Kinetic models for imidization of PMR-15

The temperature and time dependence of imidization of PMR-15 was investigated using Fourier transform infrared spectroscopy (FT-IR). A series of isothermal aging tests were performed, where the aging temperatures (65 degrees C less than or equal to T less than or equal to 300 degrees C) and aging times (3 less than or equal to t less than or equal to 150 minutes) were systematically varied. FT-IR spectrophotometry showed that imidization (measured by the changes in the imide carbonyl absorption at 1778 cm(-1)) increased with isothermal aging temperature and time. Imidization was found to be nearly completed after isothermal aging at 300 degrees C for 2.5 h. Imidization of PMR-15 was found to proceed in two stages: (i) initial imidization region characterized by a very rapid reaction and spans about 0.5 h; and (ii) final imidization region characterized by a gradual reaction, spanning about 2 h. Mathematical models were developed to describe the imidization of PMR-15. The rate constants, k(I) for regions I and II were determined to range from 0.0286 to 0.0462 min(-1) and 0.0077 to .0009 min(-1), respectively. The corresponding activation energies for imidization in the two regions were determined to be 12 kJ/mol and 9 kJ/mol, respectively.