Nanoporous low-κ polyimide films via poly(amic acid)s with grafted poly(ethylene glycol) side chains from a reversible addition-fragmentation chain-transfer-mediated process

Abstract

Thermally-initiated living radical graft polymerization of poly(ethylene glycol) methyl ether methacrylate (PEGMA) with ozone-pretreated poly[N, N′-(1,4-phenylene)-3,3′,4,4′-benzophenonetetra-carboxylic amic acid] (PAmA) via a reversible addition-fragmentation chain-transfer (RAFT)-mediated process was carried out. The chemical compositions and structures of the copolymers were characterized by nuclear magnetic resonance (NMR) spectroscopy, thermogravimetric analysis (TGA), X-ray photoelectron spectroscopy (XPS), 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 low-dielectric-constant (low-κ) polyimide (PI) films were prepared by thermal imidization of the PAmA graft copolymers under reduced argon pressure, followed by thermal decomposition of the side chains in air. The nanoporous PI films obtained from the RAFT-mediated graft copolymers had well-preserved PI backbones, porosity in the range of 5-17 %, and pore size in the range of 30-50 nm. The pores were smaller and the pore-size distribution more uniform than those of the corresponding nanoporous PI films obtained via graft copolymers from conventional free-radical processes. Dielectric constants approaching 2 were obtained for the nanoporous PI films prepared from the RAFT-mediated graft copolymers.