Phase Equilibria of a Brush-Bearing Coating Swollen with a Lubricant and Regulation of Its Composition to Facilitate Ice Shedding

Coatings that shed ice spontaneously reduce icy rain damage and improve aviation safety. A lubricant in equilibrium with a cross-linked polymer film bearing on its upper surface a covalently attached liquid polymer may serve as an outstanding ice-shedding coating. This paper reports a statistical thermodynamic treatment of such a system. First presented are the expressions for the chemical potentials of the lubricant in the coating matrix and in the grafted liquid polymer layer. The equality of the chemical potentials in these two phases is then used to compute the equilibrium volume fractions of lubricant in the phases in the lubricant-starved regime that does not feature an upper lubricant layer. Further derived are the equations for identifying the triple point when a top neat lubricant layer just begins to be secreted. Additionally presented are the equations for calculating the thickness of the upper lubricant layer in the lubricant-rich regime. Such phase transitions and system compositions at different temperatures and feed lubricant volume fractions can also be read from an ingeniously constructed phase diagram. For improved ice shedding in the lubricant-starved regime, the brush layer should be preferentially swollen by the lubricant over the matrix. This scenario can be achieved, according to the current theory, by enhancing the compatibility between the lubricant and the grafted polymer, decreasing the molecular weight of the lubricant, and increasing the cross-linking density of the coating matrix.