High Transverse Relaxation Microsphere Induced Low <i>K</i> Toughness Polyimide Films

Polyimide films have been widely used in the fields of microelectronics and aerospace due to their exceptional overall performance. However, achieving simultaneous improvements in the dielectric, toughness, and mechanical properties of PI films has been a critical and challenging problem. Herein, pyromellitic dianhydride and 4,4′-oxydianiline were used as reactive monomers to prepare polyamic acid porous microspheres (PM) (PAA–PM) and polyimide PM (PI–PM) using a multiple emulsion method. The results show that PAA–PM has high transverse relaxation times (T2) in the precursor solution, which enables high additive capacity, and its homogeneous dispersion in the precursor solution benefits from the hydrogen bond interactions between the carboxyl groups of PAA–PM and the molecular chains of the precursor. Due to the formation of a microcross-linking bridged structure between PAA–PM and the precursor after thermal imidization, the toughness of the final composite film was increased by 332.6% compared to pure PI film, while the tensile strength and elongation at break were improved by 24.5% and 71.1%, respectively. In addition, the dielectric constant decreased by 23.1% (from 3.63 to 2.79 at 1 MHz) and the incorporation of microspheres has almost no effect on thermal stability of the film (T5% of 559.46 °C). The PM effectively mitigated the deterioration of dielectric properties caused by the reduction of the free volume fraction during the thermal stretching of the films.