poly(dimethylsiloxane) composites for use in finger-sensing piezoresistive pressure sensors

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Poly(3-hexylthiophene) wrapped carbon nanotube/ poly(dimethylsiloxane) composites for use in finger-sensing piezoresistive pressure sensors Jihun Hwang a, Jaeyoung Jang a, Kipyo Hong a, Kun Nyun Kim b, Jong Hun Han c, Kwonwoo Shin c, Chan Eon Park a,* a

POSTECH Organic Electronics Laboratory, Polymer Research Institute, Department of Chemical Engineering, Pohang University of Science and Technology, Pohang 790-784, South Korea b Convergence Sensor Research Center, Korea Electronics Technology Institute (KETI), Seongnam 463-816, South Korea c Energy Nano Materials Research Center, Korea Electronics Technology Institute (KETI), Seongnam 463-816, South Korea

A R T I C L E I N F O

A B S T R A C T

Article history:

We fabricated a piezoresistive composite using multi-walled carbon nanotubes (MWCNTs)

Received 28 May 2010

as a conductive filler and polydimethylsiloxane (PDMS) as a polymer matrix, which oper-

Accepted 26 August 2010

ated in the extremely small pressure range required for finger-sensing. To achieve a homo-

Available online 19 September 2010

geneous dispersion of MWCNTs in PDMS, the MWCNTs were modified by a polymer wrapping method using poly(3-hexylthiophene) (P3HT). The percolation threshold of the composites was significantly lowered by the presence of P3HT. The electrical conductivity and piezoresistive sensitivity of the composite were found to strongly depend on the P3HT concentration. The well-dispersed P3HT-MWCNT/PDMS composite showed good piezoresistive characteristics in the 0–0.12 MPa pressure range.  2010 Elsevier Ltd. All rights reserved.

1.

Introduction

Electrically conductive polymer composites (ECPCs), consisting of a conductive filler and an insulating polymer, have received considerable attention due to their unique mechanical and electrical properties [1–3]. Recent studies have shown that the resistance of ECPCs depends on the external pressure, a phenomenon known as the piezoresistive effect [4–6]. The piezoresistivity of ECPCs, as well as their mechanical flexibility and ease of processing, make these composites promising candidates for use in cheap, flexible, and large area pressure sensors [7,8]. Among the conductive fillers considered to date, carbon nanotubes (CNTs) have been widely investigated because of their excellent electrical, mechanical, and thermal properties [9,10]. For example, the current density of individual metallic single-walled CNTs is

about 4 · 109 A/cm2, which is 1000 times higher than that of copper wires [11]. The conduction mechanism in ECPCs can be explained in terms of a conductive network of filler in a polymer matrix, where the critical filler concentration for network formation is defined as the percolation threshold [12]. To date, most studies on ECPCs have sought to lower the percolation threshold because a high filler concentration raises the product cost and changes the mechanical properties of the polymer matrix; for example the viscosity and storage modulus increase with increasing filler concentration [13]. The extremely high aspect ratio (AR) of CNTs means that they can form a conductive network in a polymer matrix with a much lower percolation threshold than is the case for other conductive fillers such as carbon black and metallic nanoparticles, which have much lower ARs [14,15]. Despite their advantages, CNTs easily aggregate and become entangled due to the

* Corresponding author: Fax: +82 54 279 8298. E-mail address: [email protected] (C.E. Park). 0008-6223/$ - see front matter  2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2010.08.048

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intermolecular van der Waals (VDW) forces between them, making it difficult to form a homogeneous dispersion of CNTs in a polymer matrix. This negative characteristic has been an obstacle to the wide use of CNTs in ECPCs. The percolation threshold of ECPCs is influenced by many factors: filler particle shape, polymer matrix viscosity, degree of filler–matrix interaction, and so on [4,9]. Among these, achieving a homogeneous dispersion of the conductive filler in the polymer matrix is crucial to lowering the percolation threshold [4]. In previous efforts to homogeneously disperse CNTs as a conductive filler in polymer matrices, many groups have covalently functionalized the CNT surface with –CH3 or – NH2 [16,17]. In most cases, however, such covalent functionalization changes the structure of the CNTs, thereby degrading their unique electrical and mechanical properties. Furthermore, functionalization causes shortening of the CNTs, which reduces the advantage of CNTs in regard to AR [18]. Therefore, the use of noncovalent functionalization, which maintains the CNT structure, has been proposed as a key strategy to achieve well-dispersed CNTs in polymer matrices with a low percolation threshold [19–21]. In the present study, we fabricated ECPCs using poly(3-hexylthiophene) (P3HT)-wrapped multi-walled carbon nanotubes (MWCNTs) as a conductive filler with a polydimethylsiloxane (PDMS) matrix without covalent functionalization of the MWCNTs.

2.

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sure equipment in combination with DC current–voltage measurements (Keithley 2500 and 236 source/measure units).

3.

Results and discussion

Since the P3HT backbones were bound to the MWCNT surfaces by p–p interactions, the MWCNTs did not aggregate in organic solvents, as shown in Fig. 1(a) and (b). We observed the P3HT-wrapped MWCNTs using high resolution scanning transmission electron microscopy (HR-STEM, JEOL JEM2100F) (Fig. 1(c)), which showed that the MWCNT surface was coated with layers of another material. Energy dispersive X-ray spectroscopy (EDS) analysis (Fig. 1(d)) revealed that sulfur atoms were present in the coating layer, implying that the MWCNT surface is wrapped by P3HT. The morphologies of fractured surfaces of the MWCNT/ PDMS composites were examined using high resolution field-emission scanning electron microscopy (HR-SEM, Hitachi S-4800). Comparison of SEM images of the pristine MWCNTs (p-MWCNTs)/PDMS composite (Fig. 2(a)) and P3HTwrapped MWCNTs (P3HT-MWCNTs)/PDMS composite (Fig. 2(b)) disclosed that the P3HT enhanced the homogeneous dispersion of the MWCNTs in the PDMS matrix. Fig. 2(a) shows that the p-MWCNTs readily aggregated, giving rise to a sphere-like filler morphology. On the other hand, the P3HT-MWCNTs (1:1 ratio) were dispersed homogeneously as

Experimental

MWCNTs (purity > 95 wt%, Product No. CM-95), prepared by a chemical vapor deposition method, were purchased from Hanwha Nanotech Co. (Korea), and were used without further purification. The MWCNTs had diameters of 10–15 nm and lengths of 10–20 lm. The PDMS (Sylgard 184, supplied by Dow Corning) is a transparent and flexible polymer with good insulating and mechanical properties but without environmental toxicity, making it suitable for use as a human finger-touch material. Regioregular P3HT purchased from Aldrich (number-average molecular weight (Mn): 26 kg/mol, polydispersity index (PDI): 1.69, electrical conductivity: 103 S/cm when doped with iodine) was used without further purification as a wrapping polymer. ECPCs were fabricated using a wet mixing method. In this method, 0.5–3 wt% of MWCNTs were dispersed in chloroform and the mixture was sonicated for 30 min using an ultra-sonicator (Sonosmasher, 700 W, 50% power). In addition, solutions comprising 0.5–3 wt% P3HT in chloroform were fully stirred, and these solutions were slowly added to the MWCNT solution. The MWCNT/P3HT mixture was then sonicated for 15 min, after which PDMS was added and the mixture was carefully stirred for 12 h at 60 C. A rotary evaporator was used to evaporate the solvent from the mixture, and 10 wt% PDMS curing agent was added and the mixture was stirred for 30 min. The mixture was coated onto an indium tin oxide (ITO) patterned glass substrate using the doctor-blade coating method, followed by curing at 80 C for 3 h and then post-curing at 120 C overnight. The resulting samples were cut into 5 · 5 · 0.4 mm3 pieces, and silver paste was coated on top of the samples to enhance the electrical contact. The piezoresistive properties were measured using uniaxial dynamic pres-

Fig. 1 – Photograph of (a) pristine MWCNTs and (b) P3HTwrapped MWCNTs in chloroform after 72 h. The concentration of MWCNTs in chloroform is 1.26 mg/ml in all cases. (c) HR-TEM image of P3HT-MWCNT (1:1 ratio) and (d) EDS data of P3HT-MWCNT, which was measured in the red circle region of (c). Scale bar of (c) is 2 nm (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.).

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Fig. 2 – SEM images of fractured surfaces of (a) a p-MWCNT/PDMS composite and (b) a P3HT-MWCNT (1:1 ratio)/PDMS composite. The MWCNT concentration in the composites was 1.5 wt%.

individual tubes in the PDMS matrix (Fig. 2(b)). This can be attributed to a substantial reduction in the intermolecular VDW forces between the MWCNTs due to the presence of the P3HTs on the MWCNT surfaces. This reduction in VDW forces caused the interaction between the P3HT-MWCNT and the PDMS matrix to become stronger than that between the P3HT-MWCNTs [8]. Further evidence supporting a strong interaction between the P3HT-MWCNTs and the PDMS matrix is found in the observation that, at the fractured surface of the P3HT-MWCNT/PDMS composite (Fig. 2(b)), only a small number of MWCNTs were pulled out from the PDMS matrix. Fig. 3(a) shows the percolation behavior of the P3HT-MWCNT/PDMS and p-MWCNT/PDMS composites. The P3HT-MWCNT/PDMS composite showed a lower percolation threshold (0.75 wt%) (the middle point of the sharply increasing region) than that of the p-MWCNT/PDMS composite

(1.75 wt%). As mentioned above, the MWCNTs in the pMWCNT/PDMS composite have sphere-like shapes due to their strong entanglement and aggregation. By contrast, the MWCNTs in the P3HT-MWCNT/PDMS composite remain as individual tubes and are homogeneously dispersed. Therefore, the system with non-entangled and well-dispersed MWCNT fillers showed a lower percolation threshold, consistent with recent reports [6,8]. The electrical conductivity of the ECPCs as a function of P3HT concentration is shown in Fig. 3(b). The conductivity decreased with increasing P3HT concentration due to the increased amount of the P3HT on the MWCNT surface, which disturbed electron tunneling between MWCNTs and hence lowered the electrical conduction among MWCNTs [17]. On the other hand, in case of lower P3HT concentration than that of MWCNTs, the MWCNT aggregation arose because of a lack of P3HT to wrap the MWCNTs.

Fig. 3 – (a) Percolation behavior of p-MWCNT/PDMS and P3HT-MWCNT (1:1 ratio)/PDMS composites. (b) Electrical conductivity of P3HT-MWCNT/PDMS composites as a function of P3HT concentration.

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Piezoresistivity measurements using the configuration shown schematically in Fig. 4(a) yielded the dependence of the relative resistance (R/R0, where R is the resistance under an applied pressure and R0 is the original resistance without pressure) on the applied pressure, as shown in Fig. 4(b). As reported previously, the change in resistance under external pressure can be explained by competition between two factors: (1) formation of new conductive networks and (2) destruction of existing conductive networks [6,8]. In the ECPCs with a sphere-like filler, application of external pressure induces the formation of new conductive networks rather than the destruction of existing networks due to the sphere-like shape and the high percolation threshold. Therefore, as the external pressure on such systems is increased, the relative resistance decreases (the negative pressure coefficient effect (NPC)) [14,15]. In contrast, for ECPCs with fiberlike fillers such as MWCNTs, which have a high AR and low percolation threshold, the destruction of the existing conductive network occurs more easily, and hence the relative resistance increases on application of an external pressure (the positive pressure coefficient effect (PPC)) [6]. In our ECPCs, which showed homogeneous dispersion and a low percolation threshold due to P3HT wrapping, representative PPC characteristics were observed at extremely small pressures, over the range 0–0.12 MPa (Fig. 4(b)). Moreover, the average distance between the MWCNTs in the composite with a 3:1 ratio of P3HT:MWCNT was longer than that of the 1:1 P3HT:MWCNT composite, as shown in Fig. 4(c) and (d). In other words, as the P3HT:MWCNT ratio was varied from 1:1

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to 3:1, the conductive network is more easily disrupted by external pressure because formation of the new conductive network occurs more rarely, and hence the piezoresistive sensitivity increases remarkably as shown in Fig. 4(b). Therefore, the P3HT concentration is a key determinant of the electrical conductivity and piezoresistive sensitivity of these ECPCs.

4.

Conclusion

In conclusion, we have fabricated a piezoresistive composite using P3HT-wrapped MWCNTs as a conductive filler and PDMS as an insulating polymer matrix. The MWCNTs wrapped in P3HT were dispersed homogeneously in the PDMS, leading to a lower percolation threshold. Electrical measurements revealed that the P3HT concentration plays an important role in the electrical conductivity and piezoresistive sensitivity of the composite. The P3HT-wrapped MWCNTs/PDMS composites described here could potentially be used as the basis for cheap and flexible finger-sensing devices.

Acknowledgements This work was supported by the IT R&D Program of MKE/KEIT (2009-S-001-01, Development of Informative & Electronic Core Technology in Company Needs) and a Korea Science and Engineering Foundation (KOSEF) Grant funded by the Korean Government (MEST) (20090079630).

Fig. 4 – (a) Schematic of the experimental setup used to determine piezoresistive properties. (b) Dependence of relative resistances on the applied pressure for P3HT-MWCNT/PDMS composites with different P3HT:MWCNT ratios. SEM images of a fractured surface of (c) P3HT:MWCNT (1:1 ratio) composite and (d) P3HT:MWCNT (3:1 ratio) composite. The MWCNT concentration in the composites was 1 wt%. Scale bar is 1 lm.

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