High performance transparent polyimides by controlling steric hindrance of methyl side groups

Highly transparent polyimides (PIs) with excellent combined properties are in great demand for flexible optical devices. In this work, transparent PIs were synthesized by fluorinated aromatic dianhydride (6FDA) and three aromatic diamines containing methyl side groups with different amount and position. These PIs possessed moderate to great optical transparency and excellent solubility. With four methyl groups at 2,2′,5,5′-position in diamine, 2255TMB–PI exhibited the best transmittance up to 82% at 400 nm. In all cases, PIs were determined to have low dielectric constants (2.84–2.99) and high glass transition temperature (above 390 °C, even up to 407 °C for 2255TMB–PI). In addition, the toughness of these PI films was also proven by a tensile strength of 130–156 MPa. In spite of the electron donating character of methyl groups, their steric hindrance was found to have significant effect on the combination properties of PIs. Spatial structures and packing morphology are thought to be the main contribution. Therefore, this work provided an easy way to prepare highly transparent polyimides with excellent combined properties by controlling steric hindrance through methyl side groups.