Smectic layer structure study

2 Relationship between molecular tilt angle and chevron layer tilt angle 

The origin of the chevron layer structure is explained by the discrepancy between the layer spacing of the SmC phase and that of the high temperature phase, which is usually the SmA phase. The layer spacing in the SmA phase, dA is fixed at the surfaces, and it decreases to do in the SmC phase because the molecules decline at the tilt angle Q from the smectic layer normal as shown in Fig. 5.1.6. This figure leads to a simple relation, 

The chevron layer structures of several SSFLCs from various FLC materials exhibiting different optical molecular tilt angles have been precisely investigated, and a correlation between the layer tilt angle of the chevron structure and the optical molecular tilt angle was confirmed. 

The layer structure was measured by using an X-ray scattering system with a goniometer at room temperature (25 °C). The measurement geometry is shown in 

The optical molecular tilt angle 6 was thus determined from Eq. (5.1.2). 

5 2005 Kohki Takatoh, Masaki Hasegawa, Mitsuhiro Koden, Nobuyuki Itoh, Ray Hasegawa and Masanori Sakamoto 

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In addition to the homopolyesters 80a-i, 81a-i and 82a-i, three classes of co PEIs were studied 83a-g [24], 84a-d and 85a-f [84]. The combination of two different alkane spacers did not significantly change the properties of the co PEIs 83a-d, when compared to 82i. However, when the difference on their length increased, a destabilization of the smectic layer-structures became evident, with the consequence that a nematic phase was formed on top of the smectic-C phase... 

Molecular orientational models of SSFLCs have previously been studied, so the orientational states of SSFLCs and their optical properties are fully understood. For example, the smectic layer structures of various SSFLCs have been studied by using higji-resolution X-ray diffraction and the relationship between the layer tilt angle and the optical molecular tilt angle has been confirmed. 

A helical director field also occurs in the chiral smectic-C phase and those smectic phases where the director is tilted with respect to the layer normal (Figure 1.13(c)). In these cases, the pitch axis is parallel to the layer normal and the director inclined with respect to the pitch axis. Very complicated defect structures can occur in the temperature range between the cholesteric (or isotropic) phase and a smectic phase. The incompatibility between a cholesteric-like helical director field (with the director perpendicular to the pitch axis) and a smectic layer structure (with the layer normal parallel or almost parallel to the director) leads to the appearance of grain boundaries which in turn consist of a regular lattice of screw dislocations. The resulting structures of twist grain boundary phases are currently extensively studied. The state of the art in this topical field is summarized in Chapter 10. 

Fujikake H, Aida T, Takizawa K, Kikuchi H, Fuji T, Kawakita M (1999) Study of smectic layer structure of polymer-stabilized ferroelectric liquid crystal with grayscale memory. Electron Commun Jpn 82(8) 1-8... 

The Tokyo Tech group assigned a C2 structure for the layers in the B2 phase, and ferroelectric packing of such layers to form a locally polar C2v macroscopic structure, as indicated in Figure 8.20. Other early workers in the field also adopted this structural model for the B2 phase. Brand et al. had discussed a C2 smectic chevron structure in their 1992 theoretical study,29 and while they seem to be referring to an all-anticlinic bilayer smectic, their actual graphic is basically identical to that shown in Figure 8.20. Furthermore,... 

If the ratio f lf2 I is greater than unity the torques induced by the symmetric and antisymmetric strain rates respectively will never cancel out and the antisymmetric pressure will never vanish. This means that the director continues rotating for ever. The liquid crystal is said to be flow unstable and complicated flow patterns arise. TTiey have been studied comprehensively both experimentally and theoretically [30]. Some nematic liquid crystals are flow stable whereas others are not. For example, 4-n-pentyl-4 -cyanobiphenyl (5CB) is flow stable whereas 4-n-octyl-4 -cyanobiphenyl (8CB) is flow unstable. The only difference between this two substances is the length of the hydrocarbon chain attached to the cyanobiphenyl skeleton. Nematic liquid crystals that are flow stable usually become flow unstable close to the nematic-smectic A transition. The reason for this is that there is an emergent layer structure in the fluid that is incommensurate with the strain rate field. 

The various orientations and conformations mesogenic groups can adopt in a layer has been studied extensively for low molar mass smectic materials, and the classification and terminology of smectic systems is entirely based on these studies. However, low molar mass smectic compound or smectic LC-side chain polymers do, of course, not allow one to elucidate the role spacers play in the layer structures of LC-main chain polymers. Therefore, poly(ester-imide)s, po-... 

Another set of problems was studied in connection with the PEIs 179,180 [136]. It was found that all these PEIs do not show an LC-phase,but form a smectic crystalline solid state with upright mesogens (smectic-E like). However, the PEIs can in principle adopt two different kinds of chain packing and layer structures as illustrated in Fig. 29. The first is that the mesogens pack in an antipar-... 

Finally, it should be emphasized that the tendency of polyimides containing aliphatic spacers to form stable layer structures of various dimensions and degrees of order can be utilized for an even wider variety of studies. A recent example is a study of the epitaxial growth of polyethylene on smectic crystallites of PEI [140]. Most likely smectic crystallites adopt a lamellar form with the large surface covered by loops of the aliphatic spacers. This hypothesis still need detailed studies and confirmation. Anyway, smectic crystalline polymers are interesting substrates for studies of epitaxial crystallizations. Furthermore, layer structures derived from long aliphatic spacers (alkanes or oligoethers) may play... 

X-ray studies were done for poly(terephtaloyl-di-n-ozybenzoat) with decamethilene flexible spacers [8], Atter several heating-cooling cycles it was found that polymer transforms into melt at T=110°C. Disorder of layer structures and transformation of system into isotropic state from liguid-crystalline occurs at temperature rise up to 190°C. When polymer is cooled at 180°C melt transforms from isotropic state into liguid-crystalline-state with the structure of smectic type. 

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