The Nematic-Isotropic Phase Transition

Depression of the nematic-isotropic phase transition temperature(Tjjj) is caused by the addition of cis-BMAB. Sudden phase transition occurs when the content of cis isomer reaches the critical... 

The Nematic - Isotropic Phase Transition. For nematic solutions kept free from moisture, the phase transformations described in the preceding were not observed, but the nematic phase could be reversibly transformed to the isotropic phase over a temperature interval Tj - Tj lOK. For the sample with w = 0.041, this transition occurred over the range T = 92 C to Tj = 101 0. For temperatures between T and Tj, the sample was biphasic, with the isotropic and nematic phases coexisting. This behavior is similar to that observed in previous studies, in which Tj - Tj is observed to be independent of w over a range of w for which Tj increases with increasing w (3,4). 

It is observed that in the nematic phase of a liquid crystal, the solvation dynamics of coumarin 503 are biexponential [184a]. The slowest time constant decreases from 1670 ps at 311.5 K to 230 ps at 373 K. The solvation time is not affected by the nematic-isotropic phase transition. Thus, it appears that the local environment and not the long-range order controls the time-dependent Stokes shift. A theoretical model has been developed to explain the experimental findings. This model takes into account the reorientation of the probe as well as the fiuctuation of the local solvent polarization. Similar results are also obtained for rhodamine 700 in the isotropic phase of octylcyanobiphenyl [184b]. 

All physical parameters mentioned above are material specific and temperature dependent (for a detailed discussion of the material properties of nematics, see for instance [4]). Nevertheless, some general trends are characteristic for most nematics. With the increase of temperature the absolute values of the anisotropies usually decrease, until they drop to zero at the nematic-isotropic phase transition. The viscosity coefficients decrease with increasing temperature as well, while the electrical conductivities increase. If the substance has a smectic phase at lower temperatures, some pre-transitional effects may be expected already in the nematic phase. One example has already been mentioned when discussing the sign of Ua- Another example is the divergence of the elastic modulus K2 close to the nematic-smecticA transition since the incipient smectic structure with an orientation of the layers perpendicular to n impedes twist deformations. 

Warner et al. [88,89] give a full description of the free energy and recover, by minimization, the spontaneous strain and the mechanical critical point. They also show that, if the network is crosslinked in the nematic phase, a memory of-the nematic state is chemically locked. This causes a rise in the nematic-isotropic phase transition temperature compared with the uncrosslinked equivalent. After crosslinking in the isotropic state, the transition temperature (on the contrary) is lowered. 

X=LILq is plotted as a function of the reduced temperature red at constant nominal stress CTn = 2.11xlO N mm . Here Lg is the loaded sample length at Tred l-OS. These results will also be used below to establish a close connection between the strain tensor and the nematic order parameter. It has also been shown that a quadratic stress-strain relation yields in the isotropic phase above the nematic-isotropic phase transition a good description of the data for ele-ongations up to at least 60% [4]. 

Static mechanical properties in the vicinity of the nematic-isotropic transition in liquid single crystal elastomers (LSCEs) have been investigated [10, 11]. In Fig. 5 the deformation L/Lq (mon) is plotted as a function of the reduced temperature red- Here Lo(mon) denotes the length of the LSCE at the phase transition temperature of the nematic-isotropic phase transition and... 

In equations (5)-(8), i is the molecule s moment of Inertia, v the flow velocity, K is the appropriate elastic constant, e the dielectric anisotropy, 8 is the angle between the optical field and the nematic liquid crystal director axis y the viscosity coefficient, the tensorial order parameter (for isotropic phase), the optical electric field, T the nematic-isotropic phase transition temperature, S the order parameter (for liquid-crystal phase), the thermal conductivity, a the absorption constant, pj the density, C the specific heat, B the bulk modulus, v, the velocity of sound, y the electrostrictive coefficient. Table 1 summarizes these optical nonlinearities, their magnitudes and typical relaxation time constants. Also included in Table 1 is the extraordinary large optical nonlinearity we recently observed in excited dye-molecules doped liquid... 

Savithramma, K. L., and Madhusudana, N. V., The nematic-isotropic phase transition application of the Andrew method, Mol. Cryst. Liq. Cryst., 62, 63-80 (1980). 

It is, however, necessary to mention that, in contrast to thin LC films, in thick LC cells the quadrupolar and other bulk contributions to the net SHG signal are not negligible and have to be properly taken into account in the data analysis. For example, SHG measurements at the nematic-isotropic phase transition of 8CB have shown that the bulk contribution to the SHG signal has an opposite phase with respect to the surface contribution [32]. 

A number of both, theoretical and experimental studies of hybrid nematic systems, have been stimulated by their possible technological applications [49] and by the physical phenomena related to the fr ustration. The thickness dependence of the nematic-isotropic phase transition temperature and the stability of ordered structures in a hybrid nematic film were studied both theoretically [50] and experimentally using a quasi-elastic scattering method... 

Fig. 3.10. Magnitude of the flexoelectric coefficient e3 versus the temperature deviation from the nematic-isotropic phase transition for (a) ClPbislOBB and for (b) 5CB. Data taken from Harden et al Note the three orders of magnitude difference in the vertical scales ...

Thermo-optic effects in liquid crystals were investigated extensively from the point of view of device applications. Nematics are particularly suitable for such purposes because in this phase the refractive indices have an unusually large temperature dependence, especially near the nematic-isotropic phase transition. This strong temperature dependence is connected to the corresponding variation of the order parameter, 5. The order parameter is a measure of the degree to which the molecules are oriented along the director. The birefringence, is... 

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. 

Study of the refractive indices of the liquid crystal 4-cyano-4 -pentylbiphenyl (K 15, SCBorPCB) over its nematic range, has shown an increase in magnitude of the thermo-optic coefficient as the nematic-isotropic phase transition is approached. From (1) it is clear that if dT/dP is constant over this range, then a commensurate reduction in operating power should result. 

Gramsbergen EF, Longa L, de Jeu WH (1986) Landau themy of the nematic isotropic-phase transition. Phys Rep Rev Sect Phys Lett 135 195-257... 

Denolf, K., van Roie, B., Glorieux, C., Thoen, Yildiz, S., and Ozbek, H. (2007) An adiabatic scanning calorimetry study of the nematic-smectic A and the nematic-isotropic phase transitions in 4-butyloxyphenyl-4 -decyloxybenzoate. Mol. Cryst. Liq. Cryst., ATJ. 3-16. 

---