A Review on Thickness Dependence of Surface Free Energy in Ultrathin Polymer Films

In recent years, thin and ultrathin polymer films coated on solid surfaces have been widely used in various industries such as coatings, adhesives, membranes, sensors, etc. However, as the film becomes thinner, the effect of the interfaces increases, so that in thicknesses below 100 nm, the structure and properties of such ultrathin films, including surface properties such as surface free energy, undergo significant changes relative to the bulk of the same polymer. Considering the importance of surface free energy as a controlling factor in many industrial and theoretical phenomena such as adsorption, wettability, lubrication, etc. in polymers, this article aims to review the changes in surface free energy of ultrathin polymer films with changing their thicknesses. Two main factors have been identified in the literatures for surface free energy changes with changing films thicknesses in ultrathin polymer films. First, the long-range forces acting on the chains from the substrate as well as on the droplet placed on the film surface in order to measure the contact angle, and second, the finite size effect that in thicknesses comparable to the gyration radius of polymer chains, along with the intra- and intermolecular entanglements, can strongly affect the conformation of polymer segments as well as the chain mobility in ultrathin films which are slightly screening out with increasing the film thickness.