a) b - École Polytechnique de Montréal

the percolation threshold. As compared in Figures 2-20a and 2-20b, by using high aspect ratio
fillers such as conductive fibers/wires, the conductivity of the polymer composite can be
achieved at a much lower filler loading level (Figure 2-20c).
Carbon-black particles are more irregularly shaped and often have long branches reaching out
from the main body of the particle. These moderately structured fillers can require anywhere
from 5 to 35% loading to reach the percolation threshold(Sichel, 1982). In carbon black filled
nitrile rubber composite, the percolation threshold lies in between 20% and 35%, while for a
short carbon fiber filled composite it ranges from 10% to 20%(Pramanik, Khastgir, & Saha,
1992). Highly shaped fillers such as carbon or stainless-steel fibers may be present in as little as
a few percent by volume in order to achieve low resistance. Gelves et al.(Gelves, Lin,
Sundararaj, & Haber, 2006) used silver and copper nanowires with a diameter of 25 nm and
aspect ratios of up to 200 and 400, respectively. Electrical percolation thresholds of 0.5-0.75
vol.% for silver nanowires and 0.25-0.75 vol.% for copper nanowires in PS nanocomposites
were found.
Figure 2-20. Schematic illustrations of conductivity establishment for polymer composites with
(a) spherical fillers, (b) high aspect ratio fillers (fiber/wire) and (c) percolation threshold (Vc) in
a resistivity vs. filler loading plot(Wong, 2003-2008)
Lin et al.(Lin, Gelves, Haber, Potschke, & Sundararaj, 2008) reported an electrical percolation
threshold between 1 and 2 vol.% for polystyrene/copper nanowire nanocomposites. The
nanowires have a diameter of around 25 nm and an average length of 1.29±0.83 μm. A very low
percolation threshold of high aspect ratio fillers for silver nanotubes is shown in Figure 2-21.
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