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

Three kinds of aromatic poly(amic acid)s that contain acetylene groups in t he main chain were cured in air at temperatures up to 400 degreesC to give intermolecular crosslinked polyimides. The crosslink reactions occurred at the internal acetylene units and the chemical structures thus generated hav e been investigated by thermal analyses (thermogravimetric analyses (TGA) a nd differential scanning calorimetry (DSC)) and high resolution solid-state C-13 cross polarization (CP)/magic angle spinning (MAS) NMR. The thermal a nalyses indicated that broad exotherms are observed above 300 degreesC for all the polyimides cured at 200, 300, and 350 degreesC, and the exothermal reactions do not eliminate volatile components from the polyimide structure . The C-13 CP/MAS NMR spectra of pyromellitic dianhydride/3,3'-diaminodiphe nyl-acetylene (PMDA/m-intA) polyimides show that the signal intensities of the acetylene carbons and the phenyl carbons directly bonded to the acetyle ne units significantly decrease after curing above 300 degreesC, which indi cates that crosslink reactions occurred during curing. In addition, a signa l 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 degreesC. From the examination of the chemical shift of the new signal , the most probable crosslink mechanism is the Diels-Alder cycloaddition th at occurs between two phenylethynyl groups and provides polycyclic aromatic structures containing biphenyl linkages. The DSC, TGA, and NMR spectra sug gest that the same crosslink reactions occur in the other two polyimides, a lthough they occur in the different temperatures ranges. The signal intensi ties in the NMR spectra indicate that 40-55% of the acetylene carbons remai ned unreacted even after curing at 400 degreesC. This situation significant ly differs from the cases of acetylene- and phenylethynyl-terminated polyim ides, in which almost no acetylene units remained after curing as a result of a variety of crosslink reactions. The crosslink reactions occurring at t he internal acetylene units are more selective and uniform due to the prefe rred layer packing of the diphenylacetylene groups and the restricted molec ular motion in the solid state. (C) 2001 Elsevier Science Ltd. All rights r eserved.