Mesophase monotropic

Apart from the parent compound 1 and its very simple alkyl derivatives, 1,3,4-oxadiazoles are solids. Solid oxadiazoles containing biphenyl or triphenyl substituents exhibit interesting properties upon heating. The symmetric 2,5-bisbiphenyl-4-yl-l,3,4-oxadiazole 38 melts into an isotropic phase showing small monotropic mesophase. By contrast, the asymmetric (hockey stick-shaped) mesogen 2-terphenyl-4-yl-5-phenyl-l,3,4-oxadiazole 39 exhibits a more stable enantiotropic liquid crystalline phase (a smectic phase as well as a nematic phase) <2001PCB8845>. 

Note 4 A monotropic mesophase can be stabilised in a mixture when, as a result of melting-point depression, a metastable mesophase becomes stabilised. Such a mesophase is distinct from an induced mesophase. 

A few series of azo and azoxy group containing liquid crystalline copolyesters were prepared by limura and coworkers and their phase transition temperatures were examined. No unusual phenomena were found, although monotropic mesophases were observed for the following copolyesters, depending on the combinations and fractions of alkylene groups ... 

Figure 10. Transformation of a virtual or monotropic mesophase of the monomeric structural unit (M ) into an enantlotroplc mesophase by Increasing the degree of polymerization. The upper part (a) describes the influence of molecular weight on the dependence between the free energies of the crystalline (G ), liquid crystalline (G ) and isotropic (G.) phases and transition temperatures. The translation of this dependence into the dependence phase transition temperature-molecular weight is shown in the lower part (b).

Case 2. The Monomeric Structural Unit Displays a Virtual or a Monotropic Mesophase The Polymer Displays a Monotropic or an Enan-tlotropic Mesophase... 

Simon and co-workers have also studied phthalocyanines with crown-ether moieties attached to the periphery, giving the materials shown in Figure 71 [135]. These compounds were reported as having a monotropic mesophase at 150°C (although melting points were not given, nor was it all clear whether 150°C represented a transition temperature) and X-ray scattering studies, showed the mesophase to be based on a two-dimensional square lattice (lattice parameter... 

Another interesting phenomenon found by Stevens et al. is the monotropic mesophase formed by the dimer and trimer of the polymer with m = 6. The dimer has a monotropic nematic phase, the trimer has a monotropic smectic phase. These metastable monotropic phases become stable enantiotropic phases with the increase of n by 1. At about the same time, Blumstein et al. (1984) found the low mass model compound of a main-chain type liquid crystalline polymer was monotropic while the mesophase of the polymer was enantiotropic. 

The incorporation of lateral chains in the series of complexes 30 had dramatic effects on the mesomorphism [59]. Indeed, almost all the complexes displayed only monotropic mesophases, essentially nematic, with a few only showing Sc phases. The only enantiotropic mesophases were obtained for m=l for both Ni and Cu complexes (Crys 239 Sc 267 N 281 I and Crys 188 Sc 243 N 267 I, respectively). 

Table 4. Phase transition temperatures (°C) of selected alkane-1,2-diols, 7a-h [78, 79]. rci=clearing temperatures of these compounds saturated with water (50 wt%), = monotropic mesophases.

Barbera et al. prepared a series of thallium(I) complexes, in which the position and number of side chains were changed systematically ((58) M = Tl). A minimum of four chains was required to induce mesomorphism in these systems (Table 37), and on the basis of X-ray diffraction, the monotropic mesophases were identified as Colii. The formation of the columns was due to the association of these half-disk compounds into dimers, and to their subsequent stacking. The dimeric nature of these thallium(I) complexes was confirmed by the analysis of the single-crystal structure of the non-mesomorphic dimethoxy analog. In the crystalline phase, the co-planar... 

Cr, crystalline phase SmA, smectic A phase Monotropic mesophase. SmX, unidentified smectic phase 1, isotropic liquid. ... 

In a new twist on the theme, double hydrogen bonds were utilized for the construction of the structure shown in Figure 3.14 [20]. The pyridine components by themselves are non-mesogenic. A monotropic mesophase is obtained in the equimolar complexes. The temperature range of the mesophase can be increased by preparing equimolar copolymeric complexes combining both pyridine components (m = 3 and m = 6) with the polymer. 

THF and chloroform. It was found from measurements using a polarizing optical microscope that PCBT and PPCHT exhibit fan-shaped and polygonal textures, respectively, in the cooling process, indicating monotropic mesophases. The typical fan-shaped texture of PCBT is shown in Figure 12.1. Furthermore, analysis of the X-ray diffraction patterns and molecular mechanics calculations demonstrated that the LC phase observed in the polymers is assignable to a smectic A2 phase. 

Case 2. The structural unit displays a virtual or a monotropic mesophase the polymer displays a monotropic or an enantiotropic mesophase. The slope of the Tic-c versus molar mass (M) is steeper than that for Tc i, and as a consequence there will exist a critical value of M below which a mesophase is not observed. This effect has been observed in main chain and side chain liquid crystalline polymers. 

Mesophases may be enantiotropic or monotropic. While enantiotropic mesophases are stable and exist above the melting temperature of the crystalline phase, monotropic mesophases are metastable and occur below the melting point of the crystalline lipid. Thus, they are formed upon cooling the isotropic melt. 

Let us first consider very briefly the influence of various parameters (i.e., nature of flexible spacer and its length, nature and flexibility of the polymer backbone and its degree of polymerization) on the phase behavior of a side chain liquid crystalline polymer. According to some thermodynamic schemes which were described elsewhere, the increase of the degree of polymerization decreases the entropy of the system and therefore, if the monomeric structural unit exhibits a virtual or monotropic mesophase, the resulting polymer should most probably exhibit a monotropic or enantiotropic mesophase. Alternatively, if the monomeric structural unit displays an enantiotropic mesophase, the polymer should display an enantiotropic mesophase which is broader. It is also possible that the structural unit of the polymer exhibits more than one virtual mesophase and therefore, at high molecular weights the polymer will increase the number of its mesophases. All these effects were observed with various polymer systems. ... 

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