Wetting autophobicity of polymer melts

Abstract

Numerical solutions are obtained for a set of self-consistent-field equations that give the thickness dependence of the Helmholtz energy of a polymer film. Helmholtz energies are calculated for a series of polymer melts in order to illustrate some of the unique wetting behaviours that are exhibited by amorphous polymers. The primary focus is on wetting autophobicity, where the Helmholtz energy shows an absolute minimum at a finite film thickness. The equilibrium situation in this case is for bulk droplets of the polymer melt to be in equilibrium with a polymer film with a thickness corresponding to the Helmholtz energy minimum. This phenomenon has important consequences, in that macroscopic de-wetting can occur in systems for which the polymer–substrate interactions are neutral. The depth of the Helmholtz energy minimum is related to the equilibrium contact angle for the droplets, and is calculated for blends of end-adsorbed polymers in matrices of homopolymers of identical composition. This quantity is an increasing function of the molecular weight of the non-adsorbed polymer and of the areal density of the adsorbed polymers at the interface. We also find that the tendency for wetting autophobicity is considerably enhanced if the molecules are able be adsorbed onto the interface from both of their ends. These calculations are also extended to block copolymer systems, where similar results are obtained.