Texture transitions, smectics

The texture transitions have been observed in smectics A many years ago [104], however, they are especially pronounced in materials with large dielectric anisotropy. A high-quality planar texture goes to a quasi-homeo-tropic optically transparent texture via intermediate structural defects (80CB, Ae = 8.2, d = 50 /xm, and voltage 30 V) [105]. Other substances were studied in [106]. A homeotropic texture can also be obtained by applying the field to the less-ordered focal-conic texture if Ae > 0 [107] (erasing a defect structure produced either by the thermal action of a laser or by electrohydrodynamic instability [108]). 

FIGURE 6.30. Homeotropic texture of a smectic A in an electric field Ae > 0. (a) Initial geometry (b) Parodi texture transition [109] (c) wave-like instability [7], experimentally observed patterns corresponding to a Parodi transition [109] (d) from a homeotropic orientation and (e) from a planar orientation. 

The texture transition can also be observed for smectic A liquid crystals with negative dielectric anisotropy [112]. In that case, the transition from a homeotropic into a planar texture occurs. The threshold of this, dielectric transition, can be modified (lowered) at the low frequencies of an applied field by the anisotropy of the electrical conductivity of a substance. 

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).

With respect to the higher temperature transition at 445 °C, there are two conflicting views of this transition, namely that the phase above 445 °C is a smectic C and the other that it is nematic. Based on high temperature X-ray diffraction studies, Yoon et al. have concluded that it is a smectic C (see Fig. 4) [28], Thus, in Fig. 4, the disappearance of the 211 peak indicates that the nematic E structure is converting to a nematic C. In our work, using polarizing optical microscopy, we have observed a nematic texture for high molar mass specimens heated rapidly to 480 °C, sheared, and then quenched. In the case of a... 

Price and Wendorff31 > and Jabarin and Stein 32) analyzed the solidification of cholesteryl myristate. Under equilibrium conditions it changes at 357.2 K from the isotropic to the cholesteric mesophase and at 352.9 K to the smectic mesophase (see Sect. 5.1.1). At 346.8 K the smectic liquid crystal crystallized to the fully ordered crystal. Dilatometry resulted in Avrami exponents of 2, 2, and 4 for the respective transitions. The cholesteric liquid crystal has a second transition right after the relatively quick formation of a turbid homeotropic state from the isotropic melt. It aggregates without volume change to a spherulitic texture. This process was studied by microscopy32) between 343 and 355.2 K and revealed another nucleation controlled process with an Avrami exponent of 3. 

The methyl-substituted ester model compound, shown below, also shows both smectic and nematic phases, as well as a smectic C-to-nematic transitional phase (18) from 169 to 170°C. Figure 3 shows two of the textures observed in this model. 

Figure 3. Liquid-crystal textures of the methyl-substituted model ester viewed through crossed polarizers, a, Smectic C-to-nematic transitional phase and b, smectic mosaic texture at 160 °C. Original magnification, 320x.

Two papers published in 1991 described the thermal properties of the diester ferrocene derivatives 8 [15] and 9 [16] (Fig. 9-4). Complexes 8 (n = 3, 4, 6, 8) were found to be non-mesomorphic and 8 (n = 10) seemed to give rise to a smectic phase (unidentified) between 85 °C and 88 °C during the second heating cycle. When 9 n = 1) was cooled from the isotropic melt at a rate of 2 °C/min, a liquid crystalline texture was observed (transition temperatures and liquid crystalline range not given) that, according to the authors, was indicative of a smectic phase. The latter was not identified. Derivative 9 n = 10) formed a nematic phase between 78 °C and 79 °C. This mesophase did not reform on cooling from the isotropic melt. 

Ferrocene derivatives 15 exhibited remarkable liquid crystal properties (Fig. 9-13). Indeed, they all gave rise to enantiotropic mesophases. Structures with n = 1 to 11 showed nematic phases. From n = 12 a smectic C phase formed. The latter was monotropic only for 15 (n = 12). The smectic C domain increased from n = 13 to n = 16, and, inversely, the nematic range narrowed. The last member of this series (n = 18) presented one smectic C phase between 159 °C and 179 °C. A nematic to smectic C transition and a focal-conic texture of a smectic C phase are presented in Figs. 9-14 and 9-15, respectively. 

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). 

There are occasions when specimens exhibit a paramorphotic texture, i.e. one that reflects order inherited from the parent phase. At a molecular level, the transition between liquid crystalline phases typically occurs via a route that requires the minimum instantaneous rearrangement of molecules. Because textures are dictated by molecular order, the immediate post-transition texture may not easily be distinguishable from its pre-transition counterpart. Stable textures that are characteristic of the new phase may require a long time, sometimes months, to form. Transitions between highly otdered smectics are especially likely to favour paramorphoses. 

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 texture of polymeric chiral liquid crystalline phases. The chiral liquid crystalline phases include the chiral smectics and the chiral nematic or cholesteric phase. Poly(7-benzyl-L-glutamate) and derivatives of cellulose are popular examples of polymers that form a chiral mesophase. Side-chain type copolymers of two chiral monomers with flexible spacers of different, lengths and copolymers of one chiral and the other non-chiral mesogenic monomers may also form a cholesteric phase (Finkelmann et al., 1978 1980). In addition, a polymeric nematic phase may be transformed to a cholesteric phase by dissolving in a chiral compound (Fayolle et al., 1979). The first polymer that formed a chiral smectic C phase was reported by Shibaev et al. (1984). It has the sequence of phase transition of g 20-30 Sc 73-75 Sa 83-85 I with the Sc phase at the lower temperature side of Sa- More examples of Sc polymers are given by Le Barny and Dubois (1989). 

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