High piezo-resistive performances of anisotropic composites realized by embedding rGO-based chitosan aerogels into open cell polyurethane foams

Tianliang Zhai ab, Letizia Verdolotti a, Saulius Kacilius c, Pierfrancesco Cerruti d, Gennaro Gentile d, Hesheng Xia *e, Mariamelia Stanzione a, Giovanna Giuliana Buonocore *a and Marino Lavorgna a
aInstitute of Polymers, Composites and Biomaterials, National Research Council, P.le Fermi, 1-80125 Portici, NA, Italy. E-mail: gbuonoco@unina.it
bGuizhou Building Material Quality Supervision Testing Center, Guiyang, 550014, China
cInstitute for the Study of Nanostructured Materials, National Research Council, 00015 Monterotondo Staz., RM, Italy
dInstitute of Polymers, Composites and Biomaterials, National Research Council, Via Campi Flgrei 34, Pozzuoli, NA, Italy
eState Key Laboratory of Polymer Materials and Engineering, Sichuan University, Chengdu, China. E-mail: xiahs@scu.edu.cn

Received 6th January 2019 , Accepted 29th March 2019

First published on 3rd April 2019


Abstract

Anisotropic aerogel-foam composites were developed by embedding a reduced graphene oxide (rGO)/chitosan aerogel directly into an open-cell polyurethane foam through an in situ bidirectional freeze-drying process. The resulting aerogel–foam composites possess both excellent compression-resilience performance and stable piezo-resistive properties due, respectively, to the excellent mechanical properties of polyurethane foams and to the presence of a chitosan-based aerogel loaded with rGO. The latter, indeed, provides outstanding electrical properties due to its conductive and parallel flat lamellar structure. It has been proven that both mechanical and piezo-resistive properties are stable even after 1000 loading/unloading cycles and a reduction of the electrical resistance of about 86% is observed upon the application of a 60% strain. The high sensitivity, long cycling life, and reliable performance over a wide strain range make this unique anisotropic aerogel-foam composite a highly promising candidate for the production of wearable sensors and healthcare monitoring devices.


Introduction

Graphene-based aerogels consisting of 3D porous conductive networks not only possess the common characteristics of organic and inorganic aerogels, such as low density, high specific surface area and high porosity,1 but also exhibit unique thermal, electrical, and adsorptive properties.2 Due to their outstanding properties, these fascinating materials are frequently proposed in several application fields such as energy storage, actuators, superabsorbent materials and sensors.2–6 Among these applications, thanks to their unique 3D conductive networks and extraordinary electrical properties, the graphene-based aerogels have great potential mainly as piezoresistive sensors. However, their mechanical properties are usually poor; indeed, due to their excessive light weight, they are often difficult to handle and generally do not fulfill the requirements for practical applications, such as high elasticity with a fast recovery rate and reasonable durability, as well as high fatigue resistance under high-strain cyclic compression. The achievement of these challenging characteristics would represent a driving force for the diffusion of functional organic or inorganic aerogels.7 In the literature, only a few papers deal with the development of a high-performance graphene-based aerogel. Kim et al.8 prepared a super-elastic graphene-based aerogel by coating a single-walled carbon nanotube-based aerogel with several layers of graphene nanoplatelets. The obtained aerogel exhibited reliable mechanical properties even after about 1 × 106 compressive cycles. Qin et al.9 reported an approach for transforming the mechanically fragile reduced graphene oxide (rGO) aerogel into super flexible 3D architectures by exploiting the concept of composite materials. The rGO/PI composites were prepared by the freeze-casting method followed by thermal annealing. The resulting material exhibited desirable electrical conductivity and excellent flexibility. Gao et al.7 prepared a chitosan-graphene oxide (CS-GO) material with super-elasticity and high fatigue resistance by a bidirectional freeze-drying process. The parallel flat lamellar structure of the obtained CS–GO material was transformed into a wavy multi-arch morphology by subsequent thermal-annealing.

Piezoresistive sensors, which transduce stress/strain into a resistance signal, are widely used in various applications, including sport motion monitoring, electronic skin, rehabilitation, and healthcare monitoring.10 In recent years, piezoresistive sensors based on light weight, flexible, conductive porous materials have been developed by using several approaches including layer-by-layer assembling carbon black coated polyurethane foams,10 synergistic multiwalled carbon nanotubes and graphene coated polyurethane foams,11 thermally induced phase separation porous graphene/thermoplastic polyurethane foams,12 and reduced graphene oxide patterned on porous poly(dimethylsiloxane).13

In this context, we propose for the first time the idea to develop graphene-based porous composites consisting of a graphene-based aerogel produced directly within an elastic polymeric foam. Indeed, due to its excellent mechanical properties, an open cell polyurethane foam (PUF) was chosen as a 3D elastic scaffold to guarantee compression-resilience performance and mechanical durability to the resulting composites. A reduced graphene oxide (rGO)/chitosan (CS) aerogel was prepared directly inside the open cells of the PUF by a simple and effective bidirectional freeze-drying process. It acted as a highly conductive 3D network, providing superior piezoresistive properties. Chitosan, a biodegradable natural biomaterial, was chosen to prepare the rGO-based aerogel since CS macromolecules stabilize the GO suspensions due to their excellent hydrophilic interactions with GO and allow the preparation of highly reliable aerogels.14 Moreover, CS also promotes the thermal reduction of GO at low temperature, presumably due to the direct redox reaction between GO and –NH2 groups along the CS macromolecular chains.15–18 The resulting graphene-based aerogel/PUF composites show an anisotropic structure, consisting of flat lamellae which are well oriented and regularly alternated to form a lamellar porous structure. In order to further improve its piezoresistive behaviour, graphene coating was also used as a coupling layer between the polyurethane foam and the graphene-based aerogel, thus enhancing the overall conductivity of the resulting material. The complex structure of the aerogel/foam composite designed in this work confers to the final material excellent mechanical properties, thus enlarging the application field of such low density functional porous materials, whose main technological limitation is the difficulty to be handled due to their excessive light weight. In addition, the observed high sensitivity, long cycling life, and reliable performance of the developed aerogel/PUF composites make them promising candidates for wearable sensors and healthcare monitoring devices.

Results and discussion

Open cell polyurethane foams have excellent compression-resilience performance, thus being an ideal elastic scaffold to host and protect graphene-based aerogels for the development of complex composite materials with excellent mechanical and piezoresistive properties. In this work, pristine PUFs and PUFs completely coated with a uniform layer of rGO (herein defined as rGO-PUFs) were used as a scaffold to prepare aerogel-based materials. The aerogel/foam composites were prepared by enabling the growth of the GO/CS aerogel directly in the PUFs via a bidirectional freeze-drying method, as schematically illustrated in Fig. 1(a). This approach, firstly proposed by Bai et al.19 and Gao et al.7 as a valuable method to prepare complex anisotropic porous structures, allows tailoring the aerogel morphology with a unique long-range lamellar structure (in Fig. S1 two examples showing the effect of the bi-directional freezing process are reported). After drying by lyophilisation, the GO-based aerogel/foam composites were subjected to a thermal annealing treatment at 200 °C to transform the GO/CS aerogel into the rGO/CS conductive aerogel.
image file: c9nr00157c-f1.tif
Fig. 1 (a) Schematic preparation of the aerogel/polyurethane foam composite. (b) Scheme of the resulting aerogel/PUF composite with the 3D orthogonal coordinates. SEM images showing the aligned lamellar structure of the aerogel/PUF composites observed from surface S(x, 0, z) at low (c) and high (d) magnifications, and the flat wall structure observed from surface S(0, y, z) at low (e) and high (f) magnifications. Scheme of the aligned lamellar structure of aerogel/PUF (g) and aerogel/rGO-PUF (i) composites. High magnification SEM images show the bonding interface between the aerogel lamellae and the polyurethane foams for the aerogel/PUF (h) and aerogel/rGO-PUF (j) composites.

Fig. 2(a–c) show the open cell morphology of the pristine PUF. The cells are somewhat spherical with the diameter in the range of 500–800 μm. The morphology of rGO-PUFs is shown in Fig. 2(d–f). Obviously rGO-PUF shows a dark black color due to the rGO coating layer deposited on the PUF surface. In fact, differently from the smooth internal surface of pristine PUF cells, a rough rGO coating layer can be clearly observed on the cell walls of the rGO-PUF cells as shown respectively in Fig. 2(c) and (f). This rGO layer enables both the enhancement of electrical conductivity and the improvement of the interfacial bonding between the PUF and the graphene-based aerogel (as discussed below). Both the pristine PUF and rGO-PUF have shown excellent mechanical stability in the compression-resilience test (Fig. 2(g and h)); indeed the hysteresis loops obtained from the 10 loading/unloading cycles are almost all overlapped. In addition, the presence of the rGO coating layer shows a significant reinforcing effect of the polyurethane porous foams since the stress at 40% strain for the rGO-PUF (39.8 kPa) is about 1.8 times higher than that of the pristine PUF (22.1 kPa).


image file: c9nr00157c-f2.tif
Fig. 2 Digital photos of the (a) pristine open-cell PUF, (d) rGO coated open-cell PU foam. SEM images of the pristine PUF and rGO-PUF at low (b and e) and high (c and f) magnification. Loading/unloading curves for the compression-resilience test up to 40% strain for (g) pristine PUF and (h) the rGO-PUF.

For the sake of convenience, a 3D coordinate system is adopted to identify the three directions within the anisotropic complex material. The X-axis identifies the freezing line along which the solution has been cooled isochronously, whereas y and z-axes identify the directions along which the freezing of the solution advances over freezing-time. In detail, the direction characterized by an ice-growth advancement at a rate of 1.2 mm min−1 is defined as the z-axis, and the direction characterized by an ice advancement at a rate of 7.5 mm min−1 is defined as the y-axis (i.e. this is the direction perpendicular to the x-axis in the plane where the freezing sample is located). Thus the surface S(x, 0, z) is the surface parallel at x and z-axes which intersects the y-axis in 0, and similarly for the surfaces S(0, y, z) and S(x, y, 0).

The schematic illustration of the resulting aerogel/PUF composite with the indication of the already described 3D coordinates is shown in Fig. 1(b). The parallel lamellar structure of the graphene-based chitosan aerogel within the polyurethane foams can be clearly seen when the composite is observed through the S(x, 0, z) surface (Fig. 1(c and d)), whereas the flat wall structure of the lamellae is observed through the S(0, y, z) surface (Fig. 1(e and f)). For the sake of comparison, the aligned structures through the surface S(x, 0, z) and S(0, y, z) for the simple rGO/CS aerogel (not included in the foam) are shown in Fig. S2.

The observed lamellar structure of the rGO-based aerogel provides unique electrical and piezoresistive properties to the aerogel/PUF composites. It is worth noting that the conductivities of aerogel/PUF composites are lower than those of the rGO/CS aerogels (Table 1) due to the porous structures of the PUF, which allow conduction only through a limited fraction of the surface area corresponding to the effective area of the holes connecting foam cells. More evidence of the lamellar structure of the rGO/CS aerogel within the PUF is given in Fig. S3.

Table 1 Densities and electrical conductivities of the rGO-PUF, aerogel/PUF and aerogel/rGO-PUF samples
  Density (g cm−3) Direction Conductivity (S m−1)
Pristine PUF 5.2 × 10−2 Isotropy
rGO-PUF 5.5 × 10−2 Isotropy 1.82 × 10−2
Aerogel/PUF 6.5 × 10−2 x- 1.18 × 10−5
y- 6.78 × 10−5
z- 9.34 × 10−5
Aerogel/rGO-PUF 6.6 × 10−2 x- 1.42 × 10−1
y- 2.20 × 10−1
z- 2.05 × 10−1
rGO/CS aerogel 8.1 × 10−3 x- 2.29 × 10−4
y- 1.27 × 10−3
z- 1.55 × 10−3


In the z-axis and y-axis directions, the aligned thin graphene-based lamellae ran throughout the whole cross section of the composite foams and provide an effective path for the conduction of electrons. While, in the x-axis direction, the electron transfer is somewhat hindered by the interlayer air-gaps between the lamellae. Thus, the electrical conductivities of either the aerogel/PUF or the aerogel/rGO-PUF composites in z-axis and y-axis directions are higher than that in the x-axis direction (see Table 1). In detail, for the aerogel/PUF composites, the conductivity in the z-axis direction is almost 7.9 times higher than that in the x-axis direction. On the other hand, the conductivity of aerogel/rGO-PUF in the z-axis direction is only 1.4 times higher than that in the x-axis direction and, due to the homogeneity of graphene-based aerogel flat lamellae, its conductivity values measured along the y-axis and z-axis directions are comparable. The electrical conductivity of the aerogel/rGO-PUF in the x-axis direction (1.42 × 10−1 S m−1) is almost four orders of magnitude higher than that of aerogel/PUF composites in the same direction (1.18 × 10−5 S m−1). Finally, the conductivities of aerogel/rGO-PUF composites in the three directions are all higher than those of the rGO-PUF composite. These results confirm the significant role of the rGO coating in improving the electrical conductivity of the aerogel-based materials and the synergistic effect due to the simultaneous presence of the graphene-based aerogel and rGO coating. This last effect is also favored by the excellent adhesion between the rGO coating and the single graphene-based lamellae which constitute the aerogel. Fig. 1(h and j) show the two different bonding interfaces observed in aerogel/PUF and aerogel/rGO-PUF. As for the aerogel/PUF composite, the lamellae grow directly on the smooth surface of the PUF (Fig. 1(h)), while, in the aerogel/rGO-PUF composite, the lamellae grow directly on the rough rGO layer deposited on the PUF surface (Fig. 1(j)) resulting in a highly effective interfacial bonding between the conductive aerogel and the conductive PUF surfaces.

Table 1 also reports the density values of the composite materials. It is shown that the density of rGO-PUF is about 5.7% higher than that of the pristine PUF. According to the density values, the rGO content in the rGO-PUF composite is about 5.5 wt%. The densities of the composites with only the aerogel and both coating + aerogel are about 25% and 27% higher than that of the pristine PUF, respectively. Considering the weight ratio of 5/5 mg mL−1 of the GO and CS in the initial solution, the content of rGO in the final composites should be around 10 wt% for the aerogel/PUF composite and 8.3 wt% for the aerogel/rGO-PUF composite, respectively. By taking into account the weight loss associated with the reduction of GO, it is worth noting that the real content of rGO in the composites is significantly lower after the reduction process.

The electrical results also confirm that the thermal reduction of GO is an effective method to achieve a satisfactory reduction of GO without damaging the porous structure of the pristine GO/CS aerogel.20 CS behaves as a chemical reducing agent for the reduction of GO,15–18 and it allows a significant decrease of the reduction temperature down to 200 °C as compared to the higher temperature needed in the traditional thermal reduction of GO. After the thermal treatment, the conductivities as well as the elastic properties of the resulting aerogel-based composites are satisfactory. Fig. 3(a) shows the electrical resistance variation and the weight loss of the GO-based chitosan aerogel as a function of thermal-annealing time. Initially, the GO/CS composite aerogel is an electrically insulating material. When the annealing temperature increases from room temperature to 200 °C, its resistance decreases to 103 MΩ. Starting from this point, the electrical resistance significantly decreases with thermal-annealing time reaching the value of 33.4 kΩ in the first 12 min at 200 °C, and 11.9 kΩ after 75 min. In addition, the weight loss of the rGO/CS aerogel after 12 min of thermal-annealing at 200 °C is only 7.5 wt%, and it is mainly ascribed to the removal of water adsorbed on hydrophilic groups present in the chitosan/GO aerogel.


image file: c9nr00157c-f3.tif
Fig. 3 (a) Electrical resistance and weight loss curves as a function of thermal annealing time at 200 °C. Deconvoluted C 1s XPS spectra of (b) GO, (c) rGO powder annealed at 200 °C for 12 min, (d) GO/CS aerogel, and (e) rGO/CS aerogel annealed at 200 °C for 12 min.

The effectiveness of the thermal treatment adopted for the reduction of GO in the presence of chitosan, in terms of time and temperature, was assessed by FTIR analysis and compared with the thermal reduction of GO without chitosan, as shown in Fig. S4(a–c). Results confirm that the oxygen-based groups present on the surface and edges of GO platelets are completely removed during the annealing process and that chitosan has a beneficial effect on enhancing the extent of thermal reduction as compared to the thermal reduction of pristine GO.

A further confirmation of the chitosan-assisted thermal reduction of GO is provided by Raman results (Fig. S5(a–c)) and by XPS analysis. Fig. 3(b–e) show the C 1s XPS spectra of the samples before and after thermal annealing. The main peaks were deconvoluted by considering the following band-assignments: 284.6 eV for non-oxygenated ring C (C–C/C[double bond, length as m-dash]C), 286.8 eV O–C– and C[double bond, length as m-dash]O bonds (alcohol and/or ether groups) and 289.4 eV for OH–C[double bond, length as m-dash]O carboxylic bonds.22,26 The C 1s XPS spectra of rGO and the rGO/CS aerogel also exhibit these oxygen functional groups. However, their peak intensities are much weaker than those in GO and the GO/CS aerogel, respectively. The C 1s/O 1s ratio of rGO powder increased from 2.7 to 7.9 after thermal annealing (i.e from GO to rGO). The effective reduction of GO in the chitosan-based aerogel is also confirmed by the C 1s/O 1s ratio for the rGO/CS aerogel which increases from 2.0 to 2.7 with the thermal annealing (i.e. from the GO/CS aerogel to the rGO/CS aerogel), while for the CS aerogel it increased slightly from 2.0 to 2.2 (Fig. S6(a and b)).

FTIR, Raman and XPS results confirm that 12 minutes of thermal annealing at 200 °C are sufficient for the reduction of GO and the formation of an effective rGO network which guarantees a significant electrical conductivity.

All the graphene-based aerogel/PUF composites were characterized in terms of piezoresistive properties in order to envisage the potential of the use of an anisotropic structure for application in the field of sensors. It is well known that as far as the piezoresistive behavior is concerned, the variation of electrical resistance with mechanical load/deformation is often caused by the competition between the breakage of existing conductive paths and the formation of additional conductive paths.21,22 Based on the results present in the literature, two opposite piezoresistive behaviors, namely “positive” and “negative” effects, can be observed and rationalized. When the breakage of conductive paths is more significant than the formation of new conductive paths, the electrical resistance increases with increasing compression strain. This phenomenon is defined as the “positive” piezoresistive effect. Alternatively, the “negative” piezoresistive behavior occurs when the resistance decreases by increasing the compression strain as a consequence of the formation of more effective conductive paths. The “positive” effect is more common for the conductive polymer-based nanocomposites, because during compressive deformation, the conductive fillers are often removed from the positions which allow the occurrence of an effective conductive network. This results in a dramatic reduction of the number of the conductive paths with the consequent increment of the electrical resistance. Differently from the common nanocomposites, the graphene-based aerogel/PUF composites prepared in this work show an anisotropic and “negative” piezoresistive effect, whose extent depends both on the composite typology (presence or absence of the rGO coating on the PUF) and the direction of the compressive strain. In fact, when the compression load is not applied, the movement of electrons in the x-axis direction is hindered by the interlayer air-gap between lamellae, whereas, during the compression, the junction gap between the adjacent lamellae decreases with the increasing load/strain, as schematically illustrated in Fig. 4(a) and (i). By reducing the gap distance, new junction points are generated along the x-axis direction, thus resulting in the formation of a more effective conductive path and an increase of the electrical conductivity. As a consequence, the electrical resistance of the samples dramatically decreases with the increase of the strain. When the load is gradually removed, the distance between the adjacent lamellae recovers to its original value, which brings about the recovery of the initial resistance for the graphene-based aerogel/PUF composite sample. The normalized electrical resistance variation, ΔR/R0 (where ΔR/R0 = (RiR0)/R0, and R0 and Ri denote the resistance of the sample without and with applied load, respectively) under repeated loading and unloading cycles at 20%, 40%, and 60% strain is shown in Fig. 4(c), while the absolute values of electrical resistance variation are shown in Table 2. The extent of the resistance variation increases with the increase of the compression strain. The piezoresistive test was also carried out under 20% strain with 0%, 20%, and 40% of pre-strain to evaluate the piezoresistive behavior in different strain regimes. Fig. 4(d) shows that the electrical resistance variation with 0% pre-strain is much smaller than that observed when 20% and 40% pre-strain is applied, being 26.4%, 57.0%, and 73.5%, respectively. These results indicate that more conductive paths form at higher strain and, as a consequence, the piezoresistive sensitivity is significantly enhanced.


image file: c9nr00157c-f4.tif
Fig. 4 Schematic illustration of the aerogel/PUF composites compressed along the (a) x-axis and (b) z-axis direction. Electrical resistance variations recorded in four loading/unloading cycles compressed in the x-axis direction (c) under 20%, 40%, and 60% strain without pre-strain, and (d) 20% strain with 0%, 20%, and 40% pre-strain. Electrical resistance variations recorded in four loading/unloading compression cycles in the z-axis direction (e) under 20%, 40%, and 60% strain without pre-strain, and (f) 20% strain with 0%, 20%, and 40% pre-strain. Stability tests under 20% strain with 20% pre-strain for 1000 loading/unloading cycles compressed in (g) x-axis direction and (h) z-axis direction. Schematic illustration and SEM images showing the changes of aerogel structures when the aerogel/PUF composite is compressed in (i) x-axis direction and (j) z-axis direction (movies available as the ESI).
Table 2 Absolute values of the electrical resistance variations of the aerogel/PUF composite along the x-axis and z-axis under different conditions of compressive strain
  Absolute electrical resistance variations, %
20% strain 40% strain 60% strain 20% strain
0% pre-strain 20% pre-strain 40% pre-strain
x-axis 26.4 60.1 85.9 26.4 57.0 73.5
z-axis 12.3 40.9 78.8 12.3 34.8 78.0


Differently from the x-axis direction, when the aerogel/PUF composite is compressed in the z-axis direction, the aligned lamellae tend to buckle, as shown in Fig. 4(b) and (j). However, the contribution to ΔR/R0 due to the formation of new junction points by buckling the graphene-based lamellae (Fig. 4(e and f)) is less significant than the contribution of new junctions by compacting the lamellae in the x-axis direction. In fact, as reported in Table 2, the electrical resistance variation obtained in the z-axis direction under 40% strain (about 40.9%) is smaller than that obtained in the x-axis direction at the same strain value (60.1%). Furthermore, the resistance variation obtained in the z-axis direction under 20% strain with 20% pre-strain (34.8%) is smaller than that obtained in the x-axis direction for the same strain regime (57.8%). The compactness and buckling of the lamellae during the cyclic compression test have been filmed by real-time SEM observation in both the x-axis and z-axis and the movies are provided in the ESI as Movie 1 and Movie 2, respectively. Selected SEM images captured from the real-time movies are shown in Fig. 4(i and j).

The dimensional stability of the aerogel/PUF composites compressed along the x-axis and z-axis directions was investigated by subjecting the samples to more than 1000 loading/unloading cycles with 20% strain and 20% pre-strain. The values of electrical resistance variation are very stable during 1000 cycles, thus implying a long working-life and a significant reliability of the measured values (Fig. 4(g and h)).

It has been proven that the electrical conductivity of aerogel/rGO-PUF composites depends significantly on the contribution of the conductive rGO layer. This layer plays a key role also in the piezoresistive properties. Fig. 5(a–c) show the electrical resistance variations for the isotropic rGO-PUF material and the aerogel/rGO-PUF composites subjected to loading/unloading compression cycles. Interestingly, both “positive” and “negative” piezoresistive effects are observed when the rGO-PUF material and the aerogel/rGO-PUF composite are compressed under 20% strain and 0% pre-strain. For a more comprehensive understanding, three complete loading/unloading cycles are re-plotted in Fig. S7(a). The electrical resistance increases with the increase of the strain from 0% to about 10%, thus realizing a “positive” piezoresistive effect. Then, when the strain further increases from 10% to 20%, a “negative” piezoresistive effect is observed, i.e. the electrical resistance decreases with the increase of the strain. This phenomenon has not been observed for the aerogel/PUF composite; thus, it has been tentatively ascribed to the presence of the rGO coating layer on the PUF surfaces. Likely from 0% to 10% strain, the rGO coating layer is temporarily deactivated by strain and this leads to an increment in the resistance. When the external load is removed, the conductive paths are able to restore their initial state. However, when the strain is higher than 20%, only the “negative” piezoresistive effect is observed. Therefore, as it is possible to observe in Fig. 5(a–c), the electrical resistance variation of the rGO-PUF and aerogel/rGO-PUF composites under 20% strain with 20% and 40% pre-strain changes monotonically with the pre-strain. As shown in Table 3, the aerogel/PUF composites compressed respectively along x-axis and z-axis directions with 20% strain and 40% pre-strain exhibit a reduction of the electrical resistance variation of about 73.5% and 78.0%, whereas for the aerogel/rGO-PUF composites this reduction is only about 46.5% and 50.1%. At 40% strain, the reduction of electrical resistance for rGO-PUF and aerogel/rGO-PUF composites is much smaller than that for aerogel/PUF composites (Fig. S7(b)).


image file: c9nr00157c-f5.tif
Fig. 5 Electrical resistance variations for loading/unloading cycles of (a) rGO-PUF composite, and aerogel/rGO-PUF composite compressed along (b) x-axis and (c) z-axis directions under 20% strain with 0%, 20%, and 40% pre-strain. Stability tests under 20% strain with 20% pre-strain for 20 cycles of (d) rGO-PUF composite, and aerogel/rGO-PUF composite compressed along (e) x-axis direction and (f) z-axis direction.
Table 3 Absolute values of the electrical resistance variations of aerogel/PUF composite along x-axis and z-axis under different conditions of strain
  Absolute electrical resistance variations, %
20% strain
0% pre-strain 20% pre-strain 40% pre-strain
a Isotropic samples.
rGO-PUFa 2.4 22.9 41.2
x-axis of aerogel/rGO-PUF 0.9 27.4 46.5
z-axis of aerogel/rGO-PUF 0.3 15.5 50.1


As obtained for the aerogel/PUF composite, also the rGO/PUF and aerogel/rGO-PUF composites exhibit excellent stability in terms of reliability of the electrical resistance variation over loading/unloading cycles. As shown in Fig. 5(d–f), a very constant electrical resistance variation is obtained when the composites are compressed for several cycles under 20% strain with 20% pre-strain. In this experiment, the change of the electrical resistance for the aerogel/rGO-PUF composite in the x-axis direction (27.4%) is higher than either the values in the z-axis direction (16.5%) or in the x-axis direction (22.6%) for the rGO-PUF composite. This result confirms that also the aerogel/rGO-PUF composite has an anisotropic piezoresistive behavior and the electrical resistance variations along the x-axis and z-axis direction are comparable between them and also similar to what is observed for the isotropic system rGO-PUF.

In order to investigate the piezoresistive behaviors of the aerogel/PUF and aerogel/rGO-PUF composites under high compressive strain, the resistance variations under 60% strain and also 40% strain with 20% pre-strain in x-axis and z-axis directions were also measured. The results are shown in Fig. S8 and S9, respectively. The electrical resistance variation under compression strain as high as 60% became similar and did not depend on the x-axis and z-axis directions. Under these conditions likely all the cells of foam are completely compressed and the anisotropic character of the aerogel is lost. However, it is worth noting that the absolute value of the electrical resistance variation for the aerogel/PUF composites is higher than that of the aerogel/rGO-PUF composites. This confirms that the aerogel with an aligned lamellar structure has a better piezoresistive sensitivity than that exhibited by the system with a rGO layer, not only at low strain but also at high strain.

The corresponding compression stress–strain curves recorded during the piezoresistive tests for the aerogel/PUF, aerogel/rGO-PUF, and rGO-PUF composites under loading/unloading compression cycles are shown in Fig. S10 and S11. In general, the compressive strength of the several composites in the z-axis direction is higher than that in the x-axis direction. This is likely ascribed to the mechanical resistance of the lamellae to the buckling. Furthermore, the compressive strength of aerogel/rGO-PUF composites is higher than that of the rGO-PUF composites.

As shown in this paper, the electrical resistance variation is a function of the compression strain. Thus, the sensitivity of the composites can be assessed by the Gauge factor, which is defined as G = (ΔR/R0)/ε, where ΔR is the relative electrical resistance variation, R0 is the initial resistance, and ε is the compressive strain. The resistance variation curves for rGO-PUF, aerogel/PUF, and aerogel/rGO-PUF composites under 20% compressive strain with 20%, and 40% pre-strain are plotted as a function of compressive strain in Fig. 6(a and b). The calculated Gauge factors are indicated for each curve. The Gauge factors of aerogel/PUF in any directions are always higher than those of rGO-PUF and aerogel/rGO-PUF composites. This indicates that the composite with the graphene-based aerogel with an aligned lamellar structure has higher sensitivity than the system with the rGO coating layer. When the samples are compressed under 20% with 20% pre-strain (Fig. 6a), the Gauge factors along the x-axis direction are higher than that along the z-axis direction, for both aerogel/PUF and aerogel/rGO-PUF composites. In fact, the highest Gauge factor of the aerogel/PUF composite in the x-axis direction (−2.83) is two times higher than that of the aerogel/rGO-PUF composites (−1.39). Since the aerogel/rGO-PUF composite is highly conductive in the z-axis direction, the Gauge factor is only −0.53 for compression in this direction. This highlights that the z-axis direction is the best direction for the electrical conduction. However, when 40% pre-strain is applied (Fig. 6b), the difference of the Gauge factors in different directions is not obvious. Especially for the aerogel/rGO-PUF composites, the normalized resistance-strain curves almost overlap with that of rGO-PUF and the Gauge factors are in the range from −1.8 to −2.1. This is because the highly conductive rGO coating layer dominates the electrical and piezoresistive behavior at high compressive strain. On the other side, the Gauge factors of the aerogel/PUF composite along x-axis and z-axis directions are −4.10 and −4.62, respectively. Compared to the gauge factor values of conventional CNT and graphene based materials reported in the literature (Table 4), the Gauge factors of the aerogel/foam composites presented in this work are generally higher and thus very promising to substantiate the potential use of graphene-based aerogel–polyurethane foam composites for piezoresistive sensors.


image file: c9nr00157c-f6.tif
Fig. 6 Electrical resistance variation (ΔR/R0) of rGO-PUF, aerogel/PUF and aerogel/rGO-PUF composites in the x-axis and z-axis directions as a function of compressive strain under 20% strain (a) with 20% pre-strain, and (b) with 40% pre-strain.
Table 4 Gauge factors of porous composites from recent papers in the literature
Composition Piezoresistive strain range (%) Extreme ΔR/R0a (%) Gauge factor Ref.
a ΔR/R0 = (RiR0)/R0, where R0 and Ri denote the resistance without and with applied pressure.
rGO based aerogel/PUF 0–60 −85.9 −4.62 This work
CNTs/PUF 0–40 −20 −0.58 Zhai et al.23 (our previous work)
Carbon black/PUF 0–60 −20 to −30 −3.1 Wu et al.10
(CNT + graphene)/PUF 0–75 80–90 1.75 Ma et al.11
Graphene/TPU foam 0–90 450–500 12.24 Liu et al.12
Graphene/PUF 0–90 −90 to −100 Wu et al.24
Graphene foam 0–60 70 to 80 1.3 Kuang et al.25
rGO/polyimide foam 0–50 −49.3 Qin et al.9
CNT sponges 0–60 −20 −0.24 Gui et al.26
CNT/Ag sponges 0–40 −40 to −60 −1.4 Zhao et al.27
Graphene monoliths 0–30 −15 to −20 −0.58 Li et al.28
Graphene/CNT aerogel 0–60 −40 −0.5 Kim et al.8
CNF aerogel 0–80 −70 −1.0 Wu et al.29


Conclusions

A novel anisotropic structure consisting of a graphene-based aerogel filled in an open cell polyurethane foam was, for the first time, designed and prepared in this work. In particular, a graphene-based chitosan aerogel consisting of a long-range parallel lamellar structure was prepared directly in the open cell polyurethane foam via the in situ bidirectional freeze-drying process. The resulting material exhibits excellent electrical and piezoresistive properties as well as low density. The graphene-based aerogel systems show an anisotropic behavior in terms of electrical conductivity which is significantly enhanced in the two directions involving the graphene-based lamellae. In addition, it has been shown that a good interfacial bonding between the aerogel and PUF is obtained when the polyurethane foam is initially coated with rGO. Although this system exhibits the highest electrical conductivity, its piezoresistivity sensitivity is lower than that of the aerogel/PUF composite not coated with rGO, regardless of the compression direction. Alongside high sensitivity, the dimensional stability of these anisotropic systems is very high and provides a repeatable and reproducible piezoresistive signal over 1000 loading/unloading cycles. The high sensitivity, long cycling life, and reliable performance over a wide strain range of the aerogel/PUF composite make it a promising candidate for strain sensors, wearable textiles and healthcare applications.

Experimental

Materials

Flake graphite (∼75 μm) was purchased from Qingdao Tianhe Graphite Co. Ltd (China). Hydrochloric acid (HCl) and concentrated sulfuric acid (H2SO4), which were all analytical-grade, were purchased from Sichuan Xilong Chemical Co. Ltd (China). Potassium permanganate (KMnO4) was purchased from Chengdu Kelong Chemical Reagent Company (China). Medium molecular weight chitosan (448877 Aldrich, with a deacetylation degree greater than 75–85%), ascorbic acid, acetic acid (ACS reagent, ≥99.7%) and silver paste were purchased from Sigma-Aldrich (Italy).

Polyurethane foams were produced by using methylene diphenyl diisocyanate (MDI), Specflex NE 134 (index of NCO ∼ 0.8) and synthetic polyol, Specflex NF 660 (OH value: 65.3–75) kindly provided by Dow chemicals, Italy, and Bio-based polyol, FF1 BiosucciniumTM, (hydroxyl value equal to 61.5 mgKOH g−1) kindly provided by Reverdia, Netherlands.

Distilled water, CH3COOK, PM40 and L6164 (kindly provided by Momentive, Italy) were used as a blowing agent, catalyst and surfactant, respectively.

Preparation of GO and GO/CS solution

The preparation of graphene oxide was described in a previous study of some of the authors.30 CS solution with a concentration of 10 mg mL−1 was prepared by dissolving CS powder (5 g) in 500 mL of 1 v/v% acetic acid solution and stirred for 10 h at room temperature. The obtained CS solution was stored at room temperature without any further treatment before utilization. GO powder (1 g) was dispersed in deionized water (100 mL) and sonicated in a bath-sonicator for 3 h to obtain a uniform GO suspension (10 mg mL−1). After sonication, the GO suspension (40 mL) and CS solution (40 mL) were mixed in a 100 mL flask and sonicated for 30 min with a probe-sonicator. The obtained uniformly dispersed GO/CS suspension was stored and then used for preparing the graphene-based aerogel.

Preparation of PUFs

Open cell polyurethane foams were prepared by adding MDI (38.6 g) to a mixture of polyols (i.e. bio-based polyol/synthetic polyol, 10 g/40 g), H2O, CH3COOK (0.1 g), PM40 (0.1 g) and L6164 (0.1 g). After stirring (for 20 s), the mixture was quickly poured into a mold where it foamed due to the production of carbon dioxide via the reaction of water with isocyanate groups. The resulting polyurethane foams were post-cured in an oven at 75 °C for 1 h. Then, the temperature of the oven was increased to 120 °C for further curing for 3 h.

Preparation of rGO-coated PUFs

The reduced graphene oxide-coated PUF foams (rGO-PUFs) were prepared by in situ thermal reduction of graphene oxide nanoplatelets deposited onto the surfaces of polyurethane foams. A centrifuge tube (50 mL) containing 4 pieces of PUF cubes (15 × 15 × 15 mm3) was placed in a vacuum desiccator. The desiccator was well sealed and vacuumed to 80 kPa for 30 min. Afterward, under negative pressure conditions, the GO/ascorbic acid premixed solution (the weight ratio of GO to ascorbic acid is 1/40 mg mg−1 in 1 mL) was allowed to flow into the centrifuge tube through a rubber pipe linked to an external liquid reservoir. When the centrifuge tube was fully filled, the system was kept under vacuum for about 30 min. After degassing, the centrifuge tube containing PUF samples and GO/ascorbic acid solution was left into an oven at 90 °C for 2 h to promote the in situ reduction of GO and the deposition of rGO on the PUF surfaces. The resulting rGO-PUF samples were taken out from the solution and washed with ethanol and deionized water several times. The obtained rGO-PUF samples were oven-dried at 90 °C for additional 3 h.

Preparation of graphene-based chitosan aerogel/PUF composites by bidirectional freeze-drying

Centrifuge tubes containing four PUF cubes (15 × 15 × 15 mm3) were placed in a vacuum desiccator which was well sealed and vacuumed to 80 kPa below atmospheric pressure for about 30 min. Then GO/CS solution (5/5 mg mL−1) was allowed to flow into the centrifuge tube, as previously described. When the centrifuge tube was fully filled, the system was kept under vacuum for 30 min. Afterwards the centrifuge tube was kept in a bath-sonicator for 30 min. A silicone rubber mold, with a cubic empty space to locate the samples, was placed on the top surface of a steel plate and the PUF cubes, completely filled with GO/CS solution, were carefully located into the silicone rubber space mold. A suitable amount of GO/CS solution was added into the silicone rubber mold to ensure that the PUF cubes were totally immersed. One end of the steel plate was immersed in liquid nitrogen to induce a temperature gradient on the plate surface. The water ice crystals grew along both the horizontal direction (y-axis, freezing rate 7.5 mm min−1) and the vertical direction (z-axis, freezing rate 1.2 mm min−1) to form parallel ice columns with long-range alignment (see Fig. 1(a) and comments about the preparation approach and the morphology of the samples). The freezing rates were calculated based on the dimension of the cubic space within the silicone rubber mold and the time necessary to freeze the cubic samples in each direction, which was measured during the freezing process. When the samples were completely frozen, they were removed by using the mold and subjected to the lyophilization process to remove the liquid phase. Afterward, the obtained aerogel/PUF composites were thermally treated in a vacuum oven at 80 kPa below atmospheric pressure firstly at 90 °C for 3 h for removing the residual acetic acid and finally at 200 °C for 12 min to promote the thermal reduction of the GO. Graphene-based aerogel/rGO-PUF samples were produced with a similar procedure by starting from polyurethane foams coated with reduced graphene oxide (rGO-PUF).

Characterization techniques

The morphology of the graphene-based aerogels and composites was investigated with a Leica model S440 (Leica Microsystems GMbH, Germany) scanning electron microscope (SEM) operating at 20 kV. Small specimens were carefully cut by the razor blade from the middle of the prepared cubic samples and gold sputtered (Emscope SC500, UK) before microscope observation.

FTIR spectra were recorded at room temperature by using a Nicolet FT-IR spectrometer (Thermo Scientific, Italy) in Attenuated Total Reflectance (ATR) mode in the spectral range of 400–4000 cm−1 with 4 cm−1 resolution and 64 scans.

X-Ray photoemission spectroscopy (XPS) analyses were carried out by using an electronic spectrometer Escalab MkII (VG Scientific, East Grinstead, UK) equipped with an Al Kα source. The photoemission spectra were collected at 40 eV pass energy in selected-area mode A3 × 10 (about 3 mm in diameter). All experimental data were processed by using the software Avantage v.5 (Thermo Fisher Scientific). More experimental details have been published elsewhere.31

Thermogravimetric analysis (TGA) was carried out by using a TGAQ5000, TA Instruments. The samples were heated in nitrogen flow up to 200 °C with a heating rate of 10 °C min−1 and then left under isothermal conditions at 200 °C for 2 h.

Raman spectra were collected at room temperature by using a micro-Raman spectrometer Renishaw inVia with a 514 nm excitation wavelength (laser power 30 mmW) in the range of 100–3000 cm−1. In order to separate the D and G bands, a curve resolving algorithm was applied by reducing the number of adjustable parameters and enhancing the fitting of the experimental profile.

A two-electrode method was used to measure the volume conductivity of the samples with a picoammeter (Agilent 34401A). Silver paste was deposited on the cross-sections of the specimens to ensure a good contact between the specimen and electrodes. The measured volume resistivity, Rv, was converted to volume conductivity, σ, according to ASTM D4496 and D257 by using eqn (1):

 
σ = dRvA(1)
where Rv is the measured resistance (Ω), A is the effective area of the measuring electrode (m2) and d is the specimen thickness (m).

The mechanical properties of material specimens (40 × 40 × 10 mm3) were measured by using an Instron 5564 universal testing instrument. The specimens were compressed along the shortest direction by applying a strain rate of 3 mm min−1.

The deformation mechanism of the foams under compression was evaluated by means of compression tests coupled to SEM analysis, using a Deben microtensile/compression stage equipped with a 20 N cell load installed on a SEM FEI Quanta 200 FEG. The compressive deformation tests were performed on small cubic specimens (about 7 × 7 × 7 mm3) within the SEM chamber applying a strain rate of 0.5 mm min−1 and imaging the samples, at 20 or 30 kV, in high vacuum mode, using an Everhart–Thornley detector (i.e. collection of images and video of compression loading/unloading cycles).

The measurement of piezoresistive properties was realized by using the set-up shown in Fig. S12. In detail, two copper meshes 20 × 50 mm2 were placed with the silver paste on both sides of the specimens (15 × 15 × 15 mm3) and connected with the picoammeter (Agilent 34401A). The assembled sample was compressed by using an Instron testing instrument and electrical and mechanical properties were simultaneously collected by taking into account several experimental conditions. In particular, the specimens were strained at 40% of their initial height, at 60%, at 20% with 20% pre-strain, and at 20% with 40% pre-strain, respectively. The electrical resistance data were recorded accurately with an interval time of 2 seconds.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the International Science & Technology Cooperation Program of China (2015DFA51110), by the Joint Laboratory for Graphene based Multifunctional Polymer Nanocomposites funded by CNR (Joint Lab Call 2015–2018) and by the Grande Rilevanza project, GRAPE-MAT project supported by MAECI (Italy). The authors thank Dr A. T. Silvestri and Mrs A. Aldi for their support in the experimental part and Dr E. Migliore for the graphical assistance.

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Footnote

Electronic supplementary information (ESI) available. See DOI: 10.1039/c9nr00157c

This journal is © The Royal Society of Chemistry 2019