# Physical Hybrid of Nanographene/Carbon Nanotubes as Reinforcing Agents of NR-Based Rubber Foam
## 1 Introduction  
In recent decades, polymer nanocomposites have received a great deal of attention, since adding only a small fraction of nanofillers to the polymer matrix significantly improves their properties [1]. This concept has been successfully applied to thermoplastics [2], elastomers and thermoset elastomers [3]. Today, a wide array of different nanofilers are available, such as nanolox [4], carbon nanotubes (CNTs) [5], graphene nanosheets (GNSs) [6, 7], nano cellulose [8], and halloysite nanotubes [9], which are frequently used in polymer matrices as reinforcing agents. CNTs show many desirable properties related to flexibility, high strength, ability to withstand transverse and torsional displacements, tensile strength and compressive strength without breaking [10]. GNSs are recognizedas one of the most effective nanomaterials because of their high potential for mechanical-property improvement due to the combination of enormous specific surface area, strong filler-matrix adhesion, and the extraordinary mechanical properties of the sp2 carbon bonding network [11]. Despite having numerous extraordinary properties, the commercialization of these two carbon nanomaterial allotropes is still limited due to their tendency to agglomerate (bundle and layer forming behavior for CNT and GNS, respectively) [12]. Rubber foam is an interesting material, as it can be used in a diverse range of applications, such as insulation, packaging, and cushioning. The foam can be fabricated according to specific requirements using different types of polymers and blowing agents. A foam's properties are controlled by its density, cell structure, and the chemistry of the raw materials used [13]. Rubber foams can be classified into two groups based on their cell structure: open-cell foams [14; 15; 16], where the gas phases (cells) are linked to each other, and closed-cell foams, in which the cells have isolated or separated cell walls [17]. The incorporation of various types of nanofiller has been a commonly used measure to improve the mechanical, thermal, and electrical properties of rubber foams [18]. However, there are still challenges in achieving a segregated filler network wherein the nanofillers are arranged to produce a 3-D network, which percolates the polymer volume. Recently, the combined use of nanofillers as a physical and chemical hybrid has exhibited a synergistic effect that led to outstanding enhanced performance of nanocomposites with respect to employing only one type of reinforcement [19; 20; 21]. Since the building blocks of a nanocomposite are of nanoscale, many interfaces exist between the two intermixed solid reinforcement phases and the superior properties arise from phase interactions at these interfaces. [PERSON] et al. showed that morphology of NR-CNT composite gradually evolves from a discontinuous set of aggregated clusters to a continuous 3-D network upon the replacement of CNT by nanographene by 50 percent (50:50 hybrid nanofiller) [22]. [PERSON] et al. suggested that the synergism of CNT and GNS is derived from two main mechanisms: CNTs act as bridges between GNSs and GNSs fill the space between CNTs [23]. [PERSON] et al. presented a predictive model to quantitatively explain the synergistic behavior of CNT/GNS hybrid, which was achieved by featuring an increased number of contacts between GNSs and CNTs [24]. The knowledge of how to control the properties of rubber foams reinforced by hybrid nanofillers is of fundamental importance in order to determine the final performance and application of the product. Although several studies on NR solid nanocomposites reinforced by hybrid nanomaterial are available [22], the literature still lacks information on their foams. Thus, the novelty of this work lies in its primary objective, which is to assess the fabrication of NR nanocomposite foams reinforced by a CNT/GNS hybrid.