Disclinations defects

Nevertheless in polymeric liquid crystals the same types of orientational defects and thus the same types of textures as present in the low mass counterparts have been observed. The textures often formed by polymers are the threaded texture, the schlieren texture and the focal conic texture of smectics. As is for low mass liquid crystals, the texture is a consequence of defects (disclinations and dislocations, refer to Chapter 1) present in the liquid crystal and is characteristic of a specific type of the phase. The texture examination has become a very useful tool in the determination of the type and nature of the polymeric liquid crystals. 

Some optical observations of defects (disclination lines) in polymeric nematic phases exhibiting threaded textures showed that a massive organization of macromolecules occurs as the temperature is raised Immediately after melting of the specimen be-... 

When L/p I, the cholesteric does not differ much from the nematic phase. No wonder therefore that optical observations for weakly twisted cholesterics reveal thick (nonsingular) and thin (singular) line defects —disclinations similar to that in the nematic phase. Moreover, in droplets of the so-called compensated cholesteric mixtures with extremely small Ljp one can observe point defects [6] which, from the topological point of view, are allowed only in a nematic phase. 

As indicated above and illustrated in Fig. 32 i, layers lie horizontally in planes and show some translation dislocations. They are oblique in polygonal fields and generally cross the focal lines at points where they are strongly hat-shaped, whereas elsewhere they are saddle-shaped and dislocations are present, often superimposed on the focal lines. In fans, the layers are vertical and all types of defect (disclinations, focal curves, dislocations) are present. 

A TN cell without having reverse twist defect (disclination) will be fabricated by giving the twist angle of less than 90° [16] or by adding a chiral agent [17]. 

As in crystals, defects in liquid crystals can be classified as point, line or wall defects. Dislocations are a feature of liquid crystal phases where tliere is translational order, since tliese are line defects in tliis lattice order. Unlike crystals, tliere is a type of line defect unique to liquid crystals tenned disclination [39]. A disclination is a discontinuity of orientation of tire director field. 

Note A nucleus can indicate either the end of a disclination line terminating at the surface of a sample or an isolated defect. 

Fig. 23. Schematic representation of the director alignments at disclinations with different values of s and (J)o,s 72 correspond to two-brush defects and s = 1 to four-brush...

The textures in homeotropic lamellar phases of lecithin are studied in lecithin-water phases by polarizing microscopy and in dried phases by electron microscopy. In the former, we observe the La phase (the chains are liquid, the polar heads disordered)—the texture displays classical FriedeVs oily streaks, which we interpret as clusters of parallel dislocations whose core is split in two disclinations of opposite sign, with a transversal instability of the confocal domain type. In the latter case, the nature of the lamellar phase is less understood. However, the elementary defects (negative staining) are quenched from the La phase they are dislocations or Grandjean terraces, where the same transversal instability can occur. We also observed dislocations with an extended core these defects seem typical of the phase in the electron microscope. 

Liquid crystals may have line defects called disclinations. The name comes from discontinuity and inclination. The director rotates about a line normal to the disclination. The strength of a disclination, S, is defined by... 

At the late stage of lamella orientation, classical topological defects (dislocations and disclinations) dominate [40, 41] (Fig. 8h and Fig. 9), and their movement and annihilation can be followed in Fig. 8h-i and Fig. 9. The latter presents an example of the apparent topological defect interactions and their transformations. Displayed are two dislocations of PMMA, which have an attractive interaction due to their opposite core sign. Therefore, in the next annealing step the dislocation is shifted... 

Hahm J, Sibener SJ (2001) Time-resolved atomic force microscopy imaging studies of asymmetric PS-b-PMMA ultrathin films dislocation and disclination transformations, defect mobility, and evolution of nanoscale morphology. J Chem Phys 114(10) 4730-4740... 

Fig. 22 Simulated images (upper panel) and SFM phase images (300 x 300 nm) (lower panel) presenting classical topological defect configurations in lying cylinders (a, e) cyl-dislocation (b, f) m-dislocation (c, g) +1/2 cyl-disclination and (d, h) +1/2 m-disclination. SB films were annealed under 70% of the saturated vapor pressure of chloroform. Reprinted from [36], with permission. Copyright 2008 American Chemical Society...

In order to account for the real 3D structure of cylindrical microdomains, we denote the configurations in Fig. 22a, e and c, g as cylinder-phase defects (cyl-dislocation and +1/2 cyl-disclination), and the configurations in Fig. 22b,f and d,h as matrix defects (m-dislocation and m-disclination). In our systems, cyl-dislocations generally develop during the early stages of film annealing when the overall defect density is high. In well-equilibrated films, cyl-dislocations are less frequent as compared to m-dislocations. 

The rich phase behavior of cylinder-forming block copolymers is reflected in the modification of classical defects by incorporation of elements of non-bulk structures such as +1/2 disclination with incorporated PL ring (Fig. 23a, d) or white dot (b, e) and —1/2 disclination with incorporated PL domain (Fig. 23c, f). Defects Fig. 23a and b are topologically equivalent, but functionally different. Their... 

Detailed analysis of defect configurations in the cylinder phase and of their evolution allowed us to conclude that representative defect configurations provide connectivity of the minority phase in the form of dislocations with a closed cylinder end or of classical disclinations with incorporated alternative, non-bulk structures with planar symmetry. Further, block copolymers show a strong correlation between the defect structure and chain mobility on both short- and long-term time scales. 

Small-angle light scattering has also been extensively applied to PLCs subject to flow [173]. As in the case of scattering dichroism, SALS patterns arise principally from fluctuations in orientation, and these arc strongest in the vicinity of disclinations, or defects in the director field. The experimental geometries used for SALS in liquid crystals normally use polarizers placed before and after the sample. The arrangements include VV scatter-... 

Parabolic focal conics are a special case of generic focal conic defects, which are composed of layers curved to form toroidal surfaces called Dupin cyclides (see Fig. 10-31). Each such structure contains a pair of disclination lines—one an ellipse and the other... 

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