Investigation of glass transition behaviours in aromatic poly(amic acid) precursors with various chain rigidities by oscillating differential scanning calorimetry

Abstract

Most aromatic poly(amic acid)s, which are precursor polymers of polyimides, are thermally converted to the corresponding polyimides via imide-ring closure formation over their glass transition temperature ( T g) ranges or below. In addition, it is very difficult to remove completely solvents used in the synthesis from their cast samples, due to the formation of complexes between the solvent molecules and the orthoamic acid groups in poly(amic acid)s. For these reasons, it is not possible to measure T g values of poly(amic acid)s with residual solvent or of poly(amic acid)s with solvent free in conventional ways. However, in the present study T g values of several poly(amic acid)s were successfully measured as a function of the content of residual solvent by the recently developed oscillating differential scanning calorimetry: poly( p-phenylene biphenyltetracarboxamic acid) (BPDA-PDA), poly( p-phenylene pyromellitamic acid) (PMDA-PDA), and poly(4,4′-oxydiphenylene biphenyltetracarboxamic acid) (BPDA-ODA) precursors. The content of residual solvent in cast poly(amic acid) samples was independently determined by proton nuclear magnetic resonance spectroscopy. By fitting the measured T g values by a modified Gordon–Taylor equation as a function of composition, T g values of the poly(amic acid)s in solvent free were estimated: 248.4°C for BPDA-PDA, 283.4°C for PMDA-PDA, and 184.4°C for BPDA-ODA precursor. The estimated T g values were correlated to the calculated Kuhn statistical segments ( L K values), which are the measures of chain rigidities: 62.2 Å for BPDA-PDA, 88.7 Å for PMDA-PDA, and 41.2 Å for BPDA-ODA: That is, high chain rigidity causes higher T g. In addition, phase diagrams of the poly(amic acid)/ N-methyl-2-pyrrolidone solvent mixtures were constructed. The number of solvent molecules bound to the chemical repeat unit were also estimated to be 1.46–2.16 in the highly dried precursors, depending on the poly(amic acid)s.