High Transparency and Colorless Polyimide Films Derived from Aliphatic-Chain Containing Diamine for Display Technology

The transparent flexible substrate is vital for flexible displays, acting as both a carrier and a support. This study presents a molecular design strategy to enhance the optical and dielectric properties of colourless polyimide (CPI) films by incorporating flexible alkyl segments. First, we designed and synthesized three diamine monomers with varying alkyl chain lengths, and then polymerized them with commercially available 6FDA (2,2′-bis(3,4-dicarboxyphenyl) hexafluoropropane dianhydride) via solution polycondensation to produce a series of colorless polyimide films. To further fine-tune the material properties, controlled copolymerization with fluorinated diamine TFMB (2,2′-bis(trifluoromethyl)-4,4′-diaminobiphenyl) was performed, resulting in advanced CPIs with superior optical clarity and dielectric properties (ε = 2.8 ∼ 3.5, tanδ = 0.01 ∼ 0.02 at 1 MHz). The TDA-CPI-2 film (18 µm) demonstrated high optical transparency ( T 400 =  82.9%, haze  =  0.28, YI = 0.73) with a UV cut-off at 340 nm. It exhibited low water absorption (1.89%) and a hydrophobic surface (contact angle of 80.84°), ensuring stable dielectric performance under varying humidity conditions. The film maintained excellent thermal ( T g = 172.4 °C, Td5% = 491.4 °C) and mechanical properties (tensile strength of 74.5 MPa, modulus of 1.37 GPa), while being solution-processable for practical applications. This work offers a versatile approach to developing high-performance CPI films for advanced optoelectronic flexible display devices. Graphical The three series of transparent and colorless polyimide films (EDA-CPI-X, HAD-CPI-X, TDA-CPI-X) exhibit exceptional optical clarity and outstanding dielectric properties (dielectric constant ε = 2.8~3.5, dissipation factor tanδ = 0.01~0.02 at 1 MHz).