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The retardation of polybutene-1 form II to form I transition in the late stage has been investigated by means of time-resolved wide-angle X-ray diffraction. Form II samples with different lamellar thickness and constituent in amorphous phase were generated via varying crystallization temperature and molecular weight and aged at room temperature for the form II to form I transition. The II to I polymorphic transition in polybutene-1 undergoes two stages, where slow nucleation and rapid growth proceed in the first stage and extremely slow secondary nucleation and growth take place in the late stage. The degree of transition reaches a plateau value in the late stage of transition, which depends highly on crystallization temperature and molecular weight that low molecular weight and high crystallization temperature always contribute to a greater degree of transition, but is less affected by the kinetics in the first stage. According to the crystal unit cell parameters of form II and form I, there should be an extension in the normal direction of lamellae and a shrinkage in the lateral direction during phase transition, which thus would induce an additional pressure in the normal direction and the tendency to expansion in the lateral direction on the residual from II crystallites. As a result, the nucleation and further growth of the II to I transition are retarded. Low molecular weight and high crystallization temperature lead to larger content of chain ends and higher chain mobility in the amorphous phase; hence, the samples are more likely to release the unfavorable factors restraining generation of form I nuclei to some extent. The transition degree in late stage is thus an intrinsic property of PB-1 samples determined by their microstructures and would not be influenced by the transition behavior in the first stage. [ABSTRACT FROM AUTHOR]

The syndiotactic polystyrene (sPS) γ-to-α/β phase transition was investigated using temperature-dependent synchrotron wide-angle X-ray diffraction (WAXD) and small-angle X-ray scattering (SAXS) measurement. The sPS γ form samples, which were obtained from acetone solvent induced crystallization followed by annealing at various temperatures (Ta), exhibited a regular manner on the selection of the α or β crystal modification during the phase transition process. As the Ta was increased from 25 to 150 °C, the weight fraction of β form in bulk sPS decreased from 55% to 0%, but that of the α form increased from 45% to 100%. In other words, the α form became the dominating crystalline modification at higher Ta. The lamellar thickness (dc) of the γ form at the initial state (before heating) ranged from 4.4 to 5.8 nm, and the weight crystallinity (Xc) was between 0.52 and 0.64. However, the dc values of the γ form at the final state (before γ-to-α/β phase transition) were very close for the samples pretreated at different Tas, which were in the range 7.0-7.4 nm. Therefore, the most plausible interpretation for the crystal modification selection during the γ-to-α/β phase transition was that the nucleation of β form was suppressed totally in the γ form with higher initial dc value (>5.8 nm). With the elevated Ta and increase of initial dc, the promoted nucleation of α form in the γ form resulted in the enhanced growth of the α crystalline phase during the γ-to-α/β phase transition. As a result, after the phase transition the crystalline phase in bulk sPS had a gradually increased weight fraction of the α form and turned out to be pure α form when Ta was increased from 25 to 150 °C. [ABSTRACT FROM AUTHOR]

In the previous report we proposed, on the basis of the detailed X-ray diffraction data analysis, the crystal structure of PLLA/PDLA stereocomplex (SC) with the space group P3, which is different from the R3c or R3c models proposed in the literature. The most important point of the change of the structure model is to explain the experimental data that the SC can be formed in the region of the L/D ratio of 70/30-30/70, not only for the L/D = 50/50 ratio. In our P3 structural model, one crystal site is occupied by the Ru and Ld (or Rd and Lu) (3/1) helices at the statistical probability determined by the L/D ratio, where R and L indicate the right-handed and left-handed helical chains and u and d show the upward and downward directionality along the chain axis, respectively. In other words, the D and L chain stems can coexist in a heterogeneous way in the SC crystalline lattice. This structural model has been supported experimentally from the totally different point of view on the basis of the vibrational circular dichroism IR spectral measurement. When the right-handed (r) and left-handed (l) circularly polarized IR beams are incident to the optically active chemical species, the ratio between the r and l band intensity changes depending on the relative content of the R and L species (IR circular dichroism). The IR circular dichroism of PLLA/PDLA blend sample was found to change its dichroic ratio depending on the L/D content in the sample. The most important finding is that the dichroic ratio change was detected not only for the amorphous IR bands but also for the crystalline bands, indicating definitely that the D and L chain stems can coexist in the crystalline region at the various L/D ratios as proposed by the X-ray structure analysis. [ABSTRACT FROM AUTHOR]

As for the crystal structure of poly(3-hydroxybutyrate) (PHB) a-form, the methyl group was said to locate at an abnormally short distance from the oxygen atom of C-O unit, resulting in the formation of CH3···O-C hydrogen bond. This idea was proposed on the basis of infrared spectra detecting a band of antisymmetric methyl C-H stretching mode [Vas(CH3)] at anomalously high frequency position and the too short CH3···O-C interatomic distances calculated from the atomic coordinates derived from the previously reported X-ray structure analysis. However, the crystal structure analyzed by the X-ray method, even the C and O atomic positions, has not yet been established confirmatively because of the appreciably small number of the observed X-ray diffraction spots. We have reinvestigated the crystal structure of PHB a-form by collecting the observed X-ray diffraction spots of about twice larger total number than before, and confirmed the positions of the C, O and H atoms in the unit cell at enough high accuracy. The finally obtained crystal structure gave the reliability factor of 14.5% for the X-ray data at 23 °C (the total number of observed reflections 70) and 16.0% (94 at -140 °C). The thus-established crystal structure was found to exhibit the shortest H···O distance of 2.62 Å, which is shorter than the value expected from the normal van der Waals distance. In order to clarify the relation of this geometry with the observed higher-frequency shift of Vas(CH3) band from the viewpoint of space group symmetry, the factor-group analysis and the normal modes calculation were performed for the PHB a-crystal by referring to the presently analyzed crystal structure information. Among the plural possibilities to cause the high-frequency shift of originally doubly degenerated Vas(CH3) mode, the normal coordinates calculation has revealed that the vibrational coupling due to the intermolecular interactions between the adjacent molecular chains may be the most significant factor rather than the effect of lowering of C3v symmetry of regular CH3 groups coming from the site group symmetry. Once the crystal structure was established well, then the 3D anisotropic elastic constants matrix was calculated for the first time about the PHB a crystal. The ultimate Young's modulus along the chain axis was only 5 GPa, which was comparable to the moduli in the direction perpendicular to the chain axis. In other words, the mechanical property of this crystal is mostly isotropic as a whole. [ABSTRACT FROM AUTHOR]

A sample of poly(ferrocenyldimethylsilane)- b-poly(2-vinylpyridine) (PFS 23- b-P2VP 230), with a short PFS block and a P2VP block 10-fold higher in degree of polymerization, forms spherical micelles when dissolved in ethanol. Over time (hours, days, and weeks), these solutions undergo a micelle sphere-to-cylinder transition eventually forming rather stiff, uniform fiber-like micelles with a core width of 10 nm, lengths between 20 and 50 µm, and approximately four polymer molecules per nm length. Here, we report the results of a combination of transmission electron microscopy, wide-angle X-ray scattering (WAXS), as well as static and dynamic laser light scattering measurements to follow the structural evolution. One key observation is the onset of partial aggregation of spherical micelles after an initial induction period (hours), so that the system as it ages consists of mixtures of free spherical micelles, micelle aggregates, and elongated structures with a high aspect ratio. Another important observation is the growth in intensity and sharpness of the WAXS peak characteristic of crystalline PFS domains as the number and uniformity of the cylindrical micelles increases. This formation of crystalline domains is the likely driving force for the structural transformation. [ABSTRACT FROM AUTHOR]

A new class of metal-containing diblock copolymers, PFS-b-P2VP (PFS poly(ferrocenyldimethylsilane), P2VP poly(2-vinylpyridine)), with a hydrophobic organometallic block and a highly polar hydrophilic organic block were synthesized via 1,1-dimethylsilacyclobutane (DMSB)-mediated sequential anionic polymerization. In the bulk state, PFS-b-P2VP copolymers with a volume fraction of PFS close to 0.20 give a morphology with cylindrical domains of PFS. A remarkable difference in the morphology of PFS-b-P2VP micelles was observed depending on the nature of selective solvent. Simply by using different alcoholic solvents either spherical or cylindrical micelles can be selectively formed from the same copolymer. WAXS experiments indicate that the crystallinity of the core-forming PFS block may be responsible for this difference. [ABSTRACT FROM AUTHOR]