# Physical Hybrid of Nanographene/Carbon Nanotubes as Reinforcing Agents of NR-Based Rubber Foam
## 3 Results and Discussion
### Compression Test  
The introduction of nano reinforcement leads to a noticeable change in the mechanical behavior of elastomeric materials under cyclic force. Figure 7 shows the effect of CNT/GNS hybrid content on the stress-strain behavior of the NR/CNT/GNS foams during a cyclic compression test. We found that the modulus and strength (i.e., stress at 50% strain) of the foams increased steadily with increasing CNT/GNS hybrid content in the NR/CNT/GNS foams. The extremely high mechanical strength of the individual CNTs provides superstrong engineering materials that can sustain stress perfectly. It is well established that nanomaterials have a high reinforcing effect in increasing the modulus of polymer foams [18]. The rubber foams can be considered as a composite of NR/CNT/GNS matrix with a dispersed gas cell phase. Therefore, the foam's compressive stress-strain modulus depends on the mechanical properties of both individual phases and morphology. On the other hand, the moduli of the foams all increased because the interconnected 3-D framework of the CNT/GNS hybrid can effectively resist deformation and withstand force. The formation of the 3-D network of CNT/GNS in a solid polymer matrix has been reported before [24; 32]. In addition, the increase in the modulus was expected due to the hardening effect of the CNT/GNS hybrid as a consequence of its stiff nature. As can be noted from Figure 7, increasing the nanofillers' content from 0 to 2 phr improved the compression behavior and increased the stress at 50% strain from 150 kPa to 440 kPa, which are related to higher modulus, smaller cell size, and robust matrices. Thus, an increased CNT/GNS hybrid content can increase the modulus of the foam via the two  
Figure 6: The variation in resilience in NR foams containing different content of GNS/CNT hybrid. possible mechanisms; the matrices' modulus increased due to the reinforcing effect of the nanofillers caused by a 3-D network of inherently stiff nanomaterial, and reduction of the foam's cell size. Figure 7: The effect of CNT/GNS hybrid content on the stress–strain behavior of the NR/CNT/GNS foams: (**a**) first cycle, (**b**) second cycle, and (**c**) third cycle. Fatigue behavior is associated with a decrease in strength under cyclic loads, localized and progressive damage to the polymer structure due to cyclic loading. This may cause fractures or incomplete cracks after a sufficient number of compressive cycles. The S2 sample displayed the best cyclic stability, which indicates that the CNT/GNS hybrid can change the dynamic molecular properties of NR and enhances its resistance to large deformation due to repeated stress. Moreover, the nanofillers could act as nanobarriers to crack propagation, thereby increasing matrix fatigue resistance. The evolution of the stress-strain hysteresis loop of each NR/CNT/GNS nanocomposite foam samples with various content of CNT/GNS is provided in the supporting information (Figures S1 to S7) for better comparison. The significant difference in the width of the loops in Figures S1 to S7 proves the important role of the hybrid filler content as a parameter to control rubber deformation and ratcheting progress over stress cycles. Over the first cycle, the rubber blend S0 sample with no filler content presented a wide hysteresis loop indicative of dramatic plastic deformation. The hysteresis loop in samples containing a higher amount of CNT/GNS became narrower than that of the pristine foam. This means that as the magnitude of CNT/GNS content increases, stress-strain hysteresis loops gain higher stiffness and modulus of elasticity. By progressing loading cycles, the area within the individual hysteresis loops gradually drops, indicating decay in ratcheting rate in composite samples. The width of the loop or the area between the loading and unloading cycle corresponds to dissipated energy in tested samples over the loading/unloading cycles leading to heat build-up in specimens [33].