Plasma polymerization
[1] Some common monomers polymerized by this method include styrene, ethylene, methacrylate, and pyridine, just to name a few.
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 limited applications.
[8] This reactor has internal electrodes, and polymerization commonly takes place on the cathode side.
The deposited polymers then propagate off the surface and form growing chains with seemingly uniform consistency.
[10] This system bypasses the requirements for special hardware involving vacuums, which then makes it favorable for integrated industrial use.
It has been shown that polymers formed at atmospheric pressure can have similar properties for coatings as those found in low-pressure systems.
However, other parameters are also important as well, such as power, pressure, flow rate, frequency, electrode gap, and reactor configuration.
[1] A heavy monomer, therefore, needs a faster flow, and would likely lead to increased pressures, decreasing polymerization rates.
[6] A narrower electrode gap also tends to increase polymerization rates because a higher electron density per unit area is formed.
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.
[13] In polymerization, both gas phase and surface reactions occur, but the mechanism differs between high and low frequencies.
As polymerization occurs, the pressure inside the chamber decreases in a closed system, since gas-phase monomers go to solid polymers.
Polymerization is likely to take place through either ionic and/or radical processes which are initiated by plasma formed from the glow discharge.
[7] In the diagram, Mx refers to the original monomer molecule or any of many dissociation products such as chlorine, fluorine and hydrogen.
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 were decomposed more readily.
Common polymers include: polythiophene,[19] polyhexafluoropropylene,[20] polytetramethyltin,[21] polyhexamethyldisiloxane,[22] polytetramethyldisiloxane, polypyridine, polyfuran, and poly-2-methyloxazoline.
Linear polymers are not formed readily by plasma polymerization methods based on propagating species.
[24] An example of a proposed structure for plasma polymerized ethylene demonstrating a large extend of cross-linking and branching is shown in Figure 4.
The amount of free radicals present varies between polymers and is dependent on the chemical structure of the monomer.
Consequently, the kinetic path length for these polymers must be sufficiently long, so these properties can be controlled to a point.
Nearly all monomers, even saturated hydrocarbons and organic compounds without a polymerizable structure such as a double bond, can be polymerized with this technique.
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 influence of process parameters on the chemical composition of the resultant polymer means it can take a long time to determine the optimal conditions.
Through the use of low-power and pressure plasma, high functional retention can be achieved which has led to substantial improvements in the biocompatibility of some products, a simple example being the development of extended-wear contact lenses.
Due to these successes, the huge potential of functional plasma polymers is slowly being realized by workers in previously unrelated fields such as water treatment and wound management.
[28] 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.
Plasma polymers have been studied as chemical sensory devices for humidity, propane, and carbon dioxide amongst others.
Plasma polymers formed from tetramethoxysilane have been studied as protective coatings and have been shown to increase the hardness of polyethylene and polycarbonate.
[29] Plasma polymer surfaces with tunable wettability and reversibly switchable pH-responsiveness have shown promising prospects due to their unique property in applications, such as drug delivery, biomaterial engineering, oil/water separation processes, sensors, and biofuel cells.
