Smectic A-isotropic phase transition

A-nematic phase transition (SNT) is of second order, while the NIT is of first order. In Figure 2.16b, the smectic order parameter discontinuously drops to zero with increasing <)> and the SNT is of first order. In Figure 2.16c, the nematic phase disappears and we only have the first-order smectic A-isotropic phase transition (SIT). Owing to McMillan theory for a pure nematogen (<)> = 0), the SNT should be second order for Tsf /T j < 0.87 and first order for larger values of 7 /T j, where shows the SNT temperature of a pure nematogen. 

Fig. 9-6. Phase diagram of ferrocenes 11. Melting point nematic/isotropic liquid transition smectic A/isotropic liquid transition A isotropic liquid/smectic A transition O isotropic liquid/nematic transition.

Fig. 11. The dependence of the transition temperatures on the number of methylene groups, n, in the flexible alkyl spacer for the CBOnO.lO series.[69] indicates interdigitated smectic A-isotropic transitions, nematic-isotropic transitions, A interdigitated smectic A-interdigitated smectic C transitions, interdigitated smectic A-nematic transitions, O intercalated smectic A-nematic transitions and A intercalated smectic A-intercalated smectic C transitions. The melting points have been omitted for the sake of clarity. SmAa Interdigitated smectic A phase SmCa interdigitated smectic C phase SmAc intercalated smectic A phase SmCca intercalated alternating smectic C phase N nematic I isotropic...

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. 

In the nematic phase, this ratio is larger than unity (R /R 1.3) (Warner and Terentjev, 1996), but after a nematic-isotropic phase transition, this ratio approaches unity as a result of the formation of a random coil of polymer chains, which makes the polymer material contract along the director axis of LCEs. In the smectic A phase, the ratio R /R is in general smaller than unity because the polymer chains are likely to exist between the smectic layers (Cotton and Hardouin, 1997). 

From DSC thermogram, optical anisotropy under polarizing optical microscope (p.o.m.) and wide-angle X-ray diffraction (w.a.x.d.) data it can be deduced that the endothermic peak at 77 °C is due to the mesomorphic-isotropic phase transition. The enthalpy of phase transition is 20.35 J/g. Hence, according to Brandon and Marmur (1996), the structure of PAC8 below 77 °C is ordered in smectic crystalline, which can be classified as smectic B (Sb) type. 

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. 

Let us first consider dodecyl-cyanobyphenyl which shows two phase transitions at 48 °C (crystalline-semectic A) and at 58.5 °C (smectic A-isotropic). The analysis of the whole spectrum is reported in the original work [112] and we focus here only on the variation with temperature of the conformational structure of the dodecyl side chain which we hope to reveal with the study of the temperature-dependent vibrational spectrum in the 1420-1280 cm" range where defect modes are expected to occur. 

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. 

P2 (Eq. 5.11) and a (Eq. 5.16) for three different values of a is shown in Eig. 5.19. Eor large a (for example a = 1.1) d is large and smectic ordering is favoured. There is thus a first-order transition on heating from the SmA phase to the isotropic phase. However, as a is lowered, a nematic phase is formed between smectic and isotropic phases. In the case o = 0.85, the transition between SmA and N phases is first order, whereas at lower o, for example a = 0.6, it is continuous (second order), as shown by the continuous decrease of a to zero (P2 also varies continuously, but there is a change of slope with respect to temperature at the transition). The McMillan theory predicts that the crossover from a first-order to a second-order transition (called a tricritical point) occurs ata = 0.98, which corresponds in the model to a ratio of phase transition temperatures 7an/ i = 0.870. 

We start with the microscopic definitions and discussion of the nematic and smectic order parameters and then proceed with some elementary information about anisotropic intermolecular interactions in liquid crystals. Then we discuss in more detail the main molecular theories of the nematic-isotropic phase transition and conclude with a consideration of molecular models for smectic A and smectic C phases. 

---