Abstract:Differences of morphological structure and properties of common polyurethane (PU(a)) and thermo sensitive polyurethanes（TSPU(b)and TSPU(c)）were investigated. Differential Scanning Calorimetry (DSC), Positron Annihilation Lifetimes（PAL）and water vapor permeability (WVP) were employed to investigate the phase transitions of polyurethanes and evaluate the temperature influence on the free volumes (FV) and the WVPs of all PU membranes. TSPU(b) with a glass transition reversible phase and TSPU(c) with a crystalline transition reversible phase showed an obvious phase separated structure and a phase transition in designed temperature range(20~50 ℃). In addition, PAL study indicated that the radius of free volume holes(R) and the fractional free volume (fv) of TSPU are closely related to the phase transition temperature (defined as switch temperature, Ts), phase transition leads to dramatic changes in R and fv. When the temperature exceeds the Ts, the fv change from 1.6% to more than 9.0%, the average radius of free volume holes change from 0.23 nm to above 0.4 nm (Rb=0.418 nm, Rc=0.467 nm) and become larger than that of the water vapor (Rw=0.4 nm).Simultaneously, the WVPs of TSPU (b) and TSPU(c) membranes also showed a dramatic increase by 130%,160% respectively and thermo sensitive characteristics.