Electrical and mechanical properties of iodine-doped highly elongated ultrahigh molecular weight polyethylene films filled with multiwalled carbon nanotubes

Highly elongated ultrahigh molecular weight polyethylene (UHMWPE) films filled with multiwalled carbon nanotubes (MWNTs) were prepared by gelation and/or crystallization from solution, and the resultant films were elongated up to 50-fold. The drawn specimens were doped by iodine successfully. Electrical conductivity of the drawn MWNT-UHMWPE films increased sharply with increasing volume fraction of MWNTs beyond the percolation threshold since conductive paths began to form. This was due to a drastic increase of mutual contact possibility of MWNTs. The higher aspect ratio of MWNTs than carbon fibers provided higher $t$ value denoting universal critical exponent by a scaling law, since the percolation threshold of the conductivity against MWNT content was lower than that against carbon fiber content. Iodine doping especially enhanced a drastic increase in electrical conductivity, but the trend of the conductivity versus the volume fraction of MWNTs was hardly affected by iodine doping. The mechanism responsible for the conductivity increase was analyzed mainly by Raman spectroscopy in terms of bond polarization. The doped iodine existed mainly as $\mathrm{I}_5{}^{-}$, which formed the charge transfer complex. It may be expected that the $\mathrm{I}_5{}^{-}$, provided an increase in charge carriers linked to the MWNTs and could be taken as bridge for the adjacent or nearly MWNTs. Incidentally, the storage modulus of the iodine-doped composite with $4.16\mathrm{vol}\%$ at $20°\mathrm{C}$ was slightly higher than $25\mathrm{GPa}$, and the corresponding electrical conductivity was approximately $0.1\mathrm{S}∕\mathrm{cm}$, which indicated development of materials with high electrical conductivity and high mechanical property.

Figure 1

(a) TEM image and (b) Raman spectrum of MWNTs.

Figure 2

(a) Electrical conductivity versus volume fraction of MWNTs. (b) Log-log plot of the conductivity against $\Phi -{\Phi }_c$ with ${\Phi }_c=1.52\mathrm{vol}\%$ according to Eq. (1).

Figure 3

Raman spectra of the iodine-doped composite films with $1.52\mathrm{vol}\%$ MWNTs using a $514.5\mathrm{nm}$ excitation: (a) pristine and (b) $2\text{months}$ after doping.

Figure 4

Raman spectra of the composite films with $1.52\mathrm{vol}\%$ MWNT doped iodine measured using a $514.5\mathrm{nm}$ excitation: (a) pristine and (b) $2\text{months}$ after doping.

Figure 5

Electrical conductivity for the films with $1.52\mathrm{vol}\%$ MWNTs during three heating cycles: (a) pristine and (b) iodine doped.

Figure 6

Electrical conductivity for the films with $4.16\mathrm{vol}\%$ MWNTs during three heating cycles: (a) pristine and (b) iodine doped.

Figure 7

Temperature dependence of storage and loss moduli of composite films at drawn ratio of 50-fold: (a), (b) with $1.52\mathrm{vol}\%$ MWNTs and (c), (d) with $4.16\mathrm{vol}\%$ MWNTs.