Synthesis, characterization, and investigation of structure-thermal cycloimidization relationship of novel poly(amide amic acid)s to poly(amide imide)s by thermogravimetric analysis Part B Polymer physics

Novel diamic acids (DAAs) and poly(amide amic acid)s (PAAs) were prepared and their thermal cycloimidization to the corresponding imide form was investigated by thermogravimetric analysis under isothermal conditions at four different temperatures, that is, at 175, 200, 225, and 260 °C for 75 min. A general equation, 18NW/RMW, where, the numerical 18 corresponds to the molecular weight of water, N is the number of water molecules, which would be eliminated per repeat unit of the PAA upon cycloimidization, W is the weight of PAA taken for TGA, and RMw is the molecular weight of the repeat unit of PAA, has been derived for the calculation of theoretical amount of weight loss of PAA upon complete cycloimidization. The degree of cycloimidization (DCI%) of PAAs to poly(amide imide)s (PAI) has been calculated from their isothermal TGA curves. The variation in DCI on temperature, time, and the structures of diamine and acid chloride, especially, with respect to meta- and para-linkages and the presence of electron withdrawing groups has been discussed. Cycloimidization occurs at faster rate in the initial stages of about 20 min, curing and then proceeds in a gradual manner and reaches almost a plateau within an hour. The DCI was more at higher temperatures, and the final values were 22-60% at 175 °C, 34-78% at 200 °C, 50-96% at 225 °C, and 85-99% at 260 °C after 75 min of heating, depending on the nature of diamine and acid chloride in the PAA. The DCI of PAAs with meta-linkages in either of diamine or diacid chloride was somewhat lower than those having para-linkages. The DCI of PAAs containing electron withdrawing group like sulfone in the diamine is somewhat higher compared with those of others. The final DCI (%) values obtained from FT-IR spectra and isothermal TGA curves were very close to each other. Further, the thermal and thermooxidative stabilities of the PAIs were discussed. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 2937-2947, 2007