Thermally Stable Colorless Copolyimides With A Low Dielectric Constant And Dissipation Factor And Their Organic Field-Effect Transistor Applications

We developed highly thermally resistant and colorless polyimides (CPIs) with an ultralow coefficient of thermal expansion (CTE) and sufficient mechanical durability as a flexible substrate for organic field-effect transistors (OFETs). The CPIs were synthesized from trans-1,4-cyclohexyl diamine (t-CHDA) with different ratios of 3,3',4,4'-biphenyltetracarboxylic dianhydride (s-BPDA) and pyromellitic dianhydride (PMDA). The effects of the composition of s-BPDA and PMDA on the thermal, mechanical, electrical, and optical properties of CPIs were investigated. The optimized CPI, PI-3 with 90 mol % s-BPDA and 10 mol % PMDA, showed a relatively high elongation at break (8%) with a low CTE of 14 ppm K--(1), a high glass transition temperature (T-g) of 340 degrees C, and a large tensile modulus (E) of 4.1 GPa, respectively. Besides, PI-3 possessed a high transparency with a light transmittance at 400 nm (T-400) of 81% and a low cutoff wavelength (lambda(cutoff)) of 349 nm. Next, the dimer diamine of DDA (Priamine 1074) was introduced into the PI-3 structure to reduce the dielectric constant and enhance the stretchability. For example, PI-6C, with 85 mol % t-CHDA and 15 mol % DDA, showed a low dielectric constant (D-k) of 2.8, a low dissipation factor (D-1) of 0.004, and a high T-400 of 86% with maintained thermal and mechanical properties. Finally, a flexible OFET device using PI-3 as the substrate and dielectric was fabricated and characterized and exhibited an outstanding performance preservation after 1000 bending cycles or the high-temperature heating test, suggesting its excellent durability. The experimental results indicate that the CPIs studied have potential applications for transparent organic electronic devices.