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]. 

One modification of polyethylene (PE), which appears to be stable at high temperatures and pressures (see Sect. 3.2), also presents a pseudohexagonal packing and a disorderd chain conformation [59],... 

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... 

If die well-crystallized PAs are heated, the two characteristic diffraction signals move together and meet at what is called the Brill temperature (Fig. 3.5).6,20 Above this Brill temperature, there is only one spacing for both directions this implies a pseudohexagonal packing, such as that observed in the poorly crystallized PA-6 at room temperature. 

Figure 3.5 Changing spacing of unit cell with temperature as measure with WAXS.20 Transition from triclinic (a-stmeture, with two spacings) to pseudohexagonal (/-structure, with one spacing) can be seen.

The dimensions of the unit in the basal plane closely approximate those for the similarly pseudohexagonal crystal hydrargillite, Al(OH)3, as well as of the hexagonal layers in two forms of silica, /3-tridymite and 0-cristo-balite. The monoclinic (pseudohexagonal) unit of structure of hydrargillite8 has a = 8.70 A, b = 5.09 A, c = 9.76 A, and 0 = 85°29, and contains 8A1(0H)3. The crystal is composed of layers of octahedra,... 

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). 

Dashed pseudohexagonal cells corresponding to the NiAs cell. 

The unit cell is pseudohexagonal, with a = 10.87 A (1.087 nm), b = 18.83 A (1.883 nm), and c = 52.53 A (5.253 nm). The chain contains 14 monomers in three turns of the left-handed helix. Antiparallel packing of the chains yields the best fit with the X-ray data and the least number of close contacts. The overall R factor is 58%, and it is 30% for the zero, third, and sixth layer reflections alone. 

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. 

In each 2 1 mixed layer, the upper tetrahedral sheet is translated by a/3 with respect to the lower one, thus creating the octahedral oxygen coordination around the cations of the intermediate sheet. Translation may occur along any positive or negative direction defined by structural axes Xj, X2, and X3 of a pseudohexagonal lattice, as shown in figure 5.43. 

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. 

Dashed pseudohexagonal cells corresponding to the NiAs cell. Numbers z coordinates (in the viewing direction)... 

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