Enhancing the separation properties of plasma polymerized membranes on polydimethylsiloxane substrates by adjusting the auxiliary gas in the PECVD processes

Abstract

Thin SiNwOxCyHz coatings were deposited from hexamethyldisilazane as a precursor in a microwave driven low pressure plasma enhanced chemical vapor deposition process, in order to investigate their suitability as silicon based separating layers in membranes for gas separation. Polydimethylsiloxane composite membranes were used as substrate, as they have a dense and defect free surface and by this provide a smooth surface to ensure a homogenous and defect free coating. To evaluate correlations between process parameters, coating properties and permeation/selectivity performance, the influence of different compositions of the feed gas (auxiliary gas and monomer) on the chemistry and structure of the coatings and subsequently on permeability were investigated. For this, auxiliary gas was varied (N2, none and Ar) and coatings/membranes were analysed regarding their structural properties with atomic force microscopy and cyclic voltammetry as well as their chemical properties with x-ray photoelectron spectroscopy. Correlations between those properties and the permeation properties were examined. The investigations reveal that coating and gas transport properties can be adjusted by changing the auxiliary gas type. Membrane selectivities could be produced that are above Knudsen selectivity, especially for the gas pairings CO2/N2 (up to 15), He/N2 (up to 9) and CO2/CH4 (up to 8) at 30 °C.

On the Mixed Gas Behavior of Organosilica Membranes Fabricated by Plasma-Enhanced Chemical Vapor Deposition (PECVD)

“… Robeson plot for He/CO 2 with a variety of organosilica membranes fabricated via PECVD by the authors (filled circles) [ 21 , 22 ] and the uncoated PDMS substrate membrane. As reference, the plot displays two commercially available polymers (Matrimid [ 36 ] and P84 [ 37 ]) with an assumed thickness of 1 µm.…”

“…It allows the fabrication of tailored coatings with regard to mechanical and chemical properties and, at the same time, enables low coating thicknesses in the nanometer range. The formation of organosilica layers by PECVD utilizing hexamethyldisiloxane (HMDSO) as a precursor results in membranes with gas separation characteristics [ 15 , 16 , 17 , 18 , 19 , 20 , 21 , 22 , 23 ]. Inorganic [ 24 , 25 , 26 , 27 ] or organic substrate membranes [ 15 , 21 , 22 , 23 , 28 , 29 , 30 , 31 , 32 , 33 ] function as support for the thin organosilica layer.…”

“…The formation of organosilica layers by PECVD utilizing hexamethyldisiloxane (HMDSO) as a precursor results in membranes with gas separation characteristics [ 15 , 16 , 17 , 18 , 19 , 20 , 21 , 22 , 23 ]. Inorganic [ 24 , 25 , 26 , 27 ] or organic substrate membranes [ 15 , 21 , 22 , 23 , 28 , 29 , 30 , 31 , 32 , 33 ] function as support for the thin organosilica layer. Besides, PECVD enables the combination of advantages of different membrane materials.…”

“…The solid line in Figure 1 represents the Robeson upper bound [ 4 ]. The filled circles show the permeation characteristics of our composite membranes with selective PECVD coatings fabricated with varying coating parameters (refer to Kleines et al [ 21 , 22 ]). Additionally, the characteristics of the used PDMS substrate for the PECVD coating and two commercially available polymers (Matrimid [ 36 ] and P84 [ 37 ]) are plotted as reference.…”

Organosilica coating layer prevents aging of a polymer with intrinsic microporosity

“…[25][26][27][28][29][30][31] Coatings are typically deposited on inorganic [32][33][34][35] or on organic substrate membranes. [27,[36][37][38][39][40][41][42] Up to now, the influence of PECVD coatings on the aging behavior of the underlying polymer has not been studied.…”