A series of segmented polyurethanes (PUs) were prepared, in which five different polyols and hexamethylene diisocyanate were used as soft segments, and 4,4'-diphenylmethane diisocyanate, hydrophilic segment poly (ethylene glycol) 200 (PEG 200), and chain extender 1,4-butanediol were used as hard segment. Morphology of the PUs was investigated using differential scanning calorimetry, wide angle X-ray diffraction, polarizing microscopy, and transmission electron microscopy. Water vapor permeability of the membranes as a function of temperature was tested accordingly. Results show that the presence of PEG200 interferes the crystallization of hard segment in these PUs, and at the same time, increases phase compatibility between soft and hard segment in the PET-PU. It leads to a lower crystal melting temperature and degree of crystallinity of soft segment in the segmented PU than those of pure polyols, and no crystallization existing in hard segment. A morphological model is proposed, that is, aggregated soft-segment-rich domains can be observed clearly in the PUs with high crystallinity in soft segment, while identifiable hard domains are well-distributed in the soft segment domains in the PU with low crystallinity. Within the temperature range of crystal melting, water vapor permeability of the PU membranes increases significantly with increase of temperature. Such temperature-sensitive property is triggered by crystal melting of soft segment. (c) 2007 Wiley Periodicals, Inc.