Controlling electrical percolation in multicomponent carbon nanotube dispersions.

Carbon nanotube reinforced polymeric composites can have favourable electrical properties, which make them useful for applications such as flat-panel displays and photovoltaic devices. However, using aqueous dispersions to fabricate composites with specific physical properties requires that the processing of the nanotube dispersion be understood and controlled while in the liquid phase. Here, using a combination of experiment and theory, we study the electrical percolation of carbon nanotubes introduced into a polymer matrix, and show that the percolation threshold can be substantially lowered by adding small quantities of a conductive polymer latex. Mixing colloidal particles of different sizes and shapes (in this case, spherical latex particles and rod-like nanotubes) introduces competing length scales that can strongly influence the formation of the system-spanning networks that are needed to produce electrically conductive composites. Interplay between the different species in the dispersions leads to synergetic or antagonistic percolation, depending on the ease of charge transport between the various conductive components.

Probing the cooperative nature of the conductive components in polystyrene/poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate)-single-walled carbon nanotube composites.

The percolation threshold of single-walled carbon nanotubes (SWCNTs) introduced into polystyrene (PS) via a latex-based route has been reduced by using conductive surfactants. The use of the conductive polymeric latex, poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS), in conjunction with SWCNTs leads to conductive composites with loadings of both constituents below their own individual percolation thresholds. The high concentration of PEDOT:PSS in the final composites raises the concern that the composite conductivity is a result of the presence of the PEDOT:PSS alone. To elucidate the cooperative nature of the two conductive components, the contribution of the SWCNTs to the overall composite conductivity is investigated by replacing the original high-quality SWCNTs with SWCNTs of a lower quality. Percolation thresholds recorded for systems utilizing the lower quality tubes stabilized with nonconductive surfactants were over 2 wt % SWCNTs (4 times that of previously reported systems). The introduction of PEDOT:PSS was, once again, found to lower the percolation threshold (to 0.3 wt %) and to increase the ultimate conductivity up to the level of a pure PEDOT:PSS/PS blend. In the PS/PEDOT:PSS-SWCNT systems, the role of the SWCNT network is proposed to be limited to the formation of a template or scaffold on which a (more or less) continuous PEDOT:PSS layer deposits. The ultimate conductivity is therefore determined by the PEDOT:PSS alone.