Figure 1.

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For any gas composition, three simultaneous processes alter the outer molecular layers of the polymer: ablation, crosslinking, and activation. The effect of each depends on the chemical nature of the gas plasma and the polymer.

1. Ablation is literally "boiling off" of the outer molecular layer of the polymer surface by the bombarding energetic plasma particles. Charged particles (free radicals, electrons, and ions) and ultraviolet photons break the covalent bonds of the polymer backbone, resulting in fragmented polymer chains of a much lower molecular weight.

As long molecules become shorter, the volatile oligomer and monomer byproducts ablate and are swept away with the outgoing vacuum pump exhaust. Ablation can be very effective in cleaning metal foils or fabrics as well as conventional polymer films, removing contaminants (such as mold-release or process oils), or in removing weak boundary layers.

2. Crosslinking is forming covalent bonds between adjacent polymer chains, ideally in an atmosphere such as argon or helium. The inert gas is ionized and the covalent polymer bonds are broken at the polymer surface, 10 to 40 Å deep.

Since there are no free radical scavengers in an inert gas, one of three things can occur in the inert plasma: The dissociated molecule can simply revert to its previous state by recombining; it can react with an adjoining free radical within the polymer chain, forming a double or triple bond; or it can form a bond with a nearby free radical on an adjacent chain (this occurrence is crosslinking). Crosslinking may strengthen certain polymers, retard the migration of additives (blooming), and/or modify the permeation characteristics.

3. Activation results when different atoms or chemical groups from the plasma are added to the surface molecules of the treated plastic surface. As with ablation, surface bombardment by energetic particles breaks the polymer chain or extracts pendant groups or atoms such as hydrogen, forming free radicals.

Similarly, high-energy UV photons emitted by excited free electrons in the plasma have sufficient energy to break carbon-carbon and carbon-hydrogen bonds in the polymer chain, also creating free radicals on the polymer surface. Active species within the cold plasma react with these sites to obtain thermodynamic stability resulting in changed chemistry at the polymer surface

With activation, the surface energy of a polymer can be increased by employing an oxygen-rich process gas, or it can be decreased by employing a process gas with a high fluorine content. Selection of the gas and the process parameters permits the polymer surface to be specifically tailored to promote or prevent wetting and adhesion.

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