Morphology and properties of porous polyimide films prepared through thermally induced phase separation

Abstract

Porous polyimide (PI) synthesized from 4,4′-oxydiphthalic anhydride (ODPA) and 4,4′-diaminodiphenyl ether (ODA) monomers is a promising material with an ultralow-dielectric constant. We report here a strategy toward engineering PI films of various porous textures using a small molecular phase dispersion agent, dibutyl phthalate (DBP), as porogen. In the presence of DBP, the ODPA–ODA poly(amic acid) solution, the precursor to PI, undergoes phase separation as N,N-dimethylacetamide (DMAc) solvent is slowly evaporated, forming spherical domains of DBP phase uniformly dispersed in the polyamidic acid matrix. Upon thermal imidization, a polyimide film with high porosity is attained after acetone extraction of DBP. It is demonstrated in this study that the porous texture of PI films can be readily engineered by tailoring the initial DBP content. The average pore size increases with increasing concentration of DBP, but was no larger than 6 μm. The PI film achieves a dielectric constant of 1.7 at an optimal porosity of 72%, This study examined the pore formation mechanism, the imidization chemistry, the surface morphology, the density, the thermal stability and mechanical properties of the formed porous PI films. Thermo-gravimetric analysis indicated that porous films retain the inherent exceptional thermal stability of polyimides, with thermal decomposition onset above 500 °C in nitrogen atmosphere.