Pinching of luminous gas phase

Because a polymer-forming luminous gas phase such as the tail-flame portion of an inductively coupled radio frequency glow discharge behaves as a fluid, the deposition mechanism can be investigated by examining the influence of the fluid mechanical aspects of luminous gas phase on the deposition rate of polymer. 

For simplicity of discussion, let us consider a simple monomer system and treat the monomer as an ideal gas. From the kinetic theory of gases, the ratio of gas-wall collisions to gas-gas collisions in the total system is proportional to the surface to volume ratio SjV for a given monomer at fixed pressure and temperature. 

The solid circles represent the deposition rate obtained at a vapor pressure of 60 mtorr (the initial pressure) and the open circles at a vapor pressure of 40 mtorr. In these experiments, the system pressure was not controlled independent of monomer flow rate. The lower pressure was obtained by a lower flow rate. At the latter vapor pressure, glow does not penetrate into the 5-mm constriction and no polymer deposition occurs within the constriction. This is due to the increased gas-wall collisions that quench the luminous reactive species. 

At a fixed flow rate of the equipment used, the slopes of two straight lines are in good agreement with the dependence of the deposition rate being approximately proportional to the square of the initial pressure, i.e., the deposition rate being proportional to the flow rate. The results show that the polymer deposition rate is indeed directly proportional to S/ V. This means that neither the gas phase reaction nor the surface reaction is the predominating factor in LCVD unless the S/V ratio is extremely high. 

It has been generally observed that polymer deposition occurs mainly on a surface exposed to glow. More precisely, the deposition rate of polymer onto a surface that does not make contact with glow is several orders of magnitude smaller than that onto a surface that contacts glow. The results outlined here clearly demonstrate that the rate of deposition of a polymer onto a surface that contacts glow is dependent on the S/pV of the glow. This factor seems to have important implications in the application of plasma polymerization, which may involve substrates of various sizes and shapes. 

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