Keywords

1 Introduction

Conductive rubber composites have received great interest over the last decade from various scientific fields as their applications in electromagnetic shielding, flexible electronic devices, thin film transistors, electrodes and sensors are almost indispensible [1–4]. The possibilities and applications of stretchable and flexible materials in the manufacture of electronic devices is tremendous. However, the basic characteristic properties of both natural and synthetic rubbers are its remarkable electrical resistance, elasticity, toughness, impermeability and adhesiveness which makes it a useful material in different fields rather than in electronics industry. Electrical properties in rubber can be achieved only by the formation of conductive networks in the matrix, which offers an electrical pathway for the conduction of electrons. It is proved that the conducting fillers (CFs) such as carbon black, nanotubes, graphene and metal particles can create an electrical pathway by the formation of filler–filler networks within the rubber matrix.

Styrene–butadiene rubber (SBR) is a synthetic rubber, also known as Buna-S, used in various sectors such as tyre, medical, footwear and automobile industries. It is produced by the copolymerization of butadiene, CH2=CH–CH=CH2 (75 %), and styrene, C6H5–CH=CH2 (25 %). Pure gum strength of this non-stereospecific, amorphous polymer is low when compared to stereospecific natural rubber (NR). Subsequently, SBR will not crystallize upon stretching. However, white and black fillers-reinforced SBR exhibit excellent strength and abrasion resistance. It is resistant to mineral oils and fats, and solvents such as aliphatic, aromatic and chlorinated hydrocarbons [5].

The primary goal of this chapter is to give an outlook into the properties of SBR and various CF composites and its challenges and potential applications in electronic devices and sensors.

2 Relevance of Stretchable Materials in Electronics

Stretchable and flexible electronic composites are considered to be a revolutionary technology for both academicians and industrialists. Conductive stretchable materials have been a research theme for a long time. However, the recent developments in the fabrication of stretchable electronic materials are remarkable which mainly focused on electronic devices and sensors from elastomeric materials. Very recently, Zhu and co-workers [6] and Huang and co-workers [7] reviewed the applications of stretchable materials for flexible electronics. The main advantage of stretchable electronics is the designing of flexible, light-weight, cost-effective materials, since the matrix material is highly inexpensive compared to conventional substrates. Moreover, the key attraction of flexible electronics is its ability to bent, fold, twist and even form into different shapes without affecting the performance and properties. Flexible electronic devices are more relevant in the manufacture of medical devices for diagnosis and treatment purposes in human bodies. The potential applications include the fabrication of smart clothing, foldable displays, robotic skins, stretchable solar cells and bioinspired devices [8–10].

3 Conductive Styrene–Butadiene Rubber (SBR) Composites

SBR is an insulator, which makes it conductive by incorporating conductive materials such as metals, carbon-based fillers and ceramics. Common conductive fillers used in elastomers include conductive carbon black, carbon nanotubes (CNTs), graphite, graphite fibres, metal particles, metal wires and powders [11–18]. Conductivity of rubber matrix can be enhanced by changing the weight percentage of these conductive fillers, and it is noteworthy that not only the conductivity but physical properties can also be modified. Rubber composites are used for various applications instead of pure rubber, in which fillers act as reinforcing agents irrespective of their conducting or non-conducting nature. Till date, the actual mechanism involved in the reinforcement of filled rubber is not fully explained, but interpreted in terms of the formation of rubber–filler networks. The inclusion of rigid filler in the soft rubber matrix restricts the segmental mobility of the polymer chain. Conductivity of SBR can be varied over a wide range by varying the CF and its dispersion in the matrix.

Properties of composites depend on several factors such as the nature of individual components, size and shape of fillers, morphology of the system and nature of the interface between the phases. Interface interaction between phases plays an important role in the strength of composite in addition to ensuring uniform dispersion of filler in the rubber matrix. In the manufacturing of rubber nanocomposites, to attain the fine dispersion of filler in the matrix is still a challenging problem for the material scientists. Various methods for the fabrication of SBR composite include solution mixing, latex route, melt compounding and two-roll mill mixing. Structure and morphology of the manufactured rubber composites can be explored using different techniques such as X-ray diffraction (XRD), solid-state nuclear magnetic resonance (NMR), Fourier transform infrared (FTIR), scanning electron microscopy (SEM) and transmission electron microscopy (TEM).

3.1 SBR/Metal, Metal Oxides and Ceramics

Conductive SBR composites are frequently manufactured by incorporating metal fillers such as copper, steel, silver, gold and aluminium. These are generally used in the form of powders or flakes. Ceramic files are also used for the fabrication of conductive composite, which includes silicon carbide, boron nitride, aluminium nitride and alumina.

Negri and co-workers [19] have synthesized magnetic nanoparticles, Fe3O4 (magnetite) with average size less than 10 nm to ensure better superparamagnetism to avoid large irreversible magnetic effects. These magnetite particles are then covered with conducting Ago to generate microparticles of Fe3O4@Ag, which exhibits both magnetic and conductive properties. During the preparation of SBR-DEG-Fe3O4 films, the formation of pseudo-chains (needles) of inorganic materials is magnetically aligned and is evident in the morphological analysis presented in Fig. 1. Magnetically aligned needle like Fe3O4@Ag can be clearly seen in the optical and SEM images. In the centre, the needles are normal to the surface; however, at the edge, it is diagonal to the surface due to the uniformity of the magnetic field in the centre and not in the edges. They have investigated the magneto- and piezoresistivity of nanostructured composite films based on SBR filled with electrically CF. Diethylene glycol is added to SBR to get considerable electrical conductivity. The formation of structured SBR-DEG-Fe3O4 films with anisotropic properties is the reason behind the conductivity, which decreases the percolation critical point to observe conductivity. These films offer piezoresistive and magneto-resistive properties. Negri and co-workers [19, 20] used Fe3O4@Ag filler in various elastomeric matrix due to its simultaneous exhibition of magnetic and electrical properties.

Fig. 1
figure 1

SBR-DEG—12 % Fe3O4@Ag composite films. a Optical photograph and b SEM image [19]. Copyright 2013. Reprinted with permission from Elsevier Ltd.

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Lee et al. [21] fabricated highly stretchable Ag nanowire–Ag nanoparticles (AgNW–AgNP)-reinforced electrically conductive styrene–butadiene–styrene (SBS) fibre by wet-spinning method and iterative process for Ag precursor absorption and reduction. During stretching AgNW-embedded SBS exhibits high initial electrical conductivity due to the presence of highly conductive fillers. According to them, AgNWs form bridges between AgNPs for electrical conductivity, which will retain up to 100 % strain after that decline of conductivity observed due to the breakdown of networks. SBS–AgNW composite fibre shows strain-sensing behaviour and is incorporated into a smart glove to perceive human motions is shown in Fig. 2.

Fig. 2
figure 2

a Images of blue-coloured LEDs connected by 0.56 wt% AgNW–AgNP embedded composite fibres. b The normalized resistance changes varying the applied strains at 0, 20, 40, 60, 80 and 100 %. c Reliability test measured the changes in the normalized resistance of the composite fibre during the stretching and releasing cycle. d Photograph of the smart glove attached to the composite fibre on each finger. e Motion detection of English letters ‘Y’, ‘S’ and ‘U’ for utilizing sign language glove and photographs of detecting each letter [21]. Copyright 2015. Reprinted with permission from John Wiley & Sons

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Titanium carbide (TiC) belongs to the class of interstitial carbides, in which the metal retains its closed packed lattice structure and C atoms occupy the octahedral interstices. TiC is extremely hard refractory materials and good conductors of electricity. Electrical and electromagnetic wave shielding properties of TiC-loaded SBR have been investigated by Sung and El-Tantawy [22]. According to them, SBR–TiC composite is a promising material for applications in self-electrical heating, temperature sensor, time delay switching and electromagnetic wave shielding effectiveness. The electromagnetic wave shielding effectiveness of the composite is due to reflection loss, consisting of the reflections from the material boundaries and surroundings. Reflection loss depends on the electrical conductivity and network structure of the conductive particles in the elastomer.

Measurement of electrical conductivity of filled elastomers gives information about the dispersion of filler in the matrix. Gwaily et al. [23] studied the effect of γ-radiation in the conductivity of Fe and Cu metal particles-filled SBR. In the report, they mentioned that without irradiation, the filled SBR behaves like an insulator, and conductivity, σ is in the range 10−12 Ω−1 cm−1. It is due to the weak interfacial interaction of metal–polymer and segregation of filler particles after moulding. Upon γ-irradiation >300 kGy, σ increases significantly due to the formation of filler–filler networks.

3.2 SBR/Carbonaceous Materials

Conducting SBR composites can be prepared by incorporating conducting carbonaceous fillers such as carbon black, carbon fibre, graphite and CNTs. Improvement in mechanical, thermal and electrical properties of composite is strongly influenced by the nature of filler, size, shape, surface area, surface chemistry, distribution within the matrix and processing techniques.

3.2.1 SBR/Carbon Black

Carbon black is one of the major reinforcing filler used in rubber compounds, especially in tyres to impart improved strength and performance. CB is also used as CF for the preparation of conducting elastomeric composite due to its low cost, good processability, availability and corrosion resistant [16, 24, 25]. Electrical properties of conducting carbon black have been used in antistatic, electromagnetic interference (EMI), electroactive pressure sensors and actuators. A special type of highly conductive novel carbon black with superhigh surface area and structure has been synthesized and incorporated in elastomeric matrix, which shows conductivity higher than that of conventional electroconductive carbon [26]. SBR filled with carbon black shows more network chains due to its strong absorbability [27], which thereby reduce the agglomeration of fillers and enhance the conductivity of elastomers. Conductivity of CB depends on three major factors: particle size, structure and surface chemistry. Fillers with high surface area and porosity provide high electrical conductivity to the elastomeric matrix [28]. Podhradska et al. [29] reported on the stability of electrical conductivity during cyclic thermal treatment of SBR/carbon black composites. They observed that the conductivity of SBR/CB composite increased from 1.1 × 10−2 to 3.0 × 10−2 Scm−1 when the temperature increased to 85 °C.

Designing of elastomeric materials with conductivity and the maintenance of mechanical performance is a challenging problem for the material scientists. The reason behind this is due to the fact that the lowering of percolation threshold concentration (PTC) is essential for better mechanical properties and good processability. Therefore, filler–filler and polymer–filler network formation is required to obtain elastomeric composite with lower PTC and higher mechanical properties. Due to the high structure of carbon black, the PTC observed is around 5–10 vol.% for conducting rubber composite system [30]. Kawazoe and Ishida [31] proposed a novel mechanism for the network formation of carbon black in an immiscible SBR/NBR blend system. They have demonstrated that the PTC of the conductive rubber composite of binary immiscible blends of SBR/NBR could change by changing the solvent. PTC reduced from 5.7 vol.% for SBR and 8.9 vol.% for NBR to 1.2 vol.% for SBR/NBR (90/10) system. In blend systems, the affinity towards CB is different for individual components, which results in the selective adsorption of filler particles on the polymer matrix. Non-uniform distribution of CB in rubber blend can be explained in terms of the molecular confinement mechanism. PTC can be controlled by the selective adsorption of CB; it reduces the aggregation of filler particles and thus localizing at the interface [32–37]. As the specific surface area of CB increases, PTC is found to be lower. Each CB differs in their electrical conductivity due to the difference in specific surface area as well as PTC. Meissner and co-workers [38] relate the effect of CB concentration and type on the electrical conductivity of SBR composite. Four different grades of CB, ISAF, HAF, FEF and La were used and found that the PTC decreases with increase in specific surface area of filler. They have theoretically correlated the conductivity of composites in terms of percolation theory. Percolation theory predicts the relation between the conductivity of composite, σ, and the volume fraction of filler, X. The equation is,

$$ \sigma = \sigma_{0} \left( {X - X_{\text{c}} } \right)^{\text{s}} $$
(1)

where σ 0 is the conductivity of filler particles, X c is the volume fraction of the filler at percolation threshold and s is the quantity determining the power of the conductivity increasing above X c. Unexpectedly, high values of s were found for SBR/CB composite and can be described by the behaviour of a single tunnel junction.

Silica-based fillers, fumed or precipitated silica, are used as reinforcing fillers in tyre industry (green tyres) because they provide better fuel efficiency and low rolling loss when compared to carbon black-filled tyres. Zhao and Wang [39] investigated the antistatic characteristics of solution-polymerized SBR composites filled with hybrid SiO2/CB. In SSBR/SiO2 composite, the static accumulation is found to be high, while in the presence of SiO2/CB in SSBR composite it exhibits antistatic property. This can be attributed in terms of the antistatic nature of CB. According to their studies, the ratio of SiO2/CB in 35/35 was the percolation threshold of the antistatic property, as the amount of SiO2 increases the surface and volume resistivity of the composite increases.

He et al. [40] used water-soluble SBR–sodium carboxy methyl cellulose (CMC) mixture as a binder for sulphur cathode, which consists of 60 wt% insulating S active material and 40 wt% conducting carbon black filler. A remarkable change in cyclic performance of sulphur cathode was observed in the presence of SBR-CMC binder than conventional poly (vinylidene fluoride) (PVDF) binder. Binder acts as both adhesion agent and dispersing agent as it accelerates the uniform dispersion of conducting CB and insulating sulphur to ensure a good electrical contact. Capacity of SBR-CMC cathode was found to be 580 mAhg−1 after 60 cycles, while that of PVDF cathode is 370 mAhg−1.

3.2.2 SBR/Carbon Nanotubes (CNTs)

CNTs have high electrical conductivity along the tube axis. The percolation threshold of polymer/CNT composites is found to be considerably lower when compared to other conventional composites. Therefore, CNTs are considered as one of the ideal conducting material for elastomers. Also, its unique properties such as high mechanical strength, aspect ratio, tubular structure and nanoscale diameter make it a versatile material for the fabrication of stretchable conductive elastomeric matrix. Significant electrical properties of composites of CNTs favour their potential applications in electrostatic dissipation, EMI shielding, super capacitors, chemical sensors and automotive and aviation industries. On incorporating CNTs in elastomers, a continuous network of filler is formed across the matrix and enables the material to a sudden transition from an insulator to a conductor.

The poor dispersion of CNTs in the polymer matrix is one of the challenging problems for material scientists. In order to overcome this facet, Paul and co-workers [41] prepared masterbatches of oxidized MWCNT with SBR using two methods, one by coagulation with acetone and another by aqueous coagulation method followed by mastication in a twin screw extruder, and then analysed the masterbatch rheology to get an insight into the good dispersion and conformation of the MWCNT in the final product. Electrical resistivity decreases with increase in the concentration of MWCNT; however, the change is much less than that expected for percolation networks [42–45]. The tubes are not actually touching each other, but the decrease in resistivity can be explained by electron hopping [46].

Li and Shimizu [47] fabricated a thermoplastic elastomer based on poly(styrene-b-(ethylene-co-butylene)-b-styrene) (SEBS) triblock copolymer and multiwalled carbon nanotubes (MWCNTs) using high-shear processing technique with a screw rotation speed of 1000 rpm. Nanocomposites with 15 wt% of MWCNT show excellent electrical conductivity and stability upon uniaxial deformation with a strain less than 50 %.

Another important factor for determining the electrical properties of composites is the processing method. Nogueira and co-workers [48] reported the electrical conductivity of composites based on MWCNT and styrene–butadiene–styrene block copolymer processed by two methods: melt mixing (extrusion) and solution casting. Composites prepared by solution casting showed higher electrical conductivity than melt mixing. SBS/MWCNT with 6 wt% prepared by melt mixing and the one with 1 wt% prepared by solution casting show the same magnitude of conductivity. Lower PTC is observed for the samples prepared by solution casting method. The reason behind the decrease in electrical conductivity of extruded sample is the decrease in aspect ratio of MWCNT on applying shearing force.

In order to prevent the breaking of CNTs during processing, a novel mixing method rotation–revolution mixer was employed for the preparation of SBR/CNT composite [49]. The obtained results indicated that the new mixing technique reduced the amount of CNT required to provide high electrical conductivity to the SBR matrix than conventional processing methods.

Materials such as CNTs and FG have high aspect ratio, which enable both electrical and mechanical percolation at low filler concentration. Heinrich and co-workers [50] prepared solution styrene–butadiene rubber (S-SBR) composite using graphene nanoplates (GnPs), expanded graphite (EG), MWCNTs and a combination of EG with MWCNT. To get an insight into the specific dispersion and reinforcement of the fillers, they compared the mechanical and electrical properties, presented in Fig. 3a. A remarkable drop in resistivity was observed for MWCNT composite at 3 wt%. This is attributed to the high aspect ratio of MWCNT, leading to the formation of filler–filler network. At higher concentration of EG and GnP, particles form networks, resulting in a conducting pathway and the resistivity of the matrix decreased. Mixed filler system exhibits a synergistic effect in the electrical resistivity of composites, shown in Fig. 3b.

Fig. 3
figure 3

Percolation of S-SBR filled with a GnP, EG, MWCNT and b 20 phr EG and various amount of MWCNT [50]. Copyright 2012. Reprinted with permission from Elsevier Ltd.

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Challenges in the design and fabrication of conducting elastomers are the homogeneous dispersion of CFs in the matrix. Graphene sheets have a tendency for restacking in the matrix, and CNTS forms entanglements. These challenges have been addressed by many researchers by using hybrid fillers in elastomers. For example, graphene and MWCNT hybrid fillers, in which MWCNT helps to form bridges between graphene platelets, provide more interface area between the filler and matrix [50, 51].

3.2.3 SBR/Graphene (GN)

Recently, graphene has attracted much attention of material scientists due to the similarities in properties to carbon nanotubes. The production of carbon nanotubes is highly expensive, so material scientists thought about an alternative and cost-effective material for potential applications. Apart from the above reason, graphene is preferred over other fillers due to high surface area, aspect ratio, mechanical properties, thermal and electrical conductivity, EMI shielding ability, flexibility and transparency [52–55]. Graphene is a single carbon layer of graphite. It is a two-dimensional one-atom-thick sp2-bonded carbon atom densely packed like a honeycomb crystal lattice [56]. In 2010, Andre Geim and Konstantin Novoselov won the Nobel Prize in Physics for their outstanding work in graphene. In 2004, they have identified single layers of graphene and other 2-D crystals [57]. Recently, many researchers reviewed the advances in the properties of graphene-based polymer composites [58–62].

Graphene and functionalized graphene (FG) are extensively used in elastomers as a reinforcing and conducting material [63]. It is a semiconductor with a zero gap and is different from conventional Si semiconductor. Graphene sheets impart percolated pathways for the conduction of electrons in insulators like other CFs such as carbon black, carbon nanotubes and carbon nanofibres. However, the advantage of graphene is its property to enable the insulator to be a conductor at a lower PTC when compared to the PTC of other fillers [64]. It is accounted in terms of the disc-like structure of graphene and can percolate easily in the polymer matrix than rod-like CNTs. Graphene is considered to be a promising candidate to replace Si in microchip electronics and in faster-switching transistor. Modified graphenes can also be used for the fabrication of materials for potential applications by enhancing the interfacial interaction between filler and matrix. Properties of alkylamine (oleylamine and octadecylamine)-modified graphene oxide (GO)–SBR composite were found to be improved significantly [65]. SBR/graphene nanocomposite fabricated by latex compounding method attains antistatic criterion (10−6 S/m) at a GN loading of 3 phr, and when the loading is 7 phr, it becomes a good electrical conductor, shown in Fig. 4 [66]. Graphite flakes are converted into graphene oxide (GO) by Hummer’s method [67].

Fig. 4
figure 4

Electrical conductivity of GE/SBR nanocomposites as a function of GE loading [66]. Copyright 2014. Reprinted with permission from Elsevier Ltd.

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Mulhaupt and co-workers [68] reported about the properties of FG and other carbonaceous materials-incorporated SBR nanocomposites prepared by aqueous dispersion blending. They have compared the performance of thermally reduced graphite oxide (TRGO), chemically reduced graphite oxide (CRGO) with that of multilayered graphene, conventional graphite carbon black and CNTs. TRGO-reinforced SBR showed improved mechanical properties and electrical conductivity when compared to other systems. Elastomers show significant improvement in mechanical, electrical and low gas permeability with functionalized graphene sheets (FGS) owing to its high aspect ratio and carbon-based structure [69, 70]. The volume resistivity of SBR (3.43 × 1015 Ω cm) reduced dramatically when compounding with electrically conducting graphene (6000 S/cm) at a vol.% of 16.5 [71].

Uniform dispersion of graphene in elastomeric matrix is still a challenge. In order to overcome this, hybrid fillers of CB-reduced graphene (RG) were prepared and blended with SBR. Agglomerated CB get adsorbed on the surface of graphene to form microstructured hybrid filler. The role of CB is to block the restacking of RG sheets even after drying. When compared to SBR/RG and SBR/CB, the SBR/CB-RG showed significant improvement in mechanical properties and reduction volume resistivity due to the strong interfacial interaction between RG and SBR [72].

4 Electronic Applications of SBR Composites

Structured elastomer composite with anisotropic physical properties can be used in flexible electronics and in electronic devices. Addition of CFs makes ‘Conductive Rubber’, which can reduce or eliminate EMI and radio frequency interference (RFI) associated with stereo systems. Depending upon the intended environment and application, one can fine tune the properties of final product by varying the filler. Higher level of shielding could be achieved with the use of highly CF. Conductive elastomers can find applications in military and various industries such as telecom, medical devices and computing. Electrically conductive graphene-based SBR can be used for the manufacture of flexible electronic devices, thin film transistors, electrodes and sensors. In the near future, graphene oxide will be used for the manufacture of stretchable and electrically conductive fabrics by incorporating in elastomers [73].

The electron transport in elastomers can be explained in terms of quantum mechanical tunnelling and percolation theory [74]. If the distance between two conductive particles is large compared to atomic dimension, then the bulk resistivity of insulator predominates. However, if the distance is in the nanometre range <100 Ao, electrons can tunnel quantum mechanically between conductive fillers, resulting in less resistivity from the insulator than expected. At lower concentration of filler, electron transport is taking place by quantum mechanical tunnelling. As the filler concentration increases, the CFs come into contact with each other to form more filler–filler networks, leading to increase in conducting pathways. At a particular concentration of filler, the resistivity of the matrix drops dramatically and is called the PTC. Elastomeric composites exhibiting magneto-piezoresistivity find potential applications in sensors, flexible devices and electronic connectors [75–81].

4.1 Electromagnetic Interference (EMI) Shielding

A major concern in electronic industry is the increasing EMI among devices. Therefore, EMI shielding is essential at least for the frequency ranges 50–60 Hz in stereo systems and for 1–100 GHz in strategic applications. Electromagnetic shielding reduces the coupling of radio waves and electromagnetic and electrostatic fields by hindering the field by conducting or magnetic materials. EMI shielding used to protect the electronic devices from radio frequency electromagnetic radiation is also called radio frequency (RF) shielding. Metals such as copper, nickel or aluminium are extensively used for the EMI shielding. Although metals are very effective for EMI shielding, certain factors such as high weight, less corrosion resistance and difficulty in processing lead to the fabrication of rubber composite materials, which can substitute metals in this particular application. Insulating polymers can be converted into conducting or semiconducting polymer composite by the incorporation of CFs. Such materials find application for EMI shielding, microwave absorption and replacing conventional metals [82]. It is essential for medical and laboratory equipments to avoid the interference of other signals. Conductive SBR composite can find application as EMI and RF shielding materials in various electronic equipments.

4.2 Piezoelectric Materials

Conductive SBR composite is coming under the category ‘Flexible and Stretchable’ materials for electronic applications. Piezoelectricity means the accumulation of electrical charge on a certain material on applying a physical stress. As discussed earlier, CFs-reinforced SBR composite becomes a piezoelectric material upon stretching [19]. Piezoelectric behaviour is considered as a reversible process, in the presence of a mechanical strain charge accumulates, similarly, in the presence of an external electric field mechanical strain occurs and charge accumulates. The gripping performance of a tyre can be improved by the incorporation of piezoelectric materials [81]. Due to the flexible nature, rubber materials can generate more energy in response to a mechanical strain. Rubber composite materials can capture energy from movement or any kind of mechanical stress four times higher than the currently available piezoelectric materials. This generated energy could be used for the charging of portable electronic devices such as mobile phone. This technique can be made use of in the charging of medical devices used inside the body (pace maker). The body movements itself can generate enough power to charge the device. Another important area which will find application of flexible piezoelectric materials is shoe soles of soldiers as a source for power. Thin films of piezoelectric materials-incorporated rubber can harness tremendous amount of energy by human body movements; therefore, ‘piezo-rubber’ can find tremendous application in the near future.

4.3 Other Applications

Rubbery materials can be used for the manufacture of medical devices for diagnostic and treatment purposes due to its bendable, curved and stretchable nature. In addition to the above-mentioned applications, SBR composites with CFs are being explored for the manufacture of LEDs, sensors, actuators, smart clothes, artificial muscles, foldable displays, robotic skin (touch sensor) and solar cells. Elastomer-like electrode is fabricated by blending of conductive polyaniline (PANI) and a soft triblock copolymer poly(styrene-co-ethylene-co-styrene) grafted with maleic anhydride (SEBS-g-MA) to be used for artificial muscles [82]. Soft materials are required for smart applications that are to accommodate large actuation strains without increasing the stiffness of the material. Here, SEBS is used to control the stiffness of the material and endow flexibility and stretchability to the material.

5 Concluding Remarks

Stretchable electronics is considered to be a fast-growing revolutionary technology. From the stiff and rigid electronic devices to bendable, curved, twisted and stretchable devices represent the progress in electronic technologies. SBR composites with CFs will benefit significantly in tyre and automobile industries and in the manufacture of medical devices, shoe soles, artificial electrodes and sensors as it can sustain large strain. By incorporating conductive fillers, not only the electrical properties but also the mechanical properties of the matrix get improved. However, one of the biggest challenging problem in the fabrication of rubber composites is the formation of uniform dispersion of filler particles in the matrix. The development of advanced technologies and strategies for optimization and dispersion of fillers for improved properties will be required in the future.