Estimation of the surface properties of styrene-acrylonitrile random copolymers from contact angle measurements

The surface free energy per unit area of a solid, gamma(S), is a fundamenta l property of materials and determines their surface and interfacial behavi or in processes like wetting and adhesion. In this study the gamma(S) of a series of styrene-acrylonitrile random copolymers is evaluated. Three diffe rent approaches are used to determine the components in which the surface f ree energy can be decomposed. Using the geometric and the harmonic mean app roach, the dispersive, gamma(d), and polar, gamma(p), components of the sol id surface free energy were determined and compared to the Lifshitz-van der Waals, gamma(LW), and acid-base, gamma(AB), components using the approach developed by C. J. van Oss et al. (1987, Adv. Colloid Interface Sci, 28, 35 ). The acid-base approach was also used to evaluate the work Of adhesion of the test liquids: water; glycerol, and thiodiglycol, It was found that the contact angles of these liquids follow closely the predictions of Cassie e quation. The evaluation of the surface free energy components on one hand a nd the relative magnitude of the work of adhesion components on the other h and, suggest that below 50% of acrylonitrile the polystyrene repeating unit s are preferentially at the surface. Above 50% of acrylonitrile the segrega tion of the low-energy homopolymer at the surface decreases. (C) 1999 Acade mic Press.