In-depth distribution of silica nanoparticles in free-standing silica/vinyl polymer nanocomposite films

Film-forming silica-based materials are potentially useful as tough, abrasion-resistant coatings and fire-retardants. However, these formulations are often solvent-based, and there is an actual tendency to replace them by more environmentally friendly, water-borne coatings. Herein we study surfactant-free and water-borne nanocomposite films consisting of silica nanoparticles (20 nm size) embedded in a vinyl polymer matrix. Highly transparent free-standing films were prepared by copolymerizing of 4-vinlypyridine (4VP) with low glass transition temperature comonomers such as n-butyl acrylate (n-BuA) or n-butyl methacrylate (n-BuMA) in the presence of an ultrafine aqueous silica sol. Films with different silica contents (ranging from 14% to 61%) were prepared with thickness around 100 um. The in-depth distribution of the nanoparticles has been studied by extracting the Si profile from Rutherford backscattering spectrometry (RBS) measurements. Standard RBS with 2 MeV He+ was performed to established the Si profile in the near surface region while a focused 3MeV Li+ ion beam (3-4 um) provided by a nuclear microprobe was used in cross-section specimens to determine the in-depth profile at larger depths.