Plasma polymer membranes from hexafluoroethane/hydrogen mixtures for separation of oxygen and nitrogen

✍ Scribed by F. Huber; J. Springer; M. Muhler

• Publisher: John Wiley and Sons
• Year: 1997
• Tongue: English
• Weight: 271 KB
• Volume: 63
• Category: Article
• ISSN: 0021-8995
• DOI: 10.1002/(sici)1097-4628(19970321)63:12<1517::aid-app2>3.0.co;2-r

✦ Synopsis

Thin plasma polymer layers were produced employing feed mixtures of hexafluoroethane and hydrogen in an rf parallel-plate reactor. The layers are intended for use in membrane-based separation of oxygen and nitrogen. The hexafluoroethaneto-hydrogen mixture ratio was varied over a wide range, whereas all other process parameters (power, pressure, substrate temperature, and total gas flow) were held constant. The plasma polymers were examined by scanning electron microscopy, Xray analysis, quantitative elemental analysis, and X-ray photoelectron spectroscopy. Permeability coefficients of oxygen and nitrogen and selectivities of the pure gases were determined. Pinhole-free plasma polymer films containing different amounts of fluorine, carbon, and hydrogen were formed. The distributions of fluorine and hydrogen in the products reflect their distributions in the feed gas. Traces of oxygen in some of the polymers are explained by the reaction of trapped radicals with atmospheric oxygen on the samples' exposure to air. Fluorine-containing carbon moieties such as CF 3 , CF 2 , and CF, and carbon moieties with fluorine 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 selectivity amounts to 3.4 at an oxygen permeability coefficient of 21 Barrer.