Structure and gas separation properties of ultra-smooth PE-CVD silicon organic coated composite membranes

The layer properties of plasma polymerized membranes produced in a microwave-excited PECVD process depend on the reactor properties, process characteristics and the properties of the substrate used. In order to be able to control gas separation properties for specific applications, thin silicon based SiOCH coatings were deposited on PDMS substrates. The influence of oxygen to monomer ratios and of microwave power input were investigated, as their variation in the process is generally known to have a significant influence on the formation of the layer structure. Correlations between process parameters, coating properties and gas separation or membrane performance were evaluated by permeation measurements, structure analysis (FESEM, AFM) and chemical analysis (XPS, FTIR). The investigations show a strong dependence of the membranes gas separation properties on the adjusted PECVD process parameters. It was possible to produce dense, ultra-smooth SiOCH coatings with ideal selectivities in the range of >40 for He/N2; 10 for He/CO2 and > 8 for CO2/N2.