Electrospun fibrous nanocomposites as permeable, flexible strain sensors

Electrospun polymer/nanoparticle composite-fiber structures are investigated as potential lightweight, compliant, porous strain sensors for noncyclic strain sensing. Such composite-fiber-based structures (mats/strips) are referred to as nanocomposites in this study. The fibers in the nanocomposites consist of, for example, poly(epsilon-caprolactone) dielectric polymer matrix with embedded electrically conductive carbon black (CB) nanoparticles. The effect of CB concentration on the electrical resistance of nanocomposite strips is studied experimentally. The fiber strips with 7 wt % or more of CB are electrically conducting in the as-spun, undeformed state and are, thus, called conductive polymer composites. The experiments demonstrate that the electrical resistance of a nanocomposite strip increases with strain it undergoes, and at sufficiently high strains, the strip became nonconductive. It is argued that the stretching process breaches the percolation structure of CB nanoparticles in the dielectric polymer matrix, thereby changing the strip electrical resistance. A two-dimensional layered-percolation model is proposed to describe the resistance increase with stretching. The model is compared to the experimental data for nanocomposite strips containing different CB loadings (7 -11 wt %), and a reasonable agreement is obtained for these CB loadings. The percolation threshold for electrical conduction of these nanocomposites is found experimentally to be lower than the corresponding theoretical two-dimensional random percolation threshold. The reduction is attributed to formation of oriented clusters of CB nanoparticles inside the fibers during the electrospinning process. The scaling-like increase of the electrical resistance toward percolation breakdown at large strains closely resembles the predictions of the percolation theory. A reproducible correlation between electrical resistance of the nanocomposite strips and their stretching suggests that they can be used as resistive noncyclic strain sensors. Such flexible permeable sensors can be useful, for example, when embedded in filters, to indicate filter overstretching due to clogging. In particular, a clogged section of a filter with an embedded sensor possesses a lower permeability, which increases pressure difference acting on this section at a fixed fluid throughput. The increased pressure difference results in local bending and stretching of the clogged section of the filter with an embedded sensor. In the present work, a formula is derived to relate permeability of a clogged filter to the strain of an embedded sensor and, in turn, to the electrical resistance of the sensor. This formula is a key element of the proposed novel resistive sensor. (C) 2008 American Institute of Physics.