Structurally tailored colorless transparent polyimides via alkyl substituent engineering and dianhydride selection: Optical, thermal with DFT insights

In this work, four biphenyl diamine monomers bearing gradient steric hindrance from methyl and tert-butyl substituents were synthesized and copolymerized with 6FDA and CHDA dianhydrides to fabricate a series of colorless and transparent polyimide (CPI) films. The regulating mechanisms of substituent steric hindrance and dianhydride backbone on the optical, thermal, surface wetting and water absorption performances of the resulting CPIs were systematically investigated. The alkyl steric hindrance was proven to effectively suppress intermolecular charge-transfer complex (CTC) formation and improve the optical transmittance of films. Methyl-modified CPIs delivered superior comprehensive optical performance, while tert-butyl-incorporated systems exhibited enhanced thermal stability and hydrophobicity. Notably, asymmetric tert-butyl substituents introduced excessive molecular torsional distortion and aggravated yellowing of the films. Combined with time-dependent density functional theory (TD-DFT) calculations, the steric hindrance-induced anti-yellowing mechanism was clarified at the electronic level. Among all samples, the Y-2TB@6FDA film achieved a balanced performance of high transparency, outstanding thermal resistance and low water absorption (L * = 95.79, T d5% = 539°C). This work offers a reliable molecular design strategy for developing high-performance CPI optoelectronic materials.