Negative interfacial energies of dynamic polymer brush interfaces: a discussion of the free energy balance

Interfacial energy is an important physical parameter for describing interfacial properties. However, quantifying the interfacial energy of a polymer-liquid interface is extremely difficult because numerous polymers reconstruct when exposed to liquids. A dynamic polymer brush is a reconstructed surface. When a hydrophobic elastomer containing amphiphilic block copolymers comes into contact with water, the copolymers spontaneously segregate at the elastomer-water interface to form a hydrophilic brush known as a dynamic polymer brush. We previously succeeded in analyzing the interfacial energy of a dynamic polymer brush interface by measuring the deformation of an ultrathin square elastomer film floating on water. However, the quantitativity remains debatable because the elastic modulus of a bulk elastomer sheet, rather than an ultrathin elastomer film, was used. In this study, we reanalyze the interfacial energy of a dynamic polymer brush system using the ultrathin-film modulus, which we recently measured via a film-on-water tensile test. Large negative interfacial energy was observed for the system of high-density stretched brushes. The free energy balance for the dynamic polymer brush system floating on water was calculated, and results validated the negative interfacial energy, which is a distinctive feature of dynamic polymer brushes.