## 3 Composite Matrices and Applications
### Polymers
Figure 3: Ashby plot of Young’s modulus vs. tensile strength that compares the mechanical properties of conventional polymer composites, including glass fiber-reinforced plastic (GFRP) and carbon fiber-reinforced plastic (CFRP), with CNTs or graphene-based polymer composites [67]. Polyvinylidene fluoride (PVDF) is also a thermoplastic polymer known for its thermal and chemical stability, thus being an excellent material for the chemical industry and electronics [70; 71; 72; 73]. It is also environmentally friendly and exhibits superior mechanical properties, thus attracting attention from the industrial and research community [71; 72; 73]. Still, its low thermal conductivity makes it undesirable in several applications [68]. To address this limitation, numerous thermal conductive fillers have been tested that resulted in increased thermal conductivity [74; 75; 76; 77; 78]. Figure 4: Electrical conductivity of PP composites as a function of CNTs weight fraction [68]. Figure 5: Flexural, tensile, and compressive properties of CNT/PP composites at 1.0 wt% CNT loading [68]. Recently, [PERSON] et al. investigated if the incorporation of MWCNTs and graphene (GE) in PVDF can improve its thermal conductivity [79]. The study successfully demonstrated a significant increase in the thermal conductivity of PVDF composites, which was achieved through a well-dispersed network of MWCNTs and graphene. Different types of MWCNTs were studied: without acidification (p-MWCNTs), with acidification (a-MWCNTs), and with both acidification and silanization (s-MWCNTs). It was found that s-MWCNTs were more uniformly dispersed compared to a-MWCNTs, and both were more efficiently dispersed than p-MWCNTs [79]. Initially, MWCNTs and GE aggregates were observed in the cross-sections, leading to poor dispersion within the PVDF matrix. However, when functionalized MWCNTs were introduced, the dispersion improved significantly, as seen in the scanning electron microscopy (SEM) images of Figure 6. Acidified MWCNTs combined with GE showed better dispersibility compared to non-functionalized MWCNTs. Silanization further improved the interfacial properties, enhancing compatibility between MWCNTs/GE and the PVDF matrix and promoting better dispersibility. Additionally, the filler content directly affected dispersion: increasing filler content improved contact between fillers and PVDF up to a point where excessive filler content led to aggregation, causing cracks and voids in the composite. Overall, functionalized MWCNTs played a crucial role in enhancing dispersion and improving thermal conductivity in the composite material [79]. The thermal conductivity and enhancement rates of PVDF composites containing either single or hybrid fillers of multi-walled carbon nanotubes (MWCNTs) and graphite (GE) were compared. GE/PVDF composites show the least enhancement in thermal conductivity due to the agglomeration of GE lamellae, reducing the contact area between fillers and the matrix. However, composites with both MWCNTs and GE exhibit significantly improved thermal conductivity. Specifically, p-MWCNTs/GE/PVDF with a 5 wt% loading demonstrates a 74.17% enhancement, outperforming composites with single fillers (see Figure 7). Modification of MWCNTs with the vinyl triethoxysilane (YDH-151) further enhances thermal conductivity, particularly with silanized MWCNTs and GE, reaching  
Figure 6: SEM images showing the dispersion states of different fillers in PVDF composites with the same content of 10 wt%. (**a**) Pure PVDF, (**b**) MWCNTs/PVDF, (**c**) GE/PVDF, (**d**) p-MWCNTs/GE/PVDF, (**e**) a-MWCNTs/GE/PVDF, and (**f**) s-MWCNTs/GE/PVDF) [79]. 1.46 W/(m\(\cdot\)K) with a 10 wt% loading and a remarkable 711.1% enhancement. These results highlight the importance of compatibility in enhancing thermal conductivity, with the s-MWCNTs/GE/PVDF composite showing superior performance compared to non-functionalized MWCNTs/GE/PVDF and p-MWCNTs/GE/PVDF composites [79]. The electromagnetic properties of PVDF have also been tailored with the incorporation of carbon nanotubes.