Thickness Dependence of Contact Angles in Multilayered Ultrathin Polymer Films

The wetting behavior of ultrathin polymer films (<100 nm) is systematically studied via dynamic contact angle measurements on a model bilayer system of cross-linked polystyrene (PS) and poly(methyl methacrylate) (PMMA) to deconvolute surface energy contributions from material property changes due to confinement and long-range influence from underlayers, i.e., wetting transparency. We find that ultrathin PMMA films show no detectable material property changes and block the influence from PS underlayers when surface wetting is dominated by short-range hydrogen bonding, while suggestive wetting transparency is observed (<10 nm thick) for systems in which the wetting behavior is dominated by van der Waals (vdW) forces. In contrast to PMMA, the wettability of ultrathin PS (<100 nm thick) is dominated by changes in material properties, such as density and refractive index, and wetting transparency is not observed. Comparison between a vdW interface potential calculation and experimental data suggests that the cooperative nature of polymers may lead to a large (∼10 nm) critical thickness for wetting transparency.