Ultra-low-κ materials based on nanoporous fluorinated polyimide with well-defined pores via the RAFT-moderated graft polymerization process

Thermally-initiated living radical graft polymerization of poly(ethylene glycol) methyl ether methacrylate (PEGMA) with ozone-pretreated fluorinated polyimide (FPI) via the reversible addition-fragmentation chain transfer (RAFT)-moderated process was carried out. The chemical composition and structure of the copolymers were characterized by nuclear magnetic resonance (NMR), thermogravimetric analysis (TGA), and molecular weight measurements. The "living" character of the grafted PEGMA side chains was ascertained in the subsequent extension of the PEGMA side chains. Nanoporous ultra-low dielectric constant (K) fluorinated polyimide films were prepared by solution casting of the graft copolymers, followed by thermal decomposition of the labile PEGMA side chains in air. The nanoporous FPI films obtained from the RAFT-moderated graft copolymers had well-preserved FPI backbones, porosity in the range of 2-10% and pore size in the range of 20-50 nm. These films also had more well-defined pores, more uniform pore size distribution, and better-retained mechanical properties than those of the corresponding nanoporous FPI films obtained from the graft copolymers prepared by the conventional free-radical process. Dielectric constants approaching 2.0 were obtained for the nanoporous FPI films prepared from the RAFT-moderated graft copolymers.