MODIFICATION OF POLYMERIC MATERIAL SURFACES WITH PLASMAS^*

by: Don M. Coates^a and Stephen L. Kaplan^b

INTRODUCTION

As adaptable as polymeric materials are in their many applications in our daily lives, there is the need to tailor the surfaces of polymers to provide yet even more flexibility in their uses. Plasma treatments offer an unprecedented spectrum of possible surface modifications to enhance polymers, ranging from simple topographical changes to creation of surface chemistries and coatings that are radically different from the bulk polymer. Furthermore, plasma treatments are environmentally friendly and economical in their use of materials .

Plasma processing can be classified into at least four categories that often overlap. These are: (1) surface preparation by breakdown of surface oils and loose contaminates; (2) etching of new topographies, (3) surface activation by creation or grafting of new functional groups or chemically reactive excited metastable species on the surface; and (4) deposition of monolithic adherent surface coatings by polymerization of monomeric species on the surface. Key features of these processes will be briefly discussed, with a rudimentary introduction to the chemistries involved and examples. Focus is placed on capacitively-coupled rf plasmas, since they are most commonly used in polymer treatment.

SURFACE PREPARATION

In many industrial and scientific processes, extremely clean surfaces are crucial. By "clean" it is usually meant that the bulk material is also the material that makes up the actual surface, not foreign, loose or otherwise mechanically unstable debris. For metal surfaces, plasma treatment can strip off surface oils and contaminants leaving the surface truly "cleaned" down to the base metal (see the article by Rie et. at., in this issue of the MRS Bulletin). 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 readily 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 low molecular 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. Therefore, the base polymer structure remains essentially intact and minimally etched if short treatment times are used. Repetitive cleavage of low molecular weight polyethylene surface contaminates produces gases that can be pumped from the system, leaving a stable surface suitable for strong attachment of adhesives:

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 to 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 begins to occur, which leads us logically to the next topic.

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^* Chapter IV of Plasma Processing of Advanced Materials, edited by George A. Collins and Donald J. Rej, MRS Bulletin, August 1996