This study aims to explore the origin of toughness in beta-phase isotactic polypropylene (i-PP). Samples of commercial-grade i-PP, both neat and nucleated with a specific beta-nucleating agent (N,N'-dicyclohexylnaphthalene-2,6-dicarboxamide), were subjected to a combination of structure-sensitive methods and fracture mechanical testing. In addition to common structural characterisation methods, solid-state NMR data were collected to obtain information about the dynamics of the polymer segments as well as the structure. In particular, temperate-induced transformation between the free and constrained amorphous phase was monitored by variable-temperature C-13 MAS NMR, whereas T-1p(H-1) relaxation was used to estimate correlations times of segmental motions. Lower thermodynamic stability of rigid amorphous fraction was found in the beta-crystalline systems. In contrast helical chains in crystalline and constrained amorphous phase were found to be more restrained in the alpha-crystalline system. Overall, the results indicate larger restrictions in chain mobility in the amorphous phase of the alpha-polymorphic PP system than in that of the beta-crystalline ones. (C) 2015 Elsevier Ltd. All rights reserved.