Preparation and characterization of organic soluble polyimides with low dielectric constant containing trifluoromethyl for optoelectronic application

Polyimide (PI) films have excellent comprehensive properties and bright prospects in the next generation of optoelectronic and microelectronic applications. PIs were synthesized by aromatic diamines and alicyclic dianhydrides considered to be the most effective method for preparing polyimides with high light transmittance and low dielectric constant. In order to improve the overall performance of polyimide, the introduction of a large trifluoromethyl group into the main chain is also considered a powerful strategy. In this article a novel diamine containing 6 trifluoromethyl groups has been designed and synthesized by Suzuki reaction. Afterwards, PIs were synthesized with fluorinated aromatic dianhydride and alicyclic dianhydride through a two-step method. The two PIs have excellent solubility: PI-a and PI-b exhibited excellent solubility in common polar solvents. Such as DMF, DMAc, NMP and DMSO at room temperature, and PI-a was soluble in low-boiling solvents such as ethyl acetate, CH2Cl2 and CHCl3 at room temperature. PI-b was soluble in low-boiling solvents acetone and CHCl3 at room temperature, which was rarely seen in other aromatic polyimides. Soluble in common organic solvents, especially low boiling point solvents, made these polymers easy to process and suitable for processing technology in solution. This was an important modification for PIs. They also showed good high temperature resistance: the decomposition temperature (T5%) of two PIs were 520 °C and 475 °C. In addition, the residual rate at 790 °C was 50% and 49% respectively. Two PI films had a very low dielectric constant 2.26 and 2.24, respectively, which was attributed to the hyperbranched structure and strong electron-withdrawing fluorine-containing groups. PI-a and PI-b also had low water absorption of 0.66% and 0.72%, respectively. The transmittance of PI-a and PI-b high transparent films at 450 nm were 74% and 82%, respectively. The significant optical modification was mainly due to the weak electron-accepting alicyclic unit and the large trifluoromethyl electron withdrawing group. This work mainly provided an effective strategy for synthesizing comprehensive excellent performance PIs with high transmittance, excellent solubility, low water absorption, high heat resistance and low dielectric constant.