## 3 Pavement Self-Sensing Technology Based on Conductive Polymer Composite Materials
### Preparation Methods of Conductive Polymer Composite Materials  
One of the pursuits of composite material research is determining ways to use the lowest possible content of conductive nanoparticles to obtain high mechanical strength and high electrical conductivity. On the one hand, good filler/polymer interfacial interactions are very important for load transfers between the filler and the polymer matrix; on the other hand, the dispersion of conductive fillers in the polymer matrix has great influence on the mechanical properties, electrical conductivity, and piezoresistive properties of composites. The more uniform and better dispersed the conductive carbon filler is in the system, the better the mechanical properties of the polymer matrix will be. However, too high a content of a conductive carbon filler can cause agglomeration, forming stress concentration points, which become defects and reduce the tensile strength of the material. However, it is difficult to realize the uniform dispersion of conductive fillers in the polymer matrix. The dispersion of conductive fillers in the polymer matrix largely depends on conditions such as its viscosity, dispersion method, shear stress, and mixing time.  
Currently, commonly used methods for the preparation of polymer-based conductive composites include mechanical methods, liquid-phase blending composite methods, chemical modification methods, solid-phase eutectic composite methods, in situ polymerization methods, mixed curing agent-assisted stacking methods, sol-gel methods, etc. The degree of dispersion of conductive fillers with large specific surface areas in polymers can be facilitated by the above methods. Mechanical methods, generally utilizing ultrasonic vibration and magnetic or mechanical stirring, can assist in the dispersion of nanoparticles in dilute solutions. Liquid-phase co-compounding methods are generally aided by mixing the conductive particles in a dilute solution, which facilitates the acceleration of the wetting and penetration processes and reduces the strength of the agglomerates. In addition, the dispersion of the conductive particles in the polymer can also be facilitated by the introduction of chemical modification groups on the surface of the conductive particles that promote compatibility or linkage branching groups with the polymer, as described in Section 3.1.2.  
[PERSON] et al. [113] found that the conductivity of CNT-filled nanocomposites prepared by different dispersion processes may differ by two orders of magnitude at the same CNT concentration. [PERSON] et al. [114] synthesized epoxy resin/CNT nanocomposites using various methods to investigate the effects of dispersion methods on their properties. Firstly, CNTs were subjected to three methods: untreated, activated in HNO\({}_{3}\), and activated with coupling agents in HNO\({}_{3}\). Then, CNTs were dispersed and mixed into the resin using asymmetric centrifugation, ultrasound, and manual mixing methods. The results show that ultrasound combined with double asymmetric centrifugation was more effective for unpretreated CNTs. Dispersion with ultrasound is no longer recommended for CNTs pretreated with HNO\({}_{3}\), as ultrasound may destroy the pretreated CNTs. [PERSON] et al. [115] dispersed mechanical mixtures of conductive hybrid materials and particles separately using a three-roll mill and subsequently dispersed them proportionally into epoxy resins, which were degassed and cured under high pressure (10 MPa). [PERSON] et al. [116] utilized a N, N-dimethylformamide (DMF) solvent to promote the dispersion of CNTs in the polymer matrix material PVDF and molded the composite material on a glass substrate to introduce a conductive silver paste, as shown in Figure 12. [PERSON] et al. [117] prepared conductive composites by depositing MWCNT on the surface of electrically insulating glass fibers by electrophoresis or impregnation, followed by embedding single or multiple MWCNT glass fibers into an epoxy matrix.