Thermal degradation kinetics of poly(benzobisoxazole imide)

Polyimide (PI-HQDPA) derived from 2,6-bis(p-aminophenyl)-benzo[1,2-d;5,4- d'] bisoxazole and 1,4-bis(3,4-dicarbosyphenoxy) benzene dianhydride (HQDPA) was synthesized via a conventional twostage method. The intermediate poly(amic acid) had inherent viscosities of 1. 70 dL/g and could be thermally converted into light yellow polyimide film. The thermal degradation of the polyimide was studied by thermogravimetric analysis (TGA) using four different methods in order to determine the actual reaction mechanisms of the decomposition process. The apparent activation energy (E) of the solid-state process was determined using Flynn-Wall-Ozawa and Friedman methods, which do not require the knowledge of the reaction mechanism. And the apparent activation energies were resulted to be 356. 36 and 370. 54 kJ/mol,respectively. So the average activation energy of the polyimide was 363.45 kJ/mol. The reaction order (n) and preexponential factor (A) were determined by Friedman method,and the results were 4. 22 and 6. 44 x 10¹⁶ s^-1, respectively. The activation energies of different mechanism models were determined by Coats-Redfern and Phadnis-Deshpande methods, the most probably results were 370.66 and 367.80 kJ/mol, respectively. Compared with the value obtained from the Ozawa method and Friedman method, the actual reaction mechanism of PI-HQDPA obeyed the nucleation and growth model, and could be described by the Avrami-Erofeev function (A₃) with an integral form of g(X) =[-ln(l-X)]³.