PDMS Nanocomposites

PDMS Nanocomposites

Available online at www.sciencedirect.com ScienceDirect Materials Today: Proceedings 17 (2019) 616–622 www.materialstoday.com/proceedings RAMM 2018...

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Available online at www.sciencedirect.com

ScienceDirect Materials Today: Proceedings 17 (2019) 616–622

www.materialstoday.com/proceedings

RAMM 2018

Properties of Stretchable and Flexible Strain Sensor Based on Silver/PDMS Nanocomposites S. Han Mina, A.M. Asrulnizamb, M. Atsunoric and M. Mariattid * a,d

School of Materials and Mineral Resources Engineering, Universiti Sains Malaysia Engineering Campus, 14300 Nibong Tebal, Pulau Pinang, Malaysia

b

Collaborative Microelectronic Design Excellence Centre (CEDEC), [email protected], Level 1, Block C, No. 10, Persiaran Bukit Jambul, 11900 Bayan Lepas, Pulau Pinang, Malaysia. c

Department of Electrical and Electronic Informaiton Engineering, Toyohashi University of Technology, Aichi Toyohashi 441-8580, Japan

Abstract Stretchable and flexible strain sensors have attracted considerable attention due to their potential applications in wearable electronics. A flexible base is an important part of flexible electronics because it provides flexible support and it functions to transfer and process mechanical and electrical signals. In this study, stretchable strain sensor is fabricated by silver nanoparticle (AgNPs) filled polydimethylsiloxane (PDMS). Amount of AgNPs in PDMS was varied at 0.10, 0.15, 0.20, 0.25 and 0.30 wt% and are labeled as Sensor 1 to 5, respectively. The performance of the stretchable sensor was observed using current-voltage characteristics and strain sensing ability. The morphology of the sensor was analyzed using Scanning Electron Microscope. The tensile strain of Sensor 1 is 110% with the gauge factor (GF) value of 268.4 and Sensor 5 with tensile strain of 130% and GF value of 109.4. The strain sensors with high GF obtained in this work conformed to the development of flexible electronic. © 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the scientific committee of the 6th International Conference on Recent Advances in Materials, Minerals & Environment (RAMM) 2018. Keywords: Silver nanoparticle; Nanocomposite; Stretchable strain sensor

* Corresponding author. Tel.: +0-604-599-5262; fax: +0-604-594-1011. E-mail address: [email protected]

2214-7853 © 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the scientific committee of the 6th International Conference on Recent Advances in Materials, Minerals & Environment (RAMM) 2018.

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1. Introduction There are various conductive materials such as metal based and carbon-based materials which can be applied for electronic applications such as strain sensor. Stretchable strain sensors can be fabricated by using low-dimensional carbons (e.g., carbon blacks (CBs), carbon nanotubes (CNTs), and graphene), Ag nanowires (NWs), Ag nanoparticles (NPs), and their hybrid micro/nanostructures as conductive traces [1]. Silicone-based elastomers (e.g., Ecoflex, polydimethylsiloxane (PDMS) and Dragon Skin) and rubbers (e.g., natural rubber and thermoplastic elastomers (TPEs)) are the most commonly used polymers as flexible support materials of the soft strain sensors [2]. AgNPs-based stretchable strain sensor provides a very efficient and highly economic approach to enable the human motion, force and pressure measurement devices for the next-generation flexible and wearable electronic systems [3]. Polydimethylsiloxane (PDMS) has stable chemical properties, biological compatibility, transparency, and good thermal stability and it can be easily adhered to the surface of electronic materials. Thus, PDMS is widely used as a flexible substrate for wearable resistive strain sensors [4]. The important performance of flexible and stretchable sensor can be characterized by different parameters such as stretchability, sensitivity or gauge factor, linearity, hysteresis, durability, response and recovery time, long time stability and electrical conductivity, etc. Strain sensor respond to the applied strain with different mechanisms, depending on the type of materials, micro/nano-structure and fabrication process [5]. Wearable electronic technology integrates electronic devices into clothing, accessories, human skin and is even implanted in vivo and realizes measurements of body sensing, data storage, and mobile computing [6]. Stretchable and flexible strain sensors have attracted considerable attention for their potential applications in wearable electronics, smart textiles, soft robotics, and structural health monitoring in recent few years [7]. Strain sensors respond to mechanical deformation by the change of electrical characteristics such as resistance or capacitance. Stretchable strain sensors were fabricated through different processes including filtration method, printing technology, transferring and micromolding methods, coating techniques, liquid phase mixing, and chemical synthesis method [8]. In the present study, stretchable and flexible strain sensors were fabricated by embedding the AgNPs conducting filler into soft elastomeric materials which is PDMS. AgNPs solution were prepared by adding different concentration of AgNPs powder into the colloids to obtain five different weight percentage of AgNPs concentration (0.1 - 0.3wt%). PDMS flexible base sensor was prepared by these five different types of AgNPs solutions. The AgNPs/PDMS sensors were characterized based on electrical conductivity, stretch/ release response under static and dynamic loads, stretchability, hysteresis performance and long-term stability. 1. Materials and Methods 1.1. Materials The silver nanoparticle powder (AgNPs < 100nm size), poly (vinyl pyrrolidone) (PVP average Mw =55000) and silicon-based elastomer polydimethylsiloxane (PDMS) type (Sylgard 182, Dow Corning USA) used in this study were purchased from Sigma-Aldrich Sdn. Bhd. Ethylene glycol (99.9%), anhydrous ethanol (> 99.8%) and isopropanol (99.8%) were purchased from Merck. All reagents were used for the synthesis of AgNPs solution without further purification. The PDMS was supplied in liquid form which consisted of Part-A and Part-B (Part-A : Base and Part-B : Curing Agent). 1.2. Synthesis of Silver Nanoparticles (AgNPs) solution AgNPs solution were prepared by modified polyol method. In this method, 50 mL of ethylene glycol was stirred at 160C for 1 h. Ethylene glycol was used as a solvent. 5 mL of 0.018 M polyvinylpyrrolidone (PVP) in ethylene glycol was added into the mixture and another heating for 30 min. PVP was used for stable complex with silver cations and also act as a stabilizer for AgNPs. After that, 5 mL of 0.185 M of AgNPs in ethylene glycol was slowly poured into the solution. Then, the solution was heated for another 2 h and quenched at 5C in a freezer to stop the reaction. After the solution was cooled, a large amount of anhydrous ethanol (5:1) was added into the solution. The

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solution was centrifuged at 3000 rpm for 5 min and washed three times with anhydrous ethanol to remove the excess PVP and EG. AgNPs solution were stored in isopropanol (IPA) for used in the fabrication process. AgNPs with concentration of 0.1 wt% was used for sensor-1, 0.15 wt% for sensor-2, 0.2 wt% for sensor-3, 0.25 wt% for sensor-4 and 0.3 wt% for sensor-5. 1.3. Fabrication of the strain sensors The strain sensor was fabricated by the following procedure: AgNPs suspensions was first drop cast onto a glass slide that was previously cleaned with acetone, ethanol, and DI water and patterned with a polyimide tape (with ∼70 × 3 mm2 rectangular pattern size). Fig. 1(a) and 1(b) showed photograph of the simple layer strain sensor and a schematic diagram of the fabricated sensors. The uniformity of AgNPs thin film is an important factor for a stable and predictable response of the sensor. After drop-casting of AgNPs solution, the glass slide was exposed to room temperature to dry the AgNPs suspension. After the solution dried, polyimide tapes were removed from the glass slide and the patterned AgNPs thin film was annealed at 160°C for 20 min. The thermal annealing is used to increase the electrical conductivity of AgNPs thin films and remove the PVP surfactant from the film. The simple structured samples were fabricated by pouring the liquid PDMS with an approximate thickness of 0.5 mm layer on the AgNPs thin film pattern and cured at 70C for 2 h. After that the cured PDMS film was peeled from the glass slide, it was characterized with mechanical and electrical tests. (a)

(b)

(c)

(d)

2 mm

10 µm

Fig. 1. (a) Photograph of simple-structured AgNPs/PDMS nanocomposite simple-structured strain; (b) sensor fabrication process of the simplestructured AgNPs/PDMS strains sensors; (c) Cross-sectional SEM image of the simple-structured strain sensor; and (d) SEM image of the surface of AgNPs embedded onto PDMS.

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1.4. Characterization of composition and Morphologies of the products The morphology and structure of as-prepared samples were characterized by using Scanning Electron Microscope (SEM). The electrical properties of samples were characterized by PRS-812 Resistance meter. Five samples were used in the measurement of electrical conductivity, and the average value was reported in the present study. The thickness of the sample’s was measured by Vanier clipper. Tensile properties of AgNPs strain sensors were evaluated using an Instron 3366 with 10 kN load cells. In order to test the strain-sensing characteristics, two ends of the samples were attached to the two grids of the machine and test with a crosshead speed of 1mm min-1. The uniform strain/ release cycles were applied to samples for 10 cycles at various strains. Dimension of the asprepared samples was 25 × 70 mm2. The relative change resistance of samples under strain testing were recorded with FLUKE 115 digital multimeter. The data are presented in the form of relative change resistance ∕ and applied strain effect of gauge factor (GF). The difference in the relative change resistance ratio between the stretching and releasing process is also an important factor to evaluate the performance of the strain sensors. In order to investigate the sensitivity of the strain sensor, the gauge factor GF of the composite was calculated by equation 1: ∕ = (1) ∕

Where, Ro is the initial resistance of the sensors, R is the relative resistance change under the deformation, Lo is the initial length of the sensor, and Lis the relative elongation of the axial specimen, respectively. 2. Results and Discussions The performance of five sensors were investigated through (1) sensor response for one cycle, (2) sensor response for multiple cycles and (3) sensor response under different strain range from 0% to 50 % with an increase step of 0.5% (set period is 1 mm/min and displacement rate is 1% for 0.3mm for each step). Fig. 1(c) demonstrates the cross-sectional SEM image of the sample. When the liquid PDMS is poured onto the AgNPs film, the liquid penetrates into the AgNPs network. When the PDMS film cured, it becomes highly cross-linked with embedded AgNPs. The sheet resistance of AgNPs/PDMS film was found increased because of the presence of PDMS. Fig. 1(d) shows the SEM image of AgNPs embedded into the PDMS surface. No significant voids were observed based on the morphology observation. The electrical conductivity values at different AgNPs concentration are summarized in Fig. 2(a). As expected, composites with different AgNPs display different electrical conducting behavior. Incorporation of AgNPs from 0.1 to 0.3 wt% increased the electrical conductivity almost five order from 0.044 to 0.16 S/m. The current-voltage characteristics of a simple-structured strain sensor under different strain ranges are shown in Fig. 2(b). The sensor exhibits an ohmic behavior regardless of applied strain and the current move decreased with increasing of the tensile strain. The response of the simple-structured sample under dynamic load is demonstrated in Fig. 3. Based on Fig. 3(a) the resistance was fully recovered for stretch/release cycles with maximum strain of ε = 2%. This shows the outstanding stretchability of prepared simple-structured strain sensors. Hysteresis becomes important when the strain sensors are under dynamic load. In this case, the sensor can be used for skin mountable wearable electronics application [2]. Large hysteresis behavior leads to the irreversible sensing performance sensor upon dynamic load. All of prepared sensors exhibit large hysteresis above 2% strain. Fig. 3(b) demonstrated the large hysteresis behavior of simple-structured under strain 50%. It can be seen that the relative change resistance increased almost linearly from (0.05 to 0.98) when the tensile strain increased to 50%. Upon release of the strain, the relative change resistance partially recovered and decreased to the relative change resistance of 0.24. At low strain ( ≤ 2%), the detachment of AgNPs from PDMS is negligible hence produce hysteresis curve. However, at 50% strain ( = 50%), the detachment of AgNPs is obvious and result in decrease AgNPs contacts hence increase in resistivity. The fracture of AgNPs simple structured sample decrease the number of electrical pathways and therefore the electrical resistance of the AgNPs thin film increases irreversibly [9].

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(b)

Fig. 2. Electrical response of the simple-structured AgNPs-PDMS nanocomposite strain sensors: (a) Conductivity of AgNPs-PDMS nanocomposite with varying wt% of AgNPs; (b) Current-voltage curves of the strain sensors at different level of strains.

(a)

(b)

Fig. 3. (a) Hysteresis curve of the AgNPs-PDMS composite simple-structure strain sensor different strain; (b) Non-hysteresis curve of the AgNPs-PDMS simple-structured strain sensor under 50% strain

The experiment result showed that the prepared sensors can be operated as high-performance strain because of their excellent stretchability with resistance linearity and negligible hysteresis was reported by Amjadi et al. [8]. The AgNPs at 0.1 wt% results in high initial resistance of the strain sensor (Ro  48) due to small amount of AgNPs used, and the GF is 8.2. However, for 0.3 wt% of AgNPs sensor, small initial resistance is observed (Ro  13.3). This causes the reduction in GF= 4.3. Table 1 shows the comparison data for the performance of simple-structured sensors. Linearity is an important parameter for stretchable strain sensors because very large strain should be accommodated by strain sensors. Nonlinearity of sensors makes the calibration process complex and difficult. Nonlinearity is the one main drawbacks of almost resistive-type strain sensors reported by Hwang et al. [10]. From the table, the GF value of sensor-1 is not stable under 15%. At early stretching part AgNPs made connection between particles and the composite film ends up having good conductivity. When the composite film was further stretched to a certain extent, AgNPs were unable to connect to each other, and the original conductive network gets destroyed. Due to the change of the internal structure, the variables changed are uncertain. Sensor-5 is stretching from 0% to 130%, the resistance variation is as high as 294.6. It has the highest value of GF which is 109.4 for 130% tension and the lowest value is 2.17 for 10% tension. Finally, it can be concluded that AgNPs with 0.3 wt% (sensor-5) showed the high sensitivity, stretchability and linearity simultaneously.

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Table 1. Performance comparison of the highly stretchable strain sensor Sensor

Stretchability (%)

Region

Gauge factor

Sensor 1 (0.1 wt%)

110

Linearity (after 15%)

268.4

Sensor 2 (0.15 wt%)

130

Nonlinearity

460.7

Sensor 3 (0.2 wt%)

60*

Nonlinearity

364.9

Sensor 4 (0.25 wt%)

110

Nonlinearity

286

Sensor 5 (0.3 wt%)

130

Linearity

109.4

*The sample is able to stretch up to 140% but the resistance can be measured at 60% stretchability.

Fig. 4(a) shows the change in the normalized resistance plotted against the strain curve with different amounts of AgNPs used. At each amount of AgNPs, the average relative resistance of the sensors was calculated by measuring the resistance of five different sensors to confirm the reproducibility. When a greater value of maximum strain is applied, it leads to an increase in the gauge factor. This is because the effect of the decreasing percolation network becomes more crucial with fewer conductive pathways reported by Hwang et al. [11]. The strain sensors with 0.3 wt% exhibited linear behavior in the strain–resistance curve, with the gauge factors of 109.4 for 130% strain. As expected, with a smaller amount of AgNPs, a higher value of the gauge factor was obtained. When subjected to a maximum strain 130%, the sensor with a relatively small amount of AgNPs (0.1 wt%) showed linear change in resistance to 110% strain with a gauge factor of 268.4. The characterization of the repeatability and durability of the sensor for 10 cycles with various ratio strains as shown in Fig. 4(b). The sensors displayed lower repeatability and the response was more varied over the entire set of cycles. The various change of the resistance indicates an increase at the early part of the strain cycles and subsequent stabilization at the letter part. This behavior is observed in all of resistive strain sensors. This result indicates the dynamic orientation of AgNPs suspended in the polymer network [12].

(a)

(b)

Fig. 4. (a)The relative resistance changes versus strain curves of the sensor with different content of AgNPs (0.1 wt% and 0.3 wt%); (b) the resistance changes at various strain rate as a function number of cycles.

Microstructures of AgNPs in PDMS substrate are shown in Fig. 5(a). Under stretching, microcracks occurred and propagated throughout the AgNPs-PDMS nanocomposite thin films. The amount of 0.1 wt% and 0.3 wt% AgNPs thin film were stretched up to over 100% and then released to 0% as shown in Fig. 5(b) and 5(c). Microcrack appeared and propagated through the AgNPs network, results in larger and longer micro-cracks and hence increase electrical resistance at higher strains. Lee et al. [13] reported that the size of the cracks can affect the electrical characteristics of the AgNPs thin films.

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(b)

(a)

1µm

(c)

1µm

1µm

Fig. 5. (a) SEM micrograph of AgNPs embedded in the PDMS substrate under magnification of × 5 k at (a) the pristine state; AgNPs/PDMS nanocomposite under magnification of × 10 k at (b) 0.1 wt% of AgNPs-PDMS strain sensor stretched at 110% and (c) 0.3 wt% of AgNPs-PDMS strain sensor stretched at 130%.

3. Conclusions In summary, the resistive type of strain sensor based on the AgNPs-PDMS composite can be prepared using a simple fabrication process. Results showed that five AgNPs-PDMS sensors exhibit high sensitivity and stretchability. The gauge factor (GF) of Sensor-5 is 109.4, and the linearity and the stretchability reaches 130%. It is found that the AgNPs/PDMS strain sensors can also be used in the low strain region with less than 2%. It means the strain sensors showed successful detection of bio signals such as a wrist pulse, and the bending of elbows and fingers; thus, demonstrating its potential application as a wearable bio-sensor. Acknowledgements The authors would like to express appreciation for the financial support by AUN/SEED-Net grant number: 6050391 and JICA as well as support from the School of Materials and Mineral Resources Engineering Campus, Universiti Sains Malaysia. References [1] S. Gong, D.T. Lai, Y. Wang, L.W. Yap, K.J. Si, Q. Shi, N.N. Jason, T. Sridhar, H. Uddin, W. Cheng, ACS Appli. Mater & interfaces, 7 (2015) 19700-19708. [2] M. Amjadi, K.U. Kyung, I. Park, M. Sitti, Advanced Funct. Mater, 26 (2016) 1678-1698. [3] J. Lee, S. Kim, J. Lee, D. Yang, B.C. Park, S. Ryu, I. Park, Nanoscale, 6 (2014) 11932-11939. [4] J. Chen, J. Zheng, Q. Gao, J. Zhang, J. Zhang, O.M. Omisore, L. Wang, H. Li, Appli. Sci, 8 (2018) 345. [5] M. Filippidou, E. Tegou, V. Tsouti, S. Chatzandroulis, Micro. Eng, 142 (2015) 7-11. [6] K.K. Kim, S. Hong, H.M. Cho, J. Lee, Y.D. Suh, J. Ham, S.H. Ko, Nano. Lett, 15 (2015) 5240-5247. [7] S. Zhang, H. Zhang, G. Yao, F. Liao, M. Gao, Z. Huang, K. Li, Y. Lin, J. Alloys. Compd, 652 (2015) 48-54. [8] M. Amjadi, A. Pichitpajongkit, S. Lee, S. Ryu, I. Park, ACS nano, 8 (2014) 5154-5163. [9] F. Xu, Y. Zhu, Advanced materials, 24 (2012) 5117-5122. [10] B.-U.Hwang, J.-H. Lee, T.Q. Trung, E. Roh, D.-I. Kim, S.-W. Kim, N.-E. Lee, ACS nano, 9 (2015) 8801-8810. [11] M. Hempel, D. Nezich, J. Kong, M. Hofmann, Nano. Lett, 12 (2012) 5714-5718. [12] J. Shintake, E. Piskarev, S.H. Jeong, D. Floreano, Mater. Sci. Forum, 3 (2018) 1700284. [13] C.-J. Lee, K.H. Park, C.J. Han, M.S. Oh, B. You, Y.-S. Kim, J.-W. Kim, Sci. Rep-UK, 7 (2017) 7959.