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.

The new crystal structure models of forms I and II of isotactic polybutene-1 have been proposed by analyzing the 2-dimensional X-ray diffraction data measured for the highly oriented samples of almost pure crystal forms. The crystal form I was found to take the hexagonal packing structure of the (3/1) helices with the space group P3̅ , different from the previously reported R3̅ c or R3c models. The right-and left-handed chains are packed alternately with the random directionality along the chain axis. The crystal form II was concluded to take the tetragonal unit cell of the (11/3) helical chains, the space group of which is P4̅ b2. The righthanded (left-handed) chains are positioned at one site with the statistical disorder of upward and downward directionality along the chain axis. The time-dependent electron diffraction measurement showed that the crystal lattices of forms I and II are related to each other with the common 110 plane boundary, as already reported by the other researchers. By referring to the crystal structures confirmed in this study, a new phase transition mechanism has been proposed for understanding this geometrical relation between these two crystal phases. The mechanism is based on a kind of soft mode concept; the mutually opposite translational movements of the right-and left-handed chains occur along the 110 plane of the tetragonal lattice of form II. The phase angle between the neighboring unit cells along the [110] direction is π. This translational lattice vibrational mode increases the amplitude and causes the softening of the original form II unit cell into a transient structure composed of the hexagonally packed pairs of right-and left-handed chains. Then, this transient structure is stabilized to the crystal form I, during which the chain conformation changes cooperatively from (11/3) to (3/1) form by a slight change in the trans and gauche torsional angles of the skeletal chains. This newly proposed transition mechanism can explain also the formation of twin structure of form I crystals.

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Detailed crystal structure analysis has been carried out for four crystalline forms (I, I′, II, and high-temperature phase, HT) of uniaxially oriented specimens from a novel polyethylene-like polymer, −[(CH 2 ) 20 −CHBr] n − on the basis of the 2-dimensional Xray diffraction patterns and polarized FTIR spectral data. This polymer has Br atoms placed regularly on each and every 21st backbone carbon. The precise Br placement along the polyethylene backbone allows drastically different chain conformation and chain packing modes between the group of forms I and I′ and the group of form II and HT phase. In forms I and I′, the molecule is fully extended adopting a planar all-trans zigzag conformation with layers of Br atoms normal to the chain axis. Conformational disorder and mismatch in relative height of Br atom between the neighboring chains distinguish form I from form I′. In forms II and HT phase, the molecular chains bend at the Br substitutional point and take a large zigzag form consisting of long methylene segmental arms. The molecular bends are caused by the generation of nonplanar gauche conformers at the C−C bonds adjacent to the CHBr groups, while the CH 2 segments maintain the all-trans conformation. The major difference between form II and HT is conformational disorder within the methylene runs. Heating at T < 65 °C under unrestrained condition causes an irreversible transition from form I′ to form I, and form I transforms irreversibly to form II in a narrow temperature range of 65−66 °C. The higher temperature heating induces the reversible and apparently continuous transition of form II to the HT phase. On the other hand, the tensile stretching at room temperature causes the irreversible transition of forms I and II to the form I′.