Monotropic liquid crystal

Bashir et al. (1998), in a study of phase transitions in a monotropic liquid-crystal polyester, found that PC and HF instruments performed differently in cooling mode. Better resolution of thermal events (i.e., better separation of peaks on the thermogram) was obtained with PC instruments. These authors pointed out that the performance of any DSC instrument might not be the same during heating as during cooling. 

Bashir, Z., Khan, N., Price, D.M. 1998. Transition-temperature differences on cooling for a monotropic liquid-crystal polyester observed in DSCs of different design. Thermochim. Acta 319, 47-53. 

Figure 2. Schematic plot of the free energy diagram illustrating the origin of a monotropic liquid crystal (see text). Here, as in Figure 1, the arrows represent cooling and heating pathways.

In order to study the general applicability of the above principle for the transformation of the monotropic liquid crystals to the enantiotropic ones by increasing the molecular weight of the monotropic liquid crystal polymers, the polymer, (3.16), reported by Ober et al. (1983), was restudied (Duan et al. 1987). 

The data obtained for 58 ( ) and 59 further emphasize the influence of the substitution pattern on mesomorphic properties. Whereas a two-aromatic ring system (plus the Cp nucleus) was found to be non-mesomorphic for 1,3-disubstitution,introduction of an additional substituent on the second Cp ring gave monotropic liquid crystals 58 ( ). This result indicated that the repulsive interactions induced by the Fc core are compensated, at least in part, by the third substituent. This allows both substituted Cp rings to interact favorably, which results in mesomorphism promotion. In a structure such as 59, the depth (bulkiness) of Fc (about 3.3 A) is, in a way, hidden by the substituents anchored on the two Cp nuclei. Therefore, the 1,1 -3-trisubstitution pattern is a stronger mesomorphic promoter than the 1,3-disubstitution one (for identical substituents). This is supported by the fact that 46( =16), the 1,3-disubstituted counterpart of 59, gave a much narrower liquid-crystalline range (42 °C) than 59. 

Figure 6.3 The Gibbs function diagram for a monotropic liquid crystal. represents the equilibrium crystal, and Gy a possible metastable crystalline form. The equilibrium pathway on heating (marked by arrows) leads to direct isotropization at Ty. Heating along pathway Gy gives a metastable enantiotropic behavior, with a mesophase at Ty and isotropization at T,ci. (Redrawn from data in [4])...

Most solid materials produce isotropic liquids directly upon melting. However, in some cases one or more intermediate phases are formed (called mesophases), where the material retains some ordered structure but already shows the mobility characteristic of a liquid. These materials are liquid crystal (LCs)(or mesogens) of the thermotropic type, and can display several transitions between phases at different temperatures crystal-crystal transition (between solid phases), melting point (solid to first mesophase transition), mesophase-mesophase transition (when several mesophases exist), and clearing point (last mesophase to isotropic liquid transition) [1]. Often the transitions are observed both upon heating and on cooling (enantiotropic transitions), but sometimes they appear only upon cooling (monotropic transitions). 

The liquid crystal properties of the complexes were characterised using polarised optical microscopy and showed a nematic phase for n = 4 and 6 and a SmA phase for n = 6, 8, 10 and 12. The mesophases were monotropic for n = 4 and 6 and enantiotropic for the others the progression from a nematic phase for shorter chain lengths to SmA at longer chain lengths is quite typical for simple, polar mesogens. 

Examination of the thermal behaviour showed that with three exceptions, all complexes showed a monotropic SmA phase with in almost all cases, melting being observed between 88 and 99 °C, with clearing between 82 and 89 °C. Of the three exceptions, 15-6,8 and 15-8,10 showed no liquid crystal phase at all, while 15-12,6 showed an additional monotropic nematic phase. A curious feature of these complexes is the apparent insensitivity of the melting and clearing points to both n and m. 

Liquid crystals, as the name implies, are condensed phases in which molecules are neither isotropically oriented with respect to one another nor packed with as high a degree of order as crystals they can be made to flow like liquids but retain some of the intermolecular and intramolecular order of crystals (i.e., they are mesomorphic). Two basic types of liquid crystals are known lyotropic, which are usually formed by surfactants in the presence of a second component, frequently water, and thermotropic, which are formed by organic molecules. The thermotropic liquid-crystalline phases are emphasized here they exist within well-defined ranges of temperature, pressure, and composition. Outside these bounds, the phase may be isotropic (at higher temperatures), crystalline (at lower temperatures), or another type of liquid crystal. Liquid-crystalline phases may be thermodynamically stable (enantiotropic) or unstable (monotropic). Because of their thermodynamic instability, the period during which monotropic phases retain their mesomorphic properties cannot be predicted accurately. For this reason it is advantageous to perform photochemical reactions in enantiotropic liquid crystals. 

Depending on temperature, transitions between distinct types of LC phases can occur.3 All transitions between various liquid crystal phases with 0D, ID, or 2D periodicity (nematic, smectic, and columnar phases) and between these liquid crystal phases and the isotropic liquid state are reversible with nearly no hysteresis. However, due to the kinetic nature of crystallization, strong hysteresis can occur for the transition to solid crystalline phases (overcooling), which allows liquid crystal phases to be observed below the melting point, and these phases are termed monotropic (monotropic phases are shown in parenthesis). Some overcooling could also be found for mesophases with 3D order, namely cubic phases. The order-disorder transition from the liquid crystalline phases to the isotropic liquid state (assigned as clearing temperature) is used as a measure of the stability of the LC phase considered.4... 

A thermotropic liquid crystal (mcsogen) is a compound that, on heating the crystal or on cooling the isotropic liquid, gives rise to mesomorphism. Liquid crystallinity occurs between the crystal and isotropic liquid states. The intermediate phases, or mesophases, can be either enantiotropic, i.e., thermodynamically stable, or monotropic, i.e., thermodynamically unstable. The solid to mesophase transition is referred to as the melting point, while the mesophase to isotropic liquid transition is referred to as the clearing point. 

Interesting liquid crystal properties resulted from the Schiff-base derivatives 11 [18]. All compounds exhibited mesomorphic behavior (Fig. 9-6). The first members of the series gave rise to enantiotropic nematic phases and the long chain derivatives exhibited enantiotropic smectic A phases. The intermediate chain length derivatives presented monotropic nematic or smectic A phases. 

None of the ferrocene derivatives 13 showed liquid crystal properties on heating. They all melted into an isotropic melt. When cooled from the isotropic liquid, the first members of the series [n = 1 — 6) exhibited a monotropic nematic phase. A representative example of a nematic schlieren texture is shown in Fig. 9-10. 

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. 

While it was assumed above that only G. is affected by thermal history, in the case of main chain polymeric liquid crystals pronounced time dependent variability in G c has recently also been observed (7,8). It was shown that the lack of equilibrium perfection in the nematic phase can lead to substantial depression of the isotropization temperature T c =T. Thus non-equilibrium mesomorphic states can also, in principle, affect the phase sequence-(enantiotropic, monotropic) in the case of polymeric liquid crystals. 

A central part of the application-oriented evaluation of liquid crystals are so-called virtual clearing temperatures, electrooptic properties, and viscosities. These data are obtained by extrapolation from a standardized nematic host mixture. 7 Af, An, and jy are determined by linear extrapolation from a 10% iv/iv solution in the commercially available Merck mixture ZLI-4792 (Tfji = 92.8°C, Af = 5.27, An = 0.0964). For the pure substances the mesophases are identified by optical microscopy and the phase transition temperatures by differential scanning calorimetry (DSC). The transition temperatures in the tables are cited in °C, numbers in parentheses denote monotropic phase transitions which occur only on cooling the sample C = crystalline, S = smectic A, Sg = smectic B, S = smectic G, S> = unidentified smectic phase, N = nematic, I = isotropic. 

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