Growth mechanism, plasma polymerization

Plasma polymerization is a very complicated process, and the overall growth rate is a function of several independent factors such as the type of discharge, reactor geometry, properties and temperature of the substrate, pressure, type and composition of the feed gas and so on. As a result, formulation of a generalized reaction mechanism is not easy. However, attempts were made to formulate the overall mechanisms of plasma polymerization, and can be applied to many cases. 

XPS data, on the other hand, showed that the ETC AT treatment of Ar + CF4 and Ar + C2F4 yielded just as good, if not better, fluorination of PET fibers than radio frequency plasma treatment with these gases [14,15]. These examples clearly demonstrate that polymerizable species in plasma polymerization are not photon-emitting species in most cases. This is in accordance with the growth and deposition mechanism based on free radicals, which account for the presence of large amount of dangling bonds in most plasma polymers. 

In the chain growth free radical polymerization of a vinyl monomer (conventional polymerization), the growth reaction is the repeated reaction of a free radical with numbers of monomer molecules. According to the termination by recombination of growing chains, 2 free radicals and 1000 monomer molecules leads to a polymer with the degree of polymerization of 1000. In contrast to this situation, the growth and deposition mechanisms of plasma polymerization as well as of parylene polymerization could be represented by recombinations of 1000 free radicals (some of them are diradicals) to form the three-dimensional network deposit via 1000 kinetic... 

In discussions of the mechanism of plasma polymerization appearing in the literature, polymerization, particularly the growth mechanism of polymer formation, is dealt with in a somewhat vague manner without any clear distinction between mechanism of polymerization and mechanism of polymer deposition. For instance, the hypothesis that plasma polymerization occurs via the polymerization of adsorbed monomer on the surface invokes the location of polymer formation rather than mechanism of polymerization that is, the mechanism of polymerization, whatever that would be, is intuitively or a priori assumed. Nevertheless, such a hypothesis constitutes an important school of thought in dealing with the polymerization mechanism. 

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. 

The following aspects of parylene polymerization [3-9] seem to have important implications in an effort to understand the growth mechanisms of plasma... 

The bicyclic RSGP mechanism shown in Figure 5.3 has an important implication for the interpretation of diagnostic data of the luminous gas phase. Namely, any species identified in the plasma phase are intermediate species of step growth polymerization but not precursors of black box plasma polymerization. 

Because of the unique growth mechanism of material formation, the monomer for plasma polymerization (luminous chemical vapor deposition, LCVD) does not require specific chemical structure. The monomer for the free radical chain growth polymerization, e.g., vinyl polymerization, requires an olefinic double bond or a triple bond. For instance, styrene is a monomer but ethylbenzene is not. In LCVD, both styrene and ethylbenzene polymerize, and their deposition rates are by and large the same. Table 7.1 shows the comparison of deposition rate of vinyl compounds and corresponding saturated vinyl compounds. 

The internal stress in plasma polymer films is generally expansive, i.e., the force to expand the film is strained by external compressive stress. According to the concept presented by Yasuda et al. [1], the internal stress in a plasma polymer stems on the fundamental growth mechanisms of plasma polymer formation. A plasma polymer is formed by consecutive insertion of reactive species, which can be viewed as a wedging process. The internal stress is related to how frequently the insertion occurs as well as on the size of inserting species. The both factors are dependent on the operational factors of plasma polymerization. 

In both the polymerizations, free radicals are the species that are responsible for the formation of bonds in the depositing materials. The growth mechanism, however, is not by the conventional chain-growth free-radical polymerization. In a conventional free-radical chain-growth polymerization, two free radicals and 10,000 monomer molecules yield a polymer with degree of polymerization 10,000, which does not contain free radicals. In contrast to this situation, in plasma polymerization and Parylene polymerization, 10,000 species with free radical(s) recombine to yield a polymer matrix that has an equivalent degree of polymerization, and contains numbers of unreacted free radicals (dangling bonds). 

Plasma polymerization coating is a unique nanofilm coating technology that yields a tight amorphous network structure of atoms contained in the monomer (Type A plasma polymers). Because of the unique growth mechanism of plasma polymerization, excellent adhesion of coating could be obtained in a practical manner. 

Growth of Plasma-Deposited Phases. Plasma polymerization (22) and anodization are already used extensively in materials processing, although the mechanisms by which these reactions occur are not well characterized. However, they have their counterparts in aqueous electrochemistry in electrochemical polymerization and anodic film growth, respectively. In the case of plasma anodization, the mechanisms of growth of the oxide film are not well understood, nor are kinetic data available for a large number of systems. Considerable research effort is required to obtain the necessary data in order to develop viable mechanisms. 

Figure 3. Schematic representation of bicyclic step-growth mechanism of plasma polymerization.

Yasuda and co-workers have identified two regimes of plasma polymerization in w4ikh the nKchanisms differ dramatically the monomer-rtefident and the energy-defkaent plasma. In addition, Yasuda has proposed a bkyclic gas phase reaction scheme based in part on his observation of a marked similarity between plasma polymerization aiHl the polymerization of paraxylene. This mechanism postulates a gas-pliase of molecules via two cyclk chain growth reaction channels of radicals and di-radicals and the potential attachment of any species to the surface by physkal or chemical proccsses. 

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