# PpyNT/NR/NBR Composites with Excellent Microwave Absorbing Performance in X-Band
###### Abstract  
To meet the comprehensive demand for flexible microwave absorbing (MA) materials, a novel MA rubber containing homemade Polypyrrole nanotube (PPyNT) is produced based on the natural rubber (NR) and acrylonitrile-butadiene rubber (NBR) blends. To achieve the optimal MA performance in the X band, the PPyNT content and NR/NBR blend ratio are adjusted in detail. The 6 phr PPyNT filled NR/NBR (90/10) composite has the superior MA performance with the minimum reflection loss value of \(-\)56.67 dB and the corresponding effective bandwidth of 3.7 GHz at a thickness of 2.9 mm, which has the merits in virtue of achieving strong absorption and wide effective absorption band with low filler content and thickness compared to most reported microwave absorbing rubber materials over the same frequency. This work provides new insight into the development of flexible microwave-absorbing materials. polypyrrole nanotube; immiscible rubber blend; impedance matching; microwave absorption +
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absorption capability to a certain degree with relatively lower filler loading, the related rubber absorbers normally face the problem of narrow EAB resulting from poor impedance matching [10]. Hybrids of magnetic materials and carbon nanomaterials are becoming the recent interest because of their dual loss mechanism and better impedance matching. Numbers of rubber-based microwave absorbers, e.g., Fe-Ni/C/NBR [11], Fe\({}_{3}\)O\({}_{4}\)/rGO/SR [3], Sr-ferrite/NBR [5], etc., have been successfully developed for application in the X-band frequency range. However, most of these products involve complicated fabrication procedures unsuitable for large-scale production. In addition, a relatively high filler loading (over 40 wt.%) [3] is required to attain sufficient absorption performance. Apart from filler properties, filler dispersion is another important factor affecting rubber absorbers' MA performance. Well-established filler networks inside the rubber matrix are favorable for excellent performance. Earlier efforts have mainly focused on the uniform dispersion of fillers in a single rubber matrix. Recently, an immiscible rubber blend has emerged as a promising alternative in the design of rubber-based microwave absorbers because of the enhanced filling efficiency resulting from the selective distribution of fillers in a specific phase [12]. In addition, the heterogeneous rubber blend composites possess more effective multiple internal scattering, including inter-filler and interphase scattering, than single rubber composite, which only has inter-filler scattering [13]. A MA rubber based on a partially immiscible blend of NR and epoxidized NR containing 30 wt.% conductive carbon blacks has been successfully developed by Liet.al [14], which shows much stronger absorption capability compared to either of single rubber-based composites containing the same amount of filler. However, this MA rubber composite is non-effective over the 8.2-12.4 GHz frequency range. Despite lots of efforts devoted to the development of MA rubber, there is still a huge blank in designing a new material system to achieve strong absorption capability and a wide effective absorption band over the frequency range of 8.2-12.4 GHz with low filler content. Polypyrrole (PPy), one of the typical intrinsically conducting polymers (ICPs), has been investigated as Dielectric loss-type MA material extensively from different aspects, including preparation method [15], morphology [16], hybrid with other materials [17], etc. However, most studies mainly focus on the MA performance of separate powders in paraffin, far from practical applications. Herein, we report a novel MA rubber by incorporating homemade PPy nanotube (PPyNT) into the immiscible blend of natural rubber (NR) and acrylonitrile-butadiene (NBR) through a simple mixing technology in the rubber industry based on our previous work [12]. By simply adjusting the PPyNT content and rubber blend ratio, the impedance matching and attenuation capability of the obtained MA rubber are tuned.