## 4 Self-Sensing Geosynthetics
### Intrinsically Conductive-Polymer-Based Geosynthetics  
Intrinsically conductive polymers are \(\pi\)-conjugated systems, meaning they feature alternating single and double bonds in their chemical structures [170; 171; 172]. The backbone of these polymers is composed of \(sp^{2}\)-hybridised carbon atoms. Consequently, one valence electron resides in the \(p_{z}\) orbital orthogonal to the three sigma bonds. Therefore, the valence electrons are delocalised and exhibit high mobility. Indeed, in intrinsically conductive polymers, this mobility is a charge transport mechanism. However, to enhance the polymer conductivity and charge transfer, the structure must be disrupted by doping to either add or remove an electron [173]. Doping changes polymer conductivity from insulating to metallic [174] by either extracting electrons from the valence band highest occupied molecular orbital (HOMO)--i.e., oxidation--or moving electrons to the conduction band lowest unoccupied molecular orbital (LUMO)--i.e., reduction. To establish a positive molecular charge, the polymer backbone can be treated with electron-deficient species. Owing to hole formation, the polymer chain is converted to \(p\)-type, and electron removal from the main chain is known as \(p\)-doping or oxidation [175]. Alternatively, to induce a negative molecular charge, the polymer backbone can be improved with electron-rich species. Adding electrons is known as reduction or \(n\)-doping [175], and the electrons convert the polymer chain to \(n\)-type. Both \(n\)- and \(p\)-doping markedly boost conjugated polymer conductivity, which is determined by the dopant characteristics and concentration and the doping time. Dopants may include smaller cations and anions such as Na+, Cl-, and ClO-4 and large polymeric species such as polystyrene and polyvinyl sulfonates. Owing to their high conductivities, doped intrinsically conductive polymers are suitable for various smart geosynthetics and electronics applications. However, their relatively high production cost should be considered. The intrinsically conductive polymer structure and charge transport mechanism due to delocalised \(\pi\) electrons are illustrated in Figure 21.  
Moreover, dopant properties considerably impact conductive polymer characteristics. Large dopant molecules can change the polymer physical properties and surface topography and increase the polymer density. Furthermore, large dopant molecules strongly attach to conjugated polymers and are more difficult to leach than smaller dopant molecules, which may easily leach. Therefore, small dopant molecules are suitable for applications requiring rapid detachment and attachment [176]. Consequently, sensitivity to environmental ionic species increases with decreasing dopant molecular size. Furthermore, the dopant molecular size affects the doped polymer solubility. For example, with increasing dopant chain length, polyaniline (PANI) solubility increases, thereby facilitating solution processing [177]. However, increasing the dopant chain length by increasing the \(d\)-spacing  
Figure 20: Schematic of CNF/glass-fibre-reinforced polymer and its piezoresistive mechanisms. and inter-chain separation reduces the polymer crystallinity. Moreover, the polymer conductivity increased with increasing dopant concentration until saturation. Finally, because doping is a reversible physical process, it can be followed by de-doping to revert to the original polymer exhibiting all its original characteristics. In these polymers, temperature and conductivity are inextricably linked. In addition, highly doped polymers are less vulnerable to temperature-induced conductivity changes than less-doped polymers [178]. Reversible doping/de-doping and the polymer conductivity dependence on concentration, temperature, and dopant molecular size have all been used to design sensitive components for sensing stress and various chemical species and for electrochemical capacitors, electrochromic displays, energy devices, and smart windows [170; 179].