Poly(urethane-siloxane) copolymers were prepared by copolymerization of OH-terminated polydimethylsiloxane (PDMS), which was utilized as the soft segment, as well as 4,4-diphenylmethane diisocyanate (MDI) and 1,4-butanediol (1,4-BD), which were both hard segments. These copolymers exhibited almost complete phase separation between soft and hard segments, giving rise to a very simple material structure in this investigation. The thermal behavior of the amorphous hard segment of the copolymer with 62.3% hard-segment content was examined by differential scanning calorimetry (DSC). Both the T-1 temperature and the magnitude of the T-1 endotherm increased linearly with the logarithmic annealing time at an annealing temperature of 100 degrees C. The typical enthalpy of relaxation was attributed to the physical aging of the amorphous hard segment. The T-1 endotherm shifted to high temperature until it merged with the T-2 endotherm as the annealing temperature increased. Following annealing at 170 degrees C for various periods, the DSC curves presented two endothermic regions. The first endotherm assigned as T-2 was the result of the enthalpy relaxation of the hard segment. The second endothermic peak (T-3) was caused by the hard-segment crystal. The exothermic curves at an annealing temperature of above 150 degrees C exhibited an exotherm caused by the T-3 microcrystalline growth. (c) 2006 Wiley Periodicals, Inc.