Structure trigonal unit cell

This polymer crystallizes in three polymorphs. The threefold helical structure packs in a trigonal unit-cell with a = 14.3 A (1.43 nm) and c = 28.7 A (2.87 nm). The 8-fold helical structure occurs in a tetragonal unit-cell with a = 13.8 A (1.38 nm) and c = 78.2 A (7.82 nm). Axial periodicity in both cases is similar [h = 9.6 A (960 pm) and 9.8 A (980 pm), respectively], but the helix twist-angle is different (120 and 45°, respectively). Distribution of the charged side-groups in these helices was discussed. An orthorhombic form, with a twofold helical structure, has a repeat of 18.6 A (1.86 nm). 

Similar frustration has been evidenced for the crystal structure of isotactic poly(2-vinylpyridine)150 161 (Figure 2.28). Also in this case three independent threefold helices are included in the trigonal unit cell and the frustration is related to a different azimuthal orientation of the chains due to the different interactions between the chains.160 Two chains (chains A in Figure 2.28) maximize their interactions at the expense of the third one (chain B). 

Brucite, Mg(OH)2, has a structure very similar to that of Cdl2. The trigonal unit cell contains one molecule, D d, P3ml, aQ = 3.142, and cQ = 4.766 A. Figure 5.59 shows the layers. Hydrogen atoms are on the outside of the POP sandwiches. Mg2+ ions are only at C positions, but alternate O layers are vacant. 

MnF4, known since 1928, has now been obtained in single crystals,740 which have a trigonal unit cell, but the structure has not yet been reported. The ultramarine blue solid is very hygroscopic, loses F2 slowly at normal temperatures and hydrolyses instantly in contact with water. 

The expected composition. WN, was observed as a brown coating on W filaments [320, 321, 322], A composition near WN, (trigonal unit cell) has been observed in the electron microscope in the thinnest regions of nitrided films, but the derived structure contains three notably short N N distances of 1.71 A with eight corresponding W N ones of 2.91 3.03 A [323], The suggested structure is reminiscent of that of fluorite, but we do not feel that this compound is very w ell established. 

CH2—CHj-bond, makingphenylene mobility and disorder likely. The crystal structure of the fully ordered a-polymorph is monoclinic (C 2/m) with parallel packing of the phenylenes, while the p -structure is known to have statistical defects in a much enlarged trigonal unit cell (P3) but the detailed nature of the defects is still in dispute >. The ultimately must, in addition have CH2—CH2—... 

The parent ZrBr type of structure appears to be preferred for (CR2 X2 type compounds (denoted 3R). The alternative is the so-called IT structure type, whose identity period along [001] of the trigonal unit cell (E-3/hI)... 

Syndiotactic poly(styrene) displays a complex polymorphic behavior that reflects the specific role played by solvents. Four crystalline forms have been reported.(289,290) The a and p forms can be obtained from the melt (or glass), depending on the crystallization conditions.(291) Both structures comprise planar zigzag chains that have the same identity period of 5.1 A. The a form has a trigonal unit cell while the p form is orthorhombic. The P form can also be produced by crys-taflization from solution.(292,293) The y and 8 structures develop after interaction with solvent. In contrast to the all trans bond orientation of the a and p structures, the chains in the y and 8 crystals adopt a ttggttgg sequence of bond orientation. Thus a helical ordered structure evolves. This structure is similar to the crystalline chain conformation of syndiotactic poly(propylene).(294) The difference between the y and the 8 polymorphs is that in the former the sample is completely dried, while the solvent is included in the 8 form. It therefore represents a clathrate type structure. The formation of these structures is, thus, solvent specific.(292,293,295,296) The... 

Epitaxy of Isotactic Poly (1-butene) Isotactic poly(1-butene) (iPBul) is an archetypical polymorphic polymer with three different structures that differ by the chain conformation, and thus by the unit-cell geometry and symmetry (cf Chapter 2).The three crystal phases could be obtained by epitaxial crystallization on appropriate substrates [48-50]. Most interesting among them are the epitaxy of Form I (trigonal unit cell, threefold helical conformation, racemic phase) and that of Form III (orthorhombic unit cell, fourfold helical geometry, chiral crystal phase). 

Similarly, in statistical isotactic polypropylene copolymers containing ethylene or butene counits, a mixture of a monoclinic and y orthorhombic [52-54] crystal phases was observed, and the proportion of the /form increased with increasing counit concentration [25, 47, 55, 56]. The /form is typically observed in propylene homopolymers when crystallized under high pressure [57,58]. In the case of copolymers, the presence of non-crystallizable counits disrupts the development of polypropylene helices, resulting in an increase in the / phase content [25, 47, 55, 56]. The /phase was also observed in isotactic copolymers of propylene and 1-hexene [59]. Additionally, propylene copolymers that contain 10-25 mol% hexene units were found to crystallize in a crystal form having a trigonal unit cell [60, 61]. This structure is isomorphous to that of poly(l-butene), which. 

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