Applicazione di film sottili per il controllo della diffusione e della permeabilità di materiali polimerici per UHV

The solubility and transport behaviour of low molecular weight substances in polymeric materials are topics of interest for many fields of science and technology. The importance and relevance of such behaviour has become more apparent in recent years because of the large number of conceivable applications, such as gas barriers or gas separation membranes in the medical or industrial field. For instance, polymers with high barrier properties are required for packaging of carbonated soft drinks, where the permeation of carbon dioxide, oxygen and water should be suppressed. Products containing fats and oils require protection against oxygen and in gas separation processes polymeric membranes presenting both high permeability and selectivity are applied. In the biomedical area, materials used in contact lenses require high oxygen permeability, surface hydrophilicity and biocompatibility. Moreover, polymers present many attractive properties for the construction of vacuum components. They are easily formed, light, robust and electrically nonconducting. However, their use for UHV (Ultra High Vacuum) applications is hindered by an important outgassing of volatile elements contained in their bulk. Furthermore, they are also prone to gas permeation when exposed to a pressure difference. An obvious improvement strategy consists in coating them with a metallic thin film to produce a hybrid material, which should combine the structural properties of a polymer with the vacuum properties of a metal. The present study is being undertaken in this frame and its main purpose is to achieve a better understanding of diffusion and permeation mechanism through polymers coated with Al thin film, for a possible application in UHV systems. For this purpose, PEEK, PET, Kapton foils and a PEEK tube have been coated with Al thin film by sputtering and by evaporation. The characterisation of the film has been carried out by Energy Dispersive X-ray spectroscopy (EDX), for the chemical composition, and by X-ray diffraction (XRD), Scanning Electron Microscopy (SEM) and Transmitted Light Microscopy for the morphology and the crystallinity. The mechanical properties have been investigated by means of the scotch-tape test and the nano-indentation. Finally, the barrier efficiency of the thin films produced has been studied by gas phase permeation and outgassing measurements. The results obtained in the present investigation have shown that Al coatings allow the permeability to be reduced at the most by two orders of magnitude. Plasma pre-treatment of the PEEK substrates have not a clear influence on the permeability reduction factor, while allows the permeability of the Kapton samples to be reduced by a factor 50. The residual permeation flux seems to be due to a fraction of about 10-4 (J) of the total surface that remains uncoated: mainly 1 to 10μm diameter pinholes, caused by atmospheric dust, have been observed. With the present study it has been proved that the value of J is not due to the nature of the substrates but it comes from the fact that the depositions have not been perfomed in clean-room conditions. Finally, finite element simulations of the effect of pinholes on the permeability agree with the esperimental measurements. In conclusion, the present study has improved the knowledge of thin films deposition on polimeric substrates and of permeation and diffusion mechanisms through hybrid structures, which are, nowadays, widely used in many fields of science and technology.