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General Characteristics of Plasma Polymers
The properties of plasma polymers differ greatly from those of conventional polymers. While both types are dependent on the chemical properties of the monomer, the properties of plasma polymers depend more greatly on the design of the reactor and the chemical and physical characteristics of the substrate on which the plasma polymer is deposited. The location within the reactor where the deposition occurs also has an effect on the resultant properties of the polymer. In fact by using plasma polymerization with a single monomer and varying the reactor, substrate, etc. a variety of polymers having different physical and chemical properties can be prepared. The large dependence of the polymer properties on these factors make it difficult to assign a set of basic characteristics, but a few common properties that cause plasma polymers to stand apart from conventional polymers do exist.

The most basic difference from conventional polymers is that plasma polymers do not contain regular repeating units. Due to the number of different propagating species present at any one time as discussed above, the resultant polymer chains are highly-branched and are randomly terminated with a high degree of cross-linking.

All plasma polymers contain free radicals as well. The amount of free radicals present is dependent on the chemical structure of the monomer. Because the formation of the trapped free radicals is tied to the growth mechanism of the plasma polymers, the overall properties of the polymer directly correlate to the number of free radicals.

Plasma polymers also contain an internal stress. If a thick layer (e.g. 1 µm) of a plasma polymer is deposited on a glass slide, the plasma polymer will buckle up and frequently crack. The curling is attributed to an internal stress formed in the plasma polymer during the polymer deposition. The degree of curling is dependent on the monomer as well as the conditions of the plasma polymerization.

Most plasma polymers are insoluble and infusible. These properties are due to the large about of cross-linking in the polymers, discussed above. Consequently the kinetic path length for these polymers must be sufficiently long, so this property can be controlled to a point.

The permeabilities of plasma polymers also differs greatly from those of conventional polymers. Because of the absence of large-scale segmental mobility and the high degree of cross-linking within the polymers, the permeation of small molecules don’t strictly follow the typical mechanisms of “solution-diffusion” or molecular-level sieve for such small permeants. Really the permeability characteristics of plasma polymers falls between these two ideal cases.

A final common characteristic of plasma polymers is the adhesion ability. However again the specifics of the adhesion ability for a given plasma polymer, such as thickness and characteristics of the surface layer, are particular for a given plasma polymer and few generalizations can be made.

Advantages and Disadvantages
Plasma polymerization offers a number of advantages over conventional polymerization and in general. The most significant advantage of plasma polymerization is the ability of it to produce polymer films of organic compounds that do not polymerize under normal chemical polymerization conditions. Nearly all monomers, even saturated hydrocarbons and organic compound without a polymerizable structure such as a double bond, can be polymerized with this technique.

A second advantage is the ease of application of the polymers as a coating versus conventional coating processes. While coating a substrate with conventional polymers requires a number of steps, plasma polymerization accomplishes all these steps in essentially one step. This leads to a cleaner and ‘greener’ synthesis and coating process, since no solvent is needed during the polymer preparation and no cleaning of the resultant polymer is needed either. Another ‘green’ aspect of the synthesis is that no initiator is needed for the polymer preparation since reusable electrodes cause the reaction to proceed. The resultant polymer coatings also have a number of advantages over typical coatings, including being pinhole free, highly dense, and easily varied thickness.

There are also a number of disadvantages relating to plasma polymerization versus conventional methods. The most significant disadvantage is the high cost of the process. A vacuum system is required for the polymerization, significantly increasing the set up price.

Another disadvantage is due to the complexity of plasma processes. Because of the complexity it is not easy to achieve a good control over the chemical composition of the surface after modification. The influence of process parameters on the chemical composition of the resultant polymer means it can take a long time to determine the optimal conditions. The complexity of the process also makes it impossible to theorize what the resultant polymer will look like, unlike conventional polymers which can be easily determined based off the monomer.

Applications
The advantages offered by plasma polymerization have resulted in substantial research on the applications of these polymers. The vastly different chemical and mechanical properties offered by polymers formed with plasma polymerization means they can be applied to countless different systems. Applications ranging from adhesion, composite materials, protective coatings, printing, membranes, biomedical applications and so on have all been studied.

A significant area of research has been on the use of plasma polymer films as permeation membranes. The permeability characteristics of plasma polymers deposited on porous substrates are different than usual polymer films. The characteristics depend on the deposition and polymerization mechanism. Plasma polymers as membranes for separation of oxygen and nitrogen, ethanol and water, and water vapor permeation have all been studied. The application of plasma polymerized thin films as reverse osmosis membranes has received considerable attention as well. Yasuda et al. have shown membranes prepared with plasma polymerization made from nitrogen containing monomers can yield up to 98% salt rejection with a flux of 6.4 gallons/ft2 a day. Further research has shown that varying the monomers of the membrane offer other properties as well, such as chlorine resistance.

Plasma-polymerized films have also found electrical applications. Given that plasma polymers frequently contain many polar groups, which form when the radicals react with oxygen in air during the polymerization process, the plasma polymers were expected to be good dielectric materials in thin film form. Studies have shown that the plasma polymers generally do in fact have a higher dielectric property. Some plasma polymers have been applied as chemical sensory devices due to their electrical properties. Plasma polymers have been studied as chemical sensory devices for humidity, propane, and carbon dioxide amongst others. Thus far issues with instability against aging and humidity have limited their commercial applications.

The application of plasma polymers as coatings has also been studied. Plasma polymers form from tetramethoxysilane have been studied as protective coatings and have shown to increase the hardness of polyethylene and polycarbonate. The use of plasma polymers to coat plastic lenses is increasing in popularity. Plasma depositions are able to easily coat curved materials with a good uniformity, such as those of bifocals. The different plasma polymers used can be not only scratch resistant, but also hydrophobic leading to anti-fogging effects.