Achieving both low thermal expansion and low birefringence for polyimides by regulating chain structures

The synthesis of polyimides (PIs) with both low coefficient of thermal expansion (CTE) and low birefringence still faces a big challenge. In this study, we prepared a new kind of PI (PI-c-P), derived from a diamine monomer (cisdiamine-DBCOD) that contains dibenzocyclooctadiene (DBCOD) units, which exhibits near-zero thermal expansion with a CTE value of 1.5 ppm/K in the temperature range of 0-150 degrees C due to the unique conformation transition of DBCOD unit. The increase in chain rigidity and cis-diamine-DBCOD content could effectively reduce the CTE values by promoting conformation transition. Cross-linking could restrict chain conformation transition and impair thermal contraction, leading to higher CTE values. Unlike the conventional low CTE PIs with high orientation or crystallization, the birefringence of DBCOD-containing PI films was in the range of 0.0021 similar to 0.0094, showing both low birefringence and low thermal expansion, this property is extremely important for its application in optical and electronic devices.