Threaded nematic texture

Fig. 4. Threaded nematic texture with homeotropic regions of polyester HTH/MEG sample at 234 °C (original magnification 300 x).

Disclinations in the nematic phase produce the characteristic Schlieren texture, observed under the microscope using crossed polars for samples between glass plates when the director takes nonuniform orientations parallel to the plates. In thicker films of nematics, textures of dark flexible filaments are observed, whether in polarized light or not. This texture, in fact, gave rise to the term nematic (from the Greek for thread ) [40]. The director fields... 

When an isotropic texture appears on cooling from an isotropic melt, it is formed from the coalescence of droplets that separate from the liquid. This mechanism is indicative of a nematic phase, and one of three possible characteristic textures may result, depending on the way the droplets form larger domains. These are the Schlieren, the threaded, or the marbled nematic textures. An example of Schlieren texture is shown in Figure II. 12. 

Other textures associated with nematic LCs include the thread-like texture and the marbled texture, both of which are caused by a strong interaction of the thin nematic layer with the surrounding walls [1,3,10]. 

Nematic phases typically show a schlieren texture between crossed polarizers if the director is oriented perpendicular to the viewing direction. One feature of the schlieren texture is the occurrence of topological point defects. At these point defects either two or four dark brushes meet. The corresponding defects are denoted as 1/2 or 1, respectively. Further characteristic textures of the nematic phase are the thread-like texture, which exhibits n disclinations parallel to the substrate, and the marble texture, in which areas of differing uniform director orientations occur. 

Anisotropic nematic, thread-like textures have been observed for both thermotropic [429, 430, 435] and lyotropic [436] LCPs. It is interesting that rheo-optical studies [437] of a slow cooled thermotropic LCP and a high density non-LCP polyethylene both exhibit sinular textures in polarized light [437]. These are shown in a nematic LCP (Fig. 5.84) and for comparison in polyethylene (Fig. 5.3). The textures appear similar except for their size scales. 

Fig. 5.85 Thick and thin regions of a thermotropic melt structure in polarized light. In a thick region (A) the fine structure is not too clear but the onset of decomposition is shown by the round bubbles. A thinner region (B) shows thread-like detail and a nematic texture with four brushes.

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... 

One well-known characteristic feature of nematic liquid crystals is the thread-like texture that can be observed with a polarizing microscope. The name nematic, derived from the Greek word "thread," reflects that feature. By examining the thin and thick thread-like structures in nematic liquid crystals, Otto Lehman i and Georges FriedeF deduced that this phase involves long-range orientational order. The first step to the interpretation of the threads as disclinations of the director field has been made by Oseen. Later Frank " derived Oseen s theory of curvature elasticity on a more general basis and presented it in a simpler form (see Appendix C.1). 

Transitions with the participation of liquid crystals sometimes show characteristic phenomena. If a nematic modification turns to a smectic A or smectic C phase, transient stripes in the form of a myelinic texture (also called chevron texture or striated texture) are often visible. Typically for the polyester prepared from di-w-propyl-/ -terphenyl-4,4" carboxylate and tetramethylene glycol, the nematic phase separates from the isotropic liquid on cooling in droplets which coalesce and form large domains. Cooling of the threaded-schlieren texture produces a transition to the smectic A phase this change is characterized by transition phenomena, mostly stripes, which broaden into larger areas ( transition bars ). 

The word nematic comes from the Greek word urjpa, meaning thread, arising from the thread-like textures often seen in nematic samples. These threads correspond to lines of singularity in the director alignment called disclinations. Such defects will be discussed below in Section 3.8 after a more detailed mathematical description of nematic liquid crystals is given in Chapter 2. 

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. 

Figure 13.11 (a) Thread-like domains in a nematic liquid crystal of thickness 100 4m viewed under crossed polarisers. (b) Schlieren texture in a nematic film of thickness 10 J,m (reproduced with... 

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)... 

The nematic phase, abbreviated as N, has the simplest structure of all of the mesophases, is very fluid, and is also the least ordered mesophase. The word nematic comes from the Greek nematos meaning thread-like—this arises from the observed optical texture of the phase between crossed polarizers i vide infra). The nematic phase is characterized by one-dimensional orientational order of the molecules by virtue of correlations of the long molecular axes, although the orientational order is not polar. There is no translational order within the nematic phase. A schematic diagram of the nematic phase is shown in Figure 5. 

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

As remarked in chapter 1, the nematic state is named for the threads that can be seen within the fluid under a microscope (fig. 1.1.6(a)). In thin films sandwiched between glass plates these threads can be seen end on. A typical example of the texture in a plane film of thickness about 10 /tm between crossed polarizers - the structures a noyaux or schlieren textures - is given in fig. 1.1.6(6). The black brushes originating from the points are due to line singularities perpendicular to the layer. In analogy with dislocations in crystals, Frank proposed the term disinclinations , which has since been modified to disclinations in current usage. 

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]. 

---