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Conductive Polymer Composites

Principal investigator: Emiliano BILOTTI
Co-investigator(s): M. Baxendale and

Transparent conductive films (top right) and conductive polymer composite fibres based on nanotubes to sense external stimuli such as gases, vapour, mechanical deformation, and temperature. The exposure of these fibres to the external stimuli results in changes in electrical properties for applications in smart textiles.Conductive polymer composites (CPC) based on carbon nanotubes (CNTs) are being used more often in critical applications because they provide electrical connectivity for applications such as smart textiles and electrostatic painting, and protection from electrostatic discharge (ESD and electromagnetic-radio frequency interference (EMI). The electronic packaging industry, for example, uses CNTs to protect sensitive electronic parts such as integrated circuits (IC) and hard disc drives (HDD) from antistatic shock during fabrication and handling, while the automobile industry uses CNTs in fuel lines to prevent the electrostatic discharge during refuelling.

Conductive polymer composites (CPCs) are conventionally made by adding carbon black, metal powder, or carbon fiber into a polymer matrix. With the polymer matrix being an insulator, the conductivity of these composites can demonstrate a sudden jump when a critical filler content is reached. This phenomenon is often described as percolation. Typically, 5–20 wt.% of conventional conductive filler (such as carbon black) is added into a polymer matrix to achieve a percolating network. Such high filler contents quite often negatively affect the mechanical properties and processability of the resulting composites. As the percolation threshold has been shown to decrease with filler aspect ratio, CNTs are one of the most interesting fillers for CPCs. Percolation thresholds for CPCs containing CNTs can be < 0.1 wt.% for low viscosity resin systems like epoxies, while percolation thresholds in melt processed systems are typically higher at ~ 1 wt.%. In contrast to mechanical reinforcement where perfect dispersion is a prerequisite, in the case of electrical conductivity the lowest percolation thresholds are often obtained when the CNTs are allowed to reaggregate (dynamic percolation). We study the effect of processing conditions on dynamic percolation to achieve ultra low percolation thresholds in melt- and solution processed polymer systems.

Due to the one-dimensional structure of CNTs, oriented CNT/polymer composite fibers or tapes generate intense interest as such oriented systems could result in high mechanical and electrical efficiencies. One of the problems in creating high strength conductive fibres are that the conductivity of such oriented CNT/polymer fibres or tapes decreases upon solid-state drawing. We recently developed a new concept for the creation of multifunctional polymer nanocomposite fibres or tapes that combines high stiffness and strength with good electrical properties and a low percolation threshold. The concept is based on a bicomponent fibre or tape construction consisting of a highly oriented polymer core and a CPC skin based on a polymer with a lower melting temperature than the core, enabling thermal annealing of these skins to improve conductivity through a dynamic percolation process while retaining the properties of the core and hence those of the fibre or tape.

Conductive polymer composites are also studied to sense external stimuli such as chemicals, gases, vapour, mechanical stress or strain, and temperature. The exposure of these CPCs to the external stimuli can result in changes in electrical properties. For example a mechanical strain applied to a CPC can result in a reduction in electrical conductivity and a clear electric signal which can be used for sensing. Applications for sensors based on CPCs are expected in a wide range of fields such as smart textiles, building and medical applications and protective clothing.