Smectic samples, textures

The mesophases of LC diols la-lg were also observed directly in polarized optical micrographs taken immediately after melting the sample. Textures were identified only by comparison with published micrographs (2 ), and are therefore tentative. A nematic texture is observed for If (Fig. 2), while more highly ordered smectic textures are observed for Ib-le and Ig (Fig. 3). 

Some materials crystallize to give a microstructure that can be mistaken for a smectic mosaic texture. A simple test involves reheating the sample as soon as the "mosaic texture" has formed if the transition is not reversed at close to the same temperature, we are dealing with crystallization (Neubert, M.E., Kent State University, personal communication, 1989). A reversible transition would be inconclusive, i.e. it would be consistent with the formation of either a crystalline or a liquid crystalline phase. However, reversibility is expected over a greater range of cooling rates in the latter case. 

The observed EM effects depend very strongly on the virgin texture and the director alignment of the smectic sample (helix developed, unwound, etc.). The linear effeo is proportional to the applied voltage. Nonlinear EM response (with respect to SHO) has also been observed and studied by Jikli et al. [90]. 

FIG. 32 Textures of Sodium Pentadecane 1-Sulfonate, (a), Crystalline phase at 80°C (b), smectic B phase at 113°C (c), smectic A phase at 250°C. Sample window 1.4 mm2, crossed Nicols. 

Preliminary room temperature x-ray data of 0.65 Me4C00-PECH indicates that the sample presents a highly ordered smectic mesophase which was not yet completely assigned. The textures seen by polarized optical microscopy are also typical of smectic phases. Due to the very high molecular weights involved, textures specific to mesophase in thermodynamic equilibrium could not be developed within a reasonable amount of time by annealing. 

Figure 7.1 Illustration of different aggregation states obtained (from left to right) by increasing temperature crystal (K), smectic C (SmC), nematic (N) and isotropic (I). Row a shows macroscopic appearance of samples in row b, short-range microscopic ordering is represented (each bar represents a molecule) thermotropic phase diagram of row c illustrates relevant transition temperatures (Tm melting temperature Tsmc-N transition temperature between SmC and N Tc clearing temperature) row d shows different texture of different states as seen through polarizing microscope (with crossed polars, isotropic phase appears black).

Note 4 A thin sample of a smectic C phase with the layers parallel to the sample surfaces gives schlieren textures with centers that have four brushes. However, a smectic C phase formed by odd-membered liquid-crystal dimers (see Definition 2.11.2.9, Note 5) has schlieren textures with two or four brushes. 

Above 110 °C, this arrangement becomes mobile, and a smectic C liquid-crystalline phase is entered. Samples cooled down from the isotropic melt (140 °C) show Schlieren and banded textures when viewed under crossed polarizers (Figure 8). These textures look similar to nematic Schlieren textures, but from the X-ray diffraction data it is clear that 12c forms a homeotropically oriented smectic C phase. In a nematic phase, the small-angle diffraction peak would be absent, and a broad scattering feature, a nematic streak , would be observed. Polymer 12c was the first example of a PPE derivative for which three states of matter, i.e. crystalline, thermotropic liquid crystalline, and a highly viscous isotropic liquid, were accessible [46]. 

In dynamic x-ray diffraction studies reported earlier (17). perpendicular equatorial and meridional arcs with the same d-spacing were observed in drawn fibers. Both pairs of arcs showed different transition temperatures. DSC of annealed samples showed a small endo therm at 120°C which occurred at the same temperature observed for the transition in the BP6L meridional diffraction arc. The endothermic transition of the annealed THF insoluble fraction corresponds to the 160°C transition of the BP6L equatorial diffraction arc. Both fractions exhibit mesophase behavior above the observed thermal transitions, and only a subtle textural change is evident at that temperature under crossed polars, indicating that these thermal transitions are due to trace amounts of crystallization. The BP6Li fraction displays characteristics of the smectic mesophase while the texture and x-ray observations of BP6Ls do not allow conclusive identification of the mesophase (presumably nematic). 

They observed that the sample with a polyion backbone formed a smectic A phase with a focal conic fan texture and a perpendicular structure. On the other hand, the material based on the neutral backbone formed a nematic phase with a schlieren texture. Once again, the presence of charges in the polymer severely influenced the polymorphism of the compounds. The authors showed the potential of these LC systems in the fields of photomemory, optical storage, and light drive display, especially because these amphiphilic polymers yield excellent azodense LC thin films [96]. 

Figure 4.21 shows a focal-conic fan texture of a smectic A phase in which is observable some defects represented by Figure 4.20(b). While Sa forms the fan-like focal-conic textures, the focal-conics formed by the Sc phase are often broken and less distinct. In addition, as stated above, Sa is uniaxial but Sc is biaxial. Sa may take a homeotropic alignment with the axis normal to the sample plane and extinct on POM with crossed polars. The two smectic phases are therefore distinguishable with POM. Nevertheless, it is desirable to include WAXS studies in order to identify the two with assurance (Section 4.3). 

Studies on low and high mass cholesteric phases have shown that the compound may form a focal-conic texture if no displacement is made on the cover slip when the sample is heated up or cooled down to the cholesteric phase. The focal-conic texture is similar to that of a smectic A, but the particles are usually too small to identify except for rare preparations. If the cover slip is displaced the focal-conic texture will give way to a homeotropic one with the optic axis normal to the plane of the preparation. The cholesteric homeotropic film reflects vivid colors having wavelength A decided by the De Vries (1951) expression A = pn where p is the pitch... 

The X-ray diffraction of polymeric liquid crystal systems and their low mass counterparts are the same in principle, but the diffraction results for the polymers are often less ideal and more difficult to interpret. In practice the oriented specimens are often preferred over the unoriented samples for an unambiguous determination. X-ray diffraction is nearly always used together with texture observations using a polarizing optical microscope. Miscibility tests are also used in some cases for confirmation. For smectic phases with higher translational and orientational orders, X-ray diffraction is the most useful (if not the only) technique for unmistakable characterization. A few examples are cited below. The details of each characterization of the various polymeric smectic phases were described by individual authors. 

Optical observations of mesophase textures have been previously discussed in detail It is to be stressed here that they are generally in excellent agreement with thermal analysis data and indicate the existence of smectic and nematic phases in samples 42 (w=2) through i (DP =4.1). On the contrary, polymer 43 7 CBPj =8.7) displayed a weakly birefringent, possibly nematic, texture. 

Our results also indicate that thermal history and sample preparation conditions can alter the morphology and texture of the polymer and also influence the melting point. The preferred orientation of the smectic layered structure of the cast films on the glass substrate is not altogether unexpected since such observations are commonly found among mesogenic compounds. 

Smectic A and C phases are the most common smectic phases. They exist in several texture variants. Similar to the nematic phase, smectic A phase can exhibit the homogenous texture. The corresponding texture of the smectic C is the schlieren texture, In this texture, the layers are parallel to the sample surface. The schlieren indicates strong distortions of the director field [1]. 

Cooling the preparation down to 153.5°C, a low-contrast front crosses the sample and we observe a texture almost identical with that of the smectic C phase. As the temperature falls even further below 153.5°C, disclination cores open up into magnificent five-pointed stars, whose description goes beyond the context of the present introduction. The sample is still in the smectic state, for the focal conics are still clearly visible. The exact nature of this phase is more subtle than those described previously. Layers are organised in a different way. Although it is still liquid and molecules are still tilted, the local order is very... 

---