Commercially available thermoplastic polymers were modified by forming semi Interpenetrated Polymer Networks (semi-IPNs) with poly(ethylene glycol) diacrylate (PEGDA). These semi-IPNs were used to prepare asymmetric flat sheet membranes. The successful formation of the semi-IPNs was confirmed by Fourier Transform Infra Red (FT-IR) spectroscopy. The property enhancement of the semi-IPN asymmetric membranes compared to commercial polymer was evaluated by characterizing thermal, morphological and gaseous permeation properties. The semi-IPNs showed two-step degradation pattern indicating the incorporation of polyethylene glycol moieties into the thermoplastic polymers. The thermal stability of all the membranes was above 360 degrees C indicating their suitability for high temperature applications. The presence of asymmetric morphology was confirmed by the scanning electron microscopy. Enhancement in gas permeation properties, particularly for the carbon dioxide transport, was observed for the synthesized semi-IPNs, over the pristine polymers. The semi-IPNs showed increase in selectivity and increase in CO2 permeation when compared to the unmodified polymers. Among the three different polymers, poly(amide-imide) exhibited higher selectivity whereas the polysulfone showed higher permeation.