Plasma polymerization mechanism

A very common and useful approach to studying the plasma polymerization process is the careful characterization of the polymer films produced. A specific property of the films is then measured as a function of one or more of the plasma parameters and mechanistic explanations are then derived from such a study. Some of the properties of plasma-polymerized thin films which have been measured include electrical conductivity, tunneling phenomena and photoconductivity, capacitance, optical constants, structure (IR absorption and ESCA), surface tension, free radical density (ESR), surface topography and reverse osmosis characteristics. So far relatively few of these measurements were made with the objective of determining mechanisms of plasma polymerization. The motivation in most instances was a specific application of the thin films. Considerable emphasis on correlations between mass spectroscopy in polymerizing plasmas and ESCA on polymer films with plasma polymerization mechanisms will be given later in this chapter based on recent work done in this laboratory. 

The inclusion of particles in a film of plasma polymer was once considered by some investigators to be a characteristic problem due to the plasma polymerization mechanism, which hampers the practical use of plasma polymers in some applications. In contrast to this view, the formation of powder or the inclusion of particles in a film is related to the polymer deposition part of polymerization-deposition mechanisms. The inclusion or elimination of particles, therefore, could be accomplished by selection of the proper operational parameters and reactor design. The data of Tiepins and Sakaoku [7] are a typical demonstration that powders can be formed nearly exclusively if all conditions are selected to favor powder formation. An important point is that the monomers used in their study were those commonly used by other investigators for the study of film formation by plasma polymerization in other words, no special monomer is needed to form powders exclusively. 

In an attempt to consider some extent of fragmentation of the monomer as well as to explain polymerization of simple organic molecules that are not considered monomers, plasma polymerization mechanisms are often explained by assuming plasma-induced precursors, which have polymerizable structures. The precursor concept is detailed in Figure 10.3. It is significantly different from the simple process described in Figure 10.2 however, it still depends on a simple deposition process from precursors to plasma polymer. This concept intuitively assumes that the structure of a plasma polymer can be predicted from the structures of precursors. 

Schematic representation of the plasma polymerization mechanism according to Yasuda. (From Yasuda, H., Plasma Polymerization, Academic Press, New York, 1985.)...

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. 

In this study, the plasma polymerization and the discharge phenomena were studied as a function of the discharge frequency. In order to support the speculation of the polymerization mechanism, infrared spectra and the dielectric properties were also measured for the samples formed in the same discharge frequency range. 

A brief description of the key plasma diagnostic techniques, which have been especially useful in delineating the gas phase processes in fluorocarbon plasmas, will be given followed by an extensive discussion of plasma etching and polymerization mechanisms. 

The Mechanism and Kinetics of Plasma Polymerization Table 6. Reaction Mechanism for Plasma-Polymerization of Unsaturated Hydrocarbons... 

It is apparent from the preceding discussion that there are differences of opinion concerning the mechanism by which plasma polymerization is initiated some... 

The results demonstrate the versatility of plasma polymerization of various monomers onto rubber fillers and vulcanization ingredients. The largest effects are seen in blends of different rubbers with unequal polarities. Substantial improvements in mechanical properties are seen in comparison with the use of unmodified fillers and curatives. 

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