For PU-2,3 membranes due to the molecule weight of PPG is lower than that of PU-1(PPG-2000), the polymeric chain of soft segment mobility is easily interrupted by hard segment which results in the higher degree of hard segment and soft segment mixing, Because the high interior cohesion of hard segment can enhance the thermal resistance of soft segment,so the Tg of soft segment in PU-2,3 is higher than Tg of PU-1 in Fig. 8(b).