Electrically electromagnetic interference shielding microcellular composite foams with 3D hierarchical graphene-carbon nanotube hybrids

Electrically electromagnetic interference shielding microcellular composite foams with 3D hierarchical graphene-carbon nanotube hybrids

Journal Pre-proofs Electrically Electromagnetic Interference Shielding Microcellular Composite Foams with 3D Hierarchical Graphene-Carbon nanotube Hyb...

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Journal Pre-proofs Electrically Electromagnetic Interference Shielding Microcellular Composite Foams with 3D Hierarchical Graphene-Carbon nanotube Hybrids Hongming Zhang, Guangcheng Zhang, Qiang Gao, Meng Zong, Mingyue Wang, Jianbin Qin PII: DOI: Reference:

S1359-835X(20)30011-7 https://doi.org/10.1016/j.compositesa.2020.105773 JCOMA 105773

To appear in:

Composites: Part A

Received Date: Revised Date: Accepted Date:

19 December 2019 7 January 2020 12 January 2020

Please cite this article as: Zhang, H., Zhang, G., Gao, Q., Zong, M., Wang, M., Qin, J., Electrically Electromagnetic Interference Shielding Microcellular Composite Foams with 3D Hierarchical Graphene-Carbon nanotube Hybrids, Composites: Part A (2020), doi: https://doi.org/10.1016/j.compositesa.2020.105773

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© 2020 Published by Elsevier Ltd.

Electrically Electromagnetic Interference Shielding Microcellular Composite Foams with 3D Hierarchical Graphene-Carbon nanotube Hybrids Hongming Zhanga, Guangcheng Zhanga*1, Qiang Gaoa, Meng Zonga, Mingyue Wanga, Jianbin Qina* a

Department of Applied Chemistry, MOE Key Lab of Applied Physics and Chemistry in Space,

School of Natural and Applied Sciences, Northwestern Polytechnical University, Xi’an, 710072, China.

Abstract Three kinds of carbon-based fillers (thermally reduced graphene oxide-carbon nanotubes (TG-CN), chemically reduced graphene oxide-carbon nanotubes (RG-CN), graphene

nanoplates-carbon

nanotubes

(GN-CN))

were

synthesized.

Their

corresponding foams were fabricated by combined process of compression-molding and batch-foaming. While GN-CN/PMMA composite foams presented the bimodal microcellular structure, composite foams with RG-CN and TG-CN hybrids still shown typical unimodal cell-size distribution. Much higher electrical conductivity and electromagnetic interference shielding efficiency (EMI SE) were achieved in the TGCN/PMMA foams, which is closely associated with high intrinsic electrical conductivity and thin hierarchical structure of TG-CN hybrids. Specifically, a prominent electrical conductivity of 2.92 S/m and an absorption-dominated EMI-SE of more than 30 dB in the X-band were achieved in the lightweight TG-CN/PMMA foams (~0.65 g/cm3) at 10 wt% loading. These results demonstrate the TG-CN hybrids are highly promising nanofillers which could endow the composite foams with superior conductive and mechanical performance towards outstanding EMI-shielding application. Key words: Polymer-matrix composites (PMCs); Microstructure; Foaming; Electrical properties 1

Corresponding author. Tel.: +86-29-88431672 E-mail address: [email protected]; [email protected]

1. Introduction In recent decades, large-scale and fast-growing development of electronic technology in civil and military fields brings great convenience to our dairy life. Nevertheless, the correspondent pernicious electromagnetic (EM) pollution caused by the electronic equipment and integrated devices have not only interrupted the normal operation of surrounding facilities, but also severely threatened the human health.1-3 In this regard, great efforts have focused on the development of high-performance EMI shielding materials.4-5 Among these EMI shielding materials, conductive polymer composites (CPCs) are considered to be realistic alternatives to conventional metalbased materials owing to the attractive qualities of chemical stability and structural flexibility. Meanwhile, the outstanding characteristics of low density and design flexibility inside CPCs bring great values for their practical use in aerospace, automobile industry and portable electronics fields.6-9 Carbon-based nanofillers, such as carbon nanofibers (CNFs)10, carbon nanotubes (CNTs)11 and graphene12, have manifested great potential in assembling advanced multifunctional materials. Among them, graphene is of great interest as a result of their unique layered structure and remarkable conductivities. Graphene is a newly twodimensional (2D) carbon allotropes which was covalently bounded by atomically thin single layer carbon atoms.13 Typically, there are three main kinds of graphene products prepared by top-down approach utilizing as nanofillers in polymer composites, Graphene nanoplates (GNPs),14 which is prepared by the direct exfoliation of graphite sheets by ultrasonic technique or electrochemical method, chemically reduced graphene oxide (RGO),13 which is obtained by reducing the graphene oxide via different chemical agents, and thermally exfoliated reduced graphene oxide (TGO),15 which is produced by heating the dry GO under high temperature. Plenty of work focused on the functionalization and surface modification of these three types of graphene sheets to avoid the undesirable re-stacking and try to facilitate the efficient connections through plane-to-plane and edge-to-edge contacts in advanced EMI shielding materials.16-17 Nevertheless, the 2D graphene sheets have difficulties in linking together within the matrix in comparison with the traditional one-dimensional (1D) CNTs, consequently

impeding the construction of conductive networks in polymer composites.4,18 For example, Liang and his co-workers prepared epoxy/functionalized-RGO composite foams, and a low EMI SE of 21 dB was obtained when 15 wt% graphene was added.19 Ling et al. used phase separation process to produce PEI/TGO nanocomposites, and the EMI SE values of resulted foams were just found to be 11 dB with a graphene loading of ~10 wt%.15 Sima et al. reported on PLA/GNPs composites, which were fabricated by melt mixing and compression molding two-steps technology, recording a SE value of 11.6 dB with a high content of 15 wt%.20 The high loading also sacrifices the mechanical performance, process-ability of graphene-based composites and inevitably increases the economic cost, thereby immensely restricting their practical applications. To explore an effective strategy to promote both dispersion state and electrical property of graphene-based composites, researchers innovatively insert onedimensional CNTs into two-dimensional graphene sheets to construct hybrid network structure.4,21-22 Three-dimensional (3D) materials comprised of CNTs and graphene have actually avoided the self-aggregation of graphene to some extent and provided abundant efficient pathways for electrons transfer.23-24 These graphene-CNT hybrids (G-CNT) have shown extraordinary electrical performance in preparing batteries,25 supercapacitors 26 and microwave absorbers. 27 For example, Sun et al. investigated the effect of super-aligned CNT-graphene composites on the electrochemical performance of S-CNT/G lithium sulfur batteries. A remarkable specific capacity before and after 100 cycles were achieved thanks to the constructed hybrid framework.25 Chen et al. reported the microwave absorbing performance of 3D absorber hybrids comprising of polar oxygen functionalized MWCNTs and graphene and got a -41dB reflection loss at 9.6Hz.27 In the noted conductive nanocomposites, three-dimensional hierarchical GCNT hybrids have also been incorporated into polymer matrix to enhance their EMI shielding capabilities.28-30 Verma et al. developed a GNPs/MWCNTs/PU composites and were succeed to obtain an EMI SE of 47dB and an electrical conductivity of 9.5×102

S/cm at 10wt%.29 In another study, Zhao and co-workers studied the EMI shielding

performance of PVDF/MWCNTs/Graphene composite films with different thickness. A high specific SE value of 1557dB·cm3/g was observed at a thin thickness of 0.1mm.30

These results are obviously superior to the single graphene-based composites which are attributed to the formation of 3D reinforcement and conducting framework. Porous CPCs possess both lower density and higher absorption-dominant EMI shielding at the same time, which could significantly reduce the materials and energy consumption and avoid the undesirable secondary microwave pollution.31-32 Unlike the traditional chemical and physical foaming agents, the nontoxic and inexpensive supercritical carbon dioxide (scCO2) is much environmental in line with the requirement for sustainable development. The moderate re-distribution and selective localization of nanofillers inside the scCO2 foamed materials are proved to be efficient in constructing the conductive network.4,33 In addition, research works have also demonstrated that the porous microcellular structure can provide more multiple reflections to eliminate EM energy.32 Wang et al. fabricated PLA/graphite foams (PLA/G-MOFIM) via mold-opening foam injection molding process. The corresponding EMI SE value reached 45 dB with 10 wt% graphite, while the EMI SE of the PLA/graphite composites formed by simple injection molding was just 18 dB.34 In our previous works, we also have successfully prepared Ni-chains/PVDF and MWCNTs/Epoxy composite foams exhibiting superior EMI shielding performance based on batch foaming process.11,41 By far, however, there are few reports on the fabrication of G-CNT/polymer composite foams, or on their EMI shielding capacities. In this research, we fully utilized the advantages of synergistic effect inside G-CNT hybrids and optimized microcellular foaming degree, and developed the G-CNT/ polymethyl methacrylate (PMMA) composite foams by a compression molding and scCO2 foaming combined technology. Above all, 3D hierarchical “line-plane” RG-CN and TG-CN hybrids were synthesized by the chemical cross-linking reaction between chlorinated-CNT and amided-GO followed by chemical reduction and thermal reduction, respectively. Meanwhile, GN-CN hybrids were constructed via a facile method of ultrasonication filtration process. Three different kinds of G-CNT hybrids were then incorporated into PMMA matrix by solution mixing and compressionmolded into nanocomposite sheets. Finally, G-CNT/PMMA nanocomposites were foamed with the physical agent in an optimized degree. The effects of various G-CNT

hybrids and their content on the final morphologies, mechanical performance, electric conductivity and EMI shieling properties of the obtained composite foams were explored. Furthermore, the detailed electrical conducting and EMI shielding mechanisms inside the G-CNT/PMMA foam were also systematically discussed.

2. Experimental section 2.1 Materials PMMA (DF23-8N) with average molecular weight of 350000 g/mol were provided by Evonik Industries AG. The MWCNTs (purity>95%, diameter 30-50 nm, length 1020 μm, density 2.2 g/cm3, surface area 200 m2g-1) were obtained from Chengdu Organic Chemicals Co., Ltd. GNPs (XF021, thickness 3-10 nm, average diameter 5-10 μm, density 2.2 g/cm3) were supplied by Nanjing XFNANO. Industrial-grade graphite were purchased from Qingdao Tianheda Co., Ltd. All the chemicals and reagents were analytical grade, used without further purification, and obtained from Aladdin chemical agent Co., Ltd. CO2 with a purify of 99.9 % was employed as the physical blowing agent. 2.2 Preparation of cross-linked GO-CNT hybrids Graphene oxide (GO) was prepared from graphite via a modified Hummers method.36 Subsequently, 600 mg ethylenediamine NH2(CH2)2NH2 was dissolved in 40 ml ethanol and dropwise put into 40 ml GO-water suspension (5 mg/ml) under continuous mechanical mixing to react for 24 h at 25 °C.37 The obtained product was washed by deionized water-ethanol for several times and the precipitation was filtered and dried at 80 ℃ for 12 h. The ethylenediamine functionalized GO were labelled as EGO. Meanwhile, MWCNTs were oxidized by a mixture of sulfuric acid and nitric acid (v/v = 3:1) at 60 °C for 6 h, and then centrifuged, filtered and freeze-dried to obtain oxidized CNT (O-CNT). The CO were chlorinated with aid of thionyl chloride (SOCl2) at 70 °C for 12 h under N2-purged reflux conditions to get F-CNT.38 The remaining SOCl2 was removed in vacuum oven. To prepare chemical bond cross-linked GO-CO, a certain mass ratio of E-GO and F-CNT (1:3) was added into 20 ml anhydrous toluene under sonication for 30 min, and then heated to 75 °C to react for another 24h with

vigorous mixing in N2 atmosphere. Finally, the black powders (GO-CNT) were then collected by vacuum filtration with aid of PTFE membrane and kept it in an oven at 70 ℃ for 24 h to remove the moisture. The synthesis process of GO-CNT was shown in Figure S1. 2.3 Preparation of RG-CN, TG-CN and GN-CN hybrids The desired RG-CN and TG-CN were obtained by chemical reduction and thermal reduction from GO-CNT, respectively. For preparing RG-CN, 0.1g GO-CNT was put into 100 ml deionized water and then add 5 ml hydrazine hydrate at 100 ℃ for 2 h to chemical reduce the GO-CNT hybrids.21 For preparing TG-CN, the dried GO-CNT powders heated to 900 ℃ under argon gas atmosphere for 1h in a tube furnace.39 The GN-CN hybrids were prepared by a facile process of ultrasonication filtration. The mass ratio of GNPs and MWCNTs was chosen as 1:3. In a typical procedure to prepare GCNT hybrids, 0.1 g GNPs and 0.3 g MWCNTs were dispersed in 100 ml deionized water and mixed vigorously for 3 h. The black powders were then collected by vacuum filtration and kept it in an oven at 100℃ for 36 h to remove the moisture. For convenience, these three types of obtained products were denoted as GN-CN, RG-CN and TG-CN, respectively. The detailed synthesis process of GN-CN, RG-CN and TGCN hybrids were shown in scheme 1. RGO and TGO were also prepared for comparison. Typically, to obtain RGO, GO (100 mg) was first added into 100ml deionized water under mechanical agitation. The suspension of GO was subsequently chemically reduced with 5 ml hydrazine hydrate at 100℃ for 2h. The resultant solids were washed with water and dried at 70℃ for 24h. To prepare TGO, the freeze-dried GO products were heated to 900℃ with argon gas atmosphere protected and held for 1h in a tube furnace.

Scheme 1 Synthetic illustration of the preparation of (A) GN-CN hybrids and (B) RG-CN and TG-CN hybrids.

2.4 Preparation of G-CNT/PMMA nanocomposites We fabricated G-CNT/PMMA nanocomposites through solution mixing followed by compression molding.4 The GN-CN hybrids were selected as an example. In brief, GN-CN hybrids were initially dispersed in 200 ml THF under continuous ultrasonication for 30 min. Quantitative amount of PMMA granules were subsequently dissolved in the pre-prepared suspension with the assistance of 60 min magnetic agitation and 30 min ultrasonication at room temperature. Then, the mixtures were poured into 100 ml methanol to facilitate precipitation. Finally, the G-CNT/PMMA composites were acquired through vacuum filtration, evaporation and compressionmolding processes. The hot-press temperature, pressure and time were 245 ℃, 10 MPa and 15 min, respectively. To explore how the hybrids content influenced the EMI performance, different weight fractions of GN-CN, RG-CN and TG-CN hybrids in PMMA matrix were prepared. (1 wt%, 2 wt%, 3 wt%, 5 wt%, 7 wt% and 10 wt%) For convenience, various nanocomposites are marked as GN-CN-x, RG-CN-x and TG-CNx, respectively, where x denotes the weight percentages. 2.5 Fabrication of lightweight G-CNT/PMMA composite foams A two-step batch foaming process was used to prepare G-CNT/PMMA nanocomposite foams. Before foaming, all the samples were dried using a vacuum oven

at 80 °C for 4 h to remove all moisture. In a typical experiment, the G-CNT/PMMA nanocomposites were cut into 25 mm × 15 mm × 2 mm sized pieces to be placed into the vessel. The pre-molded specimens were first immersed into supercritical CO2 under the certain pressure of 10 MPa and temperature of 40 ℃. After adsorbing for 16 h, the autoclave was placed into ice-water mixture for 15 min to suppress cell nucleation in this step. The CO2 was then released, and the saturated specimens were immediately transferred into the hot oil at 100 ℃ for 60 s to induce cell nucleation and cell growth. Subsequently, the specimens were put into cold water to stabilize their foam structure. 2.6 Characterizations X-ray photoelectron spectrometer, Raman spectroscopy, X-ray diffraction, Thermal gravimetric analysis: X-ray photoelectron spectrometer (XPS) spectra of the fillers were recorded with a Thermo Fisher Scientific (SID-Elemental) spectroscopy. Raman spectra were collected on a WITech CRM Raman system using a 532nm laser. The cystal structure of the hybrids was obtained by X-ray diffraction (XRD, Rigaku, model D/max-2500 system) equipped with a Cu Kα source. Thermogravimetric analyses (TGA) of the samples were tested by TGAQ 600 from 25 to 650 °C at a heating rate of 10 °C/min at N2 atmosphere. Scanning and transmission electron microscopy: The microstructures of the nanofillers and the distribution state of these fillers within the polymer matrix were characterized by field-emission scanning electron microscope (FE-SEM, Verios G4, FEI) and high-resolution transmission electron microscopy (HRTEM, Tecnai F30 G2, FEI). The cellular structure of foams was analyzed with a scanning electron microscope (SEM, TESCAN VEGA3 LMH). The specimens were freeze-fractured in liquid nitrogen, and sputter-coated with gold layer (10-20 nm) prior to SEM observations. Cell parameters, mass density and expansion ratio: Cell density and size were calculated based on the SEM images via the Image-Pro Plus software. Cell density was obtained by the following formula:32 𝑛𝑀2

𝑁𝑓 = (

𝐴

32

)

(1)

where n is the number of cells, A is the area of the SEM image and M is the

magnification factor. The cell size (d) is estimated by equation (2): 𝑑=

∑𝑑𝑖𝑛𝑖

(2)

∑𝑛𝑖

where ni is the number of the cell, di is a perimeter-equivalent diameter (i >200). The mass density of both solids (ρs) and foam (ρf) was obtained by a water-displacement method (ASTM D792). The expansion ratio is the ratio of sample density of composite to corresponding foam. Electrical conductivity and EMI shielding effectiveness (SE): The electrical conductivities below 10-6 S/m of composites and foams were determined by FischerElektronik Tera-Ohmmeter TO-3, and the conductivities above10-6 S/m were measured by four-point probe apparatus (JGR, ST2263). All the samples were cut into disk shapes with a diameter of 7.0 mm and a thickness of 2.0 mm. Characteristic EMI shielding parameters of the samples were measured by vector network analyzer (VNA) (HP8720ES, Agilent) using the wave-guide method in the Xband frequency range according to ASTM D5568-08, and the corresponding dimension of the testing samples is length of 22.86 mm, width of 10.16 mm, and thickness of 2.5 mm. The scattering coefficients (Sij) were obtained directly and used to calculate the reflectivity (R), transmissivity (T), absorptivity (A), microwave reflection (SER), absorption (SEA) and total EMI SE (SET), using the following formulas:4,34 𝑅 = |𝑆11|2 , 𝑇 = |𝑆12|2

(3)

𝐴=1―𝑅―𝑇

(4) 1

𝑆𝐸𝑇 = 10𝑙𝑜𝑔𝑇

(5)

SE𝑅 = 10 × 𝑙𝑜𝑔10(1/(1 ― 𝑅))

(6)

𝑆𝐸𝐴 = 10 × 𝑙𝑜𝑔10((1 ― 𝑅)/𝑇)

(7)

Mechanical properties: The compressive strength was measured by a SANS CMT5105 test machine with a sensor of 20 KN according to ASTM D1621‐2010 at room temperature. Cubic specimens with a dimension of 10 × 10 × 10 mm3 were used for the compression test. However, as the solids were very thin (about 2 mm), it was difficult to perform the compressive tests according to the standards. Four pieces were glued together to ensure a thickness of 8 mm. The foamed samples were then cut by a

diamond-wafering blade into 10 × 10 mm2 square pieces, and the surfaces of the samples were smoothed with fine-grade sandpaper. The compression speed used in the test was 1 mm min−1, and the maximum compression depth was 50% of the height of the specimen.

3. Results and discussion 3.1 Characterization of G-CNT hybrids Fig.1 illustrates the TEM photographs of GO, O-CNT, GO-CNT, RG-CN, TG-CN and GN-CN, respectively. GO presents the typical folded and exfoliated structure (Fig.1a) with a single-layer thickness around 1.4 nm (AFM, Fig.S2(a)). In Figure.1b, the carboxylated MWCNTs (O-CNT) hold the intact tubular nanostructure without obvious aggregation and shortening after modification. For the GO-CNT hybrids (Fig.1c), the chemical amide bonds which formed between the edges of GO and the ends of MWCNTs promote the construction of 3D hierarchical structure. Besides, the strong π-π stack interaction and hydrogen bond interaction between GO and O-CNT also prompt the tubular MWCNTs to uniformly disperse on the surface of GO. After chemical and thermal reduction, as shown in Fig. 1d and 1e, both RG-CN and TG-CN hybrids present the undamaged 3D nanostructure inherited from the GO-CNT hybrids. Furthermore, for the GN-CN hybrids (Fig.1f), the fibrous MWCNTs are randomly covering on the thick GNPs (thickness around 4.5 nm, Fig.S2(b)). Fig.1 also shows the FESEM images of the typical RG-CN, TG-CN and GN-CN heterostructures. It could find that all the heterostructures exhibit a homogeneous morphology of layer-by-layer heterostructures, and these different kinds of graphene sheets are covered with networkstructured CNTs. To better confirm the structural evolution of these hybrids, the XRD patterns of pristine Graphite, GO, RGO, TGO and GNPs without combing with MWCNTs have also been exhibited in Fig.2(d). According to the Bragg equation, graphite presents a characteristic peak around 25.88° corresponding to an interplanar distance of 0.34 nm, while GO shows an increased intra-gallery of 0.80 nm. Nevertheless, both RGO and TGO present unconspicuous peaks on their panels, suggesting efficient exfoliation of the graphene sheets.16 Conversely, GNPs display the

most compact structure with a 0.30 nm interplanar distance.

Fig.1 TEM images of (a) GO, (b) O-CNT, (c) GO-CNT, (d) RG-CN hybrids, (e) TG-CN hybrids and (f) GN-CN hybrids; FESEM images of (g) RG-CN hybrids, (i) TG-CN hybrids and (j) GN-CN hybrids

To monitor the chemical compositions of obtained hybrids, the XPS wide scan spectra of RG-CN, TG-CN and GN-CN heterostructures is presented in Fig.2(a). The general XPS spectra of G-CNT hybrids contained typical peaks only assigned to C, O, while the N peaks were just present in the spectra of TG-CN and RG-CN hybrids, suggesting that GO and CNT are successfully bonded by amide groups. Compared to GO, RG-CN hybrids exhibit a distinct decrease in the O1s peak intensity, signifying successful removal of oxygen-containing groups in GO-CNT due to chemical reduction by hydrazine treatment. More interestingly, TG-CN hybrids, which obtained by thermal annealing process, display a more significant decline in O1s peak. It is noted that the C/O atomic ratio of TG-CN is about 30.9, which is almost 6 times higher than that of RG-CN (5.7) and 2 times higher than the mechanically mixed GN-CN composite (15.4).

This obvious distinction could be attributed to strong reduction ability of hightemperature annealing. The deconvoluted C 1s spectra of these hybrids also confirmed the successful chemical bonding between GO and CNT and efficient elimination of oxygen-containing groups after chemical and thermal reduction process. Meanwhile, a weak signal of N 1s could also be found in the TG-CN hybrids even after hightemperature thermal annealing.

Fig.2 (a) XPS panels, (b) TGA results and (c) Raman spectra of GN-CN, RG-CN and TG-CN hybrids; (d) XRD patterns of pristine Graphite, GO, RGO, TGO and GNPs

To investigate the thermal stabilities and further confirm the functionalization degree of the RG-CN, TG-CN and GN-CN hybrids, the thermogravimetric analysis (TGA) were performed. The TGA results are displayed in Fig.2b. It is obvious that both RG-CN and GN-CN hybrids show low thermal stability with temperature and they start to lose weight below 100 ℃ owing to the intercalated water in RG-CN and GN-CN molecules. Moreover, both RG-CN and GN-CN hybrids exhibit significant weight loss between 100-650 ℃ which is ascribed to the removal of unstable oxygen functionalities. In this temperature region, RG-CN and GN-CN hybrids show 14.6 wt% and 4.2 wt%

loss of weight, respectively. It indicates that RG-CN possesses 14.6 wt% oxygen functional groups and GN-CN possesses 4.2 wt% oxygen functional groups, which is accordance with the above XPS results. Different from chemical reduced RG-CN hybrids, thermal reduced TG-CN shows much excellent thermal stability because oxygen functional groups have been eliminated during the annealing process, which is consistent with their high C/O ratios. Furthermore, Raman spectroscopy was utilized as another tool to detect structural or chemical bonding, as shown in Fig.2c. All these three types hybrids demonstrate two typical peaks of G band around 1564 cm-1 and D band around 1340 cm-1. The G-band is related to sp2-hybridized carbon, whereas the D band is induced by structural defects and disordered structures of sp3 carbon.39 Generally, the intensity ratio of D band to G band (ID/IG) is applied to express the defected degree of graphene. The ID/IG of GOCNT is about 1.07 and it declines to 0.82 after chemical reduction, suggesting that less functional groups and fewer defects on RG-CN hybrids. Furthermore, the ID/IG value further decreases to 0.51 for TG-CN after thermal annealing, suggesting the efficient restoration of structural defects and the transform from sp3 C to sp2 C, which is beneficial to improve the carrier mobility and EMI shielding performance.40 Meanwhile, the ID/IG of GN-CN was measured to be 0.18, which also displays great prospect in enhancing the electrical conductivities and EMI shielding capacities.

3.2 Dispersion of G-CNT hybrids in PMMA Matrix The foaming behavior and final properties of composites are greatly depending on the dispersibility of nanofillers in polymer matrix.41 To evaluate the dispersion state of G-CNT hybrids in composites, FE-SEM was first used to investigate the morphology of the composites. The microstructures of MWCNTs/PMMA composites have also been performed for comparison. All three G-CNT/PMMA composites shows uniform dispersion of G-CNT hybrids in freeze-fractured sections, while relatively plenty of aggregations appeared in the fractures of MWCNTs/PMMA composites (Fig.3a). However, GN-CN hybrids (Fig.3d) present clear withdraws in the fractures, indicating the poor compatibility of thick GN-CN with the PMMA matrix even they have been

modified with SDBS. As for RG-CN (Fig.3b) and modified TG-CN (Fig.3c) based composites, the residual oxygen groups on RG-CN, the introduced amino on modified TG-CN as well as their thin 3D thin hierarchical structure provide strong interfacial adhesion between hybrids and matrix without obvious sliding under tension. TEM photographs were then employed to further characterize the distribution of G-CNT hybrids. As shown in Fig.3e, the pristine MWCNTs is severely aggregated in the PMMA matrix. In contrast, all these three kinds of G-CNT hybrids are homogeneously and separately distributed in the composites without obvious aggregations. In addition, compared to GN-CN hybrids, both RG-CN and modified TG-CN hybrids present a much better dispersion state in polymer matrix owing to their hydrophilic functional groups and thin layer structures. 1D CNTs and 2D graphene sheets show obvious synergistic effect in dispersing in the polymer matrix. This benefits a lot to establish efficient conductive network and load transfer skeleton at low fillers content, and thereby endowing the insulating polymer matrix with superior electrical and mechanical properties.38

Fig.3 Morphology of G-CNT/PMMA composites. SEM images of (a) MWCNTs/PMMA, (b) RGCN/PMMA, (c) TG-CN/PMMA and (d) GN-CN/PMMA composites; TEM images of (e) MWCNTs/PMMA, (f) RG-CN/PMMA, (g) TG-CN/PMMA and (h) GN-CN/PMMA composites.

3.3 Morphology of G-CNT/PMMA composite foams

To observe how various G-CNTs hybrids and their content affected the final morphologies of the foams, different microcellular PMMA composite foams with a 2fold expansion ratio (relative density around 0.5) were successfully fabricated by using CO2 as the foaming agents, which was realized via the same batch-foaming process. Fig.4 and 5 exhibit the morphology and corresponding cell size distribution of the final G-CNT/PMMA composite foams. For comparison, the SEM image and cell-size distribution of pure PMMA foam are shown in Fig.S3. As shown here, while GNCN/PMMA composite foams present the bimodal microcellular structure, the composite foams with RG-CN and TG-CN hybrids show the typical homogeneous cellsize distribution. This possibly resulted from the relatively high content of CO2 absorbed by the thick GNs compared to thin RG and TG during the saturation process. These CO2 inside the thick GNs were unstable and more prone to escape from the polymer matrix when the pressure released, and thereby inducing a big-size cell in the foaming process, which have been explained in our previous work.4 Meanwhile, the high-aspect-ratio MWCNTs induced the small-sized cell inside this GN-CN/PMMA foams system. Conversely, both the thin RG-CN and TG-CN hybrids were homogeneously distributed in the PMMA matrix, they just acted as one kind of site to induce the cell nucleation.4,32-34 Therefore, RG-CN/PMMA and TG-CN/PMMA foams possess the unimodal cell-size distribution. Moreover, as compared in Fig.4, much smaller cell size and narrower cell-size distribution were observed in TG-CN/PMMA foams. Table.1 summarizes the parameters of these foams, such as the average cell size, cell density, mass density and void fraction. Notably, all these three kinds of composite foams displayed decreased average cell sizes and boosted cell densities with the increasing filler content. Particularly, both the primary and secondary cell size inside GN-CN/PMMA foams decreased at the same time. The introduction of G-CNT gradually decreased the cell sizes and increased the pore densities, mainly due to the heterogeneous nucleation effects of G-CNT hybrids in the polymer matrix. Typically, as the TG-CN hybrids raising from 1wt% to 10 wt%, the average cell size drop from 2.28 m to 0.78 m and the cell density increased from 7.16×1010 cells/cm3 to 8.44×1011

cells/cm3. Furthermore, the mass density of resultant foams decreased to around 0.65 g/cm3 and the void fraction came upon to about 0.5, which indicates that G-CNT-based composites could be employed as lightweight EMI shielding materials.

Fig.4 Cell morphology of G-CNT/PMMA composite foams. SEM images of (a1) RG-CN-1, (a2) RG-CN-2, (a3) RG-CN-3, (a4) RG-CN-5 and (a5) RG-CN-10 foams; (b1) TG-CN-1, (b2) TG-CN-2, (b3) TG-CN-3, (b4) TG-CN-5 and (b5) TG-CN-10 foams; (c1) GN-CN-1, (c2) GN-CN-2, (c3) GNCN-3, (c4) GN-CN-5 and (c5) GN-CN-10 foams.

Fig.5 Cell-size distribution of G-CNT/PMMA composite foams: (a1) RG-CN-1, (a2) RG-CN-2, (a3) RG-CN-3, (a4) RG-CN-5 and (a5) RG-CN-10 foams; (b1) TG-CN-1, (b2) TG-CN-2, (b3) TGCN-3, (b4) TG-CN-5 and (b5) TG-CN-10 foams; (c1) GN-CN-1, (c2) GN-CN-2, (c3) GN-CN-3, (c4) GN-CN-5 and (c5) GN-CN-10 foams. Table.1 The average cell size, cell density, density, relative density, void fraction and expansion ratio of the RG-CN/PMMA, TG-CN/PMMA and GN-CN/PMMA foams Samples

Cell size (m)

Cell density

Density

Relative

Void

Expansion

D1

D2

(cells/cm3)

(g/cm3)

density

fraction

ratio

RG-CN-1

2.74

--

5.04×1010

0.55

0.46

0.54

2.17

RG-CN-2

1.92

--

1.28×1011

0.56

0.47

0.53

2.13

RG-CN-3

1.79

--

1.92×1011

0.59

0.49

0.51

2.04

RG-CN-5

1.38

--

3.28×1011

0.64

0.52

0.48

1.93

RG-CN-10

0.92

--

6.22×1011

0.66

0.52

0.48

1.93

TG-CN-1

2.28

--

7.16×1010

0.57

0.48

0.52

2.08

TG-CN-2

1.52

--

2.33×1011

0.60

0.50

0.50

2.00

TG-CN-3

1.45

--

3.03×1011

0.62

0.52

0.48

1.92

TG-CN-5

0.95

--

6.04×1011

0.64

0.53

0.47

1.89

TG-CN-10

0.78

--

8.44×1011

0.65

0.53

0.47

1.89

GN-CN-1

1.86

32.96

3.28×1010

0.54

0.45

0.55

2.22

GN-CN-2

1.76

22.92

7.32×1010

0.55

0.46

0.54

2.17

GN-CN-3

1.72

21.97

8.23×1010

0.56

0.46

0.54

2.17

GN-CN-5

1.18

15.19

1.44×1011

0.57

0.46

0.54

2.17

GN-CN-10

1.07

11.99

2.35×1011

0.59

0.47

0.53

2.13

3.4 Electrical conductivity of G-CNT/PMMA composite foams As discussed above, electrical conductivity is a key factor to determine the EMI shielding capacity of composite foams.3 The volume electrical conductivity values of G-CNT/PMMA nanocomposites and its microcellular foams as a function of G-CNT hybrids species and loading were investigated and plotted. (Table.S1 shows the conversion from mass fraction to volume fraction in both solid and foamed composites.) Fig. 6(a), (b) and (c) presented the electrical conductivity of solid and foamed GNCN/PMMA, RG-CN/PMMA and TG-CN/PMMA composites, respectively. Obviously, the mounting G-CN hybrids content enhance the electrical performance of these obtained composites and foams gradually. All the composite foams exhibit far higher electrical values than solid composites at the same hybrids content. For instance, the electrical conductivity of GN-CN/PMMA foams with 3 wt% (0.69 vol%) hybrid is about 2.6×10-3 S/m, while GN-CN/PMMA composites with 3 wt% (1.7 vol%) hybrids could only reach 2.86×10-4 S/m. Similarly, TG-CN/PMMA composite foams with 3wt% (1.1 vol%) nanofillers could reach up to 3×10-2 S/m, which satisfyingly present a significant enhancement compared to that of TG-CN/PMMA solids. The results indicate that moderate microcellular foaming might promote the construction of superior conductive network inside the G-CNT/PMMA composite foams. During foaming process, biaxial stretching prompts nanofillers to slightly orient around the cells while cell-to-cell compression force nanofiller to decrease their distance. This action increases the interconnections among G-CNTs and thereby increases the electrical conductivity.

Fig.6 (a) Electrical conductivity of GN-CN/PMMA composite solids and foams with various volume fractions; (b) Electrical conductivity of RG-CN/PMMA composite solids and foams with various volume fractions; (c) Electrical conductivity of TG-CN/PMMA composite solids and foams with various volume fractions; (d) Electrical conductivity of GN-CN/PMMA, RGCN/PMMA and TG-CN/PMMA foams with different mass fractions.

As shown in the curves, all these prepared composites and foams exhibit typical percolation behaviors. The percolation threshold is calculated via power law: σ = σ0(𝜑 ― 𝜑𝑐)𝑡. where  is the composite’s conductivity,  is the concentration of G-CNT hybrids, c is the percolation threshold, and t is a critical exponent revealing the lattice dimensionality. In theory, the value of t > 1.3 and t > 2 mean two-dimensional and three-dimensional conductive networks formed in CPC, respectively.42 Both c and t of these obtained composites are fitted and shown in Fig.7. As compared in the figure, apparently, all the foamed G-CNT/PMMA composites presented higher t values than their solid systems. In this work, the value of t is 2.45, 2.21 and 2.96 for the GNCN/PMMA foam, RG-CN/PMMA foam and TG-CN/PMMA, respectively, indicating the existence of 3D conductive pathways in all these foamed composites but two-

dimensional conducive networks in GN-CN/PMMA and RG-CN/PMMA composites. Meanwhile, the critical volume fraction of each foamed composites also has different degree drop compared to that of solids. As confirmed in our previous work,4,41 microcellular foaming facilitates a better dispersion state and a high-level exfoliation of 2D Graphene and a moderate biaxial stretching and cell-to-cell compression prompt nanofillers to slightly construct conductive pathways around the cells with decreased distance. Therefore, we summarized that the reason for a higher conductivity inside composite foams is that the volume exclusion of foamed specimen and the orientation and interconnection of fillers during cell-growth-induced stretching promote an obvious decline of the average distance between G-CNTs, and the well-proportioned microcellular structures prompted an efficient conductive pathway in composite foams, thereby providing a fast electron transport channel internally.34

Fig.7 (a) The percolation threshold and the critical exponent of the G-CNT/PMMA solids and foams; (b) Electrical conductivities of MWCNTs/PMMA, GNPs/PMMA, RGO/PMMA, TGO/PMMA, GN-CN/PMMA, RG-CN/PMMA and TG-CN/PMMA foams with 10wt% loading.

Moreover, as exhibited in Fig.7(b), all the G-CNT composite foams with 10wt% loading of G-CNT hybrids show higher conductivities than that of both single-CNTs and single-Graphene based composite foams. Especially, while the electrical conductivity of TG-CN/PMMA foams comes upon to 2.92 S/m, the MWCNTs/PMMA and TGO/PMMA composite foams can only reach 0.56 S/m and 0.46 S/m. This

phenomenon could be explained by the synergistic effect of G-CNT hybrids. It is generally believed that graphene nanosheets are inclined to reaggregate during the reduction process because of the intermolecular π−π stacking attraction forces and MWCNTs have a strong tendency to bundle in polymer matrix due to the strong Vander Waals’ interactions30, thereby leading to an unfavorable effect for the electrical property in the singe-filler based composites. However, in G-CNT hybrids system, CNT could insert into the graphene nanosheets to impede the restacking during the reduction process and graphene also provide increased steric hindrance to separate the bundled CNTs. Besides, as a result of the high aspect ratio and outstanding electrical conductivity, the excess 1D MWCNTs bridges the adjacent 2D graphene, providing extra channels for the motion of both migrating and hopping electrons.23 Consequently, the hierarchical Graphene-MWCNTs structures endow the G-CNT/PMMA composite foams with an ideal electrical conductivity. The electrical property of GN-CN/PMMA, RG-CN/PMMA and TG-CN/PMMA composite foams has also been compared in Fig. 6(d) and Fig.7(b). As seen in the figure, TG-CN/PMMA foams present a higher electrical conductivity than both GNCN/PMMA and RG-CN/PMMA composite foams. As confirmed in XPS results, TGCN which obtained by thermal annealing process displays a higher C/O ratio and larger intrinsic electrical conductivity, and it is more efficient in improving electrical conductivity of composite foams while they present almost the same expansion ratio in each content. High oxidation degree and defects on RG-CN hybrids which shown in XPS and Raman results may greatly impede the movement of electrons and phonons, and thereby contributing to relatively low electrical conductivity. Apart from this, GNCN/PMMA composite foams also exhibit a bit lower conductivity than TG-CN/PMMA foams in high loadings (>5wt%), this could attribute to the thick graphene structure in GN-CN hybrids. Scheme 2 depicts the microstructure of these three kinds of nanofillers and their corresponding electron transport network in relative composites and foams. The remarkable electrical conductivity inside the TG-CN/PMMA foams is closely associated to the synergistic effect of TGO and MWCNTs and the moderate microcellular foaming process, which could provide the most efficient pathways for

electron transport.

Scheme 2 (A, B and C) Microstructure of GN-CN, RG-CN and TG-CN hybrids. (A1, B1 and C1) Electron transport network of GN-CN, RG-CN and TG-CN hybrids in composites. (A2, B2 and C2) Electron transport network of GN-CN, RG-CN and TG-CN hybrids in foams.

3.5 EMI shielding performance of G-CNT/PMMA composite foams In contrast to the pure PMMA foam that is highly electrically insulating, all these G-CNT/PMMA composite foams show good electrical conductivity, which gradually increases with the increasing G-CNT hybrids content. These values are higher than the target electrical conductivity value (∼1 S m−1) required for EMI shielding application.43 As a result of their high electrical conductivity and abundant interfaces resulting from the uniform porous structure, these as-prepared G-CNT/PMMA composite foams are expected to meet the requirements for efficient EMI shielding application. The total EMI SE of both the solid and foamed G-CNT/PMMA composites with different contents was further recorded in X-band frequency (8.2~12.4 GHz). It is obvious in Fig. 8 that EMI SE values slightly increased with the increasing frequency and were greater at a higher G-CNT hybrids content in both the foamed and solid specimens. As

compared in Fig.8, at a given G-CNT hybrids content, all the foamed composites presented higher SET values than their solid counterparts. Typically, when the TG-CN hybrids loading is 10 wt%, the average EMI SE of the foamed composites could reach 30.4 dB, which could block 99.90 % the incident microwave, while the correspondent solid composites just had an EMI SE of 21.6 dB. Similarly, the SET of GN-CN/PMMA composite foams with 10 wt% fillers could easily get a value of 25.2 dB, which was 0.4 times higher than the solid composites. These results indicate that moderate microcellular foaming plays a decisive role in promoting the EMI shielding performance. In detail, foaming enhanced the electric conductivities, and thus providing more free electrons to interact with the incident radiation.35 Meanwhile, both microcellular structures and G-CNT hybrids which located along the cell wall promoted more interfaces to enhance the multiple reflections and microwave absorptions. Besides, the more condensed and randomly oriented G-CNTs hybrids inside the composite foams could also raise the polarization losses.41

Fig.8 EMI SE as a function of frequency for GN-CN/PMMA solids (a) and foams (d); EMI SE as a function of frequency for RG-CN/PMMA solids (b) and foams (e); EMI SE as a function of frequency for TG-CN/PMMA solids (c) and foams (f).

As compared in Fig.S4, all these three kinds of G-CNT hybrids-based composite foams exhibited superior SET performance than both Graphene-based and CNT-based composite foams. Especially, while the average SET value of MWCNTs/PMMA and

TGO/PMMA foams at 10wt% loading are just 17.56 dB and 20.46 dB, respectively, that of TG-CN/PMMA foams almost can reach ~30 dB, which could meet for the requirement for commercial application. The enhanced EMI shielding performance is closely associated with the hierarchical conducive fillers, which attributes to the synergistic effect of 2D graphene and 1D MWCNTs inside the foamed system. Moreover, the EMI shielding performances of GN-CN/PMMA, RG-CN/PMMA and TG-CN/PMMA foams are compared in Fig.8. In line with the electrical conductivity results, the conductive TG-CN/PMMA foams exhibit better EMI shielding performance than RG-CN/PMMA foams, indicating the crucial role of conductivity for shielding performance. The RG-CN/PMMA foams at high RG-CN content of 10 wt% just show a lower EMI SET of 18.1dB. This could be put down to the low intrinsic electrical conductivity of RG-CN hybrids root in its incomplete reduction. Interestingly, the GNCN/PMMA foams also exhibit a relatively lower EMI SE value than that of TGCN/PMMA foams at the same content, even though they almost present the similar electrical conductivity. According to previous reports,12,44 microwave attenuation capacity is also related to the morphology of nanofiller. Compared to the thick GN-CN hybrids, the high polarity of the TG-CN could induce more polarization loss and the extremely thin and wrinkled TG-CN heterostructures as shown in scheme 2 (C2) would provide more surfaces to enhance the multiple internal reflections. Therefore, by benefiting from both the high intrinsic electrical conductivity and thin 3D hierarchical “line-plane” structure of TG-CN hybrids, TG-CN/PMMA composite foams achieve a much better EMI shielding performance. To better understand the underlying EMI shielding mechanism of these as-prepared G-CNT/PMMA foams, their average SEA and SER are counted and plotted in Fig.9. TG-CN/PMMA foams exhibit the best absorption performance among the three samples. For example, the average SEA values of GN-CN-10, RG-CN-10 and TG-CN10 foams are 22.08 dB, 16.47 dB and 28.25 dB, respectively, while the SER of these foams is almost the same and far below 5 dB. The contribution of SEA to the total SET is much larger than SER. Furthermore, the absorptivity (A), reflectivity (R), and transmissivity (T) coefficients are calculated by the equations (2) and (3).45 As shown

in Fig. 9, the reflection and absorption almost contributed the same in the GN-CN-10 foams, where 52% incident microwave was reflected back and 48% incident microwave was absorbed. Instead, as far as RG-CN-10 and TG-CN-10 foams, the A values were 0.68 and 0.61, respectively. Microwave absorption contributes more to EMI SE than reflection, indicating that TG-CN/PMMA foams and RG-CN/PMMA foams possess an absorption-dominated EMI shielding feature, which could greatly impede the undesirable secondary reflection. These results indicate that TG-CN/PMMA composite foams present the best EMI shielding performance with absorption-dominated EMI shielding feature.

Fig.9 SET, SEA and SER of the grand average SE values for GN-CN/PMMA foams (a), RGCN/PMMA foams (b) and TG-CN/PMMA foams (c) with different contents; Absorptivity (A), reflectivity (R), and transmissivity (T) coefficients at 12.4 GHz of GN-CN/PMMA foams (d), RGCN/PMMA foams (e) and TG-CN/PMMA foams (f) at different loadings.

The reflection originates from the impedance mismatching between the shielding composites and air interfaces.5,6 They greatly depend on the amount of surface charges and mobile charge carriers on the surface. Meanwhile, the absorption mechanism is closely associated to the absorber’s thickness, the ohmic loss and the polarization loss.40 Herein, the ohmic loss represents the energy consume in the current flow via conduction, hopping, and tunneling mechanisms.46 And the polarization loss is related to the composites’ interfaces, functional groups and defects. What’s more, multiple reflection

and scatteration refers to the reflection and scatteration at various surfaces or interfaces within the composites.47 Scheme 3 schematically depicts the electromagnetic waves transferring across the TG-CN/PMMA composite foams. Initially, as a result of the highly porous structure, the impedance mismatch between air and as-prepared composite foams alleviate dramatically and the incident microwave could deep penetration inside the composite foams rather than reflect at the interfacial region.6 Secondly, after biaxial stretching and orientation, the composite foams with highly conductive 3D hierarchical TG-CN hybrids possess a more condensed network to accelerate the electron transport by direct contact and hopping mechanism.34,48 Meanwhile, the supercritical fluids (SCF) treatment and foaming process also promote the in-situ exfoliation of graphene sheets. This higher level of TG-CN exfoliation could provide a higher ohmic loss when interplay with the incident microwaves, which in turn raises the overall shielding. Thirdly, the great amount of polarization loss including interfacial polarization and dipolar polarization also make great contribution to microwave absorption.49 On the one hand, the abundant insulator-conductor interfaces between TG-CN-PMMA and TG-CN-air, as revealed by TEM and SEM analysis, facilitate the charge accumulation, and thus inducing the interfacial polarization loss.41 On the other hand, the effect of nitrogen atom improving dipole polarization has been explored by density functional theory (DFT) calculations.49,50 The dipole composed of the high positive charge C atom and the nearby high negative charge N atom in TG-CN hybrids could serve as polarization center and attenuate microwave owing to dipole polarization. Besides, the residual functional groups evidenced by XPS result could also dissipate microwave energy by dipole polarization.27 Furthermore, the composite foams with microcellular structure can also significantly provide plenty of interfaces (like TGCN-PMMA and PMMA-air interfaces) for multiple reflections and scattering in various directions and reduce the escape possibility of incident microwave, thereby dissipating the trapped microwave in the form of heat.5,6,12,29,34,41,48,51-56

Scheme 3 Schematic illustration of microwave transmitting across the TG-CN/PMMA composite foams and proposed shielding mechanism: (a) Absorption-dominated shielding feature from the porous microcellular structure; (b) Ohmic loss generated from the conductive TG-CN networks; (c) Enhanced dipole polarization by nitrogen atom doping and interfacial polarization from abundant heterogeneous interfaces; (d) Multiple reflections by microcellular structure and TG-CN hybrids

3.6 Mechanical performance of G-CNT/PMMA composite foams Instructive mechanical performance is a critical index for polymeric foams to evaluate their value in practical industrial applications. The typical compressive stressstrain curves of pure PMMA and G-CNT/PMMA composite foams with 10wt% loading are shown in Fig.10 (a). The maximum compressive strain is fixed at 50%. It is obvious that all these curves behave with three stages of elastic, post-peak softening and plateau religion. Fig.10 (b) summarizes the compressive strength and modulus of these foams. Obviously, pure PMMA foam displayed week mechanical performance, and the compression strength and modulus were just 15.23 MPa and 194.34 MPa, respectively. As expected, after incorporating G-CNT hybrids, the collapse strength of composite foams had different levels of enhancement. Meanwhile, both TG-CN/PMMA and RGCN/PMMA composite foams exhibited higher compressive strength and modulus than

that of GN-CN/PMMA foam. For TG-CN/PMMA displaying the best electrical and EMI shielding performance, the introduction of 10 wt% TG-CN hybrids increased both compressive strength and modulus by 134% (~35.68 MPa) and 117% (~422.18 MPa), respectively, compared to pure PMMA foam. The significant enhancement in mechanical properties might be ascribed to the following factors. Initially, the welldispersed 3D hierarchical TG-CN hybrids architecture and their great interfacial compatibility with PMMA matrix evidenced in the SEM images play an important role in improving mechanical properties of TG-CN/PMMA foams.4,6 The MWCNTsGraphene networks which embedded in cell walls reinforced the foam matrix by inhibiting the deformation of cells and presenting the cell cracks from extending in different regions. Moreover, the uniform microcellular structures along with small cell also provide plenty of ribs and cell-walls to undergo the compression stress.4,57-60 Moreover, as shown in Fig. S5, EMI SE of TG-CN-10 foams could still maintained at 20.5 dB up to 50% compressive strain, which is high enough for commercial EMI shielding applications. All in all, the unique integration of light-weight, superior EMI shielding and excellent mechanical performance in TG-CN/PMMA composite foams would make it competent to apply in electromagnetic protection and widen its application in smart sensors.

Fig.10 (a) Compressive stress-strain curves of pure PMMA and G-CNT/PMMA composite foams; (b) Compressive strength and modulus of pure PMMA and G-CNT/PMMA composite foams.

4. Conclusion Herein, three kinds of fully carbon-based fillers (TG-CN, RG-CN and GN-CN

hybrids) with “line-plane” structure were successfully synthesized and their corresponding G-CNT/PMMA nanocomposite foams were prepared via anti-solvent precipitation, hot-pressing and batch-foaming combined process. All these kinds of GCNT hybrids, which were consisted of one-dimensional CNTs and two-dimensional graphene, exhibited better dispersion state in the polymer matrix. Moreover, the effects of various G-CNTs hybrids and their content on the final morphologies, mechanical performance, electrical and EMI shieling properties of the prepared foams were compared. Interestingly, while composite foams with thick GN-CN hybrids exhibited the bimodal microcellular structure, the composite foams with thin RG-CN and TG-CN hybrids still presented the typical unimodal morphology. The TGCN/PMMA composite foams exhibit higher electric conductivities and EMI shielding values than both RG-CN/PMMA and GN-CN/PMMA foams. The thin thickness, low oxidation degree, 3D hierarchical “line-plane” structure of TG-CN hybrids and moderate microcellular foaming enhance the ultimate electron transport ability inside the TG-CN/PMMA foams. Furthermore, the increased dipole and interfacial polarization, promoted multiple reflections along with enhanced ohmic loss raised from the higher electrical conductivity endowed the TG-CN/PMMA composite foams with an absorption-dominated shielding feature. Typically, the lightweight TG-CN-10 (~0.65 g/cm3) foams with unimodal microcellular structure exhibited a prominent electrical conductivity of 2.92 S/m and a high EMI shielding efficiency of 30.4 dB with more than 61% absorption over the X-band. The TG-CN/PMMA composite foams reinforced with the 3D hierarchical TG-CN hybrids also displayed good mechanical properties; 134 and 117% improvements were achieved for compressive strength and modulus, respectively, compared to the neat PMMA foam. These results demonstrate the TG-CN hybrids are highly promising nanofillers in carbon family and that corresponding electrically conductive TG-CN/PMMA microcellular composite foams with good electrical conductivity and EMI shielding performance while preserving good mechanical property are promising in applications of aircraft, intelligent and electronic devices.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] *E-mail: [email protected] ORCID Hongming Zhang:0000-0002-6131-2505 Guangcheng Zhang: 0000-0001-5303-6355 Mingyue Wang:0000-0002-2979-1841 Notes: The authors declare no competing financial interest.

ACKNOWLEDGEMENTS The authors are grateful to the National Natural Science Foundation of China (Grant No.51773170) and the Shaanxi Coal Joint Fund (Grant 2019JLM-24). We would like to thank the Analytical& Testing Center of Northwestern Polytechnical University for equipment supporting, Anhui Kemi Machinery Technology Co., Ltd for the 100ml high-pressure vessel and Nanjing XFNANO. Materials Tech Co., Ltd for nanofillers.

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interference

shielding

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Supporting Information for

Electrically electromagnetic interference shielding microcellular composite foams with 3D hierarchical Graphene-Carbon nanotube hybrids Hongming Zhanga, Guangcheng Zhang*a, Qiang Gaoa, Meng Zonga, Mingyue Wanga, Jianbin Qina a

Department of Applied Chemistry, MOE Key Lab of Applied Physics and Chemistry in Space,

School of Natural and Appiled Sciences, Northwestern Polytechnical University, Xi’an, 710072, China.

1. Experimental section

Fig. S1 Synthesis process of GO-CNT

2. Results and discussion

Figure. S2 AFM images of (a) GO and (b) GNPs.

Figure. S3 SEM image and cell-size distribution of pure PMMA foam.

Figure. S4 EMI SE of MWCNTs/PMMA, GNPs/PMMA, RGO/PMMA, TGO/PMMA, GNCN/PMMA, RG-CN/PMMA and TG-CN/PMMA foams with 10wt% loading.

Figure. S5 EMI SE of TG-CN-10 foams before and after 50% compression strain

Table S1 Conversion from mass fraction to volume fraction in both solid and foamed composites Mass fraction (wt%) 1 2 3 5 10

GN-CN/PMMA solid foam 0.56% 1.11% 1.70% 2.88% 5.94%

0.21% 0.43% 0.69% 1.21% 2.50%

Volume fraction (vol%) RG-CN/PMMA solid foam 0.63% 1.26% 1.91% 3.24% 7.12%

0.29% 0.59% 0.93% 1.68% 3.90%

TG-CN/PMMA solid foam 0.69% 1.41% 2.13% 3.59% 7.35%

0.33% 0.71% 1.10% 1.89% 4.21%

Table S2 Comparison of EMI SE and Specific EMI SE in 8.2~12.4GHz for the obtained GCNT/PMMA composite foams and the reported EMI shielding composites Matrix

Filler

Filler content

Thickness (mm)

EMI SE (dB)

Specific EMI SE (dB/(g/cm3))

Ref.

PC PP PLA PU PMMA PS PU ABS Epoxy foam PLLA foam PI foam PC foam PEI foam PMMA foam PMMA foam PMMA foam

MWCNTs MWCNTs GNPs rGO TGO MWCNTs+GNPs MWCNTs-rGO MWCNTs-TGO MWCNTs MWCNTs rGO GNPs TGO GN-CN RG-CN TG-CN

15wt% 7.5vol% 9.1vol% 5.5vol% 4.23vol% 2+1.5wt% 10wt% 1+10wt% 3wt% 10wt% 16wt% 0.5wt% 10wt% 10wt% 10wt% 10wt%

2.0 1.0 1.5 3.0 3.4 5.6 3.0 2.0 2.8 2.5 0.8 2.0 2.3 2.5 2.5 2.5

27.0 34.8 15.5 21.0 30.0 20.2 32.0 29.9 7.1 23.0 21.1 15.0 13.0 25.2 18.1 30.4

--------21.3 77.0 75.0 39.0 44.0 47.5 26.2 43.4

1 2 3 4 5 6 7 8 9 10 11 12 13 This work This work This work

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TOC:

CRediT authorship contribution statement Hongming Zhang: Conceptualization, Methodology, Investigation, Writing-original draft. Guangcheng Zhang: Supervision, Writing - review & editing. Qiang Gao: Investigation. Meng Zong: Methodology; Mingyue Wang: Formal analysis, Data curation. Jianbin Qin: Supervision.

Declaration of interests ☐ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

The authors are grateful to the National Natural Science Foundation of China (Grant No.51773170) and the Shaanxi Coal Joint Fund (Grant 2019JLM-24). We would like to thank the Analytical& Testing Center of Northwestern Polytechnical University for equipment supporting, Anhui Kemi Machinery Technology Co., Ltd for the 100ml high-pressure vessel and Nanjing XFNANO. Materials Tech Co., Ltd for nanofillers.