Poly(p-phenylenebenzobisoxazole) Multiphase

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Graphene-MWNT-Poly(p-phenylenebenzobisoxazole) Multiphase Nanocomposite via Solution Prepolymerization with Superior Microwave Absorption Properties and Thermal Stability Jiasong Hua, Yanxiao Li, Xiaoyun Liu, Xinxin Li, Shaoliang Lin, Jinlou Gu, Zhong-Kai Cui, and Qixin Zhuang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b11925 • Publication Date (Web): 27 Dec 2016 Downloaded from http://pubs.acs.org on December 31, 2016

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The Journal of Physical Chemistry

Graphene-MWNT-Poly(pphenylenebenzobisoxazole) Multiphase Nanocomposite via Solution Prepolymerization with Superior Microwave Absorption Properties and Thermal Stability Jiasong Huaa, Yanxiao Lia, Xiaoyun Liua, Xinxin Lia*, Shaoliang Lina, Jinlou Gua, Zhong-Kai Cuib, Qixin Zhuanga* a. The Key Laboratory of Advanced Polymer Materials of Shanghai, School of Materials Science and Engineering, East China University of Science and Technology, Shanghai, China, 200237 b. Department of Chemistry, Université de Montréal, C.P. 6128, Succ. Centre Ville, Montréal, Québec, Canada, H3C 3J7

Email address of the corresponding author: [email protected] [email protected]

ACS Paragon Plus Environment

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Abstract

This paper demonstrates both the high-performance microwave absorption properties and the superior thermal stability of Graphene/Multiwalled carbon nanotubes (MWNTs)/poly(pphenylenebenzobisoxazole) (PBO) composites synthesized via in situ polymerization of functionalized PBO precursor with Graphene Oxide/MWNTs nanocomposites, followed by hightemperature calcination. The incorporation of three-dimensional Graphene/MWNTs network significantly improved the reflection loss of PBO composites (−50.17 dB at 12.58 GHz) by over 20 times than that of pure PBO (−2.33 dB at 12.58 GHz), with a sample thickness of only 2.6 mm. The effect of Graphene/MWNTs content on the microwave absorption performance was also investigated. The Graphene/MWNTs/PBO composite shows great promise as a microwave absorber in high-temperature environments.


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Introduction With the exponential growth of electronic devices, electromagnetic pollution has become a major concern for human beings. Therefore, microwave absorption materials are highly demanded to eliminate unwanted radiation in both civilian and military fields.1-2 Furthermore, in aerospace engineering, a high-performance microwave absorber is also essential for the stealth technology.3 A broad spectrum of microwave absorbing materials, such as dielectric fillers (carbon nanotubes, graphene, TiNC, etc.)4-8 and magnetic fillers (Fe2O3, carbonyl iron, ZnO, Fe3O4, etc.),9-16 has been widely investigated. Among those, graphene has stimulated great interest owing to its unique properties e.g. high thermal conductivity, high electrical conductivity and high strength.17-19 In addition, in order for lightweight absorbing material, polymers, due to their good chemical stability and processing property, are usually utilized as the matrices. Liu et al. incorporated reduced graphene oxide in the polyaniline matrix and achieved reflection loss (RL) of −41.4 dB at 13.8 GHz.20 Zhang et al. developed RGO/MnFe2O4/PVDF composites with a maximum absorption of −29 dB at 9.2 GHz and an effective absorption range from 8.00 to 12.88 GHz.21 Although extensive research has been carried out on graphene as a microwave absorption agent, easy stacking remains challenging, which hampers the maximization of its microwave absorbing capacity. Recent studies suggest that the incorporation of carbon nanotubes into graphene can form a three-dimensional interconnected network and can effectively inhibit aggregation of graphene.2223

The 3D carbon nanohybrid has attracted great attention due to its outstanding properties and

has been exploited in a wide range of applications e.g. lithium ion batteries,24-25 supercapacitors,26-27 solar cells28-29 and electromagnetic interference shielding materials.30-32


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However, the microwave absorption properties of the 3D carbon nanohybrid have been rarely reported. Meanwhile, the majority of polymer-based microwave absorption materials exhibit poor thermostability, which has severely hindered their practical applications.33 Poly(p-phenylenebenzobisoxazole) (PBO), a highly conjugated polymer, possesses excellent thermal stability with decomposition temperature over 600 °C.34-36 In addition, attributed to its high wave-transmissivity, PBO matrix can provide propagation channels for microwave and promote the impedance matching of the whole composite, which is of great benefit to the absorption of microwave. Therefore, it is expected that the Graphene/Multiwalled carbon nanotubes (MWNTs)/PBO composite will be a great candidate as a microwave absorbing material with high-temperature resistance. Here, we present a facile synthesis of the Graphene/MWNTs/PBO composite. To promote the integration of nanoparticles and PBO matrix, acid-modified MWNTs and Graphene Oxide (GO) were functionalized to constitute better bonding with the amino groups in PBO via SOCl2. Then, functionalized MWNTs and GO were mixed together via ultrasonication to obtain the GO/MWNTs network. It is expected that the long one-dimensional MWNTs can bridge adjacent two-dimensional GO platelets and inhibit their aggregation, resulting in remarkable synergetic effects on the enhanced microwave absorption properties. Since PBO is insoluble in all kinds of organic solvents, PBO nanocomposites were prepared by the polymeric precursor method. First, GO/MWNTs/pre-PBO composites were synthesized via in situ polymerization, which enables the uniform dispersion of nanoparticles. Then, Graphene/MWNTs/PBO composites were developed through calcination of the precursor at a high temperature, which, in the meantime, led to the reduction of GO.37 To ascertain a multi-scale approach on the properties of Graphene/MWNTs/PBO composites, a variety of techniques were utilized for characterization.


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Figure 1. Schematic of the preparation of Graphene/MWNTs/PBO nanocomposites.

Experimental Materials MWNTs (20-40 nm) synthesized by a chemical vapor deposition (CVD) were purchased from Shenzhen Nanoport Company (Guangdong, China). MWNTs were modified by an oxidation reaction with concentrated nitric acid. Graphite powder was provided by Alfa Aesar (Shanghai,


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China). All the reagents, obtained from Aldrich Chemical Company (Milwaukee, USA), were of high purity and used as received. Fabrication of GO/MWNTs network GO was synthesized from graphite powder according to a modified Hummer’s method.38 GO (0.5 g) and N, N-dimethyl-formamide (DMF) (2 ml) were added to SOCl2 (100 ml). The mixture was stirred at 70 °C for 24 h to yield acyl-chloride functionalized GO (GO-Cl). The dispersion was filtrated and dried at 30 °C under vacuum for 12 h. Acid-modified MWNTs were functionalized as well to yield MWNT-COCl. For GO/MWNTs network, 50 mg GO-Cl was dispersed in 100 ml deionized water, followed by ultrasonication to obtain a brown solution. Then, 50 mg MWNT-COCl were added to the solution and stirred vigorously for 2 h. The asprepared solution was freeze-dried to obtain the GO/MWNTs network. Synthesis of 4,6-di(tert-butyldimethylsilylamino)-1,3-di(tert-butyldimethylsiloxy) benzene (TBS-DAR) 0.213 g 4,6-diaminoresorcinol dihydrochloride (DAR·2HCl) was dissolved in N-methyl-2pyrrolidone (NMP) under nitrogen. Then 1.5 g tert-butyldimethylsilyl chloride and 2 mL trimethylamine (TEA) was added to the solution. The mixture was stirred for 24 h at room temperature, followed by filtration to gather a precipitate. The obtained powder was dried at 60 °C under vacuum for 12 h. Preparation of GO/MWNTs/pre-PBO 30 mg GO/MWNTs was dispersed in 50 ml NMP using ultrasonication. Then, under a nitrogen atmosphere, 3.06 g TBS-DAR was added into the solution under magnetic stirring in an ice bath. After TBS-DAR was completely dissolved, 1.015 g terephthaloyl chloride (TPC) was added into


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the solution and stirred for 48 h at room temperature. The solution was poured into excess methanol, followed by filtration to collect the precipitate. GO/MWNTs/pre-PBO was obtained after drying in a vacuum oven at 80 °C for 24 h. Preparation of Graphene/MWNTs/PBO composites The

complete

conversion

to

Graphene/MWNTs/PBO

was

achieved

by

annealing

GO/MWNTs/pre-PBO at 500 °C for 3 h under nitrogen. The preparation process of composites was repeated for 1.5, 3.0, 4.5, 6.0, 7.5 and 9.0 wt% of GO/MWNTs loading and designated as GMP1.5, GMP3.0, GMP4.5, GMP6.0, GMP7.5 and GMP9.0, respectively. Characterization Infrared spectra were acquired on a Nicolet Magna-IR550 Fourier transform infrared (FTIR) analyzer with KBr as the non-absorbent medium. X-ray powder diffraction (XRD) was carried out using a Rigaku D/max 2550 V with Cu Kα radiation in the scattering range of 10-80°. Raman spectra were obtained at 514 nm excitation (He-Ne laser) on a Renishaw inVia Reflex Raman spectrometer with a backscattering configuration. Thermogravimetric analysis (TGA) was conducted using a DuPont model 951 thermogravimetric analyzer under a nitrogen atmosphere with a heating rate of 10 °C min-1. The surface morphology of GO/MWNTs network and Graphene/MWNTs/PBO composites was observed by field emission scanning electron microscopy (FE-SEM) (Hitachi S-4800). The detailed morphology of GO/MWNTs was also observed by transmission electron microscopy (TEM) (JEOL-2100F). The electromagnetic parameters (relative complex permittivity and permeability) were measured on the vector network analyzer (VNA), Agilent E8363B, using the coaxial transmission line method in the frequency range of 2-18 GHz. The samples were prepared with 80 wt% of Graphene/MWNTs/PBO composites and 20 wt% of paraffin wax since the paraffin


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wax barely contributes to permittivity and permeability. The obtained samples were molded into toroids with an outer diameter of 7 mm, inner diameter of 3 mm and thickness of 3 mm. The reflection loss was calculated from the measured relative permittivity and permeability according to the transmission line theory.

Results and discussion Figure 2 presents FTIR spectra of GO-Cl, MWNT-COCl, pre-PBO, GO/MWNTs/pre-PBO and Graphene/MWNTs/PBO. The spectra of GO-Cl and MWNT-COCl both display a peak at 1720 cm-1 assigned to the C=O stretching vibrations of acyl chloride group, indicating successful acylchloride functionalization. The same characteristic absorption band can also be slightly observed in the spectrum of pre-PBO (corresponding to the -COCl group at the end of the chain), but not in the spectrum of GO/MWNTs/pre-PBO. This clearly suggests that GO/MWNTs was chemically grafted onto the pre-PBO chain due to the reaction between the -COCl group of GO/MWNTs and the -NH2 group of pre-PBO. The structures of GO/MWNTs/pre-PBO and Graphene/MWNTs/PBO were well confirmed by FTIR. In the spectrum of GO/MWNTs/pre-PBO, the absorption peaks at 1636 cm-1 and 1528 cm-1 are associated with the C=O stretching vibrations and the -N-H bending vibrations of amide groups

(CONH)

in

pre-PBO,

respectively.

While

the

peak

at

1628

cm-1

for

Graphene/MWNTs/PBO can be attributed to C=N bond, a part of the benzene ring in PBO. Thus, a complete conversion to Graphene/MWNTs/PBO from GO/MWNTs/pre-PBO can be confirmed.


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Figure 2. FTIR spectra of GO-Cl, MWNT-COCl, pre-PBO, GO/MWNTs/pre-PBO and Graphene/MWNTs/PBO.

XRD patterns of GO, PBO and Graphene/MWNTs/PBO are shown in Figure 3. GO exhibits a carbon (001) diffraction peak at 2θ value of 10.20°, suggesting the successful introduction of oxygen functional groups on the GO nanosheets, increasing the GO interlayer distance.39 Two characteristic peaks appeared in the pattern of PBO. The peak at 15.86° is assigned to the ‘sideto-side’ or horizontal dimension of PBO chain. The peak at 26.91° corresponds to the ‘face-toface’ or interplanar dimension of PBO molecule.40 Compared to PBO, no new peaks are present in the pattern of Graphene/MWNTs/PBO, indicating the successful reduction of GO during the annealing process and the uniform dispersion of Graphene/MWNTs in PBO. In addition, the peak at 26.91° shifts to 25.94°, suggesting an increase of the interlayer distance, which might be ascribed to the strong chemical bonding between Graphene/MWNTs and PBO.


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Figure 3. XRD patterns of GO, PBO and Graphene/MWNTs/PBO.

Figure 4 shows the Raman spectra of GO/MWNTs, PBO and Graphene/MWNTs/PBO. For GO/MWNTs, it has two intense Raman features at 1595 cm-1 (G band) and 1350 cm-1 (D band).30 For PBO and Graphene/MWNTs/PBO composites, the characteristic peaks at 930, 1170, 1277, 1306, 1543 and 1618 cm-1 are observed, which reveals the presence of PBO structure in Graphene/MWNTs/PBO composites. This clearly suggests that Graphene/MWNTs are encapsulated in the PBO matrix, which is in good agreement with SEM studies. The new peak at 1581 cm-1 in the spectrum of Graphene/MWNTs/PBO composites should be assigned to the G band of Graphene/MWNTs with a shift of 14 cm-1. In addition, the D band is not observed in the spectrum of Graphene/MWNTs/PBO, which suggests the crystal defects of Graphene/MWNTs are repaired. The slight shift of the G band, coupled with the absence of D band, indicates the strong interaction between Graphene/MWNTs and PBO.


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Figure 4. Raman spectra of GO/MWNTs, PBO and Graphene/MWNTs/PBO.

Photographs of GO-Cl, MWNT-COCl and GO/MWNTs dispersions in NMP are shown in Figure 5. It can be seen that GO-Cl is deposited at the bottom of the NMP solvent after standing for 2 days. It might be ascribed to the Van der Waals forces of GO-Cl nanosheets, which give rise to the stacking of them. However, the MWNT-COCl is well dispersed in NMP due to the great particles-solvent interactions. Interestingly, GO/MWNTs suspension exhibits unique colloidal stability in NMP. The scheme in Figure 5 was proposed to explain the phenomenon. The long and tortuous MWNTs can connect two graphene nanosheets, inhibiting the stacking of them.41 Moreover, the hierarchical structure can increase the contact area of GO/MWNTs and polymer, which facilitates polymer chains grafting and increases the compatibility.


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Figure 5. Photographs of 1 mg/mL GO-Cl, MWNT-COCl and GO/MWNTs dispersions in NMP after standing for 2 days, at room temperature.

The morphology of GO/MWNTs network was investigated by FESEM and TEM. Figure 6a shows the corresponding FESEM image of GO/MWNTs, which shows the hierarchical structure of the composite. The red arrows designate the presence of separated graphene nanosheets connected by tortuous MWNTs, though MWNTs are inevitably entangled with each other. The TEM image in Figure 6b shows that MWNTs and graphene maintain their original morphology after acyl-chloride functionalization and the single-layer structure of graphene is also retained. Figure 7 shows the FESEM images of Graphene/MWNTs/PBO nanocomposites. It can be seen in Figure 7a that Graphene/MWNTs is well dispersed in the PBO matrix due to in situ polymerization, as designated by red arrows. Another FESEM image of Graphene/MWNTs/PBO composites is shown in Figure 7b, which suggests the hierarchical structure of the composites. As indicated by the red circle, PBO chains were grafted onto the graphene nanosheet by covalent chemical bonding. In addition, MWNTs align along the graphene surface by Van der Waals


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forces (shown by red arrows), hindering the restacking of the graphene nanosheets. We believe that the covalent bonding between polymer and Graphene/MWNTs greatly contributes to the compatibility of the three components in the ternary composite. Therefore, it is anticipated that the synergetic effects of the components will make a significant improvement on the properties of nanocomposites.

Figure 6. FESEM (a) and TEM (b) images of GO/MWNTs network.

Figure 7. FESEM (a) and (b) images of Graphene/MWNTs/PBO nanocomposites. TGA analysis was performed at a temperature range from 100 to 800 °C to obtain the thermal behavior of Graphene/MWNTs/PBO composites, as shown in Figure 8. It has been reported that PBO is stable in a nitrogen atmosphere up to 600 °C due to its oxazole ring


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structure. Therefore, the TGA curves of both PBO and Graphene/MWNTs/PBO composites do not show any features before 600 °C. The 5% weight loss temperatures of GMP1.5, GMP3.0, GMP4.5, GMP6.0, GMP7.5 and GMP9.0 are 649, 640, 647, 649, 653 and 651 °C, respectively, which are higher than pure PBO (636 °C). This improvement could possibly be ascribed to the strong chemical bonding between Graphene/MWNTs and PBO.

Figure 8. TGA curves of PBO and Graphene/MWNTs/PBO composites.

Microwave absorbing materials are characterized by their relative complex permittivity and relative complex permeability. The real part refers to the energy storage capability of electric and magnetic energy, and the imaginary part represents the amount of electric and magnetic energy loss.42-43 As shown in Figure 9, the real and imaginary parts of the effective permittivity (ε' and ε") and permeability (µ' and µ'') were measured from 2 to 18 GHz. It is observed that both permittivity and permeability are frequency dependent. In terms of permittivity, both the real part (ε') and imaginary part (ε") of pure PBO remain nearly constant. Generally, as the Graphene/MWNTs concentration increases, the value of ε' of Graphene/MWNTs/PBO


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composites increases from 2.5 to ~5. For composites in Figure 9c, it can be seen that the ε" also increases with an increasing concentration of Graphene/MWNTs. In a microwave region, the dielectric performance of the composites is primarily determined by the various polarizations e.g. interfacial, orientational and space charge polarization.44-46 Here, the incorporation of Graphene/MWNTs, as well as the in situ polymerization process, helps to generate numerous interfacial sites which cause the accumulation of charges, distorting the electric field and increasing the overall composite’s capacitance and loss, thereby increasing the complex permittivity value. Generally, the higher the concentration of Graphene/MWNTs, the higher ε' and ε". Meanwhile, it is also worth noting that both the ε' and ε" curves exhibit resonant behavior, especially the ones of GMP6.0, GMP7.5 and GMP9.0. Take GMP9.0 for example. A strong peak can be observed on the ε" curve at 12.92 GHz originating from dipole orientation polarization of the heterojunction at the interface of the Graphene/MWNTs and PBO. At frequencies below 12.92 GHz, when the dipoles can keep up with the pace of field variations, ε" continues to increase, but the storage of electric energy ε' begins to drop due to the lack of alignment between the dipoles and the external field. At frequencies above 12.92 GHz, the field is oscillating too fast to affect polarization, so both parts of a complex permittivity decrease. These phenomena are the typical characteristic of Debye relaxation.47-50 The electric loss tangent is the ratio of ε" to ε'. As seen in Figure 9e, the curves are similar to those in Figure 9c, with the values of loss tangent ranging from 0.01 to 0.75. The greater the loss tangent of the material, the greater the attenuation as the wave travels through the material. Since PBO and Graphene/MWNTs/PBO composites contain no magnetic components, there is little difference between them in permeability.


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Figure 9. The real part and imaginary part of the effective permittivity (ε'(a) and ε"(c)) and permeability (µ'(b) and µ"(d)) and the electric (e) and magnetic (f) loss tangent for PBO and Graphene/MWNTs/PBO composites.


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Microwave absorption is caused by various loss mechanisms. Hence, the cumulative absorption effects are what we are most interested in. In this paper, we used the metal-backed absorber model to calculate the normalized wave impedance of the microwave absorbing materials:

Zin =

µr 2π fd tanh( j µr ε r ) εr c

(1),

where f is the microwave frequency, d is the thickness of the absorber, c is the speed of light, εr and µr are the complex relative permittivity and permeability, respectively. Then the reflection loss (RL):

RL ( dB ) = 20 lg

Z in − 1 Z in + 1

(2).

The thickness of a sample is a crucial parameter to determine the intensity and position of the RL peak. Figure 10 shows the calculated RL curves of PBO and Graphene/MWNTs/PBO composites with different concentrations of Graphene/MWNTs. As seen in Figure 10a, pure PBO has little microwave absorbing capacity with RL values ranging from 0 to −3.5 dB. In contrast, the value of reflection loss was significantly improved with the incorporation of Graphene/MWNTs. With various Graphene/MWNTs content, as demonstrated in Figure 10b-g, the reflection loss minimum (RLmin) shifts from −7.33 dB to −50.17 dB. Enhanced absorption can be explained in terms of the incorporation of Graphene/MWNTs network, which substantially contributes to the dielectric loss. Figure 10h shows the RL value of all the samples at a thickness of 2.6 mm. It is clear that all of the GMP4.5, GMP6.0, GMP7.5 and GMP9.0 show improved reflection loss values, with GMP7.5 exhibiting the best microwave absorption


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performance. The RLmin of GMP7.5 reaches −50.17 dB at 12.58 GHz and the effective absorption bandwidth is 4.20 GHz (from 10.83 to 15.03 GHz). The major factors in achieving good microwave absorption are the transmission of incident microwave through the absorbers, and then the attenuation of the transmitted microwave by the absorber.51 In this case, as shown in Figure 11, the combination of Graphene/MWNTs and PBO enhances the matching of impedance between the surface of the absorber and free space, which improves the transmission of microwave by reducing the reflection from the surface of absorbers. On the other hand, as we all know, the conservation of energy for the electromagnetic field is indicated by Poynting theorem.52-53 It states that the total power entering the absorber goes partially into increasing the field energy stored inside the absorber and partially is lost into heat, i.e.,

(

∂u 2 + ∇ ⋅ S = − j ⋅ E − 2ω Im ε E + µ H ∂t

2

)

(3),

where

d (ωµ )  d (ωε ) 2 u = Re ε 0 E + µ0 H ω ω d d 

2

  

(4),

where〈…〉denotes the time average over the period of the carrier frequency, and S=E×H is the Poynting vector. The quantities ε 0 and µ0 are the permittivity and the magnetic permeability of vacuum, respectively. The quantities E and H are the electric and magnetic field intensities, respectively. According to the above equations, the absorption depends quadratically on the electric field intensity. Since Graphene/MWNTs/PBO composites are dielectric loss materials, the attenuation of microwave is mainly due to the strong dielectric loss derived from electron polarization and interfacial polarization of the heterojunction formed at the interface of the


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Graphene/MWNTs/PBO heterostructure.39, 54-56 Meanwhile, the Graphene/MWNTs network in PBO matrix forms a steric conductive channel, which not only promotes polarization efficiently, but also causes multiple reflections of incident microwave and thus, the incident microwaves can be scattered repeatedly and transformed into thermal energy rapidly.41 Furthermore, the residual defects and functional groups on graphene can act as polarized and scattering centers, which may further enhance the microwave absorbing capacity.


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Figure 10. Reflection loss curves of (a) PBO, (b) GMP1.5, (c) GMP3.0, (d) GMP4.5, (e) GMP6.0, (f) GMP7.5 and (g) GMP9.0, respectively, with varying thickness and (h) the reflection loss curves of the seven samples with a thickness of 2.6 mm.


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Figure 11. Microwave absorbing mechanism for Graphene/MWNTs/PBO composites. Table 1 compares the microwave absorption properties of state-of-the-art absorbers reported in the literature. Among all the composites, the Graphene/MWNTs/PBO composite exhibits the best microwave absorption ability: RLmin reaches −50.17 dB and the effective bandwidth is 4.2 GHz (RL < −10 dB, 90% absorption). Moreover, the filler loading is only 7.5% and the thickness is just 2.6 mm. All these characteristics meet the requirements (i.e., thin thickness, low density, broad effective bandwidths, strong absorption) for high-performance microwave absorption applications. Table 1 Microwave absorption properties of state-of-the-art absorbers Types of filler material

Matrix

Filler wt%

Thickness (mm)

RLmin (dB)

RGO/MnFe2O4

PVDF

5

3

-29

Effective bandwidth (< −10 dB) (GHz) 4.88

Frequency range (< −10 dB) (GHz)

Ref.

8.00-12.88

21 (2014)

TiNC

wax

-

1.32

-40.1

2.5

11.1-13.6

6 (2016)

EG

wax

14

4

-27.6

4.8

5.6-10.4

5 (2013)

Graphene

wax

0.5

5

-44.06

4.9

6.55-11.45

4 (2015)

RGO/Fe3O4

wax

45

2

-15.38

2.8

10.4-13.2

42 (2013)

RGO/α-Fe2O3

wax

8

5

-33.5

6.4

10.8-17.2

12 (2013)

Carbon/ Fe3O4

epoxy resin -

20

1.6

-32

2

10.5-12.5

14 (2015)

-

0.9-1

-35.51

60.5

57 (2015)

1 (2014)

Graphene foam Graphene foam

-

-

2

-34

14.3

6-18, 26.5-40, 75110 3.7-18

γ-Fe2O3/MWNTs

PBO

12

2.7

−32.7

2.48

11.31-13.79

58 (2016)


21


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Page 22 of 27

BaTiO3/MWNTs

PBO

12

3

−45.5

4.1

8.3-11, 15.1-16.5

2 (2015)

Graphene/MWNTs

PBO

7.5

2.6

−50.17

4.2

10.83-15.03

This work

Conclusions In summary, the multiphase nanocomposite of Graphene/MWNTs/PBO was fabricated via in situ polymerization of functionalized PBO precursor with GO/MWNTs nanocomposites, followed by high-temperature calcination. The presence of Graphene/MWNTs network in PBO plays a critical role in improving the properties. The Graphene/MWNTs/PBO composite with 7.5 wt% of GO/MWNTs loading exhibits excellent microwave absorption ability with RLmin of −50.17 dB at 12.58 GHz with a thin thickness of only 2.6 mm and the effective absorption bandwidth of 4.2 GHz (10.83-15.03 GHz, < −10 dB). The improvement of microwave absorption capacity is due to the synergetic effects of graphene, MWNTs and PBO, which enhances the polarization of the absorber. Moreover, the Graphene/MWNTs/PBO composite possesses exceptional thermal stability with the onset of thermal degradation over 600 °C. Therefore, it is believed that this work provides an effective strategy for high-performance microwave absorbers in hightemperature environments.

Acknowledgements This work was financially supported by the National Natural Science Foundation of China (51573045) and the International Collaboration Research Program of Science and Technology Commission of Shanghai (16520722000).

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TOC

3D Graphene/MWNTs/PBO composite with both superior thermal stability and excellent microwave absorption properties was obtained via in situ polymerization.


27

Poly(p-phenylenebenzobisoxazole) Multiphase

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