Isotropic phase, supercooling

The mesophase is locked-in indefinitely by simple cooling from the isotropic phase. Supercooling at Txn is from V-IO to 20 C. 

On heating from a crystalline phase, DOBAMBC melts to form a SmC phase, which exists as the thermodynamic minimum structure between 76 and 95°C. At 95°C a thermotropic transition to the SmA phase occurs. Finally, the system clears to the isotropic liquid phase at 117°C. On cooling, the SmC phase supercools into the temperature range where the crystalline solid is more stable (a common occurrence). In fact, at 63°C a new smectic phase (the SmF) appears. This phase is metastable with respect to the crystalline solid such phases are termed monotropic, while thermodynamically stable phases are termed enantiotropic. The kinetic stability of monotropic LC phases is dependent upon purity of the sample and other conditions such as the cooling rate. However, the appearance of monotropic phases is typically reproducible and is often reported in the phase sequence on cooling. It is assumed that phases appearing on heating a sample are enantiotropic. 

Note 1 The divergence temperature is the lowest limit of metastable supercooling of the isotropic phase. 

Drozd-Rzoska, A. (2009) Glassy dynamics of liquid ciystalline 4 - -pentyl-4-cyanobiphenyl (5CB) in the isotropic and supercooled nematic phase, J. Chem. Phys. 130, 234910... 

This can be also illustrated as follows a fatty man can be surrounded by 4 - 5 closely-packed fat men. But the same man can be surrounded even by 10 slim men. The latter can parallel the increasing dimensionality of space for the fat man. Consequently, the elongation of fluctuations due to the action of the strong electric field in the homogeneous phase of binary mixtures of limited miscibility is equivalent to increased dimensionality d = A. The uniaxial symmetry is natural also for the isotropic phase nematic liquid crystals. It may be considered for supercooled nitrobenzene due to intermolecular interactions (Fig. 5). ... 

ThLppjeesence of the solid phase will prevent the supercooling of a liquid (hence the ef cacy of inoculation or seeding ) and the presence of fh. .vapour phase will prevent the superheating of a liquid. Suspended transformation takes place more readily in the case of the passage of an isotropic into an anisotropic phase e.g. liquid into crystalline solid), where a definite arrangement of the molecules in a space lattice is necessary, than in the case of the transformation of one isotropic phase into another isotropic phase, where the molecules are in disarray. 

In the case of a supercooled liquid, spontaneous formation of nuclei or crystal growth centres also takes place. Since, however, an anisotropic phase, with definite molecular arrangement, must now be formed from an isotropic phase, the number of nuclei formed in a given time per unit mass of the substance will be less than in the case of transformations between isotropic phases. 

C. Similarity in Orientational Dynamics Between the Isotropic Phase of Liquid Crystals and Supercooled Liquids... 

Li et al. employed Eqs. (40) and (41) to fit the temperature-dependent OHD-OKE data on mesogens in the isotropic phase [91]. Equation (40) is identical to the one use in the analysis of the supercooled liquid data. The difference between the schematic model developed by Li et al. and the one applied to supercooled liquids is Eq. (41). The schematic mode coupling theory developed by Li et al. was found to be successful in reproducing the OHD-OKE data on three mesogens in the isotropic phase on all timescales and at all temperatures investigated [91]. 

In a recent work, the translational motion of 4- -hexyl-4 -cyanobiphenyl (6CB) was studied in the isotropic phase by atomistic molecular dynamics simulation [134], The mean-square displacement showed evidence of sub-diffusive dynamics, with a plateau that became very apparent at the lowest temperatures. A three-time self-intermediate scattering function revealed that this plateau was connected with a homogeneous dynamics that, at longer times, became heterogeneous and finally exponential. These features, which are shared by, for example, a high-density system of hard spheres, support the universal character of the translational dynamics of liquids in their supercooled regime. 

Because the isotropic-nematic transition is discontinuous, the correlation length does not diverge at the transition, however, it diverges at the supercooling limit of the isotropic phase. In the nematic phase ao = 3/4(1 + 1 — 80/9) and... 

This chapter contains the following sections Brief overview of rigid-flexible (RF) polymers Supercooling at the isotropic-mesophase transition Memory of thermal history in the isotropic phase and Memory of thermal history in the nematic-isotropic biphase and aging of the mesophase. 

S is no longer a local minimum and the energy barrier disappears. T is therefore the supercooling temperature below which the isotropic phase becomes absolutely unstable. At this temperature, 5i =8. AX.T-j = T - 2°C, there are two minima located at 52(> 0) and 53(< 0) (the minimum value is shghtiy below 0), and a maximum at = 0. 

On cooling the nematic phase can be supercooled more than 25. Smectogenic phases may exhibit a whole sequence of liquid crystal phases. An example of that is terephtal-dibutyl-aniline (TBBA). In addition to two crystalline modifications and the nematic (N) and isotropic phase (I) there are five different smectic phases. 

The positional order of the molecules within the smectic layers disappears when the smectic B phase is heated to the smectic A phase. Likewise, the one-dimensional positional order of the smectic M phase is lost in the transition to the nematic phase. AH of the transitions given in this example are reversible upon heating and cooling they are therefore enantiotropic. When a given Hquid crystal phase can only be obtained by changing the temperature in one direction (ie, the mesophase occurs below the soHd to isotropic Hquid transition due to supercooling), then it is monotropic. An example of this is the smectic A phase of cholesteryl nonanoate [1182-66-7] (4), which occurs only if the chiral nematic phase is cooled (21). The transitions are aH reversible as long as crystals of the soHd phase do not form. 

Thermotropic LCs can be further divided into (a) enantiotropic materials, in which the LC phases are formed on both heating and cooling cycles, and (b) mesotropic materials, in which the LCs are stable only on supercooling from the isotropic melt. Mesotropic LCs have been further divided into three groupings as follows ... 

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