Thermo-sensitive polyurethane (TSPU) solution containing varying concentration of in situ-generated TiO2 nanoparticles was successfully prepared via an organic inorganic hybrid technique, and the final nanocomposite membranes were formed via solution casting. Depending on the membrane formation temperature (T-mf) during casting, completely-opposite gas transport behaviors of the TSPU nanocomposite membranes were observed. When T-mf was lower than the melting point (T-m) of soft segment (PCL10000) in the TSPU, gas permeability coefficients of the nanocomposite membranes were found to increase significantly with increasing nano-TiO2 concentration. Conventional composite theory failed to explain such observation because filler particles are generally considered to create more tortuous diffusion path in the polymer for penetrants, and should thereby lead to a systematic reduction in gas permeability. This counter-intuitive phenomenon was then rationalized by postulating that rigid TSPU chain could not pack efficiently around the TiO2 nanoparticles during solvent evaporation, which resulted in polymer layer with higher free volume at the interface than the bulk polymer regions. Evidences supporting such presumption were provided by (1) density measurement that found negative deviation of actual densities of TSPU nanocomposite membranes from theoretical prediction; (2) positron annihilation lifetime spectroscopy that demonstrated increased free-volume size and concentration within TSPU upon the in situ generation of TiO2 nanoparticles. However, when T-mf was elevated above the T-m of the soft segment, flexible TSPU chain seemed able to pack around the TiO2 nanoparticles as efficiently as in the bulk polymer. In this case, gas transport behavior of the TSPU nanocomposite membranes followed the prediction of conventional composite theory. (C) 2011 Elsevier Ltd. All rights reserved.