Modification of Polymeric Surfaces With Plasmas
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MRS BULLETIN/AUGUST 1996
down to the base metal. (See the article by Rie et al. in this issue.) This is possible since metals are typically quite resistant to attack from many plasma chemistries.
In the case of polymers, the use of the word "cleaning" takes on a slightly different context since polymers are easily attacked by plasma environments. Thus polymer surfaces are not just "scrubbed" down to the base polymer but are actually altered chemically and topographically. Production of high-strength composites requires surfaces of fiber and plastic insert parts to be stripped of lowmolecular-weight, poorly attached surface-polymer residues before matrix resins are applied.1 Plasma "scrubs" surfaces of unwanted materials largely by a combination of sputtering by energetic ions and uv photolysis of covalent bonds of the surface contaminates' molecular structure. For example, the surface of polyethylene is typically contaminated with low-molecular-weight, wax-like, incompletely polymerized oligomers of ethylene, the monomer for polyethylene. These poorly adherent fragments must be removed before printing inks or adhesives can be applied. Since the fragments are on the surface and they are more easily and quickly degraded into volatile compounds than the base polymeric structure, the base polymer structure remains essentially intact and minimally etched if short treatment times are used. Repetitive cleavage of low-molecularweight polyethylene surface contaminates produces gases that can be pumped from the system, leaving a stable surface suitable for strong attachment of adhesives: Ar Plasma
* Gaseous products (e.g.,CH2 = CH 2 ,H 2 ,CH 4 ). (1)
[-CH2-]N
3 um Figure 1. Scanning electron micrographs of ultrahigh-molecularweight polyethylene precleaned in hexane (a) before and (b) after exposure to an oxygen plasma.
Noble feed gases such as argon are used since they tend to initiate cleavage without grafting to the surface. Treatment times are typically short so as to reduce further unwanted chemistry. Excessive treatment results in attack of the base polymer, which ultimately "ashes" the entire polymeric part, resulting in its destruction. A typical cleaning procedure for polyethylene would be to treat with argon at a pressure of 0.01-0.4 Torr, with a power density of -0.5 W/cm2 at 13.56 MHz rf on parallel-plate electrodes. Once contaminants are removed, a more stable polymer surface is exposed to the plasma environment so as to facilitate further reaction such as etching, grafting, or direct application of the desired surface coating. More aggressive processing can be achieved by using reactive gases or by use of electrical biasing to increase the energetics of the bombarding ions. However etching into the base polymer be-
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Modification of Polymeric Surfaces With Plasmas
gins to occur, which leads us logically to the next topic.
Etching To obtain highly adherent coatings on polymeric surfaces, more than surface cleaning is often required. Plasma can provide "microroughened" surface topographies unattainable
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