Relaxation behavior and activation energy of relaxation for polyimide and polyimide–graphene nanocomposite

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 °C 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, Tg, to higher temperature from 360 °C at 0.05 Hz to 420 °C at 100 Hz. The tan δ peak height for both α and β 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 Tg for the composite was shown to be higher than that for the matrix by about 5–10 °C. The Arrhenius equation was used to calculate the activation energy for both α and β transitions. The activation for α and β 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.