Anthony Combessis

Effect of filler auto-assembly on percolation transition in carbon nanotube/polymer composites

A series of composites with various content of multi-walled carbon nanotubes dispersed in polymer was tested for electrical properties. During isothermal annealing in the melt, dynamic percolation transition induced a tremendous increase in conductivity. The unstable structure also experienced a more than one order of magnitude reduction in percolation threshold. The insulator to conductor transition concurrently became softer, as revealed by a monotonous increase in the critical exponent, gradually departing from the universal value. These large and concomitant changes in percolation transition with annealing time were ascribed to the self-organization of the filler that favors the completion of the conductive network.

Real nanocomposites effect in carbon nanotubes / polymers

Dispersion and percolation of MultiWall Carbon NanoTubes (MWCNT) was studied in silicon oils by means of electrical measurements. After MWCNTs were homogenously dispersed with ultrasounds in silicon oil, the composite was slowly diluted to obtain a large number of filler content. Conductivity measurements were done for each filler content allowed us to determine the percolation threshold and the corresponding critical exponent with an unmatched accuracy. A decrease of percolation threshold over time has been observed over time and was correlated to MWCNT aggregation. More surprising, critical exponent also varies over time, which suggests a strong correlation with the quality of the filler dispersion in the matrix. Owing to the large number of data collected, a model derived from static percolation was established. The latter describes conductivity as a function of time and filler content.

Dynamic percolation as a tool for tailoring the electrical properties of carbon nanotube / polymer composites

The effect of dynamic percolation on the a.c. conductivity and permittivity of thermoplastic nanocomposites filled with 2.2 wt% of carbon nanotubes was studied. During isothermal annealing of the samples in the melt state a gradual and large increase of conductivity was evidenced with time. The profile of permittivity versus annealing time unexpectedly exhibited 2 maxima. This surprising feature was further studied and understood in terms of the action of two possible driving forces for dynamic percolation. The first one was attributed to the local polymer chain relaxation. The second mechanism was ascribed to the motion of the conducting filler, similar to a diffusion process.