As previously introduced in Figure 2, the electrical characterization of the starting PEO/PEDOT:PSS NFs clearly shows that the designed nanofiber mats are able to conjugate two main features: the intrinsic conductivity
, ensured by the PEDOT:PSS polymer, and optimal electron pathways along the nanofiber axis thanks to their aligned distributions.
These properties include: capacitance, electric polarizability tensor, intrinsic conductivity
, volume, gyration tensor, hydrodynamic radius, intrinsic viscosity, friction coefficient, diffusion coefficient, sedimentation coefficient, and related quantities.
It is interesting to see that [f'.sub.0] does not decrease monotonically with decreasing intrinsic conductivity
(with a dip seen at ~ 46 S/m).
According to EMI theory, filler with a higher aspect ratio and intrinsic conductivity
should have a better EMI shielding performance in the same polymer matrix.
Moreover, weak binding of the nanostructure leads to low shear modulus, and tends to reduce the intrinsic conductivity
. (30) Unlike the columnar grains that are parallel to heat flow.
This fact is already known [19, 20] and can be explained by two reasons: (1) the functionalization process must generate defects in the outer walls of the nanotubes due to the breakage of C--C bonds, decreasing their intrinsic conductivity
, and (2) the strong chemical interface formed between amino-functionalized CNTs and the epoxy matrix leads to polymer wrapping around the dispersed nanotubes, insulating the nanotubes from each other.
As EMI depends strongly on both the intrinsic conductivity
as well as the spatial distribution of conductive filler, we hypothesize that this decreased shielding performance for our materials is associated with the distribution of nanotubes throughout the coating; these SWNTs form a percolated network as evidenced by the high conductivity, but the network appears to be heterogeneous based on the EMI shielding.
(72) directly measured the conductivity of a film of raw MWCNTs prepared by evaporation of ethanol as ~ [10.sup.3] S/m, which suggests that the observed value may be near the intrinsic conductivity
of entangled MWCNTs and represent an ideal mode of conduction.
Under the proper conditions, the electrospinning process can produce submicron to nanoscale fibers by selecting a proper solvent and polymer concentration as well as controlling the intrinsic conductivity
, the viscosity, and the surface tension of the solution [4, 7].
As described by Garboczi and coworkers (19-21), the intrinsic conductivity
of the conducting filler in an insulating matrix can be defined in analogy to the intrinsic viscosity as:
The electrical conductivity of conductive polymer compounds is generally determined from the filler volume fraction and intrinsic conductivity
values of the fillers.
There does not appear to be any trend concerning the critical percolation exponent (s) or the proportionality constant ([[sigma].sub.0]), which is close to the intrinsic conductivity
of carbon black (on the o rder of 10 S/cm) at all temperatures.