The dynamic behavior of functional groups on polymer surfaces was studied with regard to the effects of aging media on the work of adhesion (W-A) and the surface free energy of the polymeric solid (gamma(A)). Films of polystyrene (PS), polydimethylsiloxane (PDMS), and a cured phenol-formaldehyde resin (PFR) were exposed to oxygen plasma. The plasma-treated samples were washed with methanol to remove the low-molecular-weight components from the surfaces and were aged in two environments, dry nitrogen gas and distilled water, at 25 degrees C for up to 500 h. The effects of changing the medium during aging were also studied. When the treated polymers were aged in nitrogen gas, the W-A value decreased with aging time. On the other hand, when the polymers were aged in water, the value of the surface free energy approached that of water, 72.8 mJ/m(2). In addition, in the case of PS and PDMS, the W-A values which decreased during aging in nitrogen increased after the medium was changed to water, and vice versa. These results suggest that the functional groups introduced by plasma treatment diffused from or to the polymer surface and that their surface concentration was optimized to minimize the free energy of the interface between the polymer and the aging medium.