Synthesis and Characterization of a Colorless Polyimide Membrane With a Low Dielectric Constant

Recently, colorless polyimide (CPI) substrates with excellent optical properties, a suitable thermal stability, and a low dielectric constant have become a key factor in the development of wearable and portable flexible electronic devices. A series of branched CPI films has been successfully prepared here by using a self-made triamine (1,3,5-triple (2-trifluoromethyl-4-aminobenzyl) benzene [TFAPOB]). By adjusting the ratio of the cyclopentanone bis-spironorbornane tetracarboxylic dianhydride (CPODA) monomer and the 4,4-(hexafluoroisopropyl)-diphthalic anhydride (6FDA) monomer, an optimal film (CBPI-10) with the highest content of CPODA was obtained. This film could achieve the highest transmittance of 90% at 550 nm and the highest glass transition temperature of 311°C. This result could be explained by the fact that CBPI-10 contained the least amount of trifluoromethyl side groups and the largest amount of alicyclic units, which resulted in the generation of the minimum free volume fraction (according to molecular dynamics simulations) and the disruption of the conjugation effect between the benzene rings. Additionally, it is worth mentioning that the highest dielectric constant of these films was only 2.17, which was significantly higher than those of traditional CPI polymers (2.61–2.72). Consequently, the present study has focused on developing a novel manufacturing scheme for branched CPIs, which is of guidance importance for the design of colorless polymers and the advancement of flexible display technology.