Wrinkling of Poly(4-Vinylpyridine) Thin Films after Ion Bombardment

Ion bombardment (IB) is an efficient method of manipulating & cleaning surfaces of materials. When the beam of ions hits the substrate, a few possibilities that depend on the substrate and bombardment conditions can occur; e.g. ion implantation, breaking of molecular bonds, and sputtering off of surface atoms resulting in surface erosion. In the specific conditions that Poly(4-Vinylpyridine) (4-VP) thin film on a silicon wafer is bombarded at 2 keV with argon ions and subsequently exposed to humidity, periodic wrinkling of the film develops over time and a decrease in surface wettability is observed. Without bombardment, 4-VP degrades in water and is relatively unaffected by the level of humidification that is sufficient for wrinkling. The potential to wrinkle is credited to this affinity for water. Polystyrene (structurally similar) was tested under identical conditions and no changes were observed. In 4-VP, bombardment results in a constant decrease in contact angle with water regardless of humidity conditions, implying bombardment is uniquely associated with a decrease in contact angle and consequently changes in the surface energies that govern it. In humidified samples, increasing absolute humidity increases the rate at which the amplitude of the wrinkles develop, and increasing film thickness proportionately increases their wavelength. This evidence and potential water signals on Raman spectra of wrinkled samples suggest that under these conditions the bombarded polymer slowly absorbs water in the air, consequently causing a uniform bi-axial stress that drives isotropic wrinkling.