Dielectric Relaxation of Epoxy Matrix Nanocomposites with Carbon Nanotubes

The frequency dependent dielectric permittivity of a commercial epoxy mixture (Dow Chemicals DER 330 ) reacting isothermally with an aromatic amine hardener (MDEA) has been measured by means of dielectric spectroscopy in the range 10 - 106 Hz. Dielectric spectra were taken at regular intervals during reaction, in the pure reactive system and in composites obtained by adding commercial carboxy-funcionalized double-walled carbon nanotubes (C-DWCNTs, Nanocyl 2100). The C-DWCNTs were stirred in the MDEA at the melting temperature of the amine (i. e. 100°C for 45 min.). The C-DWCNTs grafted with MDEA, MDEA/C-DWCNTs, were added to obtain a percentage of 0.1wt% and 1wt% respect to the DGEBA monomer maintaining a stoichiometric molar ratio (1:2.33) with MDEA. Calorimetric and rheological tests reveal how the presence of MDEA modified C-DWNTs speed up the maximum degree of cure and the gel time of the epoxy system. The dielectric response in the pure system appears to be dominated by ionic conductivity and by structural relaxation with characteristic time strongly increasing with curing time. In the nanotube loaded samples the presence of the filler originates a new and strong relaxation, which overcomes and hides the structural contribution. The characteristic parameters of the dielectric relaxation in the loaded samples are compatible with hopping dominated conduction mechanisms. An increase of DC conductivity in the final part of the reaction is also observed, which can be ascribed to dielectrophoretic migration of carbon nanotubes.