Optically transparent sulfur-containing polyimide-TiO2 nanocomposite films with high refractive index and negative pattern formation from poly(amic acid)-TiO2 nanocomposite film

A series of semialicyclic polyimides (PIs) have been successfully prepared by the polycondensation of two alicyclic dianhydrides, 1,2,3,4-cyclobutanetetracarboxylic dianhydride (CBDA) and 1,2,4,5-cyclohexanetetracarboxylic dianhydride (CHDA) with two sulfur-containing aromatic diamines, 4,4'-thiobis[(p-phenylenesulfanyl)aniline] (3SDA) and 2,7-bis(4-aminophenylenesulfanyl)thianthrene (APTT), respectively, by a two-step polymerization procedure via the soluble poly(amic acid) (PAA) precursors. Flexible and tough PI films were obtained with a tensile strength greater than 82 MPa and elongation at breaks higher than 12%. The films showed good thermal and thermal-oxidative stability without significant weight loss up to 400 degrees C, both in air and nitrogen. The glass-transition temperatures of the PIs ranged from 236.5 to 274.1 degrees C. Good optical transparency, including the cutoff wavelengths lower than 350 nm and transmittances higher than 80% at 400 nm, was achieved for all the PI films. The refractive indices of the PI films were in the range of 1.6799-1.7130 measured at 632.8 mn, which agreed well with the values calculated by the time-dependent density functional theory (TD-DFT). In addition, PAA-IIa (CHDA-3SDA) showed a good affinity with silica-modified TiO2 nanoparticles. The hybrid system with 45 wt % load of TiO2 nanoparticles, combined with the photobase generator N-[[(4,5-dimethoxy-2-nitrobenzyl)oxy]carbonyl)2,6-dimethylpiperidine (DNCDP) exhibited good photolithographic characteristics. A fine negative pattern with a resolution of approximately 4 mu m was successfully printed on the hybrid film. The cured PI-IIa-TiO2 hybrid film exhibited a refractive index of 1.8100 at 632.8 nm.