Clearing-point temperature

Figure 13.17 Alternation of clearing point temperature (°C) with alkyl chain length (n) for compounds of type 13.14.

Table 3 shows that the clearing point temperatures are lower for the optically active 1-methylheptyl 4 -(4-n-alkoxyphenylpropioloyloxy)biphenyl-... 

The Tgs of LCPs with methacrylate backbones are 30 to 60 C higher than those of polymers with acrylate backbones however, clearing point temperatures are much less sensitive to changes in backbone structure. 

Figure 9. Shear viscosity coefficients r/i, t 2 and t)3, rotational viscosity coefficient y, and isotropic shear viscosity coefficient as a function of temperature for the liquid crystal Nematic Phase V. T, Clearing point temperature.

Figure 12. Apparent viscosity measured in a capillary as a function of temperature for 4-ethoxybenzylidene-4 -n-butylaniline (20 -4) and the homologous pen-toxy compound 50 4 in the nematic and isotropic phases. T, Clearing point temperature.

On the whole, the correlations that are made concern structure versus phase transition phenomena. In particular we shall concentrate on the identity of mesophases that are formed, the persistence of the mesophase region (thermal stability of the mesophase) and the glass-transition and clearing-point temperatures (Tg and 7 ). But other properties are important also and we shall look at a few correlations between structure and, for example, thermodynamic quantities and dielectric relaxation phenomena. 

The conventional liquid state is described as the isotropic phase. The temperature at which the compound passes from the solid phase into a mesophase is described as the melting point and the transition temperature between a mesophase and an isotropic liquid is described as the clearing point. 

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). 

A closer look at the thermal behavior variation upon introduction of a second aryl ring (see Figure 8.5 for the behavior of the derivatives with a w-decyloxy chain) reveals very interesting features for the phenyl isocyanide complexes the melting and clearing temperatures decrease in the order Cl > Br > I. This is also the trend of the clearing points for biphenyl isocyanide complexes, but their melting temperatures follow the opposite trend that is, I > Br > Cl. 

Liquid crystals based on aliphatic isocyanides and aromatic alkynyls (compounds 16) show enantiotropic nematic phases between 110 and 160 °C. Important reductions in the transition temperatures, mainly in clearing points (<100 °C), areobtained when a branched octyl isocyanide is used. The nematic phase stability is also reduced and the complexes are thermally more stable than derivatives of aliphatic alkynes. Other structural variations such as the introduction of a lateral chlorine atom on one ring of the phenyl benzoate moiety or the use of a branched terminal alkyl chain produce a decrease of the transition temperatures enhancing the formation of enantiotropic nematic phases without decomposition. 

The main result of three papers respectively by Holland and Miller [91] and by Chau and Geil [96,97] is that the polymorph which results in crystallization at or close to r.t. of solutions of iPBl depends on the maximum temperature reached by the solutions (TMs), or, more precisely, on the temperature at which the solution has been equilibrated last, irrespective of the polymorph of the initial crystals. For example if samples of any of the three crystal forms are dissolved at concentrations of 0.01-0.03% in amyl acetate (clearing point 65 °C) and brought to Tms of 90 °C, kept there for 3 to 48 hours (ts) and then crystallized at temperatures (Tx) between 24 and 50 °C, then 100% form I crystals are obtained. With the same procedure but using a Tms of 120 °C, 100% of the chiral form III crystallizes. For shorter values... 

It is now time to draw all the threads together, and look at the temperature at which the thin lines intersect. It is clear from Figure 5.18 that the intersection temperature for the mixture occurs at a cooler temperature than that for the pure material, showing why the melting point temperature for a mixture is depressed relative to a pure compound. The depression of freezing point is a direct consequence of chemical potentials as defined in Equation (5.12). 

PFO 196 is clearly crystalline with a melting point temperature around 150°C, above which a nematic mesophase exists up to ca. 300°C. Nanoscale crystallinity of PFO 196 was demonstrated by x-ray diffraction (XRD) experiments (Figure 2.8) [236,237], For the crys-... 

Mesophase that is thermodynamically stable over a definite temperature or pressure range. Note The range of thermal stability of an enantiotropic mesophase is limited by the melting point and the clearing point of an LC compound or by any two successive mesophase transitions. 

Clearing point the temperature at which the mesophase transforms into an isotropic fluid. 

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