Mechanism of the Polysiloxane Concentration-Dependent Hydrophobicity in Epoxy Resin Based on a Thermodynamic Ring Model With Saturation Effect

To deeply understand the controversial results about nonpolar organics concentration-dependent hydrophobicity in polar polymers, we establish a thermodynamic ring model combined with the saturation effect of surface tension for a smooth multiphase surface. It is found that the static contact angle of the water droplet on the smooth solid surface is significantly affected by the area proportion of nonpolar dispersed phases near the three-phase contact line. The proportion of the nonpolar dispersed phase on the surface is affected by the concentration of the nonpolar dispersed phase and the particle size of the dispersed phase. Due to the high concentration and large particle size of the dispersed phase, the proportion of the nonpolar dispersed phase on the surface has obvious dispersibility, which may lead to the increase or decrease of the proportion of the nonpolar dispersed phase on the surface. When the proportion of the surface nonpolar dispersed phase is low, the static contact angle increases with the increase of the proportion of the surface nonpolar dispersed phase. However, the further increase of the proportion of nonpolar dispersed phase enhances the saturation effect of surface tension, contributing to a saturated static contact angle. Our findings afford a thorough explanation for the reported discordant results about the effect of nonpolar organics concentration on hydrophobicity in polar polymers, which is critical to rational design for polar polymers modified by nonpolar organics.