Structure-gas transport property relationships of poly(dimethylsiloxane-urethane) nanocomposite membranes

Poly(dimethylsiloxane-urethane) (PDMS-PU) nanocomposite membranes with various concentrations of polyhedral oligomeric silsesquioxanes (POSS) were synthesised and the influence of POSS molecules on gas transport properties was studied. In this, two different types of POSS nanoparticles such as CyPOSS and POSS-H were incorporated and their effect on permeation properties was studied. Nanocomposite membranes were characterized using DSC, AFM, XRD and contact angle measurements. Glass transition temperature (T_g) of nanocomposite membranes were enhanced due to the restriction of polymer matrix by the bulky POSS group. Contact angle measurements show that the nanocomposite membranes displayed a significant enhancement in the surface hydrophobicity as well as reduction in surface free energy. Both AFM and contact angle measurements show POSS molecule aggregates on the surface of membranes in the place of urethane moiety. XRD profile reveals the absence of crystalline nature of pure POSS showing high amorphous nature of the membranes. Gas permeation measurements were carried for O₂, N₂, and CO₂ gases by employing different pressures. The decrease in permeability was observed on incorporation of CyPOSS type POSS molecule for the membranes. However, the permeabilities enhanced on slight addition of POSS-H type molecule into the membranes. It was found that the nature of POSS molecules and their compatibility with polymer matrix alters the permeation properties. The selectivities were mainly dependent on the presence of urethane functional groups of membranes showing 1.9-2.4 and 7.5-14.9 for the O₂/N₂ and CO₂/N₂ gas pairs, respectively.