Gas-discharge polymerization

Abstract

A synthesis of the study that we undertook on the polymerization of monomer vapors in an alternating luminescent discharge is presented. After a brief description of the deposit apparatus, the experimental study of the deposit parameters has been given. The study relates to the influence of the discharge frequency (50 Hz to 6 MHz), of the discharge duration and its current density, of the substrate temperature, and of the monomer pressure (7×10−2 to 3 torr). Thereby it is demonstrated that the polymerization takes place in the interelectrode space rather than on the electrodes above a frequency of about 105 Hz, that the deposit thickness is a linear function of time (15 s to 10 mn), and that for a given substrate temperature and pressure it is necessary not to exceed a certain current threshold above which there is not any increase of the growth rate but rather a degradation of the forming layer. Finally it is shown that too high a pressure lowers the film growth rate as a result of the cooling of discharge electrons following an increase of the shocks in the discharge. These experimental results permit an explanation of the film formation on the substrate by the association of two phenomena: the monomer-molecule adsorption on the substrate which yields the matter to be polymerized and the electrical discharge which furnishes the necessary active particles to initiate and thereafter maintain the polymerization mechanism. In the second part of this article, an experimental and theoretical verification of this hypothesis has been undertaken. At first, the case of an dc electrical discharge is considered which permits the demonstration that the active particles are the positive ions of the discharges. Next, the multilayer adsorption phenomenon following the BET (Brunauer, Emmet, and Teller) theory is presented, and it is shown that this is in good agreement with the experimental results of the first part. Finally a radical polymerization scheme is proposed enabling the determination of an analytical expression for the polymer formation speed which agrees well with the experimental findings.