Identification of mesophases

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. 

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)... 

IDENTIFICATION OF MESOPHASES EXBIBITED BY THERMOTROPIC LIQUID CRYSTALLINE POLYMERS... 

S and N) but this is not always so (e.g. and may be confused). X ray patterns of polymers with mesogenic moieties and flexible spacers in the main chain are often too diffuse to be of help for identification of mesophases, especially in the case of unoriented... 

X-ray diffractometry, polarization microscopy and mixture testingt are important in the identification of mesophases of liquid crystals while DSC and DTA are the most valuable aids in revealing and confirming phase transition. Simultaneous polarization microscopy-DSC instruments are now commercially available. 

Other techniques used to identify the stracture of mesophases and liquid ciystalline mesophases include neutron scattering studies (usually of partially deuteriated samples) and nnclear magnetic resonance (NMR) studies (particularly useful for analysing lyotropic liquid ciystal phases). However, this introductoiy chapter will concentrate on the most simple and most commonly nsed techniques for the identification of mesophases. 

X-ray, electron scattering and thermal analysis remain the main tools in analyzing liquid crystalline order in condensed polymeric materials. The effectiveness of optical microscopy and of the compatibility rules formulated by Sackmann and Demus a decade ago and widely used in identification of mesophases formed by low molecular compounds (41, 42) is questionable in the case of... 

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... 

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). 

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. 

Continued investigations led to the identification of three main types of mesophase a smectic state smegma in Greek, meaning soap), a nematic state nema in Greek, meaning thread), and a cholesteric state observed in systrans containing molecules with a chiral craitCT. 

Hyde, S.T. (2001) Identification of lyotropic liquid-crystalline mesophases, in Handbook of Applied Surface and Colloid Chemistry (ed. K. Holmberg),... 

Each CPP was separately solubilized within the Hn for identification of its specific location and chemical interactions within the mesophase. The SAXS measurements indicated two trends in the effect of the peptides. Up to 2 wt% of PEN solubilization and initial shrinkage of the aqueous cylinders to a minimum of 53.4 0.5 A was detected. With increasing solubilization loads of PEN to 5 wt%, the shrinkage changed, and a swelling effect of the cylinders was detected until their diameter reached their original radius of 57.9 0.5 A (Fig. 12.7). 

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