Enhancement in thermal stability and transparency enabled by solvent engineering via formic acid

Although polyimides (PIs) have been widely used in various fields such as microelectronics, aerospaces, automobiles and flexible electronics, in most cases, they cannot simultaneously fulfill the requirements such as dielectric properties, thermal stability, thermoplasticity, transparency and so on. As numerous devices are increasingly being operated under extreme conditions, it is highly desirable to endow PIs with outstanding comprehensive performance. Herein, we introduce a solvent engineering approach to address this issue by using formic acid (FA). The poor solvent FA for poly (amic acid) leads to an increased degree of chain entanglements of curled-up polymer chains and loosened chain packing. As a result, PI TFMB-BPDA treated by FA is able to achieve higher T g of 381°C, lower CTE of 23 ppm K −1 , higher T 400–800 of 76.7% as well as better mechanical performance than PI TFMB-BPDA with T g of 362°C, CTE of 38 ppm K −1 and T 400–800 of 70.8%. However, this strategy exhibits varying effectiveness for different kinds of PIs. By comparing the cases of two kinds of PIs, it is found that CTE is not only related to dense chain packing but also the aggregation structure of PIs.