Correlation of friction, adhesion, wettability and surface chemistry after argon plasma treatment of poly(ethylene terephthalate)

The combination of wettability, X-ray photoelectron spectroscopy and scanning force microscopy has been used to analyse the changes to the surface after plasma treatment of poly(ethylene terephthalate) film. Calculations on contact angle data with a combination of polar and non-polar liquids have shown that argon plasma treatment considerably enhances the work of solid-(polar) liquid adhesion and the surface free energy of the films due to the creation of acidic and basic functions on the polymer surface. In contrast, Lifshitz-van der Waals (apolar) interactions decrease slightly as a consequence of plasma-induced chain-scission. We present the first study of a plasma-treated polymer by chemical force microscopy. Plasma-modified surfaces exhibit substantially higher friction than untreated material and are more easily disrupted by the movement of the tip during scanning. Friction is reduced when methyl-functionalised tips are employed. There is a correlation, on plasma treatment, between the rapid increases in surface friction probed by lateral force microscopy and surface free energy probed by wettability and X-ray photoelectron spectroscopy. The modified mechanical properties and polar group incorporation both result from scission of polymer chains and contribute to the lateral force contrast.