Dielectric properties of polymethacrylate-grafted carbon nanotube composites

The dielectric properties of a series of multi-walled carbon nanotubes on which poly(cyclohexyl methacrylate)s were densely grafted (PCHMA-CNTs) were systematically characterized over the broad frequency range of 10(-2)-10(8) Hz. PCHMA-CNT was used as an ideal polymer/CNT nanocomposite system in which the CNT is homogeneously dispersed and the polymer/CNT interface is strong. When the volume fraction of the CNT Phi(CNT) < 0.1 PCHMA-CNT exhibited a relatively high dielectric constant epsilon(r)' and a quite low dielectric loss factor epsilon(r)'', while when Phi(CNT) > 0.1 a very large epsilon(r)' was observed at low frequencies. The Cole-Cole relaxation model was applied to the impedance spectra of PCHMA-CNT in order to investigate the correlation between the tunneling conduction and the very large epsilon(r)'. Also, from thus-obtained relaxation frequencies, the inter-nanotube distance in the PCHMA-CNT system was estimated. As a result, a new mechanism was proposed for the very large epsilon(r)': the locally enhanced electric field by the tunneling conduction results in the very large epsilon(r)' at the low frequencies along with the increase in epsilon(r)''. Furthermore, the CNT was demonstrated to be five times more effective than barium titanate (BT) for the enhancement in epsilon(r)' of polymer materials because of the higher electric field enhancement effect of the CNT than that of the BT.