Preparation and properties of colorless and transparent semi‐alicyclic polyimide films with enhanced high‐temperature dimensional stability via incorporation of alkyl‐substituted benzanilide units

Abstract Two colorless and transparent polyimide (CPI) films with enhanced high‐temperature dimensional stability and solution‐processability were prepared from the methyl‐substituted benzanilide containing organo‐soluble polyimide (PI) resins. For this target, a new aromatic diamine, 2,3′‐dimethyl‐4,4′‐diaminobenzanilide (MMDABA) was synthesized. The methyl substituents were expected to endow the derived PI resins good solubility and rigid‐rod benzanilide units can efficiently reduce the coefficients of thermal expansion (CTE) of the derived CPI films. CPI‐a and CPI‐b were prepared by the one‐step thermal polycondensation from MMDABA with hydrogenated pyromellitic dianhydride for CPI‐a and hydrogenated 3,3′,4,4′‐biphenyltetracarboxylic dianhydride for CPI‐b, respectively. The derived PI resins were soluble in polar aprotic solvents. Colorless and transparent CPI‐a and CPI‐b films were prepared from the PI solutions and the subsequent 280°C curing. The CPI films showed good transparency in visible light and transmittances higher than 80% at 450 nm and yellowness index ( b* ) below 2.0. More importantly, the derived CPI‐a film showed the glass transition temperature ( T g ) of 417.5°C and the CTE value of 46.9 × 10 −6 /K in the range of 50–250°C. Apparently, incorporation of alkyl‐substituted benzanilide units achieved good balance among solution‐processability, high‐temperature dimensional stability, and optical properties.