Polyimides with an Ultralow Coefficient of Thermal Expansion Enabled by Electron-Donating Aromatic Groups

Polyimides (PIs) always serve as substrates in the fields of microelectronics, solar cells, and flexible displays, followed by the deposition of other functional layers on their surface. Therefore, PIs should be well matched with these functional layers in terms of thermal dimensional stability to ensure the device performance. Considering that copper, glass, and silicon have a low coefficient of thermal expansion (CTE), PIs with high thermal stability, especially thermal dimensional stability, are highly desirable. Besides cross-linking and the formation of hydrogen bonds, introducing rigid units is another effective approach to achieve a low CTE without issues such as low toughness or high moisture uptake. However, the monomers involving rigid moieties sometimes present low polymerization activity. Moreover, PIs with a low CTE together with a good comprehensive performance are still worth further research. To address these issues, we introduced aromatic donor groups, triphenylamine and carbazole, into the main chains of PIs. On the one hand, they can enhance the electron-donating ability of diamines and thereby the charge transfer complex (CTC) effect. On the other hand, large conjugated triphenylamine and carbazole groups are conducive to further enhancement of the intermolecular interaction and chain rigidity. Furthermore, carbazole-based diamine has a better coplanarity and higher polymerization activity. As a result, the carbazole-based PIs achieve a lower CTE, particularly carbazole-based PI CzDA-PMDA, exhibiting an ultralow CTE of 5 ppm K^–1. Moreover, PI CzDA-PMDA also yields a high T_g of 445 °C, a dielectrical breakdown strength of 193 kV mm^–1, and a tensile strength of 152 ± 11 MPa along with a tensile modulus of 4.2 ± 0.5 GPa.