Surface photopolymerization of tetrafluoroethylene

Abstract

The formation of thin (< 1000 Å) polymeric films from tetrafluoroethylene in a pre-evacuated (10^–6 Torr) system by surface-photopolymerization has been monitored by quartz crystal microbalance, mass spectrometric, and infra-red techniques. Film growth rate was found (1) to depend upon ultra-violet light intensity to the 1.6 power at wavelengths of about 2100Å; (2) to vary inversely with the surface temperature over the range 10 to 60°C; (3) to vary positively with monomer pressure from 0.5 to about 10 Torr. Depositions using selected wavelengths demonstrated that the growth could not be initiated or maintained with light above 2150 Å, the effective cutoff for absorption by C₂F₄ in the gas phase. Adsorption on the surface does not perturb the TFE molecule to the extent that the photolysis can be initiated by less energetic photons. For irradiation at wavelengths < 2150Å, film growth terminated on removal of the light soure. Approximately one monomer unit was polymerized for each photon ∼ 2000 Å incident on the deposition cell. Evacuation of the photolysis system after deposition resulted in film weight loss as high as 50 % depending upon the experimental conditions. Mass spectral analysis of gas phase by-products identified higher molecular weight species up to C₉F₁₈. Infra-red analysis of the gas phase also indicated the presence of cyclic fluorocarbons. A proposed mechanism for the surface-photopolymerization process invokes the photodissociation of C₂F₄ followed by reactions of difluorocarbene and its higher homologues both in the gas phase and on the surface. Polymeric film growth on the surface then results from interaction of the higher molecular weight components.