On the determination of the surface energetics of porous polymer materials

The solid surface tension gamma(sv) of hydrophobic polymer powders has bran determined using the capillary penetration technique. By plotting K gamma(lv), cos theta, where K is a geometric factor, Versus the liquid surface tension gamma(lv), the following values of gamma(sv) were directly derived from the curves: poly(tetrafluoroethylene) gamma(sv) = 20.4 mJ/m(2), polypropylene gamma(sv) = 30.2 mJ/m(2), polyethylene gamma(sv) = 34.4 mJ/m(2), and polystyrene gamma(sv) = 27.5 mJ/m(2). These values are in good agreement with the gamma(sv) values obtained from contact angle measurements on flat and smooth solid surfaces of the same materials. If the contact angles were first calculated from the capillary penetration experiments, which is the usual procedure applied in the literature, distinctly higher contact angles were obtained. Obviously these angles are affected by the powder morphology and are therefore meaningless contact angles in terms of a surface energetic interpretation.