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PTFE was treated with oxygen plasma, and the effects of treatment time were evaluated by XPS, SEM, and the contact angles of water and CH2l2. Advancing and receding angles were interpreted in the light of current theories on contact angle hysteresis. It was found that at short treatment time wettability reflects chemical modification of the surface, while at longer treatment times surfaces are deeply etched and contact angles are controlled by roughness. With water as the wetting liquid, the typical behavior of composite surfaces was observed.

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SYNOPSISOxygen plasma treatment of polypropylene (PP) surfaces led to introduction of oxygencontaining functionalities, with consequent improvement of surface wettability. A combination of X-ray photoelectron spectroscopy (XPS) , static secondary ion mass spectroscopy (SSIMS), and contact angle measurements (water-in-air and air-in-water) allowed us to characterize the behavior of the treated surface in contact with air (low-energy surface) and water (high-energy surface).The treated surface showed the tendency to rearrange itself to minimize its interfacial energy. When contacted with air (low-energy surface), polar groups were buried away from the polymer/air interface, while in contact with water (high-energy surface) polar groups remained at the polymer/water interface.When contacted with air, the polymer surface layer rearranged by macromolecular motions within itself, since interdiffusion with the bulk polymer seems forbidden. These motions are thermally activated and it was possible to obtain an apparent activation energy (58.1 kJ/mol) close to those reported for other vinyl polymers. I NTRO DUCT10 NPlasma treatments have long been known to increase the wettability of polyolefin Well known also is the fact that treated surfaces tend to lose wettability with time.1s6s7 In this respect, in the last few years an increasing amount of work has been devoted to polymer surface and interface dyn a m i c~.~'~ and electrical discharges or plasmas 12,13 on the chemistry of polypropylene (PP) surfaces has been extensively studied in the past. In this work, a single and reproducible oxygen plasma treatment condition was chosen, focusing on the behavior with time of the treated surface in contact with media of different surface energy, namely air and water.The effect of flame treatments X-ray photoelectron spectroscopy (XPS) and static secondary ion mass spectroscopy (SSIMS) have been used to assess the surface composition of untreated and plasma-treated surfaces. In the case of SSIMS, the use of "OZ plasmas (for the first time, to our knowledge ) provided unambiguous evidence on the oxygen plasma/surface interaction, free of contributions from contamination or other environmental effects. Water-in-air and air-in-water contact angle hysteresis measurements provided important information about the wettability and aging of treated surfaces in contact with different media. Fourier Transform Infrared in the Attenuated Total Reflectance mode (FTIR-ATR) and scanning electron microscopy (SEM) were used to detect eventual oxygen-plasma-induced changes in vibrational spectra and surface morphology, respectively.A preliminary and partial account of the contact angle and XPS parts of this work was presented earlier.14 In part I1 of this work, we will discuss the properties of interfaces of untreated and oxygenplasma-treated polypropylene surfaces with epoxy resins.15 EXPERIMENTAL Materials and ProceduresOxygen plasma treatments were performed using a parallel plate reactor, with the samples located on the water-cooled grounded el...