Open Questions that Can Bridge Intermolecular Interactions and Macroscopic Wetting/Dewetting Behaviors of Thin Films

Polymer films and coatings are ubiquitous in nanoscale applications where confined macromolecules experience greater interfacial effects, causing deviations from bulk material properties; however, the fundamental physics dictating their wetting behavior remains poorly understood. This review provides a perspective on thickness-dependent surface wettability in thin polymer films characterized by critical thickness, where surface energy and contact angle measurements deviate from bulk values. In contrast with dewetting behaviors, which are well-modeled by van der Waals (vdW) potentials to afford several nanometers of critical thickness, an unexpectedly large critical thickness of approximate to 100 nm is reported for thin polystyrene films via surface energy and contact angle measurements. This deviation in the critical thickness highlights an opportunity to modify the current theoretical understanding of the vdW potential model. In addition, possible explanations to interpret the long critical length scales are discussed: 1) challenges in sample preparations and contact angle measurements; 2) temperature dependence of surface energy; and 3) the cooperative nature of polymers. This review invites researchers to bridge the knowledge gap between macroscopic behaviors (e.g., wetting/dewetting) and molecular-level understanding (e.g., vdW potentials) in polymer thin films.