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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]

The crystal structure of poly(l-lactic acid) β form has been reinvestigated on the basis of the 2-dimensional X-ray diffraction diagram measured for the sample obtained by stretching the highly crystalline α form at a high temperature. The six helical chains of (3/1) conformation are packed in a rectangular unit cell of the space group P1 with the complicated but systematic packing mode of the upward and downward chains. This structural model is different from the previously reported trigonal model [Puiggali; Polymer 2000, 41, 8921]. The structural phase transition mechanism from the α form to the β form via the δ form has been proposed by assuming the cooperative displacements of the upward and downward helical chains as well as the conformational change. To support this mechanism, the two types of experiments were performed: (i) The highly oriented regular α form was stretched at about 165 °C to the various drawing ratios and cooled to the room temperature with the sample length fixed constantly. The X-ray diffraction data of these samples revealed the transition from the α to the mechanically deformd α (αd), to the δ form, and then to the β form depending on the drawing ratio. (ii) The α sample was suspended vertically with a constant load in the heating process, and the X-ray diffraction pattern was measured as a function of time (temperature). The original α form was found to melt at about 200 °C via the transition to the αd form and then to the δ form, followed by the recrystallization into the highly oriented β form. From these two experiments, the tension-induced structural transition from the α to the β form was found to occur via the disordering of the α form to the αd and to δ form under tension. The structural change process was derived on the basis of all of the knowledge collected from the X-ray structure analysis and the ex-situ and in-situ X-ray diffraction measurements: the molecular chains experience the cooperative translational slippage along the diagonal directions in the ab-plane to give the complicatedly mixed packing of the upward and downward chains to give the characteristic structure of the β form. At the same time, the chain conformational changes from the regular (10/3) to the disordered (10/3) and then to the (3/1) form during the disordering in the chain packing mode. [ABSTRACT FROM AUTHOR]

A novel oxonitridosilicate phosphor host Sr3Si2O4N2 was synthesized in N2/H2 (6%) atmosphere by solid state reaction at high temperature using SrCO3, SiO2 and Si3N4 as starting materials. The crystal structure was determined by a ltaetveld analysis on powder X-ray and neutron diffraction data. Sr3Si2O4N2 crystallizes in cubic symmetry with space group Pa3, Z = 24, and cell parameter a ≈ lS.6593(1) A. The structure of Sr3Si2O4N2 is constructed by isolated and highly corrugated 12 rings which are composed of 12 vertex-sharing [SiO2N2] tetrahedra with bridging N and terminal O to form threedimensional tunnels to accommodate the Sr2+ ions. The calculated band structure shows that Sr3Si2O4N2 is an indirect semiconductor with a band gap 2.84 eV, which is dose to the experimental value ≈ 2.71 eV from linear extrapolation of the diffuse reflection spectrum. Sr3Si2O4N2,xLi+ shows a typical emission band peaking at ∼600 nm under 460 nm excitation, which perfectly matches the emission of blue InGaN light-emitting diodes. For Ce3+/Li+-codoped Sr3Si2O4N2, one excitation band is in the UV range (280-350 nm) and the other in the liv blue range (380-420 nm), which matches emission of near-UV light-emitting diodes. Emission of Sr3-xSi2O4N2:xCe3+xLi+ shows a asymmetric broad band peaking at -S20 inn. The long-wavelength excitation and emission of Eu2+ and Ce3+/Lt-doped Sr3Si2O4N2 make them attractive ftr applications in phosphor-converted white light-emitting diodes. [ABSTRACT FROM AUTHOR]

The crystal structure model of stereocomplex between poly(l-lactic acid) (PLLA) and poly(d-lactic acid) (PDLA) with the L/D ratio of 50/50 was proposed by several groups [the space group P1 model by Okihara et al. (1991) and R3c or R3c model by Cartier et al. (1997)], among which the model of the space group R3c (or R3c) had been currently recognized as the most preferable one. However, the thus apparently established model cannot explain such an important experimental fact that the stereocomplex is formed not only for the PLLA/PDLA blend of the L/D ratio 50/50 but also for the blends with the L/D ratio of 70/30-30/70. We have proposed here a new model with the space group P3, which can cover the SC structures of the L/D ratio in the range of 70/30-50/50-30/70. This model can show the more quantitative reproducibility of the observed X-ray diffraction data for both the hkl and 000l reflection profiles. In particular the experimental observation of the 0003 reflection is inconsistent with the previous model of the space group R3c, which requests the appearance of only the 000l with l = 6, 12, etc. In the newly proposed P3 model the molecular chains take the regular (3/1) helical conformation; however, the packing mode is not regular. The unit cell consists of the statistically disordered arrangement of the right-handed (R) and left-handed (L) chains of the upward (u) or downward (d) directionality. For example, in the case of the stereocomplex with L/D ratio 70/30, a pair of the left-handed downward (Ld) and right-handed upward (Ru) chains is located at one lattice site at the statistical occupancy of 70/30 ratio and another pair of the left-handed upward (Lu) and right-handed downward (Rd) chains is at the adjacent lattice site at the same 70/30 ratio. The unit cell contains the three sets of these two pairs, which are connected to each other by the symmetric relation of the 3-fold rotation axis. The proposed statistical packing model of the upward and downward chains can explain such various observations as the spherulite formation or the existence of the lamellae with the chain folding structure in the spherulite, the solvent-induced change from the stereocomplex to the a form, and so on. [ABSTRACT FROM AUTHOR]

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 R3̅c 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 P4̅b2. The right-handed (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. [ABSTRACT FROM AUTHOR]

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 P3, different from the previously reported R3c 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 P4b2. The right-handed (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 p. 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. [ABSTRACT FROM AUTHOR]