Segregated Networks and Dynamic Percolation

Polymer Carbon Nanotube Composites Tbe Polymer Latex Concept Nadia Grossiord, Marie Claire Hermant, and Cor Koning Copyright 2012 Pan Stanford Publishing Pte. Ltd. 

This dry-mixing process is not favorable when dealing with high aspect ratio fillers like CNTs, due to the fact that CNTs are often aggregated and poorly separated through mechanical shear. In order to overcome this experimental hurdle, it would be best if the two constituents could first be dispersed in a liquid in which both are insoluble, and subsequently the two dispersions could be mixed to create a two-component colloidal mixture. The hydrophobicity of both CNTs and most polymers, used as composite matrix material, implies that water is a very good candidate as a carrier solvent for the mixing process. 

In all these systems, it was envisaged that the initial state [CNT configuration] within the mixed colloid system would be maintained throughout all the subsequent processing steps. It has, however, been reported that the nature of the processing steps has a large influence on the final percolation network structure for both spherical and rod-like fillers. These effects have been linked to the mobility of the matrix in the inter-particle spaces, the mobility of the filler particles [network equilibration], and the increased interaction between adjacent tubes. This led to the definition of dynamic percolation, where the network structure is not kinetically frozen, but rather seeks an equilibrated state. 

Strictly speaking, one should distinguish between the percolation threshold and the minimum CNT concentration at which network formation takes place [MCCNF]. The latter corresponds to the lowest CNT concentration at which a nanocomposite starts being conductive, or, at which a CNT network is just formed, regardless of whether the system has reached thermodynamic equilibrium or not. On the other hand, the percolation threshold is an intrinsic value of the percolating system considered [in this case, the CNTs dispersed in a PS matrix], which is assumed to be at thermodynamic equilibrium and which 

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