Investigation of glass transition behaviors in poly(amic acid) precursors of semiflexible polyimides by oscillating differential scanning calorimetry

Aromatic poly(amic acid) precursors always form complexes with dipolar aprotic solvents via strong acid/base interaction and are expected to have relatively high glass transition temperatures (T-g's) which are overlapped with or higher than the imidization temperatures, so that their T-g's could not he determined in spite of their wide usages. In the present study, the measurement of T-g was attempted fur poly(amic acid) precursors of three different aromatic polyimides synthesized in N-methyl-2-pyrrolidone (NMP) from the respective dianhydrides and diamines: poly(4,4'-oxydiphenylene pyromellitamic acid) (PMDA-ODA), poly(p-phenylene 3,3',4,4'-oxydiphthalamic acid) (ODPA-PDA), and poly(p-phenylene benzophenonetetracarboxamic acid) (BTDA-PDA). Phase transitions, as well as imidization reactions in the precursor/NMP mixtures, were measured with varying compositions by a newly developed oscillating differential scanning calorimetry Compositions in the mixtures were determined by proton nuclear magnetic resonance spectroscopy. For solvent-rich mixtures, a melting point depression of the NMP solvent was observed, whereas for precursor rich mixtures, T-g depression was detected. In particular, T-g's measured for the precursor rich mixtures were best fitted by a modified Gordon-Taylor equation as a function of composition, in order to estimate T(g)s of poly(amic acid)s in solvent free, that is, true T-g's of the precursor polymers: 207.4 degrees C for PMDA-ODA, 166.3 degrees C for ODPA-PDA, and 213.2 degrees C for BTDA-PDA precursor. The Kuhn segment length, which is a measure of chain flexibility, was estimated to be 43.3 Angstrom for PMDA-ODA, 34.6 Angstrom for ODPA-PDA, and 34.6 Angstrom for BTDA-PDA. In addition, a phase diagram was constructed Cor the PMDA-ODA precursor/NMP mixture. For the highly dried precursor samples, the chemical repeat unit, was also determined to complex with 1.4-1.7 NMP molecules, depending on the precursors.