The size and shape of free-volume holes available in membrane materials control the rate of gas diffusion and its permeability. Based on this principle, two segmented thermo-sensitive polyurethane (TSPU) membranes with functional gates, i.e. the ability to sense and respond to external thermo-stimuli, were synthesized and used for water vapor controllable permeation. Differential scanning calorimetry (DSC), positron annihilation lifetimes (PAL), water swelling and water vapor permeability (WVP) were used to evaluate how the structure of the polyurethane (PU) and the temperature influence the free-volume holes size and the water vapor permeability (WVP) of the PU membranes. DSC study reveals that TSPU with a glass transition or a crystalline transition reversible phase shows an obvious phase-separated structure and a phase transition temperature (defined as switch temperature, T-s). PAL study indicates that the free-volume holes size of TSPU is closely related to the T-s. When the temperature is higher than the T-s, the ortho-positronium (o-Ps) lifetime (tau(3)) and the average radius (R) of free-volume holes of TSPU membrane increase dramatically. As a result, the WVP of TSPU membrane shows a dramatic increase. Additionally, the water swelling and the WVP of TSPU membrane are found to depend on the inner structure of the polymer, and they also give different responses to temperature variation. When the temperature is higher than the T-s, there is a significant increase of WVP from 3.80 kg/m(2) day to 7.63 kg/m(2) day for TSPU(a) and from 4.30 kg/m(2) day to 8.58 kg/m(2) day for TSPU(b), respectively. Phase transition accompanying significant changes in free-volume holes size and WVP can be used to develop "smart membranes" with functional gates and controllable gas permeation. (c) 2006 Elsevier B.V. All rights reserved.