A highly-deformable composite composed of an entangled network of electrically-conductive carbon-nanotubes embedded in elastic polyurethane

doi:10.1016/j.carbon.2012.03.008

Carbon

Volume 50, Issue 10, August 2012, Pages 3446-3453

https://doi.org/10.1016/j.carbon.2012.03.008 Get rights and content

Abstract

The electrical resistance change of a highly extensible composite consisting of a network of entangled multi-wall carbon nanotubes in a thermoplastic polyurethane elastomer is tested. The composite is prepared by taking a non-woven polyurethane filter membrane, enmeshing it with carbon nanotubes and melding them together. Testing has shown that the material can be elongated as much as 400% during which the resistance is increased more than 270 times. It indicates favorable properties of the composite for its use as a highly-deformation strain sensing element, a strain-electric signal transducer, electromagnetic field shielding and protection against lightning. As an example of its use as a strain sensor, human knee flexion and knee cyclic movement has been monitored. This may be of use in orthopedics and rehabilitation.

Introduction

The practical use of carbon nanotube (CNT) structures demonstrates their application in the field of sensing technology as novel types of sensors [1] or as an active part of sensing structural composites [2], [3]. The interconnected CNT structures are capable of detecting macroscopic electrical resistance change induced by their deformation. The published data show that the electrical response to strain or stress is sufficient and the sensing can be performed in real time [2], [4], [5]. Moreover, the deformation process is reversible although some irreversibility is also measured as a hysteresis loop in cyclical loading tests [2], [6], [7], or as a residual resistance which remains after specimen unloading [3], [8], [9].

The CNT based strain sensitive elements may consist of either a pure CNT entangled network of buckypaper [1], [4], [5], [6], [7], [9], [11], or CNT/polymer particulate composite formed from a CNT bulk polymer nanocomposite [2], [3], or CNT/polymer thin-film composite [8], [10]. In the case of the entangled network, the resistance response to deformation is governed by nanotube interactions. The decisive role is played by local resistance in nanotube contacts and the straightening and buckling of nanotubes what affects the number of contacts between nanotubes, rather than the change of intrinsic piezoresistive properties of individual CNT [6]. Both the tensile [4], [6], and compressive [1], [7], [9] deformation of CNT entangled network was tested. In the case of tensile load, CNT network is attached to the surface of monitored tensile specimen by epoxy resin [11], or is encapsulated into the specimen [6].

The change of network electrical resistance may then be detected simultaneously with deformation by a two-point [6], or four-point method [11]. The resistance measured usually does not follow deformation linearly [6], although for small strain (up to ∼0.02) a linear relationship is found [11]. The evaluative sensing (gauge) factor which measures the network strain sensitivity and is defined as the ratio of the relative resistance change to mechanical strain, reached for encapsulated CNT networks in epoxy resin values comparable with values for metallic strain gauges (0.6–2.2) [6]. Significantly higher gauge factor value (7) is shown for a pure single-walled carbon nanotube network [5].

In this paper we study the deformation and electrical resistance change of the polymer composite, which consists of multi-walled carbon nanotube (MWCNT) entangled network of buckypaper embedded in the thermoplastic polyurethane (PU). The composite is prepared by an innovative procedure wherein the non-woven polyurethane filtering membrane and the carbon nanotube filtration cake are integrated by compression molding. The composite can sustain very large deformations, what is promising for its practical use for highly-tensile sensing elements or highly-deformable electromagnetic shielding. As an example of how reliable the composite used as a strain sensor is, a human knee flexion and its cyclic movement is tested, that may, for example, be useful in orthopedics and rehabilitation. The study is in parallel to our recent work on MWCNT entangled networks as potential resistive gas sensors for organic vapor detection [12].

Section snippets

Multi-wall carbon nanotube entangled networks

Purified MWCNTs produced by chemical vapor deposition of acetylene which were supplied by Sun Nanotech Co. Ltd., China. According to the supplier, the nanotube diameter is 10–30 nm, length 1–10 μm, with a purity of ∼90% and (volume) resistivity of 0.12 Ω cm. Further details on the nanotubes were obtained by means of the transmission electron microscopy (TEM) analysis presented in our previous paper [7]. From the corresponding micrographs the diameter of individual nanotubes was determined to be

Tensile deformation and gauge factor

In the first case, the composite specimen is deformed by the tensile stress 1.27 MPa in seven extension/relaxation cycles. The results are shown in Fig. 2 in terms of the percentage of relative resistance change, defined ΔR/R₀ = (R − R₀)/R_0, where R₀ is the electrical resistance of the measured sample before the first elongation, and R is the resistance while elongating, vs. the percentage of mechanical strain, which is the relative change in length. The periods of extension, when stress 1.27 MPa is

Conclusions

We have introduced a highly deformable composite composed of a network of electrically-conductive entangled carbon nanotubes embedded in elastic polyurethane. The composite is prepared by taking a non-woven polyurethane filtering membrane, enmeshing it with carbon nanotubes and melding them into one. This innovative procedure eliminates the laborious process that is usually used, i.e. peeling off the nanotube network from the common micro-porous (polycarbonate, nylon) filter followed by the

Acknowledgments

This work was supported by the Operational Program Research and Development for Innovations co-funded by the European Regional Development Fund (ERDF), the national budget of the Czech Republic within the framework of the Center of Polymer Systems Project (Reg. Number: CZ.1.05/2.1.00/03.0111), the Czech Ministry of Education, Youth and Sports Project (MSM 7088352101) and by the Fund of Institute of Hydrodynamics AV0Z20600510.

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A highly-deformable composite composed of an entangled network of electrically-conductive carbon-nanotubes embedded in elastic polyurethane

Abstract

Introduction

Section snippets

Multi-wall carbon nanotube entangled networks

Tensile deformation and gauge factor

Conclusions

Acknowledgments

Compos Sci Technol

Compos Sci Technol

Compos Pt B-Eng

Carbon

Scr Mater

Compos Pt A-Appl Sci Manuf

Compos Sci Technol

Compos Sci Technol

Electromechanical sensors based on carbon nanotube networks and their polymer composites

Fabrication and property prediction of conductive and strain sensing TPU/CNT nanocomposite fibres

J Mater Chem