Melting and isotropization temperatures

Originally. TOA was devised and used for analyses of chain mobility in polymers (7) and polymer blends (8.91 by monitoring birefringence disappearance during programmed heating in scratches scribed on film surfaces. Lenz (10.Ill has reported the use of TOA in the study of liquid crystal polymers, and recently we have used the technique in determining melt- and isotropization temperatures for thermotropic fully aromatic liquid crystal polymers (4.51. 

The mesomorphic properties of hydrogen-bonded mesogens have been compared [40] with those of covalently bonded compounds 16 and 17 [42] (Fig. 6 and Table 2). The melting and isotropization temperatures of the complexes are lower than those of ester compounds, which leads to wider and lower mesomorphic temperature ranges [40]. Moreover, smectic mesophases are observed for fluorinated H-bonded complex 15. 

Table 12. Representation of some typical structures of polyesters containing the flexible spacer -(CHjlio-and their melting and isotropization temperatures.

With respect to the unsubstituted polymer, the introduction of a methyl group reduces both the melting and isotropization temperatures by about 80 °C. An increase in the alkyl chain further reduces the transition temperatures, which gradually level off and reach their saturation values corresponding to the polymer homolog with n = 5. 

TABLE 2.16 Melting Point, Tm (°C), and Isotropization Temperature, I) (°C), of Poly(l,4-phenylene terephthalate) LCPs Containing Various Substituents... 

All of the present polyesters are thermotropic and formed nematic liquid crystalline phase upon melting. The isotropization temperatures, Ti, of the ordered sequence... 

Fig. 22. Dependence of the glass transition temperature T, melting point and isotropization temperature T, on the number of phenyl groups n in cycles of cyclolinear polymer Horizontal solid lines indicate the corresponding transitions for PDPhSandPDMS...

FIG. 1 Thermogram of decylammonium decyl sulfate (DeADeS) obtained by differential scanning calorimetry during heating (full line) and cooling (dashed line) scans. T, represents polymorphic transition, and T, represent the temperatures of melting and isotropization, while r and represent the temperatures of deisotropization and crystallization. The lettering a-f denotes the temperatures of diffraction patterns shown in Fig. 3. 

Glass is the name given to any amorphous solid produced when a liquid solidifies. Glasses are non-crystalline and isotropic, i.e. their physical properties are independent of the direction in which they are measured. When a glass is heated, it does not melt at a fixed temperature but gradually softens until a liquid is obtained. 

In the case of monotropic behavior, the isotropiza-tion endotherm and the corresponding thermodynamic parameters for the mesophase-isotropic transition can be obtained by isolating the mesophase when cooling from the melt and holding the temperature in a region where the transformation into the crystal is very slow... 

The second consideration, anticipated in Sect. 3.1, is that whereas Sm and Sc increase little with temperature (see Fig. 8) so that in the literature the temperature dependence of Gc and Gm is often approximated by straight lines, Sl varies moderately up to the glass transition temperature Tg but will increase substantially between Tg and the isotropization temperature. The behavior of Sl is due to the fact that, as the temperature increases, polymer chains are progressively more likely to bend sharply in the melt, whereas they are forced to remain straight in the mesophase and in crystals. The downward curvature of Gl in Fig. 8 shifts to lower temperatures with inherently more flexible chains. With very flexible polymers Tml becomes therefore smaller than the crystal-melting temperature Tcl and a stable mesophase cannot form. 

Figure 7.1 Illustration of different aggregation states obtained (from left to right) by increasing temperature crystal (K), smectic C (SmC), nematic (N) and isotropic (I). Row a shows macroscopic appearance of samples in row b, short-range microscopic ordering is represented (each bar represents a molecule) thermotropic phase diagram of row c illustrates relevant transition temperatures (Tm melting temperature Tsmc-N transition temperature between SmC and N Tc clearing temperature) row d shows different texture of different states as seen through polarizing microscope (with crossed polars, isotropic phase appears black).

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