of water clusters during vacuum deposition, thereby contributing to the formation of extrinsic hole traps. Furthermore, dispersing BZCz2 at low concentration in the host material of 4,4′‐bis(carbazol‐9‐yl)biphenyl (CBP), which exhibits dense molecular packing and minimal void formation, effectively suppresses extrinsic hole trapping in host–guest systems. These findings provide a strategy for suppressing extrinsic trap states and pave the way for highly efficient single‐layer EL devices.

Chemically cross-linked ferroelectric poly(vinylidene fluoride-co-trifluoroethylene) [P(VDF-TrFE)] films were prepared using 1,3-bis(3-aminopropyl)tetramethyldisiloxane (Bis-APTMDS), a diamine cross-linker containing a flexible siloxane segment. Fourier-transform infrared spectroscopy, X-ray photoelectron spectroscopy, and gel fraction analysis confirmed the formation of a chemically cross-linked network between P(VDF-TrFE) chains. Increasing the Bis-APTMDS content altered the surface morphology and slightly reduced the crystallinity while largely preserving the ferroelectric Form I crystalline phase. Cross-linking decreased the Young’s modulus and increased the strain at break, indicating enhanced mechanical deformability. Although the positive remanent polarization decreased with increasing cross-linker content under the same applied electric field, it recovered to a value comparable to that of uncross-linked P(VDF-TrFE) under a higher maximum applied electric field. These results demonstrate that Bis-APTMDS cross-linking can enhance the mechanical deformability of P(VDF-TrFE) films while maintaining recoverable ferroelectric polarization, providing a promising strategy for the design of mechanically compliant ferroelectric polymer films for flexible piezoelectric devices.

To enable quantitative palpation in vivo, we previously developed a catheter-type tactile sensor that uses a polyvinylidene fluoride (PVDF) film for detecting lesions based on surface changes. This study investigates the effects of the structural parameters for the sensor on the piezoelectric output. A coupled electrical-structural finite element analysis (FEA) is used to simulate the displacement of a sensor tip composed of silicone rubber layers, a PVDF film, and a plastic substrate film. The FEA results indicated that encapsulating the plastic film in rubber increased the output charge by approximately 56.5%, primarily due to enhanced strain caused by lateral expansion of the silicone rubber. Increasing the plastic film thickness and the distance between the neutral plane and the PVDF film was also found to increase the output charge. In addition, the sensor output was larger when the lower rubber layer was made thicker than the upper rubber layer. However, the material used for the substrate film was predicted to have the dominant effect on the sensor output, with the largest output being achieved for a film with a large Young’s modulus. To confirm the FEA results, we fabricated prototype sensors that used plastic, steel, and titanium films, and experimentally evaluated their performance. The results indicated that an appropriate choice of film material could increase the sensor output by 18.9 fold.

Polyurea and its derivatives exhibit excellent physical and chemical properties originating from intermolecular hydrogen bonding. Consequently, they have been used as coating materials that are resistant to heat, water, and chemicals, and in recent years, as materials for 3D printers. Their properties depend on their formation and hydrogen bonding mechanisms; however, these processes are very fast, making them challenging to study experimentally. In this study, polyurea formation via the polyaddition reaction of a diamine and diisocyanate was investigated insitu by fast time-resolved Fourier-transform infrared–attenuated total reflectance (FTIR–ATR) spectroscopy. This method enabled the analysis of the polyaddition reaction within 1 s and observation of intermolecular hydrogen bond formation. We observed the polyurea formation of different aliphatic and alicyclic monomers by recording the FTIR–ATR spectra every ~0.07 s and analyzed the reaction kinetics. In the case of aliphatic monomers, strong hydrogen bonds were formed in the initial stage of polyurea formation, whereas in the case of alicyclic monomers, weak hydrogen bonds were initially formed, which strengthened as the reaction proceeded. These findings elucidate the role of monomer structure on the formation and hydrogen bonding state of polyurea and demonstrate the effectiveness of time-resolved FTIR–ATR spectroscopy as a technique for observing very fast reactions and intermolecular interactions.

To enable quantitative palpation in vivo, we previously developed a catheter-type tactile sensor with an outer diameter of 2 mm composed of a polyvinylidene fluoride (PVDF) film for minimally invasive surgery. However, our previous studies did not evaluate the effect of the PVDF film shape on the sensor output. In this study, we fabricated three types of prototype sensors with different PVDF film shapes and sizes using a simple cutting method. One of the films had the same shape as that used in one of our previous studies. We also prepared two types of PVDF film with a wide base and a narrow tip because we assumed that the deformation of the sensor gradually decreases from the root to the tip, similar to the first mode of the natural frequency. We evaluated the frequency response of the proposed sensors by vibrating the sensor tip and compared the results with the theoretical values. It was confirmed that the sensor output increased with PVDF film size. Although this tendency was observed for both the measured and theoretical values, the measured values were smaller than the theoretical values. Moreover, the effect of film size was larger than that of film shape. Improvements in the sensor structure and the theoretical equation and better evaluation methods are required in order to optimize the film shape and size.