Polar interactions at liquid/polymer interfaces

Numerous relationships have been proposed in the literature to interpret wettability in terms of solid and liquid surface free energies. In the classical approach based on surface free energy components, the energy of interactions between the liquid and the solid is obtained from the geometric mean of the dispersion and polar contributions of the liquid and solid surface free energies. In this work, it is shown that the surface polarity of polar liquids can be modeled by the interaction of aligned permanent dipoles. A good agreement is found between the surface polarity characterized by polar component of the surface free energy of polar liquids (water, formamide and ethylene glycol) and the dipolar energy of interactions calculated from their dipole moment. At the liquid/polymer interfaces, polar interactions are better described by a simple relationship of proportionality with the polar component of the liquid surface free energy. This observation leads us to evaluate the hypothesis of induced polar interactions at liquid/polymer interfaces, the surface polarity of the solid being induced by the polar liquid in contact with the solid surface. Thus, the variation of the contact angle of a series of polar and non-polar liquids on various polymer substrates appears to be in better agreement when compared to the classical description of permanent polar interactions, so that a surface polarizability is defined for polymers. Using the surface polarizability approach rather than the polar component for the solid surface, we find also that the dispersion (non-polar) component of the polymer surface free energy is obtained with a better confidence, especially by taking into account the contact angles of both non-polar and polar liquid probes, or even by considering only polar liquid probes.