(73d) Wetting Transparency of Ultrathin Polymer Films

Recent advancements in chemical vapor deposition, surface-initiated polymerization, and others provide fine-tuning of polymeric coatings below 10 nm-thick, which becomes comparable to the length scales of molecular interactions. In these ultrathin coatings, surface wettability can be influenced by its underlayer as if it is “transparent” in terms of wettability: so-called wetting transparency. While wetting transparency has been extensively studied for graphene, little work has been done with polymeric materials, perhaps because of the structural heterogeneity of polymers. Herein, we study a model bilayer system of thin, crosslinked polymer films of poly(methyl methacrylate) and polystyrene (PS) to prevent undesired interphase diffusion or dewetting. We aim to decouple surface energy contributions from long-range van der Waals (vdW) forces and short-range hydrogen bonding interactions by performing contact angle measurements with hydrophilic (water) and hydrophobic (diiodomethane) test liquids. The thickness dependence of these contact angles will determine a critical film thickness for wetting transparency associated with short and long-range interactions. In preliminary calculations derived from existing theories of vdW potential and the Lifshitz theory, we find that the contact angle of PS films show thickness dependence when films are smaller than ~5 nm due to vdW forces from underlying substrates. This prediction cannot explain previous experimental work, which shows that water contact angle on a PS film/silicon oxide substrate starts to deviate from its bulk value at thickness ~ 100 nm. In this presentation, we will discuss a hypothesis to describe the mismatch between theory and experiments.