The phase transitions and the morphology of hard-segment domains of those siloxane-containing hard-segmented polyurethane copolymers are studied by differential scanning calorimetry (DSC). The NH-SiPU2 copolymer, which comprises a siloxane-urea hard segment and a polytetramethylene ether glycol soft segment (PTMG2000), exhibits a high degree of phase-separation and a highly amorphous structure. Therefore, NH-SiPU2 copolymer proceeds with a melt-quenching process and with various annealing conditions, to examine the morphologies and the endothermic behaviors of the siloxane-containing hard-segment domains. DSC thermograms of further annealed NHSiPU2 indicate that the first endotherm (T-1) at around 75 degrees C is related to the short-range ordering of amorphous siloxane hard-segment domains (Region 1), and the second endotherm (T-2) at around 160 degrees C is related to the long-range ordering of amorphous siloxane hard-segment domains (Region II). The DSC thermograms at annealing temperatures below and above T-1 demonstrate that both the temperature and the enthalpy of T-1 linearly increase with the logarithmic annealing time (log t(a)). This result shows that the endothermic behavior of T-1 is typical of enthalpy relaxation, which is caused by the physical aging of the amorphous siloxane hard segment. Additionally, the siloxane hard segments in Region I are movable, and can merge with the more stable Region II under suitable annealing conditions. Transmission electron microscopy shows that Regions I and II are around 200 and 800 nm wide, and that the Region I can be combined with the stable Region II, under suitable annealing conditions. (c) 2006 Wiley Periodicals, Inc.