Porosity and composition dependence on electrical and piezoresistive properties of thermoplastic polyurethane nanocompos
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Hani Naguiba) Department of Mechanical and Industrial Engineering, University of Toronto, Toronto, Ontario, Canada M5S 3G8; and Department of Materials Science and Engineering, University of Toronto, Toronto, Ontario, Canada M5S 3G8 (Received 17 March 2013; accepted 11 July 2013)
The development and characterization of pressure sensing porous nanocomposites are reported here. A thermoplastic polyurethane (TPU) was chosen as an elastomeric matrix, which was reinforced with multiwall carbon nanotubes (MWNTs) by high shear twin screw extrusion mixing. Porosity was introduced to the composites through the phase separation of a single TPU-carbon-dioxide gas solution. Interactions between MWNT and TPU were elucidated through calorimetry, gravimetric decomposition, conductivity measurements, and microstructure imaging. The piezoresistance (pressure–resistance) behavior of the nanocomposites was investigated and found to be dependent on MWNT concentration and nanocomposite microstructure. Mechanisms of piezoresistance in solid and porous nanocomposites are proposed.
I. INTRODUCTION
Multifunctional materials possess multiple desirable attributes concurrently such as improved thermal, mechanical, electrical, chemical properties, etc. Several research studies have highlighted the benefits of utilizing nanoparticles, including carbon nanotubes (CNT), dispersed in a polymer matrix to fabricate multifunctional materials.1–3 Such nanocomposites can possess improved mechanical, electrical, and/or thermal properties and have numerous applications as sensors,4–6 electrodes,7,8 actuators,9–11 structural materials,12,13 adhesives,14,15 etc. Piezoresistance is a passive phenomenon that can be used for sensing of load/pressure through variations in a material’s electrical resistance. The mechanisms of piezoresistance can vary between materials, and through various design configurations, these materials can be used for force, pressure, and flexure sensing. One mechanism is found in conventional metallic strain gages commonly used in structural strain monitoring. These sensors rely on resistance change because of a changing geometry (definable by their Poisson’s ration) due to external strain.16 Metallic strain gages possess appreciable linearity and sensitivity, measurable by gage factors (change in resistance: change in strain) as high as 2, however, they are limited to measuring strains less than 2%. Another piezoresistance mechanism is found in semiconductor-based pressure sensors where deformation of a doped semiconductor lattice a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2013.218 J. Mater. Res., Vol. 28, No. 17, Sep 14, 2013
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will result in a change of the charge carrier’s mobility and the material’s resistance. The gage factors for these devices are around 200 (recently shown to be as high as 84317). However semiconductor-based pressure sensors are also limited to small strain ranges, which are around 0.5%. A third piezoresist
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