In this study, a new family of poly(ethylene glycol) (PEG)-based membranes for CO2 separation was developed using PEG diglycidyl ether (PEGDE) cross-linked with a low-molecular-weight diamine (2,2'-(ethylenedioxy) bis(ethylamine)) and a polyamine-based ionic liquid (triethylenetetramine trifluoroacetate, [TETA][Tfa]) in a solvent-free process. A mass fraction up to 80% PEG dimethyl ether (PEGDME, average M-n similar to 250) was added to the cross-linked matrix to enhance the gas transport properties. The cross-linking reaction mechanism, thermal stability of the resulting membranes, as well as water uptakes of the formed membranes were systematically investigated. The CO2, N-2, and CH4 gas transport properties of these new membranes were studied. Both CO2/N-2 and CO2/CH4 binary mixed gas separation performances of the membranes at various humidity levels were tested. The gas permeation results showed that the free PEGDME additive acts as plasticizer in the polymeric matrix, resulting in excellent CO2/N-2 and CO2/CH4 separation properties.