Negative dielectric permittivity of PVDF nanocomposites induced by carbon nanofibers and polymer crystallization

Negative permittivity is a key characteristic of the distinctive metamaterials, a novel class of artificial materials with particular electromagnetic properties. Negative permittivity has been realized in metallic structures with special designs, but rarely achieved in polymer nanocomposites. Our recent studies discover that negative permittivity can be attained from randomly distributed carbon nanofiber (CNF) filled poly(vinylidene fluoride) (PVDF) composites over a wide frequency range, and the negative permittivity values are strongly influenced by CNF and PVDF crystalline structures. The effects of CNF on the crystallization of PVDF, and the resultant negative dielectric permittivity of CNF/PVDF composites influenced by crystallization of PVDF and CNF, are investigated. It is revealed that the introduction of CNF not only affects the dielectric permittivity directly, but also causes indirect effects to the dielectric permittivity through influencing the crystallization of PVDF. In particular, due to addition of more CNF, a α- to β-phase transformation in PVDF is found to affect permittivity of the nanocomposites. Furthermore, the permittivity of CNF/PVDF composites are increased considerably (“more negative”) with more CNF, and is affected noticeably by crystalline structures of PVDF. The lowest negative permittivity achieved is −2,500 for the nanocomposite with 5 wt% CNF at 5 kHz.