Piezoresistive polypropylene-carbon nanofiber composites as mechanical transducers

Abstract: polymeric materials have been replacing other materials in various applications, from structural to electronic components. In particular, since the discovery of conducting polymers and the beginning of the manufacture of conducting composites with carbon fillers, their use in electronics is growing up. A group of electronic components with large potential for industrial applications such as structural monitoring, biomedical or robotics are sensors based on the piezoresistive effect, fabricated from conductive polymers and/or composites. The aim of this article is to characterize the piezoresistive effect of conductive polymer composites based on polypropylene filled with carbon nanofibers, and to demonstrate a way of fabricating strain gauges from these materials, using industrial techniques. With this purpose, some films were prepared by shear extrusion, which allows the composites to be produced industrially in a standard non-expensive process. Then, both the dependence of the electrical response on the preparation conditions and on the mechanical solicitations was measured. The obtained gauge factor values, up to 2.5, and piezoresistive coefficients up to 0.0019 mm2/N, prove the viability of these materials for fabricating strain-gauges, where their main advantages are the lower price and the ability to deal with much higher deformations, when compared to metal or semiconductor strain-gauges.We acknowledge the Foundation for Science and Technology through the 3 degrees Quadro Comunitario de Apoio, the POCTI and FEDER programs and the NANO/NMed-SD/0156/2007 project. The support of Applied Sciences Inc. for generously supplying the CNFs used. We would also like to thank Carla Leer and Patrick Lake for their assistance in the production of the CNF composites. J. G. Rocha thanks the FCT for the Grant SFRH/BSAB/1014/2010

Effects of Compression and Filler Particle Coating on the Electrical Conductivity of Thermoplastic Elastomer Composites

Elastomeric polymers can be filled with metallic micro- or nanoparticles to obtain electrical conductivity, in which the conductivity is largely determined by the intrinsic conductivity of and contact resistance between the particles. Electrons will flow through the material effectively when the percolation threshold for near-neighbor contacts is exceeded and sufficiently close contacts between the filler particles are realized for electron tunneling to occur. Silver-coated glass microparticles of two types (fibers and spheres) were used as fillers in a thermoplastic elastomer composite based on styrene–ethylene–butylene–styrene copolymer, and the direct-current (DC) resistance and radiofrequency impedance were significantly reduced by coating the filler particles with octadecylmercaptan. Not only was the resistance reduced but also the atypical positive piezoresistivity effect observed in these elastomers was strongly reduced, such that resistivity values below 0.01 Ω cm were obtained for compression ratios up to 20%. In the DC measurements, an additional decrease of resistivity was obtained by inclusion of π-extended aromatic compounds, such as diphenylhexatriene. Some qualitative theories are presented to illuminate the possible mechanisms of action of these surface coatings on the piezoresistivity