It has been found that, dependent on the crystallization temperature (T c ), the disorder (R′) and order (R) phases of poly(L-lactide) (PLLA) are formed at low (T c < 100°C) and high (T c g 120°C) temperatures, respectively. In the DSC curves, the sample with R′ phase demonstrates a peculiar small exothermal peak around 160°C just prior to the melting point, while the sample crystallized at temperatures around 120°C (between 110 and 130°C) shows a double melting behavior. These distinct thermal behaviors of various PLLA samples were investigated in detail by simultaneous measurements of WAXD and DSC. It is confirmed that the small exothermal peak corresponds to the disorder-to-order (R′-to-R) phase transition, in which the chain packing of the crystal lattice becomes more compacted. In the process of the R′-to-R phase transition, the isosbestic points were found in the temperature-dependent WAXD profiles. So far, the R′-to-R transition was considered to occur apparently continuously as long as the main 200/110 peak was measured, but detailed investigation of higher 2θ peaks has revealed for the first time that the R′ phase transforms discretely to the R phase in the first-order transition mode. On the basis of the X-ray diffraction and DSC data, a kind of phase diagram concerning the R′ and R forms has been constructed reasonably.

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In order to clarify the structural changes that occur in the thermochromic phase transition of poly (3‐dodecylthiophene) [P3DT] and poly (3‐hexylthiophene) [P3HT], the temperature dependence of x‐ray diffraction and Fourier transform infrared spectra was measured. (1) Orthogonal unit‐cell parameters were determined at room temperature: a=25.83 Å, b=7.75 Å, c (fiber axis)=7.77 Å for P3DT and a=16.63 Å, b=7.75 Å, and c=7.77 Å for P3HT. A large variation of the a‐axis length between P3DT and P3HT indicates the extended trans conformation for the alkyl side chains which are oriented along the lateral a‐axis direction. (2) The interplanar spacing, intensity, and integral width of the x‐ray (h00) and (00l) reflections were found to change drastically in the transition region. (3) Polarized infrared measurements at high temperature revealed a marked increase of the gauche band intensity for the alkyl side group modes followed by a decrease in the band intensity of the thiophene ring modes. (4) The layer reflections of the x‐ray fiber diagram become diffuse at high temperatures, indicating that the transition occurs in a liquidcrystalline manner with the orientation of the main chain axes preserved but with almost no axial correlation between the neighboring main chains. These results provide experimental support for the structural model proposed earlier: as the temperature increases, the trans‐type side chains begin to disorder by introduction of gauche bonds. This disordering disrupts the regularity of the main chain conformation and decreases the effective length of the polythiophene conjugated system.

The crystal structure of poly(l-lactic acid) (PLLA) α form has been analyzed in detail by utilizing the 2-dimensional wide-angle X-ray (WAXD) and neutron diffraction (WAND) data measured for the ultradrawn sample. The WAXD data were collected using a synchrotron-sourced high-energy X-ray beam of wavelength 0.328 Å at SPring-8, Japan and the WAND data were measured using a neutron beam of wavelength 1.510 Å with a cylindrical imaging-plate camera of BIX-3 system at Japan Atomic Energy Agency. The initial crystal structure model was extracted successfully by a direct method under the assumption of the space group P212121 using about 700 X-ray reflections observed at −150 °C, the number of which was overwhelmingly large compared with the data reported by the previous other researchers and allowed us to perform more precise structural analysis. The crystal structure model obtained by the direct method was refined so that the best agreement between the observed and calculated integrated intensities was obtained or the reliability factor (R) became minimal: R was 18.2% at −150 °C and 23.2% at 25 °C. The thus-obtained chain conformation took the distorted (10/3) helical form with 21 helical symmetry along the chain axis. However, the symmetrically forbidden reflections 003, 007, 009 etc. were detected in a series of the 00L reflections, requiring us to erase the 21 screw symmetry along the molecular chain. By assuming the space group symmetry P1211, the structural refinement was made furthermore and the finally obtained R factor was 19.3% at −150 °C and 19.4% at 25 °C. Although the structural deviation from the 21 screw symmetry was only slightly, this refined model was found to reproduce the observed reflection profiles well for all the layer lines. The thus X-ray-analyzed crystal structure was transferred to the WAND data analysis to determine the hydrogen atomic positions. The R factor was 23.0% for the 92 observed reflections at 25 °C. The agreement between the observed and calculated layer line profiles was good. In this way the crystal structure of PLLA α form has been established on the basis of both the X-ray and neutron diffraction analyses.

Structural changes in the orthorhombic-to-hexagonal phase transition of polyethylene crystal has been investigated by measuring the differential scanning calorimetry, X-ray diffraction, and infrared and Raman spectra for the geometrically-constrained ultradrawn samples in the course of heating up to the melting temperature. The infrared and Raman spectral patterns characteristic of the hexagonal phase have been confirmed. In particular, the bands characteristic of the disordered short trans segments (shorter than five CH2 units) and the bands of the kink or double gauche linkages have been detected definitely. The degree of orientation of the averaged chain axis, as detected from the temperature dependence of the X-ray fiber diagram, was reserved well enough even in the hexagonal phase, while the orientational degree of the methylene units, as detected from the polarized infrared spectral measurement, was found to be lowered appreciably. This finding, combined with the observation of the trans and gauche bands, has confirmed experimentally and definitely the existence of the conformationally disordered chains in the hexagonal phase.

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By using the highly oriented 1:1 stoichiometric blend of high-molecular-weight PLLA and middle-molecular-weight PDLA species and by choosing a suitable annealing condition, we have succeeded for the first time to observe the phase transition phenomenon in the heating and cooling processes by measuring the temperature dependence of the X-ray fiber diagram and polarized IR spectra. The as-drawn sample was found to consist of the two crystalline phases; disordered α form (α‘) and stereocomplex (βc). In the heating process, the α‘ form was found to be reorganized into the oriented α phase above 120 °C, which melted around 180 °C. However, by heating this sample some more, some parts of the molten chains are found to crystallize on the surface of the βc crystallites with high degree of chain orientation. In the subsequent cooling process from the βc phase, the unoriented α crystal appeared at around 120 °C and coexisted with the highly oriented βc crystals.