## 2 Results and Discussion
### Nonlinear Rheological Behavior  
In a large number of studies, the rapid decrease in modulus upon shear at large amplitudes in the nonlinear viscoelastic regime might be attributed to the destruction of the particle-particle network of the filler in the rubber matrix [36; 37; 38], the destruction of the weak polymer-filler particle network [38; 39], the desorption of rubber molecular chains [40], and the destruction of the chemical network [36; 41], etc. However, these mechanisms could hardly explain the Payne effect of NR, SBR, and NR/SBR blends. In order to investigate the effect of blend morphology on the strain (\(\gamma\)) response of NR/SBR blends, dynamic \(\gamma\)-sweeps are performed at 100 \({}^{\circ}\)C and \(\omega\) = 1 rad/s. The normalized \(G^{\prime}\)/\(G^{\prime\prime}\)0 and \(G^{\prime\prime}\)/\(G^{\prime\prime}\)0 values for the NR, SBR, and NR/SBR blends as a function of \(\gamma\) are shown in Figure 6a,b. Here, \(G^{\prime}\)0 and \(G^{\prime\prime}\)0 are the storage and loss moduli at low \(\gamma\) in the linear viscoelastic region, respectively. For immiscible blends, their nonlinear viscoelastic response is not only related  
Figure 3: TEM micrographs of (**a**) NR/SBR (75/25), (**b**) NR/SBR (50/50), and (**c**) NR/SBR (25/75) blends. to the components in the blends, but is also related to the blend morphology. Here, the hollow symbols are the experimental results, and the lines are fitted by linear numerical addition as follows [42]:  
\[G\ ^{*}_{\mathrm{blend}}=\varphi\cdot G\ ^{*}_{\mathrm{SBR}}+(1-\varphi)\cdot G\ ^{*}_{\mathrm{NR}} \tag{1}\]  
in which \(\varphi\) is the content of SBR. Figure 4: Storage moduli \(G^{\prime}\) (open symbols) and loss moduli \(G^{\prime\prime}\) (solid lines) as a function of frequency \(\omega\) at \(\gamma\) = 0.5% and 100 \({}^{\circ}\)C for the NR, SBR, and NR/SBR blends. Figure 5: (a) Storage moduli \(G^{\prime}\) and (b) loss moduli \(G^{\prime\prime}\) as a function of frequency \(\omega\) at \(\gamma\) = 0.5% and 100 \({}^{\circ}\)C for the NR/SBR blends and NR + SBR physical blends. The linear viscoelastic regime for SBR is remarkably longer than that of NR, and the linear region for NR/SBR blends extends with the increasing SBR content. Furthermore, the fitted numerical addition results may just correspond to the modulus contribution from the NR and SBR components, and the rapid decrease in dynamic moduli for NR/SBR blends from experimental results obviously occurs at the higher \(\gamma_{\text{c}}\). Compared with their numerical addition results, the extension of the linear viscoelastic regime for the NR/SBR blend may be attributed to the contribution from the heterogeneous structure in NR/SBR blends. Here, we also investigate the difference of dynamic \(\gamma\)-response for the NR/SBR blends and NR + SBR physical blends to reveal the effect of blend morphology on their Payne effect. Figure 6c,d show the \(\gamma\) dependence of normalized \(G^{\prime}/G^{\prime}{}_{0}\) and \(G^{\prime\prime}/G^{\prime\prime}{}_{0}\) for NR/SBR blends and NR + SBR physical blends, respectively. To show the influence of blend composition and morphology on their Payne effect clearly, their normalized moduli are vertically shifted by a factor of n for the blends with different weight ratios, and the corresponding n values are labelled. For three systems with different compositions, the amplitude of the modulus decay for NR/SBR blends is much weaker than that for their corresponding NR + SBR physical blends. Here, without the incorporation of nanoparticles, the Payne effect for NR/SBR blends and NR + SBR physical blends may mainly originate from the shear-induced overstraining [43], over-orienting [39], and disentanglement activated by the convective constraint release [27; 44; 45] in the nonlinear viscoelastic region. The weakening of the Payne effect for NR/SBR blends may be related with their interface contribution of heterogeneous structure, especially in the NR/SBR 50/50 blend (co-continuous morphology).