Phase transition isotropic-cholesteric

To understand the cholesteric-isotropic phase transition regarding interactions and models of the molecules in these two phases. Clearly experiments with changes in temperature, concentration, substituents, solvents, and molecular mass will be very helpful in achieving these goals and may serve as tests for the proposed ideas and models. 

Chiral dithienylcyclopentene compounds can also be used to induce the cholesteric-isotropic phase transition [36]. A cholesteric polygonal fingerprint texture was exhibited by 10 wt% of 2 as a mesogenic dopant in a conventional achiral nematic 5CB as shown in Figure 5.6a. The cholesteric phase to isotropic transition temperature for the doped 5CB was 42 C. With UV irradiation at 310 nm (30 mW cm ) for 30 s, the sample went into the isotropic phase (Fig. 5.6b) whereas upon visible fight irradiation at 670 nm a reverse process was reached within 30 min (Fig. 5.6c). 

In support of this interpretation, the point of intersection between the phase dependent Arrhenius slopes occurs at a temperature which is several degrees lower than the macro-scopically observed cholesteric to isotropic phase transition... 

Cellulose triacetate-trifluoroacetic acid cholesteric solutions - This kind of lyotropic polymer liquid crystals undergoes a mesomorphic-isotropic phase transition upon heating. The peak is well defined but very small The determination of N for this... 

There are of course many open questions and further possibilities in the field. Some specific points were emphasized in the text. It should be remarked that up to now most researchers concentrated on the nematic phase. Although there are still many important aspects to be investigated even in this phase, the study of other mesophases looks very promising as well. We called attention already to the problem of optical reorientation in the cholesteric and smectic C phases. Regarding thermal effects we remind that the interesting point about nematics is the nearly critical behaviour near the nematic - isotropic phase transition. Similar phenomena can be expected to take place at other second-order phase transitions such as the smectic A - smectic C or some of the nematic- smectic A transitions. 

The physics of liquid crystals [97] indicate that the viscosity passes through a maximum at the point of tire transition from the isotropic to Ae LC state as a result of ordering of the object and cooperative orientation in flow for nematic systems and a large number of cholesteric systems. The existence of a maximum is usually found in flow of a liquid crystal with an uncontrollable orientation. If the coefficients corresponding to orientation of the macromolecules parallel to the direction of the velocity gradient (%) and parallel to the velocity vector (t] ) are separated out, then a more complex picture is obtained (Fig. 9.23). The maximum in the region of the nematic-isotropic phase transition for p-azoxyanisole is observed for the dependences 11(7) and A break is... 

Navard and Haudin studied the thermal behavior of HPC mesophases (87.88) as did Werbowyj and Gray (2), Seurin et al. (Sp and, as noted above, Conio et al. (43). In summary, HjPC in H2O exhibits a unique phase behavior characterized by reversible transitions at constant temperatures above 40 C and at constant compositions when the HPC concentration is above ca. 40%. A definitive paper has been recently published by Fortin and Charlet ( who studied the phase-separation temperatures for aqueous solutions of HPC using carefully fractionated HPC samples. They showed the polymer-solvent interaction differs in tiie cholesteric phase (ordered molecular arrangement) from that in the isotropic phase (random molecular arrangement). 

An analogous enhancement in the optical rotatory power of the isotropic phase of a thermotropic liquid crystal has been observed near the isotropic-cholesteric phase transition. Patel and DuPre concluded that it is attributable to short-range chiral ordering of the long axes of the macromolecules. 

A similar thermally-induced inversion of the cholesteric sense was observed for the PBLG liquid crystal in benzyl alcohol. In this solution, a gel-like opaque phase coexists with the cholesteric phase at lower temperatures. The opaque phase disappears around 70 °C, where endothermic peaks are observed in the differential scanning calorimetry curve. The value of S below 70 °C remains constant, and then changes with temperature above 70 °C. The compensation occurs at about 103 °C, and the transition from biphasic phase to the isotropic phase is observed above 150 °C in this case. The results are summarized in Fig. 12, where the reciprocal of the half-pitch is plotted against temperature. The sign of 1/S is taken as positive when the cholesteric sense is the right-handed. 

Figure 3. Trends of phase transition temperatures of poly(ester B-sulfide)s IV-m with varying number m ( ) melting, (A) smectic-cholesteric, and ( ) isotropization.

Following a similar approach to that of pyrene exclmer formation, activation energies for pyrene- 4. exciplex formation can be obtained from expression of Eqn. 9 in an Arrhenius form and differentiation by 1/T. ki+k2 are obtained from data taken in eyelohexane (32), and A3 and E3 from the lifetime taken at f CA1 - 0. E3 can also be obtained from the slope of the phase dependent dynamic Stern-Volmer plots. As seen in Table 1 the data from each method are in good agreement. The small differences in activation parameters measured in the cholesteric and isotropic phases probably reflect changes in viscosity that accompany phase transitions. 

In cholesteric 11, the Implication is that the two ends of all PnP approach one another via a similar mechanism which Is Independent of chain length (presumably a sliding motion such as route b In Fig. 6). In the Isotropic phase, all possible routes for the pyrenyl groups to collide are possible and, conceivably, one or more of these routes may require pre-equilibria of reactant and transition states with some Intermediate state. 

---