Thermo-optical properties of polyimide films with bulky side groups: Experimental and theoretical investigations

In this study, we report the design and synthesis of two novel diamine monomers: hydroxyl-containing 4-(bis(4-amino-3,5-dimethylphenyl) methyl) phenol ( HGDT ) and pyridine-functionalized 4,4’-(pyridin-4-ylmethylene) bis(2,6-dimethylaniline) ( PYDT ). These monomers were copolymerized with three dianhydrides-4,4’-(hexafluoroi-sopropylidene) diphthalic anhydride (6FDA), 1,2,3,4-cyclobutanetetracarboxylic dianhydride (CBDA), and 1,2,4,5-cyclohexanetetracarboxylic dianhydride (HPMDA) via a one-step polymerization method to fabricate seven colorless polyimide films ( PI-2-1 to PI-2-7 ). Theoretical calculations using density functional theory (DFT) and time-dependent DFT (TD-DFT) were performed to predict the optical properties of the polymers. Among binary systems, PI-2-2 exhibits the lowest values for four characteristic parameters (D = 8.603 Å, TSE = 0.99 e, t = 6.279 Å, λ max = 295.65 nm), indicating its excellent colorless transparency. Experimental results further confirm that all seven PI films exhibit high optical transmittance, ranging from 53.85%∼82.61% at 450 nm and 71.46%∼89.42% at 500 nm. Notably, PI-2-2 achieves the highest transmittance (82.61% at 450 nm, 89.42% at 500 nm), and the film exhibited the best overall performance: (i) it exhibits a 5% weight-loss temperature (T 5% ) as high as 493.12°C, T 10% of 520.72°C, and T g of 351.0°C, indicating exceptional thermal stability; (ii) remarkable optical transparency (82.61% transmittance at 450 nm, 89.42% at 500 nm) with low haze (1.89); and (iii) superior colorlessness (CIE coordinates: L* = 94.72, a* = −0.56, b* = 0.41). The incorporation of bulky pyridine side groups was found to significantly enhance both thermal stability and optical transparency, positioning these advanced PIs films as promising candidates for next-generation flexible display technologies.