Kinetic and Mechanistic Studies of Plasma Polymerization

Owing to the variety of species formed during PP under a number of often different experimental conditions, it is impossible to characterize the polymerization process in terms of a mechanism and rate as for conventional polymerization reactions. It is nonetheless valid to make general observations as to the predominant mode of polymer formation and to comment on the factors that influence this. 

Much discussion centres on whether plasma polymerization takes place by a predominantly radical or ionic mechanism. Both species will undoubtedly be present in a gas discharge the pre-eminence of either will depend upon the monomer and the reaction configuration. Evidence for the existence of radicals trapped in plasma polymer film from electron spin resonance (e.s.r.) studies is common, and the effect of ions in polymerization is demonstrated by polymerizations in electroded systems, where polymerization is predominantly at an electrode (the cathode in d.c. discharges), although this is dependent upon the frequency. Polymerization may take place at a surface, in the gas phase, or by a combination of both. 

Radical polymerization routes have been suggested by several investigations. The formation of radicals has been proposed to arise from the ionic bombardment of adsorbed species at surfaces, which then react with gas-phase radicals formed by electron impact. Using a pressure-pulsed audio 

Both ions and radicals are suggested as significant precursors in the production of pyrolytic carbon from methane. There are, of course, several pathways leading to polymerization and an overall scheme described as a Competitive Ablation Polymerization (CAP) has been set out.  

Rates of plasma polymerization as a function of the experimental conditions have been reported by a number of authors. In general, polymer deposition rates tended to increase with applied power and monomer availability as indicated by flow rate and pressure. The effect of position within the reactorand of monomer structure and gas composition on deposition rates has also been 

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Since this is the first review of PP to appear in this series it is appropriate to begin with a brief description of the technique and the characteristic features of its products. The remainder of the review is divided into four sections kinetic and mechanistic studies of plasma polymerization, product and plasma characterization, applications of plasma polymerization to achieve technological goals (including patents), and finally plasma-induced polymerization of liquid monomers. 

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