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 t heir T-g's could not he determined in spite of their wide usages. In t he present study, the measurement of T-g was attempted fur poly(amic a cid) precursors of three different aromatic polyimides synthesized in N-methyl-2-pyrrolidone (NMP) from the respective dianhydrides and diam ines: poly(4,4'-oxydiphenylene pyromellitamic acid) (PMDA-ODA), poly(p -phenylene 3,3',4,4'-oxydiphthalamic acid) (ODPA-PDA), and poly(p-phen ylene 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 di fferential scanning calorimetry Compositions in the mixtures were dete rmined by proton nuclear magnetic resonance spectroscopy. For solvent- rich mixtures, a melting point depression of the NMP solvent was obser ved, whereas for precursor rich mixtures, T-g depression was detected. In particular, T-g's measured for the precursor rich mixtures were be st fitted by a modified Gordon-Taylor equation as a function of compos ition, 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 PM DA-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 flexi bility, 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 dia gram was constructed Cor the PMDA-ODA precursor/NMP mixture. For the h ighly dried precursor samples, the chemical repeat unit, was also dete rmined to complex with 1.4-1.7 NMP molecules, depending on the precurs ors.