Search

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]

Abstract: Relationship between the chain conformation in the crystal lattice and the ultimate Young's modulus has been discussed on the basis of the crystal structural information revealed by the X-ray diffraction analysis for a series of arylate polyesters with long methylene segments (–[–COC6H4CO–O(CH2) m O–] n –). The X-ray structural analysis revealed that the molecular chains take the all-trans-zigzag conformations for all of the even-numbered polyesters and their model compounds as well as the odd-numbered polyesters with the methylene segmental length longer than (CH2)14. These chain conformations have been correlated well to the ultimate Young's modulus along the chain axis or the crystallite modulus E c , which has been estimated experimentally by the X-ray diffraction method under a constant stress and also predicted theoretically using the X-ray-analyzed crystal structures on the basis of the molecular mechanics method. The E c was found to show the minimum at around m = 4–6 and increased gradually with an increment of m and approached the crystallite modulus of polyethylene, 235 GPa (X-ray value) ∼ 316 GPa (calculate) at an infinite m value. This behavior of E c as a function of the number of methylene segmental units m was reasonably interpreted by developing the theoretical equation of E c for a simplified zigzag chain model composed of a repetition of two linear rods representing the benzene–ester and methylene segmental parts respectively. These findings may promise that the mechanical property of arylate polyester can be controlled by adjusting the methylene segmental length m. [Copyright &y& Elsevier]

Crystal structures have been determined by the X-ray diffraction analyses for a series of aromatic polyesters –[–O(CH2) m O–COC6H4CO–] n – (mGT) with the long methylene sequence (m = 9–20) and their model compounds. The X-ray diffraction spots of these polyesters were observed more or less to shift upward and downward slightly from the averaged horizontal layer lines, indicating the tilting phenomenon of the molecular chains from the draw direction, the analysis of which was helpful for the definite determination of the unit cell parameters. For example, the c axis of 10 GT was found to be tilted by about 1° from the drawn axis in the 10 tilt plane. The unit cell parameters obtained by the quantitative analysis of the tilting phenomenon were a = 4.77 ± 0.01 Å, b = 5.66 ± 0.01 Å, c (f.a.) = 20.7 ± 0.1 Å, α = 108.2 ± 0.1°, β = 122.7 ± 0.2°, and γ = 97.1 ± 0.1°. In the case of 20 GT, the tilting phenomenon was not observed in the X-ray fiber diagram. The triclinic unit cell parameters were determined for 20 GT: a = 4.59 ± 0.02 Å, b = 5.75 ± 0.02 Å, c (f.a.) = 33.3 ± 0.1 Å, α = 113.2 ± 0.2°, β = 116.6 ± 0.2°, and γ = 98.1 ± 0.2°. The space group was P for both of 10 GT and 20 GT. One chain is included in the unit cell. The chain conformation is of the all-trans-zigzag type. The reliability factor or the measure of the agreement between the observed and calculated X-ray diffraction intensity was 18.8% for 10 GT (90 reflections in total) and 18.5% for 20 GT (67 reflections). The crystal structures of 12 GT and 16 GT were found to be essentially the same as those of 10 GT and 20 GT. As for the odd-numbered mGT or 9 GT, 11 GT and 15 GT, the most plausible structure models were built up, which gave the good correspondence to the observed diffraction patterns semi-quantitatively. As the total number of methylene monomeric units was increased, the chain conformation was found to change from the trans-gauche combined form (3 GT and 5 GT) to the all-trans form (15 GT). The single crystal structure analysis of the low-molecular-weight model compounds confirmed the reasonableness of the crystal structures of the corresponding polymers. [ABSTRACT FROM AUTHOR]

At present, polylactide (PLA) film with high toughness is on expectation, but in general, it is brittle with 45 MPa tensile strength and only 3% elongation at break. The present work clarifies that isotropically small crystalline lamellae is the key microstructure to obtain super-tough PLA (180 MPa tensile strength and 80% elongation at break). The systematic study on two parameters of biaxial-stretching technique, i.e. , stretching rate and draw ratio, allows us understanding the development of microstructure, especially the crystalline phase, the regular chain packing in the crystal lattice, and the evolution of the higher-order structure. The integrated structural analyses based on Fourier transform infrared spectroscopy (FTIR), 2D-wide angle X-ray diffraction (2D-WAXD) and 2D-small angle X-ray scattering (2D-SAXS) declare that the structurally-irregular amorphous PLA starts to develop the mesophase and δ -crystal (including α -crystal) with the crystallite size of several tens of nanometers by slow biaxial-stretching (3 mm/s). At this stage, the biaxially oriented PLA (BOPLA) still shows relatively poor mechanical properties. When the stretching rate and the draw ratio increased above a certain level, i.e. , 75 mm/s and 5 × 5, respectively, the small δ -crystallites of about 10 nm size with isotropic orientation are mainly formed. This structural evolution has been found to result in the drastic increase of the toughness of BOPLA film, which is about 4 times higher than the PLA film produced by conventional stretching. In this way, the present work shows for the first time that the simple way to toughen the PLA film is to induce a well-dispersed higher-order structure consisting of many but small δ -crystallites as seen in the model case of BOPLA. [ABSTRACT FROM AUTHOR]