The structure and dynamics of segmented polyurethane networks prepared from poly(butadiene) diols (PBD) of different molecular weights M-n = 1200, 2160,2650,4690 and 10,300 g mol(-1), 4,4'-diphenylmethane diisocyanate (MDI) and poly(oxypropylene) triol (POPT) (M, = 710 g mol-1) have been investigated by dynamic mechanical analysis, dielectric relaxation spectroscopy, differential scanning calorimetry (DSC), small- and wide-angle X-ray scattering (SAXS and WAXS). Microphase separation on Danometre scale has been confirmed by SAXS in all networks. Except for the highest molecular weight of PBD, the networks consist of microdomains of similar size and composition dispersed in the soft phase of polybutadiene segments. Dielectric spectroscopy revealed the existence of three types of noncrystalline phases, each characterised by a well-developed dynamic glass transition. Absence of crystallinity has been proven by WAXS and DSC. (C) 2002 Elsevier Science Ltd. All rights reserved.