**2. Conductive polymer composites. PTC effect**

Unlike intrinsically conductive polymers (see [13–15], such as polyacetylene, polyaniline (PANI), polythiophene, polypyrrole (PPy), poly(p-phenylene vinylene), or PEDOT:PSS), the polymer composites, consisting of an electrically insulating polymer and a conductive phase, are much easier to prepare and have high chemical stability, and their electrical conductivity can be varied simply, within very wide limits, by changing the nature and the concentration of the conductive filler, changing the nature of the polymers used as a matrix, as well as by changing some technological factors (such as mixing the components, heat treatments, and/or irradiation, pressure). Common polymers are known to have intrinsically poor electrical and thermal conductivity, making them useful as insulating materials. The addition of a conductive filler to the polymer matrix gradually worsens the dielectric properties of the material, which at some point, quite suddenly, becomes conductive; the concentration value at which the insulator-conductor transition occurs is called the percolation threshold. Compounding polymers with conductive fillers has become a method to obtain a wide range of materials with different electrical properties, including those generically called conductive polymer composites. In recent years, there has also been a growing interest in making composite materials using intrinsically conductive polymers, as well as integrating them into a range of electronic devices, including sensors (see [16]).

#### **2.1 Polymeric matrices**

All categories of polymers—thermoplastics, elastomers, thermosets, synthetic polymers, or natural polymers are suitable in principle for making polymer matrices, as can be seen from the examples presented below. Elastomeric conductive composites (ECCs) based on carbon fillers are very attractive and play a significant role in the field of smart sensors due to their excellent flexibility, wide sensitivity spectrum, as well as fast response to external stimuli.

A current trend is also the growing interest in biodegradable matrices such as polylactic acid (PLA), giving to the products that incorporate them a lower environmental impact than synthetic polymers.

## **2.2 Fillers**

Combining polymers with electrically and/or thermally conductive nanofillers allows the development of polymer nanocomposites in the form of thin, light, flexible, wearable films that present interesting electrical and thermal properties for various technical, environmental, and biomedical applications. As conductive fillers for obtaining composite films applicable as sensors in general, various structures are mentioned, as for example Au, Ag nanoparticles, Au or Ag nanowires [2, 8], Ni microparticles [17], and carbon-based materials (e.g., graphene or reduced graphene oxide (rGO) [18], CNT [19] and carbon black (CB)[10], and ceramics (e.g., Mn1.71Ni0.45Co0.15Cu0.45Zn0.24O4 [20]) often together with newer manufacturing technologies such as additive manufacturing (AM) through 3D printing [21].

The appearance of electrical conductivity at the percolation threshold can be explained by a simple intuitive model, in which it is considered that each filler particle is in physical contact with the neighboring ones, thus achieving electrical contact throughout the matrix. Another mechanism for achieving conduction, which explains reaching the percolation threshold at concentrations lower than those corresponding to the physical overlap of the particles, is that of tunneling; in this case, the distance between two filler particles must be max. 1.8 nm [2, 22].

For a given polymer, the electrical resistivity of the resulting composites can be varied over a very wide range by changing the nature and concentration of the filler, as well as the production technology [23]. As the concentration of conductive filler in the polymer matrix increases, the resistivity of the material gradually decreases, reaching a critical concentration (φc) at which the insulator/conductor transition is observed for the studied composite. The value φ*c* represents the conductive percolation threshold, according to the classical theory of percolation. For concentrations of the conductive phase, φ > φ*c*, the value of φ*c* can be determined with relation (1) [24, 25]:

$$\sigma \propto (\rho - \rho\_c)\mathbf{t} \tag{1}$$

where φ is the electrical conductivity of the composite and t is an exponent that depends mainly on the size of the conductive network in the composite [10].

The decrease in resistivity and the appearance of conductive properties is determined by the formation of conductive networks inside the polymer, in which conductive particles are in electrical contact, allowing the passage of electric current from one conductive particle to another (through tunneling or jumping mechanisms). The maximum distance between particles in such a conductive path has been estimated to be several nanometers [26]. Not all conductive particles dispersed in the polymer participate in the transport of electrical charges through the sample (i.e., they are not effectively involved in conductive paths), but only a considerably smaller fraction than the filler fraction contained in the polymer (**Figure 1**). For example, in the case of CNT, it was estimated that approximately 3.3% of the particles are involved in the effective conduction of the current (being part of the so-called backbone of the electrical conduction (**Figure 2**), in fact, a conductive path similar to those in **Figure 1**, for spherical particles of CB).

The composite materials, also called hybrid materials, are biphasic or multiphase systems, which present special properties, resulting from the synergistic combination of components [1, 27]. Conductive polymer composites have, due to their remarkable properties (e.g., low density, high mechanical strength and corrosion resistance, and controllable electrical conductivity over a wide range, up to values close to the conductivity of metals), numerous applications (e.g., space constructions, automotive industry, sporting goods, electronic devices, electromagnetic shielding, energy storage (safe batteries), overcurrent protections (resettable fuses), filters, and sensors [28])

*Novel PTC Composites for Temperature Sensors (and Related Applications) DOI: http://dx.doi.org/10.5772/intechopen.110358*

#### **Figure 1.**

*Possible distribution of small spherical particles (e.g., CB) within a polymer composite: 1—electrodes; 2—polymer matrix: 3—conductive particles (CB). The red arrows indicate the conductive paths which enable the electrons to across the sample. E* ! *= electric field used to reveal the conductivity.*

#### **Figure 2.**

*Possible CNT distribution within a 2D CNT/polymer composite (e.g., a thin film sample): 1—electrodes; 2 polymer matrix; 3—conductive path (backbone of conductive network); 4—zero-current branches; 5—balanced branches; 6—isolated CNT clusters; E* ! *= electric field used to reveal the conductivity (Adaptation after [26]).*

where they have successfully replaced "traditional" materials, such as ceramics or metals [10].

The influence of the filler type, polymer matrices, and dispersion methods on the electrical properties of the resulting CPCs has been reviewed by different authors (see [29] for segregated CPCs, [11] for control methods of electrical properties, and [30] for correlating CPC electrical properties with phase morphologies such as phase segregation or co-continuity [9]).

An effective way to reduce the conductive filler content (or percolation threshold) is to make segregated composites (s-CPCs). The polymer matrix can consist of two thermoplastic, immiscible polymers, e.g., polystyrene (PS) and polypropylene (PP) [31], with the conductive filler (e.g., a carbonaceous material) being preferentially distributed in one of the polymers (PS), and the condition φ<sup>c</sup> for the respective polymer is achieved (**Figure 3**). The conductive paths are thus formed at the interface of the two polymers, the composite as a whole becoming conductive at very low concentrations of the conductive filler, compared to homogeneous composites [29]. Other possibilities for obtaining segregated CPCs are shown in **Figure 3**. Comparing

**Figure 3.**

*Three simple ways to produce segregated CPCs: a) melt blending; b) latex technique; and c) dry or solvent mixing (adapted from [29]).*

**Figures 1** and **3**, it intuitively results that the concentration required to create a network of conductive paths is substantially lower in the case of s-CPC.

Ideally, carbonaceous materials have a structure consisting exclusively of carbon, which can be found in various states of sp<sup>n</sup> hybridization, n = 1–3. We must also take into account the fact that, often, there may be heteroatoms (mainly hydrogen or other, oxygen, sulfur, and nitrogen), especially at the edges of extended carbon domains. In general, carbonaceous materials can be characterized by the state of hybridization, as well as by the ratio of the number of hydrogen to carbon atoms (H/C) [32]. Among the carbonaceous materials, those that have potential for use in composites are those that present electrical conductivity. For the sp3 hybridization state, the typical representative, diamond, does not conduct electricity because all the valence shell electrons move in well-defined directions along the σ bonds. In the case of the other types of hybridization, sp. and sp<sup>2</sup> , the electrons in the unhybridized *p*-orbitals have greater freedom of movement and, in the case of extended conjugation effects, can cause the electrical conductivity of the respective substances to appear. It follows that the appearance of electrical conduction will occur in the case of large molecules (polymers or oligomers), in which there may be a large number of conjugated bonds, leading to the formation of molecular orbitals extended on a molecular scale, in which *p* electrons can move practically free.

Sp-hybridized compounds that fall under this condition are polyynes (or carbynes), which formally originate from the polymerization of acetylene and are a linear allotropic form of carbon. However, these compounds currently have a rather theoretical importance, not existing in nature, and their synthesis and stability remain
