## 1 Introduction  
Blending different rubbers (natural or synthetic) is one of the facile and effective strategies to prepare the rubber products with a desired performance, as adjusting their chemical structure requires a lot of energy and cost to improve their properties. At present, most high-performance rubbers are multiphase/multicomponent systems, the properties of which are mainly determined by their morphology and structure. The resulting morphology can be influenced by many factors, such as blend ratio, blend composition, microstructure, and their viscosity, polarity, and mixing process [1]. Studies have found that in styrene-butadiene rubber (SBR)-based mixtures, increasing the mixing temperature leads to a finer dispersion of cyclic butylene terephthalate (CBT) oligomer compounds, which further improved the mechanical properties [2]. Hence, it is crucial to understand the relationship between their morphology/structure and properties for optimizing the performance of their final products. The molecular miscibility in most polymer blends is impossible because of the difference in surface free energy. When the phase decomposition and the combination processes [3] are in equilibrium state, the equilibrium morphology is achieved. Furthermore, the molecular weight and molecular weight difference between the two phases also affect their morphology. The smaller phases are related to similar molecular weights, and the domain size increases with the increasing molecular weight differences of the components [4]. Viscosity also affects the domain size in the same way, and the greater viscosity difference between components may result in the formation of larger domains [5; 6; 7; 8]. Natural rubber (NR), as a renewable biosynthetic polymer, is the most commonly used general rubber [9; 10; 11]. Although NR has excellent elasticity, flex resistance, and mechanical strength, its abrasion resistance and wet skid resistance is slightly poorer than other rubbers. On the other hand, SBR has excellent abrasion resistance and wet skid resistance, but its elasticity is slightly poor. Therefore, an NR/SBR blend can combine the advantages of both to improve its wear resistance, oxidative stability, and wet skid resistance and, as such, can have broad application prospects in rubber products, especially in the tire industry [12; 13]. The NR/SBR blend is also used in many other aspects, such as sealing elements [14], rubber soles [15], and the surface corrosion protection of metallic tanks [16]. Most investigations are focused on the improvement of their mechanical properties and their extensive application, but there are few in-depth scientific studies on the morphological structure evolution and nonlinear viscoelasticity of NR/SBR blends. It is well-known that NR is hardly miscible with SBR at the molecular level in most cases, and that the microstructure of SBR (including the vinyl and styrene contents) may affect the morphology of the NR/SBR blend [17; 18; 19; 20]. In previous research, the effect of domain morphology on the physical and dynamic mechanical properties of the NR/SBR or NR/BR blend and their filled compounds has been discussed preliminarily and macroscopically [21; 22; 23]. For the safe driving of the tire, it is not enough to only pay attention to their mechanical performance, and more attention should also be paid to their nonlinear viscoelastic response and morphology evolution during periodic deformation at large strain amplitude (\(\gamma\)). Rubber materials subjected to large-\(\gamma\) oscillation usually exhibit a typical nonlinear softening response, namely the Payne effect [24; 25; 26]. Storage modulus (\(G^{\prime}\)) drops with the increasing strain amplitude (\(\gamma\)) beyond the linear regime upon loading, and then partially recovers with decreasing \(\gamma\) upon unloading, while loss modulus (\(G^{\prime}\)) approaches a maximum (weak strain overshoot, WSO) or decreases synchronously with \(G^{\prime}\)[27; 28].