Abstract
Three kinds of polyimide precursors, poly(amic acid) (PAA), poly(amic ester)s (PAEs para-, meta-, and mixed isomers), and poly(amic acid) amine salt (PAS), were synthesised from pyromellitic dianhydride and oxydianiline. Thermal imidisation behaviour of polyimide precursors was investigated using TGA and DTGA. In comparison with PAA, the imidisation temperature of PAS shifted downwards while the imidisation temperatures of PAEs shifted upwards at a given extent of imidisation. All precursors showed a two step imidisation behaviour, with fast imidisation at an early stage and slow imidisation at a later stage. Kinetic parameters were obtained from the logarithmic plots of conversion v. time during isothermal curing, which followed the first order rate equation during both fast and slow imidisation processes. The imidisation rates in the fast and slow imidisation stages followed the order: PAS>PAA> PAEs. Among the isomeric PAEs, para- PAE imidised fastest while reaction rates of the meta-PAE and mix-PAE depended on the imidisation temperature used. The imidisation behaviour was related to the probable imidisation mechanisms and other factors, such as the nature of the leaving group and conformations of precursor polymer chain.
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