Helical molecular arrangement

Figure 2. Helical molecular arrangement of wool (The International Wool Secretariat, Heusden, The Netherlands)...

There are at least four crystalline forms of cellulose, based on different packing of the primary chain (Blackwell, 1982), and three forms of granular starch, based on the packing of double helices (Noel et al., 1993). The differences are largely in the unit-cell dimensions and the crystallization and precipitation temperatures. One form of starch, precipitated with alcohol, is in a symmetrical molecular arrangement and is readily dispersible in cold water (Kerr, 1950). Mannan and dextran yield different crystals at low and high temperatures, and there was not only a polymorphic difference, but a conformational difference in cellulose (Quenin and Chanzy, 1987). Curdlan appears to have three polymorphs—anhydrous, hydrated, and annealed. 

Twisting a nematic structure around an axis perpendicular to the average orientation of the preferred molecular axes, one arrives at the molecular arrangement commonly called cholesteric (Kelker and Hatz, 1980). The twisted nematic phase is optically uniaxial, however with the axis perpendicular to the (rotating) director. Such a mesophase combines the basic properties of nematics with the implications of chirality The structure itself is chiral and as a consequence, a non-identical mirror image exists as it is shown schematically in Fig. 4.6-7. Besides the order parameters mentioned before, the essential characteristics of a cholesteric mesophase are the pitch, i.e., the period of the helical structure as measured along the twist axis, and its handedness, i.e., whether the phase is twisted clockwise or anticlockwise. 

From the coincidence of the X-ray layer lines of Sa,i and Sa,2 with those of Sf Tuinstra concluded that these polymers have the same molecular conformation. Since the diffraction patterns of Sa,i and Sa,2 were much poorer than that of the exact positions of the atoms of the helices remained uncertain [54, 115]. However, the gross arrangement of the helices in the units of Sa,i and S b2 could be determined. Accordingly, the structure of Sa,i is an orthorhombic lattice which has nearly the same size as the unit cell of on a pseudo-orthorhombic setting (Table 20). In these structures the helices are arranged parallel to each other. Since the densities of both were found to be the same the unit cell was expected to be build by four helices as in the case of Sf. A maximum of interlocking of the helices was estimated for the direction of the h-axis which consists of alternating helices with opposite turns (see Fig. 18). Because of the similarity with the fibrous sulfur allotrope S, ... 

Cholesteric LCs, in which the molecules are twisted perpendicular to the helical direction, the molecular axis being parallel to the director, are used in a number of optical applications such as LC displays and electronic paper [11,12]. One of the most important properties of cholesteric LCs is that a specific wavelength can be selectively reflected by controUing the directions in which the molecules are oriented. In general, an external electric or magnetic field is used to control the cholesteric LC molecular directions, and it is important to understand the ways external fields affect the molecular arrangements and dynamics so that further developments can be made. 

We also attempted to observe second-harmonic generation in cholesteric liquid crystals. Hie negative results indicate that the molecular arrangement of the liquid crystals in a plane perpendicular to the helical axis has an over-all inversion symmetry. 

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