High-Performance Graphene Sponges Reinforced with Polyimide for

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High-performance graphene sponges reinforced with polyimide for room temperature piezoresistive sensing Jingxia Huang, Jinqing Wang, Zhigang Yang, and Shengrong Yang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b17018 • Publication Date (Web): 08 Feb 2018 Downloaded from http://pubs.acs.org on February 8, 2018

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ACS Applied Materials & Interfaces

High-performance graphene sponges reinforced with polyimide for room temperature piezoresistive sensing Jingxia Huang a, b, Jinqing Wang a, *, Zhigang Yang a, and Shengrong Yang a

a

State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics,

Chinese Academy of Sciences, Lanzhou, 730000, P. R. China. b

University of Chinese Academy of Sciences, Beijing, 100049, P. R. China

* Corresponding author. Tel: +86-931-4968076; Fax: +86-931-4968019. E-mail address: [email protected] (J. Q. Wang)

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ABSTRACT: The bulk materials of three-dimensional graphene (3DG) with high network structure and excellent electrical conductivity have many potential applications in flexible electronics, but the network structure is not very stable due to weak bonding between graphene sheets. Here, a polyimide (PI) layer was introduced on the as-prepared 3DG sponge by vacuum infiltration-curing method. The resulting 3DG/PI composite sponges with robust 3D network structure exhibited excellent electrical conductivity (3.7 S/cm), compression strength (175 kPa), elasticity and flexibility, as well as the outstanding compression sensitivity to resistance and stable piezo-resistance effect; namely, they possess a large change of resistance in response to application of small strain and low density, while the resistance change remains favorably stable after performing 300 times of compress-release cycling, which means that the prepared composite sponges can find wide applications in pressure sensing or stimulus-responsive graphene system. Table of Contents Graphic

KEYWORDS: graphene sponge, polyimide, compressibility, electromechanical performance, piezo-resistive sensing


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1. INTRODUCTION Graphene monoliths including graphene aerogel/foam/sponge are of considerable interests, because of their abundant three-dimensional (3D) porous macroscopic architectures with some unique properties of large surface area, low mass density, excellent electrical conductivity and flexibility.1 Moreover, the outstanding physical and chemical properties of 3D graphene (3DG) monoliths can satisfy the requirements arising

from

different

applications

such

as

water

purification,

catalysts,

supercapacitors, flexible electronics and energy storage devices.2 Given that, various attempts have been performed for generating high-quality 3DG bulk materials and a number of methods including chemical vapor deposition (CVD),3 wet chemistry assembly,4-7 ice template,8-9 and 3D printing10 have been reported. However, CVD-grown 3DG materials are usually brittle under low compression and can collapse after etching out the template, and their geometry, density, and porosity are not easily controllable during the self-assembly process. Also, the strength and stiffness of the as-obtained 3DG materials by wet chemistry assembly or ice template are not strong enough to bear the relative high external force due to their disordered structure. Again, no matter 3D printing or CVD method, it is difficult to achieve the large scale production of 3DG materials. In recent years, various methods have been developed to prepare 3DG/polymer composites to enhance the comprehensive properties of 3DG materials. For examples, Huang et al. fabricated 3DG/polystyrene composites by a self-assembly and hot-pressing

technique.11

Ren

et

al.

reported


a

highly

conductive


3DG/polydimethylsiloxane (PDMS) foam by a template-directed CVD method.12 However, these methods are complicated and can only be applied to prepare thermoplastic polymer or composite foam. Meanwhile, researchers have also focused on developing novel 3DG/polymer materials by assembly of GO nanosheets with polymer, direct incorporation of polymers during reduction self-assembly of GO nanosheets, and the incorporation of polymers into pre-synthesized 3DG frameworks 2. Because the introduction of polymer can impart new functionalities to the 3DG materials for widespread applications such as electronic devices, sensors, actuators, electromagnetic shielding.1-2 Among them, the incorporation of polymers into 3DG frameworks does not require the complicated functionalization and dispersion of individual GO sheets in the polymer matrix; compared with those made from the functionalized graphene/polymer composites13-14, the resulting composites displayed superior mechanical properties, electrical and thermal conductivities. For instance, Zhao et al. reported a highly compressible graphene aerogel composite of CGA/PDMS with excellent electromechanical performance.15 Yu et al. fabricated 3D reduced graphene oxide/epoxy composites (rGO/EP), which presented better electrical conductivity and mechanical properties than those prepared by ultrasonication-assisted solution mixing.16 Again, Liao et al. infused PDMS into 3DG foam, which presented the excellent pressure and strain sensing.17 However, only some typical polymers like PDMS and EP were employed in these studies; therefore, the search of polymers with unique property to be infiltrated into 3DG scaffold is absolutely necessary. Polyimide (PI) is a kind of high-performance polymers and


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exhibits excellent mechanical property, high thermal stability, good chemical resistance and processability, which has been widely applied in microelectronic, optoelectronics, adhesives, aerospace engineering, non-linear optical devices 18-19. Here, this work is designed to improve the structure stability of 3DG scaffold with high network structure and excellent electrical conductivity, while there is little effect on its conductivity involved after the precursor solution of PI was introduced into 3DG scaffold using the vacuum infiltrating-curing strategy. In detail, after 3DG was infiltrated with the ammonium salt solution of polyimide acid (PAS), the water-soluble precursor of PI, the resulting 3DG/PI composite sponges displayed robust 3D network structure, excellent electrical conductivity, superior mechanical performances in compression strength, elasticity and flexibility, as well as outstanding compression sensitivity to resistance and stable piezo-resistance effect; namely, the composite sponges possess large changes of resistance in response to applications of small strain and low density, while the change of resistance remains favorably stable after executing 300 times of compress-release cycling. These eminent properties mean that

the

prepared

composite

may

be

applied

in

pressure

sensing

or

stimulus-responsive graphene system. 2. EXPERIMENTAL SECTION 2.1. Materials. Graphite powder (≤30 µm), H2SO4 (98%), HCl (37%), K2S2O8, P2O5, H2O2 (30%), 4,4ʹ-oxydianiline (ODA, 98%), and pyromellitic dianhydride (PMDA, 99%), N,N-dimethylacetamide (DMAc, 98%) and triethylamine (TEA, 99%) were purchased from Aldrich and Sinopharm Chemical Reagent Co., Ltd and directly



used as received. Ethanol and ultrapure water (18 MΩ·cm) were used in this work. 2.2. Preparation of the Compressible 3DG Sponges. The preparation of GO colloid solution was presented in our previous papers.20-21 3DG sponges were obtained via a solvothermal method reported elsewhere.22 Firstly, GO alcoholic solutions with the designed concentrations (3, 5, 7, 10 and 15 mg/mL) were respectively obtained using isopycnic differential centrifugation by replacing water with ethanol, and then used to prepare 3DG hydrogels in a Teflon-lined autoclave at 180 ºC for 12 h. After that, the ethanol-filled hydrogels with the black cylindrical shape were removed and immersed in mixture of acetone and ethanol with the volume ratio of 1:1, and the water was slowly added into the system until the monoliths were fully immersed in water, resulting in the exchange of the ethanol absorbed inside of sponges by water. After 24 h of treatment, the solvent volume of ~1/3 was slowly removed by decanting. Thereafter, water was added again to submerge the sponge and such process was repeated about 6-8 times. Finally, the water-filled sponges were freeze-dried and named after 3DG-x, where x represents the concentration of GO alcohol solution. 2.3. Synthesis of Water-soluble PI Precursor Solution. As shown in Figure S1, the water-soluble precursor of PI, i.e., PAS, was synthesized according to the procedures reported in the literatures23-24. In brief, 2.0 g of ODA was dispersed in 30 mL of DMAc by stirring, then 2.18 g of PMDA was added into the mixture to react after being dissolved completely, and the system was stirred for another 240 min. The resultant solution was dumped into the excessive ultrapure water, followed being


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filtrated, washed, and dried. Finally, 1 g of PAS and 0.48 g of TEA were added into 18.52 mL of ultrapure water under stirring for 5 h at room temperature to obtain the PAS solution. 2.4. Fabrication of 3DG/PI Composite Sponges. The 3DG/PI composite sponges were fabricated by means of a two-step strategy including vacuum-assisted infusing and curing, and the protocol was depicted in Figure 1. Firstly, the as-prepared 3DG sponges were completely immersed into PAS solution and placed in a vacuum chamber about ~30 min for removing air bubbles enclosed in the hierarchical pores of 3DG, followed by infusing at 80 ºC for 1 h. Then, the 3DG sponges were taken out from PAS and cured by thermal annealing process at 300 ºC for 2 h in argon atmosphere to win the 3DG/PI composite sponges. Herein, several kinds of composites were fabricated and labeled as 3DG-x/PI. Additionally, pure PI sponge was also obtained by freeze-drying of PAS for comparison.

Figure 1. Schematic diagram for fabricating 3DG/PI composite sponges.



2.5. Material Characterization and Electrical Measurements. The surface morphology and microstructure of samples were respectively observed by a scanning electron microscope (SEM, JSM-5601LV, Japan), a transmission electron microscope (TEM, TF20, USA), and a nanoscope IIIa multimode atomic force microscope (AFM, Veeco, USA) in tapping mode with Si3N4 tip. The phase structure of samples was characterized by the high resolution X-ray diffraction (XRD, D8Discover25, Germany) with Cu-Kα radiation (λ=1.54059 Å). The composition and chemical structure of samples were respectively detected employing a Fourier transform infrared (FTIR) spectrometer (TENSON 27 instrument, Bruker) for both sides of the membrane using a reflection mode, a X-ray photoelectron spectroscopy (XPS, PHI-5702, USA) with monochromatic Al-Kα irradiation, and a Raman spectroscopy (LabRAM HR Evolution, France) equipped with 514 nm laser excitation. The thermal stability was determined using thermogravimetric analysis (TGA, STA 449C, Germany) over a temperature range of 25-800 °C at a heating rate of 10 ºC/min under a N2 atmosphere. The nitrogen adsorption and desorption isotherm was measured at 77 K using a Micromeritics ASAP2020 system. The electrical conductivity of samples was measured by a standard four-point probe method (RTS-9, PROBES TECH), and at least five samples were tested. 2.6. Mechanical and Electromechanical Measurements. The as-obtained 3DG hydrogel from hydrothermal process was cut into the cylinder with the same diameter and height of about 1.0 cm. After being freeze dried into a sponge-like block, it was used to fabricate the nanocomposites. Furthermore, the compressive performances of


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all sponges was assessed by an electrical universal material testing machine with a 500 N load cell (EZ-Test, SHIMADZU), the crosshead velocity was kept at 2 mm/min for loading and unloading tests under the conditions of 23 °C and 35% relative humidity. The modulus (E) was calculated from the initial linear region of stress-strain curve for loading curve. The energy loss coefficient (ξ) was defined as the loop area relative to the area under the loading curve, and the total loss strain (η) was regarded as the difference between the corresponding strain values when the stress was zero in a loading-unloading process. The ultimate stress (σu) was taken as the stress at the strain of 50%. To realize the measurement of in-situ change in resistance for the application of compression strain on samples, the sourcemeter (2450, Keithley) was coupled with the motion controller (SC300-2A, Zolix) and high precision electrical-controlling move apparatus (TSA50-C, Zolix), and the moving rate was set as 1 mm/min. 3. RESULTS AND DISCUSSION The 3DG/PI composite sponges were fabricated by the procedures of solvothermal and vacuum infiltration-curing, as illustrated in Figure 1. The final surface morphologies of the samples were observed by SEM, as shown in Figure 2 and Figure S2. Obviously, bread-like 3DG sponge with the interconnecting macropores structure was obtained from GO nanosheets, which are transparent and wrinkled with average area and thickness of ~3.91 µm2 and ~1.23 nm (see Figure S3 and S4). Then, PAS was infiltrated into the porous space of the 3DG sponge, inducing the subsequent polymerization around each individual graphene sheet by strong



interactions including the chemical interaction25 (π-π stacking interaction) and physical interactions26 (van der Waals forces and hydrogen bonding) in the process of the vacuum infiltration and thermal curing. After that, the interconnected porous and laminar structure of 3DG sponge can be still maintained in the 3DG/PI composite sponges, implying the basic structure was not destroyed during the surface modification using PI polymer, and PI layer was distributed on the surfaces of pore wall instead of filling the pores. Compared with the pristine 3DG, the surface of the composite became rougher by the direct observation of TEM images (see Figure 2e and 2f), where the graphene nanosheets became nontransparent and some wrinkles and ripples on its surface disappeared, further demonstrating the successful coating of the pore wall with the PI layer. In addition, as shown in Table S1, higher specific surface areas ranging from 134 to 218 m2/g were obtained for 3DG sponges in comparison to that of the 3DG (64-165 m2/g) obtained by the hydrothermal method27, attributing to its hierarchically porous structure with very broad size distributions ranging from tens to hundreds of micron. Apparently, the porous architecture led to the low density,28 while the stack of graphene nanosheets became compact with rising the concentration of GO and the density increased correspondingly. Moreover, after being incorporated with PI, the density of the resultant composite sponges increased to some extent, and the corresponding values are provided in Table 1.


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Figure 2. SEM images of (a) 3DG-5 (14.3 mg/cm3) and (c) 3DG-10 (20.2 mg/cm3) with the porous structure, as well as the corresponding composite sponges of (b) 3DG-5/PI (31.4 mg/cm3) and (d) 3DG-10/PI (39.7 mg/cm3). TEM images of 3DG (e) and 3DG/PI (f). The inset in (e) is the corresponding selected area electron diffraction (SAED) pattern.



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Table 1. Physical properties of 3DG/PI composite sponges prepared from the GO solutions with different initial concentrations GO concentration

3DG/PI bulk density

(mg/mL)

ρ (mg/cm3)

3DG-3/PI

3

24.0

3DG-5/PI

5

31.4

3DG-7/PI

7

35.5

3DG-10/PI

10

39.7

3DG-15/PI

15

44.3

Samples

As a representative, the thermal stability, composition and microstructure of 3DG-5/PI composite sponge were respectively investigated by XPS, Raman, FTIR, XRD and TGA, and the results were presented in Figure 3. It can be found from the XPS survey spectra (Figure 3a) of 3DG-5/PI that the appearance of characteristic peaks centered at 284.6 eV (C 1s), 532.0 eV (O 1s) and 400.0 eV (N 1s) indicates the successful polymerization after infiltrating and thermal curing.29 Moreover, the atomic ratio of carbon to oxygen (C/O) has light decrease from 6.0 for 3DG-5 to 5.1 for the composite due to the introduction of C=O from the imide groups, the results are also verified by the detailed analysis of C1s peaks in Supplementary Information (see Figure S5). In addition, the Raman spectrum of the 3DG-5/PI presented two typical peaks located at 1356 cm-1 and 1586 cm-1 (Figure 3b), which showed that the characteristic D-band of 3DG-5/PI moved to a high-frequency position compared with


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those of 3DG-5 and GO (at 1347 cm-1) because of the charge transfer between PI and 3DG-5.30-31 Meanwhile, another broad peak at 1586 cm-1 is resulting from the overlap of the G-band of 3DG-5 and the C-N stretching vibrations of the imide ring of PI at 1601 and 1612 cm-1.32 Also, the typical peaks of the FTIR spectra for the 3DG-5/PI composite sponge located at 1726 and 1773 cm-1 result from the cyclic imide C=O symmetric and asymmetric stretches (Figure 3c), while the peaks of C-N stretching vibration (1365 cm-1), C=C stretching vibration (1503 cm-1) and C=O bending vibration of the imide groups (723 cm-1) can also be observed, in agreement with the ones in the literature.33 The XRD pattern of 3DG-5/PI composite sponge also provide the same information and the detailed illustration is given in Figure S6. In addition, as shown in Figure 3d, the 3DG-5/PI presents an improved thermal stability in comparison with the PI due to the presence of thermally-stable graphene nanosheets34-35, which act as barriers to inhibit the mobility of polyimide acid chains during pyrolysis36-37, and the strong interactions between graphene nanosheets and PAS substantially influence on the mechanical properties of the nanocomposite.24



Figure 3. XPS survey spectra (a), Raman (b), and FTIR (c) spectra, as well as TGA curves (d) of the as-prepared 3DG-5/PI composite sponge and contrast samples.

To evaluate the electrical transport properties of the as-prepared 3DG/PI composite sponges, the density-dependent conductivity of samples has been investigated, as this is a critical prerequisite for bulk porous materials serving as the conductive materials. In this work, the obtained 3DG/PI composite sponges were pressed into films for electrical conductivity measurement. Obviously, the bulk density increases with rising GO concentration (Table 1 and Table S1), due to the formation of progressively denser conductive networks, thusly their conductivities are significantly enhanced, as shown in Figure 4a. The slight decrease of conductivity for 3DG scaffold could be found after being incorporated with PI; for instance, the


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composite sponge of 3DG-15/PI with the density of 44.3 mg/cm3 displays the highest electrical conductivity of 3.7 S/cm, which is slightly lower than that of 4.6 S/cm for 3DG-15 scaffold with the density of 22.7 mg/cm3. It implies that the infiltration of PI into 3DG scaffold has little influence on the interconnected conductive graphene networks and may associate with the formation of chemical bonds on the outmost graphene layers of the cellular walls.38 Such trend can also be reflected by the change of the electrical resistivity (Figure 4b); namely, the resistivity exhibits the decreasing trend as the concentration of GO increases, attributing to the obviously raised proportion of graphene in the composite.

Figure 4. (a) Electrical conductivity of samples as a function of the density. (b) Electrical resistivity of samples as a function of GO concentration.

Exhilaratingly, all of the electrical conductivities measured in this work are apparently higher than those of previously reported graphene monoliths16, 26, 28, 39-40, graphene-based polymer composites26, 41-43, and other forms of carbon materials44, highlighting the great advantage of the composite sponges for electrical applications as compared with other materials shown in Figure S7. It can be concluded that three



main features can lead to the outstanding conductivity for the composite sponges: (a) the resulting 3DG scaffold with the compact, well-interconnected network architectures and the higher bulk density than those from other reports39-40 can be served as conductive pathways; moreover, the conducting mechanism of 3DG relies on the Mott’s hopping model39, i.e., σ∝T-[1/(d+1)], where σ, T, and d represent the electrical conductivity, temperature, and dimensionality, respectively. Hence the transport of electrons in 3DG should obey 3D and two-dimensional (2D) hopping mechanisms simultaneously; that is, the dual conducting behavior of 3DG contains the 2D nature of the single graphene sheet and the interconnected graphene networks in graphene aerogel. (b) the favorable 3D conducting scaffold of graphene sponge can still be well-preserved after being incorporated with PI, and the coated PI layer can weld the cross junction between graphene nanosheets, thusly reducing the contact resistance between graphene nanosheets; (c) the almost removal of the functional groups from 3DG can be achieved after the hydrothermal and annealing processes. The excellent mechanical properties are pursued in flexible electronics;12 however, most of graphene bulks with relative poor structure stability generally induce the poor mechanical strength (see Figure S8).45-47 In order to evaluate the practical application potential of our 3DG/PI composite sponges in flexible electronics, the compressive stress-strain (σ-ε) curves of 3DG-5/PI and 3DG-15/PI composites are investigated and compared. The typical J-shaped σ-ε curves for the loading-unloading cycles at set strains of 10%, 30%, 50% can be observed, and all curves contain three regions including initial linear elastic deformation, subsequent


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plastic deformation, and final densification, as clearly presented in Figures 5a-d. In comparsion with single 3DG sponge (Figure S8), apparently, the shape of the curves is similar, and the σu values for 3DG-5/PI (~175 kPa) and 3DG-15/PI (~100 kPa) taken at the strain of 50% increased by 91% and 85%, respectively. They are even higher than those of other graphene-based polymer composites24, 45, 47, ascribing to the favourable structure stability of 3DG/PI composite sponges. On the contrary, the compressive recycle stability of 3DG-15/PI is better than that of the 3DG-5/PI (Figures 5c-d); namely, after being repeated 10 times of loading-unloading cycling, the latter exhibits significant strength degradation. However, the σ-ε curve of the 10th cycle for 3DG-15/PI is identical to that of the second cycle, except for the first cycle that has an greater strength loss (Figure 5d). Also, the ultimate stress value decreases slightly to 86% of the original level, indicating the highly reversible and durable sponge behavior.



Figure 5. Compressive σ-ε curves of 3DG-5/PI (a) and 3DG-15/PI (b) composite sponges at different strains. The insets show the relationship between ξ and ε. Mechanical stability of 3DG-5/PI (c) and 3DG-15/PI (d) after 10 times of loading-unloading cycling under the compressive strain of 50%. The insets illustrate the calculated values of ξ, E and σu at the strain of 50%. (e) Retention of the σu value at a strain of 50% and the η within 10 cycles for 3DG-15/PI. (f) The compressive σ-ε curves of 3DG-15/PI under the strain of 50% at different set strain rates. The inset is ξ. (g) The compressive schematic of 3DG/PI composite sponge.


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Subsequently, the relationship between η and cycle time for 3DG-15/PI was further analyzed (Figure 5e), which presented a slight decrease of ~2% after 10 times of

repeated compressions. Even after being compressed at a wide range of strain

rates from 10% to 1000%/min, the σ-ε curves of 3DG-15/PI are almost overlapping each other (Figure 5f). The hysteresis loop still maintained under fast deformation and could be attributed to the robust structure as well as the excellent elasticity and flexibility of 3DG scaffold. The corresponding ξ exhibited a tiny decrease from 45.3% to 44.7% due to the friction or fracture during the deformation and the unbinding of walls during the unloading process.4,8 These measurements demonstrate the superelasticity and mechanical property of 3DG/PI composite sponges resulting from the intimately synergistic effects of graphene and PI. Furthermore, the snapshots in the compressive testing of composite sponge and mechanical performance of contrast sample of PI monolith are also presented in Figure S9 for comparison. The above brilliant mechanical properties of composite sponges can be illustrated as follows: (1) the unique lamellar and network structure of 3DG scaffold, π-π interaction and mechanical force, as well as van der Waals’ force, make graphene nanosheets adhere strongly to each other, endowing the as-obtained 3DG scaffold with more rubost;48 (2) the presence of conjugated PI layer increases the strength of 3D structure via strong π-π interaction to bear a certain extra force; (3) the unique binary structure design of 3DG/PI composite combines the advantages of PI and graphene sponge, which effectively transfers load from graphene sheets to PI skeletons under mechanical deformations. PI skeletons return to its original



configuration when the load is removed, allowing 3DG scaffold to recover its initial shape,42 as depicted in the compressive schematic (Figure 5g). The 3DG/PI sponges with the unique binary structure possess not only excellent mechanical properties but also high electrical conductivity, holding great potential for applications in the field of flexible and compressible sensor. Here, the strain-induced and the bulk density-dependent variations of electrical resistance ratio ((R0-R)/R0, where R and R0 represent the resistances with and without strain, respectively) for 3DG/PI composite sponges were investigated. Before testing, the top and bottom surfaces of cylinder composite sponges were treated by the conductive silver paint, and then two copper wires were connected to an sourcemeter. In Figure 6a, the current-voltage (I-V) curves of 3DG-15/PI composite sponge at different compressive strains of 0% - 90% exhibited the good linear characteristics, and the slope of I-V curves increased with raising the strain; that is, the bulk resistance reduced, owing to the increased contact area between graphene nanosheets in 3D bulk scaffold, which provided more conductive pathways for electron transport, and further led to the decrease of resistance during the compression process (as evidenced by Figure S10). The resistance response to repeated compression cycles is also given in Figure 6b; obviously, the change of resistance variation ratio raised from about 26% to 68% when compression strain increased from 15% to 90%, implying potential application of the composite sponge in pressure-sensor field.49


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Figure 6. I-V curves (a) and resistance change (b) of 3DG-15/PI composite sponge under different applied strains for 10 cycles. I-V curves (c) and resistance change (d) of 3DG/PI composite sponges with different bulk density when repeatedly compressed up to 30% of strain for 10 cycles. (e) Piezoresistive behavior of 3DG-15/PI composite sponge when repeatedly compressed up to 30% of strains for 300 cycles.

Again, the I-V curves of 3DG/PI composite sponges at compressive strain of



30% indicate the bulk resistance decreases with the increase of bulk density (Figure 6c). Similarly, the compressive sensitivity of the composite sponges to resistance under strain of 30% reduces with rising the bulk density, as clearly shown in Figure 6d; namely, the electrical resistance ratio of 38.5% is corresponding to the bulk density of 44.3 mg/cm3, whereas the electrical resistance ratio of 88% is corresponding to the density of 24 mg/cm3. Such excellent strain sensing performances can be ascribed to the elaborate micro/nanomanufactures and potentially apply to the sensors with differentiate tiny amounts of strains caused by the songs or hertbeats.17 Impressively, the response of bulk electrical resistance is almostly constant within 10 cycles of compression (Figure 6d), further verifying their remarkable structural resilience and recoverable stability of conductive networks. In addition, the durability is a very important indicator in practical applications as the strain sensor, such as reducing the use-cost and enlarging its popularity. Here, the change of resistance remains favorably stable after 300 times of compress-release cycling under compressive strain of 30% for 3DG-15/PI (Figure 6e), implying the stable response of electrical resistance to cyclic compressive strain and electromechanical sensitivity. Therefore, it is hopeful for the applications of the prepared composite sponges in flexible electronics and strain sensor.

4. CONCLUSIONS In this work, we have demonstrated that the structure stability of 3DG sponges can be greatly improved by introducing PAS aqueous solution, precursor of PI, by


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vacuum infiltration and curing methods. Also, the resulting 3DG/PI composite sponges exhibit markedly comprehensive properties including the excellent electrical conductivity, compression performance, cyclic stability within the compression cycles of 10, as well as the prominent electromechanical properties and stable piezo-resistance effect. These outstanding properties can be attributed to the interconnecting and robust network structure of 3DG as well as the effective cooperation of graphene and PI. Again, our studies also revealed that the resistance change of 3DG-based polymer composite materials is dependent on not only the strain, but also the bulk density. Based on the above mentioned results, it is believed that the 3DG-based polymer composite sponges will find broad applications in the flexible and stretchable electronics, such as integrated circuit conductor, wearable stress sensor or strain sensor and so forth.

ASSOCIATED CONTENT Supporting Information Reaction schemes of PI and 3DG with PAS; SEM images of 3DG and 3DG/PI sponges; TEM and AFM characterization of GO nanosheets; physical property of 3DG/PI composite sponges; comparison diagram of samples for C1s peaks in the XPS spectra; XRD patterns of 3DG scaffold and 3DG/PI composite sponge; electrical conductivity of 3DG bulks and 3DG/PI composite sponges in comparison with the other reports; compressive performance of 3DG and PI sponges; snapshots in compressive testing of 3DG/PI composite sponges; and electromechanical



performance of 3DG-15 monolith.

ACKNOWLEDGEMENTS We gratefully acknowledge the funding support from the National Natural Science Foundation of China (Grant Nos. 51675514 and 51575507).

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252x100mm (150 x 150 DPI)


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High-Performance Graphene Sponges Reinforced with Polyimide for

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