Surface characterization of oxyfluorinated isotactic polypropylene films: Scanning force microscopy with chemically modified probes and contact angle measurements

The surface of isotactic polypropylene (IPP) films modified by oxyfluorination was studied by a combination of complementary techniques to elucidate the effect of the modification on the chemical composition, surface energy, and morphology. The elemental composition was measured by X-ray photoelectron spectroscopy (XPS), and the surface free energy was characterized by contact angle measurements with different liquids. Following the approach by Good and co-workers, the acid-base characteristics of the modified polymer surfaces were estimated. Subsequently, the modified IPP films were investigated by scanning force microscopy (SFM). In tapping mode SFM, the sample topography was imaged and the surface roughness was quantitatively determined. Gold-covered SFM probes modified with carboxylic acid and methyl terminated self-assembled monolayers of thiols were used to determine the pull-off force distributions in ethanol. The interaction of the treated polymer film surfaces with chemically functionalized SFM tips was shown to correlate with the surface tensions obtained by contact angle measurements. In particular, pull-off forces measured with carboxylic acid functionalized tips in ethanol depended approximately linearly on the basic part of the surface free energy of the polymer film surface.