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a) b - École Polytechnique de Montréal

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2.3.1.1 Conducting Polymer Composite Materials (CPCM)<br />

The <strong>de</strong>velopment of efficient conductive polymer composites materials(CPCMs) remains an<br />

important en<strong>de</strong>avor in light of growing energy concerns. Conductive and anti-static compounds<br />

have been commercially prepared for more than 80 years. A CPCM consists of a random<br />

distribution of conducting filler throughout an insulating polymer; this is of interest for several<br />

application fields. It is crucial to choose a suitable filler-polymer pair in the production of<br />

CPCM. A good balance between filler-polymer and filler-filler interactions can lead to the<br />

formation of a continuous network, but filler particles tend to stick together if the filler-filler<br />

interactions dominate and aggregates rather than forming network of particle-contacting chains.<br />

On the other hand, a good adhesion between polymer and filler results in an insulating layer<br />

around the filler particles and prevents formation of current-conducting chains. Many conductive<br />

polymer composites exhibit percolation characteristics(Chung, Sabo, & Pica, 1982; Flandin,<br />

Bréchet, & Cavaillé, 2001).<br />

The most universal conducting filler for this purpose is carbon black, because dispersion of<br />

conducting carbon black particles in synthetic polymers is a very efficient way of providing them<br />

with semi-conductive and antistatic properties. From the economical and technical viewpoint,<br />

<strong>de</strong>creasing the carbon black percolation threshold as much as possible in the blend is <strong>de</strong>sirable.<br />

Consequently, the percolation theory is the most a<strong>de</strong>quate for mo<strong>de</strong>lling conductivity of CPCM.<br />

It has been observed that insulator-conductor transition in polymer-filler composites <strong>de</strong>pend on<br />

the aggregation(Masao Sumita, Kayaki, & Miyasaka, 1986), size, size distribution, structure and<br />

porosity of the conducting particles(Verhelst, Wolthuis, Voet, Ehrburger, & Donnet, 1977), on<br />

rheological properties of polymer components(Miyasaka et al., 1982b), and on processing<br />

conditions.<br />

The curve of conductivity versus filler concentration is S-shaped (Figure 2-18), which clearly<br />

<strong>de</strong>monstrates a relatively narrow filler loading range during which a small increase in loading<br />

will result in a drastic increase in conductivity. This change implies some sud<strong>de</strong>n changes in the<br />

dispersing state of conductive particles at a critical point (percolation threshold), i.e. the<br />

coagulation of particles to form networks which facilitates the electrical conduction through the<br />

composites. Figure 2-18 shows the conductivity of a thermoset system of multiwall-nanotube/<br />

46

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