Steady-state relaxation kinetics observed on fluoropolymers modified by ambient air plasma treatment

Abstract

Fluoropolymers were treated by an ambient air plasma at 18.7 watt of RF power, 1.0 torr of vacuum pressure, and 0.5-2.0 min of exposure time. The surface wettability and composition were characterized by contact angle measurements and X-ray photoelectron spectroscopy, respectively. The plasma-modified fluorinated surfaces exhibited two different relaxation kinetics based on contact angle (theta (A) ) changes of water: i) an increase of cos theta (A) for fully fluorinated (perfluorinated) polymers with nonpolarity, ii) a decay of cos theta (A) for partially fluorinated polymers with polarity. A steady-state relaxation model was successfully applied to two different contact angle changes on the plasma-modified fluorinated surfaces. The initial polar fraction, f (0), was fitted as 0.03-0.2 for fully fluorinated polymers and 0.5-0.7 for partially fluorinated polymers, respectively. After the plasma treatment, the fully fluorinated polymers exhibited the relative increase of final polar fraction to initial one (i.e., f (a)/f (0)=1.2-2.4), but partially fluorinated polymers exhibited the relative decrease of final polar fraction to initial one (i.e., f (a)/f (0)=0.7-0.8). The continuous decrease of water contact angles on plasma-modified perfluorinated surfaces might be attributed to the further interactions of generated polar groups with atmospheric environment, while the increase of water contact angles on partially fluorinated surfaces are mainly attributed to the recovery of pristine surface by chain relaxation mechanism.