## 3 Composite Matrices and Applications  
### Polymers  
Polymers are macromolecules composed of repeating monomers linked by covalent bonds. Their widespread application, customizable properties, and easy manufacturing process have made them indispensable in modern society [54]. However, there are several limitations to how polymers can be utilized in some sectors because of their lack of ductility and electrical and thermal conductivity. Researchers have been investigating ways to improve mechanical properties, such as the incorporation of mechanically superior additives [55, 56, 57]. Traditionally, carbon and glass fibers have been used as fillers for polymers to enhance their mechanical performance [58, 59, 60, 61, 62]. Although these conventional methods improve the mechanical properties of the polymers, CNT-based polymers have shown exceptional potential, as seen in Figure 3. MWCNTs with consistent alignment, orientation, and surface defects can also be used as fillers, with the developed nanocomposites showcasing exceptional mechanical performance and improved thermal conductivity, which are particularly beneficial for automobile tires and other transportation applications, as they result in enhanced fuel efficiency and improved fatigue resistance [63, 64, 65, 66, 67]. A study led by [PERSON] et al. investigated the properties of carbon nanotube (CNT)/polyppropylene (PP) composites and the improvement of electromagnetic interference (EMI) shielding effectiveness and several mechanical properties [68]. The author used the solid-phase molding method to restrict the diffusion of CNTs and maintained the polymer in a solid phase during the molding process. Elevated pressure and temperature were used to fabricate the CNT/PP composites with a segregated structure [68]. Figure 2: Multi-walled CNTs (MWCNTs) [51]. PP granules were mixed with water/ethanol, and CNTs were dispersed in ethanol with ultrasonication. Then, the two mixtures were combined and further ultrasonicated. After filtration, the CNT-coated PP granules were dried and compression-molded. This process resulted in CNT/PP-I. For comparison, CNT-coated PP granules were molded under regular solid-phase conditions (CNT/PP-II) and regular molding conditions (CNT/PP-III). Additionally, CNT/PP composites were prepared using solution mixing (CNT/PP-IV) and melt mixing (CNT/PP-V). The researchers found that in CNT/PP-I, carbon nanotube paths are more tightly packed compared to CNT/PP-II, which suggests that higher pressure and temperature help create better conductive CNT networks in the solid-phase molded composites. However, when the molding temperature surpasses the melting point of PP, some CNTs start to diffuse into the melted polymer regions due to lower viscosity, thus disrupting the segregated structure [50; 68]. Among all the different compositions, CNT/PP-I shows the highest electrical conductivity. For instance, at 2.0 wt%, CNT, CNT/PP-I, and CNT/PP-II reach an electrical conductivity of 70 S/m, which is significantly higher than CNT/PP-III and CNT/PP-IV. It is also much greater than CNT/PP-IV by 7 orders of magnitude, as seen in Figure 4.  
Compared to CNT/PP-II with a regular segregated structure, CNT/PP-I demonstrates significant enhancements in compressive, tensile, and flexural properties, including a 133% increase in compressive strength and a 65% increase in modulus (Figure 5). These improvements were caused by the elimination of voids and compacting of the segregated structure in CNT/PP-I. Overall, CNT/PP-I shows superior mechanical properties compared to the other CNT/PP composites. As mentioned previously, one of the major drawbacks of polymers is their lack of thermal conductivity. However, some thermoplastic polymers are able to address this challenge to some extent. Polyethylene, polypropylene, polyvinyl chloride, polystyrene, polyamides, polyesters, and polyurethanes are some known examples of such polymers [69].