Surfaces: The Invisible Universe (The Plasma Chemistry of Polymer Surfaces – Review)

We interrupt the normal flow of essays on the invisible universe of surfaces to present a short review of a newly available volume which should be of strong interest to all who are interested in the modification of polymer surfaces using plasma technology. The volume in question is:

THE PLASMA CHEMISTRY OF POLYMER SURFACES: Advanced techniques for Surface design

By: Jörg Friedrich, (Wiley-VCH Verlag GmbH & Co. KgaA, Weinheim, Germany, 2012)

By way of disclosure we should point out that Prof. Friedrich is a good friend and colleague of ours and a long time participant in the MST CONFERENCES symposia series on Polymer Surface Modification and Metallized Plastics.  We can therefore personally vouch for his outstanding expertise in the topic under discussion.

The volume is indeed a major undertaking to categorize and review the truly vast topic of plasma processing as applied to polymer surfaces.  The table of contents gives a glimpse of the wide scope of topics covered:

• Interaction between Plasmas and Polymers
• Plasma
• Chemistry and Energetics in Classic and Plasma Processes
• Kinetics of Polymer Surface Modification
• Bulk, Ablative and Side Reactions
• Metallization of Plasma-Modified Polymers
• Accelerated Plasma-Aging of Polymers
• Polymer Surface Modification with Monosort Functional Groups
• Atmospheric Pressure Plasmas
• Plasma Polymerization
• Pulsed-Plasma Polymerization

 

The volume is clearly directed toward the polymer chemist with emphasis on the creation and behavior of “functional groups” which tend to dominate the chemical behavior of polymer surfaces. The ubiquitous hydroxyl (-OH) group is a prime example. The introduction of such groups to a highly hydrophobic polypropylene surface, which repels water and most common inks, can almost magically turn it into a hydrophilic surface which can be easily written on with a ball point pen.  There are of course many more such “functional groups” and the volume goes into great detail as to how they can be created, how they affect the surface chemistry and how they behave over time.

The wide scope of the volume makes it impossible to review all of the topics listed above in any detail so we will focus instead on the chapter on Atmospheric Pressure Plasmas which is likely to be of most interest to readers of this column.

The section on atmospheric plasmas begins with the initially astonishing assertion that more than 99% of the universe is in the plasma state.  To us earthlings bound to a highly condensed rock of a planet this seems hardly credible as our local experience of the plasma state is confined to relatively rare phenomena such as lightning, auroras and flames.

However, simply looking up at the sky during daylight reveals the sun which is a seething cauldron of plasma gasses weighing more than 100 million times the mass of the earth.  The sun of course is just one of a near infinitude of stars that make up the universe so the 99% estimate is actually highly conservative.

Having established the ubiquity of the plasma state in the universe the author then goes on to review a wide range of recent work on the use of atmospheric plasmas for the surface modification of polymers.  Of particular interest is the comparison of atmospheric plasma jets, low pressure oxygen plasma and dielectric barrier discharge plasma.  Figure 10.1 from the text is particularly interesting.  It compares the relative effectiveness of these techniques in introducing oxygen into the surface of polypropylene as a function of application time.  Interestingly the data show that after about 6 seconds the atmospheric plasma jet technique delivers nearly twice the oxygen concentration to the surface than either the low pressure oxygen or dielectric barrier method.  This easily explains why the plasma jet approach is highly favored in manufacturing applications where a rapid process is necessary to achieve required manufacturing volumes.

A second most interesting application is the use of atmospheric plasma for polymer deposition onto surfaces.  This is an apparent attempt to apply a polymer coating to a surface which in conventional low pressure plasma processes is carried out by the plasma polymerization method.  By the conventional approach plasma activated monomer species are allowed to condense on the target surface where they can then react to form polymer networks.  This approach to applying a polymer coating to a surface faces a number of problems due to the high reactivity of the plasma activated monomers.  One on the surface they could react in a wide variety of ways giving rise to a complicated morphology which is difficult to understand.  In addition the activated monomers tend to condense not only on the target substrate but on all the vacuum chamber surfaces.  Over time this can give rise to coated chamber surfaces which can flake off and cause a contamination problem.

In order to circumvent the problems encountered with the conventional plasma polymerization method the atmospheric pressure approach injects a fully formed polymer species into the plasma stream as an aerosol.  The plasma gas activates both the polymer and the target surface allowing the polymer to form a strongly adhered coating.  Thus the prefabricated polymer goes down as a complete entity and there is no vacuum chamber to be coated and build up possible contamination effects.

These and other innovative plasma techniques are reviewed in some detail in this quite comprehensive volume.  We can highly recommend this book as a reference work for all those engaged in the research and development of plasma surface treatment technologies.

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