The surface tension of polymer liquids

An experimental and theoretical review of the surface tension of polymer liquids is presented. New experimental methods for polymer surface tension measurement are reviewed. Numerous theoretical and empirical approaches are briefly described. Strong emphasis is placed on the accuracy and limitations of the thermodynamic information which are used as input to many of if not all these theories. It is shown that, by using accurate thermodynamic bulk properties for these liquids including polar polymers, one can use a simple corresponding states principle to describe the temperature and molecular weight dependences of the surface tension with no adjustable parameters to within a few per cent, even though the actual surface tensions may vary by a few hundred per cent. Any small inaccuracies in the predictions arise from conformational entropic effects or because one cannot accurately compute the 'real' cohesive energy density of a polymer liquid from thermodynamic properties alone. The results of recent measurements on high molecular weight polymers are compared with previous work on lower molecular weight liquids. We use the polyethylene and polytetrafluoroethylene oligomeric series to illustrate the uses and limitations of thermodynamic data for the prediction of surface tension. The temperature and molecular weight dependences of the surface tensions of these polymers are dominated by the dependence of the bulk thermodynamic properties on temperature and molecular weight. Limited discussion is also given to studies of copolymers and blends using surface tension and complementary techniques.