Poly(ethylene terephthalate)/polyethylene glycol (PETG) copolymers with molecular weight of PEG varied from 800 to 20,000 g/mol were synthesized by melt macromolecular interesterification method. The results of contact angle and water adsorption capacity reveal that the molecular weight of PEG has a positive effect on the hydrophilic behavior of PETG copolymers. The interaction between water molecule and PETG copolymer was investigated through each relaxation time component by the Carr-Purcell-Meiboom-Gill sequence of low-field nuclear magnetic resonance. The analysis of T-2 component with different states shows that with the increase of molecular weight of PEG, the water adsorption capacity increases due to the microphase separation behavior of PETG copolymers. This result was also supported by the endothermic behavior of H2O/PETG mixtures measured by differential scanning calorimetry (DSC). Water molecule was tightly bound with PEG flexible chains at first stage, and then water molecules could swell the PETG copolymers, additional water permeating into the swelling copolymers. (C) 2015 Society of Plastics Engineers