Direct Measurement of Molecular Level Adhesion Forces between Biaxially Oriented Solid Polymer Films

We report the first measurements of the solid surface energy of a polymer film by means of the Israelachvili surface forces apparatus (SFA). To this end, poly(ethylene terephthalate) (PET) films were generated in 3.5–6 pm thick samples, which could be silvered and used in the SFA. Adhesive contact pull-off forces were measured in air for unmodified PET as well as for PET modified by direct fluorination. The solid surface energy of the polymer films was determined by application of the Johnson-Kendall-Roberts (JKR) theory or the Derjaguin-Muller-Toporov (DMT) theory. Measurements of the critical wetting tension as well as surface analysis by XPS were done on the PET surfaces. The value of solid surface energy for untreated PET predicted by the DMT theory agrees well with the critical wetting tension determined by contact angle measurements, but the details of the shape of the contacting interface as well as the radius at pull-off are best represented by the JKR theory. With increasing fluorination of the PET surface, the wettability by polar liquids increases while the SFA solid surface energy decreases. This is ascribed to a weak boundary layer generated by the fluorination.