Ultralow-Dielectric-Constant Films Prepared from Hollow Polyimide Nanoparticles Possessing Controllable Core Sizes

Polyimide (PI) is a candidate material that is showing promise for use in a wide range of applications in several advanced technologies. In this study, we employed the reprecipitation method and subsequent imidization to prepare unique hollow PI nanoparticles (NPs) by blending a suitable polymer porogen with the PI precursor in the form of individual NPs. The hollow structures were induced through phase separation between the PI and the porogen, a process that was influenced by the compatibility between the two polymers. We then assembled multilayered films by depositing the hollow PI NPs electrophoretically onto a substrate. After spin-coating a solution of the PI precursor and then subjecting the system to thermal treatment, the individual PI NPs were bound to their adjacent neighbors, thereby improving the film strength to some extent. We obtained dense, uniformly packed films having controlled thicknesses in the range from 500 nm to 10 μm. This strategy provided films in which air voids existed between and within the composite PI NPs; as a result, the dielectric constant reached as low as 1.9.