Tuning the Wettability of a Thin Polymer Film by Gradually Changing the Geometry of Nanoscale Pore Edges.

Abstract

Controlling wetting of solids by liquids attracts attention because of its scientific and technological importance. In this paper, the wettability of a highly uniform porous poly(methyl methacrylate) film on a silicon substrate containing a high density of randomly distributed self-similar pores was gradually tuned by changing the shape of nanometric crownlike structures around the pores. Fine-tuning the topography of these thin films was performed by isothermal annealing. The equilibrium contact angle of a water droplet placed on the surface of the films could be varied from 72 to 102°. The contact angle changes were assumed to be a consequence of changes in surface topography in the nanoscale. A simple method of a quantitative description of the change of the topography of these films was developed. Critical dimensions of these films were determined in horizontal and vertical directions relative to the surface plane. The slope coefficient (SC) describing how sharp the structures are, is defined as the ratio between the critical dimensions: the root-mean-square roughness σ and the autocorrelation length ξ. For SC > 0.08, the contact angle increased proportionally to the value of SC, whereas for SC < 0.08, the contact angle proportionally decreased. At the highest SC values, the contact angles were 6-10% higher than those predicted for flat porous surfaces using the Cassie-Baxter equation. We suggest that this discrepancy is due to the capillary tension caused by the submicron-scale undulation of the triple line, which was found to be proportional to the height of the crownlike pore edges and the value of SC. The same effect is responsible for the linear dependence of the contact angle on the SC value.