Synthesis and characterization of optically transparent semi-aromatic polyimide films with low fluorine content

Abstract

A series of novel semi-aromatic polyimides with low fluorine content were prepared from alicyclic dianhydrides, i.e. 1,2,4,5-cyclohexane-tetracarboxylic dianhydride (CHDA) and bicyclo[2.2.2]octa-7-ene-2,3,5,6-tetracarboxylic dianhydride (BCDA), and three aromatic diamines containing ortho-substituted methyl and pendent phenyl groups, respectively. These polyimides exhibited excellent solubility even though their fluorine content lower than 10 wt%. Chemical resistance to MEK and aqueous alkali of films was also satisfied for application as flexible substrates. The semi-aromatic polyimides showed outstanding heat resistance with Tg values up to 387 °C and good mechanical strength. These films had excellent optical transparency with cutoff wavelengths of 282–286 nm, transmittance >89% at 500 nm and yellowness indices <14.8. Especially, PIBF-3 and PIBF-6 were extremely transparent and very light color with thickness of 70 μm. Their improved optical transparency and elimination of coloration was mainly attributed to weakened electrophilicity of alicyclic dianhydrides, steric hindrance and distorted non-coplanar caused by ortho-substituted and bulky pendent groups.