Nanoporous Low-Dielectric Constant Polyimide Films via Poly(amic acid)s with RAFT-Graft Copolymerized Methyl Methacrylate Side Chains

Abstract

Poly[N,N′-(1,4-phenylene)-3,3′4,4′-benzophenonetetracarboxylic amic acid] (PAmA) with grafted poly(methyl methacrylate) (PMMA) side chains (PAmA-g-PMMA) was synthesized via thermally induced graft copolymerization of methyl methacrylate (MMA) with ozone-pretreated PAmA in the reversible addition-fragmentation chain-transfer (RAFT)-mediated process. The graft copolymers were characterized by nuclear magnetic resonance (NMR), elemental analysis, X-ray photoelectron spectroscopy (XPS), thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), and molecular weight measurements. The "living" character of the grafted PMMA side chains was ascertained in the subsequent extension of the PMMA side chains. The nanoporous low dielectric constant (low-κ) polyimide (PI) films were obtained by thermal imidization of the PAmA-g-PMMA films in argon, followed by thermal decomposition of the PMMA side chains in air. The nanoporous PI films obtained from the RAFT-mediated PAmAg-PMMA had well-preserved PI backbones, porosity in the range of 5-20%, and pore size in the range of 5-15 nm. The pores were smaller, and the pore size distribution was more uniform than those of the corresponding nanoporous PI films obtained via graft copolymers from the conventional free radical process. Dielectric constants approaching 2.1 were obtained for the nanoporous PI film with a porosity of about 20%.

Living Radical Polymerization by the RAFT Process

“…[228] Most work has used ATRP or NMP, though papers on the use of RAFT polymerization have begun to appear. [229][230][231][232][233][234][235] The first to apply RAFT in this context were Tsujii et al [229] and Brittain and coworkers. [230,231] Recent papers describe RAFT polymerization from plasma-treated Teflon surfaces [232] and ozonolyzed polyimide films.…”

“…[230,231] Recent papers describe RAFT polymerization from plasma-treated Teflon surfaces [232] and ozonolyzed polyimide films. [235] The approach used in these and most other studies [230][231][232][233][234][235] has been to immobilize initiator functionality on the surface by chemical or plasma modification and use this to initiate polymerization in the presence of a dithioester RAFT agent. Tsujii et al [229] have indicated that some difficulties arise in using RAFT for grafting from particles because of an abnormally high rate of radical-radical termination caused by a locally high concentration of RAFT functionality.…”

Nanoporous polymeric materials: A new class of materials with enhanced properties

“…Polyimides (PI) are a favored material because of their high thermal stability, low stress coefficient of thermal expansion, low dielectric constant, high resistivity, and high dielectric breakdown [120,[130][131][132][133][134][135]. For instance, Lee et al [130] managed to develop a nanoporous PI films (thickness: 200 lm) with pore sizes from 10 to 40 nm, a porosity around 20%, and a dielectric constant of 2.25 (a reduction of 31% with respect to the solid).…”

Synthesis and characterization of flexible and high-temperature resistant polyimide aerogel with ultra-low dielectric constant

“…By this methodology, various ultra low-k PIs have been reported [10][11][12][13][14]. However, the attempts further reducing the k values of PIs below 1.5 seem to be more challenging because the conventional methods could only achieve a very limited air loading (<60%, volume ratio).…”