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Novel Electrically-Conductive Porous PDMS/Carbon Nanofibre Composites for Deformable Strain-Sensors and Conductors Shuying Wu, Jin Zhang, Raj B. Ladani, Anil R. Ravindran, Adrian P. Mouritz, Anthony J. Kinloch, and chun H. wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b00847 • Publication Date (Web): 11 Apr 2017 Downloaded from http://pubs.acs.org on April 15, 2017

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ACS Applied Materials & Interfaces

Novel Electrically-Conductive Porous PDMS/Carbon Nanofibre Composites for Deformable Strain-Sensors and Conductors Shuying Wu,†, ‡ Jin Zhang,§ Raj B. Ladani,† Anil R. Ravindran, † Adrian P. Mouritz,† Anthony J. Kinloch,∥ and Chun H. Wang‡,* †

Sir Lawrence Wackett Aerospace Research Centre, School of Engineering, RMIT University,

GPO Box 2476, Melbourne, VIC 3001, Australia ‡

School of Mechanical and Manufacturing Engineering, University of New South Wales, Sydney,

NSW 2052, Australia §

Australian Future Fibres Research and Innovation Centre, Institute for Frontier Materials,

Deakin University, VIC 3220, Australia ∥Department

of Mechanical Engineering, Imperial College London, London, SW7 2BX, UK

*E-mail: [email protected]

KEYWORDS: Porous nanocomposites, stretchable, strain-sensor, conductor, piezoresistivity, carbon nanofibres, polydimethylsiloxane

ABSTRACT Highly flexible and deformable electrically-conductive materials are vital for the emerging field of wearable electronics. To address the challenge of flexible materials with a relatively high electrical conductivity and a high elastic limit, we report a new and facile method to prepare porous polydimethylsiloxane/carbon nanofibre composites (denoted by p-PDMS/CNF). This method involves using sugar particles coated with carbon nanofibres (CNFs) as the templates. The resulting three-dimensional porous nanocomposites, with the CNFs embedded in the PDMS 1 ACS Paragon Plus Environment


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pore walls, exhibit a greatly increased failure strain (up to ~ 94%) compared to that of the solid, neat PDMS (~48%). The piezoresistive response observed under cyclic tension indicates that the unique microstructure provides the new nanocomposites with excellent durability. The electrical conductivity and the gauge factor of this new nanocomposite can be tuned by changing the content of the CNFs. The electrical conductivity increases, while the gauge factor decreases, upon increasing the content of CNFs. The gauge factor of the newly-developed sensors can be adjusted from approximately 1.0 to 6.5 and the nanocomposites show stable piezoresistive performance with fast response time and good linearity in ln(R/R0) versus ln(L/L0) up to ~ 70% strain. This tunable sensitivity and conductivity endow these highly stretchable nanocomposites with considerable potential for use as flexible strain-sensors for monitoring the movement of human joints (where a relatively high gauge factor is needed) and also as flexible conductors for wearable electronics (where a relatively low gauge factor is required).

1. INTRODUCTION Highly flexible and stretchable electrically-conductive materials are a key to future wearable electronics with a diverse range of applications such as electronic skins,1 human-friendly interactive electronics,2 and wearable health monitors.3-4 Recently, elastic and conductive polymer nanocomposites have been extensively investigated due to their excellent deformation capability compared to the traditional rigid metallic strain-sensor gauges and conductors.2 One common technique is to incorporate electrically conductive nanomaterials, such as nanowires,4-5 carbon nanotubes (CNTs),6-7 carbon nanofibers (CNFs),8 metal nanoparticles9 and graphene10, into flexible polymers to achieve a relatively high elastic limit and electrical conductivity. In

2 ACS Paragon Plus Environment

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particular, carbon nanomaterials with excellent mechanical and electrical properties are emerging as a leading solution for fabricating flexible strain-sensors and conductors. However, due to the strong π-π interactions in such carbon nanomaterials, it remains a challenge to disperse such nanomaterials uniformly in polymeric matrices. This leads to the need for a relatively high concentration of carbon nanomaterials to be added in order to reach the percolation threshold and so achieve sufficient electrical conductivity for their use as flexible strain-sensors and conductors.8

To avoid the dispersion problem and to improve greatly the electrical conductivity of the polymer nanocomposites, researchers have recently shown that arranging the carbon nanomaterials into three-dimensional (3D) interconnected structures is a promising strategy.11-15 By selecting an appropriate polymer matrix (e.g. polydimethylsiloxane (PDMS)), stretchable strain-sensors based on 3D-structured graphene have been developed by our group and other researchers.11-14 Such nanocomposites strain-sensors have shown high sensitivity with the gauge factor being as high as 99, which is significantly higher than the typical value of about 2 for metallic foil strain-sensor gauges. Nonetheless, a relatively poor deformation capability has been observed (i.e. the maximum failure strain being approximately 30%).11 Additionally, the complicated fabrication process associated with their manufacture limits their applications.

Recent research has revealed that making elastomeric polymers porous improves their deformability and that their failure strain greatly exceeds that of their solid (i.e. non-porous) counterparts.16-18 Using a photolithography technique,16 Park et al. showed that 3D nanostructured pores can be created in a PDMS polymer which enhances the fracture strain by



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225 % compared with the solid counterpart. Following this pioneering work, porous PDMS polymers have been fabricated using a 3D printing technique17 or nickel foam as the template.18 Nevertheless, the thickness of the porous PDMS using the photolithography technique is limited to the micrometer scale, while the 3D printing technique and nickel-foam procedures require time-consuming etching processes and involve the use of toxic solvents. Hence, it is of great importance to explore new cost-effective approaches to fabricate porous polymer nanocomposites.

Herein, we demonstrate a novel strategy for creating stretchable and conductive porous PDMS/CNF nanocomposites (denoted by p-PDMS/CNF) by using CNF-coated sugar particles as the templates. CNFs with a high-aspect-ratio are used due to their high electrical conductivity (i.e. ~ 103 S/cm) and relatively low cost (i.e. they are typically three and five hundred times cheaper than multi-wall or single-wall carbon nanotubes, respectively).19-22 Recent work has demonstrated that conductive nanomaterials can be incorporated into porous polymer substrates via dip-coating.17-18, 23-25 However, detachment of the nanomaterials from the substrates may occur under cyclic loading-unloading, leading to an ineffective stress transfer from the polymer substrate to the nanomaterials, and thereby a reduction in their long-term durability.25 In the present work, the CNFs, which are originally on the surfaces of sugar particles, are transferred and embedded in the pore walls of the porous PDMS, which ensures good adhesion between the CNFs and the polymer. In comparison with the nanocomposites prepared by dip-coating, these new polymer nanocomposites demonstrate a significantly higher durability. Furthermore, the sensitivity of these new p-PDMS/CNF nanocomposites is tunable by simply varying the content of the CNFs. The potential applications of these conductive nanocomposites as a strain-sensor


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for detecting movement of the finger joint and as a stretchable conductor in a LED circuit are demonstrated.

2. EXPERIMENTAL SECTION 2.1 Materials The carbon nanofibres (Pyrograf®-III, grade PR-24-XT-HHT) used were supplied by Applied Sciences Inc. The CNFs had a diameter of about 70–200 nm and a length of 50–200 µm. The Sylgard 184 silicone elastomer base and curing agent were supplied by Dow Corning Co. Ltd (Australia). Brown sugar with an average particle size of about 225 µm was used. Isopropanol (IPA) was sourced from Sigma-Aldrich. Graphene oxide (GO) was synthesized using an improved Hummers' method.14, 26 The high-purity silver-paste (Silver Paste Plus™) was from SPI Supplies (USA).

2.2 Preparation of Porous PDMS/CNF (p-PDMS/CNF) Nanocomposites Using CNF-coated Sugar-Particle Templates The p-PDMS/CNF nanocomposites were prepared by the following procedure (illustrated in Figure 1): firstly, the CNFs were dispersed in IPA with GO as the dispersant by using a Sonics VCX-750 Vibra Cell Ultra Sonic Processor (40% amplitude, pulse duration: 5 second on, 5 second off). GO is used as dispersant since it has been previously utilized as a two-dimensional surfactant to disperse carbon nanotubes in water.27 The mass ratio of CNF:GO was 30:1 and there were no visible aggregates after sonication for 30 minutes. Sugar particles were added to the dispersion (3 mg/mL) to get a slurry-like mixture, which was then placed in a vacuum oven to remove the IPA. Subsequently, the CNF-coated sugar was compacted into a thin sheet



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template with dimensions of 40 mm (length) × 9 mm (width) × 2 mm (thickness) using a hydraulic press operated at a pressure of 2 MPa. Sugar particles were selected as the porecreating agent since they are eco-friendly and easily removed at a later stage, i.e. simply by dissolving them in water.

The PDMS prepolymer was mixed with the curing agent at a weight ratio of 10:1. This precursor mixture was then magnetically stirred for 10 minutes and degassed for 30 minutes, after which the mixture was poured onto the CNF-coated sugar template. Owing to its low viscosity and low surface energy, the PDMS prepolymer infiltrated gradually into the 3D interconnected pores between the CNF-coated sugar particles. To facilitate the infiltration, the template infiltrated with the PDMS prepolymer was placed in a vacuum oven for 1 h, which was subsequently heated at 65 oC for 2 h for curing. Afterwards, the excess PDMS polymer around the edges was trimmed away to expose the sugar particles, which were removed by warm water under sonication. It was found that a negligible concentration of CNFs were removed during the sugar dissolving and sonication processes. Finally, porous p-PDMS/CNF nanocomposites, with interconnected CNFs in the pore walls, were obtained by drying in a fume hood for 24 h. Different p-PDMS/CNF nanocomposites were prepared by changing the weight content of the CNFs (i.e. 0.1, 0.3, 0.5, 0.7, 1.4, and 2.8 wt% of CNFs). Porous, neat PDMS polymer sheets were also prepared following the same procedure but using a sugar-particle only template which was made by pressing the sugar particles into a 2 mm thin sheet using the hydraulic press at the same pressure as above (i.e. 2 MPa). Also, 2 mm thin sheets of the solid, neat PDMS polymer were prepared.


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2.3 Preparation of Porous PDMS/CNF (dc-PDMS/CNF) Nanocomposites Using Dip-coating For comparison, another type of porous PDMS/CNF nanocomposite, denoted by dc-PDMS/CNF, was prepared using a typical dip-coating method.17-18, 20-22 Firstly, thin sheets of porous, neat PDMS were prepared as described above. The CNFs were then coated onto the porous PDMS by dipping the porous PDMS into a dispersion of CNFs in IPA, with GO as the dispersant, and then drying the material in a vacuum oven. Before dip-coating, the porous PDMS was treated in an oxygen plasma chamber (Zepto, Diener Electronics with 60W power under 0.5 mbar air pressure) for 3 minutes to increase the polar nature of the exposed surfaces. This plasma treatment is essential for the deposition method to be successful due to the superhydrophobic nature of the PDMS. To be consistent, the dc-PDMS/CNF nanocomposite was prepared by repeating the dipcoating and drying process several times, so that the dc-PDMS/CNF nanocomposite contains 0.3wt% of CNFs.

Figure 1. Schematic illustration of the preparation methods for the two different types of porous PDMS/CNF nanocomposites. 7 ACS Paragon Plus Environment


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2.4 Characterization. The morphologies of the p-PDMS/CNF nanocomposites were examined using a scanning electron microscope (FEI Nova NanoSEM). The nanocomposite specimens were cryogenically fractured in liquid nitrogen and then coated with a thin layer of gold prior to observation. The degree of porosity of the porous PDMS prepared by using the sugar particles was estimated to be ~ 79% based on the ratio between the density of porous (0.23 g/cm3) and solid PDMS (1.1 g/cm3). This value is similar to that reported by Zhang et al.28

A tensile testing machine (Instron Model 4466) was employed to characterize the mechanical properties. The PDMS/CNF nanocomposites were tested under quasi-static tension and also under cyclic tensile loading-unloading cycles under displacement control. The test specimens had dimensions of 40 mm (length) × 6 mm (width) × 2 mm (thickness). For the quasi-static tension test, a strain rate of 8%/min was used. The cyclic tests were conducted by applying a sinusoidal cyclic load at a frequency of 0.08 Hz. The electrical resistance was measured using a digital multimeter (34465A, Keysight Technologies). Thin copper wires were attached to the two ends of the specimens using a conductive silver-paste adhesive. The response time was measured by rapidly (40 mm/s) stretching the sensor to 40% strain and holding for 30 s. The sample was then unloaded at the same rate back to 0% strain and held for 30 s before starting the next cycle.

3. RESULTS AND DISCUSSION 3.1 Microstructure and Mechanical Properties of the Porous PDMS/CNF Nanocomposites Figure 2a shows a SEM image of the sugar particles, from which it is estimated that their average size is approximately 225 µm. Figure 2b is a SEM image of the sugar particles coated


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with CNFs. From the insert taken at a relatively high magnification, it is seen that the CNFs are uniformly coated onto the surfaces of the sugar particles and form a network. When the liquid PDMS prepolymer is cast onto the CNF-coated sugar templates, the PDMS infiltrates into the interconnected pores of the CNF-coated sugar particles, owing to the relatively low viscosity and low surface energy of the PDMS prepolymer.29 After curing the PDMS, the CNFs are entrapped within the PDMS, near the surfaces of pores, forming a nanocomposite layer of CNFs and PDMS. The SEM image (Figure 2c) shows the microstructure of the p-PDMS/CNF nanocomposites and clearly reveals that the sugar particles are removed by the water and that interconnected pores are formed. The high-magnification SEM image (Figure 2d) shows that majority of the CNFs are embedded just below the surface of the PDMS, with some CNFs being visible on the surface. The contact and close proximity between the CNFs in the p-PDMS/CNF nanocomposites give rise to a relatively high electrical conductivity, as discussed below. By contrast, for the nanocomposites prepared by the dip-coating method (i.e. the dc-PDMS/CNF), although the porous structure is similar, the CNFs are only loosely attached to the surface of the pore walls, as indicated by the SEM image in Figure 2e.



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0.6 0.4 0.2 0.0 0

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Figure 2. SEM images of (a) raw sugar particles; (b) CNF-coated sugar particles (the insert is a magnified image from the region marked by the cycle); (c) p-PDMS/CNF nanocomposite (0.3 wt% of CNFs); (d) is an image taken from the pore wall marked by the circle in (c); (e) a dcPDMS/CNF nanocomposite (0.3 wt% of CNFs); (f) quasi-static tensile stress-strain curves of the porous and solid, neat PDMS polymers and the two PDMS/CNF nanocomposites.


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The porous, neat PDMS polymer exhibits a significantly higher tensile breaking strain (of ~150%) than the solid, neat PDMS polymer (of ~48%) (Figure 2f). The underlying mechanism for the increased fracture strain has been attributed to local rotations of narrow structural elements (i.e. the pore walls) in the 3D network relative to the stretching direction under the tensile deformation, which leads to a lower strain acting in the pore walls.16-17 This observation was based on finite element modelling studies, which indicate that the applied strain is distributed in the 3D structure of the porous material, resulting in less actual strain on the interconnecting material.16-17 Now, the introduction of the CNFs into the porous PDMS increases the stiffness of the resulting nanocomposite but lowers the fracture strain. Interestingly, the pPDMS/CNF nanocomposite shows a higher modulus but a lower fracture strain than the dcPDMS/CNF nanocomposite. Specifically, the modulus and fracture strain for the p-PDMS/CNF nanocomposite are approximately 314 kPa and 87%, respectively, while the dc-PDMS/CNF nanocomposite exhibits a modulus of ~ 115 kPa and a fracture strain of ~ 120%. The electrical conductivity of the dc-PDMS/CNF nanocomposite was measured to be ~ 0.007 S/m, whilst the p-PDMS/CNF nanocomposite shows conductivity nearly five times higher of ~ 0.033 S/m. These different properties may be attributed to the different microstructures, i.e. the CNFs are loosely coated on the surface of the PDMS pore walls in the dc-PDMS/CNF nanocomposite, whilst the CNFs are embedded and uniformly dispersed within the PDMS pore walls in the p-PDMS/CNF nanocomposite.



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3.2 Comparison of the Piezoresistivity of the Two Types of Porous PDMS/CNF Nanocomposites 3.2.1 Quasi-static Tensile Loading To evaluate the strain-sensing capability of the two types of PDMS/CNF nanocomposites, they were firstly subjected to quasi-static tensile loading (see Figure 3). For both nanocomposites, which contained 0.3 wt% of CNFs, the relative change in the resistance, ΔR/R0, increases continuously with the applied strain, ε. The resistance of the dc-PDMS/CNF nanocomposites increases rapidly after being stretched to ~60% strain and reaches values beyond the measurement capacity of the digital multimeter used. As shown in Figure 3a, the values of ΔR/R0 for both types of PDMS/CNF nanocomposites show a nearly linear dependence on the applied strain up to around 20%-30% strain. However, under larger deformations, nonlinear responses are observed. It is interesting to note that better linearity is obtained by expressing the experimental data using natural logarithms (Figure 3b). Therefore, instead of the commonly used linear ratio of (ΔR/R )/(ΔL/L ) to define the gauge factor, GF,30 the sensitivity of the strain-sensor under large deformations may now be expressed quantitatively by the gauge factor, GFld, defined as: GF =

(/ ) (/ )

(1)

where R0 and L0 are the initial resistance and length while R and L are the resistance and length of the strain-sensor under an applied strain, ε. The value of GFld obtained by linear regression analysis of the data according to eq.(1) is 6.4 (with a R2 = 0.997) and 4.1 (with a R2 = 0.999) for the dc-PDMS/CNF and p-PDMS/CNF nanocomposites, respectively. Thus, the dc-PDMS/CNF nanocomposite exhibits a higher sensitivity than the p-PDMS/CNF nanocomposite, which is


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likely to be due to the different microstructures of the strain-sensors and piezoresistivity mechanisms, as discussed below.

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Figure 3. Comparison of the variation in resistance under quasi-static loading for the two different types of PDMS nanocomposites containing 0.3 wt% of CNFs: (a) ΔR/R0 versus strain and (b) ln(R/R0) versus ln(L/L0). (The insert in (a) is the enlarged view of the rectangular region.)

3.2.2 Cyclic Loading and Unloading To investigate the physical durability of these two types of PDMS/CNF nanocomposites as strain-sensors they were subjected to cyclic loading and unloading (i.e. stretching and release cycles). Figure 4 shows the resistance changes of the nanocomposites being stretched to 30% of strain and then gradually unloaded to 0%. From Figures 4a and 4c it can be seen that the value of the relative resistance change, ΔR/R0, steadily increases upon stretching and then gradually decreases during unloading. After the first cycle, the resistance (unloaded to 0% strain) increases irreversibly by ~35% for the p-PDMS/CNF and ~113% for the dc-PDMS/CNF nanocomposite, respectively (Figure 4c). However, in the subsequent stretching-releasing cycles, both of these



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two nanocomposites essentially recover their resistance after releasing (Figure 4a), indicating excellent durability.

Figure 4b shows the value of ΔR/R0 at the maximum strain of 30% as a function of the number of cycles of loading and unloading. For the dc-PDMS/CNF nanocomposite, each consecutive cycle generates a regular, continuous downward drift of the relative resistance change, ∆R/R0. This downward drift is more noticeable in the first few cycles and slows down as the number of cycles increases. Indeed, for the dc-PDMS/CNF nanocomposite, this downward drift in the value of ∆R/R0 is never arrested. In contrast, for the p-PDMS/CNF nanocomposite, the value of ∆R/R0 decreases somewhat during the initial few cycles but then rapidly stabilizes all the way up to 10,000 cycles (the maximum number of cycles employed in the present tests). These experimental measurements demonstrate that strain-sensors fabricated from the dc-PDMS/CNF nanocomposite, where the CNFs are deposited on the surface of the pores, are not as durable and hence as stable as those fabricated from the p-PDMS/CNF nanocomposite, where the CNFs are embedded in the pore walls of the PDMS.


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Figure 4. Comparison of the piezoresistivity of the two types of PDMS/CNF nanocomposites containing 0.3 wt% of CNFs: (a) ∆R/R0 under cyclic tension with a maximum strain of 30% and a minimum strain of 0% (the insert in (a) is ∆R/R0 for the first ten cycles); (b) ∆R/R0 at an applied strain of 30% versus cyclic numbers; (c) ∆R/R0 versus ε curves corresponding to the first cycle in (a).

3.2.3. Mechanisms of the Piezoresistive Behaviour The observations noted above, and hence the differences in the piezoresistive behaviour, of these two PDMS/CNF nanocomposites may be attributed to their different microstructures. The conductive layer in the p-PDMS/CNF nanocomposite shows a similar microstructure to PDMS



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nanocomposites containing uniformly dispersed conductive nanomaterials.8, 31 By contrast, the structure of the dc-PDMS/CNF nanocomposite is similar to those prepared by depositing a conductive nanomaterial layer onto a PDMS substrate.32-33 Electrons can tunnel through a polymer matrix if the distance between the adjacent CNFs is less than the tunneling distance (of the order 5–50 nm).34 During stretching, the PDMS experiences a much larger deformation in the direction of stretching and the CNFs may slide past each other and move apart, leading to enlarged gaps. The resistance, R, is thus increased in the stretching direction due to an increase in the tunneling resistance. A further increase in the applied strain may result in total loss of contact, leading to a further increase in the resistance. During the initial stretching, the buckling of the CNFs may occur due to the Poisson’s effect in the transverse direction of stretching. Some of the CNFs may therefore buckle out of the stretching plane and fracture (Figures 5a-b).5, 29, 35 Thus, the resistance may not be completely recovered after releasing the strain back to 0%. After the initial few cycles of loading-unloading, the rate of buckling and breaking of CNFs would be expected to diminish and the piezoresistivity would now arise from the reversible movements of the inter-CNFs gaps.

By contrast, microcracks are observed in the CNF layer in the dc-PDMS/CNF nanocomposites after multiple cyclic loading and unloading (Figures 5c-d), which is believed to play an important role in the piezoresistive behaviour of these nanocomposites. Before stretching, as shown in Figure 2e, a densely packed layer of CNFs, containing no microcracks, is lying on the surface of the PDMS pore walls. After stretching, microcracks appear, leading to an increase in the resistance. Upon unloading, the PDMS substrate relaxes, causing the shrinkage, or even complete closure, of these microcracks. During subsequent cycling, the reversible


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opening/closing of these microcracks causes the resistance to increase and decrease. The plasma treatment was employed to introduce some functional group (oxygen-containing groups) to enhance the adhesion between the CNFs and the PDMS substrate. However, the adhesion is likely to remain relatively weak and some CNFs are detached from the surface of the PDMS in the dc-PDMS/CNF nanocomposite after repeated stretching (as indicated by the dashed oval in Figure 5d), which limits the stress transfer from the PDMS to the CNFs. Therefore, the relative resistance change in such nanocomposites shows a continuous decline up to 10,000 cycles. The changes in the microstructure of these two PDMS/CNF nanocomposites after cyclic tension are compared and illustrated schematically in Figure 5e.



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Figure 5. Typical SEM images for (a) and (b) the p-PDMS/CNF and (c) and (d) the dcPDMS/CNF nanocomposites after being stretched between 0% and 30% for 200 cycles; (e) schematic illustration of the morphology changes after cyclic loading. (Arrows in (a) and (b) indicate the buckling, fracture, and detachment of the CNFs in the p-PDMS/CNF while those in (c) and (d) indicate the microcracks in the CNFs layer on the pore wall surface of the dcPDMS/CNF. 18 ACS Paragon Plus Environment

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3.3 Piezoresistivity of the p-PDMS/CNF Nanocomposites and its dependence on the CNF Content As the above results show, the p-PDMS/CNF nanocomposites exhibit a superior durability, and hence stability, compared to the dc-PDMS/CNF nanocomposites. These properties are key characteristics and requirements of effective and useful flexible strain-sensors and conductors. In the present section, the piezoresistivity of the p-PDMS/CNF nanocomposites will be discussed in more detail. Figure 6a shows the voltage, V, versus current, I, characteristics of a p-PDMS/CNF nanocomposite, containing 0.3 wt% of CNFs, at different applied strains. It can be seen that the slope of the V versus I curves increases with the applied strain, indicating that the electrical resistance depends on the applied strain. Figure 6b presents the relative resistance change, ∆R/R0, over ten cycles, as the maximum applied strain is increased from 10% to 50%, with the minimum applied strain always being 0%. The relative resistance change, ∆R/R0, versus strain with a maximum strain of 10%, 30%, and 50% pertinent to the first and tenth cycle of Figure 6b, are presented in Figure 6c. It may be seen that the value of ΔR/R0 increases by ~ 60% to 445% when the maximum applied strain is increased from 10% to 50%. Moreover, there is no significant difference in the relationships between the relative resistance change, ∆R/R0, versus strain for the first and tenth cycle, indicating excellent durability of the p-PDMS/CNF nanocomposite in this respect.



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50%

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10% 0.1

0.2 0.3 Strain (ε)

0.4

0.5

Figure 6. (a) The voltage, V, versus current, I, plot of a p-PDMS/CNF nanocomposite (0.3 wt% CNFs) under different levels of applied tensile strain; (b) the relative resistance change, ∆R/R0, during 10 cycles of loading-unloading and (c) the relative resistance change, ∆R/R0, versus strain (for both loading and unloading) for the first and tenth cycle in (b). (The maximum applied strain is indicated with the minimum applied strain always being 0%)

It is widely recognized that the gauge factor of nanocomposite strain-sensors is affected by the number of conductive pathways,36 which can be altered by changing the concentration of CNFs in the nanocomposite. Figure 7a presents the electrical conductivity of the p-PDMS/CNF nanocomposites containing different concentrations of CNFs. Upon increasing the concentration


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of CNFs from 0.1 to 2.8 wt%, the electrical conductivity increases continuously from 0.01 to 4.2 S/m. At a low concentration of CNFs, a sparse conductive network is formed, leading to a low electrical conductivity. Upon increasing the CNF content, a much denser network of CNFs is formed, resulting in an increased electrical conductivity. The tensile strain-stress curves of the pPDMS/CNF nanocomposites containing various contents of CNFs are given in Figure S1. It is seen that the failure strain varies from 72% to 94%, with an initial increase upon increasing the CNF concentration up to 0.7 wt% but a decrease is observed thereafter (Figure 7b). In contrast, the modulus and tensile strength increase continuously upon increasing the concentration of CNFs. Similar results have been recently reported for porous PDMS/CNT nanocomposites.37 The enhanced modulus and tensile strength properties may be attributed to the reinforcing effects of CNFs or CNTs with relatively high modulus and strength. However, an excessive concentration of CNFs or CNTs, reduces the failure strain, which may result from an effective increase in the degree of crosslinking of the PDMS due to the increased concentration of CNFs or CNTs.

Figure 7c presents the representative relative resistance change, ∆R/R0, under quasi-static loading for the p-PDMS/CNF nanocomposites containing various concentrations of CNFs. It should be noted that, regardless of the content of CNFs, the piezoresistive response of the nanocomposites is characterized by a linear increase up to a certain applied strain and an exponential increase thereafter. Figure 7d shows a plot of ln(R/R0) versus ln(L/L0), see eq. (1), and by fitting the data in the linear range the value of GFld can be calculated. The average value of GFld increases from 1.0 to 6.5, upon decreasing the CNF content from 2.8 to 0.1 wt%, as shown in Figure 7a. The scatter in the measured values of the gauge factor, indicated as the error



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bars in Figure 7a, was obtained by measuring at least three specimens for each type of sensor. As an example, Figure S2 presents the variation of the performance of three specimens with a concentration of 0.1 wt% CNFs. The variation in the gauge factor may be mainly ascribed to slight variations in the microstructures of the nanocomposites, resulted from the inhomogeneities in the size and shape of the sugar particles (i.e. the templates) and the coating of the CNFs on the sugar-particle surfaces.


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4 2 0 0

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4 6 8 10 Stretching cycles Figure 7. (a) The electrical conductivity and gauge factor, GFld, (b) failure strain, (c) and (d) the

resistance change versus strain under quasi-static loading, and (e) relative resistance change, ∆R/R0, over 10 cycles of loading-unloading (with a cyclic maximum applied strain of 40% and a minimum applied strain of 0%) of the p-PDMS/CNF nanocomposites containing different concentrations of CNFs. 23 ACS Paragon Plus Environment


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The nanocomposites with different concentrations of CNFs were also subjected to cyclic loading-unloading cycles between 0% to 40% strain. Figure 7e shows the relative resistance change during the first ten cycles. For the p-PDMS/CNF nanocomposites containing 0.1, 0.3, 0.5, 0.7, 1.4 and 2.8 wt% of CNFs, the relative resistance changes, ∆R/R0, were 620%, 320%, 229%, 182%, 98%, and 46%, respectively, at the maximum applied strain of 40%. The relative resistance change exhibited a slight reduction during the first few cycles but then stabilized quickly, indicating an excellent stability.

The nanocomposite with the highest sensitivity, which contained 0.1 wt% of CNFs, was further subjected to cyclic loading under various cyclic maximum applied strains from 10% to 50% (Figure 8). Figure 8a shows the relative resistance change in the first ten cycles while Figure 8b shows the relative resistance change, ∆R/R, for the loading and unloading curves pertinent to the first cycle. The value of ∆R/R0 increases from 62% to 1115% upon increasing the applied strain from 10% to 50%. Also, a maximum strain of 40% was applied to the sensor at a ramping rate of 40 mm/s (i.e. 260 %/s) and then maintained for 30 s to investigate the response and relaxation properties. (It should be noted that 40 mm/s is the highest ramping rate that can be achieved by the equipment employed.). Figure 8c shows the relative resistance changes and strain in three cycles and Figure 8d indicates the response and relaxation time of the sensors. The response time was estimated to be around 0.160 s, representing a latency of 0.006 s when considering the time required for the sensor to ramp to 40% strain (0.154 s). This indicates a very rapid response time for the p-PDMS/CNF nanocomposite. However, inherent creep behavior was observed with a sharp overshoot and a relatively long relaxation time (i.e. 3.982 s with a latency of 3.828 s) to reach a value within 10% of the baseline value. This may be attributed to the


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viscoelasticity of the PDMS.38 However, this relaxation behavior is markedly faster than the 100s recovery time observed for thermoplastic elastomer/carbon black composites39 and the 50s recovery time for sensors based on single-walled carbon nanotube films in a PDMS matrix.38



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Figure 8. Response of the p-PDMS/CNF nanocomposite (0.1 wt% CNFs) being cycled to various levels of maximum applied strain, with the minimum applied strain always being 0%: (a) the relative resistance change under 10 cycles of loading-unloading and (b) the relative resistance change versus strain curves pertinent to the first cycle in (a); (c) and (d) the response and relaxation properties of the p-PDMS/CNF nanocomposite (0.1 wt% CNFs). (Note: (d) is the detail of the response and relaxation times taken from the third cycle in (c).)


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3.4 Potential Applications It is noteworthy that the strain sensors developed in the present work demonstrate tunable sensitivity (ranges from 1.0 to 6.5) and stable piezoresistive performance (as indicated in Figure S3) with good linearity in the relationship between ln(R/R0) and ln(L/L0) up to ~ 70% strain. Compared to the recently reported stretchable strain sensors listed in Table S1 and the review paper by Amjadi et al.,40 it is seen that the newly-developed nanocomposites are very promising for the application as stretchable electronics. More importantly, the use of CNFs-coated sugar particles as templates is a novel, simple, and cost-effective strategy, which can be easily scaled up. The high sensitivity and stretchability of porous PDMS/CNF sensors offer great potential for a wide range of novel applications. Two such examples are presented below.

3.4.1 Strain-sensors for Detecting Movements in Human Joints To demonstrate the potential applications in wearable devices for detecting and monitoring the movement of human joints (e.g. the bending of fingers), the p-PDMS/CNF nanocomposite containing the lowest content of CNFs (i.e. 0.1 wt%), which gives the highest piezoresistive sensitivity (with GFld of 6.5), was employed to characterize human joint movements. The flexible strain-sensor was attached onto the index finger of a glove by bonding the two ends with an adhesive. Five repeated bending/relaxation cycles (with the bending angle from 0 to 45° and then 0 to 90°, followed by 0 to 45° again) were applied to the index finger. Figure 9a shows the detected relative resistance change during the bending motion of the finger. It is clearly seen that the bending and relaxing of the finger can be detected by the increase and decrease of the relative resistance change and the degree of bending can be quantitatively monitored by the change in the strain-sensor’s resistance. By comparing the response of the strain-sensor to different applied



strains (Figures 8a-b) with the data from bending of the finger, the strain from bending the finger can be estimated (Figure 9b). For instance, when bent to 45° and 90°, strains of approximately 12.8% and 42.5% are generated. These values are in very good agreement with the estimates from a photographic analysis and the values reported in the literature.2, 34 Thus, this p-PDMS/CNF nanocomposite is a very promising wearable stretchable strain-sensor.

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Ι ⇔ ΙΙΙ

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Figure 9. (a) Response of a p-PDMS/CNF nanocomposite strain-sensor (0.1 wt% CNFs) under five repeated bending/relaxation cycles when attached to an index finger (the insert illustrates the bending of the index finger to different angles); (b) the estimated strain based on the relative resistance change.

3.4.2 Stretchable Conductive Materials Maintaining high electrical conductivity under large deformations is essential in the development of wearable electronics. This requires that electrical conductors show a low sensitivity to an applied strain, contrary to the requirements for strain-sensors. Herein, the p-PDMS/CNF nanocomposite containing 2.8 wt% of CNFs was selected for further study as a stretchable


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conductor, since it possesses the lowest gauge factor amongst the nanocomposites investigated with a GFld of ~1.04. A simple circuit was constructed to connect an LED with the p-PDMS/CNF nanocomposite and a Keithley source meter with a stabilized output of 10 V. Figure 10a shows a photograph of the illuminated LED before applying any strain. It is readily observed from Figure 10b that the LED did not show any noticeable brightness degradation, even when the nanocomposite was stretched to ~50% strain. For this p-PDMS/CNF nanocomposite containing 2.8 wt% of CNFs, its initial electrical resistance was 500 Ω, which was increased by 53.2% to 766 Ω at 50% strain. Thus, when this conductor undergoes a relatively large deformation of 50%, it is capable of maintaining the required potential drop to activate the LED light. It should be noted that the conductivity of the p-PDMS/CNF nanocomposites is relatively low compared to metallic and some previously reported polymer nanocomposites-based stretchable conductors.18, 29, 41

However, the operating voltage needed for this simple demonstration was not unduly high

and future work will study further increasing the electrical conductivity by increasing the concentration of the CNFs employed and/or by introducing more conductive materials such as silver (or gold) nanoparticles or nanowires.

Figure 10. Photograph of an illuminated LED in a circuit connected using a p-PDMS/CNF (2.8 wt% CNFs) conductor at (a) 0% and (b) 50% strain. 29 ACS Paragon Plus Environment


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4. CONCLUSIONS Stretchable porous polydimethylsiloxane/carbon nanofibre (denoted by p-PDMS/CNF) composites have been developed via a new, cost-effective method using CNF-coated sugar particles as the templates. The successful transfer of the CNFs from the sugar surface into the PDMS, and subsequent removal of the sugar particles, yielded porous structures with the CNFs embedded within the pore walls of the PDMS. The interconnected CNF network provided electrically conductive paths that, when subjected to mechanical deformation, gave rise to a change in the electrical resistance. These new p-PDMS/CNF nanocomposites exhibit more stable piezoresistive behaviour than those prepared by a conventional dip-coating method (termed dcPDMS/CNF nanocomposites). These observations have been explained by demonstrating that the piezoresistivity of the p-PDMS/CNF nanocomposites arises from the variation of the tunneling resistance and contact resistance, whilst that of the dc-PDMS/CNF nanocomposites is largely due to the formation of microcracks in the CNF layer.

The electrical conductivity and piezoresistive sensitivity of these p-PDMS/CNF nanocomposites can be tuned by simply varying the content of the CNFs. Upon decreasing the concentration of CNFs from 2.8 to 0.1 wt%, the electrical conductivity of the nanocomposite decreased continuously from 4.2 to 0.01 S/m, while the gauge factor, GFld, increased from 1.0 to 6.5. It was also found that the p-PDMS/CNF nanocomposites exhibited stable piezoresistive performance with a fast response time and a good linearity in terms of ln(R/R0) versus ln(L/L0) up to ~ 70% strain. The p-PDMS/CNF nanocomposites have been demonstrated to be effective as strainsensors for the quantitative monitoring of the bending of a human finger and as stretchable conductors. It is believed that such a simple and cost-effective fabrication protocol provides new


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insights into the design of 3D, highly-elastic conductive polymer nanocomposites for use as flexible and stretchable strain-sensors and conductors for future wearable electronics.

ASSOCIATED CONTENT Supporting Information. Additional figures and table include: 1) The tensile stress-versus strain properties of pPDMS/CNF nanocomposites containing different concentrations of CNFs; 2) The relative resistance change from replicate specimens of the same p-PDMS/CNF nanocomposite (0.1 wt% of CNFs); 3) The relative resistance change of p-PDMS/CNF nanocomposites (0.3 wt% of CNFs) over 10 cycles of loading-unloading (with a cyclic maximum applied strain of 70% and a minimum applied strain of 0%); 4) A table summarizing the sensing performance of recently reported strain sensors.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest.



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ACKNOWLEDGMENTS The authors are grateful for the financial support received from the Australian Research Council’s Discovery Grant (DP140100778).

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Carbon Nanofiber

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