Flexible and anisotropically conductive sandwich lm by assembly of liquid silicone rubber and cobalt coated glass ber composites via magnetic eld inducement

In this work, we have prepared electromagnetic cobalt coated glass ber (Co@GF) composite via electroless plating method rstly. And then a conductive sandwich exible lm consisted of Co@GF composite and liquid silicone rubber (RTV-2) is successfully formed, through tape casting method at room temperature. Based on the perfect coating and excellent electrical conductivity of Co@GF composite, the resultant RTV-2/Co@GF/RTV-2 sandwich exible lm shows high electrical conductivity of 0.264 Ω·cm and can stretch to 100% ( of 4.40 Ω·cm) without obvious fracture. When the magnetic eld was applied to the curing process, the electromagnetic Co@GF composite aligned automatically in the RTV-2 matrix on account of its ferromagnetic nature. The as-prepared lm shows anisotropy in electrical performance. The volume resistivity parallel to the magnetic eld direction is about two times lower than that of perpendicular direction. The maximum difference in volume resistivity (ρ ∥ of 0.768 Ω ·cm and ρ ⊥ of 1.549 Ω ·cm) is obtained at magnetic eld intensity of 800 mT. In addition, a certain magnetic eld intensity at 100mT is helpful to improve the electrical conductivity of the as-obtained sandwich lm. The anisotropic RTV-2/Co@GF/RTV-2 sandwich exible lm would be considered as promising exible electronic sensor where discrepant sensing sensitivity is required in orthogonal directions.


Introduction
The rapid development of portable, wearable and implantable exible electronics has attracted tremendous attention. Those optimized and superior characteristics will facilitate the development of electronic devices with different functionalities, such as exible displays [1], smart mobile devices, implantable biosensors [2], etc. As a series of performance such as excellent electrical conductivity, robust construction, super exibility and sensitivity [3][4][5][6] have been improved. One new challenge is to create anisotropic exible materials [7,8] which possessed discrepant sensing sensitivity in different directions. To date, some strategies have been proposed for the alignment of llers in matrix, basically applying external elds including shear ow [9], magnetic [10] and electric eld [11]. The magnetic eld inducement method shows irreplaceable superiorities, intelligent control and different orientations at arbitrary directions. In such approach, the conductive component is required to have response to the external magnetic eld, particularly magnetic metals such as Co and Ni. For instance, Xu et al. suggested the magnetic eld inducement processing to align nickel coated glass ber in polypropylene matrix along the magnetic direction and observed obvious anisotropy in electrical and electromagnetic interference (EMI) shielding performance [12]. Knaapila et al. also used magnetic eld to align nickel particles into stringlike assemblies in urethane oligomer mixtures and create a Ni particle/polymer composite with anisotropic electrical conductivity [13]. It is well known that Ni and Co metals are easily oxidized at high temperature and bare environment. To solve this problem, we need choose a novel forming method at low temperature, and also the nally conductive llers must not be exposed to the air. According to common exible matrix materials, liquid silicone rubber [14,15], thermoplastic polyurethane (TPU) [16], dimethylsiloxane (PDMS) [17,18], and epoxy [19,20] so on, liquid silicone rubber must be the best choice, owing to its enough exibility and mechanical property, most of all, it can be molded with a desired size at room temperature. In addition, some reports about sandwich-like structure [21][22][23] conductive lm indicated its unique advantages, such as structure stability, persistent electrical conductivity and lower llers consumption.
Glass ber possesses excellent chemical and thermal stability, low-cost, large aspect ratio and high mechanical strength. Electroless plating is a kind of surface treatment technology, usually to make nonelectromagnetic ller electromagnetization [24,25]. Therefore, it is promising to realize the magnetic inducing alignment of such llers in exible matrix.
In this work, we reported a novel strategy, namely, magnetic-assistant tape casting method to prepare conductive exible lm comprising cobalt coated glass ber (Co@GF) as conductive ller and liquid silicone rubber (RTV-2) as exible matrix. The resulting RTV-2/Co@GF/RTV-2 sandwich lm showed high exible and excellent conductivity, which stretched to 100% without severe damage of mechanical property and conductivity. After magnetic inducing, the as-prepared lm exhibited signi cant response to the magnetic eld during the curing process and thus oriented automatically in RTV-2 matrix. A signi cant difference in volume resistivity between parallel and perpendicular to the direction of magnetic eld has been observed. And appropriate orientation contributed to form a more perfect conductive network, so the excellent volume resistivity (r ∥ of 0.153 Ω·cm and r ⊥ of 0.211 Ω·cm) has been achieved at magnetic eld intensity of 100 mT.

Synthesis of Co@GF composite
Co@GF composite, with volume resistivity of 4.2×10 -3 Ω·cm, saturation magnetization (Ms) of 21.4 emu/g and coercivity (Hc) of 382.7 Oe, was prepared via electroless plating technology, which similar to our previous work [26]. Glass ber (density: 2.5 g/cm 3 , diameter: 15 μm, and length: 40-200 μm) was used as the core material. All other materials, solvent and reagent were obtained from commercial sources and used without further puri cation. The process parameters and conditions of electroless cobalt plating are presented in Table 1. the schematic illustration is provided in Fig. 1. First, 2 g of RTV-2, 5 g of xylene (C 8 H 10 ), and 0.06 g of tetraethyl orthosilicate (TEOS) were mixed homogeneously under vigorous stirring. C 8 H 10 was worked as diluent solvent, and TEOS was served as curing agent in the liquid silicone rubber system. Subsequently, the diluent silicone rubber solution was poured into the die plate (PMMA, r=10cm), and then cured at room temperature for 45 min. The obtained lm was marked as the pure RTV-2 lm. The RTV-2/Co@GF/RTV-2 sandwich exible lm was composed of pure RTV-2 lm and Co@GF/RTV-2 composite lm. The Co@GF/RTV-2 composite lm was prepared by the same method using Co@GF as a ller. Typically, 3 g of RTV-2, 7.5 g of C 8 H 10 , and 0.09 g of TEOS were mixed homogeneously under vigorous stirring. After that, different content of Co@GF ller (0.5~3.0 g) was added into the mixed solution with mechanical agitation for 5 min. Finally, the resulting mixed solution was poured cautiously onto the pure RTV-2 lm, which the surface must be hard enough to make sure that the mixed solution owed by gravity. After curing at an ambient temperature for 24 h, the RTV-2/Co@GF/RTV-2 sandwich exible lm was fabricated.

Preparation of anisotropic conductive exible lm
The anisotropic conductive exible lm was prepared via magnetic-assistant tape casting method at room temperature, the schematic illustration is presented in Fig. 6a. Herein, two kinds of anisotropic conductive exible lm were prepared, sandwich lm (RTV-2/Co@GF/RTV-2) and simple lm (Co@GF/RTV-2), respectively. The foregoing conductive RTV-2/Co@GF/RTV-2 exible sandwich lm was induced magnetic eld ranging from 0 to 800 mT during the curing process, which so called anisotropic sandwich lm. In order to observe the orientation of ller more clearly, a simple lm was prepared. The preparation method was similar to Co@GF/RTV-2 composite lm, just also being induced magnetic eld (0~800 mT) during the curing process. The process parameters of anisotropic conductive exible lm are presented in Table 2. Notes: The data before and after" + "are the process parameters of pure RTV-2 lm and Co@GF/RTV-2 composite lm, respectively.

Characterizations and measurement
XRD pattern was recorded on a Bruker D8 focus diffractometer using CuKα radiation (λ = 0.15406 nm) with the 2θ angle ranging from 30 to 85 o . SEM images were taken by a Hitachi SU-1500 scanning electron microscope, and volume resistivity values were obtained by a SB120 four-point-probe instrument, and the tensile strain-volume resistivity curved line was tested by virtue of the self-regulating manual stretching equipment based on SB120 four-point-probe instrument.

Results And Discussion
SEM has been used to characterize the morphology of the as obtained Co@GF composite in the typical synthesis. It could be seen from Fig.2a that the uniform and compact cobalt layer formed on the surface of the glass bers, the length-diameter ratio was about 2~14. A close-up view of the single composite ber revealed that the cobalt coating was consisted of nanoparticle which tightly adhered to glass ber ( Fig.2b). Powder XRD has been used to determine the crystal structure and chemical composition of the resultant product. Fig. 2c displayed the XRD pattern of the as-prepared product. Five diffraction peaks index to (100), (002), (101), (102) and (110) planes of the hexagonal close-packed cobalt crystals (JCPDS Card no. 05-0727). No peaks due to the impurities of cobalt oxides or hydroxides could be observed, which indicates that high quality of cobalt layer on glass ber has been achieved. The magnetic properties of Co@GF composite obtained from the typical synthesis were investigated at room temperature, as exhibited in Fig. 2d. it can be con rmed that the Co@GF composite showed typical ferromagnetic behavior with saturation magnetization (Ms) of 21.4 emu/g and coercivity (Hc) of 382.7 Oe separately. The volume resistivity value of the as-obtained Co@GF composites was as low as 4.2×10 -3 Ω·cm. It proved that uniform and compact Co layer on the surface of glass ber was successfully prepared, both possessing outstanding electrical conductivity and magnetic property.
The conductive RTV-2/Co@GF/RTV-2 sandwich exible lm has been prepared at different content Co@GF composites of 0.5 g, 1.0 g, 1.5 g, 2.0 g, 2.5g and 3.0 g. Fig.3 showed the SEM morphology of sample with 2.5 g of Co@GF composites. From Fig. 3a, it can be found that a sandwich structure lm was successfully prepared, which Co@GF composites were embedded between two layers of silicone rubber. The middle layer also contained small amounts of silicone rubber which used to bond conductive llers. In an enlarged view (Fig. 3b), Cobalt layer coating on the surface of glass bers kept intact and barely peeled off, which could carry high density of current and result in high electrical conductivity. Thus the cobalt layer formed a continuous conductive network successfully, attributing to its well distribution at the interface between glass ber and liquid silicone rubber. The exciting innovative point of such sandwich lm is that some advantages such as stability of electrical conductivity, highly exible property, low content of llers, high tolerance to stretch and excellent mechanical stability could be found, presented in Fig.1 a, b and c. As shown in Fig.4a, volume resistivity of conductive RTV-2/Co@GF/RTV-2 sandwich exible lm notably decreased with the increasing Co/GF composites content, owing to the gradually formed conductive network. The volume resistivity of sandwich lm was 4.409 Ω·cm at 0.5 g of Co@GF content because of the relatively low content of Co@GF composites, for which could not achieve e cient touch and form conductive network in exible matrix. When the content of Co@GF composites increased gradually, the volume resistivity of sandwich lm could achieve 0.757 Ω·cm at 1.0 g of Co@GF content which indicated the formation of conductive network. After that the volume resistivity of the sandwich lm decreased slowly as the increasing Co@GF composite content from 1.5 g to 3.0 g. The sandwich lm with 2.5 g Co@GF content possessed the lowest volume resistivity of 0.264 Ω·cm, which exhibited superior conductivity. To further investigate the relationship between volume resistivity of the sandwich lm and tensile strain ranging from 0 to 100%, the stretchable performance test has been showed in Fig.4b. At different Co@GF content, the volume resistivity decreased rstly and then increased with increasing tensile strain. This phenomenon could be explained by that certain tensile stress (less than 20%) contributed to contact of Co@GF composite with each other, and further improved the electrical conductivity of the obtained exible lm. Besides, the amount of Co@GF composites directly affected the tensile conductivity of the sandwich lm. The less llers, the more sensitive of the tensile conductivity.
And the more llers, the more stable of the tensile conductivity. Considering the above analysis, the optimum dosage of Co@GF composite must be 2.5 g, both possessing excellent conductivity and exibility (0.264 Ω·cm at tensile strain of 0% and 4.40 Ω·cm at tensile strain of 100% ).
As described in Tab.2 and Fig.5a , anisotropic conductive RTV-2/Co@GF/RTV-2 sandwich lm and Co@GF/RTV-2 simple lm have been prepared by magnetic induced method, the SEM images have presented in Fig.5c and Fig.5d. In order to explore the in uence of magnetic eld on the electrical performance of lm, r ∥ and r ⊥ were measured, as displayed in Fig. 5b. It was obvious that the volume resistivity of sandwich lm was 1~2 orders of magnitude lower than simple lm. In addition, a signi cant difference in volume resistivity between r ∥ and r ⊥ have been observed, which was strongly depended on magnetic intensity. In order to explain the mechanism of conductive anisotropy, SEM images of Co@GF composites in simple lm under different magnetic intensity have been showed in Fig.6. It could be seen that Co@GF composites were gradually orientated along with the magnetic eld direction. However, a high magnetic eld strength was easy to form discontinuous conductive network, thus the volume resistivity increased and it gradually transfered to non-conducting material (Fig. 6f).
So a sandwich lm has been suggested to solve this problem, the detail data of volume resistivity showed in Fig. 5b. The excellent volume resistivity (r ∥ of 0.153 Ω·cm and r ⊥ of 0.211 Ω·cm) was achieved at magnetic eld intensity of 100 mT. This phenomenon could be interpreted as following point, since Co@GF composites showed appropriate orientation parallel to magnetic eld direction, a more perfect conductive network have been formed in the parallel direction, so r ∥ displayed an downtrend after inducement. Co@GF composites were easy to form interconnected network because of the high aspect ratio, thus cobalt coated on the surface of glass bers could form an e cient conductive network in aligned direction. However, r ∥ and r ⊥ increased when the magnetic eld intensity further increased, the maximum difference in volume resistivity (r ∥ of 0.768 Ω·cm and r ⊥ of 1.549 Ω·cm) was obtained at magnetic eld intensity of 800 mT, the corresponding SEM images showed in Fig. 7. Obviously, the Co@GF composites, paralleled to the direction of magnetic eld, oriented in alignment. It was understandable that much higher alignment would reduce the interconnection of adjacent Co@GF to some extent, which resulted in the increasing volume resistivity consequently. This result was consistent with Xu et al.' report [12]. Comparing with simple lm, sandwich lm exhibited much higher electrical conductivity based on the stability of conductive network.

Conclusions
In summary, a novel structure of conductive sandwich exible lm consisted of Co@GF composite and liquid silicone rubber (RTV-2) have been successfully prepared via tape casting method at room temperature, which possessed excellent electrical conductivity of 0.264 Ω·cm and could stretch to 100% without serious mechanical property and conductivity damage. At different content of Co@GF composite, a same experimental rule have been found, which the volume resistivity value decreased rstly and then increased with increasing tensile strain ranging from 0 to 100%. This phenomenon could be explained by that certain tensile stress (less than 20%) contributed to contact of Co@GF composites, and further improved the electrical conductivity of the obtained exible lm. When different magnetic eld intensity (0~800mT) was applied to the curing process, the electromagnetic Co@GF composites aligned automatically in the RTV-2 matrix. The as-prepared lm showed anisotropy in electrical performance. A signi cant difference in volume resistivity between r ∥ and r ⊥ have been observed, the excellent volume resistivity (r ∥ of 0.153 Ω·cm and r ⊥ of 0.211 Ω·cm) was achieved at magnetic eld intensity of 100 mT.
This result revealed that appropriate orientation contributed to form a more perfect conductive network, so both r ∥ and r ⊥ displayed a downtrend after inducement. By contrast with the simple lm, the anisotropic RTV-2/Co@GF/RTV-2 sandwich exible lm highlighted some advantages such as superior electrical conductivity, less ller content, more stable structure and excellent weatherability. So it would be considered as promising exible electronic sensor where discrepant sensing sensitivity is required in orthogonal directions.