Using ADSA and the determination of solid surface tensions of polymers
Contact angles of liquids on polymer surfaces are widely used to predictwetting and adhesion properties of these solids by calculating theirsolid-vapour surface tension. While the theory is based on the equilibrium ofan axisymmetric sessile drop on a flat, horizontal, smooth, homogeneous,isotropic, and rigid solid, it is generally found in practice that a wholerange of contact angles is accessible experimentally causing wetting or contactangle hysteresis. The reason is that contact angle phenomena are verycomplicated. Contact angles on polymer surfaces are not only influenced by theinterfacial tensions according to Young´s equation but also by many otherphenomena, such as surface roughness, chemical heterogeneity, sorption layers,molecular orientation, swelling, and partial solution of the polymer orlow-molecular constituents in the polymer material. These effects have to beconsidered when contact angle measurements are used to calculate the solidsurface tension of polymers.
In recent papers it was shown that using axisymmetric drop shape analysis-profile (ADSA-P) for measuring contact angles (at very low advancing velocity of the three-phase contact line) (see Fig.1a) very complex contact angle responses for certain solid-liquid systems were revealed and "meaningful" contact angles could be distinguished from "meaningless" ones. We define an advancing contact angle as "meaningful" when the low-rate dynamic advancing contact angle is constant while the three phase contact line advances over the solid surface(see Fig. 1b). In this case, we assume that the solid-vapour surface tension is not changed due to the contact with the liquid. "Meaningless" contact angles are also experimentally measured advancing contact angles, but in contradiction to "meaningful" ones they show other patterns which can be revealed by ADSA-P: stick/slip behaviour, a decrease or an increase of the advancing contact angle were observed. In these cases, we assume that the solid surface is changed during the contact with the liquid. Therefore, these contact angles cannot be used in conjunction with Young`s equation. Following this concept, we investigate systematically the influence of the chemical structure of polymers and copolymers on their surface energetic properties. By combining contact angle measurements with other surface sensitive methods, such as X-ray photo electron spectroscopy (XPS), zeta potential measurements, and atomic force microscopy (AFM), a better understanding of the molecular causes of macroscopic wetting and adhesion properties is the aim of our studies.
Fields of Research
- Correlations between the surface energetic properties of polymers determined by contact angle measurements and their molecular structure (chemical composition)
- Effect of roughness on contact angle hysteresis
- Wetting of heterogeneous surfaces of block copolymers and polymer blends
- Dr. Karina Grundke
- Kathrin Pöschel