# PpyNT/NR/NBR Composites with Excellent Microwave Absorbing Performance in X-Band
## 3 Results and Discussion
### MA Performance of PPyNT/NR/NBR Composites
#### 3.2.2 Optimization of the NR/NBR Blend Ratio
As shown in Figure 4d, the Cole-Cole curves of the composites with NR/NBR = 90/10, 70/30, and 50/50 are closer to a straight line overall despite the appearance of several small semicircles, suggesting the predominant role of conduction loss in the dielectric loss for these samples. Especially the steeper slope of the line for the NR/NBR 70/30 composite indicates its higher conduction loss. On the contrary, polarization loss dominates dielectric loss of the NR/NBR of 30/70 and 10/90 composites, which show an undulant line with a slope approximately 0 and an obvious semicircle with a large radius, respectively. As we discussed above, a PPyNT/NR droplet-NBR matrix phase structure was formed in the composites with lower NR content. The conduction network formed by PPyNT was interrupted, which in turn contributes negatively to the conduction loss [24]. Figure 5 shows the three-dimensional \(RL\) plots of the composites with different NR/NBR ratios. The \(RL_{\text{min}}\), EAB, and the corresponding thickness are also summarized and presented. The results reveal that composite with higher NR content has better MA performance. The \(RL_{\text{min}}\) of the composite with the NR/NBR ratio of 10/90 and 30/70 is \(-\)18.25 dB at 3.2 mm (Figure 5a) and \(-\)19.28 dB at 2.3 mm (Figure 5b), respectively, while \(RL_{\text{min}}\) of the composites with rich NR content are all below \(-\)30 dB (Figure 5c-e). In particular, the \(RL_{\text{min}}\) of the composite with an NR/NBR ratio of 90/10 can reach \(-\)56.67 dB at 2.9 mm (Figure 5e), and the corresponding EAB can achieve 3.7 GHz (covering 86% of the X-band). The synergy between attenuation capability and impedance matching accounts for the observed variation in MA performance with a blend ratio of NR/NBR. Figure S5 (see Supplementary Materials) shows the attenuation constant (\(\alpha\)) of different composites, where the order of \(\alpha\) is (NR/NBR) 70/30 \(>\) 90/10 \(>\) 50/50 \(>\) 30/70 \(>\) 10/90, is consistent with that of dielectric loss tangents (Figure 4c). The \(|Z_{\text{in}}/Z_{0}|\) mappings of the composites are presented in Figure S6. Apparently, poor impedance matching is observed for the composites with lower NR content. The increase in NR content improves impedance matching. In particular, the superior impedance matching is observed for the composite with NR/NBR ratio of 90/10, which shows the largest area much approaching 1 in \(|Z_{\text{in}}/Z_{0}|\) mapping. Therefore, combined with its attenuation capability, 6 phr PPyNT filled NR/NBR (90/10) composite shows the best comprehensive MA performance. Compared with other rubber-based microwave absorbers reported in the literature (Table 1), it has the merits of achieving strong absorption and a broad effective absorption band with low filler content and thickness. Figure 4: The real \(\epsilon^{\prime}\) (**a**) and imaginary \(\epsilon^{\prime\prime}\) (**b**) parts of the complex permittivity, and the dielectric loss tangent factor tan\(\&\) (**c**); Cole-Cole curves (**d**) of 6 phr PPyNT filled NR/NBR composites with different ratios of NR/NBR. Based on the above discussion, a possible microwave absorbing mechanism for the 6 phr PPyNT filled NR/NBR (90/10) composite is briefly outlined in Figure 6. Firstly, an excellent impedance matching achieved with optimal composite composition facilitates the entry of more electromagnetic waves into the composite. Secondly, the conductive pathways formed by PPyNT in the continuous NR phase are beneficial for electron migrating or hopping, inducing conduction loss. Thirdly, the dipolar polarization originated from PPyNT, and interfacial polarization induced from the abundant heterogeneous PPyNT/NR interface also makes some contributions to attenuate electromagnetic energy. Fourthly, the PPyNT and NBR droplets are distributed in the NR matrix, providing multi-scattering and reflection sites to prolong the propagation path of electromagnetic waves and, in turn, benefit from energy attenuation.