Piezoresistive and Mechanical Characteristics of Graphene Foam

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Piezoresistive and mechanical characteristics of graphene foam nanocomposites Shashikant P Patole, Siva Reddy, Andreas Schiffer, Khalid Askar, B. Gangadhara Prusty, and Shanmugam Kumar ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b02306 • Publication Date (Web): 05 Feb 2019 Downloaded from http://pubs.acs.org on February 6, 2019

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ACS Applied Nano Materials

Piezoresistive and Mechanical Characteristics of Graphene Foam Nanocomposites Shashikant P. Patole1, †, Siva K. Reddy1, Andreas Schiffer2, Khalid Askar1, B. Gangadhara Prusty3, and S. Kumar1,*

1Department

of Mechanical and Materials Engineering, Khalifa University of Science and Technology, Masdar Institute, Abu Dhabi 54224, UAE

2Department

of Mechanical Engineering, Khalifa University of Science and Technology, Abu Dhabi, 127788, UAE

3ARC

Training Centre for Automated Manufacture of Advanced Composites, School of

Mechanical and Manufacturing Engineering, UNSW Sydney, NSW 2052, Australia.

†

Currently at the Department of Physics, Khalifa University of Science and Technology,

Abu Dhabi, 127788, UAE. *

Corresponding author, E-mail: [email protected]

ACS Paragon Plus Environment

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KEYWORDS. Graphene foam nanocomposites, piezoresistivity, energy absorption, pressure sensors, hysteresis performance.

ABSTRACT: Here, we report the piezoresistive and mechanical characteristics of threedimensional (3D) graphene foam (GF)-polydimethylsiloxane (PDMS) nanocomposites processed by a facile two-step approach. A polyurethane (PU) foam with graphene embedded (and aligned) in the pore walls is pyrolyzed and then impregnated with PDMS to form a GF-PDMS nanocomposite, resulting in a slit-like network of graphene embedded in the viscoelastic PDMS matrix. The interconnected graphene network not only imparts excellent electrical conductivity (up to 2.85 S m-1, the conductivity of PDMS is 0.25×10-13 S m-1) to the composite but also enables ultrasensitive piezoresistive behavior. For an applied compressive strain of 10% we report a 99.94% reduction in resistance, with an initial gauge factor of 178 and note that this value is significantly higher


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than those reported in the literature. Cyclic compression-release tests conducted at different strain amplitudes demonstrate that both the mechanical and piezoresistive response of the GF-PDMS are fully reversible up to a maximum strain amplitude of 30%. The facile processing, recoverable and reversible response over 1000 cycles, good hysteresis performance over a range of strain-rates and energy absorption characteristics open new opportunities for GF-PDMS nanocomposites in various applications such as soft robots and human-machine interface technologies.

1. INTRODUCTION Senses are transducers from the physical world to the realm of the mind where we interpret the information, creating our perception of the world around us.1 Touch, sound, smell, taste, and sight are the five senses which help humans to interact with their surroundings. There is a huge demand and associated challenges in developing artificial sensors which can emulate the human senses. These sensors can help machines to interact with the surrounding effectively and can help disabled people to


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envisage the realistic prosthesis.2-4 An artificial skin is an example in which the embedded artificial pressure sensors generate signals to reflect the intensity and the location of the pressure imposed on the surface of the skin5. The pressure sensing mechanism of these sensors includes capacitive sensing,6 transistor sensing,7 piezoresistive sensing,8-13 and piezoelectric sensing.14 Among these, the piezoresistive sensors, that transduce the pressure imposed on the sensor to resistance signal, have been widely used owing to their attractive advantages such as ease of fabrication, lowcost, and easy signal collection.15 Conventionally, polymers and elastomeric rubbers with conductive carbon derivatives, such as carbon black and carbon nanotubes, are used as the sensing elements for piezoresistive sensors.11-13, 16-18 However, these conductive sensors are insensitive, unstable, and difficult to reproduce in both medium(10-100 kPa suitable for object manipulation) and low-pressure regimes (~10 kPa, comparable to gentle touch), 19 limiting their specific applications, such as to artificial skin.


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Conductive foams are considered as alternative materials for the fabrication of piezoresistive pressure sensors owing to their combinational electronic conductivities and mechanical flexibilities.14 Recently, several conductive foams with high electronic conductivities and good mechanical properties have been reported. For example, carbon nanotube and graphene foams constructed via a chemical vapor deposition process;20-21 silver-carbon nanocable/carbon nanotube foams synthesized using hydrothermal method;22 bacterial cellulose foam fabricated by the freezer drying and pyrolysis of pellicles;23 and graphene foam obtained using solution dip coating and thermal reduction of graphene oxide on the backbone of commercial polyurethane foams for pressure sensors.24-30 Although these newly developed conductive foams have been used as flexible conductors and pressure sensors, their low-resistance sensitivity to the pressure imposed and complexity in the fabrication process have hindered their application in many devices.

Many different kinds of bio-inspired sensors have been developed in recent years, including gas sensors,31-32 torsion sensors,33 and, in particular, strain sensors 34-37.


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Inspired by the spiders‘ sensory system, Kang et al.35 prepared slit-like ultra-sensitive strain sensors, which are highly sensitive to strain and vibration, benefiting from the disconnection-reconnection process in the slit. Wu et al.36 used carbon black coated polyurethane sponge as a pressure sensor for human-machine interface platform, which exhibited significant advantages such as low cost, ease of fabrication, and versatility. Yao et al.37 created a graphene wrapped polyurethane sponge pressure sensor with high sensitivity and cycling stability. However, the connected crack junctions experience finite conductance and non-connected crack junctions experience zero conductance resulting in a sudden jump in the conductance values when pressure is applied. Most sensors based on pure cracking mechanism exhibit a limited detection range due to easy breakage of conductive pathways, inhibiting their application in large deformation detections.

In this work, we demonstrate a straightforward and highly efficient strategy for the synthesis of novel pressure sensors using graphene foam (GF)-PDMS nanocomposites. The GF is obtained by soaking PU foam into the commercial graphene crystals (GC)


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dispersed ethanol mixture and subsequently carbonizing GC-PU foam. A highly crystalline GC with the basal plane extending more than tens of microns imparts unique morphological, thermal, and electrical properties to the GF. Further, the PDMS impregnation locks the three-dimensional GC-slit network inside the viscoelastic PDMS providing extra solidity and robustness to the fragile GF network. In a simple demonstration of human-machine interface, the GF-PDMS strain sensor detects the pressure exerted by a finger and converts it into an electrical signal. Compared to the other works, our method is simple, versatile, and cost-effective. The present work is novel and different from the previous related reports in various aspects. The starting material used itself is crystalline graphene flakes (not the graphene oxide) and there is no reduction in the synthesis process. The temperature used for the PU removal in the present work is just above the degradation temperature of PU (i.e. 250 oC) which is quite acceptable temperature (compared to 1000 oC used in previous works) for the industrial scale production and use. The detailed comparison of the current approach with those described in the literature is provided in Table S1 (Supporting Information).


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2. NANOCOMPOSITE SYNTHESIS AND CHARACHTERIZATION 2.1 Synthesis of Graphene foam/PDMS nanocomposites

As sketched in Fig. 1a, the graphene foam/PDMS nanocomposites are synthesized by a two-step approach: (a) preparation of Graphene foam by carbonizing graphene crystal adsorbed PU foam, followed by (b) PDMS impregnation into the Graphene foam. The details of this process are as follows.

(a) Preparation of graphene foam:

Highly crystalline graphene crystals (GC) in powder form were obtained from Graphene

CrystalTM KAUST-Saudi Arabia. The details of the graphene crystals are provided in the patent.38 The GC are graphene flakes with less than 10 layers containing defect free graphene lattice (sp2 hybridized C-C bonding) over the full range of basal plane. The flake size is less than 100 µm. PU foam sheets (pore size ≈ 0.7 mm) were cut into small cuboids of size 2.5 cm x 2.5 cm x 4 cm. They were thoroughly washed with ethanol


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(purchased from Sigma Aldrich) to remove any impurities adsorbed and further dried in air at room temperature. In glass beakers, 40 ml (30 g) ethanol was used to prepare different GC concentration samples. In a typical experiment, GC were weighed with respect to the ethanol weight to obtain different weight% (wt%) samples. GC were dispersed in ethanol for 60 min by tip-sonication. The dried PU foam pieces were dipped into the GC-ethanol solution (see Fig. 1a). The foam was squeezed and soaked several times into the GC-ethanol to obtain a uniform adsorption of GC. The beakers were kept in a fume hood for two days to evaporate the ethanol. During the ethanol evaporation, the uniform coating of GC into the foam was ensured by squeezing and rolling the foam inside the solution for several times. The GC adsorbed PU foam (GCPU) was further heated in a muffle furnace to carbonize the PU (see Fig. 1a). The heating was performed in open air with a very slow heating rate. In a typical experiment, muffle furnace was heated up to 100 °C within 2 h and held at that temperature for 1 h and subsequently it was heated up to 200 °C within 2 h and held at that temperature for 1 h. The temperature was further increased to 250 °C within 1 h and held at that temperature for 1 h. The slow heating for carbonization is very important to avoid the


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collapse of foam cell structure due to cracking of volatile species in the PU. It also prevented collapse of the GC network due to shrinkage of PU foam during heating. After carbonization, the samples were cooled to room temperature by switching off the muffle furnace. Carbonization turned the GC-PU foam into graphene foam (GF). In a control experiment, neat PU foam was heated in a similar way to obtain the carbon foam (CF).

(b) PDMS impregnation:

Sylgard® 184 grade PDMS was purchased from Dow Corning. This silicone-based elastomer consists of two parts: PDMS base and a curing agent. Both the base and curing agent were combined in a weight ratio of 10:1 and mixed thoroughly in a Teflon beaker. The mixture was degassed for 2 h to remove air bubbles, while the GF samples were carefully transferred into separate paper cups. The bubble free mixture of PDMS base and curing agent was poured into these paper cups. The pouring rate was kept minimal to ensure that the PDMS impregnates into the GF completely (see Fig. 1a). The PDMS impregnated GF was degassed for 4 h at room temperature to remove any trapped air bubbles. After degassing, samples were heated at 90 °C for 12 h in a


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vacuum furnace to cure the PDMS. The final GF-PDMS samples were prepared by cutting the excess PDMS surroundings from cured block. In a similar manner, PDMS impregnated PU foam (PU-PDMS) and CF-PDMS were obtained.

2.2 Material characterization

The foam structure was observed under the optical microscope (Olympus BX51M) with a 5x magnifying lens. GC were characterized under a transmission electron microscope (TEM, FEI Titan G2 80-300 ST). Monochromator and image corrector were used to acquire high-resolution TEM (HR-TEM) images at 80 kV. The detailed microstructure of the foam was observed under the scanning electron microscope (SEM, FEI Nova Nano). Spectroscopic characterizations were carried out using Fourier transform infrared (FTIR) spectroscopy in an attenuated total reflection (ATR) mode using Bruker Vertex 80v FTIR. Raman confocal spectroscopy was carried out using WITec confocal Raman spectrometer with an excitation wavelength of 532 nm. Thermal behavior of the composite was characterized using thermogravimetric analysis (TGA, NETZSCH STA 449 F3 Jupiter) and differential scanning calorimetry (DSC NETZSCH High


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Temperature DSC) under inert atmosphere (N2 gas). TGA was performed from room temperature to 1000 °C at a ramp rate of 10 °C/min and DSC was performed from room temperature to 600 °C at a ramp rate of 10 °C/min.

2.3 Mechanical and piezoresistive testing

Mechanical tests were carried out using a Zwick-Roell universal testing machine. All samples were tested first under uniaxial monotonic quasi-static displacement-controlled compression and subsequently, selected samples were tested cyclically under displacement-controlled compression. The displacement-controlled static- and cyclicloadings were carried out at a displacement rate of 1 mm/min. A 2.5 kN load cell was used. Piezoresistivity tests were performed by measuring the in situ change in electrical resistance using Tektronix DMM 4050 multimeter during compression tests. The deformation of the samples during loading was recorded using CCD camera with a spatial resolution of 2448 x 2048 pixels, Schneider Xenoplan 2/3” (11 mm) lens, and two light source lamps.


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3. RESULTS AND DISCUSSION 3.1 Structure and morphology

We examine the structure and morphology of the GF/PDMS composites developed by the two-step method described in Section 2.1. It is interesting to note that without GC adsorption the carbonization of PU foam results in a carbon foam (CF) at the expense of around 80% volume shrinkage as shown in Fig. 1b and Fig. S1 (Supporting Information). The FTIR studies (Fig. S3) revealed that the strong peaks of PU foam at 3296 cm-1 (N–H stretching vibrations), 2971 and 2867 cm-1 (asymmetric and symmetric stretching vibrations of CH2, respectively), 1717 cm-1 (stretching vibrations of hydrogen-bonded C=O group), 1530 cm-1 (urethane N–H binding and C–N stretching), 1085 cm-1 (C–O–C) are completely absent in CF. This molecular loss results in the shrinkage of the PU foam. On the other hand, in spite of similar molecular loss, the GC adsorbed PU foam (GC-PU) does not shrink noticeably after carbonization, retaining its original shape and volume (Fig. 1b). The continuous network of GC over the PU prevents its shrinkage. The resulting GF is an ultra-lightweight and fragile


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aerogel which possesses the minimum density of 1.2 mg cm-3. The GF density can be tailored by changing the GC concentration in ethanol or by choosing a PU foam of different pore density (see Section 2.1). Optical microscopy images (Fig. 1b) reveal that GC are adsorbed onto the foam uniformly along the polyhedral cell edges forming a 3D GC network. To enable robustness and strength to the GF without compromising its inherent thermal and electrical properties, PDMS was impregnated by the vacuum infusion method. The resultant GF-PDMS nanocomposite is electrically conductive showing conductivity up to 2.85 S m-1 due to the interconnected network of GC in the PDMS matrix.

The surface morphology and crystal structure of GC are shown in Fig. 2. GC are in flake form with the basal plane extending more than tens of microns (Figs. 2a-c).38 A TEM image of a single graphene layer at its edge is shown in Fig. 2d, while Figs. 2e and 2f give insight into the atomic structure of a GC, showing a defect-free graphene lattice over the full range of basal plane. Note that a defect-free lattice is beneficial in terms of electrical and thermal conductivity because it facilitates the sp2 hybridized C-C bonding


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which is a key factor for the superior properties of graphene. This is typically not observed in the lattices of reduced graphene oxide in which missing carbon atoms compromise the sp2 hybridized C-C bonding.39-40 The GC used in this study possess high crystallinity which can also be seen from the Raman spectra provided in Fig. S2 (Supporting Information), showing minimal D band intensity compared to the G band and 2D band intensities. Moreover, FTIR spectra reveal that the basal plane of GC is free from hydroxyl and oxygen species (Fig. S3, Supporting Information). Furthermore, our TGA results show that the GC possess high thermal stability with only 0.6 wt.% weight loss up to 1000 °C (Fig. S4, Table S2, Supporting Information).

The detailed surface morphology of the PU foam cell walls after adsorption of the GC (GC-PU) is shown in Fig. 2g-i. It is clear that GC are adsorbed uniformly onto the cell walls of the PU foam, and most of the GC are arranged at an angle to the cell wall surface, resulting in a flaky surface morphology. The carbonization of GC-PU foam by prolonged heating removes the PU from the cellular structure and results in a 3D network of GF as shown in Figs. 2j-l. It should be noted that the perpendicular


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attachment of the GC flakes onto the surface of the cell walls remains intact after carbonization, resulting in a substantial increase in surface area. After impregnation with PDMS, the three dimensional GC slit-like network is locked inside the rubbery polymer, resulting in a continuous and flexible network of GC (Figs. 2m-o).

3.2 Mechanical and piezoresistive response

In Fig. 3a compressive stress vs. strain measurements are shown for GF-PDMS (0.5 wt% GC), PU foam, PDMS, PU-PDMS and CF-PDMS; the inset of Fig. 3a shows the typical nonlinear compressive stress-strain response of the neat PU foam. It can be seen that PDMS impregnation into the PU foam (PU-PDMS) results in a stiffer response and in a decrease of densification strain compared to the neat PU foam, as expected. It is also clear that the GF-PDMS nanocomposite has a stiffer compressive response than the neat PDMS, indicating that the presence of the GC network within the PDMS has a significant reinforcing effect on the mechanical response of our nanocomposite. Interestingly, the response of the GF-PDMS in Fig. 3a is reminiscent of the in-plane


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compressive response of hexagonal metallic honeycomb foam.41 This type of material has a load-displacement response characterized by a relatively sharp initial rise to a load maximum followed by an extended load plateau which is terminated by a sharp rise in load. The observed deformation mechanism is schematically illustrated in Fig. S6 (Supporting Information) and briefly described in the following. In the first part of the response, deformation is uniform and linear elastic. Following the load maximum, deformation tends to localize to one row of cells which collapses at a dropping overall load until the walls of each cell come into contact. Contact arrests further deformation in the collapsed row of cells and causes spreading of deformation in the adjacent rows (see movies S1, S2, and S3 in Supporting Information). Under displacement-controlled loading, this row-by-row collapse can continue, with relatively small changes in the required load, until all cells in the specimen are collapsed. The load required for further compression of the material increases sharply beyond this point. Overall, in the elastic regime, the GF-PDMS composite has the highest elastic modulus compared to the PU foam, PU-PDMS, PDMS, and CF-PDMS (see also Fig. S7, Supporting Information). The measured elastic moduli are 3.8, 599, 5000, 2128 and 7695 kPa for PU foam, PU-


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PDMS, PDMS, CF-PDMS and GF-PDMS, respectively. We note that compared to other graphene foam composites, carbon nanotube foam and carbon black PU foam composites (Table S1, Supporting Information), our GF-PDMS nanocomposite shows the highest elastic modulus.

Figure 3b presents the corresponding specific energy absorption capacities deduced from Fig. 3a by calculating the area under each stress-strain curve. It is clear that the GF-PDMS has superior specific energy absorption (2.8 MJ/mm3) compared to the PU foam (0.001 MJ/mm3), PU-PDMS (0.27 MJ/mm3), PDMS (0.737 MJ/mm3) and CFPDMS. (0.738 MJ/mm3), The obtained values are comparable to those reported in the literature21, 26 and demonstrate the possibility of incorporating the GF-PDMS composite in impact and blast resistant structural applications.

Compressive stress-strain responses for GF-PDMS composites with varying GC concentration are shown in Fig. 3c. It is seen that an increase in the GC weight fraction from 0.3 wt% to 0.5 wt% leads to an increase in both elastic stiffness and yield strength; however, as the GC weight fraction is further increased to 0.7 wt% and 1 wt%,


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respectively, a decrease in elastic stiffness and yield strength is observed. We argue that, in the latter case, the additional GC are not perfectly attached to the cell walls and form pockets within the PDMS matrix that weaken the mechanical response of the GFPDMS.

However, a continuous network of GC in GF-PDMS improves electrical conductivity which changes with the applied compressive strain, showing a strongly pronounced piezoresistive behavior (see Fig. 3d). Graphene foam composites usually exhibit an increase in resistance with strain (tensile or compression) since the application of an increased stress breaks the continuous 3D network of graphene in the foam contributing to higher resistance compared to the unstrained resistance R0.37 The reduction in resistance ΔR with increasing strain, as observed in our 3D graphene foam composites, is attributed to significant alignment of GC.42 It should be noted that in the case of an increase in piezoresistivity, the normalized change in resistance, ΔR/R0, is always in the range

0  R / R0  

0  R / R0  1

, whereas in the case of a decrease in piezoresistivity, it is

(see Section SI 1, Supporting Information). As shown in Fig. 3d, the -


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ΔR/R0 values of a GF-PDMS with 0.5 wt% GC concentration change from 0% to 99.94% within the initial 10% strain, showing its excellent performance as a piezoresistive material. With further increase in the strain up to 45%, -ΔR/R0 changes to 99.65%. Apart from small fluctuations in the -ΔR/R0 values in the strain range between 35% to 45% (not clearly visible in Fig. 3d), where collapse of the cellular structure commences, the GF-PDMS maintains its electrical conductivity up to the onset of densification. As shown in Fig. 3e, the -ΔR/R0 vs. strain measurements for a GF-PDMS with 0.5 wt% GC can be well approximated by an exponential fitting curve



R / R0  100 1  exp   n 



(1)

with the fitting parameters set to λ = 6.77 and n = 1.61. Note that both -ΔR/R0 and ε are given in % in eq. (1). Although the curve fit has been performed over the entire range of compressive strains considered in the experiments (up to ≈ 70 %), only a short range of strain is shown in Fig. 3e for illustration purposes. Despite the inherent nonlinearity in the piezoresistive response of the GF-PDMS composite, eq. (1) can be


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re-arranged and used to determine the applied strain (in %) for any value of measured change of resistance ΔR via

 ln  R /100 R0  1        . (2) 1/ n

The ability of a strain sensor to detect very small deformations or vibrations in its surroundings strongly depends on the value of its gauge factor, defined as the initial slope of the -ΔR/R0 vs. strain curve. For the nonlinear trends observed here, the initial gauge factor Gi, can be approximated by calculating the secant modulus of eq. (1) at ε = 0.5% (within the nearly linear portion of the curve), giving Gi = 178 for the case shown in Fig. 3e. We note that this value of the gauge factor is significantly higher than those reported in the literature for other graphene foam and carbon black PU foam composites (see Table S1, Supporting Information), suggesting that our GC-PDMS composite has superior sensitivity to detect small deformations or vibrations as compared to similar nanocomposites reported in the literature. Furthermore, Fig. 3e shows that the value of -ΔR/R0 changes from 0 to 98% within 10 kPa of applied


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pressure, comparable to gentle touch of human skin19. In fact, notable changes in ΔR/R0 already occur in the pressure range of 200 – 600 Pa, suggesting that our GFPDMS composite is suitable for applications that require strain sensing at very low pressure levels such as heart rate measurement and human speech recording35-36. In Fig. 3f we present similar information as in Fig. 3e but for a GF-PDMS composite with 0.7 wt% GC. It can be seen from Fig. 3f that the change of -ΔR/R0 with compressive strain is significantly lower compared to the 0.5 wt% case (Fig. 3e), reporting an initial gauge factor of Gi = 27 (chosen here as the secant modulus at 3% strain since this range better represents the nearly linear portion of the curve). Again, the -ΔR/R0 vs. strain response is well approximated by the exponential fitting curve given in eq. (1) with the fitting parameters λ = 0.154 and n = 2.21. The decrease of Gi with increasing GC concentration can be attributed to the increasing number of contact points between adjacent GC flakes in the unstrained percolating network. When the GC concentration is reduced, the GC flakes on the cell walls are loosely connected, resulting in a flaky network of GC in the PDMS matrix whose electronic percolation is low in the unstrained


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state but increases significantly with applied compressive strain as adjacent GC flakes are brought into close contact.

3.3 Response to cyclic loading

The mechanical and electrical reversibility aspects of GF-PDMS are tested by compressive loading-unloading cycles at different strain amplitudes. Figure 4a presents the measured stress and ΔR/R0 profiles over 10 compression cycles performed with 10%, 20% and 30% strain amplitudes at a strain rate of 0.017 s-1, showing a high degree of reversibility in the mechanical and piezoresistive response up to strain amplitudes of 30%. The corresponding stress-strain curves for the 1st and 10th cycle are plotted in Fig. 4b, showing increasing hysteresis with increasing strain amplitude due to viscoelastic dissipation in the rubbery PDMS. The effect of stress relaxation on the piezoresistive performance of GF-PDMS is shown in Figure 4c. The stress relaxation test was performed by repeating ten compression cycles according to the following protocol: linear increase of strain up to 30% at a rate of 0.0017 s-1, maintaining a strain


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of 30% for a duration of 60 s, unloading at a strain rate of 0.0017 s-1 and keeping the sample at zero strain for 10s. It can be seen from Fig. 4c that the induced stress slightly decreases in each cycle while the strain is kept fixed at 30% due to viscoelastic stress relaxation in the PDMS matrix. The measured ΔR/R0 profiles, on the other hand, remain constant during this period and do not seem to be affected by the occurrence of stress relaxation, demonstrating good performance of the GF-PDMS composite as a strain sensor in terms of stability.

Another important aspect in evaluating the performance of strain sensors is their long term durability, especially in situations where the material experiences continuous and prolonged cyclic loading. To probe the long term durability of our nanocomposite, we subjected one sample of GF-PDMS to 100 compression cycles at a strain rate of 0.0017 s-1, and a second sample to 1000 compression cycles at a strain rate of 0.017 s-1; in both these tests, the strain amplitude was limited to 30%. In Fig. 4d, we present stressstrain curves for the first and last cycle of the test conducted with a strain rate of 0.0017 s-1, showing negligible stress degradation during this cyclic test. Furthermore, Fig. 4e


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shows that, even after subjecting the GF-PDMS composite to 1000 compression cycles at a strain rate of 0.017 s-1, a degradation in the induced peak stress of no more than 1.84% is detected, suggesting that the GF-PDMS composite is able to sense changes in strain over a prolonged period of cyclic loading and is therefore suitable for cyclic strain sensing applications (see also Fig. S8, Supporting Information). Fig 4e also demonstrates good hysteresis performance at higher strain-rate (up to 1000 cycles) as the system doesn‘t have sufficient time to dissipate energy.

Figure 4f shows, for the case of a GF-PDMS composites with 0.7 wt% GC, compressive stress-strain curves and the corresponding normalized change of resistance -ΔR/R0 with compressive strain acquired during the 1000th cycle of a cyclic compression test performed at three different strain rates (0.0017 s-1, 0.017 s-1, 0.17 s-1); only the monotonic loading parts of the curves are shown for the sake of clarity. We also include exponential fitting curves (eq. (1)) for the measured -ΔR/R0 vs. strain data, and their fitting parameters and gauge factors are listed in Table 1. It can be seen that the GFPDMS composite exhibits a stiffer compressive response when the strain-rate is


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increased owing to the fact that a lower rate of strain allows stress relaxation processes to take place in the rubbery PDMS during the loading phase. Such relaxation processes are also found to affect the piezo-resistive response of the GF-PDMS, reporting initial gauge factors of 120.1, 16.8 and 14.3 for strain rates of 0.0017 s-1, 0.017 s-1 and 0.17 s1,

respectively. Furthermore, the measured -ΔR/R0 values at a strain rate of 0.17 s-1

reach a plateau of 71 % instead of nearly 100% for the cases of 0.0017 s-1 and 0.017 s1,

respectively. This can be explained by irreversible damage processes that

accumulate in the cellular GC network during prolonged cyclic loading at a sufficiently high strain-rate. Such damage processes result in breakage of the percolating GC network and limit the electrical conductivity at high levels of strain.

In Fig. 5a we construct a performance map by plotting the stress degradation after 1000 cycles against the Young’s modulus, and including a data point for our GF-PDMS nanocomposite as well as for other recently reported 3D foam–like materials. Compared to other metals,43 polymer,44 graphene foam,45-49 carbon foam,50-51 carbon nanotube foam52-55 and their composites,56 the mechanical response of our material remains fully


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elastic up to large deformations (30% strain) allowing complete recovery without signs of plasticity, damage or fatigue.

We have tested the applicability of our GF-PDMS nanocomposite in the humanmachine interface. In a simple demonstration, a small sample of GF-PDMS nanocomposite, with two electrical wires attached to it, was finger-pressed and the change in the resistance was monitored by a multimeter. As shown in Fig. 5b, the strain sensor converted the pressing-holding-releasing pressure sequence into an electrical signal, demonstrating its suitability for strain sensing applications in soft robots and human-machine interfaces.

4. CONCLUSION We have reported the synthesis of GF-PDMS nanocomposites with excellent mechanical, thermal and electrical properties. The GF-PDMS nanocomposite also exhibits higher Young’s modulus and energy absorption capacity compared to PUPDMS, PDMS, CF-PDMS, and other carbon based foam composites. The amount of GC used in the GF influences the mechanical and electrical properties of the GF-PDMS


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nanocomposite. A continuous network of GC in the GF-PDMS nanocomposite gives electrical conductivity which changes with the applied compressive strain exhibiting strongly pronounced piezoresistive behavior. The electrical conductivity of the GFPDMS shows excellent sensitivity to the applied strain in the low-strain regime (10%). Loading-unloading cyclic tests performed at different strain amplitudes demonstrate that both the mechanical and piezoresistive response of the GF-PDMS are highly reversible up to strain amplitudes of 30%. The cyclic test over 1000 cycles shows only 1.8% degradation in the stress peak, and 0% degradation in the electrical performance demonstrating long-term stability and durability of GF-PDMS nanocomposite for strain sensing applications. In a simple demonstration of a human-machine interface, the strain sensor from GF-PDMS nanocomposite detects the pressure exerted by a finger and converts it into an electrical signal. It is expected that the GF-PDMS nanocomposite strain sensor will find numerous applications in soft robots and human-machine interfaces.


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FIGURES

Figure 1. Overview of graphene foam (GF)-PDMS nanocomposite synthesis: (a) Schematic diagram for the preparation of GF and GF-PDMS nanocomposite; the PU foam acts as a template for the graphene crystal adsorption. (b) Photographs (top) and optical microscope images (bottom) of PU foam, carbon foam (CF), GC-PU polymer nanocomposite, GF after pyrolysis in air at 250°C, and the GF-PDMS nanocomposite; the scale bars in photographs are 5 mm and in the micrographs they are 500 µm. (c) A typical GF-PDMS composite block (1 wt% GF) showing two probe resistance of 69.3 Ω.


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Figure 2. Morphological charachteristics of GC, GC-PU, GF, and GF-PDMS: (a, b) SEM images of GC, (c,d) TEM images of GC acquired at 300 kV, and (e,f) aberration corrected HR-TEM images of GC acquired at 80 kV (Inset in (f) shows the atomic-resolution image. Individual carbon atoms appear white in the image.). SEM images acquired at various magnifications showing the morphology of (g-i) GC-PU, (j-l) GF, and (m-o) GF-PDMS. The arrows in (h), (i), (k), and (l) indicate the perpendicular arrangement of GC, and arrows in (m), and (n) indicate the GC network inside PDMS.


32

a

10

Stress (MPa)

b

GF-PDMS CF-PDMS PU-PDMS PDMS PU foam

12

8 6 4 2

0

50

100

0 0

20

40

60

80

Strain (%)

Stress (MPa)

1.5 1.0 0.5 0.0

Sample code

2

100

10

80

8

60

6

40

4

20

2

0

0

10

20

30

40

50

60

Strain (%)

70

80

0

f

0.30

100

-R/R0 (%)

60

0.15

40

0.10

20

0.05

0

0.00 1

2

20

30

40

Strain (%)

50

60

3

4

5

70

0.8 100

0.7 0.6

Fitting curve:

80

0.5

60

0.4

40

0.3 0.2

20

0.1 0

Stress (MPa)

0.20

0

10

0.25

Fitting curve:

80

0

90

-R/R0 (%)

0

e

PDMS PU-PDMS CF-PDMS GF-PDMS

Stress (MPa)

4

2.0

PU foam

d

PDMS 0.3 wt% 0.5 wt% 0.7 wt% 1 wt%

6

0.5 wt%

2.5

-R/R0 (%)

c

3.0

100

Stress (MPa)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Energy absorbed (MJ/mm3)

Page 33 of 54

0.0 0

Strain (%)

2

4

Strain (%)

6

8

10

Figure 3. Mechanical and piezoresistive performance of GF-PDMS nanocomposites: (a) Compressive stress-strain curves and (b) energy absorption per unit volume for PU foam,


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PDMS, PU-PDMS, CF-PDMS, and GF-PDMS (0.5 wt% sample); the inset in (a) shows a magnified curve of the PU foam’s response. (c) Compressive stress-strain responses for GF-PDMS nanocomposites for a range of GF concentrations in weight %. (d) Compressive

stress-strain

curve

and

change

in

-ΔR/R0 with compressive strain for a 0.5 wt% GF-PDMS nanocomposite. (e) Same information as in (d) plotted over a shorter strain range; an exponential fitting curve is included to approximate the -ΔR/R0 vs. strain response. (f) Similar information as in (e) but for a GF-PDMS nanocomposite with 0.7 wt% GC.


34

b

-100

Stress (MPa)

-50 0 1.5 1.0 0.5 0.0 30 20 10 0

0.75

Strain rate = 0.017 s-1

0.50

3

4

5

6

7

Compression cycles

8

9

0.00

10

0

d

5

0 1.4 0.7 0.0 30

10

15

Strain (%)

20

25

30

1st cycle 100th cycle

1.5

-50

1.2

Strain rate = 0.0017 s-1 0.9 0.6

60 s

0.3

15 10 s

0

1

1.5

0.0 2

3

4

5

6

7

Compression cycles

8

9

10

0

f

st

1 cycle th 1000 cycle

1.2

Stress (MPa)

2

-100

0.9

th

10 cycle

1.00

Stress (MPa)

Strain (%) Stress (MPa) R/Ro (%)

1

0

e

st

1 cycle

1.25

0.25 0

c

1.50

5

10

15

20

2.0

1.5

80

0.6 0.3

60

1.0

40 0.0017 s -1 0.017 s -1 0.17 s

20

0.0

0

5

10

15

30

100

Strain rate = 0.017 s-1

0

25

Strain (%)

-R/R0 (%)

a

20

25

Stress (MPa)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Strain (%) Stress (MPa) R/Ro (%)

Page 35 of 54

30

Strain (%)

0

5

10

15

Strain (%)

20

25

-1

0.5

0.0 30

Figure 4. Cyclic strain sensing performance of GF-PDMS nanocomposites (0.7 wt% GC): (a) stress, strain and ΔR/R0 histories for 10 compression cycles with 30%, 20% and 10% strain amplitude; (b) corresponding stress-strain curves for the first and last cycle. (c) stress, strain and ΔR/R0 histories for a cyclic compression test with 60 s holding time at


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30% strain and 10 s holding time at zero strain. (d) Stress-strain curves for the 1st and 100th cycle during a 100 cycle fatigue test with 30% strain amplitude and 0.0017 s-1 strain rate. (e) Stress-strain curves for the 1st and 1000th cycle during a 1000 cycle fatigue test with 30% strain amplitude and 0.017 s-1 strain rate. (f) Compressive stress and -ΔR/R0 responses plotted as functions of the compressive strain for three different strain rates; exponential fitting curves are included for the ΔR/R0 data.


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b

50 40%, [26]

40

Cycles = 1000

Maximum compressive strain %, [Reference number]

30

20

50%, [51] 15%, [46]

Graphene foam composites Graphene foams CNT foams CNT/Graphene foams

60%, [45]

PU foam 50%, [26] 50%, [52]

10

50%, [44]

1000 800

Resistance (Ohm)

a Stress degradation (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


600 Push 400

Release

200

95%, [51] 60%, [39]

0

0

30%, [This work]

0 500

7000

8000

Hold 0

Young’s modulus (KPa)

20

40

Time (s)

60

80

100

Figure 5. (a) Comparison of stress degradation and Young’s modulus of GF-PDMS nanocomposite and other foam-like nanocomposites. Note that the stress degradation of other materials would be different with different densities or loading directions (anisotropy) and we chose the least value (best performance) as their relaxation value. (b) A demonstration showing the detection of finger tip pressure using GF-PDMS pressure sensor. The change in the resistance (absolute values) are plotted against the time. The background photograph shows the experimental setup.


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TABLES.

Table 1. Initial gauge factor, Gi, and fitting parameters, C, λ and n, for -ΔR/R0 vs. strain measurements acquired at three different strain rates (see Fig. 4f).



Fitting curve: R / R0  C 1  exp   n 



Strain rate (s-1)

C

λ

n

R2

Gi

0.0017

100

1.28

0.474

0.92

120.1

0.017

100

0.07

1.99

0.98

16.8

0.17

71

0.028

2.99

0.99

14.3


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ASSOCIATED CONTENT

Supporting Information.

The following files are available free of charge. Details of piezoresistance limits, a comparison table, SEM, FTIR, Raman, TGA and compressive cyclic test plots for 1000 cycles (PDF) Compression test movies (AVI)

AUTHOR INFORMATION

Corresponding Author *Corresponding Author E-mail: [email protected].

Present Addresses †Department of Physics, Khalifa University of Science and Technology, P.O. Box 127788, Abu Dhabi, UAE.

Author Contributions


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All authors contributed to manuscript preparation. All authors have given approval to the final version of the manuscript.

Funding Sources S.K and S.K.R would like to thank Abu Dhabi Education Council (ADEC) for providing the research grant (EX2016-000006) through “the ADEC Award for Research Excellence (A2RE) 2015”. S.K and S.P.P gratefully acknowledge the financial support from the Abu Dhabi National Oil Company (ADNOC) under Award No:

EX2016-000010. B.G.P.

Acknowledges financial support from the ARC Training Centre for Automated Manufacture of Advanced Composites (IC160100040), supported by the Commonwealth of Australia under the Australian Research Council’s Industrial Transformation Research Program

S. Kumar and Shashikant P. Patole : Supported by ADNOC under Award No: EX2016000010 S. Kumar and Siva K. Reddy: Supported by ADEC under the Award No: EX2016-000006 Andreas Schiffer: No funding source to declare Khalid Askar: No funding source to declare


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B. Gangadhara Prusty: Supported by ARC Training Centre for Automated Manufacture of Advanced Composites (IC160100040)

ACKNOWLEDGMENT The authors thank ‘Graphene Crystal’ KAUST for providing the GC samples, Dr. Tejendra Gupta and Dr. Muhamad Arif for their help in sample preparation and characterizations, and Prof. Brian L. Wardle (MIT) for helpful discussion.

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BRIEFS Graphene foam nanocomposites.

SYNOPSIS The synthesis and characterization of a novel graphene foam-PDMS nanocomposite is presented. The compressive mechanical deformation of the nanocomposite imposes an intriguing decrease in resistance. It can be utilized for highly sensitive, reproducible and zero fatigue strain sensor with possible applications in human-machine interface.

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Figure 1. Overview of graphene foam (GF)-PDMS nanocomposite synthesis: (a) Schematic diagram for the preparation of GF and GF-PDMS nanocomposite; the PU foam acts as a template for the graphene crystal adsorption. (b) Photographs (top) and optical microscope images (bottom) of PU foam, carbon foam (CF), GC-PU polymer nanocomposite, GF after pyrolysis in air at 250°C, and the GF-PDMS nanocomposite; the scale bars in photographs are 5 mm and in the micrographs they are 500 µm. (c) A typical GF-PDMS composite block (1 wt% GF) showing two probe resistance of 69.3 Ω. 1322x541mm (96 x 96 DPI)


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Figure 2. Morphological charachteristics of GC, GC-PU, GF, and GF-PDMS: (a, b) SEM images of GC, (c,d) TEM images of GC acquired at 300 kV, and (e,f) aberration corrected HR-TEM images of GC acquired at 80 kV (Inset in (f) shows the atomic-resolution image. Individual carbon atoms appear white in the image.). SEM images acquired at various magnifications showing the morphology of (g-i) GC-PU, (j-l) GF, and (m-o) GF-PDMS. The arrows in (h), (i), (k), and (l) indicate the perpendicular arrangement of GC, and arrows in (m), and (n) indicate the GC network inside PDMS.



Figure 3. Mechanical and piezoresistive performance of GF-PDMS nanocomposites: (a) Compressive stressstrain curves and (b) energy absorption per unit volume for PU foam, PDMS, PU-PDMS, CF-PDMS, and GFPDMS (0.5 wt% sample); the inset in (a) shows a magnified curve of the PU foam’s response. (c) Compressive stress-strain responses for GF-PDMS nanocomposites for a range of GF concentrations in weight %. (d) Compressive stress-strain curve and change in -ΔR/R0 with compressive strain for a 0.5 wt% GF-PDMS nanocomposite. (e) Same information as in (d) plotted over a shorter strain range; an exponential fitting curve is included to approximate the -ΔR/R0 vs. strain response. (f) Similar information as in (e) but for a GF-PDMS nanocomposite with 0.7 wt% GC. 1435x1442mm (96 x 96 DPI)


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Figure 4. Cyclic strain sensing performance of GF-PDMS nanocomposites (0.7 wt% GC): (a) stress, strain and ΔR/R0 histories for 10 compression cycles with 30%, 20% and 10% strain amplitude; (b) corresponding stress-strain curves for the first and last cycle. (c) stress, strain and ΔR/R0 histories for a cyclic compression test with 60 s holding time at 30% strain and 10 s holding time at zero strain. (d) Stress-strain curves for the 1st and 100th cycle during a 100 cycle fatigue test with 30% strain amplitude and 0.0017 s-1 strain rate. (e) Stress-strain curves for the 1st and 1000th cycle during a 1000 cycle fatigue test with 30% strain amplitude and 0.017 s-1 strain rate. (f) Compressive stress and -ΔR/R0 responses plotted as functions of the compressive strain for three different strain rates; exponential fitting curves are included for the ΔR/R0 data. 1556x1502mm (96 x 96 DPI)



Figure 5. (a) Comparison of stress degradation and Young’s modulus of GF-PDMS nanocomposite and other foam-like nanocomposites. Note that the stress degradation of other materials would be different with different densities or loading directions (anisotropy) and we chose the least value (best performance) as their relaxation value. (b) A demonstration showing the detection of finger tip pressure using GF-PDMS pressure sensor. The change in the resistance (absolute values) are plotted against the time. The background photograph shows the experimental setup. 1381x525mm (96 x 96 DPI)


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Piezoresistive and Mechanical Characteristics of Graphene Foam

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