The time-dependent morphology development in a segmented polyurethane, which was prepared by the reaction of equimolar amounts of 1,4-phenylene diisocyanate (pPDI) and poly(tetramethylene oxide)glycol of (M.) of 975 g/mol, was investigated. No chain extender was utilized during the synthesis, and the resultant monodisperse hard segments constituted 14 wt % of the copolymer. Time-dependent microphase separation and morphology development was studied at room temperature by using solvent-cast films which were heated above the hard segment melting temperature, 55 degrees C, to erase the semicrystalline microphase morphology. Atomic force microscopy showed that, following heat treatment, the hard phase first developed into short rods within 30 min, followed by a growth period during which the short rods grew longer and eventually into a well-defined percolated structure. Morphology development was also followed by FTIR spectroscopy. While the intensity of the free C=O peak at 1730 cm(-1) decreased, the intensity of the hydrogen-bonded C=O peak at 1695 cm-1, which was not present in the original annealed sample, increased with time and began to plateau in similar to 24 h. A time-dependent increase in the storage modulus of the copolymer, following heat treatment, was also noted. This latter change could be described by the Avrami equation, yielding an Avrami exponent of 0.55. Because of the similarity of the copolymer's morphology to that of short fiber reinforced polymer composites, selected models developed for predicting the modulus of such composites could reasonably estimate the initially surprisingly high ambient temperature storage modulus of the copolymer of 0.9 x 10(8) Pa.