Nanoporous polyimides derived from highly fluorinated polyimide/poly(propylene oxide) copolymers

Abstract

Porous, low dielectric constant polyimide films have been made by a "nanofoam" approach. The pore sizes generated in the polymer films are in the tens of nanometers range, hence the term "nanofoams". The nanoporous foams are generated by preparing triblock copolymers with the majority phase comprising polyimide and the minor phase consisting of a thermally labile block. Films of the copolymers are cast and then heated to effect solvent removal and annealing, resulting in microphase separation of the two dissimilar blocks. The labile blocks are selectively removed via thermal treatments, leaving pores the size and shape of the original copolymer morphology. The polyimide derived from 2,2-bis(4-aminophenyl)hexafluoropropane (6FDAm) and 9,9-bis(trifluoromethyl)xanthenetetracarboxylic dianhydride (6FXDA) was used as the matrix material for the generation of nanofoams, and specially functionalized poly(propylene oxide) oligomers were used as the thermally labile constituent. The synthesis and characterization of the copolymers were performed and the process for obtaining nanofoams was optimized. The foams were characterized by a variety of techniques including thermal gravimetric analysis (TGA), transmission electron microscopy (TEM), dynamic mechanical thermal analysis (DMTA), density, small-angle X-ray scattering (SAXS), refractive index, and dielectric constant measurements. Thin-film, high-modulus nanoporous films with good mechanical properties and dielectric constants ∼2.3 have been synthesized by the copolymer/nanofoam approach.