Controlling the Pore Structure of Polyimide Films Prepared by Exposure to High-Pressure CO2 and UV Light

Porous polyimide is a promising low-permittivity material for reducing the attenuation of high-frequency signals. Previously, the suitability of porous polyimide films for flexible printed circuits was tested by subjecting them to cover-layering and copper plating. These treatments resulted in pore collapse and infiltration, respectively, indicating the necessity of using closed-pore films. Herein, closed-pore porous polyimide films were prepared at various CO2 gas pressures, tertiary amine methacrylate monomer concentrations, and pre-baking times, revealing the key role of pre-baking time in maximizing porosity while preserving closed pores. In contrast to our previous study, the formation of closed pores was explained by a novel mechanism featuring CO2 bubble nucleation as a key step.