Particle mobility and macroscopic magnetorheological effects for polyurethane magnetic elastomers.

The relationship between the particle mobility and magnetorheological effect was investigated for polyurethane magnetic elastomers containing carbonyl iron particles with various cross-linking densities or plasticizer concentrations. The storage modulus at 0 mT increased and the on-field modulus at 500 mT decreased with the cross-linking density. The critical magnetic field where the storage modulus starts to rise up increased with the cross-linking density, indicating that the movement of magnetic particles is depressed by the cross-linking points of the polyurethane network. Magnetic elastomers with various plasticizer concentrations revealed that the storage modulus at 0 mT decreased and the on-field modulus at 500 mT increased with the plasticizer concentration. The critical magnetic field decreased with increasing plasticizer concentration, indicating that a dense polyurethane network prevents magnetic particles from moving. It was found that the change in the modulus due to the magnetic field can be scaled by the storage modulus at 0 mT as well as the critical magnetic field. Thus, there is a certain correlation between the macroscopic modulus of elasticity (storage modulus at 0 mT) and the microscopic mobility of magnetic particles reflected in the critical magnetic field.

Extremely Long Chains of Magnetic Particles via Large Plastic Beads Observed in Bimodal Magnetic Elastomers.

The relationship between the magnetorheology of bimodal magnetic elastomers with high concentrations (60 vol %) of plastic beads with diameters of 8 or 200 µm and the meso-structure of the particles was investigated. Dynamic viscoelasticity measurements revealed that the change in storage modulus of the bimodal elastomer with 200 µm beads was 2.8 × 105 Pa at a magnetic field of 370 mT. The change in the storage modulus for monomodal elastomer without beads was 4.9 × 104 Pa. The bimodal elastomer with 8 µm beads hardly responded to the magnetic field. In-situ observation for the particle morphology was performed using synchrotron X-ray CT. For the bimodal elastomer with 200 µm beads, a highly aligned structure of magnetic particles was observed in the gaps between the beads when the magnetic field was applied. On the other hand, for the bimodal elastomer with 8 µm beads, no chain structure of magnetic particles was observed. The orientation angle between the long axis of the aggregation of magnetic particles and the magnetic field direction was determined by an image analysis in three dimensions. The orientation angle varied from 56° to 11° for the bimodal elastomer with 200 µm beads and from 64° to 49° for that with 8 µm beads by applying the magnetic field. The orientation angle of the monomodal elastomer without beads changed from 63° to 21°. It was found that the addition of beads with a diameter of 200 µm linked the chains of magnetic particles, while beads with a diameter of 8 µm prevented the chain formation of the magnetic particles.

Voids induce wide-range modulation of elasticity for magnetic elastomers II.

The magnetic response of dynamic modulus was investigated for polyurethane-based magnetic elastomers densely packed with magnetic particles with different diameters (7 µm and 235 µm). The density indicated that voids were created at volume fractions of magnetic particles above 0.47 (87 wt%) for 7 µm and 0.44 (85 wt%) for 235 µm. At volume fractions below these critical volume fractions, no apparent increase in the storage modulus was observed for the magnetic elastomers by applying a magnetic field of 500 mT. At above the critical volume fractions, dramatic increase in the storage modulus was observed; the maximum changes in the storage modulus were 8.0 MPa and 6.0 MPa, which corresponds to the relative changes in the modulus (ΔG/G0) of 74% and 11%, for magnetic elastomers of 7 µm and 235 µm, respectively. These results strongly indicate that the creation of a space enables the movement of magnetic particles in the elastomer resulting in the formation of chain structure. It was also found that the creation of a space in the matrix of elastomers can be detected by the critical strain, the amplitude of Payne effect, or the critical magnetic field since significant changes in these parameters were found at the critical volume fraction for both magnetic elastomers. SEM images displayed a clear difference in the creation process of voids that magnetic particles of 7 µm suddenly formed many macroscopic voids at the critical volume fraction, meanwhile, magnetic particles of 235 µm formed gaps around magnetic particles. The storage modulus for both magnetic elastomers changed perfectly in response to the magnetic field even after the on-off switching of the magnetic field with 20 cycles.

In Situ Observation of the Movement of Magnetic Particles in Polyurethane Elastomer Densely Packed Magnetic Particles Using Synchrotron Radiation X-ray Computed Tomography.

In situ observation of the migration and structure formation of magnetic particles in polyurethane elastomers was carried out by X-ray computed tomography using synchrotron radiation. The mean diameter of the magnetic particles was 7.0 µm, and the volume fraction was ϕ= 0.24 at its maximum. The exposure time was 100 ms/frame, and the pixel size was 0.458 µm/pixel. The orientation angle and the volume fraction of the maximum aggregate were analyzed using commercial software for image analysis. The orientation angle for magnetic elastomers with ϕ = 0.24 was approximately 55° at 0 mT and decreased remarkably with the magnetic field. At magnetic fields above 150 mT, the orientation angle gradually decreased with the field and showed a constant value of 38° at 300 mT, suggesting that magnetic particles move and form a chain-like structure although the chains do not align perfectly in the direction of the magnetic field. On the other hand, the volume fraction of the maximum aggregate was constant at magnetic fields below 100 mT, and it significantly increased with the field, indicating that magnetic particles were connected to each other and developed into a macroscopic structure with anisotropy. Dynamic viscoelastic measurements revealed that the storage modulus of the magnetic elastomers cannot be simply scaled by the orientation angle. It was also found that the volume fraction of the maximum aggregate is a good parameter for explaining the huge increase in the storage modulus. The dynamic movement of magnetic particles when a magnetic field of 300 mT was switched on and off was also successfully observed. When the field was switched on, magnetic particles connected instantly and their aggregates were rapidly elongated in the direction of the magnetic field. When the field was switched off, some of the connections between aggregates were broken; however, most of the aggregates did not return to the original position even 5 min after being switched off.

Voids induce wide-range modulation of elasticity for magnetic elastomers.

The relationship between the magnetorheological effect and the void ratio for a polyurethane magnetic elastomer with voids was investigated using a dynamic viscoelastic measurement under a magnetic field of 500 mT. The magnetic elastomer contains iron particles with a diameter of 235 µm at a concentration of 85 wt% (volume fraction: 0.43). The void ratio defined using the volume of vacancies in the non-filled volume of magnetic particles was increased by reducing the amount of polyurethane up to a maximum void ratio of 0.56. The storage modulus of the magnetic elastomer without voids was 1.5 × 105 Pa at 0 mT and 3.1 × 105 Pa at 500 mT, respectively; that is, no significant change in the modulus was observed. The storage modulus at 0 mT for the magnetic elastomer was independent of the void ratio, while the storage modulus at 500 mT increased in proportion to the void ratio. At a void ratio of 0.56, the storage modulus for the magnetic elastomer was 5.6 × 105 Pa at 0 mT and 6.1 × 106 Pa at 500 mT, respectively; that is, the magnetic elastomer demonstrated a significant change in the storage modulus on the order of MPa. This strongly indicates that production of voids enables movement of magnetic particles in the elastomer. Under both strains of 10-4 and 1, a significant and reversible response of storage modulus was observed after the first application of magnetic field even though the magnetic field was applied for 20 cycles, meaning that the change in the modulus is perfectly reversible although the elastomer contains many voids. SEM/EDX observations revealed that the area composed of carbon decreased with the increasing void ratio while the area composed of iron remained unchanged.

Chain Structure in a Cross-Linked Polyurethane Magnetic Elastomer Under a Magnetic Field.

The morphology of magnetic particles with a size of 7.0 µm was observed for magnetic elastomers with a concentration of magnetic particles of 70 wt% using an X-ray microscope remolded into high resolution. Computed tomography images revealed that magnetic particles were distributed isotopically in the absence of a magnetic field, but they formed a chain structure in the polyurethane network under a magnetic field of 270 mT. It was also established, by image analysis, that magnetic elastomers had an anisotropic structure under the magnetic field.

Elastic Properties of Magnetorheological Elastomers in a Heterogeneous Uniaxial Magnetic Field.

Magnetorheological elastomers (MREs) are stimulus-responsive soft materials that consist of polymeric matrices and magnetic particles. In this study, large-strain response of MREs with 5 vol % of carbonyl iron (CI) particles is experimentally characterized for two different conditions: (1) shear deformation in a uniform magnetic field; and (2), compression in a heterogeneous uniaxial magnetic field. For condition (1), dynamic viscoelastic measurements were performed using a rheometer with a rotor disc and an electric magnet that generated a uniform magnetic field on disc-like material samples. For condition (2), on the other hand, three permanent magnets with different surface flux densities were used to generate a heterogeneous uniaxial magnetic field under cylindrical material samples. The experimental results were mathematically modeled, and the relationship between them was investigated. We also used finite-element method (FEM) software to estimate the uniaxial distributions of the magnetic field in the analyzed MREs for condition (2), and developed mathematical models to describe these phenomena. By using these practicable techniques, we established a simple macroscale model of the elastic properties of MREs under simple compression. We estimated the elastic properties of MREs in the small-strain regime (neo⁻Hookean model) and in the large-strain regime (Mooney⁻Rivlin model). The small-strain model explains the experimental results for strains under 5%. On the other hand, the large-strain model explains the experimental results for strains above 10%.

Effect of Water Absorption on Electric Properties of Temperature-Resistant Polymers.

The effects of water absorption on the electric resistivity and dielectric constant of polyimide (PI) and poly(ethylene terephthalate) (PET) were investigated, and the mechanism of deterioration in electrical insulation properties was discussed. The polyimides are poly(oxydianiline pyromellitimide) (PMDA-ODA) and poly(para-phenylene diamine biphenyltetracarboxydiimide) (BPDA-PDA). These polymer films were immersed in pure water for various immersion times at room temperature, and the water absorption ratio was evaluated. The electric resistance for these films was measured at room temperature using a high-resistance meter, and the dielectric constant at room temperature was measured using an LCR meter in a frequency range of 200 kHz to 2 MHz. The absorption ratios at equilibrium absorption for PMDA-ODA, BPDA-PDA, and PET were 2.7, 2.5, and 0.5%, respectively. The critical volume fraction of the percolation threshold of electric conductivity due to water absorption was 0.034 for both PMDA-ODA and BPDA-PDA. On the other hand, PET did not show a significant decrease in the resistivity. For both PIs and PET, the dielectric constant observed could be explained by a series model of the respective capacitances of pure water and polymer. Actually, the resistivity of samples cut from the edges of the film after water absorption was almost the same value as that in the dry state. These results suggest that the absorbed water molecules are not uniformly dispersed in the film but are localized at the edges of the film even after the absorption equilibrium has been reached.

Fabrication of Apparatus Specialized for Measuring the Elasticity of Perioral Tissues.

On the human face, the lips are one of the most important anatomical elements, both morphologically and functionally. Morphologically, they have a significant impact on aesthetics, and abnormal lip morphology causes sociopsychological problems. Functionally, they play a crucial role in breathing, articulation, feeding, and swallowing. An apparatus that can accurately and easily measure the elastic modulus of perioral tissues in clinical tests was developed, and its measurement sensitivity was evaluated. The apparatus is basically a uniaxial compression apparatus consisting of a force sensor and a displacement sensor. The displacement sensor works by enhancing the restoring force due to the deformation of soft materials. Using the apparatus, the force and the displacement were measured for polyurethane elastomers with various levels of softness, which are a model material of human tissues. The stress measured by the developed apparatus increased in proportion to Young's modulus, and was measured by the compression apparatus at the whole region of Young's modulus, indicating that the relation can be used for calibration. Clinical tests using the developed apparatus revealed that Young's moduli for upper lip, left cheek, and right cheek were evaluated to be 45, 4.0, and 9.9 kPa, respectively. In this paper, the advantages of this apparatus and the interpretation of the data obtained are discussed from the perspective of orthodontics.

Anomalous Magnetorheological Response for Carrageenan Magnetic Hydrogels Prepared by Natural Cooling.

The effect of the cooling rate on magnetorheological response was investigated for magnetic hydrogels consisting of carrageenan and carbonyl iron particles with a concentration of 50 wt.%. For magnetic gels prepared via natural cooling, the storage moduli at 0 and 50 mT were 3.7 × 104 Pa and 5.6 × 104 Pa, respectively, and the change in the modulus was 1.9 × 104 Pa. For magnetic gels prepared via rapid cooling, the storage moduli at 0 and 50 mT were 1.2 × 104 Pa and 1.8 × 104 Pa, respectively, and the change in the modulus was 6.2 × 103 Pa, which was 1/3 of that for the magnetic gel prepared by natural cooling. The critical strains, where G' is equal to G″ on the strain dependence of the storage modulus, for magnetic gels prepared by natural cooling and rapid cooling, were 0.023 and 0.034, respectively, indicating that the magnetic gel prepared by rapid cooling has a hard structure compared to that prepared by natural cooling. Opposite to this, the change in the storage modulus at 500 mT for the magnetic gel prepared by rapid cooling was 1.6 × 105 Pa, which was 2.5 times higher than that prepared by natural cooling. SEM images revealed that many small aggregations of the carrageenan network were found in the magnetic gel prepared by natural cooling, and continuous phases of carrageenan network with large sizes were found in the magnetic gel prepared by rapid cooling. It was revealed that magnetic particles in the magnetic gel prepared by rapid cooling can move and form a chain structure at high magnetic fields by breaking the restriction from the continuous phases of carrageenan.

Development of a Method for Peeling Off Paper from Celluloid Pictures for Animation Films.

During the storage of celluloid pictures for animation films over half a century, an interleave paper adhered to acrylic paint. The purpose of this study is to establish a methodology to cleanly remove the paper from the paint. A layered film, a replica of the celluloid pictures, adhered with paper was prepared and immersed in water or ethanol. The effect of these solvents on the peeling behavior was investigated using a peel test. The maximum peel force for the dry layered film in was distributed at ~0.5 N, independently of the peel speed. The peel force was significantly reduced after the layered film was immersed in pure water or ethanol. A morphological observation revealed that the dry paper was peeled off via the cohesive failure of the paper. After the layered film was immersed in pure water, the paper was also peeled off via cohesive failure. The layered film immersed in ethanol was peeled off at the paper/paint interface. To clear the effect of the volume change in the paint on peel behavior, the relative volume was determined via image analysis. The relative volume of paint was 1.56 in pure water and 1.37 in ethanol. It can be considered that the large difference in the volume of paint induces a large shear stress at the paint/paper interface.

Mechanical Properties and Reinforcement of Paper Sheets Composited with Carboxymethyl Cellulose.

The mechanical properties for paper sheets composited with glucose (Glc), methyl cellulose (MC), and carboxymethyl cellulose (CMC) were investigated. The paper composites were prepared by immersing paper sheets in aqueous solutions of these materials and drying at 100 °C for 30 min. The stress-strain curves for these paper composites were measured by a uniaxial tensile apparatus with a stretching speed of 2 mm/min. The breaking stress and strain for untreated paper were 24 MPa and 0.016, respectively. The paper composites demonstrated stress-strain curves similar to the untreated paper; however, the breaking point largely differed for these composites. The breaking strain and breaking stress for the Glc composite slightly decreased and those for the MC composite gradually increased with the concentration of materials composited. Significant increases in the mechanical properties were observed for the CMC composite. The breaking stress, breaking strain, and breaking energy for the 3 wt.% CMC composite were 2.0-, 3.9-, and 8.0-fold higher than those for untreated paper, respectively. SEM photographs indicated that the CMC penetrated into the inner part of the paper. These results strongly suggest that the mechanical improvement for CMC composites can be understood as an enhancement of the bond strength between the paper fibrils by CMC, which acts as a bonding agent. It was also revealed that the breaking strain, breaking stress, and breaking energy for the CMC composites were at maximum at the first cycle and decreased gradually as the immersion cycles increased.

Magnetic Response Detects the Strength of Carrageenan Network.

The effect of carrageenan concentration on the magneto-rheological effect of magnetic gels with a magnetic particle concentration of 50 wt.% was investigated under a magnetic field of 50 mT by dynamic viscoelastic measurements. The change in the storage modulus for magnetic gels due to the magnetic field was 3.0 × 103 Pa at a carrageenan concentration of 1.0 wt.% and increased with the concentration. The modulus change showed a maximum of 2.3 × 104 Pa at ~2.0 wt.% and became lower at higher concentrations. This is an interesting phenomenon, which was first observed in this study. The critical strain, the strain where the storage modulus intersects the loss modulus in the strain dependence of the modulus, was much higher than that for carrageenan gels, indicating a strong interaction between the magnetic particles and carrageenan. At 0 mT, the critical strain for the magnetic gels increased remarkably with decreasing the concentration, indicating that magnetic gels have a structure that does not flow easily at concentrations below 1.75 wt.%. It is considered that the structure hardly flows, hindering the movement of particles. At high concentrations, the SEM photographs showed both a particle network of magnetic particles and a dense carrageenan network. It can be considered that the movement of magnetic particles was hindered due to these factors at high concentrations.

Flowability of Gel-Matrix and Magnetorheological Response for Carrageenan Magnetic Hydrogels.

The relationship between rheological features in the absence of a magnetic field and magnetic response was investigated for κ-carrageenan magnetic hydrogels containing carbonyl iron particles. The concentration of carrageenan was varied from 1.0 to 5.0 wt%, while the concentration of carbonyl iron was kept at 70 wt%. The magnetic response revealed that the change in storage modulus ΔG' decreased inversely proportional to the carrageenan concentration. A characteristic strain γ1 where G' equals to G″ was seen in a strain range of 10-3. It was found that ΔG' was inversely proportional to the characteristic stress at γ1. Another characteristic strain γ2 where the loss tangent significantly increased was also analyzed. Similar to the behavior of γ1, ΔG' was inversely proportional to γ2. The characteristic stresses at γ1 and γ2 were distributed at 80-720 Pa and 40-310 Pa, respectively. It was revealed that a giant magnetorheology higher than 1 MPa can be observed when the characteristic stresses at γ1 and γ2 are below approximately 240 Pa and 110 Pa, respectively.

Magnetorheological Response for Magnetic Elastomers Containing Carbonyl Iron Particles Coated with Poly(methyl methacrylate).

The magnetorheological response for magnetic elastomers containing carbonyl iron (CI) particles with a diameter of 6.7 µm coated with poly(methyl methacrylate) (PMMA) was investigated to estimate the diameter of secondary particles from the amplitude of magnetorheological response. Fourier-transformed infrared spectroscopy revealed that the CI particles were coated with PMMA, and the thickness of the PMMA layer was determined to be 71 nm by density measurement. The change in the storage modulus for magnetic elastomers decreased by coating and it was scaled by the number density of CI particles as ΔG~N2.8. The diameter of secondary particle of CI particles coated with PMMA was calculated to be 8.4 µm. SEM images revealed that the CI particles coated with PMMA aggregated in the polyurethane matrix.

Efficient Chain Formation of Magnetic Particles in Elastomers with Limited Space.

The magnetic response of the storage modulus for bimodal magnetic elastomers containing magnetic particles with a diameter of 7.0 µm and plastic beads with a diameter of 200 µm were investigated by varying the volume fraction of plastic beads up to 0.60 while keeping the volume fraction of the magnetic particles at 0.10. The storage modulus at 0 mT for monomodal magnetic elastomers was 1.4 × 104 Pa, and it slightly increased with the volume fraction of plastic beads up to 0.6. The storage modulus at 500 mT for bimodal magnetic elastomers at volume fractions below 0.25 was constant, which was equal to that for the monomodal one (=7.9 × 104 Pa). At volume fractions of 0.25-0.40, the storage modulus significantly increased with the volume fraction, showing a percolation behavior. At volume fractions of 0.40-0.60, the storage modulus was constant at 2.0 × 105 Pa, independently of the volume fraction. These results indicate that the enhanced increase in the storage modulus was caused by the chain formation of the magnetic particles in vacancies made of plastic beads.

Evocalcet prevents ectopic calcification and parathyroid hyperplasia in rats with secondary hyperparathyroidism.

BACKGROUND: Elevated parathyroid hormone (PTH) levels in secondary hyperparathyroidism (SHPT) lead to vascular calcification, which is associated with cardiovascular events and mortality. Increased PTH production is caused by the excessive proliferation of parathyroid gland cells, which is accelerated by abnormal mineral homeostasis. Evocalcet, an oral calcimimetic agent, inhibits the secretion of PTH from parathyroid gland cells and has been used for the management of SHPT in dialysis patients. We observed the effects of evocalcet on ectopic calcification and parathyroid hyperplasia using chronic kidney disease (CKD) rats with SHPT. METHODS: CKD rats with SHPT induced by adenine received evocalcet orally for 5 weeks. The calcium and inorganic phosphorus content in the aorta, heart and kidney was measured. Ectopic calcified tissues were also assessed histologically. To observe the effects on the proliferation of parathyroid gland cells, parathyroid glands were histologically assessed in CKD rats with SHPT induced by 5/6 nephrectomy (Nx) after receiving evocalcet orally for 4 weeks. RESULTS: Evocalcet prevented the increase in calcium and inorganic phosphorus content in the ectopic tissues and suppressed calcification of the aorta, heart and kidney in CKD rats with SHPT by reducing the serum PTH and calcium levels. Evocalcet suppressed the parathyroid gland cell proliferation and reduced the sizes of parathyroid cells in CKD rats with SHPT. CONCLUSIONS: These findings suggest that evocalcet would prevent ectopic calcification and suppress parathyroid hyperplasia in patients with SHPT.

Rheopectic Behavior for Aqueous Solutions of Megamolecular Polysaccharide Sacran.

The rheopectic behavior of sacran aqueous solutions, a natural giant molecular polysaccharide with a molecular weight of 1.6 × 107 g/mol, was investigated. When a low shear was applied to 1.0 wt.% sacran solution, the shear viscosity increased from 7.2 to 34 Pas. The increment in the viscosity was enhanced as the shear rate decreased. The shear viscosity was independent of the time at a shear rate of 0.8 s-1; simultaneously, thixotropic behavior was observed at shear rates higher than 1.0 s-1. A crossover was observed at 0.15 wt.% for the concentration dependence of both the viscosity increase and zeta potential, which was the vicinity of the helix transition concentration or gelation concentration. It was clear that the molecular mechanism for the rheopexy was different at lower and higher regions of the crossover concentration.

Particle Size in Secondary Particle and Magnetic Response for Carrageenan Magnetic Hydrogels.

The relation between the number of magnetic particles and the change in storage modulus induced by a magnetic field was investigated for weak hydrogels containing carbonyl iron, iron oxide, and barium ferrite particles with different diameters in primary particles while maintaining the magnetization of magnetic particles. The change in storage modulus exhibited a power dependency against the number of magnetic particles, which was nearly independent of the magnetic particles. The change in storage modulus was successfully scaled by the reduced number of magnetic particles using the diameter of secondary particles. Microphotographs revealed that iron oxide and barium ferrite particles form aggregations while carbonyl iron particles are well dispersed in carrageenan gels. The diameter of secondary particles determined by a relation between the change in storage modulus and the reduced number of magnetic particles showed similar values with those observed in microphotographs.

Magnetic Elastomers with Smart Variable Elasticity Mimetic to Sea Cucumber.

A magnetic-responsive elastomer consisting of magnetic elastomer and zinc oxide with a tetrapod shape and long arms was fabricated mimetic to the tissue of sea cucumber in which collagen fibrils are dispersed. Only the part of magnetic elastomer is active to magnetic fields, zinc oxide plays a role of reinforcement for the chain structure of magnetic particles formed under magnetic fields. The magnetic response of storage modulus for bimodal magnetic elastomers was measured when the magnetic particle was substituted to a nonmagnetic one, while keeping the total volume fraction of both particles. The change in storage modulus obeyed basically a mixing rule. However, a remarkable enhancement was observed at around the substitution ratio of 0.20. In addition, the bimodal magnetic elastomers with tetrapods exhibited apparent change in storage modulus even at regions with a high substitution ratio where monomodal magnetic elastomers consist of only magnetic particles with less response to the magnetic field. This strongly indicates that discontinuous chains of small amounts of magnetic particles were bridged by the nonmagnetic tetrapods. On the contrary, the change in storage modulus for bimodal magnetic elastomers with zinc oxide with irregular shape showed a mixing rule with a substitution ratio below 0.30. However, it decreased significantly at the substitution ratio above it. The structures of bimodal magnetic elastomers with tetrapods and the tissue of sea cucumber with collagen fibrils are discussed.