Sensitivity of surface free energy analysis methods to the contact angle changes attributed to the thickness effect in thin films

Abstract

AbstractThe surface tension of thin, uncured epoxy films spun‐cast onto oxidized silica wafers was analyzed as a function of their thickness in the range of 5 to 75 nm. Based on contact angle measurements with polar and apolar liquids, the surface free energies of various films were analyzed and compared using Lifshitz–van der Walls/acid–base (LW/AB), harmonic mean (of Wu), geometric mean, and Zisman approaches. Results of harmonic mean and Zisman surface tension analyses, in spite of significant changes in contact angle attributed to changes in thickness, did not show significant changes. However, geometric mean and LW/AB methods did reveal limited changes in free energy of epoxy surface attributed to variations in thickness. The surface tension components showed a similar trend to that of contact angle changes. It seems that it is better to use surface free energy analysis methods, which use several liquids with different polarities, to reveal the accumulative effect of different group free energy densities on total surface free energy. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 93: 1972–1980, 2004