We investigated the morphology of a series of thermoplastic polyurethanes (TPUs) based on polyisobutylene (FIB) diol. The TPUs were synthesized using PIB diol with the molecular weight of 2000 g/mol as the soft segment, with different chemical structures and weight fraction of the hard segment. Thermal properties and microphase separated structures of these TPUs were characterized by thermal analysis and X-ray scattering techniques and were compared with those of polyether-based TPUs. Our results showed significant phase mixing between the soft segment and hard segment in PIS-based TPUs. The degree of microphase separation of PIB-based TPUs was found to be comparable to or even lower than that of TPUs based on poly(tetramethylene oxide) (PTMO) with similar composition, depending on the chemical structure of the hard segment. Comparison of the morphology before and after post-annealing suggested that while most of the microphase separated structure of PIB-based TPUs has formed during thermal processing, post-annealing further promotes the phase mixing between HS and SS. (C) 2013 Elsevier Ltd. All rights reserved.