Surface modification of poly(dimethylsiloxane) by atmospheric pressure high temperature plasma torch to prepare high-performance gas separation membranes

In this study, an atmospheric pressure high temperature plasma torch (APHTPT) was used to modify poly(dimethylsiloxane) (PDMS) membranes in order to form a thin SiOx layer on the membrane surface. The chemical properties of the PDMS and APHTPT-treated membranes, as well as the conversion of polysiloxane, were determined by X-ray photoelectron spectroscopy (XPS) and energy-dispersive X-ray (EDX) microanalysis. The microstructure of the membranes as a function of depth was obtained with a variable monoenergy slow positron beam (VMSPB) spectroscopy. Results of XPS and EDX indicated that the conversion of polysiloxane to silica was favored with increasing plasma power. VMSPB data demonstrated that the treated membrane exhibited an asymmetric organic–inorganic hybrid structure. Based on the analysis of the treated membrane, the free-volume size increased with the depth and showed a bi-modal distribution. The gas permeation properties of O2, N2, and CO2 were tested. The change in the applied plasma power was found to have a great effect on the membrane gas separation performance. The membrane selectivity for O2/N2 and CO2/N2 increased from 2.11 to 4.93 and from 9.54 to 17.19, respectively, after treatment at 10 kW. The APHTPT-treated PDMS membrane was found to have the advantages of both organic and inorganic membranes, leading to outstanding gas separation performance.