Polyhedral Oligomeric Silsesquioxane Nanocomposite Thin Films via Layer-by-layer Electrostatic Self-Assembly

Fabrication of ultrathin polymer composite films with low dielectric constants has been demonstrated. Octa(aminophenyl)silsesquioxane (OAPS) was synthesized and assembled with poly(acrylic acid) (PAA) and poly(styrene sulfonate) (PSS) via a layer-by-layer electrostatic self-assembly technique to yield nanoporous ultrathin films. The OAPS was soluble in water at pH 3 or lower, and suitable pH conditions for the OAPS/PAA and OAPS/PSS assemblies were determined. The multilayer formation process was studied by contact angle analysis, X-ray photoelectron spectroscopy, atomic force microscopy, quartz crystal microgravimetry, UV−vis spectroscopy, and ellipsometry. The multilayer growth was found to be steady and uniform, and the analysis of the film surface revealed a rough topography due to OAPS aggregates. The incorporation of porous OAPS molecules into the thin films significantly lowered their dielectric constants. The OAPS/PAA multilayer thin film thus prepared exhibited a dielectric constant of 2.06 compared to 2.58 for pure PAA film. The OAPS/PAA multilayer film was heated to effect cross-linking between the OAPS and the PAA layers, and the transformation was verified by reflection−absorption Fourier transform infrared spectroscopy. The stability of the film was found to significantly improve by the cross-linking via amide formation.