PLASMA POLYMER MEMBRANES FROM HEXAFLUOROETHANE HYDROGEN MIXTURES FOR SEPARATION OF OXYGEN AND NITROGEN/

Thin plasma polymer layers were produced employing feed mixtures of he xafluoroethane and hydrogen in an rf parallel-plate reactor. The layer s are intended for use in membrane-based separation of oxygen and nitr ogen. The hexafluoroethane-to-hydrogen mixture ratio was varied over a wide range, whereas all other process parameters (power, pressure, su bstrate temperature, and total gas flow) were held constant. The plasm a polymers were examined by scanning electron microscopy, X-ray analys is, quantitative elemental analysis, and X-ray photoelectron spectrosc opy. Permeability coefficients of oxygen and nitrogen and selectivitie s of the pure gases were determined. Pinhole-free plasma polymer films containing different amounts of fluorine, carbon, and hydrogen were f ormed. The distributions of fluorine and hydrogen in the products refl ect their distributions in the feed gas. Traces of oxygen in some of t he polymers are explained by the reaction of trapped radicals with atm ospheric oxygen on the samples' exposure to air. Fluorine-containing c arbon moieties such as CF3, CF2, and CF, and carbon moieties with fluo rine atoms exclusively in secondary positions are present. A method of calculating crosslink density using the analytical data is described. The oxygen permeability coefficient and the selectivity of the plasma polymers increase as the hexafluoroethane content of the feed gas is raised. This behavior is attributed to growing solubility selectivity as a result of the rising fluorine content of the polymers. Maximum se lectivity amounts to 3.4 at an oxygen permeability coefficient of 21 B arrer. (C) 1997 John Wiley & Sons, Inc.