Dynamic relaxation characteristics of Matrimid^® polyimide

The dynamic relaxation characteristics of Matrimid^® (BTDA-DAPI) polyimide have been investigated using dynamic mechanical and dielectric methods. Matrimid exhibits three motional processes with increasing temperature: two sub-glass relaxations (γ and β transitions), and the glass-rubber (α) transition. The low-temperature γ transition is purely non-cooperative, and displays an identical time-temperature response to both the dynamic mechanical and the dielectric probes with a corresponding activation energy, E_A = 43 kJ/mol. The β sub-glass transition shows a more cooperative character as assessed via the Starkweather method. Comparison of dynamic mechanical and dielectric data for the β process suggests that the dynamic mechanical test (E_A = 156 kJ/mol) is sensitive to a broader, more strongly correlated range of sub-glass motions as compared to the dielectric probe (E_A = 99 kJ/mol). Time-temperature superposition was used to establish mechanical master curves across the glass-rubber (α) relaxation, and these data could be described using the Kohlrausch-Williams-Watts function with an exponent value, β_KWW = 0.34. The corresponding shift factors were used as the basis of a cooperativity plot for the determination of dynamic fragility. The relation between fragility index (m = 115) and β_KWW for the Matrimid polyimide was in good agreement with the wide correlation reported in the literature.