# Carbon Allotropes/Epoxy Nanocomposites as Capacitive Energy Storage/Harvesting Systems
## 3 Results and Discussion
### Thermal and Mechanical Characterization
The systematic increase in the tensile modulus values is attributed to the intrinsic stiffness and rigidness of the nanoparticles and to the interfacial bonding between the matrix and the nanoinclusions, which affects the effectiveness of the load transfer from the polymer matrix to nanofillers [26; 27]. It can be seen that the addition of very small amounts of carbon nanotubes, with very large aspect ratio and stiffness, in the polymer matrix leads to the largest modulus enhancement of all examined systems. In the case of nanodiamond-filled nanocomposites, the Young's modulus increases until the 5 phr specimen, where it obtains the maximum value and then diminishes significantly, indicating that a higher content of nanodiamonds favors their aggregation. The macroscopic properties of heterogeneous polymer nanocomposites are governed by the local stress distribution around the inclusions [28] that form a network that is able to bear the mechanical load. The tensile strength of the nanocomposites decreases with filler content for all examined systems, exhibiting lower values of strength than the neat epoxy, as depicted in Figure 3. Nanoparticles seem to act as stress raisers, which is a common problem in particulate-filled composites [29]. The nanocomposites with MWCNT and GnP nanoparticles maintained higher values of tensile strength compared to the systems with 0D nanoreinforcents. Figure 4 depicts the variation of the storage modulus and loss modulus (insets) as a function of temperature for all examined systems. The maximum values of the storage modulus follow the trend of the results for the elastic modulus. The increase of E\({}^{\prime}\) values with filler content is ascribed to the effective load transfer from the polymer matrix to the filler, as a consequence of the fine dispersion and strong adhesion of filler in the matrix. Similarly to static tests, there is a significant increase in the storage modulus values with the addition even of a low amount of MWCNTs. This behavior reflects the intrinsic stiffness of carbon nanotubes and the formation of a load-carrying network inside the polymer matrix. There are several studies in the literature [30; 31; 32] reporting a sudden increase in stiffness for similar systems and suggesting the concept of a percolation threshold for the mechanical properties of polymer nanocomposites in an attempt to explain the high ranges of mechanical properties and the level of nanoparticle networks in polymer nanocomposites. However, this does not seem to be the case in the examined systems, as theoretical and computational results [30; 31; 32] indicate that such a threshold lies further beyond the electrical threshold, which in the examined MWCNTs system is 4.71% \(w/w\)[16]. A sharp decline in the storage modulus values suggests the transition of the polymer matrix from the glassy to the rubbery state, where the nanocomposites lose their load retention  
Figure 3: (**a**) Young’s modulus; (**b**) tensile strength versus filler content for all examined nanocomposites. capacity. These step-like transitions are distinguished by the formation of peaks in the loss modulus spectra.