Model of plasma-polymerized

For the conditions shown in Fig. 8, it was estimated that the electron density was 8x 10 cm" . This means that kj = 2.5 x 10" cm /s, a value in good agreement with measured rate coefficients for dissociation of small molecules by electron impact . The gas phase propagation rate coefficient kp was also found to be in very good agreement with values determined for conventional butadiene polymerization. The agreement of the adjusted parameter values with those measured independently lends further support to the validity of the proposed model of plasma polymerization. 

Figure 16. Postulated model of plasma-polymerized ethylene film (56)...

Since plasma contains electrons, ions, photons, radicals, and excited molecules, it becomes important to identify the reactive species controlling the propagating process of the polymerization. A number of workers have reported on kinetic models of plasma polymerization. Our current xmderstanding of the chemical and physical mechanism of the process remains limited because the extreme complexity of the plasma environment resists efforts toward a generalization and characterization. The bulk of the research has been concentrated on establishing the dependence of the macroscopic and spectroscopic properties of the product on the major process variables, e.g., rf power, monomer type, and gas flow rate. 

Surface reactions are extremely important. They are promoted by bombardment with electron ions, and photons. Ions and electrons had been taken into account in the first models of plasma polymerization, e.g., by Williams and Hayes [23], who worked with a 10-kHz frequency applied to parallel plate electrodes. They proposed that the monomer is adsorbed on the surface of the electrodes. By ion and electron bombardment from the discharge, a part of the monomer is converted into surface... 

Fig. 20. XPS survey spectra of plasma polymerized acetylene films as a function of reaction time with a model rubber compound. The reaction times were (A) 0, (B) 15, (C) 35, and (D) 65 min. Reprinted by permission of Gordon and Breach Science Publishers from Ref. [23].

Positive SIMS spectra obtained from plasma polymerized acetylene films on polished steel substrates after reaction with the model rubber compound for times between zero and 65 min are shown in Fig. 44. The positive spectrum obtained after zero reaction time was characteristic of an as-deposited film of plasma polymerized acetylene. However, as reaction time increased, new peaks appeared in the positive SIMS spectrum, including m/z = 59, 64, and 182. The peaks at 59 and 64 were attributed to Co+ and Zn, respectively, while the peak at 182 was assigned to NH,J(C6Hn)2, a fragment from the DCBS accelerator. The peak at 59 was much stronger than that at 64 for a reaction time of 15 min. However,... 

The physical characteristics of a discharge and the manner in which it is sustained can have a profound effect on the kinetics of plasma polymerization . Therefore, we shall review these topics here, with specific emphasis on the characteristics of plasmas sustained between parallel plate electrodes. This constraint is imposed because virtually all efforts to theoretically model the kinetics of plasma polymerization have been directed towards plasmas of this type. Readers interested in broader and more detailed discussions of plasma characteristics can find such in referen-... 

In the present section we shall review the attempts which have been made to model quantitatively the kinetics of plasma polymerization. The assumptions underlying each model will be discussed as well as the extent to which the predictions of the theoretical models fit the experimental data. At tne end of this section it will be shown why the initial assumptions made in developing kinetic models depend on the conditions used to sustain the plasma. 

The number of attempts to model the kinetics of plasma-polymerization has been limited thus far. Nevertheless, these efforts have been useful in demonstrating the role of different processes in initiating polymerization and the manner in which the physical characteristics of the plasma affect the polymerization rate. It is anticipated that future modeling efforts will provide more detailed descriptions of the polymer deposition kinetics and thereby aid the development of a better understanding of the interactions between the physical characteristics of a plasma and the chemistry associated with polymer formation. 

Figure 35.12 The effect of plasma polymerization coating applied on Gore-Tex graft in baboon model.

A number of workers have reported on kinetic models for plasma polymerization. Williams and Hayes (36) first suggested that the reaction occurred exclusively on solid surfaces within the reaction zone. Initially, monomer is adsorbed onto the electrode surface, where a portion is converted to free radical species after bombardment by ions and electrons produced in the plasma. Surface radicals then polymerize with adsorbed monomer to yield the thin film product. Based on this scheme, Denaro, et. al. derived a simple rate expression which showed reasonably good agreement with deposition rate data at various pressures and power levels (16). It is, however, unrealistic to assume that the plasma polymerization reactions occur exclusively on the surface. A more likely mechanism is that both gas phase and surface reactions proceed simultaneously in plasma polymer formation. 

H. Yasuda, "Adhesion of Plasma Polymerized Films (A Model Study on Water Sensitivity of Adhesion)," in Adhesion Aspects of Polymeric Coatings, K. L. Mittal, Editor, p. 193, Plenum, New York (1981). 

Tsai et al. have also used RAIR to investigate reactions occurring between rubber compounds and plasma polymerized acetylene primers deposited onto steel substrates [12J. Because of the complexities involved in using actual rubber formulations, RAIR was used to examine primed steel substrates after reaction with a model rubber compound consisting of squalene (100 parts per hundred or phr), zinc oxide (10 phr), carbon black (10 phr), sulfur (5 phr), stearic acid (2 phr). 

Many applications of XPS to problems in adhesion science have been reported in the literature. One interesting example is provided by the work of Tsai et al. on the use of XPS to investigate reactions between model rubber compound and plasma polymerized acetylene films that was discussed above [22,23], Consideration of that system permits some interesting comparisons to be made regarding the type of information that can be obtained from RAIR and XPS. 

The Auger depth profile obtained from a plasma polymerized acetylene film that was reacted with the same model rubber compound referred to earlier for 65 min is shown in Fig. 39 [45]. The sulfur profile is especially interesting, demonstrating a peak very near the surface, another peak just below the surface, and a third peak near the interface between the primer film and the substrate. Interestingly, the peak at the surface seems to be related to a peak in the zinc concentration while the peak just below the surface seems to be related to a peak in the cobalt concentration. These observations probably indicate the formation of zinc and cobalt complexes that are responsible for the insertion of polysulfidic pendant groups into the model rubber compound and the plasma polymer. Since zinc is located on the surface while cobalt is somewhat below the surface, it is likely that the cobalt complexes were formed first and zinc complexes were mostly formed in the later stages of the reaction, after the cobalt had been consumed. 

Negative SIMS spectra obtained from plasma polymerized acetylene films on polished steel substrates as a function of reaction time with the model rubber compound are shown in Fig. 45. The most important changes observed in the... 

Model 4. As a result Lam et al. concluded that Model 3 best describes the plasma polymerization kinetics of styrene. 

Figure 8 illustrates a comparison between measured and computed rates of butadiene polymerization in an rf plasma sustained at 13.56 MHz. A perfect fit is achieved by adjusting the rate coefficients appearing in the model to the following values ... 

These apparent contradictions can be rationalized in terms of a model which incorporates plasma-induced polymerization along with depolymerization. PBS has long been known to exhibit a marked temperature-dependent etch rate in a variety of plasmas. This is clearly seen in the previously published Arrhenius plots (3,7) for two different plasma conditions (Figure 1). This dependence is characteristic of an etch rate that is dominated by an activated material loss as would occur with polymer depolymerization. The latter also greatly accelerates the rate of material loss from the film. Bowmer et al. (10-13) have shown in fact that poly(butene-l sulfone) is thermally unstable and degrades by a depolymerization pathway. A similar mechanism had been proposed by Bowden and Thompson (1) to explain dry-development (also called vapor-development) under electron-beam irradiation. 

Figure 11.1 Model of a composite membrane, bending because of a stress in the thin layer deposited by plasma polymerization onto a flexible polymeric substrate layer and substrate have thickness d and D and Young s module e and E, respectively.

Lam and coworkers (4 ) developed kinetic models in which initiation of monomer by electron impact is followed by propagation and termination. They showed that activation of monomer in the gas phase followed by propagation and termination on the electrode surface gave an excellent description of the plasma polymerization process. The predicted functional dependence of deposition rate/On pressure (p) and current density (J) is R p 3. ... 

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