Abstract:
We have explored the possibility of acquiring information on the molecular nature of some novel polymer films by following Zisman’s approach of determining the critical surface tension yc which is considered to reflect the molecular composition of the solid surface. This led us to obtain equilibrium contact angle data, using both a series of pure liquids and various alcohol solutions, not only with the polymer films but also with solid surfaces of polystyrene and polymethylmethacrylate.
Lower values of yc were obtained with the solutions than with the pure liquids; these lower values are attributed to the preferential adsorption of the alcohol molecules at both the polymerliquid and the polymer-vapour interfaces. The value of yc depends on the alcohol used, and is relatively independent of the solid : it is inferred that the alcohol is adsorbed with the hydroxyl group towards the polymer surface.
It is concluded that in certain cases, the value ofyc obtained using solutions cannot be used as being characteristic of the solid (as has been suggested by Zisman), and that changes at the solidvapour interface cannot be neglected when interpreting contact angle data. Several sets of data reported in the literature are discussed from this viewpoint. The Gibbs adsorption isotherm is applied to the contact angle data and the results add further weight to the conclusions regarding the occurrence of adsorption at both interfaces.
