Silica/polyimide hybrids and their dielectric properties. I. Preparation with an improved sol-gel process with poly(amic acid) as the precursor

A series of hybrid polyimide-silica hybrids films were prepared by a sol-gel process via the blending of tetrathoxysilane and a coupling reagent in a pyromellitic anhydride/4,4'-oxydianiline (ODA) based poly(amic acid) solution in N,N-dimethylacetamide, the casting of the films, the evaporation of the solvent, and thermal imidization, in that order. Fourier transform infrared, transmission electron microscopy, atomic force microscopy, and thermogravimetric analysis were used to characterize the structure of the obtained hybrids. The dispersion of silica in the hybrids was suggested to be in two states: a dominant network structure and minor discrete aggregate particles. On the basis of the network structure, the mechanical behavior of the hybrids was interpreted. An investigation of the dielectric properties revealed that the dielectric constant of the hybrids increased slowly with the concentration of SiO2. For hybrid films containing 30 wt % SiO2, the constant increased to 4.2 from the value of 3.2 for pure polyimide. Moreover, the incomplete hydrolysis and decomposition of tetrathoxysilane and the coupling reagent were the main factors contributing to the thermal stability and the uncertainty of the dielectric constant of the hybrids. (c) 2005 Wiley Periodicals, Inc.