Sulfur-Rich Polyimides Containing Bis(3-(trifluoromethyl)phenyl)thiophene for High-Refractive-Index Applications

Abstract

Three series of sulfur-containing polyimides with the bis(3-(trifluoromethyl)phenyl)thiophene moiety were reported. The synthesis of high-refractive-index polyimides was carried out using a nucleophilic substitution reaction. For that, two new sulfur-containing aromatic amino monomers were developed, one bifunctional and one trifunctional. The monomer and polymer structures were successfully characterized by NMR and Fourier transform infrared (FTIR) spectroscopic methods. Free-standing polyimide films of ∼10 μm were prepared, and they were found to have high thermodynamic stability with Tg as high as 257 °C and Td5% above 500 °C. The films were flexible as well as tough with up to 16 ± 7% elongation at break, with a Young's modulus of 2.6 ± 5 GPa. Indication for melt processability of one of the polyimides was also demonstrated at a temperature of 30 °C above its Tg. Thin films with thickness in the nanometer range were coated over modified Si-wafers. Increase in the sulfur and aromatic contents in the polymer backbone resulted in the increase of their refractive index; thus, the polymers synthesized in this work exhibited refractive index as high as 1.76 at 589 nm. The transmittance and birefringence of the polyimides were found to improve by modification in the chemical structure. The tuneable optical properties and good dynamic stability of the polyimides make them potential candidates in optoelectronic applications.