Mitsuyoshi Onoda

Actuator based on doping/undoping-induced volume change in anisotropic polypyrrole film

We propose an actuator using the cylindrical polypyrrole (PPy) film, with an anisotropy of morphology and conductivity, can be polymerized electrochemically in a Teflon slender pipe. The PPy actuator bends and reverts in a regular direction upon electrochemical undoping/doping. The bending speed becomes faster as the undoping potential is increased, but the magnitude of displacement depends not on the undoping potential but on the amount of charge or dopant concentration. The bending behavior strongly depends on the type of supporting salt in the electrolyte solutions, suggesting that cation transport into and out of the PPy plays a key role in the bending behavior and the size of the cation is a critical factor. It was also found that, the shape of the transient reduction current strongly depends on the type of cation and that the current does not follow the diffusion law. A bending mechanism based on the cation insertion and extraction, as well as on the anisotropic volume change upon doping/undoping, is proposed.

Anomalous dependences of luminescence of poly (3-alkylthiophene) on temperature and alkyl chain length

Abstract Photo-luminescence of poly(3-alkylthiophene) film is found to increase with increasing temperature and after indicating maximum at a melting point it again decreases in the liquid phase. Photo-luminescence also increases with increasing alkyl chain length. These results are discussed in terms of the changes of the effective conjugation length due to the introduction of a large torsion angle between neighbouring thiophene rings and the inter-chain interaction which influence on the dynamics of excited spices and recombination.

Dependence of Absorption Spectra and Solubility of Poly(3–alkylthiophene) on Molecular Structure of Solvent

Solvatochromic shifts of spectra of solutions of poly(3-hexylthiophene) and poly(3-docosylthiophene) have been analyzed in terms of Taft linear solvation energy relationships. Color changes of solutions of the former at 25°C are primarily due to changes in normal solute-solvent interactions. On the other hand, color changes of solutions of poly(3-docosylthiophene) appear to be due to configurational changes of the polymer chain. Solubility of the polymer is affected by solvation of the alkyl groups, electron donating or Lewis basicity properties of the solvent, polarity, and molecular volume of solvent molecules.

Fabrication of self‐assembled multilayer heterostructure of poly(p‐phenylene vinylene) and its use for an electroluminescent diode

Multilayer heterostructure thin films of electroactive polymers such as poly(p‐phenylene vinylene) (PPV)‐precursor and sulfonated polyaniline (SPAn) have been successfully fabricated via a process based on the spontaneous self‐assembly of conjugated polyion on a substrate. We have prepared SPAn‖PPV multilayer heterostructure light‐emitting diodes (LEDs) using this electrostatic self‐assembly technique and discussed the electroluminescent (EL) properties. The SPAn‖PPV monolayer LED emits greenish‐yellow light, but the SPAn‖PPV multilayer emits bluish‐green light and there is a slight shift between the two spectra of these devices. These results can be interpreted in terms of the confinement effect of carriers in the superlattice structure consisting of a SPAn‖PPV multilayer system.

Effect of Polymer Elongation on the Absorption Spectrum of Poly(3-alkylthiophene)

A remarkable absorption spectral change has been observed in ethylene propylene elastomer (EPR) films containing elongated poly(3-alkylthiophene). This change is explained in terms of the formation of new torsioned bonds between thiophene rings on the main polymer chains. This interpretation is also consistent with the anisotropy of the polarization dependence of absorption.

Heterostructure Electroluminescent Diodes Prepared from Self-Assembled Multilayers of Poly(p-phenylene vinylene) and Sulfonated Polyaniline.

Multilayer heterostructure thin films of electroactive polymers such as poly( p-phenylene vinylene) (PPV), -precursor and sulfonated polyaniline (SPAn), have been successfully fabricated by a process based on the spontaneous self-assembly of conjugated polyion on a substrate. We have prepared SPAn/PPV multilayer-heterostructure light-emitting diodes (LEDs) using this electrostatic self-assembly technique and discussed the electroluminescence (EL) properties. The thickness of each bilayer is about 20 A. The LED device composed of 10 bilayers has a turn-on voltage of about 1.4 V and greenish-yellow light was clearly observed under normal room illumination even at low driving voltage. These results are discussed in terms of radiative recombination of the singlet polaron-excitons formed by injection of electrons and holes and the formation of polaron-excitons in PPV layers of SPAn/PPV heterostructures.

Nanostructured Conjugated Polymer Films by Electrophoretic Deposition

The electrophoretic deposition of nanostructured films from colloidal suspensions of a conjugated polymer poly(3-octadecylthiophene) (PAT18), as well as the preparation of the suspension by a simple mixing method, is reported. A colloidal suspension of PAT18 was prepared by pouring a toluene solution of the polymer into acetonitrile, which is not a solvent for the polymer. It was found that the suspension obtained was stable for more than two weeks, and the stability was related to the concentration of the polymer therein. In comparison with a toluene solution of PAT18, the colloidal suspension shows a considerable red-shift in the optical absorption and photoluminescence spectra, as well as a quenched photoluminescence, indicating the solidification of PAT18 in the suspension. The electrophoretic deposition of a PAT18 film from the colloidal suspension was successfully carried out. The film shows almost the same optical absorption and photoluminescence spectra as those of a spin-coated film. On the other hand, an atomic force microscopy study revealed a nanostructured surface morphology of the electrophoretically deposited films. Such nanostructured films are expected to be promising materials for electrochemical and sensor applications.

Light‐emitting diodes using n‐type conducting polymer: Poly(p‐pyridyl vinylene)

A yellow‐orange electroluminescent (EL) diode using an n‐type conducting polymer, poly(p‐pyridyl vinylene) (PPyV), as an emitting layer has been demonstrated and its EL properties are discussed. The EL emission of this device can be interpreted in terms of radiative recombination of the singlet polaron exciton formed by the injection of electrons and holes. Although the EL intensity of the PPyV light‐emitting diode is weak, yellowish‐orange EL emissions were clearly observed under normal room light. Changing from the PPyV EL device to the PPV‖PPyV EL device using poly(p‐phenylene vinylene) (PPV) as a hole‐transporting layer increased the maximum quantum efficiency from 0.0065% to 0.08% photons per electron. Detailed electrical and optical properties for these diodes with aluminum as the electron‐injecting contacts are presented and an improved quantum efficiency for light emission has been achieved.

Large Change of Electrical Conductivity and Absorption Spectrum of Poly(3-alkylthiophene) at the Solid-Liquid Phase Transition

Electrical conductivity of poly(3-alkylthiophene) such as poly(3-dodecylthiophene) and poly(3-docosylthiophene) decreases in a step-wise manner at the melting point. The absorption spectra also changes rapidly at the phase transition. These phenomena are discussed in terms of the increased energy band gap in the liquid state due to the decrease of the co-planarity of thiophene rings accompanied by remarkable conformation changes. Large hysteresis was also observed in the temperature dependence of conductivity and absorption spectra. Thermochromism in the premelting region can be explained by effective shortening of the conjugation length due to the introduction of torsion of the thiophene rings at several points on a conjugated chain.

Three-Color Polymer Light-Emitting Devices Patterned by Maskless Dye Diffusion onto Prepatterned Electrode

A simple technique of maskless dye diffusion into polymer films on prepatterned electrodes is developed and three-color polymer light-emitting devices were successfully fabricated on a glass plate. In this method, prepatterned electrodes beneath precoated receiver film are utilized as heaters for activating dye molecules. Upon doping with three types of dye, 1,1,4,4-tetraphenyl-1,3-butadiene, coumarin 6 and Nile red into poly(N-vinylcarbazole) by the present diffusion method, blue, green and red emissions were obtained. It is found that the onset voltage of emission does not depend on the type of dye. The maskless dye diffusion technique can be applicable to a variety of devices such as photovoltaic devices.