In this work, the rheological and structural changes induced by uniaxial extensional flows are studied for two aromatic TPUs: an elastomeric (soft) material composed both of hard and soft segments and an amorphous glass (hard) one composed almost exclusively by hard segments. The uniaxial extensional viscosity was measured on a Sentmanat fixture (SER) at single temperature of 175 degrees C, showing that at high strain rates both materials initially strain-soften before eventually strain-hardening at higher Hencky strains. This effect is smoother and occurs at lower strain rates for the soft TPU. Optical microscopy, atomic force microscopy and X-ray scattering show this behavior to be related with structural changes induced by the flow. Strain-softening is attributed to the orientation of hard domains in the flow direction. In the soft TPU strain-hardening is caused by the stretching of entangled soft segments but in the hard one it appears to be associated with microcracks and with the development of a hitherto unreported hierarchical structure in the molten material. (C) 2014 Elsevier B.V. All rights reserved.