# Physical Hybrid of Nanographene/Carbon Nanotubes as Reinforcing Agents of NR-Based Rubber Foam
## 3 Results and Discussion
### Morphology  
To investigate the effect of the CNT/GNS hybrid on the cell structure of the NR nanocomposite foam samples, the morphology of the samples was studied using an optical microscope. Figure 2 displays images of the surface of NR/CNT/GNS foam nanocomposite samples. The qualitative investigation by optical microscopy showed that increasing the CNT/GNS hybrid content increased cell number and cell density while decreasing cell size and cell wall thickness. These changes in cell structure can be interpreted by the competition between the pressure of the released blowing agent and the rubber module that resists the growth of the bubble. The former is known as the driving force of the foaming process and the latter plays the role of inhibiting the force of the foaming process [30]. While these cavities have become more spherical in shape and closer to each other with the addition of CNT/GNS hybrid nanoparticles, they became more uniform overall. These results could be related to both the nucleation and the viscosity effect of the nanoparticles, and it is likely that the dispersed nanoparticles may act as nucleating sites that facilitate the bubbling process, consequently leading to smaller cells [30].  
Figure 2: Cell morphology of NR/CNT/GNS nanocomposite foams with different CNT/GNS hybrid content: (**a**) 0 phr, (**b**) 0.1 phr, (**c**) 0.25 phr, (**d**) 0.5 phr, (**e**) 1 phr, (**f**) 1.5 phr, and (**g**) 2 phr.  
Figure 3 shows the quantitative result of the measured inner cell diameter (or the cell size) of the NR/CNT/GNS foams prepared with various hybrid nanofiller contents as histograms. The cell diameters ranged from 200 to 1000 \(\upmu\)m, depending on CNT/GNS loading. The cell size distribution curve's peak in the S0 sample was at 610 \(\upmu\)m, but it shifted toward lower values with an increase in the CNT/GNS hybrid content (i.e., 110 \(\upmu\)m for the S2 sample). Moreover, the cell size distribution of the S20 sample had a much narrower distribution compared to the S0 sample. The distribution of the NR/CNT/GNS foam cells' inner diameters was quantified by calculating the polydispersity index (PDI), based on Equation (2) [31]:  
\[\textbf{PDI}=\frac{\textbf{D}_{w}}{\textbf{D}_{n}} \tag{2}\]  
where \(\textbf{D}_{n}=\frac{\sum\textbf{n}_{i}\textbf{D}_{i}}{\frac{1}{n}\textbf{n}_ {i}}\) and \(\textbf{D}_{w}=\frac{\sum\textbf{n}_{i}\textbf{D}_{i}^{2}}{\sum\textbf{n}_{i} \textbf{D}_{i}}\) are the number and the weighted average of the cell size, respectively.  
The PDI value close to unity indicates a very uniform cell size distribution [31], which is essential for developing a product with isotropic mechanical properties. Figure 3 shows that greater uniformity in the cellular structure of the NR/CNT/GNS foams can be obtained by increasing CNT/GNS hybrid content. As previously noted, the NR foam compound's initial modulus and cure rate increased by adding higher amounts of CNT/GNS hybrid nanofiller. In other words, the inhibiting force, which resists the foam cells' expansion, has increased by increasing the hybrid nanofiller loading, while the total amount of expansion is the same. Thus, the average cell size decreased, and the cell size distribution became narrower.  
Figure 4 shows the relationship between the hybrid particle content and the average cell size. Additionally, t a significant reduction in the average cell size in NR foams  
Figure 3: Cell size distribution of NR/CNT/GNS nanocomposite foams.  
reinforced by hybrid nanoparticles was observed. As mentioned, these results could be related to both the nucleation and the viscosity boosting effect of the nanoparticles. Overall, it was clear that with the addition of the hybrid nanoparticles, the cell size gradually decreased from 800 \(\upmu\)m to 80 \(\upmu\)m.