The glass transition processes of the soft-segments (SS) in a series of poly(urethane urea) (PUU) elastomers were studied by solid-state NMR. Two SS fractions, rigid (SSr) and mobile (SSm), can be discerned by time-domain wideline separation, C-13-H-1 dipolar dephasing, and frequency-switched Lee-Goldberg spin-locked T-1p relaxation experiments. At increasing temperature, part of the SSr population turns into SSm, while the decay constants of both fractions only see moderate changes. The extent of the population exchange is greater for samples with more phase mixing. This population exchange can be interpreted as the glass transition in the SS-rich domains. Comparison between T-1p relaxations on the xy plane and along the magic angle indicates that the domain sizes of SSr and SSm are likely less than 2-3 nm. SSr is not completely rigid, but possesses both a fast, anisotropic chain rotation associated with the beta relaxation and a slower motion with more isotropic nature at ca. 10(5) s(-1). It is predicted that at high strain rates, most of SSr and part of SSm would become rigid, resulting in dynamically induced strengthening and toughening. (C) 2016 Elsevier Ltd. All rights reserved.