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Graphene-elastomer composites with segregated nanostructured network for liquid and strain sensing application Yong Lin, Xuchu Dong, Shuqi Liu, Song Chen, Yong Wei, and Lan Liu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b08587 • Publication Date (Web): 23 Aug 2016 Downloaded from http://pubs.acs.org on August 27, 2016

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ACS Applied Materials & Interfaces

Graphene-elastomer composites with segregated nanostructured network for liquid and strain sensing application Yong Lin, Xuchu Dong, Shuqi Liu, Song Chen, Yong Wei, Lan Liu* College of Materials Science and Engineering, Key Lab of Guangdong Province for High Property and Functional Macromolecular Materials, South China University of Technology, Guangzhou 510640, P. R. China * Corresponding author. E-mail: [email protected] Tel: +86 20-87114857.

ABSTRACT One of critical issues for the fabrication of desirable sensing materials has focused on the construction of an effective continuous network with a low percolation threshold. Herein, graphene-based elastomer composites with segregated nanostructured graphene network were prepared by a novel and effective ice-templating strategy. The segregated graphene network bestowed on the natural rubber (NR) composites a ultralow electrical percolation threshold (0.4 vol%), 8-fold lower than that of the NR/graphene composites with homogeneous dispersion morphology (3.6 vol%). The resulting composites containing 0.63 vol% graphene exhibited high liquid sensing responsivity (6700), low response time (114 s) and good reproducibility. The unique segregated structure also provides this graphene-based elastomer (containing 0.42 vol% graphene) with exceptionally high stretchability, sensitivity (gauge factor ~139) and good reproducibility (~400 cycles) of up to 60% strain under cyclic tests. The fascinating performances highlight the potential applications of graphene-elastomer composites with an effective segregated network as multifunctional sensing materials.

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KEYWORDS: elastomer, graphene, composites, segregated network, liquid sensing, strain sensing 1. INTRODUCTION Electrically conductive polymer composites have generated considerable attention over the recent decades because of their good processability, highly tunable properties and structures, low energy consumption and processing cost1-4. Owing to their exceptional response to various external stimuli such as strain5,6, organic solvents7, pressure8,9, or temperature10, conductive polymer composites have been exploited as promising sensing materials. It is well known that the desirable sensing materials not simply have a desired conductivity, but also synchronously possess high sensitivity and good reproducibility. Nowadays, it remains a big challenge to balance these performances in the conductive composites. Very recently, the construction of a continuous conductive network has been one of most efficient strategies to achieve a good balance of electrical behavior and sensing performance. For example, Zhang et al.11 developed latex assembly technology to prepare a 3D hierarchical conductive natural rubber-carbon black (CB) composites with an electrical percolation threshold of 1.65 vol%, high liquid sensing responsivity (30~4427), and good reproducibility. And Dang et al.12 developed the conductive graphene/epoxy composite with a percolation threshold, i.e. ~1.31 vol%, exhibiting a gauge factor of ~45. However, relatively high conductive filler content (i.e. high percolation threshold) is employed to construct an effective continuous conductive network, unfortunately leading to severe damage to the mechanical performance, a reduction of sensing responsivity to


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the external stimuli, and processing difficulty. It is therefore a current significant challenge to prepare highly conductive composites with ultralow electrical percolation threshold for enhanced sensitivity of sensing materials without sacrificing any electrical conductivity and mechanical performance. To date, a great deal of work has shown the enormous potential of graphene (GE) in the conductive polymer composites1,3,4,13 because of its excellent electrical and mechanical performance, and high specific surface area as well as ease of scalable synthesis

from

inexpensive

graphite14,15.

Recent

works

have

successfully

demonstrated the construction of a continuous conductive GE network in the elastomer

composites

via

several

approaches,

such

as

self-assembly16,17,

templated-assisted assembly18, pre-construction19,20. Despite these significant attempts, much attention has not been paid to design and construct a conductive GE network in an elastomer matrix so far. More recently, ice-templating, known as freeze drying, has been recognized as one of the most environmental-friendly method to fabricate the porous interconnected aerogels21,22. Following early studies have demonstrated that phase separation resulting from the frozen process can contribute to the rejection of filler particles from the growing ice, then accumulation between the growing ice crystals, and thereby forming a continuous 3D network22,23. Ice-templating strategy has been feasibly utilized to fabricate the porous CNTs-based24, GE-based25 and clay-based26 materials. Of particular interest is that the obtained porous structure resulted from the ice-templating method potentially allows the conductive fillers to construct segregated



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network in the filled composites with less filler content. We thus believe that constructing segregated network in polymer composites by ice-templating strategy will be a promising and effective route to fabricate the high performance sensing materials. In this work, we demonstrate the fabrication of conductive elastomer composites with segregated nanostructured graphene network by the combination of ice-templated strategy and latex flocculation method. Expectedly, the resultant segregated NR/GE composites exhibited desirable electrical conductivity with a ultra-low electrical percolation threshold (0.4 vol%), high liquid sensing responsivity (highly up to ~56700), and good reproducibility when exposed to organic solvents, performances of which exhibit very competitive with the previously reported sensing materials. Importantly, because of the segregated nanostructured GE network, the fabricated strain sensors exhibited high stretchability (up to ~110%), good sensitivity (gauge factor, ~139) and reproducibility (~400 stretching/releasing cyclic tests) even when strained up to 60%. These excellent performances make the segregated NR/GE composites potential candidates for liquid and strain sensing materials. As far as I know there is no prior report on the combination of ice-templating process and latex flocculation method for constructing segregated GE network in the high performance sensing materials. 2. EXPERIMENTAL SECTION 2.1 Materials. NR latex (NRL) with a total solid content of 60 wt% was supplied by Maoming Shuguang Rubber Farm. Octyl phenol 10 (OP-10) was supplied by Sinopharm Chemical Reagent


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Co.

Ltd.

Rubber

additives

such

as

N-cyclo-hexylbenzothiazole-2-sulphenamide

zinc (CZ),

oxide

(ZnO),

stearic

2,2’-dibenzothiazole

acid

(SA),

disulfide

(DM),

2-Mercaptobenzimidazole (MB) and sulfur (S) were obtained from Guangzhou Longsun technology Co., Ltd. All the rubber ingredients were industrial grade and were used as received. 2.2 Preparation of the vulcanized aqueous suspensions. The formulation of the NR composite is listed in Table S1. All the rubber ingredients with a content and 300 mL water were subjected to the ball milling, and the stable and homogeneous aqueous dispersions were finally obtained. Herein, the ball milling conditions were identified as 2 h of balling time and 400 rpm/min of balling intensity. The ball milling process was carried out in QM-3SP4 Planetary Ball Mill (Nanjing NanDa Instrument Plant, China). 2.3 Preparation of NR composites. Graphene oxide (GO) suspensisons were firstly synthesized according to our previous work5. Then, NRL and vulcanized aqueous suspension (the crosslinking agent sulphur and other rubber additives) was blended with the GO suspensions under vigorous stirring for 2 h. Once thoroughly mixed, the mixtures were poured into the cylindrical polystyrene vials and immediately frozen in a cold trap (-80 °C). The frozen samples were transferred to a LGJ-10 Vacuum Freezing Dryer (Beijing Songyuan Huaxing Technology Develop Co. Ltd, China) for 2-3 days to completely remove the ice, and then NR-GO aerogels were ultimately obtained. The obtained aerogels were subsequently subjected to reduction at 100 °C in the hydrazine hydrate steam for 3h to obtain the NR-GE aerogels. Then, appropriate amount of the mixtures of NRL and vulcanized aqueous suspension was added to fill the pores of NR-GE aerogels. After that, the wet compounds were vacuum dried in an oven at 45 °C for 12 h. Finally, the dried solid compounds were compress molded at 143 °C for cure time



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(T90) under a pressure of 10 MPa. The fabricated composites with segregated network were marked as S-NRGE-x. For comparison, NR/GE composites with homogeneous dispersion morphology were prepared following the conventional latex compounding technology, coded as H-NRGE-x, where x represents the volume fraction of GE in the rubber. In the experiments, GE content was controlled to be 0, 0.21, 0.42, 0.63, 0.84, 1.66 and 3.27 vol% in the composites. 2.4 Characterization. The transmission electron microscopy (TEM) and scanning electron microscopy (SEM) images of the composites were collected on a JEOL2100 microscope and a Nova NANOSEM 430, respectively. Nitrogen permeability of NR composites was tested in a gas permeability tester (VAC-V2, Labthink Instruments). All the samples were circular-shaped specimens with 50 mm diameter and 0.5 mm thickness. The dependence of the elastic modulus (G’) on the strains of the uncured NR compounds was measured on a rubber processibility analyzer (Alpha, RPA 2000). Tensile and tear test experiments were performed using an UCAN UT-2060 instrument with a cross head speed of 500 mm/min, following ASTM D 412 and ASTM D 624, respectively. The electrical conductivity of all samples was collected by a two-point measurement with a resistance meter according to our previous work20. A rectangle strip samples were used for measurements, and the dimension (length, width, thickness) of the specimens was 30 mm×6 mm ×0.5 mm. The stretching-releasing of the samples and cycling tests were conducted at room temperature using a CK-50HB digital force gauge (Fuzhou Aipu Instruments Co., Ltd) with a adjustable strain rate, and then the resistance changes of the samples were synchronously recorded using a TEGAM 1740 micro ohmmeter.


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For liquid sensing measurements, a rectangle strip samples (30 mm×6 mm×0.5 mm) were used for all the composites with different GE content. Specifically, for the immersing process, the tested samples were quickly immersed into an organic solvent at room temperature, and the corresponding resistance changes were real-time recorded using a TEGAM 1740 micro ohmmeter. The copper sheets were connected tightly on both ends of samples with clamps. For the drying process, the tested samples were quickly lifted up and then dried in hot air, and the resistance was also real-time recorded. Notably, the immersing-drying cyclic tests were also performed as the aforementioned methods. Positron annihilation lifetime spectroscopy (PALS) measurements were performed using a conventional fast-fast coincidence spectrometer. The Na-22 source was sandwiched between pieces of samples. The detectors are the plastic scintillators with a time resolution of about 290 ps. We analyze the positron lifetime spectra using PATFIT program27 and CONTIN program28.

3. RESULTS AND DISCUSSION The ice-templating strategy for fabricating NR composites with segregated network is schematically illustrated in Figure 1. (1) preparation of a homogeneous suspension containing GO and NR latex (NRL) by means of mechanically mixing (Figure 1a); (2) anisotropic freezing of the mixed NRL/GO suspension (Figure 1b); (3) freeze-drying of the frozen NR/GO samples and chemically reduction for obtaining NR/GE samples (Figure 1c), and (4) the crosslinked S-NRGE composites with segregated network fabricated by filling process (the porous NR compounds repeatedly filled with NRL), flocculation and then vulcanization (Figure 1d).

Morphology and Nanostructured Network. The morphology of the GE



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sheets in NR matrix is observed by TEM micrography. Evidently, an intact segregated network is constructed throughout the S-NRGE composites (Figure 2a,b), where the GE sheets are located at the interfaces of NR phase to form a compactly segregated structure. It should be noted that the growth of ice crystals can repel NR and GO from the solidifying water as the mixed suspension is frozen, and the ice crystals between NR particles and GO sheets act as templates for the formation of the 3D segregated networks. Hereon, the NR phase can also be regarded as bridges to link the GO sheets, and is valuable in weakening GE aggregation behavior. A clear comparison of the TEM microphotographs for H-NRGE composites (fabricated by conventional latex compounding technology) are demonstrated in Figure 2c,d, the GE sheets are homogeneously dispersed in the NR matrix, instead of being intactly interconnected throughout the whole matrix. In addition, the SEM observations (Figure S1) can further coincide with the above results. As proved by Ruoff et al.29 and Lin et al.19, the segregated structure is in favour of the formation of a continuous conductive network with a lower content of GE. We then investigated the different morphologies of GE sheets in rubber matrix on the electrical properties of NR composites. Figure 3a shows the conductivity of the NR composites as a function of GE content. Expectedly, the percolation threshold of the segregated S-NRGE composites is only 0.40 vol%, 8-fold lower than that of homogeneous H-NRGE composites, which is as high as 3.60 vol%. Note that the electrical conductivity of S-NRGE composites exhibits very comparable to that of H-NRGE composites. For instance, the conductivity of the H-NRGE-0.84 composites


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is ~3.65×10-9 S/m, while it can reach up to ~0.0354 S/m for the S-NRGE-0.84, dramatically enhanced by 7 orders of magnitude. For further analyzing the relationship between filler network and electrical behavior in the S-NRGE and H-NRGE composites, a power law equation30 is rationalized in terms of a classical percolation theory σ=σ0(φ-φc)τ, where σ and σ0 are the electrical conductivity of the samples and the conductive filler, respectively. φ is the filler volume fraction. φc is the percolation threshold, and τ is the critical exponent. It has been proposed that the τ, which largely depends on the dimensionality of composite, further reflects the fillers network structure in the composites31,32. Particularly, the value of τ lower than 2.1 indicates a polymer-bridged filler network in the composites, whereas the value of τ higher than 3.75 suggests the construction of a developed filler network. As computed in Figure 3b, the value of τ is 1.91 for H-NRGE composites, and 3.82 for S-NRGE composites, which manifest the establishment of a bridged rubber-GE network in the H-NRGE composites and a developed segregated GE networks in the S-NRGE composites, respectively. Furthermore, a comparison of mechanical properties of S-NRGE and H-NRGE as a function of GE content is employed as shown in Figure S2, and the results show maintained tensile strength of S-NRGE composites. It is observed that the elongation at break of the S-NRGE composites remain the similar levels with that of the H-NRGE at the same GE content, demonstrating maintained tensile flexibility of S-NRGE composites. These enhanced properties turn out that the highly conductive and segregated S-NRGE composites with ultralow electrical percolation threshold are obtained without sacrificing any



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mechanical performance. Currently, the GE-based polymer composites with high electrically conductivity had been prepared by several approaches6,19. As a careful comparison, one can be found that the electrical performance of our composites is better than those reported previously in view of the balance of the percolation threshold and the corresponding electrical conductivity, and some representative results are tabulated in Table S2. The superior electrical conductivity in our samples can be attributed to the construction of a compactly segregated GE network in the rubber matrix by ice-templating strategy. Furthermore, the superiority of segregated network to homogeneously dispersed morphology in improving the performance of composites can be also verified by the great difference in gas barrier property of S-NRGE and H-NRGE composites, and the results are clearly displayed in Figure 3c. Furthermore, PALS was employed to analyze the rubber chains segmental motion in the composites at molecular level aiming to verify the morphology difference for these two NR composites. Figure 3d,e shows the ortho-positronium (o-Ps) lifetime (τ3) and o-Ps intensity (I3) of NR composites as a function of GE content. This decrease in the τ3 with increasing GE content strongly suggests a decrease of the average size of free volume cavities, where o-Ps is localized and annihilated. This variation is mainly attributed to that the incorporation of GE sheets into the NR matrix can not only significantly restrain the motion of NR chains due to the physical entanglements, also result in an occupancy or “annihilation” of the abundant pores33. It is evident in Figure 3e that a more significant reduction of the ortho-positronium intensity (I3)


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exhibits in the S-NRGE composites when compared with that of H-NRGE composites. Especially, a sharp falling of I3 from S-NRGE-0.21 to S-NRGE-0.42 is clearly observed. This interpretation is in accordance with the experimental results, such as electrical conductivity measurements and gas permeability measurements discussed above. Such phenomenon can mirror an evolution of the GE network in the composites due to the construction of the segregated network. Specifically, the chains segmental motion can be greatly restricted when a compactly GE network is constructed in the composites, which effectively contributes to an increase of the immobilized rubber-GE interfacial region, and thereby dramatically reduces the free-volume concentration. Additionally, one can be found that the enhancement of the elastic modulus (G′) for the S-NRGE is very superior to that for the H-NRGE composites, as shown in Figure 3f. This result greatly demonstrates the construction of a more developed segregated GE network in the S-NRGE composites. Accordingly, it is reasonably concluded that constructing a continuous segregated network in the composites is of crucial importance for achieving a desirable performance.

Liquid Sensing behavior. The liquid sensing performances of the composites were measured by immersing the samples into organic solvents. Hereon, toluene serves as an analyte for evaluating the solvent sensing performance. The resistance changes (R/R0) during the measurements effectively mirror the responsivity values of the composites (R/R0, where R0 is the initial resistance of the samples at room temperature and R is the real-time resistance of the composite when immersed into the toluene), and the immersed time corresponding to the obtained maximum R/R0 value



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represents the response time. We tuned the GE content for analyzing the effect of constructed segregated network on the liquid sensing properties, as shown in Figure 4a,b,c. During the immersion, the resistance slowly increases at first and then rises quickly for all the composites, observations of which are closely related to the destruction of the segregated conductive network due to the swelling effect of NR matrix in organic solvent. For S-NRGE-0.42 composites, the responsivity value is 234 at the response time of 56 s, while the responsivity of S-NRGE-0.63 reaches to 6700 at the response time of 114 s. Surprisingly, the responsivity reaches up to 56700 at a longer response time of 350 s. This phenomenon can be rationalized by the fact that a more developed conductive GE network at high conductive filler content can contribute significantly to the higher responsivity of composites at the expense of response time. For better evaluating the liquid sensing sensitivity of the constructed segregated conductive network to organic solvent stimuli, we chose the plot of R/R0 versus time at initial immersing process to study. It has been admitted that the higher the rate of resistance change is, the faster the response of the materials to liquid stimuli will be7. Hence, the linear slope of R/R0-time curves is employed to evaluate the sensitivity of composites responding to the stimuli. As shown in Figure 4a’,b’,c’, linear slopes of S-NRGE-0.42, S-NRGE-0.63 and S-NRGE-0.84 composites are 0.57, 0.30, and 0.14, respectively. Interestingly, the time points (τ) of abrupt resistance change are located at 12.6, 41.6 and 131.1 s, respectively. These results indicate that the segregated S-NRGE composites with low filler content exhibit a good sensitivity to solvent


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stimuli. During the drying process, a sharp drop in the resistance is observed immediately at the beginning of drying, and a good reversibility of the resistance change is clearly reflected for the both S-NRGE-0.63 and S-NRGE-0.84 composites, mainly attributed to the reconstruction of the conductive GE networks after the removal of solvent in NR. However, it is found that the resistance of S-NRGE-0.42 composites does not return back to the initial value even 2 min after the drying, showing a poor reproducibility. This phenomenon is mainly ascribed to the irreversible destruction and reconstruction of conductive networks in the S-NRGE composites within low GE content much near the electrical percolation threshold. In view of a promising and desirable sensing materials with high responsivity, low response time, good reproducibility

and

comparative

conductivity,

our

fabricated

S-NRGE-0.63

composites exhibit very competitive for the application in liquid sensing material, which simultaneously possess relatively low response time (114 s), high responsivity (6700) and comparative electrical conductivity (~10-3 S/m). Aiming to have a visual understanding of the liquid sensing behavior of S-NRGE-0.63 composites, we investigated the illumination changes of LED devices to mirror the conductivity variation. As demonstrated in Figure S3, during the immersing process, the illumination gradually fades over time, whereas the LED light can be lighted up and the luminance gradually strengthens over time. These illumination changes effectively give a proof of the evolution of destruction and reconstruction during the immersing-drying process, and indicate the liquid sensing capacity of S-NRGE



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composites. Evidently, our S-NRGE composites not only possess a comparable electrical conductivity at low filler content, but also display the higher liquid sensing properties when compared with other reported composites, the results are tabulated in the Table S2 and Table S3. Subsequently, the reproducibility of liquid sensing behavior of segregated S-NRGE-0.63 composites when exposed into toluene was examined, as displayed in Figure 4d. Ten cycles of immersing-drying process are reproducibly detected, and the results exhibit mostly good response and reproducibility in each cycle. In addition, Figure 4e shows that some different organic solvents can be effectively detected by this liquid sensor based on the S-NRGE composites. The response time for toluene, xylene, dichloromethane, n-hexane, petroleum ether and tetrahydrofuran are 114, 185, 223, 271, 342 and 394 s, respectively, and the responsivity values for all the tested solvents are more than 5965. Such results suggest that the S-NRGE composites have a good liquid sensing sensitivity for these organic solvents. The difference in liquid sensing behavior is due to the combined effect of interaction parameters between the NR and the tested organic solvents, molar volume and boiling temperature7. Table S4 displays the interaction parameters χ and characteristics of solvents used in this work. Obviously, the similar interaction parameter between the NR and the organic solvents (e.g. toluene, xylene) can facilitate the solvents to swell the NR matrix and undermine the conductive paths easily. Also, the solvent with smaller molar volume and lower boiling temperature (e.g. dichloromethane, tetrahydrofuran) is good for the solvent molecules to permeate into the NR, resulting in the destruction of the conductive


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networks, and thereby exhibiting faster response (low response time). Notice that the solvent temperature significantly affects the liquid sensing performance of composites, as depicted in Figure 4f, the high solvent temperature effectively speeds up the solvent penetration into the matrix, and thus causing great damage of the conductive pathways. Importantly, it is with this great difference in liquid sensing behavior that S-NRGE composites exhibit a promising potential in the detection and distinction of different organic solvents. To better understand the observed phenomenon about liquid sensing behavior, a mechanism about the evolution of segregated conductive networks in an immersing-drying process is proposed in Figure S4. In the original state, S-NRGE composites exhibit a compactly segregated conductive GE network at low GE content. When the composites are immersed into organic solvents, the solvent molecules gradually penetrate into the materials and swell the NR matrix, leading to the disconnection of conductive paths and thus a rapid increase in the resistance. During the drying process, the segregated conductive network can be effectively reconstructed because of the desorption of the solvents. Due to the irreversible destruction of conductive GE network originated from the swelling history and residual solvents in matrix, the full-recovery of the conductive network and performance still remains a big challenge up to now.

Strain Sensing properties. Considering that rubber materials potentially exploit their practical application, the strain sensing properties of materials should arouse concerns. To evaluate the reproducibility of the fabricated strain sensors after



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cyclic stretching/releasing process, the strain sensors were then measured with repetitive stretching of 400 cycles up to 30% strain. As shown in Figure 5a,b,c, the resistance of all the sensors almost remains unchanged during and after 400 cycles, indicating a good stability and reproducibility. Taking a closer view of these curves (Figure 5a’,b’,c’), a conspicuous ‘shoulder peak’ is observed for S-NRGE-0.42 and S-NRGE-0.63 composites, while no ‘shoulder peak’ is found for S-NRGE-0.84 composites. Such difference can be ascribed to the co-existence of the destruction and reconstruction of the conductive network in the cyclic stretching/releasing. An easier reconstruction of the conductive paths in the composites with high GE content efficiently maintains the abundant interconnected points between GE sheets and quickly restores its original conductive network while being stretched and released, respectively. This aforementioned different ‘shoulder peak’ phenomenon has also been reported in the NR/GE5, TPU/GE34, CNT/HDPE35, and MWNT/TPU36. It is noted that the strain gauge factor (∆R/(εR0), ε is the applied strain) is employed to evaluate the sensitivity of strain sensors. As demonstrated in Figure 5d, the gauge factor of S-NRGE-0.42 is computed to be ~139 with a high stretchability of ~60% strain. Compared with other reported strain sensors based on elastomer materials37-46, the fabricated strain sensor displays a relatively higher gauge factor, and the detailed contrasting results are clearly shown in Figure 5e. We therefore believe that the S-NRGE composites with low GE content (near the percolation threshold) exhibit a great potential for application in highly stretchable and sensitive strain sensors. Specifically, at low strain, the ∆R/R0 for S-NRGE-0.42 composites exhibits a


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relatively flat response to the applied strain, result of which is attributed to that the destruction and reconstruction of the conductive GE network coexist in the stretching process. With further increasing the applied strain, the drastic movement of rubber chains leads to the severe disruption of segregated conductive networks (appearing as orange rectangle in Figure 6). Thus, the destruction of the conductive network is dominant, so a significant increasing ∆R/R0 is observed. Herein, the mechanism by which deformation of S-NRGE composites can greatly affect the segregated conductive network is illustrated in Figure 6. As discussed in Figure 5d, it is believed that the strain amplitude has significant effect on the strain sensing behavior of the composites. Then the resistance-strain behavior as a function of strain amplitude under cyclic tests is investigated, as shown in Figure 7a. All the samples exhibit a good recoverability and reproducibility in the cyclic tests with 30%, 60% and 90% strain, respectively. And no fracture and electrical shot are observed during the cyclic tests. Figure 7a’ displays the ∆R/R0 variation following the 1st cycle to the strain amplitude of 30%, 60% and 90%. The gauge factors are about 6.9, 10.0 and 22.2 for the corresponding strain amplitude of 30%, 60% and 90%, respectively. This different sensitivity is mainly due to the full extent of damage to the interconnected points between GE sheets at various strain amplitudes upon stretching process. The higher strain amplitude will cause a more significant decrease of contact junctions between GE sheets, disconnecting the electronic pathway, and thus resulting in the serious breakage of segregated conductive network. In the releasing process, the conductive GE networks are of



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difficulty to be fullly recovered to original state due to the the relaxation of the rubber chains and the slipping of GE sheets, leading to a higher irreversible resistance. As seen from Figure 7a’, a highly reversible change in ∆R/R0 can be achieved upon 30% strain, whereas the ∆R/R0 is imperfectly restored with high strain amplitude of 60, 90%. Besides strain amplitude, also strain rate has a great influence on the strain sensing behavior of NR composites. Similar to the resistance-strain sensing behaviour under different strain amplitude, good recoverability and reproducibility by cyclic tests are achieved upon 30% under different strain rates, as shown in Figure 7b. It is evident that an obvious ‘shoulder peak’ clearly appears at a high strain rate (e.g. 9 min-1, 18 min-1), whereas no conspicuous ‘shoulder peak’ is observed at a low strain rate of 4.5 min-1. Based on the aforementioned discussion, this different ‘shoulder peak’ phenomenon is closely associated with the destruction and reconstruction of segregated conductive network. At low strain rate, the sensors have a plenty of time to repair and reconstruct the conductive channels, thus no evident ‘shoulder peak’ is detected. Figure 7b’ exhibits the ∆R/R0 variation following the 1st cycle to the different strain rates. The gauge factors at the strain rate of 4.5, 9.0 and 18 min-1 are estimated to be about 6.8, 22.9 and 57.5, respectively, results of which manifest that the higher the strain rate is, the higher the sensitivity has. This phenomenon can be interpreted by the fact that a higher strain rate means a larger stress applied on the materials1, so it is much easier to make the segregated network to be destructed, leading to greater variation in electrical resistance. It needs to point out that a higher


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strain rate gives rise to the more breakage of GE interconnected points by disrupting the conductive network, causing larger irreversible resistance. Additionally, we then attached this fabricated sensor to the knee for monitoring the large-scale body motions, such as bending and squatting. The tested sensor can be gradually stretched as the skin and muscle around the knee tightens. As shown in Figure 8a,b, the sensor based on S-NRGE-0.42 composites exhibits a good responsivity and reproducibility to tensile strain variation derived from the knee motions. Such distinct performance caused by different body motions provides the possibility of distinguishing the various motions using the fabricated sensor. Therefore, in view of the promising strain sensing performance, such as high stretchability, high sensitivity over a wide tensile strain range, and good recoverability and reproducibility by cyclic stretching, the S-NRGE composites with a novel segregated conductive GE network have great potential application in strain sensing fields, such as human motion detection, damage monitoring and strain gage, etc. Furthermore, the tunable liquid sensing performance of this S-NRGE composite can be achieved, which makes it very competitive for the application in liquid sensing materials. 4. CONCLUSIONS In this work, the fabrication of NR composites with segregated nanostructured GE network by an effective ice-templating process was demonstrated. The establishment of segregated GE network throughout the composites dramatically enhanced the electrical conductivity and reduced the electrical percolation threshold. The



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percolation threshold of the segregated S-NRGE was 8-fold lower than that of the H-NRGE composites with homogeneous dispersion morphology. Remarkably, the S-NRGE composite containing 0.63 vol% GE simultaneously possessed the high responsivity (6700), low response time (114 s) and good reproducibility, as well as a required electrical conductivity (0.008 S/m), which makes it suitable for the detection and distinction of different organic solvents. Notably, the liquid sensing performance of S-NRGE composites can be effectively tuned by manipulating the segregated conductive networks using different GE contents. Moreover, the strain sensing behavior of segregated S-NRGE composites was also investigated, the results demonstrated that the prepared strain sensors showed high sensitivity (gauge factor, ~139) and reproducibility (~400 cyclic tests) even when strained up to 60%, which exhibited very competitive for practical applications in the large-scale body motion detection. These high performance composites opens up a novel and simple strategy to fabricate high performance sensing materials based on segregated GE-elastomer composites for multifunctional applications. ASSOCIATED CONTENT Supporting Information. SEM observations for the S-NRGE and H-NRGE composites; Photographs of illumination changes for segregated S-NRGE-0.63 composites during immersing and drying processes; Comparison of mechanical properties of S-NRGE and H-NRGE as a function of GE content; Schematic of microstructural development for the segregated S-NRGE composites during the immersing-drying process; The experimental formula for preparation of NR


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composites; The comparison of electrical percolation threshold (φc) and conductivity for elastomer-GE composites previously reported; Comparison of liquid sensing performance of polymer composites previously reported; Interaction parameters χ and characteristics of solvents and NR used in the present work. The Supporting Information is available free of charge on the ACS Publications website. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Notes: The authors declare no competing financial interest. ACKNOWLEDGEMENTS This work is financially supported by National Basic Research Program of China (No. 2015CB654700 (2015CB654703)), National Natural Science Foundation of China (No. 51573053) and Science and Technology Planning Project of Guangdong Province (No. 2014A010105022). REFERENCES (1) Yan, D. X., Pang, H., Li, B., Vajtai, R., Xu, L., Ren, P., Wang, J., Li, Z. Structured Reduced Graphene Oxide/Polymer Composites for Ultra-Efficient Electromagnetic Interference Shielding. Adv. Funct. Mater. 2015, 25, 559-566. (2) Duan, S., Yang, K., Wang, Z., Chen, M., Zhang, L., Zhang, H., Li, C. Fabrication of Highly Stretchable Conductors Based on 3D Printed Porous Poly(dimethylsiloxane) and Conductive Carbon Nanotubes/Graphene Network. ACS Appl. Mater. Interfaces 2016, 8, 2187-2192.



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Figure 1. Schematic showing the formation mechanism of the graphene-elastomer composites with segregated network by ice-templating. When a homogeneous GO/NRL/vulcanized aqueous suspension (a) is frozen, GO/NRL particles are squeezed out at the boundary of growing ice crystals (b), finally forming a continuous segregated network. The segregated network effectively retains when the ice is removed by freeze drying (c). The obtained porous NR-GO compounds are chemically reduced and then filled with the mixture of NRL and vulcanized aqueous suspensions, followed by thermal flocculation, then the crosslinked S-NRGE composites with segregated network are fabricated by vulcanization process (d).


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Figure 2. TEM images of segregated S-NRGE-0.42 composites (a and b) and homogeneous H-NRGE-0.42 composites (c and d).



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Figure 3. (a) The electrical conductivity as a function of GE content for NR composites. (b) The fitted results of experimental data of NR composites according to the percolation law. (c) Nitrogen permeability of NR composites as a function of GE content, measured at 23 °C and 0% relative humidity. (d-e) The o-Ps lifetime τ3 (d) and o-Ps intensity I3 (e) as a function of GE content for NR composites. (f) Dependence of the G′ of the uncured NR compounds on the strain.


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Figure 4. (a-c) Responsivity change versus time during immersing-drying process for the S-NRGE-0.42 (a), S-NRGE-0.63 (b) and S-NRGE-0.84 composites. Thereinto, a’, b’ and c’ are corresponding to the enlarged images for a, b and c, respectively. (d) Responsivity change of the segregated S-NRGE-0.63 composites during repeated immersing process in toluene. (e-f) Responsivity change of the segregated S-NRGE-0.63 composites: (e) exposed to different organic liquids, (f) during immersing process in toluene at different solvent temperature.



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Figure 5. (a-c) Plot of the ∆R/R0 for segregated S-NRGE-0.42 (a), S-NRGE-0.63 (b) and S-NRGE-0.84 (c) composites under 400 stretching/releasing cycles of 30% at strain rate of 9.0 min-1. (a’-c’) The plot of the ∆R/R0 for cycles 333-338. (d) Plot of the ∆R/R0 for S-NRGE-0.42 as a function of applied strain (strain rate of 9.0 min-1). (e) The comparison of gauge factor for strain sensors based on elastomer composites previously reported. Therein,

a

Polydimethylsiloxane;

b

c

Polyurethane; Thermoplastic elastomer.

Figure 6. Schematic for the evolution of a conductive network in segregated S-NRGE composites during stretching process. NR phase is represented by blue circular particles, the conductive network by the black lines and electrons by e-.


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Figure 7. (a, a’) Plot of the ∆R/R0 for segregated S-NRGE-0.63 composites, up to different strain amplitudes at the strain rate of 4.5 min-1, during the cyclic loading (cycles 100-104) (a) and the 1st cycle (a’). (b, b’) Plot of the ∆R/R0 for S-NRGE-0.63 composites, up to 30% strain at different strain rates, during the cyclic loading (cycles 100-104) (b) and the 1st cycle (b’).



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Figure 8. Photograph of the fabricated sensor based on S-NRGE-0.42 composites attached to the knee (The inset in a) and the plot of the ∆R/R0 for various knee motions: (a) bending, (b) squatting.


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Table of Contents

A facile ice-templating strategy was firstly reported to fabricate a highly stretchable, sensitive and multifunctional sensing materials based on graphene-rubber composites with a novel segregated network.


GrapheneâElastomer Composites with Segregated ... - ACS Publications

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