Crystal crystalline state

In order to reach a crystalline state, polymers must have sufficient freedom of motion. Polymer crystals nearly always consist of many strands with a parallel packing. Simply putting strands in parallel does not ensure that they will have the freedom of movement necessary to then find the low-energy con-former. The researcher can check this by examining the cross-sectional profile of the polymer (viewed end on). If the profile is roughly circular, it is likely that the chain will be able to change conformation as necessary. 

Physical Properties. Table 3 Hsts physical properties of stereoregular polymers of several higher a-olefins. Crystal ceU parameters of these polymers ate available (34—36). AU. stereoregular polyolefins have helix conformations ia the crystalline state. Their densities usually range from 0.90 to 0.95 g/cm. Crystalline PMP, however, represents an exception its density is only 0.812—0.815 g/cm, lower even than that of amorphous PMP (0.835—0.840 g/cm ), thus making it one of the lowest densities among plastics. 

J. Cognard. Lubrication with liquid crystals. In G. Biresaw, ed. Tribology and the Liquid-Crystalline State. Washington, DC American Chemical Society, 1990, pp. 1 7. 

Certain monomers crystallize in a conformation such that they can be zipped together without changing the symmetry of the crystal lattice. In the crystalline state, the arrangement of monomers is strictly determined by crystal packing. Polymerization is usually initialed by irradiation with UV, X- or y-rays and is assumed to proceed by a radical mechanism. For example, muconic acid esters (25, 27, 29) and ammonium salts (26,28, 30) can be stcrcospccifically polymerized in the crystalline state to high conversion.224-227228 This form of... 

Two approaches to the attainment of the oriented states of polymer solutions and melts can be distinguished. The first one consists in the orientational crystallization of flexible-chain polymers based on the fixation by subsequent crystallization of the chains obtained as a result of melt extension. This procedure ensures the formation of a highly oriented supramolecular structure in the crystallized material. The second approach is based on the use of solutions of rigid-chain polymers in which the transition to the liquid crystalline state occurs, due to a high anisometry of the macromolecules. This state is characterized by high one-dimensional chain orientation and, as a result, by the anisotropy of the main physical properties of the material. Only slight extensions are required to obtain highly oriented films and fibers from such solutions. 

The few references in the literature indicated that glycerol had only been obtained in the crystalline state by chance. Inquiry among places storing large quantities of glycerol finally revealed some crystals at the plant of the Giant Powder Company at Manoose Bay, BC. 

The resonance splitting of intramolecular modes in the crystalline state is often called Davydov splitting or factor group splitting . In contrast to the static field effects of the crystal, this splitting is due to the dynamical interaction of the constituents in the primitive cell... 

Liquid crystals form a state of matter intermediate between the ordered solid and the disordered liquid. These intermediate phases are called mesophases. In the crystalline state the constituent molecules or ions are ordered in position and orientation, whereas in the liquid state the molecules possess no positional and orientational ordering. Liquid crystals combine to some extent the properties of both the crystalline state (optical and electrical anisotropy) and the liquid state (fluidity). 

In 1978, Bryan [11] reported on crystal structure precursors of liquid crystalline phases and their implications for the molecular arrangement in the mesophase. In this work he presented classical nematogenic precursors, where the molecules in the crystalline state form imbricated packing, and non-classical ones with cross-sheet structures. The crystalline-nematic phase transition was called displacive. The displacive type of transition involves comparatively limited displacements of the molecules from the positions which they occupy with respect to their nearest neighbours in the crystal. In most cases, smectic precursors form layered structures. The crystalline-smectic phase transition was called reconstitutive because the molecular arrangement in the crystalline state must alter in a more pronounced fashion in order to achieve the mesophase arrangement [12]. 

With regard to the molecular structure of CCHn, both cyclohexyl rings having a chair conformation are substituted in the equatorial positions and the alkyl chain is completely extended in the all-trans conformation. The cyclohexyl rings are nearly coplanar. The crystal structures of the investigated CCHn show that various types of molecular overlapping are present in the crystal. The molecular packing in the crystalline state is quite different in all three compounds. 

It is interesting that all three compounds show small angle reflections in the liquid crystalline state which indicates the formation of associates with a length of about twice the molecular length (for CCH5 17.5 A in the crystal phase I, 31.2 A in the Sb phase, and 27.2 A in the nematic phase) [73]. 

In 1986, Walz and Haase [148] presented the crystal structure of the mesogenic hydrocarbon compound l,2-bis-(4 -pentylcyclohexyl)ethane. The compound exhibits a smectic B phase over a remarkably broad range of temperature. To our knowledge, this is the only crystal structure determination of a mesogenic hydrocarbon compound up to now. Since this compound does not contain any polar groups, the arrangement in the crystalline state is... 

The compounds crystallise in noncentrosymmetric space groups namely PI, P2i, C2, and P2i2i2i (but with priority of P2i) due to the chirality of the molecules. Most of the compounds have a tilted layer structure in the crystalline state. The tilt angle of the long molecular axes with respect to the layer normal in the crystal phase of the compounds is also presented in Table 18. Some compounds show larger tilt angles in the crystalline state than in the smectic phase. In the following only the crystal structures of some selected chiral liquid crystals will be discussed. 

Concerning the question whether the detailed knowledge of the structural features in the crystalline state enables the prediction of the type and the properties of the liquid crystalline phases formed from the crystals, the following points should be mentioned. 

A compound which displays liquid crystal properties is referred to as a mesogen and said to exhibit mesomorphism. Liquid crystals may be considered either as disordered solids or ordered liquids, and their properties are very dependent on temperature and the presence or absence of solvent. In thermotropic liquid crystals the phases of the liquid crystals may be observed to change as the temperature is increased. In lyotropic liquid crystals the ordered crystalline state is disrupted by the addition of a solvent, which is very commonly water. For these systems temperature changes may also be... 

While crystal structures of rubredoxins have been known since 1970 (for a full review on rubredoxins in the crystalline state, see Ref. (15)), only recently have both crystal and solution structures of Dx been reported (16, 17) (Fig. 3). The protein can be described as a 2-fold symmetric dimer, firmly hydrogen-bonded and folded as an incomplete /3-barrel with the two iron centers placed on opposite poles of the molecule, 16 A apart. Superimposition of Dx and Rd structures reveal that while some structural features are shared between these two proteins, significant differences in the metal environment and water structure exist. They can account for the spectroscopic differences described earlier. 

Demus, D., Goodby, J., and Gray, G. W., Handbook of Liquid Crystals, New York Wiley-VCH, 1998 Biresaw, G., Tribology the Liquid-Crystalline State," ACS Symposium Series 441, Lavoisier, 1990. 

Peptoids based on a-chiral aliphatic side chains can form stable helices as well [43]. A crystal of a pentameric peptoid homooligomer composed of homochiral N-(1-cyclohexylethyl)glycine residues was grown by slow evaporation from methanol solution, and its structure determined by X-ray crystallographic methods. In the crystalline state, this pentamer adopts a helical conformation with repeating cis-... 

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