Dual-processable semi-aromatic fluorinated polyimides with outstanding optical transparency and low dielectric constants prepared from in situ [2 + 2] thermal polymerization

Abstract

Developing a highly efficient synthetic strategy has been a crucial issue in the preparation of semi-aromatic polyimides with N-alkyl substituted imide segments. In this work, a class of new imides containing bis-functional trifluorovinyl-ether (TFVE) substitutes were successfully synthesized via hydrosilylation from N-allyl substituted diimides. Polymerizations were then realized through directly thermo-induced [2π + 2π] cycloaddition between TFVE groups to generate semi-aromatic polyimides with perfluorocyclobutyl (PFCB) linkers. The obtained polyimides exhibited good thermostability with a 5% weight loss temperature higher than 480 °C. They also showed excellent processability and good solubility in common organic solvents. All the polyimides could be fabricated either into flexible free-standing polymer films or robust polymer sheets. Moreover, these semi-aromatic polyimides showed excellent transparency. Polyimide sheets with a thickness of 2 mm exhibited an optical transmittance of 70–75% at 450 nm, with a low cut-off wavelength of 280–340 nm. In particular, 1,2,4,5-cyclohexanetetrcarboxylic dianhydride-derived polyimide (HPMDA-PFCB) was colorless with a high transmittance of 75% even at 400 nm. Owing to the presence of N-alkyl and PFCB structures, these polyimides also exhibited low dielectric constants (2.41–2.51) under high frequencies ranging from 18 GHz to 26.5 GHz. Additionally, the polyimide films showed favorable mechanical properties with a storage modulus in the range of 1.2–2.2 GPa, obtained from DMA measurements. All the results indicate that this work provides a special pathway to prepare well-defined semi-aromatic polyimides with excellent transparency and good insulating properties, which presents potential application as flexible dielectrics in microelectronics.