Vapor-phase plasma polymerization

Figure 1.3 clearly demonstrates the luminous gas phase created under the influence of microwave energy coupled to the acetylene (gas) contained in the bottle. This luminous gas phase has been traditionally described in terms such as low-pressure plasma, low-temperature plasma, nonequilibrium plasma, glow discharge plasma, and so forth. The process that utilizes such a luminous vapor phase has been described as plasma polymerization, plasma-assisted CVD (PACVD), plasma-enhanced CVD (PECVD), plasma CVD (PCVD), and so forth. 

Any chemical reaction that yields polymeric material can be considered polymerization. However, polymerization in the conventional sense, i.e., yielding high enough molecular weight materials, does not occur in the low-pressure gas phase (without a heterogeneous catalyst). With a heterogeneous catalyst, polymerization is not a gas phase reaction. Therefore, the process of material deposition from luminous gas phase in the low-pressure domain might be better represented by the term luminous chemical vapor deposition (LCVD). Plasma polymerization and LCVD (terms explained in Chapter 2) are used synonymously in this book, and the former... 

In order to find the domain of LCVD, it is necessary to compare various vacuum deposition processes chemical vapor deposition (CVD), physical vapor deposition (PVD), plasma chemical vapor deposition (PCVD), plasma-assisted CVD (PACVD), plasma-enhanced CVD (PECVD), and plasma polymerization (PP). All of these terms refer to methods or processes that yield the deposition of materials in a thin-film form in vacuum. There is no clear definition for these terms that can be used to separate processes that are represented by these terminologies. All involve the starting material in vapor phase and the product in the solid state. 

In a plasma polymerization, the substrate is generally not heated, nor is the vapor heated. The chemical activation is done by the interaction of gas phase molecules with plasma (luminous gas) or by the generation of plasma of the starting material. In other words, activation of the starting material occurs in the vapor (plasma) phase, and the substrate is merely the collector of the product unless the substrate is used as an electrode. 

Plasma polymerization Vapor phase by plasma Substrate in the same chamber Ts < TV AE < 0... 

LPCAT polymerization Vapor phase in a separate chamber by plasma Substrate in a separate chamber Ts < Tv AE<0... 

Plasma polymerization is a vapor phase-activated (by plasma) A/D-coupled CVD. Parylene polymerization is a vapor phase-activated (by thermal process) A/D-decoupled CVD. Cascade arc torch (CAT) polymerization is a vapor phase-activated (by plasma) A/D-decoupled CVD. General CVD is a surface-activated (by thermal process) A/D-coupled CVD. HWCVD is a hot wire activated A/D-decoupled CVD. Coupled and decoupled refer only to the spatial separation. 

Plasma can be utilized in the polymerization of monomer liquid. In this case, no substrate is employed, and monomers are typically organic compounds with an olefinic double bond (monomer for chain growth polymerizations). In a typical case, the vapor phase of a monomer liquid in a sealed tube is used to create plasma. The duration of plasma is generally very short (on the order of a few seconds). After plasma exposure, the tube is shaken to mix plasma-induced reactive species with the monomer and is kept at a constant temperature (polymerization temperature) for a prolonged period. 

Comparing the terms plasma chemical vapor deposition and luminous chemical vapor deposition, the dilference exists in the meaning of plasma and luminous gas and its implications to the nature of chemical reactions that occur in the gas phase. Without referring the details of the difference, however, the process could be described either plasma polymerization (plasma CVD) or luminous CVD in all practical purposes. 

So far as the growth mechanism is concerned, parylene polymerization is the closest kin of plasma polymerization. It is advantageous to go into a little details of parylene polymerization in order to understand how vacuum phase deposition of material occurs starting from a vapor phase material. 

When monomer vapor is introduced into the reaction system, some monomers will be adsorbed or sorbed by a porous substrate. The partition between vapor phase and sorbed phase is dependent on the adsorbing capability of a porous substrate. For instance, when a porous glass tube is used as a substrate, nearly 100% of the monomer fed into a closed system is adsorbed, and it is difficult to establish a steady-state flow of monomer vapor until the substrate is saturated with the monomer, which takes several hours at the flow rates generally used in plasma polymerization. 

When an organic vapor, such as methane, in low pressure (e.g., less than 1 torr) is subjected to an electromagnetic field, the electrical breakdown of the gas occurs, yielding a glow the color of which is characteristic to the gas. In the luminous gas phase, methane is activated and forms a polymeric deposition in the form of a coating on the surface of substrate placed in the glow. This process is termed plasma polymerization because the luminous gas phase or glow indicates the presence of plasma, and the process does not proceed without plasma. The strict definition of plasma is (at least partially) ionized gas, which maintains the electrical neutrality as a whole. The luminous gas phase in which plasma polymerization takes place, however, is not plasma in the strict sense. 

Polymerizations have also been carried out in the vapor phase, in plasmas, enzymatically, - and in compressed carbon dioxide. Poly-siloxanes have also been obtained by acid leachate from chrysotile asbestos. ... 

Polyhydroxy acids are another group of biopolymers. Since polylactic acids, polyglycolic acid, and poly(citric acid) are classified as thermoplastic polyesters (saturated), they lack reactive functional groups for surface reactions. Moreover, any chemical manipulation to create activation sites results in hydrolysis of the ester bonds. The only reported successful methods for functionalization of polyhydroxy acids are blending them with ECPs, or using a plasma polymerization process [29]. Prior to the plasma polymerization process, surface activation or ionization of these biopolymers must be carried out, which is acquired by means of vapor phase deposition, laser deposition, microwave or synchrotron radiation [30], pulsed arc, pulsed combustion, spark, or friction induction [30], electron beams, plasma induction, corona, photons, ion beams, and X-rays [25]. 

Gancarz et al. [90] compared the three different approaches to modify PS membranes with AA through plasma-initiated graft polymerization (1) grafting in solution, the plasma-treated polymer membrane was exposed to air for 5 min and dipped into a deaerated aqueous solution of monomer (2) grafting in vapor phase, when Ar plasma treatment on polymers was completed, a monomer vapor was introduced into the chamber and (3) plasma polymerization of monomer vapors in a plasma reactor. It was shown that modified PS membranes prepared in a vapor phase possessed the highest flux. 

After a long reaction time, polymers with exceptionally high molecular weight can be synthesized by plasma-induced polymerization. Since only brief contact with luminous gas phase is involved, plasma-induced polymerization is not considered to be LCVD. However, it is important to recognize that the luminous gas phase can produce chemically reactive species that trigger conventional free radical addition polymerization. This mode of material formation could occur in LCVD depending on the processing conditions of LCVD, e.g., if the substrate surface is cooled to the extent that causes the condensation of monomer vapor. 

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