Tetsu Mitsumata

Magnetically Tunable Elasticity for Magnetic Hydrogels Consisting of Carrageenan and Carbonyl Iron Particles

A new class of magnetoelastic gel that demonstrates drastic and reversible changes in storage modulus without using strong magnetic fields was obtained. The magnetic gel consists of carrageenan and carbonyl iron particles. The magnetic gel with a volume fraction of magnetic particles of 0.30 exhibited a reversible increase by a factor of 1400 of the storage modulus upon a magnetic field of 500 mT, which is the highest value in the past for magnetorheological soft materials. It is considered that the giant magnetoelastic behavior is caused by both high dispersibility and high mobility of magnetic particles in the carrageenan gel. The off-field storage modulus of the magnetic gel at volume fractions below 0.30 obeyed the Krieger-Dougherty equation, indicating random dispersion of magnetic particles. At 500 mT, the storage modulus was higher than 4.0 MPa, which is equal to that of magnetic fluids, indicating that the magnetic particles move and form a chain structure by magnetic fields. Morphological study revealed the evidence that the magnetic particles embedded in the gel were aligned in the direction of magnetic fields, accompanied by stretching of the gel network. We conclude that the giant magnetoelastic phenomenon originates from the chain structure consisting of magnetic particles similar to magnetic fluids.

Magnetic polyurethane elastomers with wide range modulation of elasticity

A new magnetorheological elastomer that demonstrates drastic and reversible changes in dynamic modulus in air was obtained. The magnetic elastomer consists of polyurethane and carbonyl iron particles. The magnetic elastomer with a volume fraction of 0.29 exhibited a reversible increase by factors of 277 of the storage modulus and 96 of the loss modulus upon a magnetic field of 500 mT. The elastomer underwent high mechanical toughness with a braking strain exceeding 0.8, and demonstrates the giant magnetorheological behavior for half a year after the synthesis without degradation.

Magnetism and viscoelasticity of magnetic elastomers with wide range modulation of dynamic modulus

Magnetism, dynamic viscoelasticity, mechanical property, and durability of magnetic elastomers consisting of polyurethane and carbonyl iron particles were investigated. Magnetic measurements revealed that the magnetic permeability of the magnetic elastomers can be explained by the linear combination of the magnetic permeability of magnetic particles and polyurethane. Dynamic viscoelastic measurements showed that the magnetic particles are randomly dispersed in the elastomer. On applying a magnetic field of 500 mT, the magnetic elastomers demonstrated a drastic change in the dynamic modulus; the storage modulus increased from 6.5 kPa to 1.6 MPa, and the loss modulus increased from 3.6 kPa to 0.16 MPa. This drastic change in the dynamic modulus was observed at all frequencies from 0.01 Hz to 3 Hz. The critical volume fraction showing the transition from a random dispersion to a chain structure decreased significantly with the magnetic field. Compression tests revealed that the magnetic elastomers exhibited high mechanical toughness with high breaking strain exceeding 0.8. Durability tests showed the magnetoviscoelastic behavior of the magnetic elastomers was maintained for 1.5 years after the synthesis without degradation.

Particle Dispersibility and Giant Reduction in Dynamic Modulus of Magnetic Gels Containing Barium Ferrite and Iron Oxide Particles

The particle dispersibility of barium ferrite and iron oxide magnetic particles in carrageenan gels was investigated, and the influence of the dispersibility on the giant reduction in the dynamic modulus of the gels was discussed. The gels containing barium ferrite demonstrated giant reductions in the storage Youngs modulus on the order of 10 (5) Pa due to magnetization; however, small reductions in the storage modulus of less than 10 (4) Pa were observed for the gels containing iron oxide. The storage modulus of gels with barium ferrite did not follow the Krieger-Dougherty equation above volume fractions of 0.06, indicating the heterogeneous dispersion of the magnetic particles; however, the modulus of the gels with iron oxide satisfied the equation at all volume fractions, suggesting the random dispersion of the particles. It was noted that the gels with barium ferrite demonstrated enhanced nonlinear viscoelasticity and a large value of the loss tangent, while the gels with iron oxide exhibited weak nonlinear viscoelasticity and a small value of the loss tangent. Magnetic measurements indicated high values of remanent magnetization for barium ferrite and low values for iron oxide. After magnetization at 1 T, the magnetic gels with barium ferrite became elongated parallel to the magnetic field and shrunk perpendicular to the field. In contrast, the magnetic gels with iron oxide did not undergo a marked deformation. These results strongly indicate that the giant reduction in the storage modulus requires both enhanced nonlinear viscoelasticity and magnetostriction which originate from the particle dispersibility. The relationship between the dispersibility of magnetic particles and the giant reduction in the storage modulus is discussed using rheological and morphological data.

Cyanobacterial Polysaccharide Gels with Efficient Rare-Earth-Metal Sorption

The cyanobacterial polysaccharide sacran, which contains carboxylate and sulfate groups, was extracted from Aphanothece sacrum , and the metal sorption behavior of sacran was investigated. Heterogels, where the sacran chains were trapped by polyvinyl alcohol networks, were prepared and immersed in NdCl3 solutions to shrink and cloud due to Nd binding. These heterogels had the ability to sorb excessive amounts of Nd ions, more than the stoichiometric ratio of 1:3 (sacran anion/Nd). Furthermore, the sacran-containing gels sorbed Nd ions under highly acidic conditions below pH 2 more efficiently than alginate-containing gels. We speculated that the strong Nd condensation effect of the sulfate groups in sacran under the acidic conditions may enhance the Nd sorption to the carboxylate groups.

Gelation Behavior by the Lanthanoid Adsorption of the Cyanobacterial Extracellular Polysaccharide

The self-organization behavior of an extracellular polysaccharide (sacran) extracted from the cyanobacterium Aphanothece sacrum in response to lanthanoid ion adsorption was investigated. Consequently, cryogenic TEM images revealed that sacran could be cross-linked by Nd(3+) trivalent ions and formed a fibrous nanostructural network containing water. Furthermore, sacran adsorbed trivalent metal ions at a 3:1 ratio, which was the theoretical ionic adsorption and showed more efficient adsorption than alginate based on electric conductivity titration. The critical gelation concentrations, Cg, where sacran formed tough gels upon metal ion binding were estimated. The Cg for trivalent metal ions was lower than that for divalent ions, and the Cg for lanthanoid ions was particularly low at 10(-3) to 10(-4) M, changing every four elemental numbers. The extracellular matrix of Aphanothece sacrum, sacran, may adsorb metal ions to create fibrous nanostructures that reinforce the jelly matrix.

Anisotropic swelling in hydrogels formed by cooperatively aligned megamolecules

Sacran is a supergiant cyanobacterial polysaccharide with an extremely high absolute molecular weight that exceeds 107 g mol−1 (molecular length: over 30 μm). Sacran forms milli-scaled orientation domains in aqueous liquid crystalline (LC) state, even in trace concentrations i.e. 0.3 wt%. Aqueous sacran films that are cast from a LC state and annealed between 70–140 °C form self-standing sheets composed of oriented hydrogels. When sacran films swell, they experience changes in size that are 70 fold higher in relation to thickness than those that occur in relation to width. Either an increase in film thickness or a decrease in sacran chain length reduces swelling anisotropy, demonstrating that stress that occurs during drying can be effectively used to propagate the cooperative alignment of LC chains on a micrometer sized scale comparable with the thickness of self-standing films.

Nonmagnetic particles enhance magnetoelastic response of magnetic elastomers

The elastic modulus for bimodal magnetic elastomers has been investigated by compression measurements under large deformation. The bimodal magnetic elastomers consist of carbonyl iron magnetic particles and zinc oxide nonmagnetic particles. The Youngs modulus for monomodal magnetic elastomers was 8.94 × 104 Pa at 0 mT and 1.65 × 105 Pa at 320 mT, respectively. The relative change in the Youngs modulus for monomodal magnetic elastomer was 1.8, and it was raised to 5.8 only by mixing with the nonmagnetic particles of 9.6 vol. %. It is considered that the modulus enhancement originates from the stress transfer by the additional chains of magnetic particles via nonmagnetic particles. The electric resistivity analysis revealed that 27% of magnetic particles in a strand of chains were replaced by nonmagnetic particles. It was shown in the present study that the bimodal magnetic elastomers endured against a compression load of 30 N.

Enhancement of magnetoelastic behavior of bimodal magnetic elastomers by stress transfer via nonmagnetic particles

The magnetoelastic behavior of bimodal magnetic elastomers consisting of magnetic particles, carbonyl iron, and nonmagnetic particles, zinc oxide, has been investigated by dynamic viscoelastic measurements. The storage modulus of bimodal magnetic elastomers increased under a magnetic field of 500 mT. The change in the storage modulus was enhanced by adding nonmagnetic particles at volume fractions above a certain volume fraction of 0.02, indicating the occurrence of stress transfer by a chain structure of magnetic particles via nonmagnetic particles. The critical volume fraction at 500 mT determined by percolation analysis was nearly independent of the diameter of nonmagnetic particles. However, at low magnetic fields below 160 mT, the critical volume fraction was found to decrease with the particle diameter. Substituting magnetic particles with nonmagnetic ones, the change in the storage modulus of bimodal magnetic elastomers monotonically decreased with the substitution ratio of magnetic particles. The mechanism of the enhanced magnetoelastic response for bimodal magnetic elastomers is discussed.

High-Power Actuators Made of Two-Phase Magnetic Gels

The magnetic field-induced elongation of magnetic gels consisting of poly(vinyl alcohol) (PVA) and magnetic particles was analyzed. The elongation increased as the concentration of magnetic particles increased, and inversely it decreased as the cross-linking density increased. We propose a simple theoretical expression of the elongation taking into consideration the elastic modulus of magnetic gels. We have fabricated a high-power actuator made of magnetic gels consisting of a thin magnetic-gel phase and a thick PVA-gel phase. A maximum strain of 33% was achieved while sustaining a constant stress of 1.3 kPa using a magnetic field of 900 mT. The magnetic gel showed a maximum stress of 6.8 kPa and a strain of approximately 10%. Magnetic gels consisting of two gel phases have great potential for application in practical actuators that can be driven by a magnetic field.