# Physical Hybrid of Nanographene/Carbon Nanotubes as Reinforcing Agents of NR-Based Rubber Foam
## 4 Conclusions  
In this study, the effect of incorporating GNS/CNT on the curing behavior, the foam morphology, and the mechanical and thermal properties of the NR foams was investigated. The curing characteristics showed that the addition of a hybrid nanofiller in the NR reduced the cure start time (T\({}_{\text{s}}\)) and sped up the overall curing reaction by limiting the polymer chains' mobility. It was also observed that the GNS/CNT incorporation resulted in greater hardness and lower resilience of the NR composites, which was attributed to the good interaction between the polymer matrix and the physical hybrid of CNT/GNS. The compression test results showed that the addition of the nanomaterials increased the compressive strength of all formulations. The CNT/GNS hybrid nanofillers provided the natural rubber with better resistance to large deformation when exposed to repeated stress cycles (fatigue). The DMTA results revealed that the addition of CNT/GNS nanohybrid into the natural rubber matrix provided higher storage modulus in the foam samples due to the nanohybrid material's inherent high hardness and modulus. It was also shown that the peak of the loss factor in nanocomposites decreased with increasing nanofiller content and slightly increased the glass transition temperature (T\({}_{\text{g}}\)). The TGA study indicated that the addition of GNS/CNT hybrid caused higher thermal stability in the NR foams by acting as a delaying agent in the degradation of the rubber matrix. With the increase in CNT/GNS content, the residual weight increased by up to 10%. The morphological results from optical microscopy also illustrated that increasing the CNT/GNS hybrid nanofiller content produced more cells with smaller cell sizes and cell wall thickness. Samples with lower nanofiller content have elongated and irregular cells. Contrarily, formulations with higher GNS/CNT hybrid loading levels resulted in cells that were more spherical and closer in size to each other with an overall improved uniformity and narrower distribution. The following are available online at [[https://www.mdpi.com/article/10.3390/polym13142346/s1](https://www.mdpi.com/article/10.3390/polym13142346/s1)]([https://www.mdpi.com/article/10.3390/polym13142346/s1](https://www.mdpi.com/article/10.3390/polym13142346/s1)). Figure S1. Evolution of stress-strain hysteresis loops for NR nanocomposite foam samples with 0 phr of physical hybrid of CNT/GNS at different cycles. Figure S2. Evolution of stress-strain hysteresis loops for NR nanocomposite foam samples with 0.1 phr of physical hybrid of CNT/GNS at different cycles. Figure S3. Evolution of stress-strain hysteresis loops for NR nanocomposite foam samples with 0.25 phr of physical hybrid of CNT/GNS at different cycles. Figure S4. Evolution of stress-strain hysteresis loops for NR nanocomposite foam samples with 0.5 phr of physical hybrid of CNT/GNS at different cycles. Figure S5. Evolution of stress-strain hysteresis loops for NR nanocomposite foam samples with 0.5 phr of physical hybrid of CNT/GNS at different cycles. Figure S5. Evolution of stress-strain hysteresis loops for NR nanocomposite foam samples with 1 phr of physical hybrid of CNT/GNS at different cycles. Figure S6. Evolution of stress-strain hysteresis loops for NR nanocomposite foam samples with 1.5 phr of physical hybrid of CNT/GNS at different cycles. Figure S7. Evolution of stress-strain hysteresis loops for NR nanocomposite foam samples with 2 phr of physical hybrid of CNT/GNS at different cycles. Conceptualization, E.E. and A.V. ; methodology, E.E. and A.V. ; software, S.S., E.E. and A.V. ; validation, E.E., A.V. and T.H.M. ; formal analysis, S.S.; investigation, S.S. and E.E. ; resources, E.E. and A.V. ; data curation, S.S.; writing--original draft preparation, S.S.; writing--review and editing, E.E., A.V., G.N. and T.H.M. ; visualization, E.E., A.V. and T.H.M. ; supervision, E.E., A.V. and E.S., G.N. ; project administration, E.E., S.S. and A.V. ; funding acquisition, E.E., G.N. and E.S.