Isotropic-mesophase transition

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 LC nature of diols la-lg was demonstrated by DSC (Fig. 1). Two first order transitions were observed. The lower transition temperature appeared to be the crystalline-mesophase transition (T, ), and the higher transition temperature the mesophase-isotropic transition (T ). Other transitions were not evident in the DSC. The thermal data revealed an odd-even spacer effect for Tj . 

Fig. 15. DSC-trace of a mesophase macromolecule with flexible spacers in the main chain. Heating rate 50 K/min. Cooling rate 50 K/min. The mesophase glass crystallizes on heating at about 345 K. and melts then at about 385 K. The reversible mesophase-isotropic transition is at about 320 K. Drawn after data from Ref. 27)...

Fig. 6.1.3. Quadrupole splittings for the aromatic and a-aliphatic vj deuterons of deuterated hexa-n-hexyloxytriphenylene (THE6) as functions of temperature (r— 7 ) in the mesophase region, where 7 is the mesophase-isotropic transition point. The open circles correspond to measurements on neat THE6-ard, and THE6-adi2 separately, while the filled circles correspond to a 2 1 mixture of the two isotopic species. The scale on the upper right-hand side gives the orientational order parameter of the aromatic part. The curve at the bottom gives the ratio of the quadrupole splittings for the a-aliphatic and aromatic deuterons (Goldfarb, Luz...

Small molecule liquid crystals suffer a lowering of the mesophase-isotropic transition temperature upon lateral substitution, i,e. replacement of hydrogen on an aromatic ring. As can be seen from the compounds below the same trend is observed for both polymer and the chemically analogous small molecule. It is known that for small molecule mesogens the transition temperature, governed by the relation Tni - A H/AS, decreases upon lateral substitution primarily due to an entropic effect. The... 

Data in Tables 1 and 2 show maximum mesophase-isotropic transition temperatures for 1 2 and 1 3 binary combinations respectively. Temperatures in these Tables are given only if the mixed mesophase is more thermally stable than either of the pure components. It is seen that along a column or along a row there is a general trend for a maximum value of smectic-isotropic transition temperature to occur when the alkoxy chain lengths of both components are equal or nearly so. 

There is a wealth of information in Tables 1 and 2 and there are numerous ways in which the data could be presented. We have chosen to use four plots for this purpose (Fig. 1). In each of these plots we have considered one homologous series as the basis set (primary series) and then plotted, as a function of alkoxy chain length in this basis set, maximum mesophase-isotropic transition temperatures of binary mixtures of this primary series with selected members of the other (secondary) series - a short-, a medium- and a long-chained member of this second series. 

Figure 19. (a) Pressure-temperature diagram of benzene hexa-n-hexanoate. The mesophase-isotropic transition line extrapolated to atmosphere pressure yields a virtual transition temperature of 89 °C. (From Chandrasekhar et al. [63], reproduced by permission of Academic Press), (b) Miscibility diagram of benzene hexa-n-hexanoate and the heptanoate. The virtual mesophase-isotropic transition temperature for the hexanoate is 89 °C, in agreement with the value obtained from the pressure-temperature diagram (a). From Billard and Sadashiva [64], reproduced by permission of the Indian Academy of Sciences). 

The second hypothesis is based on morphological observations and kinetic studies of the growth of mesomorphic lamellae and the integral kinetics of the liquid-mesophase isotropic transition. According to the data from optical studies. 

NMR is not the best method to identify thennotropic phases, because the spectmm is not directly related to the symmetry of the mesophase, and transitions between different smectic phases or between a smectic phase and the nematic phase do not usually lead to significant changes in the NMR spectmm [ ]. However, the nematic-isotropic transition is usually obvious from the discontinuous decrease in orientational order. NMR can, however,... 

Note Mesophase X should be stable at lower temperatures than phase Y. For example, the wewai/c-isotropic transition temperature would be denoted as Tni. 

JVo Trans-lcis-isomer ratio in initial monomers Yield, % in 1 at 25°C, dl/g MxlO 3 Tmelb °c Mesophase-isotropic melt transition Tel °C... 

The homopolymer showed an enantiotropic nematic mesophase, whereas the diblock copolymer generated microphase-separated lamellae, in which the SCLCP block possessed a nematic-isotropization transition similar to the homopolymer (Table 17). Upon heating, the nematic microphase decreased continuously in the nematic phase from 38.5 nm to 27 nm and showed a constant value of about 26 nm after the nematic-isotropization transition. Therefore, materials in which these block copolymers are macroscopically aligned are expected to show reversible contraction in one dimension, making this polymer system an interesting candidates for an artificial muscle or actuator. 

Typical polarized optical micrographs of the molten samples were shown in Figure 1. Microscopic analyses were earned out at temperatures in the range 25 - 390 C, and threaded textures, a characteristic optical texture of nematic mesophase, were observed above LCITs over the investigated composition range. The nematic to isotropic transitions were not observed up to 390 C. 

The aspect ratio of the p-methoxycinnamate moiety was extended by synthesizing the phenyl ester of p-methoxycinnamate (see Scheme IV). Microscopy studies did not reveal any l.c. mesophase in monomer 9. However, the resulting homopolymer IX possessed l.c. order. Microscopy observations on polymer IX revealed a very stable homogeneous nematic texture. The DSC results showed a Tg at 21°C and a nematic to isotropic transition at 168°C. 

The polymers in the (4.2) series with different R groups all are noncrystalline but each form a nematic phase when heated to above Tg. The nematic phase is thermally very stable so that no endothermic isotropization transition is shown in the curve. Instead the signals for chemical decomposition will occur if the heating is continued to 300 °C or above depending on R. In such cases the characterization of the mesophase relies more on POM and X-ray scattering. 

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