Plasma polymerization, reactive

The reaction mechanisms of plasma polymerization processes are not understood in detail. Poll et al [34] (figure C2.13.6) proposed a possible generic reaction sequence. Plasma-initiated polymerization can lead to the polymerization of a suitable monomer directly at the surface. The reaction is probably triggered by collisions of energetic ions or electrons, energetic photons or interactions of metastables or free radicals produced in the plasma with the surface. Activation processes in the plasma and the film fonnation at the surface may also result in the fonnation of non-reactive products. 

Surface modification of a contact lens can be grouped into physical and chemical types of treatment. Physical treatments include plasma treatments with water vapor (siUcone lens) and oxygen (176) and plasma polymerization for which the material surface is exposed to the plasma in the presence of a reactive monomer (177). Surfaces are also altered with exposure to uv radiation (178) or bombardment with oxides of nitrogen (179). Ion implantation (qv) of RGP plastics (180) can greatiy increase the surface hardness and hence the scratch resistance without seriously affecting the transmission of light. 

Abstract Plasma polymerization is a technique for modifying the surface characteristics of fillers and curatives for rubber from essentially polar to nonpolar. Acetylene, thiophene, and pyrrole are employed to modify silica and carbon black reinforcing fillers. Silica is easy to modify because its surface contains siloxane and silanol species. On carbon black, only a limited amount of plasma deposition takes place, due to its nonreactive nature. Oxidized gas blacks, with larger oxygen functionality, and particularly carbon black left over from fullerene production, show substantial plasma deposition. Also, carbon/silica dual-phase fillers react well because the silica content is reactive. Elemental sulfur, the well-known vulcanization agent for rubbers, can also be modified reasonably well. 

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. 

The terms luminous chemical vapor deposition and plasma polymerization are used synonymously in this book. Dealing with mechanism of reactions that lead to formation of solid deposition, PP is used according to the traditional use of the term. When dealing with the formation of reactive species and other operation and processing aspects, LCVD is preferentially used. 

When plasma polymerization is carried out in a flow system, in which glow covers the entire cross-section of reactor with respect to the direction of monomer flow, the monomer molecules coming into the reactor first encounter the luminous gas phase. It is very unlikely that the molecules pass through the luminous gas phase without interacting with it and reach the relatively narrow zone in which IG or DG, located near the electrode surface, occurs. Therefore, the mode of activation that occurs in LPCAT without the influence of ionization is important in terms of the creation of chemically reactive species in LCVD. The creation of reactive species by the luminous gas is the mechanism considered here. 

Plasma polymerization is initiated via the dissociation of molecules caused by varieties of energetic species in the luminous gas phase as described in Chapter 4. It is important to recognize that the reactive species created in the luminous gas phase are not initiators of plasma polymerization. Some species, e.g., free radicals, could be initiators of some monomers that have specific functional groups under special conditions, e.g., in the off period of pulsed glow discharge and in the nonglow zone of a reactor (remote plasma). In most cases, the reactive species created in luminous gas phase are reactive building blocks of LCVD. 

In parylene polymerization, the thermal cracking of the dimer (starting material) creates monomeric diradicals. All starting materials are converted to the reactive species, i.e., diradicals. No specific initiator for the chemical structure of starting material is formed. The situation is close to that of plasma polymerization. The comparison of plasma polymerization and radiation polymerization, and the comparison of the two vacuum deposition polymerizations (parylene polymerization and plasma polymerization) enable us to construct an overall view of material formation in the luminous gas phase. 

Parylene polymerization proceeds with well-defined chemical species, whereas plasma polymerization proceeds via variety of not-well-defined chemical species, which are created in the luminous gas phase. The reactive species for parylene polymerization is para-xylylene, which has features of (1) difunctional (e.g., diradicals), (2) reactive but relatively stable, and (3) highly selective reactivity (see Fig. 2.1). The exact nature of reactive species involved in glow discharge polymerization is not well known however, (1) they are not exclusively bifunctional, (2) they are highly reactive, and (3) consequently they have very low selectivity. The difference in the stability or the selectivity of reactive species is reflected in the distinctively different characters of polymer depositions of these two processes. 

Highly reactive (unstable) and nonselective species tend to react with any surface on which the species strike and form a polymer deposition with a high level of bonding or adhesion to the surface. Because of this aspect, plasma polymerization tends to form a thin film with a good adhesion with various kinds of substrate materials. Because of nonselective reactivity, the reactive species of plasma polymerization have poor penetration into small cavities such as those of porous structures. Reactive species tend to react with wall material at the entrance of a cavity rather than penetrating into the cavity (which requires that the species not react with the wall at the entrance). 

Because of the chemical structure insensitive (nonspecific) nature of plasma polymerization illustrated above, the structure of the monomer appears to have relatively little influence on the polymerization characteristics as well as on the characteristics of plasma polymers. This is largely true in the context of selective reactivity due to the chemical structure of monomers in conventional polymerization (i.e., monomer vs. nonmonomer). However, the influence of monomer structure, as classified by five types in Table 7.2, is actually accentuated when the operational conditions are varied. In this context, therefore, the chemical structure of a monomer is a key factor in its deposition characteristics and also in determining the properties of the deposition. 

As far as plasma polymerization and plasma treatment of materials, particularly organic polymers, are concerned, the luminous gas phase (low-pressure plasma) can be divided into three major groups based on the mode of consumption of the gas used to create the plasma (1) chemically nonreactive plasma (2) chemically reactive plasma and (3) polymer-forming plasma. The terms chemically reactive and chemically nonreactive are based strictly on whether the gas used in glow discharge is consumed in chemical processes yielding products in the gas phase or being incorporated into the solid phase by chemical bonds. 

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. 

The distribution of polymer deposition observed in the plasma polymerization of acetylene at different flow rates (and different system pressures under plasma conditions) is shown in Figure 20.2. It should be noted that acetylene is the fastest polymerizing hydrocarbon and the system pressure decreases on the inception of glow discharge. In this particular configuration of reactor, the monomer does not pass the radio frequency coil, and presents a typical case in which the creation of chemically reactive species occurs at the boundary where the monomer meets the luminous gas phase, i.e., activation by luminous gas, not by ionization. 

Modification of Textile Fibers. The reaction of hydrophobic chemicals with textile fibers offers the possibUity of permanent repeUency without alteration of the other physical properties of fibers. However, the disadvantages caused by complex processing, and resultant higher costs of carrying out chemical reactions on fiber in commercial textile plant operations, have limited the commercial appHcations. The etherification and esterification of ceUulose have been most effective in terms of achieving durable water repeUency (32,33). Radiation grafting of reactive repeUents onto fibers has been studied as a potential commercial process (34,35), as has modification by plasma polymerization of gas monomers or plasma initiated polymerization of Hquid monomers (36). 

Parylene polymerization proceeds with well-defined chemical species, whereas plasma polymerization proceeds via a variety of not well-defined chemical species, which are created in the luminous gas phase. The reactive species for Parylene polymerization is pora-xylylene, which is bifunctional (i.e., biradicals), moderately reactive and relatively stable, and highly selective in chemical reactivity to its own structure. 

Highly reactive (unstable) and nonselective species involved in plasma polymerization coating tend to... 

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