Tohru Shiga

Deformation and viscoelastic behavior of polymer gels in electric fields

“Smart” polymer gels actively change their size, structure, or viscoelastic properties in response to external signals. The stimuli-responsive properties, indicating a kind of intelligence, offer the possibility of new gel-based technology. The article attempts to review the current status of our knowledge of electromechanical effects that take place in smart polymer gels. Deformation and the mechanism of polyelectrolyte gel behavior in electric fields are first studied experimentally and then theoretically. In particular, the swelling or bending is discussed in detail. Particulate composite gels whose modulus of elasticity can vary in electric fields are revealed as a new smart material. The driving force causing varying elastic modulus in electric fields is explained by a qualitative model based upon polarized particles. Finally, applications of the two electromechanical effects are presented.

A Li–O2/CO2 battery

A new gas-utilizing battery using mixed gas of O(2) and CO(2) was developed and proved its very high discharge capacity. The capacity reached three times as much as that of a non-aqueous Li-air (O(2)) battery. The unique point of the battery is expected to be the rapid consumption of superoxide anion radical by CO(2) as well as the slow filling property of the Li(2)CO(3) in the cathode.

Deformation of ionic polymer gel films in electric fields

Deformation of ionic polymer gel films in electrolyte solutions was studied under the influence of electric fields. The ionic polymer gel films used were poly(vinyl alcohol)-poly(sodium acrylate) composite gel films with a thickness of the order of submillimetres. The vibration of gel films in a.c. electric fields, has been observed for the first time. It was suggested that the vibration behaviour was based on a differential swelling. The vibration of the ionic gel films was roughly analysed as a mechanical bending of a uniform cantilever beam by sinusoidally varying forces.

Catalytic Cycle Employing a TEMPO-Anion Complex to Obtain a Secondary Mg-O2 Battery.

Nonaqueous Mg-O2 batteries are suitable only as primary cells because MgO precipitates formed during discharging are not decomposed electrochemically at ambient temperatures. To address this problem, the present study examined the ability of the 2,2,6,6-tetramethylpiperidine-oxyl (TEMPO)-anion complex to catalyze the decomposition of MgO. It was determined that this complex was capable of chemically decomposing MgO at 60 °C. A catalytic cycle for the realization of a rechargeable Mg-O2 electrode was designed by combining the decomposition of MgO via the TEMPO-anion complex and the TEMPO-redox couple. This work also demonstrates that a nonaqueous Mg-O2 battery incorporating acrylate polymer having TEMPO side units in the cathode shows evidence of being rechargeable.

Electroviscoelastic effect of polymeric composites consisting of polyelectrolyte particles and polymer gel

The dynamic viscoelasticity of polymeric composites consisting of silicone gel and polymethacrylic acid cobalt(II) salt (PMACo) particles was studied in d.c. electric fields. It was measured by applying sinusoidally varying shear strain or compressive strain to the composites. It was found that the electric fields enhanced the storage and loss moduli of the composites, and changed the loss tangent (electroviscoelastic effect). The amount of the electroviscoelastic effect was influenced by the content of absorbed water in the PMACo particle, the fraction of PMACo particles, and the intensity of the electric field. It also depended on the amplitude and frequency of the applied strain. The increments of the compressive moduli due to the electroviscoelastic effect were much larger than those of the shear moduli.

Electrically Driven Polymer Gel Finger Working in the Air

Using ionic polymer gel that demonstrated an electric field-associated bending mo tion in aqueous solutions, an electrically driven polymer gel finger has been constructed. The material used for the finger was a hybrid gel of a cylindrical poly(vinyl alcohol)-poly(sodium acrylate) composite gel (PVA-PAA gel) rod and PVA gel film containing sodium carbonate as a skin. The working gel finger has been designed by equipping the PVA gel film of the hybrid gel with two platinum wires as electrodes. When a dc voltage was applied, the finger bent smoothly toward the positive electrode at a relatively high speed in the air. The bending of the finger occurred through a differential swelling of the PVA-PAA gel in the hybrid gel. A prototype of a microrobot hand with two gel fingers has also been designed. The hand grasped or released a piece of paper in response to electric signals.

Deformation Behaviors of Polymer Gels in Electric Field

A polymer gel is a crosslinked polymer network swollen in a liquid medium. Polymer gels, solid-liquid coexistent materials, are candidate biomimetic materials. Recently, their mechanical strength has become very close to that of living muscle. In a soft structure of gels, as shown in Fig. 1, the motion of polymer networks and the diffusion of ions take place easily by an external stimulus. The large volume or shape change, induced by supplying thermal, chemical or electrical energy, is an inherent nature of swollen gels.1‒4 Therefore, polymer gels have various possibilities as advanced functional polymers.

Solid-state solar cells consisting of polythiophene-porphyrin composite films

Sandwich-type solid-state solar cells using polythiophene-porphyrin composite films were fabricated. A spin-coated film of poly(3-dodecylthiophene) (P3DT) was fabricated on a gold electrode. Next, an electropolymerized polythiophene film was superimposed on the surface of the spin-coated P3DT film by electrochemical polymerization of bithiophene (BiTh) with repeated redox cycles in the 0–+2 V region. Then, tetrathienylporphyrin (TThP) was further assembled on the polythiophene-modified electrode by using the same electrochemical polymerization procedure (1 or 10 cycles), to obtain polythiophene-porphyrin-modified gold electrodes. Finally, an aluminum electrode was deposited on the polythiophene-porphyrin modified gold electrode by vacuum deposition, forming the sandwich-type solid-state solar cells. The morphological characterizations of the films were carried out by scanning electron microscopy. The thickness of the organic layer decreased from ~5 µm m to ~3 µm by performing TThP polymerization. The amount of porphyrin moiety in the composite film was larger for the modified electrode after 10 cycles of electrochemical TThP polymerization than for that after 1 cycle of TThP polymerization. The resultant photocurrent increased with scanning cycle of TThP polymerization in the 400–600 nm region. The combination of polythiophene and porphyrin in the electrochemically modified electrode is one of the useful systems for photocurrent generation.

Fluorescence from poly(N-vinylcarbazole) in uniaxially stretched polymer films

Steady-state and time-resolved fluorescence properties of poly(N-vinylcarbazole) (PVCz) dispersed in a polystyrene (PS) cast film were studied under tensile loadings at room temperature. The excited monomer emission of PVCz located around 350 nm decreased with increasing applied tensile strain from 0 to 0.8%. The strain enhanced the emission which was ascribed to the partial-overlap excimer of PVCz in a 360–430 nm region. The emission due to the full-overlap excimer of PVCz between 430 and 500 nm was unchanged by the action of the tensile loadings. The ratio of fluorescence intensities at 375 nm and 345 nm I375/I345 was proportional to the applied strain. The time-resolved fluorescence study indicated that the lifetimes of the excited monomer and of the partial-overlap excimer were not affected by the strain. The obtained results mean that the strain applied to the PS matrix increases the partial-overlap conformation of two adjacent carbazolyl chromophores in a PVCz chain and suggest that PVCz is a useful probe for detecting residual strains in polymer matrices.

Electroplastic behavior of doped poly(3-hexylthiophene)

Dynamic viscoelasticity of poly(3-hexylthiophene) (P3HT) doped with iodine or FeCl 3 under the influence of electric fields was studied. The doped P3HTs underwent a large decrease in elastic modulus under small electric fields on the order of 1 dc V/mm. The electric fields enhanced the loss tangent. The glass transition temperature decreased as the intensity of the applied fields was increased. This electroplastic behavior was observed also in ac excitation of less than 1 kHz. It was detected in both crystalline and amorphous P3HTs. Joule heating was a main process in the electroplastic behavior of doped P3HTs. We measured FTIR and X-ray diffraction spectra under electric fields to examine the possibility of other processes. The X-ray diffraction analysis suggested a possibility of another process caused by the formation of intra-planar packing of thiophene rings.