Wettability of chemically modified polymers: Experiment and theory

The wettabilities of four polymers, polytetrafluoroethylene (PTFE), ultra-high molecular weight polyethylene (UHMW-PE), polyetherurethane (PEU) and polydimethylsiloxane (PDMS) have been studied experimentally. Using PBS as the wetting fluid, advancing and receding contact angles have been measured for virgin surfaces and for materials treated in O-2-, Ar-, N-2- and NH3-plasmas. In the case of PTFE, further surface modi-fication was produced by storage of virgin and plasma treated specimens in air or PBS for one month. These experimental measurements are presented on 'wettability plots' which facilitate interpretation in terms of current theoretical understanding. Wettability plots show contact angle measurements as a function of a parameter designated X-M which quantifies the surface composition as a weighted average of chemically distinct surface domains. The theoretical wettability of smooth surfaces with molecular scale heterogeneities can be drawn as a line (zero hysteresis) on such wettability plots and provides a helpful datum for analysing the hysteretic wetting behaviour of real surfaces in terms of contact line pinning by surface heterogeneities. An experimental method based on analysis of XPS spectra for estimating X-M is presented. This enables wettability plots (contact angles vs. X-M) to be plotted for the four materials studied. These plots elucidate the variation of the wettability of a given material as a function of surface treatment, and the comparative behaviour of different materials.