Multifunctional Bicontinuous Composite Foams with Ultralow

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Functional Nanostructured Materials (including low-D carbon)

Multifunctional bi-continuous composite foams with ultralow percolation thresholds Jiabin Xi, Yingjun Liu, Ying Wu, Jiahan Hu, Weiwei Gao, Erzhen Zhou, Honghui Chen, Zichen Chen, Yongsheng Chen, and Chao Gao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b06017 • Publication Date (Web): 30 May 2018 Downloaded from http://pubs.acs.org on May 31, 2018

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ACS Applied Materials & Interfaces

Multifunctional bi-continuous composite foams with ultralow percolation thresholds Jiabin Xi1,†, Yingjun Liu1,†, Ying Wu1,4, Jiahan Hu1, Weiwei Gao1, Erzhen Zhou1,3, Honghui Chen2, Zichen Chen3, Yongsheng Chen2 & Chao Gao1,* 1

MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Department of Polymer Science and Engineering, Zhejiang University, 38 Zheda Road, Hangzhou 310027, China

2

The Centre of Nanoscale Science and Technology and Key Laboratory of Functional Polymer Materials, State Key Laboratory and Institute of Elemento-Organic Chemistry, College of Chemistry, Nankai University, Tianjin, 300071, China.

3

Department of Mechanical Engineering, Zhejiang University, 38 Zheda Road, Hangzhou 310027, China

4

School of Material Science and Engineering, University of Science and Technology Beijing, 30 Xueyuan Road, Beijing 100083, China

E-mail: [email protected] †The authors contributed equally to this paper.

ABSTRACT

Integrating ultralight weight and strong mechanical performance into cellular monolith is a challenge unresolved yet. Here, we propose a skeleton-assisted self-assembly method to design ultralight bi-continuous composite foams (BCCFs) with high mechanical robustness and ultralow percolation thresholds. Polymer foam was employed as the skeleton to support assembled graphene networks, forming BCCFs with high tensile strength (~80 KPa) and breakage elongation (>22.2 %). The 1

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paraffin and polydimethylsiloxane infiltrated BCCFs show a record low percolation threshold of 0.006 vol% and a relatively high electrical conductivity of 0.81 S m-1 at a low graphene content of 0.216 vol%. The BCCFs demonstrate high and adjustable microwave absorbing (MA) properties. The effective absorption bandwidth (EAB, reflection loss ≤ -10 dB) for BCCFs with a low graphene loading of 3.4 mg cm-3 achieves 9.0 GHz at a thickness of 4 mm, and it further covers 13.6 GHz considering the adjustability of preferred absorption band. The BCCFs with extremely low graphene load of 0.14 mg cm-3 were further used for durable and efficient oil-adsorption, which can adsorb >60 times of their own weight. The facile fabrication of bi-continuous composite foams opens the avenue for practical applications of high-strength, multifunctional, and productive graphene-based foams.

Keywords: Bi-continuous structure, Graphene foam, Percolation threshold, Microwave absorption, Oil adsorption.

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INTRODUCTION Multifunctional graphene aerogels (GAs) assembled by graphene sheets are widely used as oil adsorption,1,2 super capacitors,3 fuel cells,4 electromagnetic interference (EMI) shielding5,6 and microwave absorption (MA)3,7,8 owing to their high hydrophobicity, high specific surface area and high electrical conductivity, etc. However, the poor mechanical performance of GAs originating from the extremely weak physical interconnections9,10 has not been solved until now. Besides, the productivity of GAs is mediocre because of the time and energy-consuming freeze-drying method.2-5,7 Integrating the multifunctional performances, high mechanical properties and productivity is the key approach for GAs to realize practical applications. Graphene/polymer foams composites show better mechanical properties due to the covalently cross-linked polymer skeleton. Concerning the low cost and high productivity, they serve as the efficient alternatives of GAs in some applications such as oil-adsorption11,12 and super capacitors.13-15 However, in some other fields, such as EMI shielding and microwave absorption, the performance of graphene/polymer foams composites can not compete with GAs, probably because that the distribution of graphene in graphene/polymer foams composites

11-18

is quite different from GAs

2-5,7,19-20

, and that graphene/polymer foams composites can not fully respond to

electromagnetic waves. For example, graphene/PU composite foams showed EMI shielding effectiveness (SE) of 12.4 dB at a thickness of 20 mm,17 while better SE of ~16 dB was obtained by GAs with identical graphene content and reduction method at 3



much lower thickness (2.5 mm).5 Designing graphene foams integrating merits of low filler content, high mechanical robustness and multifunctional performances is still a challenge. Here, we designed robust, cost-effective and productive bi-continuous composite foams (BCCFs) with two synergistic networks: melamine skeleton and graphene framework, which combine high mechanical robustness and multifunctional performances. The BCCFs show high tensile strength of ~80 KPa and breakage elongation of >22.2%, much better than that of GAs (~14 KPa and 0.76%, respectively). BCCFs also demonstrate good elasticity, recovering to 70% of their original height after 100 times of cyclic compression. BCCFs show high electrical conductivity (0.81 S m-1) at low graphene volume fraction (0.216 vol%) and fulfill record low percolation threshold (Φc) of 0.006 vol%. Benefitting from the electrically conductive graphene network and high elasticity, BCCFs show high and adjustable MA properties at low graphene filler content (3.40 mg cm-3). The effective absorption bandwidth (EAB, reflection loss (RL) ≤ -10 dB) 21,22 achieves 9.0 GHz (9.0-18 GHz), which is among the best considering the broad EAB and low filler content. The BCCFs are further used for oil-adsorption at extremely low filler content of 0.14 mg cm-3, which is even below the lower limit for freestanding GAs.1 The design of BCCFs will give a deep insight on further researches to design graphene foams with high mechanical strength and functional properties. RESULTS AND DISSICUSSION

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The BCCFs were fabricated via a facile and productive skeleton-assisted self-assembly method including two steps: immersing assembly followed by chemical reduction (Figure 1a). Firstly, commercial melamine foams (MFs, 10.00 mg cm-3) were immersed into graphene oxide (GO) aqueous solution, repeatedly squeezed and vacuumed for 3 min. In this process, the GO sheets efficiently penetrate into the inner structure of MFs, so the final products show similar microstructures on the surface and at the center (Figure S1). After drying at 60 °C overnight, GO-MFs were obtained. Secondly, the GO-MFs were chemically reduced by hydrazine vapor at 90 °C for 12 h, obtaining BCCFs. Two kinds of GO were used in the assembly process: giant GO (gGO, Figure 1b) and small GO (sGO, Figure 1c) sheets with sizes of ~50-100 µm and ~10 µm, respectively. The BCCFs are designated as gBCCF-x and sBCCF-x, accordingly, where x is the concentration of GO aqueous solutions applied for the immersing assembly. GAs, labelled as GA-x, were prepared by freeze drying process as a counterpart. The graphene filler content (mg cm-3) and volume fraction (vol%) of BCCFs are proportional to the original concentration of the GO solutions, as shown in Table S1. The BCCFs exhibit low densities owing to the low density of MFs and the low filler content of graphene. With increasing GO concentrations from 0.1 to 7 mg mL-1 employed in immersing assembly process, the densities range from 10.07 to 14.76 mg cm-3. Thanks to the simple and scalable synthesis processes, BCCFs with a size up to 15×6×5 cm3 (Figure 1d) were fabricated.

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Figure 1. (a) Schematic illustration of the fabrication processes of BCCFs, including the immersing assembly and the chemical reduction; SEM images of (b) gGO sheets, and (c) sGO sheets; and (d) digital camera image of a 15×6×5 cm3 sBCCF-0.5 sample.

BCCFs fabricated by gGO and sGO showed different morphologies. The graphene sheets in gBCCF-0.2 tend to form micro belts along with the melamine skeleton, probably formed by the shearing actions of melamine skeleton to graphene sheets in immersing assembly process (Figure 2a). When graphene fillers increase, BCCFs show an edge-supporting structure where the cellular MFs are covered by many stretched graphene micro slices assembled by graphene sheets (Figure 2b,c). 6


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Randomly scattered and curled graphene sheets among the waists of MFs skeletons are observed for sBCCFs despite the filler content, forming a waist-supporting structure (Figure 2d-f).

Figure 2. SEM images of (a) gBCCF-0.2 (b,c) gBCCF-5 (d) sBCCF-0.2 (e,f) sBCCF-5 at different magnifications.

The graphene in BCCFs construct a continuous and freestanding network, which can be observed by subjecting BCCFs to 1000 °C to destroy the melamine skeleton (Figure S2). MFs significantly contracts because of the degradation of the melamine, while the contraction degree decreases obviously with increasing graphene contents, because the high-temperature resistant graphene framework is freestanding and can support the BCCF construction even when the melamine skeleton is destroyed. Therefore, BCCFs show a bi-continuous microstructure, continuous cellular polymer skeleton and graphene framework, which contributed to excellent mechanical and multifunctional performances, respectively.

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The continuous graphene framework endows the BCCFs with ultralow percolation thresholds. BCCFs were filled with paraffin and polydimethylsiloxane (PDMS), respectively, to test the direct current (DC) conductivity. At extremely low graphene volume fraction (0.003 vol%) the BCCFs are insulating. When graphene volume fraction reaches 0.006 vol% for gBCCFs and 0.015 vol% for sBCCFs, the electrical conductivity suddenly increases (Figure 3a,b). Therefore, Φc = 0.006 vol% for gBCCFs and Φc = 0.015 vol% for sBCCFs. With identical graphene load, gBCCFs show higher electrical conductivities than sBCCFs because of the better electrically conductive graphene networks in the edge-supporting structure (Figure 2b,c).

Figure 3. Electrical conductivities of (a) BCCF/paraffin and (b) BCCF/PDMS. (c,d) show the linear fitting results according to the percolation theory.

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Table 1. Percolation thresholds of different graphene-based composites Maximum Percolation

Materials

t value

threshold (vol%)

electrical conductivity (S

Reference

m-1) gBCCF/paraffin

0.006

1.34±0.08

0.53 (0.216 vol%)

This work

gBCCF/PDMS

0.006

2.10±0.21

0.81 (0.216 vol%)

This work

Graphene/PIa

0.03

3.8±0.29

94 (5 vol%)

23

rGO/PPb

0.033

Graphene/UHMWPEc

0.070

1.26±0.06

~0.1 (0.6 vol%)

25

0.075

2.58

3.49 (1.1 vol%)

26

Graphene/PS

0.1

2.74±0.20

1 (2.5 vol%)

27

Graphene/PCe

0.14

4.05±0.58

5.12 (2.2 vol%)

28

Graphene/PVC@PVAcf

0.15

~1 (3.5 vol%)

29

Graphene-PVDFg

0.18

Graphene-PP

0.41

PLA modified graphene/PSd

a

b

PI: polyimide;

polyethylene;

d

24

3.81 0.028 (1.64 vol%)

PP: polypropylene;

PS:

polystyrene;

30

e

c

31

UHMWPE: ultra high molecular weight

PC:

polycarbonate;

f

PVC@PVAc:

vinyl

chloride/vinyl acetate copolymer; gPVDF: polyvinylidene fluoride.

The Φc of 0.006 vol% is the lowest among graphene-based composites reported in the literature (Table 1).23-31 The reason is that graphene sheets in gBCCF-0.2 tend to form a conductive network along with the melamine skeleton, so the melamine skeleton act as a template which helps to construct a conductive network at low graphene load (Figure 2a). The ultrahigh aspect ratio (D/l, where D is the average lateral size and l is the average thickness of the filler units) of giant graphene sheets 9



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further helps to lower the percolation threshold, because computational studies have demonstrated that the percolation threshold is inversely related to the aspect ratio owing to the increased exclude volume fraction among fillers.32,33 To quantitatively evaluate the percolation threshold of BCCFs, a structural model of BCCFs with cubic cells (Figure S3) is proposed, which was used to calculate the lower limit of density of graphene foams.1 According to the structural model, the Φc is inversely correlated to the lateral size (w) of graphene sheets (Equation 1), proving that BCCFs with ultra-large graphene sheets tend to show low percolation thresholds.

=

300 × vol % (1)

Where l is the thickness of graphene sheets (0.334 nm). Taking the lateral size of giant graphene sheets as 50 µm, the calculated Φc of gBCCFs reaches 0.002 vol%. Our experimental result (0.006 vol%) is higher than the theoretical value, because the graphene sheets overlap each other, and some of them may stack to form few-layer graphene sheets. Therefore, the melamine skeleton and the employ of giant graphene sheets are the key reasons for the low percolation thresholds. According to the percolation theory, beyond the Φc, the DC conductivity shows the following relationship to conductive filler content (Equation 2):23,34,35

= σ [( − )/(1 − )]

(2)

where σ is the DC electrical conductivity of the composites, σf is the intrinsic conductivity of the conductive filler, is volume fraction of the conductive fillers and t is the universal critical exponent. Theoretically, the t value only depends on the geometric accumulation – the dimensionality and the distribution of conductive 10


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fillers.

23

Higher t values signify higher contribution of conductive fillers to

macroscopic electrical conductivity. Fitting results show that the conductivity behavior of BCCFs meets well with the percolation theory (Figure 3c,d). The fitted values of t are 1.34±0.08 and 2.10±0.21 for gBCCF/paraffin and gBCCF/PDMS, respectively, which are low among former reports (Table 1). We attribute the low t value of gBCCF composites to the constrained distribution of graphene sheets. Comparing Figure 2b with 2a, the additional graphene fillers beyond Φc tend to form the micro slices that are constrained by the melamine skeleton, hence limiting the connection possibility among the graphene sheets and further limiting t values. However, considering the ultralow percolation threshold, the BCCFs still fulfill high electrical conductivity at low graphene filler content (Table 1). Maximum conductivities of 0.53 and 0.81 S m-1 are achieved for gBCCF-7 filled paraffin and PDMS, respectively with low graphene filler content of 0.216 vol% (Figure 3a, b). After compositing with guest components, the samples remain uniform considering their macro appearances (Figure 4a), and the microstructure (Figure 4b,c) remains the unchanged comparing with BCCFs without polymer matrix. Hence the facile fabrication of graphene/melamine/PDMS ternary composites throws a way to prepare conductive composites with extremely low graphene filler content.

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Figure 4. (a) Camera digital images of MF and gBCCFs filled with PDMS; and (b,c) SEM images of the gBCCF-3/PDMS at different magnifications.

Figure 5. (a) Tensile strength and breakage elongation of gBCCFs, MFs and GAs; (b) digital camera images of the gBCCF-3 during mechanical compression; (c) compressive stress–strain curves; and (d) elastic recovery of gBCCF-3 within 100 cycles.

The BCCFs manifest excellent mechanical robustness and compressed elasticity (Figure 5a, Figure S4). The tensile strength of BCCFs (~80 KPa) is far better than

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GAs (~14 KPa for GA-11), and is similar to MFs regardless of the graphene filler content (Figure 5a). With the addition of graphene, the breakage elongation decreases from ~32.9 % (MFs) to ~22.2% (gBCCF-7), because of the relatively low breakage elongation of the graphene framework. Nevertheless, it is still much higher than GAs (0.76% for GA-11). The gBCCFs exhibit good elastic properties (Figure 5b-d and Movie S1), which degraded gradually during cyclic compression. After compressed for 100 cycles, the BCCFs quickly recovers to ~70% of its original height (Figure 5d), and further recovers to ~80% within half an hour.

To reduce the adverse effects of electromagnetic waves, MA materials are used to absorb unwanted electromagnetic waves and transform them into heat.36 According to the electromagnetic theories, two critical characteristics for high-performance MA materials are the matched impedance and the high electromagnetic loss ability, ensuring the penetration of microwave into the material interface and the conversion of microwave into heat, respectively. The graphene foam show excellent MA performances,21 because the foam structure has been proved to show matched impedance to free space and high dielectric loss ability, attributing to the reticulate distributed

graphene

sheets

that

can

act

as

tremendous

resistance-inductance-resistance coupled circuits.21,37,38 The graphene framework in BCCFs show similar microstructure to graphene foams, so BCCFs are expected to show high MA performance.

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Figure 6. Real and imaginary permittivity of (a,b) gBCCFs, and (c,d) sBCCFs; RL curves of (e) sBCCF-7 (f) gBCCF-3 and (g) gBCCF-5 at different thicknesses; and (h) comparison of EAB as a function of filler contents. In (h), the black circles are carbon materials or conductive polymer, green triangles represent conductive polymer-magnetic

material

composites,

blue

inverted

triangles

suggest

carbon-magnetic material composites, and magenta diamonds stand inorganic dielectric or magnetic materials. 14


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To evaluate MA performances, BCCFs are made into coaxial ring samples to test the complex permittivity. The real (ε’) and imaginary permittivity (ε’’) are given in Figure 6a-d. The value of ε’’ signifies the dielectric loss ability of a material, which significantly relies on the microstructure and graphene content of BCCFs. Larger graphene contents lead to higher permittivity owing to the contribution of conductive graphene sheets to dielectric losses of wave energy. The gBCCFs show higher permittivity than sBCCFs, because the stretched micro graphene slices in the edge-supporting structure can preferably respond to electromagnetic waves, thus demonstrating increased dielectric loss. The MA performance, or RL is often given by equations (3) and (4):

Z = ∙ tanh$% ∙ 2'()*+ ,+ √*. ,. ∙ /0

6 784

RL = 20log4+ 56 7 : 94

(3) (4)

where Z’ is the input impedance, µr is the relative complex permeability (µr = 1 for materials without magnetic component), εr (= ε’+iε’’) is the relative complex permittivity, µ0 (= 4π×10-7 H m-1) is the vacuum permeability, ε0 (≈ 8. 854×10-12 F m-1) is the vacuum permittivity, f is the wave frequency and t is the sample thickness.7,36,39-48

Generally, optimal MA performance exists when the permittivity is moderate, because that too high permittivity causes impedance mismatch, and that too low permittivity means weak loss ability of wave energy.7,22,40,41,49,50 The calculation results are given in Figures 6 and S4. Some of the RL curves show one peak, which red shifts with increasing thickness, because of the quarter wavelength interference 15



effect.46-48 Some of the RL curves show two peaks (e.g., Figure 6e at thickness of 10 mm), among which the peak at higher frequency is attibuted to three-quarters of wavelength interference. MA properties differentiate a lot with graphene content and microstructure, because of their different wave impendance and dielectic loss ability. The sBCCFs and gBCCF-1 show moderate MA properties owing to relatively low dielectric losses (Figure S5a-c). MA performance is similar for sBCCF-7 and gBCCF-3, with EAB reaching 8.4 GHz (4.3-7.6, 12.8-17.9 GHz) for sBCCF-7 (Figure 6e) and 8.1 GHz (4.3-9.1, 14.7-18 GHz) for gBCCF-3 at thickness of 10 mm (Figure 6f). This signifies that gBCCFs are more graphene-saving to fulfill high MA performances. The gBCCF-5 performs the best at a thickness of 4 mm, showing a peak intensity of -35 dB at 12.7 GHz and an EAB of ~9.0 GHz (9.0-18 GHz) (Figure 6g). By comparison, gBCCF-7 only shows modest MA performance (Figure S5d), because too high permittivity results in impedance mismatch. The gBCCFs exhibit higher MA properties than sBCCFs, because the edge-supporting structure with stretched graphene micro slices can better respond to the electromagnetic waves. Therefore, the edge-supporting microstructure of gBCCFs is favored for microwave absorption. Optimal MA performance of gBCCFs is achieved with appropriate graphene content, because it makes a balance between impedance match and dielectric loss ability, as with former reports.7,22,40,41,49,50 MA parameters of BCCFs and former reports are given in Figure 6h and Table S2. Considering the low filler content and broad EAB, BCCFs in this study rank only next to the GAs reported before.8,21 Besides, the low densities of BCCFs (0.120 g cm-3 for gBCCF-3 and 0.134 16


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g cm-3 for gBCCF-5) make them suitable for lightweight applications, such as stealth aircrafts.

Figure 7. (a,b) Real and imaginary permittivity of gBCCF-3 with different compression ratios; (c) RL curves of gBCCF-3 with different compression ratios at original thickness of 10 mm; and (d) EAB of gBCCF-3 at different compression cycles at thickness of 10 mm.

The MA performances of BCCFs can be tailored by mechanical compression, caused by the changed permittivity and sample thicknesses, according to the quarter wavelength interference effect.46-48 Increased compression ratio (R) results in larger permittivity (Figure 7a-b and Figure S6) due to the compression induced increment of graphene contents and loss ability of BCCFs. RL at each R can be calculated by equation (3) and (4) by substituting corresponding permittivity and thickness. As shown in Figure 7c, with the increasing R, the preferred absorption frequency band 17



gradually blue shifts, which can be explained by quarter wavelength interference. Therefore, the absorption frequency bands with different R together cover broader frequency bandwidth than non-adjustable materials. For example, the gBCCF-3 with original thickness of 10 mm, the absorption bandwidth covers 13.7 GHz (4.3-18 GHz, Figure 7c). For gBCCF-5 with original thickness of 6 mm, the absorption bandwidth covers 11.9 GHz (5.1-17.0 GHz, Figure S7). The BCCFs demonstrate long lifetime as well. After a 100-time compression, the EAB of gBCCF-3 at an original thickness of 10 mm remain ~6 GHz (Figure 7d, the correlated permittivity curves are given in Figure S8).

Figure 8. (a) Water contact angles of BCCFs; (b) digital camera images of the oil-adsorption process of sBCCF-0.2 toward methylbenzene which was dyed by oil red O; (c) adsorption capacities at room temperature of sBCCF-0.2 for various organic liquids in term of weight gain; (d) digital camera image of a 15×6×5 cm3

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sBCCF-0.2 full of liquid paraffin which was dyed by Sudan black; and digital camera images of (e) sBCCF-0.2 and (f) GA-11 after stirring in water.

As shown in Figure 8a, the hydrophobicity of BCCFs is enhanced with increasing graphene contents due to the hydrophobicity of the graphene sheets. The sBCCFs show higher hydrophobicity at low graphene contents (0.14 mg cm-3) than gBCCFs (Figure 8a), probably owing to the uniformity of the graphene fillers. Since the sGO is smaller in size, it tends to homogenously dispersed in the melamine foam with lower graphene filler content than gGO. The gBCCFs and sBCCFs turn to be hydrophobic at graphene contents of 0.034 and 0.014 mg cm-3, respectively. With the hydrophobicity, the porous structure and the excellent mechanical performances, BCCFs are promising in oil-adsorption applications (Figure 8b, Movie S2). The sBCCFs-0.2 can absorb over 60 times of their own weight for involved organic solvents and ~140 times adsorption for dichloromethane with relatively high density (Figure 8c). Remarkably, the filler content of sBCCF-0.2 is only 0.014 mg cm-3, which is even below the lower limit for freestanding GAs (0.016 mg cm-3),1 so our BCCFs is more graphene-saving than GAs in oil-adsorption applications. With high mechanical strength, the 15×6×5 cm3 sBCCF-0.2 remained freestanding after saturate absorption of liquid paraffin (Figure 8d). After violent stirring in liquids (Figure 8e, Movie S3), the sBCCF-0.2 remained intact while the GA-11 with similar density (7.5 g cm-3) broke into pieces (Figure 8f, Movie S4), which demonstrated the potentials of sBCCF-0.2 in practical applications. Moreover, the high elasticity allows the BCCFs to be repeatedly used by extruding the absorbed oil. 19



The BCCFs demonstrate high mechanical performances and excellent functional properties such as high electrical conductivity, low percolation threshold, high microwave absorption property and high oil-adsorption performance, making BCCFs are robust, cost-effective and productive multifunctional materials.

CONCLUSION We designed robust, cost-effective and productive BCCFs with a bi-continuous microstructure. The cellular melamine skeleton provides the BCCFs with the mechanical robustness, and the continuous graphene framework offers the multifunctional performance. The BCCFs show an ultralow percolation threshold (0.006 vol%), high microwave absorption performances (EAB = 9.0 GHz) as well as high oil-adsorption performances (adsorbing >60 times of their own weight). The fabrication process is simple and energy-saving, contributing to the easy scale-production of BCCFs. We believe the facile fabrication of the bi-continuous structure would open the avenue to design high-strength, multifunctional, low-cost and productive graphene-based foams.

METHODS Materials and preparation: The sGO and gGO aqueous solutions (Gaoxi Tech., http://www.gaoxitech.com) were diluted into different concentrations (0.1, 0.2, 0.5, 1, 3, 5 and 7 mg ml-1). The MFs were immersed into the diluted GO solutions and squeezed by hand for several times, followed by vacuum treatment for 3 min. The GO 20


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infiltrated MFs were dried in an air circulation oven at 60°C overnight, obtaining GO-MFs. The BCCFs were obtained by the chemical reduction of the GO-MFs in hydrazine vapor at 90 °C for 12 h. Thermal annealing of BCCFs were conducted in a tube furnace (GSL-1400X, HeFei Kejing Materials Technology Co. LTD, China) at 1000 °C for 2 h with a heating and cooling rate of 5 °C min-1 to demonstrate the freestanding characteristics of graphene networks. BCCFs were floated on PDMS (Sylgard 184, DowCorning, base agent: curing agent = 10:1, w/w) solutions and were fully infiltrated within 0.5 h via capillarity. BCCF/PDMS composites were obtained after curing at 60 °C for 24 hours. GAs were prepared by a freeze-drying process as a counterpart. Typically, gGO solutions with different concentrations were frozen at -80 °C in a refrigerator and freeze-dried at 1 Pa for 24 h, followed by hydrazine vapor reduction at 90 °C for 12 h. Characterization: The graphene filler contents in BCCFs were determined by the weight loss after chemical reduction considering the 32% weight loss of graphene fillers after chemical reduction. The volume fraction of graphene and the density of BCCFs were determined using the density of graphene and MFs as 2.2 and 0.01g cm-3, respectively. Densities of materials with high graphene contents were measured by the electronic balance (ME104 precision, METTLER TOLEDO), which confirmed the accuracy of the calculation. Scanning electron microscope (SEM) images were taken on a Hitachi S4800 field-emission SEM system. The electrical conductivity was 21



measured by cyclic voltammetry method with a CHI660e electrochemical workstation (Shanghai chenhua instrument co. LTD). Mechanical tests were performed on a universal testing machine (RGM-6002T, ShenZhen REGER Instrument co. LTD, China) using rectangular-shaped samples. The permittivity was tested by a vector network analyzer (ZNB-40, Rohde & Schwarz, Germany) through a pair of 50 Ω double-shielded coaxial cables, which were connected to a standard sample holder containing the test samples. To obtain the permittivity at different compaction ratio, BCCFs were compressed and infiltrated with paraffin to prevent elastic recovery, and the BCCF/parrafin were cut into cylindrical specimens (outer diameter = 7 mm, inner diameter = 3 mm, and thickness = 2 mm) for permittivity tests. For the permittivity tests at different compression cycles, the BCCFs were cut into the cylindrical specimens (outer diameter = 7 mm, inner diameter = 3 mm, and thickness = 4 mm) without infiltrating the paraffin, ensuring the specimens can recover to the original shape after cyclic compression. Weight gain of oil-adsorption performance was characterized by the weight difference before and after the oil absorption of rectangular BCCFs with a size of 10×10×10 mm3. The contact angels were measured on a goniometer (KRUSS, DAS 100) after 2 µl water droplets were applied on specimens.

ASSOCIATED CONTENT Supporting Information Microstructure of the surface and interior of gBCCF-3 (Figure S1); linear contraction

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ratios of BCCFs after 1000 °C treatment (Figure S2); the structural model of gBCCFs for calculation of percolation thresholds (Figure S3) and a brief introduction of calculation methods; the strain-stress curves (Figure S4), reflection loss curves (Figure S5) of BCCFs; permittivity (Figure S6) and reflection loss (Figure S7) of BCCFs at different compression ratios; permittivity of gBCCF-3 at different compression cycles (Figure S8). Parameters, including graphene filler content, graphene volume fraction and density of each BCCF sample (Table S1) and microwave absorption performance comparison of BCCFs and former reports (Table S2).

AUTHOR INFORMATION Corresponding author *Email: [email protected]

Author Contributions J. Xi, Y. Liu contributed equally to this work. J. Xi and Y. Liu designed the experiments. All the authors discussed the results and commented on the manuscript. Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS

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This work is supported by the National Natural Science Foundation of China (Nos. 21325417, 51533008, 51603183), National Key R&D Program of China (Grant No. 2016YFA0200200), Fundamental Research Funds for the Central Universities (Nos.

2017QNA4036,

2017XZZX008-06),

the

China

Postdoctoral

Science

Foundation (2017M620241) and the National Postdoctoral Program for Innovative Talents (No. BX201700209).

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Multifunctional Bicontinuous Composite Foams with Ultralow

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