The crystal structure and phase transition behavior of poly(nonamethylene terephthalamide) (PA9T, -[CO-C6H4-CONH-(CH2)(9)-NH](n)-) and its related model compounds (ATm,C6H5-CONH-(CH2)(m)-NHCO-C6H5 (m = 7-12)) have been investigated by carrying out the temperature dependent measurements of wide-angle X-ray diffraction (WAXD), small-angle X-ray scattering (SAXS) and infrared spectra along with the DSC measurement. The crystal structure of PA9T at room temperature was determined by analyzing the X-ray diffraction diagram measured for the uniaxially-oriented sample. The nonamethylene segmental part takes the zigzag conformation, which was supported by the quantitative analysis of the methylene progression bands observed in the infrared spectra. The temperature dependence of WAXD, SAXS and infrared spectra of PA9T revealed the existence of the 2-staged order-to- disorder phase transitions: at room temperature this polymer exists as phase I, the crystal structure of which was analyzed as mentioned above. In the relatively low and wide temperature region (190-270 degrees C for the unoriented sample), phase I transforms to phase II, in which the methylene conformation is disordered, the aromatic amide part is slightly rotated and the intermolecular hydrogen bonds become weaker but the hydrogen bonds are still kept alive. In the temperature region immediately below the melting point (270-310 degrees C for the unoriented sample), the phase II is melted once and recrystallizes into phase III, where the intermolecular hydrogen bonds are mostly broken in addition to the further disordering of the methylene segments and aromatic amide groups. The thus-occurring violent thermal motions cause a remarkable increase of the long period in this transition region. The molecular chains are still highly oriented and so the phase III may be assumed to be in a kind of liquid crystalline state. The crystal structural changes detected in these phase transitions of PA9T were confirmed by the analysis of the Xray diffraction and IR spectral data collected for the low molecular-weight model compounds. (C) 2017 Elsevier Ltd. All rights reserved.