Polypropylene/carbon nanotube nano/microcellular structures with high dielectric permittivity, low dielectric loss, and low percolation threshold

Nano/microcellular polypropylene/multiwalled carbon nanotube (MWCNT) composites exhibiting higher electrical conductivity, lower electrical percolation, higher dielectric permittivity, and lower dielectric loss are reported. Nanocomposite foams with relative densities (rho(R)) of 1.0-0.1, cell sizes of 70 nm-70 mu m, and cell densities of 3 x 10(7)-2 x 10(14) cells cm(-3) are achieved, providing a platform to assess the evolution of electrical properties with foaming degree. The electrical percolation threshold decreases more than fivefold, from 0.50 down to 0.09 vol.%, as the volume expansion increases through foaming. The electrical conductivity increases up to two orders of magnitude in the nanocellular nanocomposites (1.0 > rho(R) > similar to 0.6). In the proper microcellular range (rho(R) approximate to 0.45), the introduction of cellular structure decreases the dielectric loss up to five orders of magnitude, while the decrease in dielectric permittivity is only 2-4 times. Thus, microcellular composites containing only similar to 0.34 vol.% MWCNT present a frequency-independent high dielectric permittivity (similar to 30) and very low dielectric loss (similar to 0.06). The improvements in such properties are correlated to the microstructural evolution caused by foaming action (biaxial stretching) and volume exclusion. High conductivity foams have applications in electromagnetic shielding and high dielectric foams can be developed for charge storage applications. (C) 2014 Elsevier Ltd. All rights reserved.