The relaxation behavior of polyimide and its nanocomposite containing 10 wt % of graphene was studied by using the dynamic mechanical spectrometer. Dynamic mechanical analysis of polyimide and its composite was performed as a function of temperature and frequency in the temperature range of 25-480 8C and frequency range between 0.05 and 100 Hz. The effect of increasing frequency of testing from 0.05 to 100 Hz is a significant shift from the glass transition temperature, T g , to higher temperature from 360 8C at 0.05 Hz to 420 8C at 100 Hz. The tan d peak height for both a and b transitions decreased with increasing test frequency from 0.24 at 0.05 Hz to 0.08 at 100 Hz, due to increasing restriction to chain motion. At any given testing frequency, the T g for the composite was shown to be higher than that for the matrix by about 5-10 8C. The Arrhenius equation was used to calculate the activation energy for both a and b transitions. The activation for a and b transitions for the composite and polyimide matrix were determined to be 688 and 537 kJ/mol and 313 and 309 kJ/mol, respectively, indicating that a significant increase in the energy barrier to chain relaxation occurred as a result of reinforcement of polyimide with low weight fraction of graphene. V C 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016, 133, 43684.

The electrical properties of polyimide and the composite at different volumes fractions were studied in the frequency range 200-20 kHz and in the temperature range 30-200 8C. Increasing the volume fraction of graphene up to 10%, resulted in an extremely large increase in the dielectric constant, which indicates the composites remarkable ability to store electric potential energy under the effect of alternating electric field. An increase in dielectric constant was also observed with increasing temperature and decreasing frequency. The outstanding dielectric properties of polyimide graphene nanocomposites are attributed to the large volume fraction of interfaces in the bulk of the material. The measured increase in dielectric constant with increasing temperature was attributed to the segmental mobility of the polymer chains. The AC conductivity for polyimide and the composites was calculated from the loss factor and a remarkably high conductivity was obtained for the composites due to the formation of conducting paths in the matrix by the graphene sheets. Also this study showed that the thermal conductivity of the composites increased sharply with increasing graphene concentration.

Dynamic mechanical spectroscopy is used to investigate the variation of the glass rubbery transition temperature ( Tg) of graphene/polyimide nanocomposites. The Tg was obtained as the temperature corresponding to the peak of the tan δ versus temperature curve for the alpha transition. Cole–Cole curve was constructed for the matrix and the composites composite at 1 Hz and a hemispherical curve was obtained suggesting that the constituents have similar relaxation behavior. The time–temperature superposition principle was used to model the behavior of the nanocomposites at lower frequencies and longer times. Frequency sweep was performed in the range of 0.05–100 Hz at different temperatures. The storage modulus curves obtained at different temperatures (isothermal), as a function of frequency, were shifted horizontally to construct a continuous master curve. Both the generalized reduced gradient method and the Williams–Landel–Ferry (WLF) equation were used to calculate the activation energy for glass–rubber transition. The universal constants C1 and C2 were determined using the WLF equation. The dependence of relaxation time on temperature was verified.