  Carboxylic functionalization (-COOH groups) of carbon nanotubes is known to
improve their dispersion properties and increase the electrical conductivity of
carbon-nanotube - polymer nanocomposites. We have studied experimentally the
effects of this type of functionalization on the thermal conductivity of the
nanocomposites. It was found that while even small quantities of carbon
nanotubes (~1 wt%) can increase the electrical conductivity, a larger loading
fraction (~3 wt%) is required to enhance the thermal conductivity of
nanocomposites. Functionalized multi-wall carbon nanotubes performed the best
as filler material leading to a simultaneous improvement of the electrical and
thermal properties of the composites. Functionalization of the single-wall
carbon nanotubes reduced the thermal conductivity enhancement. The observed
trends were explained by the fact that while surface functionalization
increases the coupling between carbon nanotube and polymer matrix it also leads
to formation of defects, which impede the acoustic phonon transport in the
single wall carbon nanotubes. The obtained results are important for
applications of carbon nanotubes and graphene flakes as fillers for improving
thermal, electrical and mechanical properties of composites.
