Mesophase identification

X-ray scattering is a very powerful tool for mesophase identification and can provide a great deal of information it is often the only unequivocal means of phase identification. The experiment is rather simple in principle and relies on the faet that mesomorphic structures are periodic and can therefore diffract a schematic cartoon of the experiment is shown in Figure 2. 

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. ... 

The sign of S is positive when the brushes turn in the same direction as the rotated polarizers, and negative when they turn in the opposite direction. Mesophase identification by this procedure requires that... 

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 ... 

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. 

Identification of mesophases by microscopy is subject to a great deal of subjective interpretation. It is advisable, therefore, to use this method in conjunction with other types of characterization, such as X-ray diffraction or thermal analysis. 

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. 

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). 

The identification of mesophases usually involves three principal techniques namely hot-stage polarizing optical microscopy, differential scanning calorimetry and small angle X-ray scattering. 

The structures and corresponding calculated MI values are given in Table 7 (nos. 1-5). Mesophases were observed for scores of MI=12 and 13 but not for the odd structures (MI range 9.5-11). In addition, a series of imide-amide oligomers were synthesised with molecular weights above 1000, in which the terminal groups were nadic anhydride. Mis ranged from 8.3 to 10.7 but no positive identification of mesophases were made in any of these (Table 7, nos. 6-9)... 

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... 

Once a material is synthesized, it is necessary to establish which mesophases it forms and at what temperatures the transitions occur. Two techniques are used routinely in all laboratories for this purpose, namely polarized optical microscopy and differential scanning calorimetry, and it is important that these are used in conjunction with one another. In addition, the technique of X-ray scattering is often used to give unequivocal phase identification when microscopy cannot do so, and also to provide additional insights into the structures adopted. These three primary techniques will now be described in more detail. 

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). 

Hyde, S. T. Identification of Lyotropic Liquid Crystalline Mesophases. In Handbook of Applied Surface and Colloid Chemistry, Holmberg, K., Ed. Wiley Chichester UK, 2002 Chapter 16, pp 299-332. 

In this discussion at attempt will be made to describe in greater detail the structure and motion for a larger number of condis crystals. A special effort will be made to point-out the differences between condis crystals on the one hand, and liquid and plastic crystals on the other. It seems reasonable, and has been illustrated on several examples, that molecules with dynamic, conformational disorder in the liquid state show such conformational disorder also in the liquid crystalline and plastic crystalline states The major need in distinguishing condis crystals from other mesophases is thus the identification of translational motion and positional disorder of the molecular centers of gravity in the case of liquid crystals, and of molecular rotation in the case of plastic crystals. 

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. 

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