Terphenyl-based colorless and heat-resistant polyimides with a controlled molecular structure using methyl side groups

Abstract

Colorless polyimide (CPI) films have promising prospects for optoelectronic devices. However, balancing optical and thermal properties remains a major challenge from a molecular design perspective. In this work, a methyl regulation strategy is proposed and verified, in which the rod-like and conjugated p-terphenyl is chosen as the skeleton core to guarantee the thermal and mechanical properties of polyimides, while the formation of its charge transfer complexes (CTC) is restricted by the conformational transformation and steric hindrance of molecular chains arising from methyl groups, which ensure its transparency. The substitution position and amount of methyl side groups in the diamines are found to play a decisive role in the transparency and heat resistance of polyimides, regulating the average transmittance in the visible region of 84–88% and the glass transition temperature (T_g) in the range of 396–413 °C. 23HMTD-6FDA with six methyl groups and substitutions at the 2,2′′,3,3′′positions show the best comprehensive performance, with a yellowness index (YI) of 1.66, a T_g of 413 °C, a tensile strength of 158 MPa, and a tensile modulus of 3.4 GPa. The design strategy of methyl side groups is proved to be an effective approach for enhancing various properties of CPI films to accommodate the photoelectric engineering demands.