Thermoplastic segmented polyurethanes (TPUs) are an important class of thermoplastic elastomers with a two-phase microstructure arising from the thermodynamic incompatibility between hard (HSs) and soft segments. This microphase separation observed on cooling from a homogeneous state is often combined with the solidification of either or both types of segments. In this study, the structuring mechanism of two TPUs with HSs based on 4,4'-diphenylmethane diisocyanate and 1,4-butanediol was investigated from rheological measurements. Hence, in addition to the structuring temperature influence, the effect of an applied preshear flow in the melt polymer was analyzed, in particular. The results clearly show an enhancement of the solidification kinetics by the preshear. Indeed, the measured structuring time can be reduced by more than 1 decade. Rheo-optical microscopy observations coupled with a shearing hot stage corroborated these results and showed the modification of the microstructure by the shear. (C) 2009 Wiley Periodicals, Inc. J Polym Sci Part 13: Polym Phys 48: 190-201, 2010