Nonaromatic Polyimides Derived from Cycloaliphatic Monomers

Abstract

A novel tetracarboxylic dianhydride bearing a cycloaliphatic structure, (4arH,8acH)-decahydro-1t,4t:5c,8c-dimethanonaphthalene-2c,3c,6c,7c-tetracarboxylic 2,3:6,7-dianhydride (DNDA), was synthesized, and its structure was determined using X-ray crystallographic analysis. Polycondensation of DNDA or bicyclo[2.2.1]heptane-2-endo,3-endo,5-exo,6-exo-tetracarboxylic 2,3:5,6-dianhydride (BHDA) with a cycloaliphatic diamine, bis(aminomethyl)bicyclo[2.2.1]heptane (BBH) was carried out in HMPA. A viscous poly(amic acid) solution was obtained only when BBH was slowly added to the HMPA solution of the dianhydride, whereas the reverse adding order of the monomers resulted in precipitation due to salt formation. The poly(amic acid) solutions afforded nonaromatic polyimides as flexible and free-standing films after being cast and cured. The polyimide films were soluble in organic solvents such as DMAc and were colorless with a cutoff at wavelength shorter than 235 nm. The average refractive index of the BHDA−BBH polyimide film was 1.522, and the optical anisotropy, the difference between in-plane and out-of-plane indices, was nearly zero. The dielectric constant that was estimated from the refractive index according to the modified Maxwell equation was 2.55. The polyimides have good thermal stability with the 5% weight-loss temperatures around 450 °C. The polyimide films exhibit higher Tg's over 290 °C. The high-temperature stability may be caused by the introduction of polyalicyclic structures, which results in an increase in the polymer main chain rigidity and less probability of main-chain scission.