Optically Transparent Sulfur-Containing Polyimide−TiO2 Nanocomposite Films with High Refractive Index and Negative Pattern Formation from Poly(amic acid)−TiO2 Nanocomposite Film

Abstract

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 °C, both in air and nitrogen. The glass-transition temperatures of the PIs ranged from 236.5 to 274.1 °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 nm, 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 µm 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.