Keywords

3.1 Polymers

Polymers are a generic term for any molecule, organic or synthetic, composed of repeating subunits. Formed by a polymerization process, or the chemical reactions used to join constituent monomers into a polymer chain, polymers play important roles in modern daily life. For example, plastics are synthetic (or sometimes semi-organic) polymers composed of hydrocarbon monomers [1]. Even deoxyribonucleic acids (or DNA) that forms the basis of human life are polymers comprised of nucleotide monomers [2]. In their unprocessed or raw state, polymers often assume the shape of long, flexible strings, much like a cooked spaghetti noodle [3], and are not particularly useful in the construction of practical products. Further processing of raw polymers, such as by introducing small amounts of selected additives, can change the bulk characteristics of those polymers into useful engineering materials.

3.2 Elastomers

Elastomers are a class of polymers characterized by occasional cross-linking of the long-chain polymers, such as by covalent cross-linking [4]. The stiffness of an elastomer is governed by the number of polymer cross-links, such as increasing cross-link density results in stiffer elastomers [5]. The cross-linking process can include the addition of a cross-link promoter (or “cross-linker”) to the raw polymer and then exposing the combined materials to either a source of energy, such as gamma radiation, or chemical activation, such as by mixing the combined materials with peroxide [6]. The elasticity of an elastomeric material is characterized by its ability to “move” or otherwise reconfigure its amorphous structure under an applied load, yet return to its original structural configuration after the load is removed due to the “memory” of the material introduced by the cross-linking process [7].

3.3 Compounding

The process of compounding can further refine the properties of an elastomer through the addition of fillers or additives that can impart special characteristics to the elastomer, such as color, impermeability, or oil resistance [8]. Fillers can also be added to “reinforce” an elastomer, such as to improve tensile strength, abrasion resistance, and extension moduli. For example, carbon black, a paracrystalline form of carbon with a high surface-area-to-volume ratio [9], is a common elastomer reinforcement filler [10]. Owing to surface chemistry, where a large percentage of carbon atoms are available to interact with the polymer matrix, carbon black is hypothesized to increase elastomer tensile strength through physical adsorption with the elastomeric polymer and covalent bonding at cross-linking points [11]. Another example of a carbon-based elastomer filler is the carbon nanotube (or CNT).

3.4 Carbon Nanotubes (CNT)

Originally synthesized and identified in 1952 [12] and subsequently “rediscovered” in 1991 [13], the CNT is a fullerene or a carbon-based material forming a molecular mesh that assumes a cylindrical shape [14]. See Fig. 3.1.

Fig. 3.1
A 3 D illustration of a cylindrical carbon nanotube of large molecules consisting of a hexagonal arrangement of hybridized carbon atoms formed by rolling up a single sheet of graphene.

Molecular structure of a single-walled carbon nanotube (SWCNT) [15]

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Physically, the diameter of a CNT can vary in the range of about 0.4–40 nanometers (nm) with lengths from 0.14 nm to 55.5 cm [16]. See Fig. 3.2. CNT display unusual tensile strength (50+ GPa) as compared to carbon steel (540 MPa) [17]. Single-walled CNT are predicted to have an electrical resistance of ∼6 kΩ independent of length and diameter [17].

Fig. 3.2
A micrograph presents a magnified view of a carbon nanotube resembling a very thin smooth tube, placed over a nearly 1000 times larger diameter human hair strand.

Scale of a CNT (thin tube) compared to a human hair (thick tube; diameter = ∼50 nm) [18]

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3.5 CNT Elastomers

Apart from their enhanced mechanical properties, elastomers doped with CNT fillers also exhibit advantageous electrical properties. While elastomers are typically considered an insulating material unable to conduct electrical charge, CNT-doped elastomers act as conducting materials, capable of transmitting a current through the elastomer in response to an applied voltage [19]. Electrical conductivity is induced as the CNT filler is dispersed throughout the insulating elastomer at a density sufficient to create a conducting network through the elastomer, such as by direct contact of CNTs creating a conduction path or by CNTs in close proximity to each other in a process known as “tunneling” [20]. In an example, CNT elastomers can be formed into the shape of a sensor, such as a resistance strain gauge. As load is applied to the CNT elastomer strain gauge, the elastomer material deforms, interrupting current transmission paths by “breaking” connections between conducting CNT in the elastomer matrix, increasing the electrical resistance of the strain gauge [21, 22]. As the load is released, the elastic nature of the CNT elastomer will cause the gauge to return to its undeformed state, reestablishing the electrical CNT connections and reducing electrical resistance of the strain gauge.

3.6 Factors Affecting CNT Elastomer Conductivity

Figures 3.3 and 3.4 illustrate the effect of CNT concentration on conductivity within an elastomer matrix [23]. In illustration (a) of Fig. 3.3, a low concentration of well-dispersed CNT does not allow for contact between CNT particles, resulting in low electrical conductivity shown in Fig. 3.4 at location a. In illustration (b) of Fig. 3.3, increased CNT concentration brings CNT particles into closer proximity and promotes “tunneling” effects [21], where electrical current can pass without physical connection between CNT particles and increases conductivity as shown in Fig. 3.4 at location b. In illustration (c) of Fig. 3.3, increased CNT concentration allows CNT particles to contact one another and provides a continuous electrical pathway through the elastomer (shown as red CNT particles in illustration (c)) to achieve the so-called percolation threshold shown in Fig. 3.4 at location c. Note that the percolation threshold can be considered the CNT concentration that will lead to an abrupt increase in electrical conductivity (or correspondingly, a sharp decrease in electrical resistance) through the elastomer. In illustration (d) of Fig. 3.3, further increases in CNT concentration lead to multiple flanking electrical paths (shown as blue CNT particles in illustration (d)) and ultimately, electrical saturation of the elastomer shown in Fig. 3.4 at location d.

Fig. 3.3
Four illustrations of electrical conductivity stages of C F S in polymers. a. The conductivity is low since there are only a few C S Fs. b. Some large clusters connected by C S Fs are formed by tunneling. c and d. Due to increased C N Ts, elastomers and multiple flanking electrical paths are formed.

(left) CNT concentration in the elastomer matrix can affect conductivity [23]

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Fig. 3.4
A line graph of electrical conductivity from polymer to filler versus filler fraction between 0 and 1. It plots a line that exponentially increases at a, b, till point c at phi subscript intersecting the vertical percolation threshold, and increases concavely down to point d.

(right) A graph of filler fraction to electrical conductivity [23]

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In addition to CNT concentration, volumetric distribution of the CNT filler in the elastomer matrix has been reported to affect conductivity [24]. Due at least in part to surface chemistry, CNT particles tend to agglomerate prior to elastomeric compounding to form “balls” or clumps of CNT [25] thought to be drawn together by ionic bonding. In general, compounding techniques that generate a more even dispersion of CNT particles through the elastomer matrix will result in a larger number of conduction paths to increase CNT elastomer conductivity and potentially lower the amount of CNT filler required to achieve the percolation threshold.

The use of CNT elastomers in sensor applications have been thoroughly explored in the literature. For example, reference to pressure sensing applications include work on “smart” skin, such as for haptic sensing applications [26], and sandwich-type piezoresistive pressure sensors [27], such as for measurement of mechanical loads. Similarly, considerable effort has been directed to strain sensing applications including sensors for human motion detection [28,29,30], roadway assessment [31,32,33], and general strain measurement purposes [34,35,36]. Many of these studies report excellent results, including substantially linear response and little to no hysteresis characteristics [37]. However, even with the benefit of more than 30 years of academic research and discussion, the authors are unable to find any commercial products utilizing CNT elastomers.

At least two issues have been suggested to explain the lack of widespread CNT elastomer adoption – stability and repeatability [21]. Stability refers to fluctuations in sensor resistance not associated with mechanical loading whereas repeatability is characterized as the “repeatable resistance-strain performance” of a sensor [21]. Similar to their foil strain gauge counterparts, CNT strain gauges can experience environmental factors that affect the stability of resistance measurements, such as temperature sensitivity [38] and humidity [39], as well as factors unique to CNT elastomers, such as chemical effects [40] and particle size effects [41]. One source of undesirable resistance repeatability is manifested with the presence of hysteresis [42], which has been suggested to indicate the degradation of the CNT elastomer while in use. It is hypothesized the as least some of the challenges related to stability and repeatability can be addressed with improved CNT elastomer processing.

3.7 Factors Affecting CNT Elastomer Stability and Repeatability

There are several factors that can be controlled, or otherwise influenced, at the elastomer compounding stage that may improve the stability and repeatability of CNT elastomer response.

3.7.1 Polymer Matrix

Proper selection of the polymer matrix optimized for a particular characteristic might reduce stability challenges associated with CNT elastomers. Viscosity of the polymer matrix might be selected based upon the type of compounding force used [23]. For example, relatively inviscid polymers can be selected where a high-speed shear mixer is intended for compounding, whereas relatively viscous polymers might be better suited for sonication dispersion. Similarly, selection of a polymer with an appropriate molecular weight can be considered to facilitate CNT tunneling in polymers with low weight-faction CNT loads. Further, compounding fillers can be selected to control moisture content [41], which has been shown to cause considerable variation in the measured resistance of unstrained structural cement applications [43].

3.7.2 CNT Particles

Several aspects of CNT particles can affect the stability and repeatability characteristics of CNT elastomers. The size, shape, and aspect ratio of CNT particles have been noted as variables that can affect the measured resistance of a CNT elastomer [23]. Molecular functionalization, or the process of adding new capabilities or properties to a material by changing the surface chemistry of the material, can be applied to CNT, such as to improve CNT dispersion in an elastomer matrix [13, 44] or improve conductivity [45]. Further, the type of CNT used, such as single-walled carbon nanotube (SWCNT), multi-walled carbon nanotube (MWCNT), or thin-walled carbon nanotubes (TWCNT), can affect CNT elastomer performance. For example, several studies have suggested CNT elastomers can demonstrate piezo-electric characteristics with the use of MWCNT particles compounded into an elastomer matrix [46, 47].

3.7.3 Compounding Variables

The methods of CNT dispersion can also play a significant role in the stability and repeatability of CNT elastomers. CNT loading of an elastomer, or the amount of CNT particles required to achieve a percolation threshold, can adjust the conductivity of a CNT elastomer. For example, increasing CNT loading can potentially improve overall repeatability by increasing the volume fraction of CNT in contact with the elastomer matrix. However, additional CNT loading will also increase bulk stiffness of the elastomer, such as to reduce sensitivity of the CNT elastomer to displacement. The mechanical compounding process can also affect stability and repeatability. For example, two common compounding techniques, shear mixing and sonication, induce CNT dispersion in substantially different ways. The effects of mixing speed, mixing time, and other variables have been reported to impact the electrical properties of the CNT elastomer [23]. Curiously, curing temperature and mixing conditions have also been identified as significant variables influencing the formation of the CNT conducting network [48], which could affect the stability and repeatability of CNT elastomer materials.

3.8 Conclusions

CNT elastomers are materials with interesting electrical properties that offer many opportunities for sensor applications. However, despite extensive research and a long history of publications, few (if any) commercial products utilize CNT elastomers as a component in their construction. Two characteristics of CNT elastomers, the stability of electrical resistance response and the repeatability of electrical resistance response in elastomers subjected to dynamic loading, have been identified as potentially limiting factors to the use of CNT elastomers in sensor applications. It is hypothesized that improved control of the CNT elastomer compounding process could play a role in increasing the stability and repeatability of CNT elastomer material response.