Enhanced electromagnetic wave absorption of engineered epoxy nanocomposites with the assistance of polyaniline fillers

In this work, the engineered polyaniline (PANI)/epoxy composites reinforced with PANI-M (physical mixture of PANI spheres and fibers) exhibit significantly enhanced electromagnetic wave absorption performance and mechanical property. Due to the synergistic effect of PANI fillers with different geometries, the reflection loss of 10.0 wt% PANI-M/epoxy could reach − 36.8 dB at 17.7 GHz. Meanwhile, the mechanical properties (including tensile strength, toughness, and flexural strength) of PANI/epoxy were systematically studied. Compared with pure epoxy, the tensile strength of epoxy with 2.0 wt% PANI-M was improved to 86.2 MPa. Moreover, the PANI spheres (PANI-S) and PANI fibers (PANI-F) were prepared by the chemical oxidation polymerization method and interface polymerization method, respectively. The characterizations including scanning electron microscope, Fourier transform infrared spectra, and X-ray diffraction were applied to analyze the morphology and chemical and crystal structures of PANI filler. This work could provide the guideline for the preparation of advanced engineered epoxy nanocomposites for electromagnetic wave pollution treatment.


Introduction
With the rapid rollout and extensive usage of 5G wireless communication systems and high-frequency electronic equipments, the electromagnetic wave pollution has become a serious threat to the environments and our daily life [1][2][3][4][5]. The electromagnetic wave interference not only affects the precision performance of sensitive electronic devices, but also human health [6][7][8][9] The electromagnetic wave absorption plays an important role in mitigating the risks induced by the electromagnetic radiation pollution [10][11][12]. Therefore, the development of high-performance electromagnetic wave absorption materials is of great significance to ensure the safe operation of equipment and human health.
Due to the high conductivity and good electromagnetic wave permeability, metal and magnetic materials are the traditional materials for electromagnetic wave absorption [13,14]. However, the drawbacks of those metal and magnetic materials including poor corrosion resistance and high density restrict the practical application [15,16]. In recent years, conductive polymers and their nanocomposites have received widespread attention for electromagnetic wave absorption because of the versatility, light weight, good corrosion resistance, and adjustable electrical conduction performance [17][18][19][20][21][22][23][24][25][26]. Among them, polyaniline (PANI) shows great potential for electromagnetic wave absorption [27][28][29][30]. For instance, Zhang et al. fabricated the interwoven cellulose/PANI nanofiber composites by depositing PANI on cellulose's surface, and minimum reflection loss can reach − 49.24 dB [31]. Wang et al. reported the carbon nanotube/polyaniline composites with a reflection loss of −41.5 dB [32]. Kulkarni et al. synthesized α-MnO 2 nanorod-PANI nanocomposites using polymer coating and grafting methods. The minimum reflection loss of α-MnO 2 -NH 2 -PANI nanocomposites has been improved to −30.79 dB at 14.5 GHz [33]. Although PANI composites show good electromagnetic wave absorption property, the poor mechanical property of these powder composites limits their value for practical application. Hence, it is important to design the composites with electromagnetic wave absorption property and mechanical property simultaneously.
Epoxy, as an engineered thermosetting material, can be applied in different fields such as marine, aerospace, and electronic devices [34][35][36][37][38][39][40][41]. The insulated epoxy is transparent to electromagnetic wave, which can be used as matrix for electromagnetic wave absorption material. Compared with paraffin wax, epoxy shows better mechanical property and chemical resistance [41][42][43][44][45]. As previously reported, the minimum reflection loss of the carbon nanofiber/epoxy composites is up to 12.6 dB [46]. In our former work, we demonstrated that PANI could react with epoxy by forming the C-N covalent bond, leading to a uniform initiation of electrical conduction network in epoxy for electromagnetic wave absorption. So, the PANI/epoxy composites show great potential for electromagnetic wave absorption. Vigneras et al. reported the electromagnetic wave absorption property of epoxy nanocomposites with different morphology of PANI fillers [47]. However, the synergistic effect of different PANI fillers on the electromagnetic wave absorption and mechanical property of PANI/epoxy composites has not been reported elsewhere.
In this work, the PANI/epoxy composites with improved mechanical property and electromagnetic wave absorption property were reported. The PANI fillers with different morphologies (sphere and fiber) were prepared by the chemical oxidation polymerization (COP) method and interfacial polymerization (IP) method, respectively. The characterizations including scanning electron microscope, Fourier transform infrared spectra, and X-ray diffraction were applied to analyze the morphology and chemical and crystal structures of PANI fillers. The effects of PANI fillers on the mechanical properties (including toughness, flexural strength and tensile strength) and electromagnetic wave absorption performance are systemically studied. This work would provide the guideline for designing epoxy-based composites for electromagnetic wave pollution treatment.

Synthesis of PANI fillers
The PANI fiber (PANI-F) was prepared through the interfacial polymerization method at room temperature. The molar ratio of aniline/APS/PTSA was 8:2:25. Firstly, 8 mmol APS was added into 100 ml PTSA aqueous solution (1 mol/L) as solution 1, and 32 mmol aniline solution dissolved in 100 ml Scheme 1 Molecular structure of Epon 862 and DETDA chloroform as solution 2. Second, solution 1 was rapidly added into solution 2 and remained there for 2 h for polymerization of aniline monomers. Then, the product was treated by vacuum filtration and washed with ethanol and deionized water several times to remove organic solvent, oligomers, and additional acid. After that, the obtained final products were dried in the oven at 50 °C for 12 h.
The PANI sphere (PANI-S) was prepared by the COP method. For solution A, 30 mmol PTSA and 18 mmol APS were added into 200 mL deionized water and then treated by sonication and mechanical stirring for 1 h in ice-water bath. The 36 mmol aniline was added into 50 ml deionized water as solution B. Then, solution B was mixed in solution A for polymerization under mechanical and ultrasonic stirring for 2 h in an ice-water bath. The product was treated by vacuum filtration, washed with ethanol and water several times, and dried at 50 °C. To make the same amount of proton acid doping level for PANI fillers, the obtained PANI-S was soaked in ammonia water (1 mol/L) for 10 min, then washed several times with water and dried at 50 °C. After that, the product was soaked in PTSA solution (1 mol/L) for 10 min and washed several times with water to remove acid. The final obtained samples were dried at 50 °C for 12 h.

Fabrication of PANI/epoxy composites
Firstly, the PANI fillers were immersed in epon monomers overnight. Then, the above mixture was mechanically stirred (500 rpm) at room temperature for 1 h. After that, curing agent DETDA was added (weight ratio of epon monomer to curing agent is 100 to 26.5), then treated with mechanical stirring for additional 1 h. Then, the above suspension was mechanically stirred at 70 °C for 2 h at a speed of 200 rpm. Finally, the solutions were poured into the silicone molds and cured at 120 °C for 5 h, then cooling to ambient temperature naturally. In this work, The pure epoxy, epoxy with 10.0 wt% PANI-F, epoxy with 10.0 wt% PANI-S, and epoxy with 2.0, 6.0, and 10.0 wt% PANI mixtures (PANI-M) which includes PANI-F and PANI-S (the mass ratio of PANI-F to PANI-S was 1:1 in PANI-M) were prepared.

Characterization
FT-IR spectra were acquired on Vertex 70 using the attenuated total reflectance (resolution: 4.0 cm −1 ). XRD of PANI fillers was recorded on D/max2200PC X-ray diffractometer with Cu Kα radiation (λ = 1.5418 Å; scan rate: 6°min −1 ). SEM was carried out using a FEI Verios 460 microscope to observe the morphology of the fabricated PANI samples and the fractured surfaces of PANI/epoxy nanocomposites. Mechanical properties of pure epoxy and epoxy nanocomposites were measured at least 3 times at room temperature including tensile test and bending performance test. Tensile measurements were carried out using the AI-7000GD unidirectional tensile testing machine (crosshead speed: 1.00 mm/min). The samples were bone-like according to ASTM (D412-98a, 2002) requirement. Bending measurements were characterized at a speed of 0.50 mm/min by strength in three-point bending using the 1036PC unidirectional tensile testing machine. The sample dimensions were 60 × 12 × 3 mm, which were designed according to the GB/T requirement. Dielectric properties and volume resistivity of the samples were conducted by Agilent E4980AL in the range of 5 × 10 3 to 1 × 10 6 Hz at room temperature. The reflection loss of epoxy nanocomposites was obtained through a vector network analyzer, E5071C, Agilent Technology, in the frequency range of 2-18 GHz under air condition at room temperature. The sample was shaped as a ring, with the inner and outer diameter of 3.04 and 7.00 mm, respectively. Figure 1A and B exhibit SEM images of different PANI-F and PANI-S prepared by two different methods. The average diameter of PANI-F and PANI-S is 105 and 170 nm measured by nanomeasurer software, respectively. The rough surface of PANI-S is due to the insufficient time for aniline monomer to grow along the molecular chain under sonication treatment and mechanical stirring. For the FT-IR of PANI-F and PANI-S in Fig. 1C, the characteristic peaks at 1558 and 1488 cm −1 are attributed to C = C stretching of the quinoid rings and bezenoid rings [28,48]. The peak at 1295 cm −1 is assigned to the C-N stretching vibration of the secondary amine [49], indicating the conducting emeraldine salt form of PANI originating from the bipolaron structure [50]. The peak 1240 cm −1 is related to the C-N stretching vibration of the polaronic structure. The peak at 796 cm −1 is attributed the C-H out-of-plane bending vibrations of the benzenoid ring [51]. The peaks located at 1240 and 1124 cm −1 are assigned to C-H bendings of the benzenoid ring and the quinonoid ring, respectively [52]. The XRD diffraction is shown in Fig. 1D. There are three diffraction peaks at around 14.8°, 19.8°, and 25.4° correspond to the (010), (100), and (110) crystallographic planes of the partially crystallized PANI nanostructure [53]. All the above results confirm that PANI-F and FANI-S were successfully synthesized by two different methods  Table 1. The PANI filler could reduce the flexural strength and toughness of epoxy, but enhance its Young's modulus, demonstrating that stiffness is enhanced by sacrificing the flexural strength and toughness. It is worth noticing that the PANI-F/epoxy nanocomposites show larger tensile strength, flexural strength and toughness than epoxy with PANI-S and PANI-M fillers when the loading level of PANI fillers is same.

Mechanical property
SEM image of the fracture surface after tensile test is applied to study the effect of PANI fillers on tensile strength (Fig. 3). The pure epoxy displays a relatively smooth fracture surface with "river-like" pattern, indicating a typical brittle fracture because of rapid propagation of the cracks [56] (Fig. 3a). However, compared with pure epoxy, the fracture surface of epoxy nanocomposites reinforced with PANI fillers become much rougher. For 2.0 wt% PANI-M/epoxy, the PANI fillers are well dispersed in the epoxy matrix (Fig. 3b), and the "river-like" pattern disappeared which indicates the strong covalent bond between PANI and epoxy matrix [57]. This formed covalent bond cannot only enhance the compatibility, but also enhance interfacial adhesion between the two phases [58]. Therefore, the tensile strength of 2.0 wt% PANI-M/epoxy is larger than that of pure epoxy. With increasing the PANI filler loading (Fig. 3c-f), the agglomeration of PANI fillers are observed in the epoxy nanocomposites, which lead to the deceasing of tensile strength. When the PANI loading is 10.0 wt%, it is observed that the PANI-F/epoxy show much rougher fracture surface than epoxy with PANI-M or PANI-S, indicating PANI-F makes more contribution to obstruct the propagation of the cracks [26]. Thus, the PANI-F/epoxy nanocomposites show higher tensile strength when the PANI nanofiller loading is 10.0 wt%.

Electromagnetic wave absorption performance
In general, the real parts (ε′ and μ′) and imaginary parts (ε′′ and μ") represent the storage and loss of electrical and magnetic energy, respectively. The values of electromagnetic parameters (ε′, ε′′, μ′, and μ") of PANI/epoxy nanocomposites are shown in Fig. 4. The positive dielectric constant of PANI/epoxy composites is due to interfacial polarization formed at the interface between PANI fillers and epoxy matrix. The charge carriers were hindered by the epoxy resin, resulting in the accumulation of space charge carriers at the interface between PANI fillers and epoxy matrix [59]. In Fig. 4a-c, the ε′ increases with increasing PANI-M content at the same frequency. For PANI/epoxy composites with 10.0 wt% PANI fillers, Fig. 4c-e, it is obvious to observe that the ε′ increase with increasing PANI-F content, which is due to the larger specific surface area of PANI-F. And ε′′ exhibits the same variation pattern as ε′. The μ′ and μ″ of the PANI/epoxy is about 1 and 0, demonstrating magnetic loss does not make contribution to convert the electromagnetic wave energy to heat.
Generally, the electromagnetic wave absorption property of a material is related to the complex permittivity and permeability [60]. The reflection loss (RL) can be expressed as Eq. 1 [61]:   where Z in is the effective input impedance, and Z 0 is the impedance in free space. Z in is given by Eq. 2 [62]: where c, f, and d stand for light velocity, frequency of electromagnetic wave, and thickness of the absorber, respectively. The RL of with PANI-S/epoxy, PANI-F/epoxy, and PANI-M/epoxy is shown in Fig. 5. Figure 5 exhibits the RL of PANI/epoxy with different PANI fillers. It can be seen that the 10.0 wt% PANI-M/epoxy nanocomposites achieve a minimum RL value of −36.8 dB at 17.7 GHz, and the bandwidth (RL below -10 dB) is 0.3 (7.5-7.8 GHz), 1.5 (11-12.5 GHz), and 1.5 GHz (16.5-18 GHz). The 10.0 wt% PANI-F/epoxy nanocomposites achieves a minimum RL of −19.8 dB at 11.6 GHz and a bandwidth (RL below −10 dB) is 0.5 (7.0-7.5 GHz), 0.9 (10.1-12 GHz), and 1.5 GHz (16-17.5 GHz). However, for the 10.0 wt% PANI-S/ epoxy nanocomposites, the RL is larger than −10 dB from 2 to 18 GHz. For practical application, RL should be less than -10 dB at least. Generally, a good impedance matching is the prerequisite for good electromagnetic wave absorption materials, and a larger difference between permittivity and permeability would lead to a worse impedance matching [59]. In this work, with increasing PANI-M loading, the difference between them is larger in the PANI-M/epoxy. So, the 10 wt% PANI-M/epoxy shows worse impedance matching. On the other hand, the 10.0 wt% PANI-M/epoxy displays better dielectric loss. Hence, the 10.0 wt% PANI-M/epoxy shows better electromagnetic wave absorption performance than epoxy with lower loading of PANI-M. When the PANI filler's loading is 10.0 wt%, although PANI-M/epoxy shows worse impedance matching than PANI-S/epoxy, the PANI-M/epoxy still exhibits better electromagnetic wave absorption performance which may be due to the synergistic effect of PANI-F and PANI-S on the interfacial polarization and dielectric loss. The similar phenomenon is observed in polyvinylpyrrolidone@multi-walled carbon nanotubes/ graphene composites for electromagnetic wave absorption [63].

Conclusion
In this study, epoxy resin was used as the matrix, and PANI with different morphologies was used as the reinforcing material.

Conflict of interest The authors declare no competing interests.
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