Mesophases isotropization

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)...

Synthesis of the phenolic bromide building block 40 was similar to the that of previously reported monomer 48 but the alkyl chain spacer unit was shortened in an attempt to increase the nematic mesophase isotropization temperature. 50 Construction of the related building blocks 41 and 42 followed similar pathways the latter were prepared to create hyperbranched macromolecules with a broader mesophase. 50 ... 

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... 

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...

Many mesophase isotropic solution transitions have been studied. Here we can only show a few examples. 

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... 

The thermal behavior of liquid crystalline main chain polymers is more complicated. Depending on thermal history, different transitions may occur. The DSC curves of samples quenched from the melt may show the glass transition (positive ACp), cold crystallization (exotherm), melting (endotherm)and the mesophase-isotropic liquid transition (endotherm)[ Fig. I6]. ... 

The polymers were formed by condensation of 4,4dihydroxy-benzene and 2,2 -dimethyl-4,4 dihydroxyazoxybenzene with various diacid chlorides acting as flexible spacer groups Polydisperse homopolymers and copolymers, sharp fractions of homopolymers and mixtures of polydisperse polymers with a low mass mesogen were investigated. Supercooling at the mesophase-isotropic and solid-mesophase transitions, sharpness of the nematic-isotropic transition (range of N+I biphase), polymer crystallization from the mesophase melt, and enhancement of crystallinity upon addition of a low mass nematic, were studied. 

BROOKS AND TA YLOR STRUCTURE IN THE CARBONACEOUS MESOPHASEreieis to the structure of the anisotropic spheres which precipitate from isotropic PITCH during pyrolysis. The structure of the spheres consists of a lamellar arrangement of aromatic molecules in parallel layers, which are perpendicular to the polar axis of the sphere and which are perpendicular to the mesophase-isotropic phase interphase. 

Fig. 4. Entropy change for the mesophase isotropic liquid transition versus number of carbon atoms in the alkyl groups of diesters.

Determination of maxima in the mesophase-isotropic curves for each binary combination was made either by microscopical observation of a Kofler contact preparation of the pure components using a Thomas micro hot stage or by a technique described previously using a Thomas-Hoover melting point apparatus. 

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. 

Table 2. Maxima in Mesophaser lsotropic Transition Temperature ( C) obtained by Contact Method for each mixture shown. A number indicates a Mesophaser Isotropic temperature higher than the Mesophaser Isotropic temperature of either pure component. All values refer to Smectic A Isotropic Transitions except those in parentheses which refer to Nematic "Isotropic Transitions.

Some chiral liquid crystals, as temperature is decreased, exhibit the mesophases isotropic cholesteric —> smectic-A. Because of the propoly shown by Equation (1.70), there is no... 

FIG. 4 Concentration dependence of the heat capacity jumps in the dimethyldodecyl-phosphine oxide-water system at the transitions lamellar mesophase + isotropic liquid isotropic liquid (curves 1 and 2) and crystal -I- isotropic liquid —> isotropic liquid (curve 3). (From Ref. 2.)... 

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