Threaded texture

Thin films of the solutions between microscope slides were sheared by applying even pressure on a coverslip while sliding it approximately one cm. The anisotropy appeared to increase as measured by increases in the birefiingence. Solutions containing 10-16% (w/w) cellulose developed a threaded texture and the mesophases were stable with time and oriented in the direction of shear. These observations, while not definitive, suggested a cholesteric to nematic transition occurred on shearing. 

Typical polarized optical micrographs of the molten samples were shown in Figure 1. Microscopic analyses were earned out at temperatures in the range 25 - 390 C, and threaded textures, a characteristic optical texture of nematic mesophase, were observed above LCITs over the investigated composition range. The nematic to isotropic transitions were not observed up to 390 C. 

The thermotropic liquid crystalline PECs could be synthesized from HBA, DHBP, DPC. Threaded textures of PECs, a characteristic of the nematic phase, were observed over the investigated composition range. 

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. 

The threaded texture is also often observed for nematics especially when a thicker film is examined. In this texture disclination lines show up as dark lines swimming in the sample with thermal motion. The lines are observable without polars (Figure 4.17) but the thread contrast will be higher when crossed polars are used. Because of the stopless molecular reorientation with thermal motion in the sample the old dark lines may disappear and new ones may form. In cases when the two ends are anchored on the surfaces the ends will stay where they have been but the thread may swing about. 

Fig. 9a,b. Microscopic observations in polarized light of the textures of the aqueous clay suspensions a nematic threaded texture of a bentonite suspension (concentration 0.044 g/cm magnification 50x) b detail of a 1/2 disclination hne (arrow) in a laponite suspension (concentration 0.034 g/cm magnification lOOx) (Reprinted from [50], copyright (1996) from the American Chemical Society)... 

Fig. 21. Threaded texture of a lo mol/1 suspension of LiMOjSej in NMF in the nematic phase (courtesy JC Gabriel)...

Figure 6. Thin and thick threads in a typical low molecular weight liquid crystal threaded texture of p-methoxybenzylidene-p-n-butylaniline (MBBA).

The threaded texture changes to the Schlieren texture when the lines are all perpendicular to the sample boundaries. This texture observed between crossed polarizers displays dark brushes (also called black... 

It should be noted that twist lines are visible in threaded textures while wedge lines are observed in Schlieren textures. The continuous transformation of a wedge S line into a wedge -S line involves the passage through an intermediate twist 5 line [19]. 

As a rule, thin lines of strength 1/2 or singular lines of strength 1 are seen in the threaded textures of nematic thermotropic MCPs [25,57,58]. Rare cases of thick lines have been observed. As we have already pointed out, the reason for this is that the elastic anisotropy is large in these systems. The disclinations with 5= l/2 were also reported to be the most abundant in the Schlieren textures of nematic copolyesters [59-65]. 

Integral lines have been observed in the threaded texture [25] of a random copolyester prepared by the Eastman Kodak Company from 40 mol% poly(ethylene tereph-thalate) and 60 mol% jt -acetoxybenzoic acid and commercially known as X-7G ... 

The shearing behavior of MCLCPs depends on the initial state of the material. When molten MCLCPs exhibiting a threaded texture in the quiescent state are subjected to shear, the threads first extend and curve and disclination loops distort within the shear. However, at a rather well defined shear strain multiplication of the defects occurs [83, 84, 96, 97]. These defects appear as short, dark curled entities which flow along the streamlines whilst continually changing their shapes, and the overall view of them during shear resembles a vat of... 

Fig. 1. Photomicrograph of nematic phase from copolyester, prepared from a 50/25/25 (mol %) mixture of terephthalic acid, pyro-catechol and methylhydroquinone. Threaded texture (crossed polarizers).

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-... 

The thread texture usually observed in thick layers... 

Mostly thermotropic polyesters show a nematic phase with a typical appearance that is called a marbled texture, a Schlieren texture, a threaded texture, or, a nematic droplets depending on the thickness and temperature of the polyester. Only in a few cases, they exhibit low-order smectic phases [100], which are identified through the observations of fan-shaped and broken fan textures in their LC phases. No report of higher-order smectic phases has been observed in this class of pol)nners. It is worth mentioning that nematic texture can simply be frozen into a glassy state in the case where there is no interference from the crystallization process. An intriguing property of an LC polyester is its biphase structure over... 

Fig. 2.16. Threaded texture. Crossed polarizers. Original magnification x 200.

In some preparations, the disclination lines do not lie perpendicularly, but more or less horizontally, their ends being attached to the glass surfaces and the other parts floating freely in the LC. The term threads is usually used to describe these disclination lines and the corresponding texture is the threaded texture. 

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