A model for diffusion-driven hydrophobic recovery in plasma treated polymers

Plasma treatment is widely used for polymer surface modification and, in particular, for the improvement of its adhesive properties. After such treatment the surface energy increases and therefore adhesion increases. However, the change of surface properties is not permanent. Following the treatment, the surface undergoes a relaxation process, called hydrophobic recovery, after which it restores, at least partially, its original low surface energy. Different mechanisms can be responsible for the observed hydrophobic recovery including the diffusion of polar groups and molecular reorientation. We propose a model for the hydrophobic recovery due to a combination of two thermodynamically non-equilibrium processes: diffusion and molecular reorientation. The model explains the mechanism of the hydrophobic recovery and fits well the available experimental data.