Identification of the Mesophases

The liquid crystalline phases are sometimes difficult to identify unequivocally, but several techniques can be used to provide information on the nature of the molecular organization within the phase. If used in a complementary fashion, these techniques can provide reliable information on the state of order of the mesogenic groups. 

FIGURE 11.12 Photomicrographs of the nematic Schlieren texture that can be observed using a polarizing microscope. (Reproduced from Noel, C., Synthesis characterization and recent developments of liquid crystalline polymers, Makromol. Chem. Macromol. Symp., 22, 95, 1988. With permission.) 

The SchhCToi textures show large dark brush patterns, corresponding to the extinction zones where the mesograis arc ahgned perpendicular to the glass slide. Also noticeable are the points where two or four of these brushes meet if the texture shows points where only two brushes meet, this is an unambiguous indication of a nematic phase. It should be noted that if a Schheren texture shows only points where four brushes meet, then this represents a smectic C texture, or its chiral modification. 

The chiral nematic phases can show a planar Grandjean textnre, with oily streaks caused by defects, but they can also show strong reflection colors, depending on the pitch of the helical structure within the phase. 

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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). 

In the following Sections it will first be discussed that there is a good chance that, particultirly for different condis crystals, a full spectrum of increasingly more cooperative transitions may be possible. Conformational disorder that can be introduced without much cooperation from neighboring groups appears gradually, without first order transition. Such condis states are difficult to identify by thermal means, and microscopic technique must be used for the identification of dynamic disorder. In the major body of the review. Sections 3-6, many condis crystal examples of small molecules and macromolecules have been treated in sufficient detail to allow clear identification of the mesophases, or at least permit an educated guess of the phase-nature. 

Liquid crystal polymers exhibit the same liquid crystalline phases and mesophases exhibited by low molar mass mesogens. However, the identification of the mesophases generated by polymers is usually far more difficult than for low molar mass materials (see Chapter 9). Usually, the nematic phase is readily characterised but smectic phases, especially the highly ordered analogues, are often uncharacterised and simply denoted S Many liquid crystal polymers, like conventional polymers, exhibit a glass... 

The study of blends of polymeric liquid crystals with low-molecular liquid crystals of known mesophase types, aiming at identification of polymeric mesophases, is at its very beginning there are only a few works concerning polymers with mesogenic groups in the main chain 67 69) and in the side chains as well 70 74). in view of the importance of such investigations, note that the principle of miscibility is thoroughly developed for low-molecular liquid crystals, whose molecules are similar in their sizes the justifiability of its application to the blends of polymers with low-molecular liquid crystals is not equally evident, as the molecular sizes of the components differ substantially. 

Identification of the molecular structures most conducive to the formation of needle coke or the spinning of mesophase fiber is a task on which Japanese workers have been particularly active... 

The mesophase exhibiting a homeotropic texture can still shew stir opalescence, which is a method of identification particularly suited for thermotropic polymers. This somewhat crude method of characterizing a liquid crystalline material is performed by shearing a thin film of the mesophase and looking for momentary appearances of turbidity in the otherwise transparent melt. No microscope is need to observe stir opalescence, but simple shearing of the homeotropic melt between crossed-polars can also reveal the mesophase. 

Structures. The three types of mesomorphic phases are usually identified by using the polarizing microscope (11). The different types of lyotropic smectic mesophases can also be distinguished through their microscopic textures (26) by careful and detailed study. Using the microscope for such identifications is sometimes made more difficult by the tendency of the mesophases to orient when placed in contact with a surface. This orientation, of course, changes the microscopic textures. A mesophase in the presence of a crystalline phase may also be difficult to identify by microscopy. NMR can provide distinguishing characteristics which are unique for several mesophases. The use of the NMR characteristics for phase... 

Miscibility is a very useful method often used in conjunction with microscopy. The simplest use of the technique is to bring two materials together on the cover slip in their mesophases in a contact preparation the identity of the mesophase of one of the materials should already be known. If the two materials are co-miscible, then both have the same mesophase (at the temperature in question) and this can then be a useful method of phase identification. Unfortunately, if the two materials are immiscible then no information is obtained as two materials in the same phase are not necessarily miscible (e.g., water and chloroform which are both isotropic). 

Cho and Lim compared the effect of a variety of lateral substituents on the thermal behavior of peripherally octasubstituted, metal-free phthalocyanines and their copper complexes. Despite being an interesting study, the characterization of the mesophases was tentatively made by optical microscopy and thus some doubts concerning mesophase identification still persist. The results of this study are gathered in Table 8 along with some metal-free and copper complexes discussed above to allow comparison. The free base with a chiral chain exhibited a texture resembling that of the cholesteric phase, whereas that of the copper complex was not identified. Compounds substituted with chiral chains were room-temperature liquid crystals, whatever the length and number of asymmetric carbon atoms, and the columns described a helical twist. ... 

Between crossed polars these defects appear as dark lines or brushes with curved or irregular shapes that correspond to extinction positions of the director and molecular long axes. Thus, the director can be either parallel or perpendicular to the polarizer and analyzer. The brushes tend to cover the specimen in rather a continuous way, indicating the liquid-like nature of the mesophase. The points where the brushes meet are called singularities in the texture (see Figure 3A). For nematic phases two forms of schlieren defect are found, one where two brushes meet at a point and one where four brushes meet. All tilted smectic phases (C, I, F, and ferrielectric C), except for the antiferroelectric phase, exhibit four brush singularities. Therefore, this provides a simple way of distinguishing between smectic and nematic phases. It should be noted that phases such as smectics A and B(hexatic) and crystal phases B(crystal), E, G, H, J, and K do not exhibit schlieren textures and so this narrows down the possibilities for phase identification. 

Other smectic mesophases are much less commonly encountered than the and phases which is, to some extent, a reflection on the relative emphasis of research on and materials, especially for ferroelectric devices. Accordingly, the identification of the and phases has been emphasised. However, other smectic phases (see Chapter... 

If a transition between mesophases has been missed by optical microscopy, then DSC may reveal the presence of a transition at a particular temperature or vice versa. After DSC, the material should be examined very carefully by optical microscopy to provide information on phase stmcture, to check that transitions have not been missed by DSC and hopefully to lead to the likely identity of the mesophases. Accordingly, optical polarising microscopy and differential scanning calorimetry are significant, complementary tools in the identification of the types of mesophases exhibited by a material. 

A major question in the liquid crystal field relates to the identification of the various mesophases. Several methods may be used (26). 

Identification of the nematic mesophase through polarized light microscopy is easier than that of the smectic mesophase, because specific defects take a linear form. The nematic mesophase is characterized by a large number of textures Schilieren texture is one of the most common nematic textures with defect centers with two arms, nematic droplets often occur from the isotropic liquid in the form of drops, string texture consists of a disclination type - line and appears as thin lines, and marble texture consists of several zones with different orientations of the director, inducing different color interferences [5]. 

In principle, one could generalize the considerations of Section 7.2.4 to a study of block copolymer mesophase ordering, by suitable identification of the order parameter of the transition. For the lamellar mesophase (Fig. 7.2), this order parameter is written in terms of concentration waves for the local concentration of component A, for instance... 

The structures of the various lyotropic mesophases mentioned so far have been elucidated over the years primarily using low-angle X-ray diffraction. An X-ray diffraction pattern of a liquid crystal provides information not only on the state of organization of the hydrocarbon chains but also on the crystallographic lattice of the micellar structure. It must be emphasized, however, that often the X-ray method alone cannot define the absolute structure of a liquid crystal phase because too few diffraction lines are observed. In these cases, a knowledge of the position and extent of the mesophase region in the phase diagram, measurements by other techniques (NMR, optical microscopy), and information such as the size, shape and chemical nature of the surfactant are necessary before a reliable identification can be made. 

For spin-f nuclei, dipolar interactions may be modulated by intramolecular (DF, reorientation etc.) and/or intermolecular (TD) processes. In general, the intra- and inter-molecular processes can produce quite different Tj frequency dispersion curves. In practice, NMR field cycling experiments are often needed to extend the frequency domain from those employed in conventional spectrometers to a lower frequency range (i.e., the kHz regime) for unambiguous separation (and identification) of different relaxation mechanisms. The proton spin relaxation by anisotropic TD in various mesophases has been considered by Zumer and Vilfan.131 133,159 In the nematic phase, Zumer and Vilfan found the following expression for T ... 

Thus, these mesostructures are predominantly lamellar, and identified as conventional (parabolic) lamellar phases, although they may in fact be hyperbolic. Indeed, unless v/al is exactly unity, a planar interface (lamellar mesophase) incurs a bending energy cost hyperbolic sponge monolayers or bilayers or mesh monolayer mesophases are favoured if v/al differs from unity. It is likely then that many "lamellar"" phases in fact adopt a hyperbolic geometry. Careful neutron-scattering studies of a lamellar phase have revealed the presence of a large number of hyperbolic "defects" (pores within the bilayers) in one case [16]. (An example of this mis-identification of hyperbolic phases in block copolymers is discussed in section 4.10.)... 

Random copolyesters based on bromoterephthalic acid, methyl hydroquinone, and hexane diol have been synthesized. Their mesophase properties were studied by differential scanning calorimetry, optical microscopy, realtime X-ray diffraction and melt rheology. At low molecular weight these copolymers exhibit triphasic behavior, where two mesomorphic phases coexist with an isotropic phase. Fractionation based on solubility in THF enables the identification of two components. Simple statistical arguments are employed to model the polymerization reaction and account for the observed phase behavior. 

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