Preparation of fluorinated polyemide/POSS nanocomposites with ultra-low dielectric constant by graft copolymerization

Abstract

Nanocomposites of fluorinated polyimides (FPI) with covalently grafted polymethacrylate side chains containing polyhedral oligomeric silsesquioxane (R7R′Si8O12 or POSS) units were prepared by thermally-initiated free-radical graft polymerization of methacrylcyclopentyl-POSS (MA-POSS) with the ozone-pretreated FPI. The chemical composition and structure of the FPI with grafted methacrylcyclopentyl-POSS side chains were characterized by nuclear magnetic resonance (1H-NMR) and X-ray diffraction (XRD). The morphology of the POSS/FPI nanocomposite films was observed by field-emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM). A layer-by-layer structure was revealed, which supports the formation of an ordered architecture by POSS crystallites in the FPI matrix, as the result of self-assembled POSS units of MA-POSS side chains. The POSS/FPI nanocomposite films had both lower and tunable dielectric constants, in comparison with that of the pristine FPI films. Dielectric constants (κ's) of about 2.5 to 2.1 were obtained. The reduction of dielectric constant was most likely due to a combined contribution of the nanoporosity of the POSS units and the external porosity introduced by the grafting of MA-POSS to the FPI chains.