Colorless and transparent semi‐alicyclic polyimide films with intrinsic flame retardancy based on alicyclic dianhydrides and aromatic phosphorous‐containing diamine: Preparation and properties

Semi‐alicyclic colorless and transparent polyimide (PI) films with intrinsically flame‐retardant features, excellent optical transparency, and good thermal stability have been designed and successfully prepared. For this target, an aromatic diamine with lateral diphenylphosphine oxide substituent, 2,5‐bis[(4‐aminophenoxy)phenyl]diphenylphosphine oxide (BADPO) was polymerized with two alicyclic dianhydrides, including hydrogenated pyromellitic dianhydride (HPMDA) and hydrogenated 3,3′,4,4′‐biphenyltetracarboxylic dianhydride (HBPDA), respectively, via a one‐step high‐temperature polycondensation procedure to afford two PIs, PI‐a (HPMDA‐BADPO), and PI‐b (HBPDA‐BADPO). The derived PI films maintained the intrinsic good properties for semi‐alicyclic PI films, including excellent optical transparency in the ultraviolet–visible light region with the cutoff wavelength ( λ cut ) around 322 nm and optical transmittance at the wavelength of 450 nm ( T 450 ) higher than 83.0%. In addition, the PI films exhibited good thermal stability with the 5% weight loss temperatures ( T 5% ) higher than 460.0°C and glass transition temperatures ( T g ) higher than 235.0°C. More importantly, the developed PI films exhibited intrinsic flame retardancy with the limited oxygen indices (LOI) of 41.1% for PI‐a and 28.8% for PI‐b. What is more, the PI films showed the flame retardancy class of UL94 VTM‐0 and low thermal energy and smoke release during combustion.