Semiaromatic Polyimides based on Bis(Aminomethyl)Bicyclo[2.2.1]Heptane

Abstract

Semiaromatic polyimides with a polyalicyclic structure were prepared by the polycondensation of bis(aminomethyl)bicyclo[2.2.1]heptane (BBH) with aromatic dianhydrides. The corresponding poly(amic acid)s possessed an inherent viscosity range of 0.60–1.00 dl g⁻¹. The polyimides were soluble in organic polar solvents and most of them formed flexible, free-standing films. The polyimides had good thermal stability with no significant weight loss up to approximately 400 °C and possessed a 5% weight loss temperature range of 428–465 °C. These polyimide films showed a tensile modulus range of 1.4–3.0 GPa, a tensile strength range of 62–115 MPa, and an elongation at break range of 5.4–6.1%. The semiaromatic polyimide films exhibit a cut-off wavelength around 322–362 nm and are entirely colourless, while the cut-off wavelengths of PI(PMDA-DDE) and PI(BPDA-DDE) films were 426 and 376 nm respectively. The origin of coloration in the aromatic polyimides and the lack of colour in the semiaromatic polyimides were elucidated from the standpoint of molecular orbital (MO) calculations of the model compounds using semiempirical AM1 and INDO/S methods. The calculated λ_max of the M(PMDA-BBH) was 371 nm, and the first excited state is mainly characterized by an n−π* transition. Based on both the MOs and the difference in electron density between the ground and the first excited states, the first excited state is attributable to light-induced polarization in the five-membered imide ring. The λ_max of the M(PMDA-DDE) model for a representative aromatic polyimide was calculated to be 392 nm. From the charge difference map, it is theoretically evident that the coloration in the aromatic polyimides caused intramolecular charge transfer between the diamine and dianhydride moieties.