Assessment of acid-base character of polymer surfaces from contact angle and other surface chemical data

The performance of many products depends on the adhesion or abhesion of component materials. In the pharmaceutical industry, for instance, the adhesion between drugs, excipients, delivery devices and packaging materials may affect product performance. Fowkes, in particular, has stressed the importance of acid-base chemistry at interfaces to adhesion. Over the years, several investigators have proposed models to describe the acid-base nature of interfaces. Good and van Oss have championed the notion of calculating surface free energy components from contact angle data. Their components, gamma(+) and gamma(-), reflect respectively electrophilicity and nucleophilicity of surfaces. Chang and Chen have suggested another model for determining the acid-base character of surfaces. Chang and Chen's model, in contrast to that of Good and van Oss, is able to account for acid-acid and base-base repulsions. Fowkes has offered an approach dissimilar to both that of Good and van Oss and that of Chang and Chen which incorporates the ideas of Drago or Gutmann regarding measurement of electrophilicity and nucleophilicity. Those employing inverse gas chromatography (IGC) to assess surface acid-base character have often used Fowkes' approach. In each of the above models different scales for electrophilicity and nucleophilicity are used. Because certain models are more easily associated with particular experimental approaches and because certain materials lend themselves to investigation by particular experimental methods, it would be useful to know if the various possible approaches give the same general assessment of surface acid-base character. In this study we have measured the contact angles of various liquids on poly (methyl metyhacrylate) [PMMA], poly (vinyl chloride) [PVC] and polyethylene [PE]. The heats of adsorption of both phenol and pyridine have been measured on PMMA and PVC. The results of this work suggest that a given model may produce results which suggest different qualitative assessments of surface acid-base character than that obtained using another model. It is clear that knowledge of the precise relations between the various theoretical quantities calculated using the above models would be particularly helpful when comparing the conclusions based on different models. At present it does not appear that an unambiguous model independent assessment of surface acid-base character can be made.