Thermal analysis and solid-state 13C NMR study of crosslink in polyimides containing acetylene groups in the main chain

Three kinds of aromatic poly(amic acid)s that contain acetylene groups in the main chain were cured in air at temperatures up to 400°C to give intermolecular crosslinked polyimides. The crosslink reactions occurred at the internal acetylene units and the chemical structures thus generated have been investigated by thermal analyses (thermogravimetric analyses (TGA) and differential scanning calorimetry (DSC)) and high resolution solid-state 13C cross polarization (CP)/magic angle spinning (MAS) NMR. The thermal analyses indicated that broad exotherms are observed above 300°C for all the polyimides cured at 200, 300, and 350°C, and the exothermal reactions do not eliminate volatile components from the polyimide structure. The 13C CP/MAS NMR spectra of pyromellitic dianhydride/3,3′-diaminodiphenylacetylene (PMDA/m-intA) polyimides show that the signal intensities of the acetylene carbons and the phenyl carbons directly bonded to the acetylene units significantly decrease after curing above 300°C, which indicates that crosslink reactions occurred during curing. In addition, a signal that can be assigned to the C1-carbon of biphenyl structure newly appears after curing, and it grows as the curing temperature increases from 350 to 400°C. From the examination of the chemical shift of the new signal, the most probable crosslink mechanism is the Diels–Alder cycloaddition that occurs between two phenylethynyl groups and provides polycyclic aromatic structures containing biphenyl linkages. The DSC, TGA, and NMR spectra suggest that the same crosslink reactions occur in the other two polyimides, although they occur in the different temperatures ranges. The signal intensities in the NMR spectra indicate that 40–55% of the acetylene carbons remained unreacted even after curing at 400°C. This situation significantly differs from the cases of acetylene- and phenylethynyl-terminated polyimides, in which almost no acetylene units remained after curing as a result of a variety of crosslink reactions. The crosslink reactions occurring at the internal acetylene units are more selective and uniform due to the preferred layer packing of the diphenylacetylene groups and the restricted molecular motion in the solid state.