Plasma-induced modification of films containing polytetrafluoroethylene (PTFE) Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/0v8382895

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  • Polymer samples of polytetrafluoroethylene (PTFE) and a PTFE-glass weave (RF- 35P) are exposed to low pressure, non-equilibrium glow discharge plasmas for enhanced wettability as measured by static contact angles. Plasma treatments are performed in two parallel plate RF plasma systems, a downstream microwave plasma and a barrel etcher using feed gases composed of H₂, N₂, Ar, He, and 0₂. Surface analysis of the topography and chemical composition of treated samples is performed by atomic force microscopy (AFM), attenuated total reflection infrared (ATR-IR) spectroscopy, x-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectroscopy (ToF SIMS). Optical emission spectroscopy is used to correlate wettability to reactive species in the plasma, and plasma parameters to species emission. In the parallel plate plasma systems, the contact angle can change from approximately 95° to 5° with treatment while treatments in the downstream and barrel etcher systems do not result in a contact angle change. The difference in plasma performance is attributed to ion bombardment. Plasmas composed of 20 to 80% H₂O in (H₂ + N₂) give the best wettability improvement. Plasma exposure significantly reduces the surface fluorine content followed by incorporation of nitrogen, oxygen and hydrogen, apparently as amino, hydroxyl and carbonyl functional groups with evidence of an amide. The incorporation of oxygen likely originates through peroxy radicals subsequent to plasma exposure. It is proposed that using a higher applied power creates a more reactive surface. A larger percentage of H₂ in the plasma tends to passivate the surface, leading to a smaller wettability improvement which is correlated to the atomic hydrogen concentration in the plasma. The addition of He or Ar into the H₂/N₂ plasma tends to dilute or weaken the plasma effect on wettability. To a much greater extent, the addition of oxygen also decreases the wettability. This latter effect is attributed to etching reactions which result in a more hydrophobic surface. An etch rate of approximately 0.2 μ/min is observed, and this is the first report of PTFE etching with 0₂/H₂/N₂. The distance between the parallel plate electrodes (gap) is a significant factor for the wettability of treated polymers, while power, pressure and flow rate are not. The optimal storage conditions to retain the wettability of H₂/N₂ plasma-treated RF-35P are low pressure and low temperature. The following model of PTFE surface modification is proposed. Ion bombardment creates reactive sites that initiate surface reactions. Reactive species from the H₂/N₂ plasma modify the surface through incorporation of amino, hydroxyl and carbonyl surface groups. These new groups increase the polymer wettability. Atomic hydrogen in the mixture is required to improve the wettability, but too high of a concentration will passivate the surface and lessen the wettability improvement. If oxygen is present, the modified surface is etched away leaving a refreshed, more hydrophobic surface.
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