In order to prepare polyimide materials with excellent optical transparency and UV barrier, the effects of the number and substitution position of −CF3 groups on polyimide properties were investigated. Researchers synthesized four novel diamine monomers (BPA-O, BPA-NH2, BPAF-O, BPAF-NH2) and synthesized eight high-performance colorless polyimides (PI-1 ∼ PI-8). On the one hand, the strong electron-withdrawing effect of −CF3 restricts electron movement within the molecular chain, weakening the CTC effect of the molecular chain and thereby enhancing the transparency of the film. On the other hand, the ortho-substitution of −CF3 on the imide ring induces greater twisting of molecular chains, disrupts their close packing, and thereby exerts a positive effect on enhancing the optical transparency of the film. The high bond energy of the C-F bond and the introduction of the flexible group −O- act as an “energy buffer,” suppressing UV-induced main chain breakage and achieving high UV barrier. All CPI films exhibit excellent transparency and thermal stability, with a cut-off wavelength range of 325 nm-381 nm and a transmittance range of 78.7 %-88 % at 550 nm. They have good optical properties, with Td5% distributed in the range of 511.5 °C − 533.7 °C, and the glass transition temperature (Tg) distributed in the range of 197.3 °C-234.4 °C, demonstrating good thermal stability. Additionally, the cut-off wavelength is at a relatively high level, reaching up to 381 nm.
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