Wetting transparency is a unique phenomenon for ultrathin materials in which surface wettability of a coating is influenced by its underlayer as the coating thickness approaches length scales of molecular interactions. While wetting transparency has been extensively studied for graphene, little work has been done with polymeric materials. In this study, we created a model bilayer system of thin, crosslinked, polymer films of poly(methyl methacrylate) and polystyrene (PS) to afford uniform film thickness, tunable chemistry, and 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. Thickness dependence of these contact angles will determine a critical film thickness for wetting transparency that is associated with short and long range interactions. In preliminary calculations derived from existing theories of vdW potential and Hamaker constants, we find that 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 poster, we will discuss a hypothesis to describe the mismatch between theory and experiments.

Wetting transparency of ultrathin polymer films

ACS Spring Meeting 2021

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