Phase pseudohexagonal

A conformationally disordered mesomorphic form is present, for instance, in the high-temperature phase I of PTFE. In this form, a long-range 3-D order is present only in the periodic pseudohexagonal placement of the chain axes [49]. In fact intramolecular helix reversals would produce the conformational disorder [50-52] and a complete intermolecular rotational disorder would be also present [49, 52,53]. 

With concern to the high internal mobility of the molecules in the high temperature solid state phase, some parallelism to n-alkanes can be stated. In the pseudohexagonal (rotator) phase the latter are also characterized by fast molecular motions. For discrimination and according to Pfitzer 14) and Dale 13) in the following the term pseudorotator phase is used for the mobil crystalline state of cyclic molecules. 

Pseudo-affine deformation scheme 96, 97 Pseudohexagonal (rotator) phase 67 Pseudorotator phase 67 Pulsed NMR techniques 30... 

Droit s original 4 1 5 phase has been studied by X-ray diffraction (Nowacki Silverman, 1961, 1962) and found to have a rhombohedral layer structure. The 1 1 1 phase was also found to have a layer structure, which consisted of pseudohexagonal layers of zinc atoms separated by ordered layers comprising oxygen and chlorine atoms (Feitknecht, Ostwald Forsberg, 1959). This fundamental structure was apparently found for both of the crystalline modifications in which this phase has been found to occur, namely the monoclinic and the orthorhombic (Sorrell, 1977). 

Examples of mesomorphic forms characterized by disorder in the conformation of the chains have already been described in Section 2.6. For instance, a mesomorphic form is present in the high-temperature form I of polytetrafluoro-ethylene.106,107 In this phase the chains are in disordered conformation due to the presence of helix reversals along the chains.108-110 Moreover, intermolec-ular disorder is also present due to the random rotations of the chains around the chain axes.109 A long-range three-dimensional order is present only in the pseudohexagonal placement of the chain axes.107,109... 

Dowell and Rinfret [1.9] demonstrated that the phase at temperatures above -160 °C consist of small crystals with = 400 A° in size and having cubic and pseudohexagonal structures. 

Turning to the low temperature transition of the homopolymer of PHBA at 350 °C, it is generally accepted that the phase below this temperature is orthorhombic and converts to an approximate pseudohexagonal phase with a packing closely related to the orthorhombic phase (see Fig. 6) [27-29]. The fact that a number of the diffraction maxima retain the sharp definition at room temperature pattern combined with the streaking of the 006 line suggests both vertical and horizontal displacements of the chains [29]. As mentioned earlier, Yoon et al. has opted to describe the new phase as a smectic E whereas we prefer to interpret this new phase as a one dimensional plastic crystal where rotational freedom is permitted around the chain axis. This particular question is really a matter of semantics since both interpretations are correct. Perhaps the more important issue is which of these terminologies provides a more descriptive picture as to the nature of the molecular motions of the polymer above the 350 °C transition. As will be seen shortly in the case of the aromatic copolyesters, similar motions can be identified well below the crystal-nematic transition. 

Variations of k must be compared with variations in a Perfection Crystalline Index (IPC) obtained according to Dumbleton s method, from the 010 and 100 peaks (7). IPC increases rapidly after heat treatments. An increase of this index is proof of a progressive transformation from a pseudohexagonal phase in the initial sample (disturbed as seen from the k value) decreasing to give a less disturbed triclinic phase after thermal treatment ... 

Metal-metal eutectics have been studied for many years due to their excellent mechanical properties. Recently, oxide-oxide eutectics were identified as materials with potential use in photonic crystals. For example, rodlike micrometer-scaled microstructures of terbium-scandium-aluminum garnet terbium-scandium per-ovskite eutectics have been solidified by the micro-pulling-down method (Pawlak et al., 2006). If the phases are etched away, a pseudohexagonally packed dielectric periodic array of pillars or periodic array of pseudohexagonally packed holes in the dielectric material is left. 

Fig. 24 Continuous-flow laser-polarized Xe NMR spectra of Xe/He mixtures flowing in the TPP pseudohexagonal phase at atmospheric pressure and room temperature. The time for diffusion of the optically polarized xenon into the TPP channels was set by the recycle delay to 200 ms. The components of the CSA tensors 622 and 633) were calculated by a numerical fitting procedure. (Adopted from [67] with permission)...

With iron it appears certain that the pure reduced metal is not the phase of highest activity when the metal is exposed to synthesis gas at reaction temperatures, the activity slowly increases, indicating that the active phase is still being formed. Indeed, Pichler (5) reported that a pseudohexagonal carbide FcjC is the active phase. Besides carbide formation the oxidation of the catalyst surface is of importance. As Eqs. (1)—(3) show, hydrogen is consumed and H2O formed. With respect to the oxidation-reduction equilibrium of the catalyst metal M,... 

The experimental data associated with this surface are actually more revealing than we have suggested (Gibbs et al. 1990, Abernathy et al. 1992). On the basis of systematic X-ray reflectivity studies, it has been deduced that a sequence of phases are found as a function of temperature these phases are shown schematically in fig. 9.12. In particular, two related pseudohexagonal phases have been foimd which differ by a small relative rotation in addition to a high-temperature disordered phase in which the pseudohexagonal symmetry is lost. However, despite the apparent abruptness and reversibility of the transition between the two distorted hexagonal phases, it remains unclear whether or not these structures constitute true equilibrium phases. 

An unusual feature of the heneicosanol measurements was a second scattering peak, which appeared to arise at high pressure at temperatures below 16°C. It was attributed to a weakly first-order phase transition analogous to the rotator Il-to-rotator I transition in lamellar crystalline n-alkanes with n = 23,25. In the rotator II phase, rapid reorientation of the chain around its axis leads to a pseudohexagonal structure. When the chains can no longer reorient, the symmetry of the structure is reduced to a uniaxially distorted hexagonal strueture. 

In a subsequent paper, Lin et al. ° report that the pseudohexagonal structure is not an equilibrium phase. They put forward the hypothesis that it arises because of the uniaxial compression of the film at the start of the experiments. Measurements made at temperatures between 10 and 20°C show that three peaks can be observed immediately after compression, one at... 

From cyclooctatetracontane orthorhombic crystals with less well defined folds grow from the melt and have a pseudohexagonal condis-phase transition before transition into the isotropic melt. This condis phase is closely related to the paraffin and polyethylene condis phase and has similar dynamics. 

Pseudohexagonal Rotator Phase of n-Alkanes Molecular-Dynamics Calculation for Tri-... 

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