Biobased and Colorless Polyimides With High Biomass Content and Transparency Derived From Camphor-Based Alicyclic Diamines.

Polyimides (PIs) are widely used as advanced substrates due to their excellent thermal and mechanical properties. However, conventional petroleum-based aromatic PIs suffer from intrinsic coloration arising from charge-transfer complex (CTC) formation, limiting their optoelectronic applications. In this study, two camphor-derived alicyclic diamines (CA and CM) were synthesized, along with a reference isosorbide-derived diamine (IS). These monomers were polymerized with 1,2,3,4-cyclobutanetetracarboxylic dianhydride (CB) and a tetracarboxylic dianhydride featuring a cyclopentanone bis-spironorbornane structure (Cp) to prepare six biobased CPIs. Their favorable optical transparency arises from the combined contribution of the non-conjugated dianhydride structures and the alicyclic camphor-derived diamine structures. The structure-property relationships of the biobased CPIs were systematically investigated. The CA series exhibits limited film-forming ability because of its highly rigid molecular structure. In contrast, incorporating a methylene spacer into the CM series enhanced chain flexibility, leading to improved film-forming ability and overall film quality. Among the prepared CPIs, CM-CB exhibited the most balanced performance, achieving 60% biomass content, optical transparency (T400 = 82% and YI = 3), and a glass transition temperature above 300°C. These results suggest that camphor-derived diamines can serve as useful biobased units for developing CPIs with balanced optical transparency and thermal properties.