Electric Conductivity and Dielectric-Breakdown Behavior for

Article pubs.acs.org/JPCB

Electric Conductivity and Dielectric-Breakdown Behavior for Polyurethane Magnetic Elastomers Shuhei Sasaki,†,‡ Yuri Tsujiei,†,‡ Mika Kawai,†,‡ and Tetsu Mitsumata*,†,‡ †

Graduate School of Science and Technology, Niigata University, Niigata 950-2181, Japan ALCA, Japan Science and Technology Agency, Tokyo 102-0076, Japan

‡

ABSTRACT: The electric-voltage dependence of the electric conductivity for cross-linked and un-cross-linked magnetic elastomers was measured at various magnetic fields, and the effect of cross-linking on the electric conductivity and the dielectric-breakdown behavior was investigated. The electric conductivity for uncross-linked elastomers at low voltages was independent of magnetic fields and the volume fraction of magnetic particles, indicating the electric conduction in the polyurethane matrix. At high voltages, the electric conductivity increased with the magnetic field, showing the electric conduction via chains of magnetic particles. On the other hand, the electric conductivity at low voltages for cross-linked elastomers with volume fractions below 0.06 was independent of the magnetic field, suggesting the electric conduction in the polyurethane matrix. At volume fractions above 0.14, the electric conductivity increased with the magnetic field, suggesting the electric conduction via chains of magnetic particles. At high voltages, the electric conductivity for cross-linked elastomers with a volume fraction of 0.02 was independent of the magnetic field, indicating the electric conduction through the polyurethane matrix. At volume fractions above 0.06, the electric conductivity suddenly increased at a critical voltage, exhibiting the dielectric breakdown at the bound layer of magnetic particles and/or the discontinuous part between chains. loss factor by magnetic fields depending on the strain amplitude, i.e., negative response at low strains and significant positive response at high strains.14 Besides the rheological response mentioned above, the electric conductivity of magnetic elastomers also alters in response to the magnetic fields. So far, many papers describing the electric conductivity for magnetic elastomers have been reported.15−24 Recently, it is widely and acceleratingly developed that the electric properties for magnetic elastomers are applied to sensors, e.g., highly stretchable electrodes or lighting devices,18 pressure- or strain-sensitive magnetic elastomers,19 and graphite doped magnetorheological plastomers.20 However, the basic property of electric conductivity for magnetic elastomers is not fully understood, particularly in the presence of high electric fields. We consider that there are three types of conduction mechanisms for magnetic elastomers depending on the electric voltages or the volume fraction of magnetic particles. At low voltages or low volume fractions of magnetic particles, the electric current flows in the insulative polyurethane matrix having an electric conductivity of ∼0.1 nS/ cm. At intermediate voltages, the electric current flows in the polyurethane matrix partially through the chains of magnetic particles without showing the dielectric breakdown. At high voltages or high volume fractions of magnetic particles, the

1. INTRODUCTION Soft materials responsive to external stimuli, such as temperature, pH, and electric fields, have attracted considerable attention as the next-generation actuators, devices with virtual reality, or soft robots, etc. A magnetic elastomer is a soft material responsive to magnetic fields and consists of polymeric matrixes and magnetic particles. When magnetic fields are applied to magnetic elastomers, the viscoelastic properties alter in response to the magnetic fields.1−5 It can be considered that the magnetic particles align in the direction of the magnetic fields and make a chain structure within the elastomer. The chain structure of the magnetic particles increases the viscoelasticity of the magnetic elastomer due to the stress transfer among the magnetic particles. So far, we have reported a new class of magnetic hydrogels6 that exhibit drastic and reversible changes in the dynamic modulus without using strong magnetic fields. The iron particle moves within the cross-linked hydrogel and makes a structure with the alignment of magnetic particles (chain structure).7 In addition, we succeeded in fabricating magnetic elastomers containing plasticizers that exhibit a wide range modulation of dynamic modulus in response to weak magnetic fields.8−10 The relative change in Young’s modulus for a monomodal magnetic elastomer is 1.8, and it is raised to 5.8 only by mixing it with the nonmagnetic particles of 9.6 vol %. The bimodal magnetic elastomer endures against a compression load of 30 N and thus can be used in actuators working at high loads.11−13 The bimodal magnetic elastomer exhibits an amphoteric response of © XXXX American Chemical Society

Received: December 22, 2016 Revised: February 1, 2017

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DOI: 10.1021/acs.jpcb.6b12875 J. Phys. Chem. B XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry B

3. RESULTS AND DISCUSSION Figure 1 shows the strain dependence of the electric conductivity for cross-linked magnetic elastomers with a

electric conduction occurs through the chains with the dielectric breakdown occurring at the bound layer of magnetic particles and/or the discontinuous part between chains. In the present study, we measured the electric-voltage dependence of the electric conductivity for cross-linked and uncross-linked magnetic elastomers in the presence of various magnetic fields. The effect of cross-linking on the electric conductivity or the dielectric-breakdown behavior for the magnetic elastomers is discussed.

2. EXPERIMENTAL PROCEDURE Synthesis of Magnetic Elastomers. Polyurethane elastomers and magnetic elastomers were synthesized by a prepolymer method. Poly(propylene glycol) (Mw = 2000, 3000), prepolymer cross-linked by toluene diisocyanate, a plasticizer (dioctyl phthalate, DOP), and carbonyl iron (CI) particles were mixed by a mechanical mixer for several minutes. The median diameter of CI particles was 2.5 ± 0.2 μm determined by a particle size analyzer (SALD-2200, Shimadzu). The magnetic particles were produced by hydrogen reduction, and the minimum weight fraction of iron for CI particles is 99.0. The saturation magnetization for CI particles was evaluated to be 245 emu/g by a SQUID magnetometer (MPMS, Quantum Design). The mixed liquid was poured in a silicon mold and cured in an oven for 20 min at 100 °C without applying magnetic fields. Note that magnetic fields were applied only at electrical or rheological measurements. The magnetic elastomers presented here contain a large amount of plasticizer, similarly to organogels. The weight concentration of DOP was defined by the ratio of DOP to the matrix without magnetic particles, and it was fixed at 65 wt %; DOP/(DOP + matrix). Magnetic elastomers with volume fractions of 0.02 (10 wt %) to 0.27 (70 wt %) were prepared in this study. Electric Conductivity Measurements. The electric conductivity for magnetic elastomers in the presence of magnetic fields was measured by the two-terminal method using a high resistance meter (DSM-8104, HIOKI) at room temperature. The electrode was made of beryllium−copper alloy, and the voltage was varied from 10 V to 1 kV. The data of electric resistance was measured after 1 s when the electric voltage was applied to the magnetic elastomer, because the resistance showed a time-dependent behavior due to the electrode polarization of conductive impurities. The magnetic field was continuously swept up to 500 mT, and the data were acquired after 15 s when the magnetic field reached the target intensity. The sample was a disk of 20 mm diameter and 2.0 mm thickness. The electrode was set between magnetic poles of an electromagnet. The resistance parallel to the direction of magnetic fields was measured at room temperature. Rheological Measurements. Dynamic viscoelastic measurements were also carried out using a rheometer (MCR301, Anton Paar) at 20 °C. The storage modulus at 1 Hz was measured at a strain of 10−4 in the presence of a pulsatile magnetic field of 500 mT. The sample was a disk of 20 mm diameter and 1.5 mm thickness. Relaxation NMR. The interaction between polyurethane and CI particles was evaluated by the relaxation rate of protons using a 13 MHz NMR spectrometer (Acron area, Xigo nanotools). The relaxation decay curves were measured, and the transverse spin−spin relaxation time was determined by hypothesizing a single relaxation process. The spin−spin relaxation rate R2 of protons within poly(propyrene grycol) (PPG) molecules is inversely related to their mobility.25

Figure 1. Strain dependence of electric conductivity at 0 and 500 mT for cross-linked magnetic elastomers with a volume fraction of 0.27.

volume fraction of 0.27. The elastomer was intentionally compressed by electrodes to find the effect of the compression on the electric conductivity for cross-linked magnetic elastomers. At 0 mT, the electric conductivity for magnetic elastomers was independent of the strain at applied voltages of 30 and 1000 V. At 500 mT, the electric conductivity increased with increasing strain at both applied voltages, showing the increment in conductive passes within magnetic elastomers. Similar strain dependence of the electric conductivity, i.e., the conductivity was almost independent off the strain, was observed at all volume fractions of magnetic particles. In the present study, the strain was kept at 0.01 for all measurements. Therefore, the compression-dependent conductivity, which is called the piezo-resistance effect, can be omitted. Figure 2a shows the electric-voltage dependence of the electric conductivity for un-cross-linked magnetic elastomers with ϕ = 0.02 at various magnetic fields. At low voltages, the electric conductivity was almost independent of the magnetic field. At high voltages, the electric conductivity significantly increased with the magnetic field, indicating the dielectric breakdown at the bound layer of magnetic particles and/or the discontinuous part between chains. Figure 2b demonstrates the electric-voltage dependence of the electric conductivity for cross-linked magnetic elastomers with ϕ = 0.02 at various magnetic fields. The electric conductivity at high voltages was not raised, although high magnetic fields were applied. Thus, the electric conductivities at 0 mT for cross-linked and uncross-linked magnetic elastomers were independent of the electric voltage. This strongly indicates that there is no effect of electric fields on the morphology of magnetic particles; that is, magnetic particles do not move in the cross-linked and uncross-linked magnetic elastomers by the electric fields. The electric conductivity at 0 mT, 30 V for un-cross-linked and cross-linked magnetic elastomers was 1.14 ± 0.06 nS/cm and 345 ± 0.1 pS/cm, respectively. The electric conductivity decreased by 1 / 3 due to the cross-linking (cure) of polypropylene glycol and prepolymer. It may be that the mobility of conductive impurities, e.g., catalyst, was reduced by an obstacle effect due to the cross-linked structure. The electric conductivity for un-cross-linked and cross-linked polyurethane B


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Figure 2. Electric conductivity for (a) un-cross-linked and (b) cross-linked magnetic elastomers at various magnetic fields as a function of voltage. The volume fraction of CI particles is 0.02.

Figure 3. Electric conductivity for (a) un-cross-linked and (b) cross-linked magnetic elastomers at various magnetic fields as a function of voltage. The volume fraction of CI particles is 0.27.

elastomers without magnetic particles was 1.56 ± 0.01 nS/cm and 225 ± 2 pS/cm, respectively, which were insensitive to the voltage studied here. The electric conductivity for un-crosslinked magnetic elastomers decreased with the volume fraction of magnetic particles; on the other hand, the electric conductivity for cross-linked magnetic elastomers was independent of the volume fraction of magnetic particles. A possible reason for this conflicting behavior for both volume and crosslinking effects is mentioned in Figure 6. Figure 3a depicts the relationship between electric conductivity and voltage for un-cross-linked magnetic elastomers with ϕ = 0.27. At low voltages, the electric conductivity was almost constant, although high magnetic fields were applied. At high voltages, the electric conductivity significantly increased with the magnetic field, indicating the dielectric breakdown at the bound layer of magnetic particles and/or the discontinuous part between chains. Figure 3b shows the relationship between electric conductivity and voltage for cross-linked magnetic elastomers with ϕ = 0.27. The electric conductivity at 0 mT was independent of the voltage similar to that for cross-linked elastomers with ϕ = 0.02. However, it increased clearly at approximately 300 V even at 50 mT, which is an indication of the dielectric-breakdown behavior. Surprisingly, the electric conductivity increased with the magnetic field even at low voltages. The dielectric-breakdown

behavior was observed at high voltages as well as those for uncross-linked elastomers with volume fractions of 0.02 and 0.27. Magnetic particles are locked in the cross-linked elastomers; however, it can be considered that they can rotate about their mean position under the magnetic fields in order to get a preferred arrangement. At 30 V, the electric conductivity at 0 mT for un-cross-linked and cross-linked magnetic elastomers was 702 ± 23 and 346 ± 24 pS/cm, respectively. Similar to the cross-linked magnetic elastomers with ϕ = 0.02, the electric conductivity decreased by 1/2 due to the cross-linking, suggesting an obstacle effect by the cross-linking, as mentioned in Figure 2b. The left figures in Figure 4 indicate the relationship between electric conductivity and magnetic field for un-cross-linked magnetic elastomers. The electric conductivity at 30 V was constant, although the magnetic field was increased. It can be considered that the electric conductivity occurs through the polyurethane of the matrix, not the chain of magnetic particles. The electric conductivity at 1000 V increased with the magnetic field. The electric conductance is considered to occur via chains of magnetic particles, not through the polyurethane of the matrix. The right figures in Figure 4 show the relationship between electric conductivity and magnetic field for cross-linked magnetic elastomers. In Figure 4b, both electric conductivities C


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Figure 4. Electric conductivity for (left) un-cross-linked and (right) cross-linked magnetic elastomers at voltages of 30 and 1000 V as a function of magnetic fields. The volume fraction of CI particles is (a, b) 0.02, (c, d) 0.06, (e, f) 0.14, and (g, h) 0.27.

at 30 and 1000 V were constant, although the magnetic field was increased, suggesting that the electric current passes through the polyurethane of the matrix. Even at 1000 V, the electric conductivity was not raised, indicating that magnetic particles are not adjacent to each other even at 500 mT as to be taken place the dielectric breakdown. In Figure 4c, the electric conductivity at 30 V was independent of the magnetic field; however, the conductivity at 1000 V gradually increased with the magnetic field. Parts f and h of Figure 4 show the relationship between electric conductivity and magnetic field for cross-linked magnetic elastomers with ϕ = 0.14 and ϕ = 0.27, respectively. Both electric conductivities at 30 and 1000 V apparently increased with the magnetic field. This indicates that the electric current at voltages of 30 and 1000 V flows via chain structures of magnetic particles; however, the dielectric breakdown did not occur at 30 V. Thus, the electric conductivity at 30 V for un-cross-linked magnetic elastomers was constant even at high magnetic fields. For un-cross-linked magnetic elastomers, i.e., magnetic fluids, magnetic particles form a chain structure in the presence of magnetic fields. The field-insensitive conductivity indicates that the electric voltage with 30 V is not sufficient for hopping the potential gap at the discontinuous part between chains of magnetic particles. Additionally, it is found from the data at

1000 V, 500 mT that the electric conductivity for un-crosslinked magnetic elastomers with ϕ = 0.02 was comparable to that with ϕ = 0.27. This means that the electric current due to the dielectric breakdown passes through a certain chain with lowest resistance. Figure 5a demonstrates the magnetic response on the storage modulus for un-cross-linked magnetic elastomers with volume fractions of 0.02 and 0.27. The absolute change in the storage modulus ΔG′ for un-cross-linked magnetic elastomers with ϕ = 0.02 was (2.86 ± 0.30) × 103 Pa. ΔG′ was calculated by subtracting the off-field modulus G0′ at 0 mT from the on-field modulus G500 ′ at 500 mT. The relative change in the storage modulus (=G′500/G′0) was calculated to be ∼160. For un-crosslinked magnetic elastomers with ϕ = 0.27, the value of ΔG′ was ′ /G0′ of (4.26 ± 0.11) × 106 Pa, which corresponds to G500 ∼9900. Thus, these un-cross-linked magnetic elastomers exhibited a significant magnetorheological effect by the magnetic field, indicating that a chain structure was formed at 500 mT in the un-cross-linked magnetic elastomers. Figure 5b exhibits the magnetic response on the storage modulus for cross-linked magnetic elastomers with volume fractions of 0.02 and 0.27. The value of ΔG′ for cross-linked magnetic elastomers with ϕ = 0.02 was (2.21 ± 0.01) × 103 Pa, ′ /G0′ of ∼1.25. The value of ΔG′ for which corresponds to G500 D


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The Journal of Physical Chemistry B σun ‐ cross = ϕσm + (1 − ϕ)σPU

(1)

where σm and σPU represent the electric conductivity for magnetic particles and polyurethane, respectively. The solid line in the figure exhibits the electric conductivity obtained from eq 1. It was found that the experimental values of the electric conductivity for un-cross-linked elastomers can be well explained by the above equation. The value of σm was determined to be 1.0 × 10−11 S/cm, which is negligibly small compared to σPU. Here, the first term on the right side in eq 1 can be omitted, and we obtained the following equation σun ‐ cross ≈ (1 − ϕ)σPU

(2)

This means that the decrease in σun‑cross is due to the decrease in conductive carriers in the polyurethane matrix. On the other hand, the electric conductivity for cross-linked magnetic elastomers slightly increased with the volume fraction of magnetic particles. Also, for cross-linked magnetic elastomers, the conductive carriers should decrease with the volume fraction of magnetic particles. This contradictory phenomenon seen in the cross-linked elastomers is considered to originate from a structure having high electric conductivity, for example, random dispersion or heterogeneous dispersion of magnetic particles in the matrix, as schematically illustrated in the inset of Figure 6. Figure 7 shows the absolute change in the electric conductivity for un-cross-linked and cross-linked magnetic

Figure 5. Storage modulus for (a) un-cross-linked and (b) cross-linked magnetic elastomers with volume fractions of 0.02 and 0.27.

cross-linked magnetic elastomers with ϕ = 0.27 was (3.09 ± 0.29) × 106 Pa, which corresponds to G′500/G′0 of ∼1230. Thus, a clear magnetorheological effect was not seen in the crosslinked magnetic elastomer with ϕ = 0.02; however, the crosslinked magnetic elastomer with ϕ = 0.27 demonstrated a significant magnetorheological effect by a magnetic field of 500 mT. This means that a chain structure was not formed at 500 mT only for cross-linked magnetic elastomer with ϕ = 0.02. Figure 6 shows the electric conductivity at 0 mT, 30 V for un-cross-linked and cross-linked magnetic elastomers as a

Figure 6. (left) Electric conductivity for un-cross-linked and crosslinked magnetic elastomers at voltage of 30 V at 0 mT as a function of the volume fraction of magnetic particles. (right) Schematic illustrations representing the dispersibility of magnetic particles in cross-linked magnetic elastomers.

Figure 7. Change in the electric conductivity at 30 V for un-crosslinked and cross-linked magnetic elastomers by a magnetic field of 500 mT as a function of the volume fraction of magnetic particles.

elastomers at 30 V as a function of the volume fraction of magnetic particles. The absolute change in electric conductivity Δσ is obtained from the following formula

function of the volume fraction of magnetic particles. At ϕ = 0, the electric conductivity for cross-linked elastomers was lower than that for un-cross-linked elastomers probably due to the viscosity increase of the liquid in the cross-linked network, as mentioned in Figure 3b. The electric conductivity for un-crosslinked magnetic elastomers decreased with the volume fraction of magnetic particles; that is, it decreased with the volume fraction of polyurethane. It can be considered that the number of conductive carriers in the polyurethane decreased with the volume fraction of magnetic particles. The electric conductivity for un-cross-linked magnetic elastomers was fitted by the following equation showing the parallel connection of electric conductivities

Δσ = σ500mT − σ0mT

(3)

σ0mT and σ500mT are the electric conductivities at 0 and 500 mT, respectively. Δσ for un-cross-linked magnetic elastomers was independent of the volume fraction of magnetic particles, indicating that the electric current flows only in the polyurethane matrix. On the other hand, Δσ for cross-linked magnetic elastomers increased remarkably with the volume fraction of magnetic particles, suggesting that the electric conductance takes place via chain structures of magnetic particles without the dielectric breakdown at the bound layer of E


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dependence of the electric conductivity for a magnetic elastomer. We defined the critical voltage and the criticalelectric conductivity as the intersection point of two lines by which the electric conductivities at low and high voltages were fitted. No critical voltage was observed for cross-linked magnetic elastomers with ϕ = 0.02. Besides this, all un-crosslinked and cross-linked magnetic elastomers exhibited a tendency that the critical voltage decreased with the magnetic field. It can be considered that the distance of the discontinuous part between chains became short at high magnetic fields. An anomalous behavior was seen in the critical voltage that the uncross-linked magnetic elastomers demonstrated high critical voltages compared to those for cross-linked ones. The critical voltage also revealed that the cross-linked elastomers have a structure with conductive passes consisting of magnetic particles. The other anomalous behavior seen in the critical voltage is that un-cross-linked magnetic elastomers with ϕ = 0.27 exhibited a high critical voltage compared to that for uncross-linked magnetic elastomers with ϕ = 0.02. It is quite natural to consider that the dielectric-breakdown voltage decreased with increasing volume fraction of magnetic particles, because the distance of the discontinuous part between chains becomes shorter with increasing volume fraction of magnetic particles. Magnetic particles in un-cross-linked magnetic elastomers with ϕ = 0.02 might move easily in the un-crosslinked matrix. Figure 9b exhibits the magnetic-field dependence of the critical electric conductivity for cross-linked and un-cross-linked magnetic elastomers with various volume fractions of magnetic particles. The critical electric conductivity for un-cross-linked magnetic elastomers was insensitive to the magnetic field. On the other hand, the critical electric conductivity for cross-linked magnetic elastomers increased with the magnetic field. This phenomenon may be caused by the fact that the chain structure is formed effectively for cross-linked magnetic elastomers. Figure 10 shows the critical voltage for cross-linked and uncross-linked magnetic elastomers at various magnetic fields as a function of the electric conductivity at 30 V. The critical voltage was inversely proportional to the electric conductivity at 30 V. Shall we summarize the dielectric-breakdown behavior for magnetic elastomers with and without cross-linking? Figure 11a illustrates the schematic illustrations representing the electric conductance at low voltages within cross-linked and un-cross-

magnetic particles and/or the discontinuous part between chains. Figure 8 shows the relationship between the electric conductivity at 30 V and the molar ratio of isocyanate to the

Figure 8. Electric conductivity at 30 V for un-cross-linked and crosslinked magnetic elastomers at various magnetic fields as a function of a molar ratio of [NCO]/[OH]. The volume fraction of CI particles is 0.27.

hydroxy group [NCO]/[OH] for cross-linked magnetic elastomers with ϕ = 0.27. At magnetic fields below 200 mT, the electric conductivity for cross-linked magnetic elastomers was lower than that for the un-cross-linked one, which is probably due to the viscosity increase by cross-linking, as described in Figure 6. At magnetic fields above 300 mT, the electric conductivity increased with the molar ratio of [NCO]. At 500 mT, the conductivity significantly increased with the molar ratio of [NCO], suggesting that the passes with high electric conductivity appeared in the cross-linked magnetic elastomers. Otherwise, the thermal fluctuation of magnetic particles might be depressed by the cross-linking, resulting in an increase in the frequency of electron hopping. Figure 9a depicts the magnetic-field dependence of the critical voltage for cross-linked and un-cross-linked magnetic elastomers with various volume fractions of magnetic particles. The inset of Figure 9a shows a typical example of the voltage

Figure 9. (a) Critical voltage and (b) critical conductivity for un-cross-linked and cross-linked magnetic elastomers with various volume fractions of magnetic particles as a function of magnetic fields. Inset: Definition of the critical voltage and conductivity in this study. F


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able to change their locations and form a chain-like structure by magnetic fields, although the particles are fixed in the crosslinked matrix. The movement of magnetic particles occurs without any destruction of polyurethane matrix; i.e., the deformation takes place within the elastic limit of the matrix. Actually, the storage or loss modulus completely recovered to the original modulus when magnetic fields were cut off. In fact, there is a strong interaction between magnetic particles and polyurethane matrix. Relaxation NMR revealed that the relaxation time of protons for poly(propyrene grycol) (PPG) was determined to be 60.2 ms, and the relaxation time of protons for PPG containing CI particles with ϕ = 3.2 × 10−4 was determined to be 49.9 ms. Accordingly, the specific relaxation rate R2sp of protons for the liquid of PPG/CI was calculated to be 0.21. This indicates that the mobility of the PPG chain was reduced by the adhesion of PPG chains onto CI particles probably due to the strong interaction between them. Figure 11b shows the schematic illustrations representing the electric conductance at high voltages within cross-linked and un-cross-linked magnetic elastomers. The electric current for un-cross-linked elastomers flows via chains of magnetic particles with the dielectric breakdown occurring at the bound layer of magnetic particles and/or the discontinuous part between chains. The electric conductance for cross-linked elastomers with ϕ = 0.02 occurs within the polyurethane matrix. On the other hand, the electric conductance for crosslinked elastomers at volume fractions above 0.06 takes place via chains of magnetic particles, showing the dielectric-breakdown behavior.

Figure 10. Critical voltage for un-cross-linked and cross-linked magnetic elastomers at various magnetic fields as a function of the electric conductivity at 30 V.

4. CONCLUSIONS The electric conductivity for cross-linked and un-cross-linked magnetic elastomers in the presence of high electric fields was investigated at various magnetic fields and electric voltages. Three types of conduction mechanisms were observed in the electric conductivity for the magnetic elastomers depending on the applied voltages or the volume fraction of magnetic particles, i.e., electric conduction through polyurethane matrix, electric conduction via chains of magnetic particles without the dielectric breakdown, and electric conduction via chains of magnetic particles with the dielectric breakdown at the bound layer of magnetic particles and/or the discontinuous part between chains. An interesting phenomenon was observed at low electric voltages that the electric conductivity increased with the magnetic field only for cross-linked elastomers. The electric conductivity in the presence of magnetic fields increased with the ratio of cross-linker, suggesting that the cross-linking contributes high electric conductivity. It was also found at high electric voltages that the critical voltage for crosslinked magnetic elastomers was lower than that for un-crosslinked ones. The evidence strongly suggests that the conductive passes of magnetic particles are efficiently formed by magnetic fields in the cross-linked elastomers, probably due to the dispersibility of magnetic particles. The results obtained here would help to elucidate the structure of magnetic particles in a cross-linked matrix in the presence of magnetic fields.

Figure 11. Schematic illustrations representing the chain structure and electric conductivity for un-cross-linked and cross-linked magnetic elastomers at (a) low and (b) high voltages.

linked magnetic elastomers. The electric current for un-crosslinked elastomers passes within the polyurethane matrix independently of the volume fraction of magnetic particles or the strength of magnetic fields, probably due to the conductivity of impurities such as catalyst. The electric conductance for cross-linked elastomers at volume fractions below 0.06 occurs within the polyurethane matrix. On the other hand, at volume fractions above 0.14, the electric conductance takes place via chains of magnetic particles without the dielectric breakdown. As we reported previously,7 magnetic particles in polysaccharide gels having a storage modulus of ∼10 kPa align in the direction of magnetic fields. Due to the movement of magnetic particles, there arises a local deformation of the network; i.e., one side of the network around magnetic particles is compressed and the other side is elongated. Pang et al. reported that the magnetic plastomers having an elastic modulus of MPa order demonstrated dramatic changes in the electric resistance by bridging the noncontact particle chains by graphite powders.20 An et al. reported that swollen physical gels with a storage modulus of ∼10 kPa order exhibit storage modulus enhancement due to “chaining” and “clustering” processes in the gel.26 Thus, magnetic particles are

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AUTHOR INFORMATION

Corresponding Author

*Phone/Fax: +81 (0)25 262 6884. E-mail: [email protected]. ac.jp. G


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The Journal of Physical Chemistry B ORCID

(17) Yu, M.; Ju, B. X.; Fu, J.; Liu, S. Z.; Choi, S. B. Magnetoresistance Characteristics of Magnetorheological Gel under a Magnetic Field. Ind. Eng. Chem. Res. 2014, 53, 4704−4710. (18) Kim, S.; Byun, J.; Choi, S.; Kim, D.; Kim, T.; Chung, S.; Hong, Y. Negatively Strain-Dependent Electrical Resistance of Magnetically Arranged Nickel Composites: Application to Highly Stretchable Electrodes and Stretchable Lighting Devices. Adv. Mater. 2014, 26, 3094−3099. (19) Bica, I.; Anitas, E. M.; Bunoiu, M.; Vatzulik, B.; Juganaru, I. Hybrid magnetorheological elastomer: Influence of magnetic field and compression pressure on its electrical conductivity. J. Ind. Eng. Chem. 2014, 20, 3994. (20) Pang, H.; Xuan, S.; Liu, T.; Gong, X. Magnetic field dependent electro-conductivity of the graphite doped magnetorheological plastomers. Soft Matter 2015, 11, 6893. (21) Bica, I. Influence of the magnetic field on the electric conductivity of magnetorheological elastomers. J. Ind. Eng. Chem. 2010, 16, 359−363. (22) Xu, Y.; Gong, X.; Liu, T.; Xuan, S. Magneto-induced microstructure characterization of magnetorheological plastomers using impedance spectroscopy. Soft Matter 2013, 9, 7701−7709. (23) Ausanio, G.; Iannotti, V.; Ricciardi, E.; Lanotte, L.; Lanotte, L. Magneto-piezoresistance in Magnetorheological elastomers for magnetic induction gradient or position sensors. Sens. Actuators, A 2014, 205, 235−239. (24) Mietta, J. L.; Tamborenea, P. I.; Negri, R. M. Anisotropic magnetoresistivity in structured elastomer composites: modelling and experiments. Soft Matter 2016, 12, 6430−6441. (25) Cattoz, B.; de Vos, W. M.; Cosgrove, T.; Crossman, M.; Prescott, S. W. Manipulating Interfacial Polymer Structures through Mixed Surfactant Adsorption and Complexation. Langmuir 2012, 28, 6282−6290. (26) An, H.; Sun, B.; Picken, S. J.; Mendes, E. Long Time Response of Soft Magnetorheological Gels. J. Phys. Chem. B 2012, 116, 4702.

Tetsu Mitsumata: 0000-0002-1355-6775 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are very grateful for Ms. J. Ikeda of Nihon Rufuto Co. Ltd. for relaxation NMR measurements. This research was partially supported by Sasaki Environment Technology Foundation, NAGAI N-S Promotions Foundation for Science of Perception, and UNION TOOL foundation.

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REFERENCES

(1) Xu, Y. G.; Gong, X. L.; Xuan, S. H. Soft Magnetorheological Polymer Gels with Controllable Rheological Properties. Smart Mater. Struct. 2013, 22, 075029. (2) Gong, X. L.; Xu, Y. G.; Xuan, S. H.; Guo, C. Y.; Zong, L. H.; Jiang, W. Q. The Investigation on The Nonlinearity of Plasticine-Like Magnetorheological Material under Oscillatory Shear Rheometry. J. Rheol. 2012, 56, 1375−1391. (3) Xu, Y. G.; Gong, X. L.; Xuan, S. H.; Li, X. F.; Qin, L. J.; Jiang, W. Q. Creep and Recovery Behaviors of Magnetorheological Plastomer and Its Magnetic-Dependent Properties. Soft Matter 2012, 8, 8483− 8492. (4) Sun, T. L.; Gong, X. L.; Jiang, W. Q.; Li, J. F.; Xu, Z. B.; Li, W. H. Study on The Damping Properties of Magnetorheological Elastomers Based on Cis-Polybutadiene Rubber. Polym. Test. 2008, 27, 520−526. (5) Chertovich, A. V.; Stepanov, G. V.; Kramarenko, E.; Yu; Khokhlov, A. R. New Composite Elastomers with Giant Magnetic Response. Macromol. Mater. Eng. 2010, 295, 336−341. (6) Mitsumata, T.; Abe, N. Magnetic-Field Snsitive Gels with Wide Modulation of Dynamic Modulus. Chem. Lett. 2009, 38, 922−923. (7) Mitsumata, T.; Honda, A.; Kanazawa, H.; Kawai, M. Magnetically Tunable Elasticity for Hydrogels Consisting of Carrageenan and Carbonyl Iron Particles. J. Phys. Chem. B 2012, 116, 12341−12348. (8) Mitsumata, T.; Ohori, S.; Honda, A.; Kawai, M. Magnetism and Viscoelasticity of Magnetic Elastomers with Wide Range Modulation of Dynamic Modulus. Soft Matter 2013, 9, 904−912. (9) Chiba, N.; Yamamoto, K.; Hojo, T.; Kawai, M.; Mitsumata, T. Wide Range Modulation of Dynamic Modulus and Loss Tangent for Magnetic Elastomers Containg Submilimeter Magnetic Particles. Chem. Lett. 2013, 42, 253−254. (10) Mitsumata, T.; Ohori, S. Magnetic Polyurethane Elastomers with Wide Range Modulation of Elasticity. Polym. Chem. 2011, 2, 1063−1067. (11) Ohori, S.; Fujisawa, K.; Kawai, M.; Mitsumata, T. Magnetoelastic Behavior of Bimodal Magnetic Hydrogels Using Nonmagnetic Particles. Chem. Lett. 2013, 42, 50−51. (12) Mitsumata, T.; Ohori, S.; Chiba, N.; Kawai, M. Enhancement of magnetoelastic behavior of bimodal magnetic elastomers by stress transfer via nonmagnetic particles. Soft Matter 2013, 9, 10108−10116. (13) Nagashima, K.; Kanauchi, S.; Kawai, M.; Mitsumata, T.; Tamesue, S.; Yamauchi, T. Nonmagnetic Particles Enhance Magnetoelastic Response of Magnetic Elastomers. J. Appl. Phys. 2015, 118, 024903. (14) Nagashima, K.; Kawai, M.; Mitsumata, T. Amphoteric Response of Loss Factor for Bimodal Magnetic Elastomers by Magnetic Fields. Chem. Lett. 2016, 45, 1033−1034. (15) Kchit, N.; Bossis, G. Electrical resistivity mechanism in magnetorheological elastomer. J. Phys. D: Appl. Phys. 2009, 42, 105505. (16) Martin, J. E.; Anderson, R. A.; Odinek, J.; Adolf, D.; Williamson, J. Controlling percolation in field-structured particle composites: Observations of giant thermoresistance, piezoresistance, and chemiresistance. Phys. Rev. B: Condens. Matter Mater. Phys. 2003, 67, 094207. H


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