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Abstract The crystal structure of the orthorhombic polyoxymethylene (POM) has been refined by analyzing the synchrotron X-ray diffraction data measured for a micrometer-size single crystal of moth-type morphology. The 3-dimensional elastic and compliance constants tensors were calculated for both the trigonal and orthorhombic phases on the basis of the thus-obtained accurate structure information. The crystal structure information allowed us also to discuss the solid-state phase transition behavior from the orthorhombic to the trigonal phase occurring at around 78 °C. For this purpose, we have performed the temperature-dependent synchrotron X-ray scattering measurement for a tiny moth-type single crystal and revealed the geometrical relation between these two crystals. The free energy was calculated as a function of temperature for these two accurately-analyzed crystal forms using a density functional theory (DFT). The result showed that the orthorhombic phase is thermodynamically more stable than the trigonal phase in a low temperature region, and the stability was reversed at about 225 K. This temperature corresponds to the phase transition point from the orthorhombic to trigonal phase, which is however considerably lower compared with the actually-observed transition point. The large discrepancy might be ascribed to the additional contribution of the entropy term due to the structural disorder and/or the thermal motion of chains with a large amplitude, which was not taken into account in the DFT calculation. Graphical abstract Image 1 Highlights • Single crystal structure of orthorhombic polyoxymethylene was refined with synchrotron X-ray diffraction data. • Ultimate mechanical properties of orthorhombic and trigonal polyoxymethylene were calculated. • Phase transition from orthorhombic to trigonal form of polyoxymethylene was clarified experimentally and theoretically. [ABSTRACT FROM AUTHOR]

The crystal structure and phase transition behavior of poly(nonamethylene terephthalamide) (PA9T, -[CO-C 6 H 4 -CONH-(CH 2 ) 9 -NH] n -) and its related model compounds (AT m ,C 6 H 5 -CONH-(CH 2 ) m -NHCO-C 6 H 5 ( 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 °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 °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 X-ray diffraction and IR spectral data collected for the low molecular-weight model compounds. [ABSTRACT FROM AUTHOR]

Detailedcrystal structure analysis has been carried out for fourcrystalline forms (I, I′, II, and high-temperature phase, HT)of uniaxially oriented specimens from a novel polyethylene-like polymer,−[(CH2)20–CHBr]n– on the basis of the 2-dimensional X-ray diffractionpatterns and polarized FTIR spectral data. This polymer has Br atomsplaced regularly on each and every 21st backbone carbon. The preciseBr placement along the polyethylene backbone allows drastically differentchain conformation and chain packing modes between the group of formsI and I′ and the group of form II and HT phase. In forms Iand I′, the molecule is fully extended adopting a planar all-transzigzag conformation with layers of Br atoms normalto the chain axis. Conformational disorder and mismatch in relativeheight of Br atom between the neighboring chains distinguish formI from form I′. In forms II and HT phase, the molecular chainsbend at the Br substitutional point and take a large zigzag form consistingof long methylene segmental arms. The molecular bends are caused bythe generation of nonplanar gaucheconformers atthe C–C bonds adjacent to the CHBr groups, while the CH2segments maintain the all-transconformation.The major difference between form II and HT is conformational disorderwithin the methylene runs. Heating at T< 65 °Cunder unrestrained condition causes an irreversible transition fromform I′ to form I, and form I transforms irreversibly to formII in a narrow temperature range of 65–66 °C. The highertemperature heating induces the reversible and apparently continuoustransition of form II to the HT phase. On the other hand, the tensilestretching at room temperature causes the irreversible transitionof forms I and II to the form I′. [ABSTRACT FROM AUTHOR]