# 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  
Based on the optimal PPyNT content obtained above, the blend composition is further optimized by changing the NR/NBR ratio from 10/90 to 90/10. Obviously, the phase morphologies of 6 phr PPyNT filled NR/NBR composites are great related to the ratio of NR/NBR. A matrix-droplet structure with NR phase as droplet and NBR as matrix is found in the samples with NR/NBR ratio of 10/90 (Figure S4a, see Supplementary Materials) and 30/70 (Figure S4b, see Supplementary Materials). The size of the NR phase increases with increasing the content of NR, and a typical co-continuous structure is formed in the composite while the ratio of NR/NBR reaches 50/50 (Figure S4c, see Supplementary Materials). With further increasing the NR/NBR ratio to 70/30 (Figure S4d, see Supplementary Materials) and 90/10 (Figure S4e, see Supplementary Materials), the NR and NBR phases change to matrix and droplet, respectively, and the size of the droplet NBR phase decreases with increasing the NR content as well. The alteration in blend ratio has little effect on the selective localization of PPyNT in the NR phase. The \(\epsilon^{\prime}\), \(\epsilon^{\prime\prime}\) and tan\(\delta_{\epsilon}\) for 6 phr PPyNT filled NR/NBR composites with different NR/NBR ratios are shown in Figure 4a-c. The values of \(\epsilon^{\prime}\) range from 8.11-7.68, 8.38-7.66, 7.94-7.05, 5.95-5.12 and 7.24-6.26 for NR/NBR ratio of 10/90, 30/70, 50/50, 70/30 and 90/10 respectively. The values of \(\epsilon^{\prime\prime}\) for the corresponding composites range from 2.46-2.20, 2.54-2.59, 2.93-2.77, 3.27-2.75, and 3.06-2.85, respectively. Accordingly, the capability of dielectric loss (tan\(\delta_{\epsilon}\)) for the composites follows the order (NR/NBR): 70/30 \(>\) 90/10 \(>\) 50/50 \(>\) 30/70 \(>\) 10/90. It is widely accepted that dielectric loss stems from conduction loss and polarization loss [23]. In the case of the dielectric loss primarily originating from the conduction loss, the values of \(\epsilon^{\prime\prime}\) will decline with increasing frequency [30]. While the polarization loss is present, fluctuation and resonance peaks in \(\epsilon^{\prime\prime}\) curves can be observed [31]. It is noted that the \(\epsilon^{\prime\prime}\) values of the composites with NR/NBR = 90/10, 70/30, and 50/50 show an overall descending trend with increasing frequency accompanied by multiple peaks centered around 8.8, 9.5, 10.3, 11.1, and 11.9 GHz. Moreover, the decline of \(\epsilon^{\prime\prime}\) with frequency is more obvious for the NR/NBR = 70/30 composite, and simultaneously resonance peaks in \(\epsilon^{\prime\prime}\) are less visible for this sample. As for the two composites with lower NR/NBR ratio, i.e., 30/70 and 10/90, the values of \(\epsilon^{\prime\prime}\) almost lose the frequency dispersion feature except for the increment caused by resonance peaks. In particular, a strong resonance peak is observed at 8.6 GHz for the NR/NBR = 10/90 composite. These results suggest that polarization loss is present for all composites. This polarization loss may come from dipolar polarization caused by intrinsic dipoles of PPyNT [32] and interface polarization caused by structural heterogeneity in the composites (abundant interface between PPyNT and NR matrix) [23]. The conduction and polarization losses contribute to dielectric loss differently with varying NR/NBR ratios. The Cole-Cole curves based on Debye theory (expressed by Equation (S5), see Supplementary Materials) [9] are employed to better understand the individual contribution from this two-loss mechanism of different samples. It is reported that one semicircle in the Cole-Cole curve corresponds to the process of a Debye relaxation, and a straight line stands for the conduction loss [33]. The radius of the semicircle and the slope of the straight line are related to the intensity of the relaxation process and the capacity of conduction loss, respectively [34].