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Plasma Polymerization (Glow Discharge Polymerization) uses plasma sources to generate a gas discharge that provides energy to activate or fragment gaseous or liquid monomer, often containing a vinyl group, in order to initiate polymerization. Polymers formed from this technique are generally highly branched and highly cross-linked, and adhere to solid surfaces well. The biggest advantage to this process is that polymers can be directly attached to a desired surface while the chains are growing, which reduces steps necessary for other coating processes such as grafting. This is very useful for pin hole free coatings of 500 Å to 1 micron thickness with solvent insoluble polymers.

Introduction/Background
In as early as the 1870s “polymers” formed by this process were known, but these polymers were initially thought of as undesirable byproducts associated with electric discharge, with little attention being given to their properties. It was not until the 1960’s that the properties of these polymers could be useful. [Goodman] It was found that flawless thin polymeric coatings could be formed on metals. By selecting the monomer type and the energy density per monomer, known as Yasuda parameter, the chemical composition and structure of the resulting thin film can be varied in a wide range. These films are usually inert, adhesive, and have low dielectric constants. The 1970’s brought about many advances in plasma polymerization, in which many different kinds of monomers were polymerized, and a probable mechanism of the polymerization process was determined. Since this time most attention devoted to plasma polymerization has been in the fields of coatings, but since it is difficult to control polymer structure, it has very limited applications.

Basic Operating Mechanism
Glow discharge is a technique in polymerization which forms free electrons which gain energy from an electric field, and then lose energy through collisions with neutral molecules in the gas phase. This leads to many chemically reactive species, which then lead to a plasma polymerization reaction. The electric discharge process for plasma polymerization is the “low-temperature plasma” method, because higher temperatures cause degradation. These plasmas are formed by a direct current, alternating current or radio frequency generator.
 * Glow Discharge

There are a few designs for apparatus used in plasma polymerization, one is the Bell (static type), in which monomer gas is put into the reaction chamber, but does not flow through the chamber. It comes in and polymerizes without removal. This type of reactor is shown on the right. This reactor has internal electrodes, and polymerization generally takes place on the cathode side. All devices contain the thermostatic bath, which is used to regulate temperature, and a vacuum to regulate pressure. Operation: The monomer gas comes into the Bell type reactor as a gaseous species, and then is put into the plasma state by the electrodes, in which the plasma may consist of radicals, anions and cations. These monomers are then polymerized on the cathode surface, or some other surface placed in the apparatus by different mechanisms of which details are discussed below. The deposited polymers then propagate off the surface and form growing chains with seemingly uniform consistency.
 * Types of reactors

Another popular reactor type is the flow through reactor (continuous flow reactor), which also has internal electrodes, but this reactor allows monomer gas to flow through the reaction chamber as its name implies, which should give a more even coating for polymer film deposition. It has the disadvantage of forming what is called “tail flame,” which is when polymerization extends into the vacuum line.

A third popular type of reactor is the electrodeless. This uses an RF coil rapped around the glass apparatus, which sends uses a radio frequency generator to form the plasma inside of the housing without the use of direct electrodes. The polymer can then be deposited as it is pushed through this RF coil toward the vacuum end of the apparatus. This has the advantage of not having polymer building up on the electrode surface, which is desirable in polymerizing onto other desirable surfaces.

Electron energy of 1-10 eV is required, with electron densities of 109 to 1012 per cubic centimeter, in order to form the desired plasma state. The electron temperatures are also not equal to the gas temperatures and have a ratio of Te/Tg of 10 to 100, so that this process can occur at near ambient temperatures, which is advantageous because polymers degrade at high temperatures. This entails non-equilibrium plasmas.
 * Physical Process Characteristics

The kinetic rate of these reactions depends mostly on the monomer gas. However, other parameters are also important as well, such as power, pressure, flow rate, frequency, electrode gap and reactor configuration. Low flow rates usually only depend on the amount of reactive species present for polymerization, whereas high flow rates depend on the amount of time that is spent in the reactor. Therefore, the maximum rate of polymerization is somewhere in the middle.
 * Kinetics

The fastest reactions tend to be in the order of triple-bonded > double-bonded > single bonded molecules, and also lower molecular weight molecules are faster than higher ones. So acetylene is faster than ethane, and ethane is faster than propene.

Increased pressure tends to decrease polymerization rates, with uniformity being controlled by constant pressure. At pressures greater than 1 torr, oligomers are formed on the electrode surface, and the monomers also on the surface can dissolve them to get a low degree of polymerization forming a oily substance. At low pressures, the reactive surfaces are low in monomer and facilitate growing high molecular weight polymers.

The rate of polymerization depends on input power, until power saturation occurs and the rate becomes independent of it. A narrower electrode gap also tends to increase polymerization rates because a higher electron density per unit area is formed. Polymerization rates also depend on the type of apparatus used for the process. In general, increasing the frequency of alternating current glow discharge up to about 5 kHz increases the rate due to the formation of more free radicals, after which inertial effects inhibit polymerization. A second maximum in frequency occurs at 6 MHz, where side reactions are overcome again and the reaction occurs through free radicals diffused from plasma to the electrodes.

Synthetic Routes
Plasma contains many species such as ions, free radicals and electrons, so it is important to look at what contributes to the polymerization process most. The first suggested process by Westwood et al. was that of a cationic polymerization, since in a direct current system polymerization occurs mainly on the cathode. However, more investigation has led to the belief that the mechanism is more of a radical polymerization process, since radicals tend to be trapped in the films, and termination can be overcome by reinitiation of oligomers. Other kinetic studies also support this theory.

In polymerization, both gas phase and surface reactions occur, but mechanism differs between high and low frequencies. At high frequencies it occurs in radical intermediates, whereas at low frequencies polymerization happens mainly on surfaces. As polymerization occurs, the pressure inside the chamber decreases in a closed system, since gas phase monomers go to solid polymers. An example diagram of the ways that polymerization can take place is shown on the right in Figure 2, wherein the most abundant pathway is shown in blue with double arrows, with side pathways shown in black. The ablation occurs by gas formation during polymerization. Polymerization has two pathways, either the plasma state or plasma induced processes, which both lead to deposited polymer.

Polymers can be placed on many substrates other than the electrode surfaces, such as glass, other organic polymers or metals, when either a surface is placed in front of the electrodes, or placed in the middle between them.

The actual polymerization is likely to take place through many ionic and radical processes, which are initiated by plasma formed from the glow discharge. There are many propagating species present at any given time as shown in Figure 2 at the right, which shows two different pathways for the polymerization to take place. The first pathway is a monofunctionalization process, which means it is analogous to a standard free radical polymerization mechanism (M•). The second pathway refers to a difunctional mechanism, which by example may contain a cationic and a radical propagating center on the same monomer (•M•)(Parylene polymerization), which means the polymer can grow in multiple directions by multiple methods off one species. Therefore, we would have a very rapid step-growth polymerization. In the diagram, Mx refers to the original monomer molecule or any of many dissociation products such as chlorine, fluorine and hydrogen. The M• species refers to those that are activated and capable of participating in reactions to form new covalent bonds. The •M• species refers to an activated difunctional monomer species. The subscripts i, j, and k show the sizes of the different species involved. Even though radicals represent the activated species, any ion or radical could be used in the polymerization. As can be seen here, plasma polymerization is a very complex process, with many parameters effecting everything from rate to chain length.

Common Monomers/Polymers
As can be seen in the monomer table, many simple monomers are readily polymerized by this method, but most must be smaller ionizable species because they have to be able to go into the plasma state. It must be noted that though monomers with multiple bonds polymerize readily, it is not a necessary requirement, as ethane, silicones and many others polymerize also. There are also other stipulations that exist. Yasuda et al. studied 28 monomers and found that those containing aromatic groups, silicon, olefinic group or nitrogen (NH, NH2, CN) were readily polymerizable, while those containing oxygen, halides, aliphatic hydrocarbons and cyclic hydrocarbons where decomposed more readily. It is also possible to incorporate N2, H2O, and CO into copolymers of styrene.
 * Monomers

Plasma polymers can be though of as a type of graft polymers since they are grown off of a substrate. These polymers are known to form nearly uniform surface deposition, which is one of their desirable properties. Polymers formed from this process often cross-link and form branches due to the multiple propagating species present in the plasma. This often leads to very insoluble polymers, which gives an advantage to this process, since hyperbranched polymers can be deposited directly without solvent.

Common polymers include: polythiophene,  polyhexafluoropropylene,  polytetramethyltin,  polyhexamethyldisiloxane,  polytetramethyldisiloxane, polypyridine, polyfuran, and poly-2-methyloxazoline.
 * Polymers

The following are listed in order of decreasing rate of polymerization: polystyrene, polymethyl styrene, polycyclopentadiene, polyacrylate, polyethyl acrylate, polymethyl methacrylate, polyvinyl acetate, polyisoprene, polyisobutene, and polyethylene.

Nearly all polymers created by this method have excellent appearance, are clear, and are significantly cross-linked. Linear polymers are not formed readily by plasma polymerization methods based on propagating species. Many other polymers could be formed by this method.

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.