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Broadband dielectric/electric properties of multiwalled carbon nanotubes (MWCNT) polymethylmetacrylate composites were studied in a wide temperature range (25-400 K). It was established that the percolation threshold is lower in composites with the shortest MWCNT, due to the better distribution of the MWCNT in the polymer matrix. The value of the dielectric permittivity and the electrical conductivity below the percolation threshold is also higher in composites with the shortest MWCNT. Below the percolation threshold the dielectric dispersion in composites is mainly related with the β relaxation in the polymer matrix. The length of the MWCNT and its concentration have also the huge influence on polymer matrix molecular dynamics, the relaxation time is shortest in composites with the higher MWCNT concentration and in composites with the shortest MWCNT. At low temperatures the conductivity of composites above the percolation threshold is governed by an electron tunneling between MWCNT clusters. At higher temperatures the electrical transport occurs also due to the finite electrical conductivity of the polymer matrix. [ABSTRACT FROM AUTHOR]

The dielectric properties of polymethylmetacrylate (PMMA) composites filled with CVD made multiwalled carbon nanotubes (MWCNT) of different mean outer diameters ( d ∼ 9 nm and 12-14 nm) were investigated in the temperature range from 300 to 420 K and in a wide frequency range (20 Hz to 1 MHz). Below the percolation threshold the temperature dependence of the complex dielectric permittivity of the investigated composites is mainly caused by β relaxation in pure PMMA matrix and the dielectric permittivity is found to be higher in composites with thicker nanotubes. The activation energy of β relaxation increases with carbon nanotube concentration. The percolation threshold is lower in composites with both thinner and oxidized carbon nanotubes. The established influence of both the MWCNT mean outer diameter and their oxidation on the broadband dielectric characteristics can be exploited for the production of effective low-cost electromagnetic and/or antistatic coatings working at different temperatures. [ABSTRACT FROM AUTHOR]