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sensors Article Enhancement of MCF Rubber Utilizing Electric and Magnetic Fields, and Clarification of Electrolytic Polymerization Kunio Shimada Faculty of Symbiotic Systems Sciences, Fukushima University, 1 Kanayagawa, Fukushima 960-1296, Japan; [email protected]; Tel.: +81-24-548-5214 Academic Editors: Vamsy P. Chodavarapu and Andreas Hütten Received: 10 February 2017; Accepted: 31 March 2017; Published: 4 April 2017 Abstract: Many sensors require mechanical durability to resist immense or impulsive pressure and large elasticity, so that they can be installed in or assimilated into the outer layer of artificial skin on robots. Given these demanding requirements, we adopted natural rubber (NR-latex) and developed a new method (NM) for curing NR-latex by the application of a magnetic field under electrolytic polymerization. The aim of the present work is to clarify the new manufacturing process for NR-latex embedded with magnetic compound fluid (MCF) as a conductive filler, and the contribution of the optimization of the new process for sensor. We first clarify the effect of the magnetic field on the enhancement of the NR-latex MCF rubber created by the alignment of magnetic clusters of MCF. Next, SEM, XRD, Raman spectroscopy, and XPS are used for morphological and microscopic observation of the electrolytically polymerized MCF rubber, and a chemical approach measuring pH and ORP of the MCF rubber liquid was used to investigate the process of electrolytic polymerization with a physical mode. We elucidate why the MCF rubber produced by the NM is enhanced with high sensitivity and long-term stability. This process of producing MCF rubber by the NM is closely related to the development of a highly sensitive sensor. Keywords: sensor; electrolytic polymerization; magnetic field; magnetic cluster; natural rubber; magnetic compound fluid (MCF); magnetic fluid; isoprene; filler; electrical resistivity; oxidation-reduction reaction 1. Introduction Sensors are currently used in many fields to detect force, temperature, pressure, etc. However, in robotics, sensors must have the mechanical durability to withstand immense or impulsive pressure, as well as large elasticity or extensibility. Certain sensors should be suitable for installation as an outer layer on robots, or for assimilation into an artificial skin. These characteristics are not, of course, limited to the sensors in robotics; sensors with mechanical durability are also required in other engineering fields. Regarding the concept of a skin sensor, there has been only a few general investigations [1]. The paradigm is that the sensor should be based on the effective control or management of human-machine systems. One new natural-contact biosensor has been designed as an interface between humans and machines for nonintrusive and real-time sensing. Although this sensor utilizes microelectromechanical systems (MEMS) or nanoelectromechanical systems (NEMS) to enable miniature sensors, flexibility remains a fundamental problem. Therefore, as described above, there is a need for a suitable rubber material with the durability to withstand impulsive, immense or repeated force while maintaining large elasticity and extensibility [2]. Researchers have examined many materials in the search for a sensor made of rubber: silicon oil rubber [3–8], chloroprene [9], styrene-butadiene [10], acrylonitrile butadiene [11], nitrile butadiene Sensors 2017, 17, 767; doi:10.3390/s17040767 www.mdpi.com/journal/sensors Sensors 2017, 17, 767 2 of 18 rubber [12], and natural rubber (NR-latex) [13–16]. Shimada proposed a new sensor made of silicon-oil rubber that uses magnetic compound fluid (MCF) as filler (called MCF rubber) [17]. MCF was devised in 2001 as a colloidal fluid containing Fe3 O4 sphere particles on the order of 10 nm, coated by oleic acid and other metal particles such as Ni, Fe, and Cu, on the order of 1 µm. These particles are dispersed in a solvent such as water, kerosene, or silicone oil to compound a magnetic fluid (MF), which is an ordinary intelligent fluid responsive to the magnetic field with Fe3 O4 particles. When a magnetic field is applied, the Fe3 O4 particles play a bonding role among the metal particles, causing numerous metal and Fe3 O4 particles to aggregate [18]. Due to these magnetic clusters, MCF is useful for many engineering applications [19]. The mechanism of electric conductivity of silicon-oil type MCF rubber is explained mainly by the percolation theory [20] and tunnel theory [21]. Percolation threshold is significant factor in the former. Therefore, in the case of a small mass concentration of MCF rubber, it is possible that electric conductivity is demonstrated by this theory. On the other hand, for a large aggregation of particles with large aspect ratio, the latter theory can better explain the electric conductivity of MCF rubber. Electrons jump between metal particles such as Fe3 O4 and Ni through the barrier of non-conductive materials such as silicon-oil rubber and oleic acid coated around Fe3 O4 . Silicon-oil type MCF rubber has a large electrical resistivity, however, which does not fulfill the needs of a skin sensor. In order to obtain sensitivity at a shearing motion, such as touching a body and scrubbing, a skin sensor requires high sensitivity, as shown by a previous investigation. When two electrodes are set between skin rubber and supplied with voltage, the electric current flowing inside the rubber can be measured. The electrical conductivity changed by the force applied on the skin rubber is related to the degree of sensitivity. In order to achieve the requirement of high sensitivity, we can enhance the sensitivity of MCF rubber using NR-latex. NR-latex type MCF rubber is made by solidification under a constant temperature (the drying method (DM)) and the application of a magnetic field. The DM has also been used to produce ordinary, commercial, pressure-sensitive electrically conductive rubber (PSECR). The application of the magnetic field causes the Fe3 O4 and metal particles to aggregate, and magnetic clusters are created in NR-latex type as well as silicon-oil type MCF rubber. In general, water involved in NR-latex plays an important role in electric conductivity, which is based on ionic conduction. Therefore, owing to the filler and the water role, electric conductivity is superior to that of PSECR. Furthermore, as shown in a previous investigation, NR-latex type MCF rubber has been evaluated the following technical cases as a physical value: surface roughness of a hard plate surface with coarse or smooth roughness in the range of Ra = 0.19–20.86 µm, which is related to finishing in precision machinery industry; softness of a soft surface of texture of clothes, paper, gel, etc.; geometric shape of concave and convex surface less than 5.7 mm in height, which is related to the evaluation of a biological surface such as a fish, insect, etc. NR-latex type MCF rubber made by the DM, however, has a problem in that deterioration occurs in a short period of about a week. In addition, it has electrical resistivity on the order of 1 Ω·m [22], which does not provide high enough sensitivity to fulfill the requirements for a skin sensor. Shimada has proposed a new method (NM), which involves solidifying rubber using both magnetic and electric fields. The rubber liquid is poured between electrodes, and a magnetic field is applied. NR-latex without MCF is created as a thin film, and its thickness does not increase even with an extended application of the electric field. With NR-latex MCF rubber, however, since the magnetic field is applied along the same direction as the electric field line, the thickness increases substantially due to the magnetic clusters. When the electric field or magnetic field is used alone, the electrical property is less sensitive than with both, and the mechanical property is less extensible. These characteristics have been shown in the case of a narrow electrode gap of 1 mm in previous studies [22]. It was also clarified that NR-latex MCF rubber made with NM maintains a smaller electrical resistivity on the order of 10−2 − 10−3 Ω·m from its creation to a period of more than a year. The NM is a noteworthy and effective production method for avoiding deterioration due to aging. Sensors 2017, 17, 767 3 of 18 However, how enhancement of MCF rubber thickness is affected by various strengths of the electric and magnetic fields has not yet been clarified. While the effect of the magnetic field on electrolytic polymerization is significant, parameters including the magnetic field distribution and magnitude have not yet been elucidated. In particular, the phenomenon of how MCF rubber is enhanced by both fields at a large electrode gap remains unknown. Investigating the enhancement of MCF rubber at a large electrode gap is related to clarifying the configuration of the enhancement. Therefore, in the present paper, we first investigate the effect of the magnetic field on the metamorphosis of MCF rubber by electrolytic polymerization. On the other hand, magnetic clusters in the filler play a very significant role in the solidification of MCF rubber with the NM. Controlling the applied magnetic field allows the magnetic clusters to grow freely in the filler. In terms of whether the filler is magnetic or conductive, there have been many studies on the use of polymers such as polymethyl methacrylate (PMMA). Carbon nanotubes is another well-known filler [23]. The properties of a sensor can be controlled by the alignment of the carbon nanotube with a magnetic field. The idea and concept of embedding fillers of carbon nanotubes in a polymer is similar to that of the MCF rubber. Only the material of the solvent and substrate is different. There has been research that looked at carbon nanotube in NR-latex [24–28]. Most of these studies, however, did not consider the alignment of the filler with the aid of a magnetic field. As for NR-latex with magnetic particles other than carbon nanotube, there have also been some studies [29–33]. A few dealt with the alignment of the filler with the help of a magnetic field. However, investigation of filler alignment with the help of a magnetic field under electrolytic polymerization such as the NM has not been clarified. Therefore, the present paper contributes to the optimization of a new manufacturing process for an NR-latex embedded with MCF as a conductive filler by adding electric and magnetic fields. From mentioned above, the mechanisms behind the enhancement, long-term stability and high sensitivity of electric conductivity have not been elucidated, nor the process of electrolytic polymerization of NR-latex type MCF rubber been clarified. In our previous study [22], we could only guess at the physical model. Clarification of this is related to the development of high-sensitivity sensors, or skin sensors if you will. Therefore, the second aim of the present study is to investigate the physical model, and the processes of enhancement of electrolytically polymerized MCF rubber by morphological and microscopic examination, including observations by SEM, XRD, Raman spectroscopy and XPS, as well as by chemical analysis including measurement of pH and ORP of the MCF rubber liquid. 2. Effect of Magnetic Field on Metamorphosis of MCF Rubber It is clear from the results of previous studies [22] that the thickness of MCF rubber solidified by electric and magnetic fields depends on the magnetic field. However, the effect of magnetic field distribution and magnitude on the enhancement of solidified MCF rubber is not clear. In our previous study [22], we applied permanent magnets to the inside of plates to form the outer surface of each plate. The liquid in the MCF rubber consisted of 12-g Ni powder, with particles on the order of µm and pimples on the surface (No. 123, Yamaishi Co., Ltd., Noda, Japan), 3-g water-based MF with 50 wt % Fe3 O4 (M-300, Sigma Hi-Chemical Co., Ltd., Tsutsujigasaki, Japan), 16-g NR-latex (Rejitex Co., Ltd., Atsugi, Japan), and 31-g water, poured into a container as shown in Figure 1. The mass concentration of MCF rubber liquid was 21.7 wt % of Ni and Fe3 O4 particles. Electrodes were immersed in the liquid. Permanent rectangular magnets, 10 mm × 15 mm in size, and 5-mm thick, were applied from outside the container. An electric field held constant at 6 V voltage and 2.7 A electric current was supplied between the plates for 30 min. The plates were held apart at a constant thickness with a spacer to create an electrodes gap of 19 mm, and a 40 mm × 40 mm square made of stainless steel. In the case of NR-latex only, and the application of an electric field without a magnetic field, the NR-latex created an anode plate as a thin film, as shown in Figure 2a. In contrast, compounding MCF into the NR-latex with an electric but not magnetic field increased the thickness of the MCF rubber to 2 mm at the electrode Sensors 2017, 17, 767 4 of 18 surface, double the NR-latex alone, as shown in Figure 2b. The enhancement of the MCF solidified rubber at the lower side was due to the sedimentation of the MCF at the bottom of the container. This enhancement did not depend on the magnetic field, as it was not applied. This phenomenon is investigated later in the subsequent figure from these figures. Sensors 2017, 17, 767 Sensors 2017, 17, 767 4 of 18 4 of 18 Figure 1. Schematic used to investigate the effect of magnetic field Figure Schematic diagram diagram of of experimental experimental apparatus apparatus Figure 1. 1. Schematic diagram of experimental apparatus used used to to investigate investigate the the effect effect of of magnetic magnetic field field distribution and and magnitude magnitude on on the the solidification solidification of of MCF MCF rubber rubber liquid liquid by by the the NM. NM. distribution distribution and magnitude on the solidification of MCF rubber liquid by the NM. (a) (a) (b) (b) (c) (c) (d) (d) Figure 2. Photographs of electrolytically polymerized rubber: (a) NR-latex without application of a Figure 2. 2. Photographs of polymerized rubber: (a) without application ofofa Figure Photographs of electrolytically electrolytically polymerized (a)NR-latex NR-latex without application permanent magnet; (b) MCF rubber without application rubber: of a permanent magnet; (c) MCF rubber with permanent magnet; (b) MCF rubber without application of a permanent magnet; (c) MCF rubber with aapplication permanentofmagnet; (b) MCF rubber application a permanent (c) MCF rubber a permanent magnet; (d)without image from viewingoftransversely tomagnet; the electrode of (c). NRapplication of a permanent magnet; (d) image from viewing transversely to the electrode of (c). with of a permanent magnet; (d) image from viewing transversely to the polymerization electrode ofNR(c). latex application or MCF rubber solidified and attached to an anode electrode after electrolytic latex or MCF rubber solidified and attached to anananode electrolytic polymerization NR-latex MCF rubber solidified attached anodeelectrode electrodeafter after under theorconditions in which the and electrodes aretoimmersed transversely inelectrolytic a containerpolymerization (see Figure 1). under the the conditions conditions in in which which the the electrodes electrodes are are immersed immersedtransversely transverselyin inaacontainer container(see (seeFigure Figure1). 1). under Meanwhile, by compounding MCF into the NR-latex and applying both an electric and magnetic Meanwhile, by compounding MCF into the NR-latex and applying both an electric and magnetic field Meanwhile, (Magnet 2 in 1), the thickness of the the NR-latex MCF rubber grew significantly along and the magnetic byTable compounding MCF into and applying both an electric field (Magnet 2 in Table 1), the thickness of the MCF rubber grew significantly along the magnetic line, as 2shown in 1), Figure 2c. NR-latex MCFgrew generally forms along as a thin film, and its field (Magnet in Table the thickness of thewithout MCF rubber significantly the magnetic field field line, as shown in Figure 2c. NR-latex without MCF generally forms as a thin film, and its thickness doesinnot increase even with a longer of theas electric field. Therefore, the line, as shown Figure 2c. NR-latex without MCFapplication generally forms a thin film, and its thickness thickness does not increase even with a longer application of the electric field. Therefore, the application of the magnetic field is necessary to create MCF magnetic clusters. Since the magnetic application of the magnetic field is necessary to create MCF magnetic clusters. Since the magnetic field is applied along the same direction as the electric field line, as may be seen in the figure, the field is applied along the same direction as the electric field line, as may be seen in the figure, the thickness of the MCF rubber grows substantially. The NM is related to so-called electric thickness of the MCF rubber grows substantially. The NM is related to so-called electric polymerization. The curing of NR-latex is promoted by the alignment of magnetic clusters in the filler polymerization. The curing of NR-latex is promoted by the alignment of magnetic clusters in the filler due to the application of a magnetic field under electrolytic polymerization. Sensors 2017, 17, 767 5 of 18 does not increase even with a longer application of the electric field. Therefore, the application of the magnetic field is necessary to create MCF magnetic clusters. Since the magnetic field is applied along the same direction as the electric field line, as may be seen in the figure, the thickness of the MCF rubber grows substantially. The NM is related to so-called electric polymerization. The curing of NR-latex is promoted by the alignment of magnetic clusters in the filler due to the application of a magnetic field under electrolytic polymerization. Sensors 2017, 17, 767 Sensors 2017, 17, 767 5 of 18 5 of 18 Table 1. Magnetic field strength of Magnets 1–4. Table 1. Magnetic strength Magnets 1–4. 3 Magnet 1field Magnetof Magnet Table 1. Magnetic field strength of2 Magnets 1–4. Location Magnet 1 34.5 Magnet 248.3 Magnet at b or c in Figure 1 (mT) 80.1 3 Location Magnet 1 Magnet 2 Magnet 3 at b or c inat Figure 1 (mT)1 (mT) 34.5 48.3 28.1 80.1 a in Figure 22.0 43.4 at b or c in Figure 1 (mT) 34.5 48.3 80.1 at a in Figure 1 (mT) 22.0 28.1 43.4 at a in Figure 1 (mT) 22.0 28.1 43.4 Location Magnet 4 107Magnet 4 Magnet 4 55.3 107 107 55.3 55.3 Magnet 2 used to create the NR-latex shown in Figure 2c had a magnetic field strength of 188 mT, Magnet 2 used to create the NR-latex shown in Figure 2c had a magnetic field strength of 2 used to create theone NR-latex shown 107.5 in Figure had a magnetic strength of based Magnet on previous studies [22]; the at the surface mT in2cthe case of a singlefield magnet. The size 188 mT, based on previous studies [22]; the one at the surface 107.5 mT in the case of a single magnet. based on previous [22];2c thecorresponded one at the surface in the case magnet. of188 themT, solidified MCF rubberstudies in Figure d = 8107.5 mm mT (diameter), e =of13a single mm (diameter), The size of the solidified MCF rubber in Figure 2c corresponded d = 8 mm (diameter), e = 13 mm The solidified MCF rubber in Figure 2c corresponded d = field, 8 mmMCF (diameter), = 13 mm f= 1.5size mm,ofgthe = 13 mm in Figure 3. Before application of the electric rubbere liquid was (diameter), f = 1.5 mm, g = 13 mm in Figure 3. Before application of the electric field, MCF rubber (diameter), f = 1.5 mm, g = 13 mm in Figure 3. Before application of the electric field, MCF rubber attracted to the electrodes attached to Magnet 2 on the outside of the container, as shown in Figure 4. liquid was attracted to the electrodes attached to Magnet 2 on the outside of the container, as shown liquid attracted tothe the magnetic electrodesfield attached to Magnet on the outside of the container, as shown Ifin both awas magnet with strength 1302 mT in used previous studiesstudies [22] with Figure 4. If both a magnet with the magnetic field of strength of used 130 mT in previous [22] a in Figure 4. If both a magnet with the magnetic field strength of 130 mT used in previous studies [22] single magnetmagnet of 84.7ofmT (Magnet 1) were used, the same solidified MCF rubber would with asurface single surface 84.7 mT (Magnet 1) were used, the same solidified MCF rubber wouldbe with a single surface magnet of 84.7 mT (Magnet 1) were used, the same solidified MCF rubber would created. However, if magnetic field strengths ofof312 3),or or490 490mT mTwere were be created. However, if magnetic field strengths 312mT mTand and177.7 177.7 mT mT (Magnet (Magnet 3), be created. However, if amagnetic field strengths of mT 312 (Magnet mT and 177.7 mT (Magnet 3),rubber or 490 would mT were used, [22] together with surface magnet of 191.3 4), solidified MCF not used, [22] together with a surface magnet of 191.3 mT (Magnet 4), solidified MCF rubber would not used, [22] together with a surface magnet of 191.3 mT (Magnet 4), solidified MCF rubber would not form. 1–4 are are listed listedin inTable Table1.1.We Wefound foundthat thatMCF MCFrubber rubber form.The Themagnetic magnetic field field strength strength of of Magnets Magnets 1–4 form. not Thesolidify magnetic field too strength of Magnets 1–4field are listed in Table 1. We found thatmagnetic MCF rubber would when large of a magnetic strength was applied. When field would not solidify when too large of a magnetic field strength was applied. When magnetic field would not solidify when too50large of a magnetic field strength wasHowever, applied. this When magnetic fieldof strength was less than about mT, MCF rubber liquid solidified. was in the case strength was less than about 50 mT, MCF rubber liquid solidified. However, this was in the case of strength was less than about 50 mT, MCF rubber liquid solidified. However, this was in the case of 21.7 MCF rubber rubberliquid liquidwas wasconsidered consideredtotodepend dependonon 21.7wt wt%%particles. particles.As Asaaresult, result, the the solidification solidification of MCF 21.7 wt % particles. As a result, the solidification of MCF rubber liquid was considered to depend on magnetic field strength. magnetic field strength. magnetic field strength. Figure3. 3. Schematic diagram of MCF rubber solidified using The for position for Figure Schematic diagram of MCF rubber liquidliquid solidified using the NM.the TheNM. position measuring Figure 3. Schematic diagram of MCF rubber liquid solidified using the NM. The position for measuring magnetic field distributions shown in Figure 5 is also delineated. magnetic field distributions shown in Figure 5 is also delineated. measuring magnetic field distributions shown in Figure 5 is also delineated. Figure 4. Photograph of MCF rubber liquid attracted to the electrodes before the application of electric Figure 4. Photograph of MCF rubber liquid attracted to the electrodes before the application of electric Figure field. 4. Photograph of MCF rubber liquid attracted to the electrodes before the application of field. field. electric In the case of Magnet 2, the magnetic field distribution at position <a> and <b> in Figure 3 was In the case of Magnet 2, the magnetic field distribution at position <a> and <b> in Figure 3 was measured; the results are shown in Figure 5. The yellow bands in Figure 5 correspond to the positions measured; the results are shown in Figure 5. The yellow bands in Figure 5 correspond to the positions at which the solidified MCF rubber increased as in Figure 2c. These results suggest that the MCF at which the solidified MCF rubber increased as in Figure 2c. These results suggest that the MCF rubber grows at the position between the maximum magnetic field strength and the maximum rubber grows at the position between the maximum magnetic field strength and the maximum magnetic field gradient. It is considered that this is due to the aggregation of the Ni and Fe3O4 magnetic field gradient. It is considered that this is due to the aggregation of the Ni and Fe3O4 Sensors 2017, 17, 767 6 of 18 In the case of Magnet 2, the magnetic field distribution at position <a> and <b> in Figure 3 was measured; the results are shown in Figure 5. The yellow bands in Figure 5 correspond to the positions at which the solidified MCF rubber increased as in Figure 2c. These results suggest that the MCF rubber grows at the position between the maximum magnetic field strength and the maximum magnetic field gradient. It is considered that this is due to the aggregation of the Ni and Fe3 O4 particles by the force of the magnetic field strength and field gradient. The permanent magnet used had a circular maximum magnetic field and field gradient. Therefore, the solidified MCF rubber was cylindrical in shape (Figure 3).17, 767 Sensors 2017, 6 of 18 (a) (b) Figure 5. Magnetic field distribution of Magnet 2 corresponding to the location labeled <a> and <b> Figurein 5.Figure Magnetic field distribution of Magnet 2 (a,b) corresponding to the location labeled <a> and 3. <b> in Figure 3. On the other hand, the enhancement of the solidified MCF rubber can be perceived at the lower side of the solidified MCF rubber shownofinthe Figure 2c, as well as rubber in the case application of lower On the other hand, the enhancement solidified MCF canwithout be perceived at the magnetic field shown in Figure 2b, due to the sedimentation of the particles in the MCF rubber. side of the solidified MCF rubber shown in Figure 2c, as well as in the case without application Therefore, large mass concentration is another factor that enhances the solidification of MCF rubber. of magnetic field6a shown Figure 2b,solidified due to the sedimentation of was the attracted particlesoninthe theanode MCFofrubber. Figure shows in MCF rubber by the NM; this rubber Therefore, large concentration another factor enhances the solidification of MCF of rubber. Magnet 2 inmass an MCF rubber liquidiswith 50 wt % largerthat mass concentrations of particles, consisting Figure shows3-g MCF rubber and solidified by theInNM; this particles rubber was onliquid the anode of 12-g Ni6apowder, MF (M-300) 12-g NR-latex. contrast, in theattracted MCF rubber in 2 were wt %.liquid The solidified Figure 6 was larger than that in Figure 2c, MagnetFigure 2 in an MCF21.7 rubber with 50 MCF wt %rubber largerinmass concentrations of particles, consisting of reaching from anode to cathode with g = 19 mm. Thus, the higher the mass concentration, the larger 12-g Ni powder, 3-g MF (M-300) and 12-g NR-latex. In contrast, particles in the MCF rubber liquid the solidified MCF rubber. in Figure 2 were 21.7 wt %. The solidified MCF rubber in Figure 6 was larger than that in Figure 2c, reaching from anode to cathode with g = 19 mm. Thus, the higher the mass concentration, the larger the solidified MCF rubber. Sensors 2017, 17, 767 7 of 18 Sensors 2017, 17, 767 Sensors 2017, 17, 767 7 of 18 7 of 18 (a) (a) (b) (b) (c) (c) Figure 6. Photographs of solidified MCF rubber with large mass concentration: (a,b) attracted on the Figure of Figure 6. 6. Photographs Photographs of solidified solidified MCF MCF rubber rubber with with large largemass massconcentration: concentration: (a,b) (a,b) attracted attracted on on the the anode; (c) between the electrodes. anode; (c) between the electrodes. anode; (c) between the electrodes. The solidified MCF rubber in Figure 6a,b, which shows the rubber between the electrodes, has The ininFigure 6a,b, which shows thethe rubber between theis electrodes, has the The solidified solidified MCFrubber rubber Figure which shows rubber between the electrodes, has the same wire-likeMCF shape as that shown in6a,b, Figure 2c. Where the magnetic field applied, needlesame wire-like shape as that in Meanwhile, Figure 2c. Where theoutside magnetic is field applied, needle-shaped the same wire-like shape asshown that shown in Figure Where the magnetic isfield, applied, shaped magnetic clusters are created. in2c.areas of field the magnetic suchneedleas the magnetic clusters are created. Meanwhile, in areas outside of the magnetic field, such as the lower shaped magnetic clusters are created. Meanwhile, in areas outside of the magnetic field, such asside the lower side of the solidified MCF rubber shown in Figure 1b, no needle-shaped magnetic clusters of the solidified MCF rubber shown in Figure 1b, no needle-shaped magnetic clusters form, and the lowerand sidethe of particles the solidified MCFinto rubber shown inNote Figure 1b,if no clusters form, aggregate a dense state. that the needle-shaped electrode gap ismagnetic small, like the 1particles aggregate into a dense state. Note that if the electrode gap is small, like the 1-mm gap used in form, and the particles aggregate into a dense state. Note that if the electrode gap is small, like 1mm gap used in previous studies [22], the solidified MCF rubber reaches from anode to cathodethe and previous studies [22], the solidified MCF rubber reaches from anode to cathode and becomes a solid, mm gap used in previous studies [22], the solidified MCF rubber reaches from anode to cathode and becomes a solid, 1-mm thick plate. In that case, the needle-like magnetic clusters are confined between 1-mm thick plate. In as that case, theour needle-like are confined the electrode becomes a solid, 1-mm thick plate. In that case, magnetic the needle-like magnetic clustersbetween are confined between the electrode plates, shown in previous study [22].clusters plates, as shown in our previous study [22]. the electrode plates, as shown in our previous study [22]. When the MCF rubber liquid does not include MF, the phenomena of the electrolytically When MCF liquid not MF, of electrolytically When the the MCF rubber rubber liquid does does not include include MF, the phenomena phenomena of the theliquid electrolytically polymerized configuration is different, as shown in Figure 7, the where the MCF rubber consisted polymerized configuration is different, as shown in Figure 7, where the MCF rubber liquid consisted polymerized configuration is different, as shown in Figure 7, where the MCF rubber liquid consisted of 12-g Ni powder, 16-g NR-latex and 34-g water. Magnet 2 was used here. The black area is Ni and of 12-g Ni powder, 16-g NR-latex and 34-g water. Magnet 2 was used here. The black area is of 12-g Ni powder, 16-g NR-latex and 34-g water. Magnet 2 was used here. The black area is Ni Ni and and the white or pale yellow is NR-latex. Even if the Ni particles were aggregated by the application of the white or pale yellow is NR-latex. Even if the Ni particles were aggregated by the application of the white or pale yellow is NR-latex. Even if the Ni particles were aggregated by the application of the magnetic field, the solidified MCF rubber would not grow. This suggests that MF is the important the field, rubber not grow. the magnetic magnetic field, the the solidified solidified MCF rubber would would notrubber. grow.This Thissuggests suggeststhat thatMF MF is is the the important important factor in the enhancement of theMCF solidification of MCF factor factor in in the the enhancement enhancement of of the thesolidification solidificationof ofMCF MCFrubber. rubber. Figure 7. Photograph of solidified MCF rubber without MF. Figure Figure 7. 7. Photograph Photograph of of solidified solidified MCF MCF rubber rubber without without MF. MF. Thus, we have demonstrated that MCF rubber liquid with 21.7 wt % can be solidified at less than wemagnetic have demonstrated demonstrated thatand MCF rubberliquid liquid with 21.7wt wt% % can canthe be solidified solidified atmagnetic less than than Thus, we have that MCF rubber 21.7 be at less aboutThus, 50-mT field strength enhanced at thewith position between maximum about 50-mT magnetic field strength and enhanced at the position between the maximum magnetic about 50-mT magnetic field strength and enhanced at the between the maximum field strength and maximum magnetic field gradient. Theposition rubber grows substantially withmagnetic a higher field strength strength and maximum maximum magnetic fieldrubber gradient. TheThe rubber grows substantially with ais a higher higher field and magnetic field gradient. The rubber grows substantially with mass concentration of particles in the MCF liquid. MF in the MCF rubber liquid a very mass concentration of particles in the MCF rubber liquid. The MF in the MCF rubber liquid is a very important factor in the enhancement of solidified MCF rubber with a wire-like shape. important factor in the enhancement of solidified MCF rubber with a wire-like shape. Sensors 2017, 17, 767 8 of 18 mass concentration of particles in the MCF rubber liquid. The MF in the MCF rubber liquid is a very important factor in the enhancement of solidified MCF rubber with a wire-like shape. 3. Mechanisms and Process of Electrolytically Polymerized MCF Rubber In this section, we first describe the electrolytic polymerization model of MCF rubber, because all the following investigations are based on this model. Figure 8 shows the physical model of the solidification of MCF rubber by electrolytic polymerization proposed in our previous study [22]. Using Sensors 2017, 17, 767 8 of 18 this model can help to clarify the mechanisms behind the MCF rubber enhancement, long-term stability 3. Mechanisms andthe Process of Electrolytically Polymerizedfield, MCF Rubber and high sensitivity. With application of the magnetic the magnetic clusters align along the In this section, first describe theaelectrolytic polymerization MCFelectrodes, rubber, because lines of the magnetic field,we which runs in transverse direction model to theoftwo as indicated the following investigations are based on this model. Figure 8 shows the physical model of the by E in the all figure. The C=C are arrayed in the same direction as the magnetic clusters. As a result, solidification of MCF rubber by electrolytic polymerization proposed in our previous study [22]. solidification of this themodel MCFcan rubber guessed to expand in the direction of the applied magnetic field. Using help tois clarify the mechanisms behind the MCF rubber enhancement, long-term When a magnetic is not applied, the bonds undergo solidification, and the direction stability field and high sensitivity. With theC=C application of simply the magnetic field, the magnetic clusters align along the lines is of random. the magneticAs field, which runs a transverseof direction to the two electrodes, as of the polymerization a result, theinthickness the solidified MCF rubber and the indicated by E in the figure. The C=C are arrayed in the same direction as the magnetic clusters. As a solidification effect are very small. At the cathode, the surface of the MCF rubber becomes rugged, result, solidification of the MCF rubber is guessed to expand in the direction of the applied magnetic as shown infield. [22], duea to the tipfield of the magnetic clusters. Cationic polymerization When magnetic is notgrowing applied, the C=C bonds simply undergo solidification, and the occurs at direction of the polymerization is random. As a result, the thickness of the solidified MCF rubber and the anode, and anionic polymerization at the cathode (A and F in Figure 8). With the application of the solidification effect are very small. At the cathode, the surface of the MCF rubber becomes rugged, electricity, each polyisoprene of a particle in the NR-latex is guessed to be mutually polymerized to as shown in [22], due to the tip of the growing magnetic clusters. Cationic polymerization occurs at bind between the ends of C=C bonds in aatcoordinated way, many C=C be of formed in a the anode, and anionic polymerization the cathode (A and F inand Figure 8). With the bonds application long chain, electricity, as indicated by B in Figure 8. At the time, the C=C bonds arepolymerized vulcanized each polyisoprene of a particle in thesame NR-latex is guessed to be mutually to by radical bind between the ends C=C in a 8. coordinated way, and many C=C bonds be formed in aC=C long bonds are polymerization, as shown byofG inbonds Figure If the NR-latex includes sulfur S, the chain, as indicated by B in Figure 8. At the same time, the C=C bonds are vulcanized by radical vulcanized by radical polymerization, as shown by C in Figure 8. In addition, from the results of polymerization, as shown by G in Figure 8. If the NR-latex includes sulfur S, the C=C bonds are the last Section 2, where we polymerization, showed that as enhancement of the8. solidified needle-shaped MCF rubber vulcanized by radical shown by C in Figure In addition, from the results of the where we rubber showed that enhancement the solidified MCF rubber occurred depends onlast theSection MF in2, the MCF liquid, it can beofguessed thatneedle-shaped radical polymerization depends on the MF in the MCF rubber liquid, can beacid guessed that radical as a result of the carboxyl group COOH of theitoleic coating Fe3 polymerization O4 , bondingoccurred C=C, and Fe3 O4 , as a result of the carboxyl group COOH of the oleic acid coating Fe3O4, bonding C=C, and Fe3O4, as as shown by D in Figure 8. shown by D in Figure 8. Figure 8. Physical model of electrolytic polymerization of the MCF rubber by the NM [22]. Note that Figure 8. Physical model of electrolytic polymerization of the MCF rubber by the NM [22]. Note that G G in the figure was added in the present study. in the figure was added in the present study. As shown in our previous study [22], the magnetic clusters that align along the magnetic field form needle-like structures. Figure 9a is an SEM image of part of a magnified magnetic cluster As shown in our previous study [22], the magnetic clusters that align along the magnetic field involved in the electrolytically polymerized solid MCF rubber of the image shown in our previous form needle-like structures. is an SEM image of a(M-300) magnified magnetic study [22]. The MCFFigure rubber 9a liquid consisted of 12-g of Ni,part 3-g MF and 12-g NR-latex.cluster MCF involved in the electrolytically polymerized solid MCF rubber of the image shown in our previous study [22]. The MCF rubber liquid consisted of 12-g Ni, 3-g MF (M-300) and 12-g NR-latex. MCF rubber was Sensors 2017, 17, 767 9 of 18 Sensors 2017, 17, 767 9 of 18 rubber was electrolytically polymerized by a 30-min application of 6 V and 2.7 A, a 1-mm space between electrodes, and Magnet 2. The molecules of the NR-latex coiled around the magnetic clusters. Sensors 2017, 17, 767 9 of 18 electrolytically polymerized by aby30-min 6 V and 2.7 A, a9a. 1-mm between electrodes, Figure 9b shows the mapping X-ray application diffraction of (XRD) of Figure The space electrolytic polymerized and Magnet 2. The molecules of the NR-latex coiled around the magnetic clusters. Figure 9b shows the rubber was electrolytically polymerized by a 30-min application of 6NR-latex, V and 2.7 A, 1-mm space MCF rubber presents a complicated configuration of intertwined Fea3O 4 and Ni, indicating between and(XRD) Magnet 2.not The molecules of theelectrolytic NR-latex coiled around the magnetic mapping X-rayelectrodes, diffraction Figure The polymerized MCF clusters. rubber presents a that theseby ingredients of NR-latex doof exist9a. independently. 9b shows theof mapping by X-rayNR-latex, diffraction (XRD) Figure 9a.indicating The electrolytic complicatedFigure configuration intertwined Fe3 O4ofand Ni, thatpolymerized these ingredients of MCF rubber presents a complicated configuration of intertwined NR-latex, Fe3O4 and Ni, indicating NR-latex dothat not exist independently. these ingredients of NR-latex do not exist independently. (a) (a) (b) (b) Figure 9. (a) SEM photograph (5000× magnification of 5000) of microspores of magnetic clusters Figure 9. (a)inside SEMsolidified photograph (5000(b) × XRD magnification of 5000) of microspores of magnetic clusters inside MCF rubber; mapping of (a). Figure 9. (a) SEM photograph (5000× magnification of 5000) of microspores of magnetic clusters solidified MCF rubber; (b) XRD mapping of (a). inside solidified MCF rubber; (b) XRD mapping of (a). Figure 10a,b present SEM and XRD images of the anode side surface, corresponding to A in Figure 8, and Figure 10c,d are those of the cathode side, corresponding to F in Figure 8. The MCF Figure 10a,b present and XRD images the anode surface, corresponding to is thatSEM shown in Figure 10bof shows on theside anode-side surface, the quantity Figurerubber 10a,bsample present SEM and XRD9. Figure images of thethat anode side surface, corresponding to A A in in of Ni and Fe 310c,d O 4 particles involved in the NR-latex is larger than that of C. In contrast, Figure 10d Figure 8, and Figure are those of the cathode side, corresponding to F in Figure 8. The MCF Figure 8, and Figure 10c,d are those of the cathode side, corresponding to F in Figure 8. The MCF shows that onshown the cathode-side surface, the quantity of C is larger thanthe thatanode-side of Ni and Fe3O 4 particles. rubber is Figure 10b shows shows that on on surface, the quantity quantity rubber sample sample is that that shown in in Figure Figure 9. 9. Figure 10b that the anode-side surface, the The electrolytic polymerization of NR-latex is thus thought to be different at the anode and cathode of Ni and Fe O particles involved in the NR-latex is larger than that of C. In contrast, Figure 33Oof of Ni and Fe 44 particles involved NR-latex is larger than that of C. In contrast, Figure 10d 10d sides the MCF rubber. This is in thethe typical distinction. shows that on the cathode-side surface, the quantity of particles. shows that on the cathode-side surface, the quantity of C C is is larger larger than than that that of of Ni Ni and and Fe Fe33O O44 particles. The The electrolytic electrolytic polymerization polymerization of of NR-latex NR-latex is is thus thus thought thought to to be be different different at at the the anode anode and and cathode cathode sides of the MCF rubber. This is the typical distinction. sides of the MCF rubber. This is the typical distinction. (a) (b) (a) (b) (c) (d) Figure 10. Photographs of (a) anode-side and (c) cathode-side surfaces of solidified MCF rubber by SEM (2000× magnification); (b) XRD mapping on (a); (d) XRD mapping on (c). (c) (d) Figure 10. Photographs of (a) anode-side and (c) cathode-side surfaces of solidified MCF rubber by Figure 10. Photographs of (a) anode-side and (c) cathode-side surfaces of solidified MCF rubber by SEM (2000× magnification); (b) XRD mapping on (a); (d) XRD mapping on (c). SEM (2000× magnification); (b) XRD mapping on (a); (d) XRD mapping on (c). Sensors 2017, 17, 767 10 of 18 SensorsWe 2017,next 17, 767investigate 10 of 18 the MCF rubber by chemical analysis. Regarding the electrolytic polymerization of NR-latex, there have been only a few invaluable reports [34,35]. In contrast, there Sensors 2017, 17, 767 10 of 18 haveWe been investigations of NR-latex as analysis. a composite without chemical polymerization nextseveral investigate the MCF rubber by chemical Regarding the electrolytic polymerization [13–16]. The former [34,35] useful their description of contrast, theRegarding results of the FT-IR spectra, etc. We there next have investigate the rubberofby chemical analysis. electrolytic of NR-latex, beenare only aMCF fewbecause invaluable reports [34,35]. In there have been several However, the structure of as thea electrolytic polymerization ofpolymerization NR-latex has not been clarified. To[34,35] select polymerization of NR-latex, there have been onlychemical a few invaluable reports [34,35]. InThe contrast, there investigations of NR-latex composite without [13–16]. former have beenanalytical several investigations of NR-latex as atocomposite without chemical polymerization the optimum instrument, it is important consider that MCF rubber involves magnetic are useful because of their description of the results of FT-IR spectra, etc. However, the structure of [13–16]. The former are useful because of theirby description of the results ofelectric FT-IR spectra, etc.thus material, 0.1-mm thick,[34,35] and created instantaneously application of anthe field.analytical We the electrolytic polymerization of NR-latex has not beenthe clarified. To select optimum However, the structure of the electrolytic polymerization of NR-latex has not been clarified. To select selected Raman spectroscopy and XPSthat (X-ray spectroscopy) as effective measures for instrument, it is important to consider MCFphotoelectron rubber involves magnetic material, 0.1-mm thick, and the optimum analytical instrument, it is important to consider that MCF rubber involves magnetic MCF rubber. created instantaneously byand thecreated application of an electric field. We thus of selected Raman material, 0.1-mm thick, instantaneously by the application an electric field.spectroscopy We thus Raman spectroscopy spectra were taken from the anode-side surface of the MCF rubber and selected XPS (X-ray photoelectron asphotoelectron effective measures for MCF Raman spectroscopyspectroscopy) and XPS (X-ray spectroscopy) asrubber. effective measures for electrolytically polymerizedspectra under were the conditions described in the last Section MCFrubber used Raman spectroscopy taken from the anode-side surface of 2; thethe MCF MCF rubber. consisted of 12-g Ni, 3-g MF (M-300) and 12-g NR-latex, and Magnet 2. Figure 11 compares MCF electrolytically under the described in the surface last Section the MCF Raman polymerized spectroscopy spectra wereconditions taken from the anode-side of the2;MCF rubberused rubber liquid before the application of electric and magnetic fields with that of MCF rubber of the electrolytically polymerized under the described the last2.Section thecompares MCF used consisted of 12-g Ni, 3-g MF (M-300) andconditions 12-g NR-latex, andinMagnet Figure2;11 MCF anode-side surface solidified by the DM with a magnetic field. It is well known that NR-latex contains consisted 12-g Ni, MF (M-300) 12-gand NR-latex, and fields Magnetwith 2. Figure 11MCF compares MCF rubber liquidofbefore the3-g application of and electric magnetic that of rubber of the molecules polyisoprene, asbyshown in Figure 12. characteristic peaks at 1663 and 1710 rubber of liquid before the application ofwith electric andThe magnetic with thatare of that MCF rubber ofcontains thecm−1 anode-side surface solidified the DM a magnetic field. fields It is well known NR-latex −1 (C=C stretching for carbon ofaisoprene); 2852, and 2960 2 stretching anode-side surface solidified by the bonds DM with magnetic field. It is2915 wellpeaks known that NR-latex molecules of polyisoprene, asdouble shown in Figure 12. The characteristic are atcm 1663(CH andcontains 1710 cm−1 −1 (C-H molecules of polyisoprene, as shown in Figure 12. The characteristic peaks are at 1663 and 1710 cm−1 at vibration of C-CH 2CH3 of isoprene); 3036 cm stretching vibration of isoprene); in addition, − 1 (C=C stretching for carbon double bonds of isoprene); 2852, 2915 and 2960 cm (CH2 stretching −1 (CH −1 −2 (C=C stretching for carbon double bonds of isoprene); 2852, 2915 and 2960 cm 2 stretching 1000 cm of (C-C of joint between each isoprene). −1 Comparison of the peak at 3036 cm of C-H, which vibration C-CH 2 CH3 of isoprene); 3036 cm −1 (C-H stretching vibration of isoprene); in addition, at −1 of C=C vibration of C-CHto2CH 3 of isoprene); polymerization, 3036 cm (C-H stretching vibration isoprene); in at of does not contribute the electrolytic with the at of 1663 andcm 1710 cm − 1 −2 addition, 1000 cm (C-C of joint between each isoprene). Comparison ofpeaks the peak at 3036 of C-H, which −1 (C-C of joint between each isoprene). Comparison of the peak at 3036 cm−2 of C-H, which 1000 cm electrolytically polymerized MCF rubber, and at 1000with cm−1the of peaks C-C are and cm presented in −1 of C=C doesdoes not not contribute to to the electrolytic polymerization, at estimated 1663 and 1710 −1 of C=C contribute the electrolytic polymerization, with the peaks at 1663 and 1710 cm of Table 2. Results indicate the typical that thecm quantity of C=C and C-C bonds increase by −1 of electrolytically MCFcharacteristic rubber,and andatat 1000 C-C estimated presented electrolytically polymerized polymerized MCF rubber, 1000 cm−1 of of C-C areare estimated andand presented in in electrolytic polymerization. Table 2. Results indicate the typical characteristic that the quantity of C=C and C-C bonds increase by Table 2. Results indicate the typical characteristic that the quantity of C=C and C-C bonds increase by electrolytic polymerization. electrolytic polymerization. Figure 11. Raman spectroscopy spectraof ofthe the electrolytically electrolytically polymerized MCF rubber of anode-side Figure 11. Raman spectroscopy spectra polymerized MCF rubber of anode-side surface indicated as “Polymerized solid”: MCF rubber liquid before the application of electric and MCF rubber rubber liquid liquid before the application of electric and surface indicated as “Polymerized solid”: MCF magnetic fields as “Liquid”; MCF rubber of anode-side surface solidified by the DM with a magnetic magnetic fields as “Liquid”; MCF rubber of anode-side surface solidified by the DM with a magnetic field as “Dried solid”. field as “Dried solid”. Figure 12. Scheme of polyisoprene structure of NR-latex. Figure 12. Scheme of polyisoprene structure of NR-latex. Figure 12. Scheme of polyisoprene structure of NR-latex. Sensors 2017, 17, 767 11 of 18 Table 2. Comparison of spectra from Figure 11. Location Sensors 2017, 17, 767 at 1663 and 1710 cm−1 of C=C Electrolytically polymerized MCF rubber 55.88 MCF rubber liquid before Table 2. Comparison of spectra from Figure 11. 13.18 electrolytic polymerization Location at 1663 and 1710 cm−1 of C=C MCF rubber solidified by Drying 13.73 Electrolytically polymerized MCF rubber 55.88 Method with magnetic field MCF rubber liquid before electrolytic polymerization MCF rubber solidified by Drying Method with magnetic field 13.18 13.73 at 1000 cm−1 of C-C 11 of 18 12.94 3.21 at 1000 cm−1 of C-C 3.36 12.94 3.21 3.36 The electrolytically polymerized MCF rubber of the anode-side surface in Figure 11 was analyzed The 13 electrolytically theelectrolytically anode-side surface in Figure under 11 wasthe by XPS. Figure presents thepolymerized results. TheMCF MCFrubber rubberofwas polymerized analyzed by XPS. Figure 13 presents the results. The MCF rubber was electrolytically polymerized same experimental conditions as in Figure 11. Figure 13 shows a comparison with rubber solidified by under the same experimental conditions as in Figure 11. Figure 13 shows a comparison with rubber the DM. solidified by the DM. (a) (b) Figure 13. XPS spectra of MCF rubber of anode-side surface: (a) electrolytically polymerized MCF Figure 13. XPS spectra of MCF rubber of anode-side surface: (a) electrolytically polymerized MCF rubber; (b) MCF rubber solidified by the DM with a magnetic field. rubber; (b) MCF rubber solidified by the DM with a magnetic field. Comparing polarization of the spectra of C to those of C=C (green line in Figure 13) and C-C (blue and yellow lines), the of C-C blue in of Figure is increased that13) in Figure Comparing polarization of quantity the spectra of C to line those C=C13b (green line infrom Figure and C-C 13a.yellow Therefore, thethe quantity of C-C bonds increases electrolytic Figures (blue and lines), quantity of C-C blue line in by Figure 13b is polymerization. increased from From that in Figure1113a. Sensors 2017, 17, 767 12 of 18 Therefore, the quantity of C-C bonds increases by electrolytic polymerization. From Figures 11 and 13, Sensors 2017, 17, 767 12 of 18 it can be deduced that by electrolytic polymerization of the MCF rubber, some polymerization occurs. The quantity ofby C=C and C-Cpolymerization bonds is related to the electro-chemical reaction, which and 13,changes it can bein deduced that electrolytic of the MCF rubber, some polymerization − ] and [H+ ] by in turn is based on the oxidation-reduction reaction. This can be evaluated by [OH occurs. measuring values redox of potential and pH. Thethe changes in of quantity C=C and(ORP) C-C bonds is related to the electro-chemical reaction, which +] by is well known surface particles have negative charge in condition. inItturn is based onthat the NR-latex oxidation-reduction reaction. Thisa can be evaluated by their [OH-]initial and [H + measuring theinvalues of redox potential (ORP) and When the water NR-latex reacts as in Equation (1),pH. hydrogen ions H attract some anionic NR-latex It is well known that NR-latex surface particles have charge inmay theirbe initial condition. particles, neutralized particles are aggregated, and a parta negative of the NR-latex solidified, which + attract some anionic NR-latex When the water in NR-latex reacts as in Equation (1), hydrogen ions H does not mean vulcanization. This phenomenon is equivalent to solidification by drying in the air (i.e., neutralized particles are aggregated, and a part of the NR-latex may be solidified, which the particles, DM): does not mean vulcanization. This phenomenon H2 O → His+ equivalent + OH− to solidification by drying in the air (1) (i.e., the DM): Therefore, the quantity of [OH− ] is greater than that of [H+ ]. As a result, NR-latex is alkaline. (1)[H+ ], 2O → By H+electrolytic + OH− This is before the application of the electricHfield. polymerization, the [OH− ] and or the ratio of oxidation and reduction, is altered. Thus, by electric-chemical measurement of ORP Therefore, the quantity of [OH−] is greater than that of [H+]. As a result, NR-latex is alkaline. This andispH, we canapplication determineofthe of the polymerizationthe of[OH the−MCF rubber based +], or the before the the reaction electric field. By electrolytic electrolytic polymerization, ] and [H on the physical model shown in Figure 8. We therefore investigated the ORP and pH of the MCF ratio of oxidation and reduction, is altered. Thus, by electric-chemical measurement of ORP and pH, rubber under the application of electric magnetic fields. The liquid in MCF the MCF rubber consisted we can determine the reaction of theand electrolytic polymerization of the rubber based on the of 12 g-Ni powder, 3-gshown MF (W-40) and 8. 24-g was pouredthe into a container, shown Figure 1. physical model in Figure WeNR-latex; therefore itinvestigated ORP and pH ofasthe MCFin rubber Theunder mass the concentration of electric the MCF liquid was 33.8 wt % andrubber Fe3 O4consisted particles.ofORP application of andrubber magnetic fields. The liquid inof theNi MCF 12 g-and powder, 3-gsettled MF (W-40) and 24-g NR-latex; itimmersed was poured container, asliquid. shown in Figure21.was pH Ni meters were between the electrodes ininto the aMCF rubber Magnet The mass of the MCF rubber liquid was 33.8 wt constant % of Ni and 4 particles. ORPsupplied and applied fromconcentration outside the container. The electric field was held at 6FeV,3Oand 2.7 A was pH meters were settled between the electrodes immersed in the MCF rubber liquid. Magnet 2 was between the plates. The plates were held apart at a constant thickness using a spacer with a 19-mm applied gap, fromcreating outside the container. The electric field held constant at 6 V,14 and 2.7 A was supplied electrodes a 40 mm × 40 mm square ofwas stainless steel. Figure shows the results. between the plates. The plates were held apart at a constant thickness using a spacer with First, we consider the NR-latex liquid. As mentioned, in its initial condition, the pH aof19-mm NR-latex electrodes gap, creating a 40 mm × 40 mm square of stainless steel. Figure 14 shows the results. liquid is alkaline as shown in Figure 14b. This state is the same as that of the MCF rubber liquid (see First, we consider the NR-latex liquid. As mentioned, in its initial condition, the pH of NR-latex Figure 14d). At the anode, by the application of the electric field, OH− from Equation (1) changes by liquid is alkaline as shown in Figure 14b. This state is the same as that of the MCF rubber liquid (see radial reaction with indicating by as follows: − Figure 14d). At the anode, by the application of the electric field, OH from Equation (1) changes by radial reaction with indicating by as follows: − · − OH → OH + OH ・ OH− → OH + e− (2) (2) However, thethe Equation (2)(2) reaction of of does notnot occur at at thethe initial condition, However, Equation reaction does occur initial condition,leaving leavingthe theNR-latex NRliquid alkaline. latex liquid alkaline. (a) Figure 14. Cont. Sensors 2017, 17, 767 13 of 18 Sensors 2017, 17, 767 13 of 18 (b) (c) (d) Figure 14. Changes in ORP, pH, temperature, electric current and voltage over time during Figure 14. Changes in ORP, pH, electric current andMCF voltage over timeinduring electrolytic polymerization by temperature, the NM: NR-latex liquid in (a,b); rubber liquid (c,d). electrolytic polymerization by the NM: NR-latex liquid in (a,b); MCF rubber liquid in (c,d). At the initial condition, the quantity of reductant [red] is larger than that of oxidizer [ox]. Therefore, the condition, ORP of thethe NR-latex is negative, in the initial ORP in[ox]. Figure 14a. At the initial quantityliquid of reductant [red]asis shown larger than that of oxidizer Therefore, the ORP of the NR-latex liquid is negative, as shown in the initial ORP in Figure 14a. Equation (2) Sensors 2017, 17, 767 Sensors 2017, 17, 767 Sensors 2017, 17, 767 14 of 18 14 of 18 14 of 18 Equation (2)application occurs the of electric the [OH−] decreases, becoming + ]. Sensors 2017, 17,the 767 14 ofsmaller 18 occurs with theapplication electricoffield, andelectric [OH −]field, decreases, becoming smaller thansmaller [OH Equation (2) occurs withwith theof application the field, and and [OH−] decreases, becoming +]. Therefore, ORP changes from negative to positive, as shown by A in Figure 14a (hereafter, than [OH + Therefore, Therefore, from negative to positive, byasAshown in Figure 14a than [OH ORP changes from negativeas to shown positive, by A in(hereafter, Figure 14aReaction (hereafter, Sensors 2017,].ORP 17, 767changes 14 ofA). 18 Equation (2) occurs with the application of as theshown electricby field, and [OH−] decreases, becoming smaller Reaction A). The pH becomes acidic, A in Figure 14b. In other words, the oxidation The pH becomes as shown by Aasinshown Figure by 14b.AIn words, oxidation process occurs at Reaction A). The acidic, pH becomes acidic, inother Figure 14b. the In other words, the oxidation thanprocess [OH+]. Therefore, ORP changes from negative to as shown occurs at anode the anode by application the application ofpositive, the electric field. by A in Figure 14a (hereafter, the anode by the application of the electric field. process occurs at the by the of the electric field. Equation (2) occurs with the application of the electric field, and [OH−] decreases, becoming smaller + +, and −] becomes Reaction Next, A). The pH becomes acidic, as shown by A in to Figure 14b. [OH In other words, higher the oxidation anionic NR-latex particles are attracted −]] becomes Next, anionic NR-latex particles arenegative attracted to H and [OH− higher thanthan [H++].[H ]. ]. NR-latex particles H++ , H higher [H thanNext, [OH+].anionic Therefore, ORP changes from to positive, as shown bybecomes A in Figure 14a than (hereafter, process occurs at the anode by the application of the electric field. Therefore, the pH of NR-latex the NR-latex liquid changes to alkaline, as shown by in Figure 14b (hereafter, Therefore, theThe pHpH of the the NR-latex liquid changes toalkaline, alkaline, asshown shown by inBFigure Figure 14b (hereafter, Therefore, the pH of liquid to as by BB in 14b Reaction A). becomes acidic, aschanges shown by A in Figure 14b. −In other words, the(hereafter, oxidation +]. Next, anionic NR-latex particles are than attracted to H+,then and changes [OH ] becomes higher than [H Reaction B); the [red] becomes larger [ox]. ORP to negative, as shown by Reactionoccurs B); the the [red] becomes larger than [ox]. [ox]. ORP then changes changes to to negative, negative, as as shown shown by by BB inB in Reaction B); becomes larger than ORP then in process at[red] the anode by the application of the electric field. Therefore, the pH of the NR-latex liquid changes to alkaline, as shown by B in Figure 14b (hereafter, Figure 14a. Figure 14a. anionic NR-latex particles are attracted to H+, and [OH−] becomes higher than [H+]. Figure 14a. Next, Reaction Next, B); the becomes larger than liquid. [ox]. ORP then itchanges toNR-latex, negative,the as pH shown by B results in we[red] consider the MCF rubber Because involves and ORP Next, we we consider consider the MCF rubber liquid. Because involves NR-latex, theFigure pH and and ORP results Next, MCF rubber Because itit involves NR-latex, the pH ORP results Therefore, the pH of thethe NR-latex liquidliquid. changes to alkaline, as shown by B in 14b (hereafter, Figure 14a. described can be compared to those of MCF rubber liquid, as shown in Figure 14c,d. These are the describedB); canthe be[red] compared to those those ofthan MCF[ox]. rubber liquid, as shownto innegative, 14c,d. Theseby areBthe the described can be compared to of MCF rubber liquid, shown in Figure 14c,d. These are Reaction becomes larger ORP then as changes as shown in Next, we consider the MCF rubber liquid. Because it involves NR-latex, the pH and ORP results reactions atanode. the anode. Meanwhile, at the cathode, undergoes a radial reaction reactions at Meanwhile, at at thethe cathode, the oleic acidoleic undergoes a radial as follows: reactions at the the anode. Meanwhile, cathode, the the oleic acidacid undergoes a reaction radial reaction as as Figure 14a. described can be compared to those of MCF rubber liquid, as shown in Figure 14c,d. These are the follows: follows: Next, we consider the MCF rubber liquid. Because it involves NR-latex, the pH and ORP results reactions at the anode. Meanwhile, at the cathode, the oleic acid undergoes a radial reaction as described can be compared to those of MCF rubber liquid, as shown in Figure 14c,d. These are the follows: (3) reactions at the anode. Meanwhile, at the cathode, the oleic acid undergoes a radial reaction(3) as(3) follows: (3)MF is The possibility of this reaction has already been presented [36,37]. Figure 7 indicated that The possibility of this reaction has already been presented [36,37]. Figure 77 indicated that MF is The possibility of this reaction has already been presented [36,37]. Figure indicated that MF is on an important factor in the enhancement of the solidification of MCF rubber. Therefore, we focus (3) an important factor in the enhancement of the solidification of MCF rubber. Therefore, we focus on an important in of the solidification of MCF rubber. Therefore, we focus on the Therole possibility of the this reaction haspolymerization. already been presented [36,37]. Figure 7 indicated that MF is the offactor oleic acid inenhancement electrolytic the role of oleic acid in electrolytic polymerization. role of oleic acid in electrolytic polymerization. an important factor in(3)the enhancement of thethan solidification of MCF rubber. Therefore, we focus on In Equation [red] becomes larger [ox], the ORP of MCF rubber liquid becomes negative, In (3) than [ox], the ORP of rubber liquid becomes negative, The possibility of thisbecomes reactionlarger has+already been presented [36,37]. Figure 7 indicated that MF is In Equation Equation (3) [red] [red] becomes larger [ox], the ofMCF MCF rubber liquid becomes negative, −] becomes the role of oleic acid in electrolytic polymerization. and [OH higher than ]. than Therefore, theORP pHthe of the NR-latex liquid becomes alkaline. At −− +].[H and [OH ] becomes higher than [H Therefore, the pH of NR-latex liquid becomes alkaline. At + an important factor in the enhancement of the solidification of MCF rubber. Therefore, we focus on andthis [OH ] becomes higher than [H ]. simultaneously Therefore, theORP pH of the NR-latex liquid becomes alkaline. In Equation (3) [red] becomes larger than [ox], the of MCF rubber liquid becomes negative, time, Reactions A and B occur at the cathode. Since the data of Figure 14 are this time,ofReactions B occur simultaneously at the cathode. SinceSince the data of Figure 14 are the role oleic acid A in and electrolytic polymerization. At this Reactions A and B[H occur simultaneously at the the cathode. the data of Figure 14 −] becomes +]. Therefore, and [OHtime, higher than the pH of NR-latex liquid becomes alkaline. At between the electrodes, they are measured as a mix of the reactions at the anode and cathode. between the electrodes, they are measured as a mix of the reactions at the anode and cathode. In Equation (3) [red] becomes larger than [ox], the ORP of MCF rubber liquid becomes negative, are Therefore between the electrodes, arein measured asofaORP ofcathode. the the anode and cathode. this time,− Reactions A summarized and Bthey occur simultaneously atmix theand Sinceinat the data of Figure 14 are they are the results pHreactions shown Figure 14c,d, respectively. +]. Therefore, Therefore are summarized results of ORP ORP andof pH shown in Figure Figure 14c,d, respectively. and [OH ]they becomes higher thanin [Hthe the pH theshown NR-latex liquid14c,d, becomes alkaline. At Therefore they are summarized in the results of and pH in respectively. between From the electrodes, they arewe measured as a mix of the reactions at thepolymerization. anode and cathode. the above results, can construct the process of electrolytic As shown From the above results, we canconstruct constructthe theprocess process of electrolytic polymerization. As shown this time, Reactions A and Bwe occur simultaneously at the cathode. Since the data of Figure 14 are From the above results, can of electrolytic polymerization. As shown in Therefore they are summarized in the results of ORP and pH shown in Figure 14c,d, respectively. in previous the previous Section 2, enhancement of solidified the solidified MCF rubber with needleor wire-like shapes in the Section 2, enhancement of the MCF rubber with needleor wire-like shapes between the electrodes, they are measured as a mix of the reactions at the anode and cathode. the by previous 2, enhancement of the solidified MCF with needleor wire-like shapes by the From theSection above results, we can construct process of electrolytic polymerization. As shown electrolytic polymerization is due to effect the the effect of therubber magnetic It particularly depends on by electrolytic polymerization is due to the ofthe the magnetic field.field. particularly depends on the Therefore they are summarized in the results ofof ORP and pH shown inItItFigure 14c,d, depends respectively. electrolytic polymerization is due to the effect magnetic field. particularly on the in the previous Section enhancement of the solidified MCF rubber or wire-like shapes MF involved in MCF the2,MCF rubber liquid. The oleic acid coating Fe3with O4 ofneedleMF is thought to play a role in MF involved the rubber The oleic acid coating 3O4 of MF polymerization. is thought to play role in From thein above results, weliquid. can construct the process of Fe electrolytic Asaashown MFelectrolytic involved in the MCF rubber liquid. The oleic acid coating Fe O of MF is thought to play role in 3 4 by polymerization is due to the effect of the magnetic field. It particularly depends on the the solidification. In addition, as previously shown, C-C bonds increase, and changes in and pH and ORP the solidification. In addition, as previously shown, C-C bonds increase, and changes in pH ORP in previous Section 2, enhancement of the solidified MCF rubber with needleor wire-like shapes thethe solidification. InMCF addition, asthe previously shown, C-C bonds and changes pH and ORP MF involved in the rubber liquid. The field, oleic acid coating Feincrease, 3O4 of MF is thought toinplay a role in occur by the application of electric confirming electrolytic polymerization of MCF rubber. occur by application of confirming electrolytic polymerization MCF rubber. by electrolytic polymerization iselectric due to field, the effect of the magnetic field. It particularlyof depends on the occur by the the application of the the field, confirming electrolytic polymerization ofpH MCF rubber. the solidification. In addition, as electric previously shown, C-Celectrolytic bonds increase, and changesprocess in and ORP as Based on these results, we hypothesize that the polymerization proceeds Based on these results, we hypothesize that the electrolytic polymerization process proceeds as MF involved in the MCF rubber liquid. The oleic acid coating Fe 3 O 4 of MF is thought to play a role in Based on these results, we hypothesize that the electrolytic polymerization process proceeds as follows. occur by the First, application the electric field,ofconfirming electrolytic polymerization of MCFtorubber. follows. at anode, the of anode, the anion the NR-latex particle (Figure 12)thought is thought change as follows. First, at the the anion of the NR-latex particle (Figure 12) is to change as the solidification. In addition, as previously shown, C-C bonds increase, and changes in pH and ORP First, at anode, the anion of the NR-latex (Figure 12)polymerization is thought to change as proceeds follows: as Based onthe these results, we hypothesize thatparticle the electrolytic process follows: follows: occur by the application of the electric field, confirming electrolytic polymerization of MCF rubber. follows. First, at the anode, the anion of the NR-latex particle (Figure 12) is thought to change as Based on these results, we hypothesize that the electrolytic polymerization process proceeds as follows: follows. First, at the anode, the anion of the NR-latex particle (Figure 12) is thought to change(4) as(4) (4) follows: (4) From Equation (4) derive we derive Equation From Equation (4) we Equation (5). (5). From Equation (4) we derive Equation (5). From Equation (4) we derive Equation (5). (4) (5) (5) From Equation (4) we derive Equation (5). (5)(5) on possibility of reaction the reaction of Equation (5) has already presented The The possibility of the of Equation (5) has already beenbeen presented [36].[36]. The The firstfirst termterm on ・ the right in Equation is rewritten Brsimplicity, for simplicity, the polyisoprene shown in(5) Figure the right sideside in Equation (5) is(5) rewritten as ・as Br for and and the polyisoprene shown in Figure The possibility of the reaction of Equation (5) has already been presented [36]. The first term on to is RH is rewritten as follows 12 to12RH rewritten as follows to R.to R. ・ Theside possibility of the(5) reaction of Equation alreadyand been presented [36]. shown The first on the right in Equation is rewritten as Br (5) for has simplicity, the polyisoprene interm Figure The possibility of the reaction of Equation (5) has already been presented [36]. The first term on · thetoright side in Equation (5) is rewritten as Br for simplicity, and the polyisoprene shown in Figure 12 12 RH is rewritten as follows to R. the right side in Equation (5) is as ・Br for simplicity, and the polyisoprene shown in Figure (6) (6) to RH is rewritten as follows torewritten R. 12 to RH is rewritten as follows to R. (6) From the right in Equation (5)derive we derive Equation (7)presented as presented a previous study From the right sideside in Equation (5) we Equation (7) as in a in previous study [37].[37]. ・ (6) (6) + RH →RBr → RBr + H・ ・ ・ +Br RH → Br H R→H RBr (7) (7) From the right side in EquationBr(5) we derive Equation (7) +asHpresented in a previous study [37]. Br(5) + RH → Br REquation H → RBr H presented in a previous study [37]. (7) From the right side in Equation we derive (7)+as ・ ・ ・ Br + RH → Br R H → RBr + H・ (7) Sensors 2017, 17, 767 15 of 18 From the right side in Equation (5) we derive Equation (7) as presented in a previous study [37]. Sensors 2017, 17, 767 · . Br + RH → Br R H → RBr + H· Sensors 2017, 17, 767 15 of(7) 18 15 of 18 RBr is created by radical asas shown in RBr radical polymerization, polymerization,as asshown shownbelow belowininEquation Equation(8). (8).Therefore, Therefore, shown Figures 1111 and 13,13, C-C bonds increase and MCF rubber liquid is vulcanized as as shown byby B and GG in in Figures and C-C bonds increase and MCF rubber liquid is vulcanized shown B and RBr is created by radical polymerization, as shown below in Equation (8). Therefore, as shown Figure 8. 8. in Figure in Figures 11 and 13, C-C bonds increase and MCF rubber liquid is vulcanized as shown by B and G in Figure 8. (8) (8) (8) Here, Here, the the following following reaction reaction may may occur, occur, Here, the following reaction・ may occur,・ OH· H· ++ OH ・ → → ・ H22O H (9) (9) (9) + side OH in Equation → H(3) 2O and ・Br react as follows, H right Next, at the cathode, the radicals of the Next, at the cathode, the radicals of the right side in Equation (3) and · Br react as follows, Next, at the cathode, the radicals of the right side in Equation (3) and ・Br react as follows, (10) (10)(10) Therefore the NR-latex and oleic acid are vulcanized as shown by D in Figure 8. We have already demonstrated that the polymerization of NR-latexasisshown different theFigure anode8.and cathode Therefore theelectrolytic NR-latex and oleic acid are vulcanized by at D in We have already sidesdemonstrated of the MCF rubber surface (Figure 10). Therefore, electrolytic polymerization of the MCF rubber that the electrolytic polymerization ofasNR-latex is different at 8. theWe anode cathode Therefore the NR-latex and oleic acid are vulcanized shown by D in Figure haveand already is different theMCF anode and cathode sides, as10). shown in Equations (8) and (10). sides ofatthe surface (Figure Therefore, electrolytic polymerization the cathode MCF rubber demonstrated that therubber electrolytic polymerization of NR-latex is different at the anodeofand The vulcanization phenomenon of MCF rubber liquidinisEquations similar to (8) that of (10). conductive polymers the anode and(Figure cathode sides, as shown and sidesisofdifferent the MCFatrubber surface 10). Therefore, electrolytic polymerization of the MCF rubber such as polyacetylene. Equations (2) and (5) describe howliquid an electron occurs andof moves through πThe vulcanization rubber is (8) similar to that conductive polymers is different at the anode and phenomenon cathode sides,ofasMCF shown in Equations and (10). conjugation as π-electron, like the carrier of dopant in the C=C bond. As shown by B and G in Figure such as polyacetylene. Equationsof (2)MCF and rubber (5) describe an electron and moves through πThe vulcanization phenomenon liquidhow is similar to thatoccurs of conductive polymers 8, bonds of C=C and C-C are connected like a of network. Therefore, electric conductivity isand enhanced conjugation as π-electron, like the carrier dopant in the C=C bond. As shown by B G in Figure such as polyacetylene. Equations (2) and (5) describe how an electron occurs and moves through −3 Ω·m, as shown in our previous study [22]. As a and electrical resistivity reaches on the order of 10 8, bonds ofasC=C and C-Clike are connected a network. Therefore, electric enhanced π-conjugation π-electron, the carrierlike of dopant in the C=C bond. Asconductivity shown by Bisand G reference, polyphenylene-based conductive plastic doped withas ASF 5 is on the order of 10−2 Ω·m. On −3 Ω·m, and electrical resistivity reaches on the order of 10 shown in our previous study [22]. in Figure 8, bonds of C=C and C-C are connected like a network. Therefore, electric conductivityAs a −7 the other hand,polyphenylene-based ordinary metals such as silver and copper are on order of 10 By the −the 3 Ω reference, doped with ASF the Ω·m. order of previous 10−2NM, Ω·m. On is enhanced and electrical resistivityconductive reaches onplastic the order of 10 ·m,5 is as on shown in our we obtained a soft and highly conductive material with high sensitivity that could be applicable to NM, −7 the other ordinary metals such as silver and copper are on the order of 10 Ω·m. By the study [22]. As hand, a reference, polyphenylene-based conductive plastic doped with ASF is on the order 5 sensors. In addition, because the bonds of C=C and C-C produced by the electrolytic polymerization −2 obtained softother and highly material with high sensitivity that could be applicable of 10we Ω·m. Ona the hand, conductive ordinary metals such as silver and copper are on the order of to have large bonding force, it makes the MCF rubber long-term stability of electric conductivity. − 7 addition, theabonds of C=C and C-C produced by the electrolytic polymerization 10 sensors. Ω·m. ByInthe NM, webecause obtained soft and highly conductive material with high sensitivity that have large bonding force, it makes the MCF rubber long-term stability of electric conductivity. could be applicable to sensors. In addition, because the bonds of C=C and C-C produced by the 4. Conclusions electrolytic polymerization have large bonding force, it makes the MCF rubber long-term stability of 4. Conclusions Our aim was to develop a new manufacturing process for NR-latex as a soft rubber embedded electric conductivity. with MCFOur as magnetic using the NM. The effect of the magnetic field on as theametamorphosis of aim was filler to develop a new manufacturing process for NR-latex soft rubber embedded 4. Conclusions MCFwith rubber byaselectrolytic polymerization and the process electrolytic were of MCF magnetic filler using the NM. The effect of theofmagnetic fieldpolymerization on the metamorphosis clarified. Our aim was by to develop a new manufacturingand process for NR-latex as a soft rubber embeddedwere MCF rubber electrolytic polymerization the process of electrolytic polymerization first rubber canThe be enhanced bymagnetic electrolytic polymerization. The MF in withWe MCF as clarified magnetichow fillerMCF using the NM. effect of the field on the metamorphosis of clarified. MCF rubber liquid is a very important factor in the enhancement of wire-shaped solidified MCF MCF rubber electrolytic andcan thebe process of electrolytic polymerization were clarified. Weby first clarified polymerization how MCF rubber enhanced by electrolytic polymerization. The MF in rubber. magnetic field distribution is also significant. field gradient andThe strength WeThe first clarified how rubber can be enhanced electrolytic polymerization. MF inmust MCFMCF MCF rubber liquid isMCF a very important factor in by theMagnetic enhancement of wire-shaped solidified be considered in order to obtain large solidified MCF rubber. Specifically, MCF rubber liquid with rubber liquidThe is a magnetic very important factor in theisenhancement of wire-shaped solidified MCF rubber. Themust rubber. field distribution also significant. Magnetic field gradient and strength 21.7 wt %field can distribution be solidified less than about 50-mT MCF magnetic fieldSpecifically, strength, and enhanced at the with magnetic also significant. Magnetic field gradient and strength must be considered be considered in orderisat to obtain large solidified rubber. MCF rubber liquid position the fieldabout strength andmagnetic maximum magnetic field gradient. This in order to large solidifiedmagnetic MCF Specifically, MCF rubber liquid with 21.7 wt % can be 21.7between wtobtain % can bemaximum solidified at lessrubber. than 50-mT field strength, and enhanced at the enhanced MCF rubber depends on the aggregation of magnetic clusters aligned along the magnetic position between the maximum magnetic field strength and maximum magnetic field gradient. This field enhanced line, likeMCF needles. clarified thataggregation this enhancement of clusters MCF rubber electrolytic rubberWe depends on the of magnetic alignedby along the magnetic polymerization based on radical shown in Equation The production NRfield line, islike needles. Wepolymerization, clarified that as this enhancement of(3). MCF rubber by of electrolytic latex polymerization MCF rubber by is the NM could be related to production of a highly sensitive sensor. based on radical polymerization, as shown in Equation (3). The production of NRSEM and rubber XRD images revealed electrolytically polymerized rubber presents a latex MCF by the NM couldthat be related to production of a highlyMCF sensitive sensor. complicated configuration of intertwined NR-latex, Fe3O4 and Ni. They also clarified electrolytic SEM and XRD images revealed that electrolytically polymerized MCFthat rubber presents a Sensors 2017, 17, 767 16 of 18 solidified at less than about 50-mT magnetic field strength, and enhanced at the position between the maximum magnetic field strength and maximum magnetic field gradient. This enhanced MCF rubber depends on the aggregation of magnetic clusters aligned along the magnetic field line, like needles. We clarified that this enhancement of MCF rubber by electrolytic polymerization is based on radical polymerization, as shown in Equation (3). The production of NR-latex MCF rubber by the NM could be related to production of a highly sensitive sensor. SEM and XRD images revealed that electrolytically polymerized MCF rubber presents a complicated configuration of intertwined NR-latex, Fe3 O4 and Ni. They also clarified that electrolytic polymerization of NR-latex is different at the anode and cathode sides of the MCF rubber surface. Therefore, electrolytic polymerization of MCF rubber is also different at the anode and cathode sides, as shown in Equations (8) and (10). Raman spectroscopy and XPS showed that the quantity of C=C and C-C bonds increases by electrolytic polymerization. Meanwhile, the chemical approach revealed that the pH and ORP of MCF rubber liquid is altered by electrolytic polymerization. Based on these results, we were able to propose a hypothetical process for electrolytic polymerization of MCF rubber, as shown in the physical model. We clarified why NR-latex MCF rubber produced by the NM has high sensitivity and long-term conductive stability. Therefore, we showed that the NM may be useful in the development of a high-sensitivity sensor. Because the new production of NM uses small quantity of voltage, electric current and magnetic field strength in the application of electric and magnetic fields, it brings about such an easy implementation that the sensing fields would develop. 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