Nematic-to-smectic-A transition

Lubensky T C 1983 The nematic to smectic A transition a theoretical overview J. Chim. Phys. 80 31-43... 

At a normal chiral nematic to smectic A transition, the helical ordering of the chiral nematic phase collapses to give the layered structure of the smectic A phase. However, for a transition mediated by a TGB phase, there is a competition between the need for the molecules to form a helical structure due to their chiral packing requirements and the need for the phase to form a layered structure. Consequently, the molecules relieve this frustration by trying to form a helical structure, where the axis of the helix is perpendicular to the long axes of the molecules (as in the chiral nematic phase), yet at the same time they also try to form a lamellar structure, as shown in Fig. 21. These two... 

The nematic to smectic A phase transition has attracted a great deal of theoretical and experimental interest because it is tire simplest example of a phase transition characterized by tire development of translational order [88]. Experiments indicate tliat tire transition can be first order or, more usually, continuous, depending on tire range of stability of tire nematic phase. In addition, tire critical behaviour tliat results from a continuous transition is fascinating and allows a test of predictions of tire renonnalization group tlieory in an accessible experimental system. In fact, this transition is analogous to tire transition from a nonnal conductor to a superconductor [89], but is more readily studied in tire liquid crystal system. 

Ferrocene derivatives 15 exhibited remarkable liquid crystal properties (Fig. 9-13). Indeed, they all gave rise to enantiotropic mesophases. Structures with n = 1 to 11 showed nematic phases. From n = 12 a smectic C phase formed. The latter was monotropic only for 15 (n = 12). The smectic C domain increased from n = 13 to n = 16, and, inversely, the nematic range narrowed. The last member of this series (n = 18) presented one smectic C phase between 159 °C and 179 °C. A nematic to smectic C transition and a focal-conic texture of a smectic C phase are presented in Figs. 9-14 and 9-15, respectively. 

Chen J-H and Lubensky TCI 976 Landau-Ginzburg mean-field theory for the nematic to smectic C and nematic to smectic A phase transitions Phys.Rev. A 14 1202-7... 

Now that we have discussed all of the structures of the phases that play a part in the formation of twist grain boundary phases, let us now consider the events that can occur at a nematic to smectic phase transition. At a normal chiral... 

The analogues of the smectic A" and G" phases, the TGBA" and TGBG phases, have been detected at the chiral nematic to smectic A" and the chiral nematic or TGBA" to smectic G" transitions [16, 17]. Two helical forms of the TGB phases are possible, one where the helical structural arrangements of the blocks or sheets of the smectic phase are rational, i.e., the number of blocks required to form a 360° rotation of the helix is a whole number, in which case the phase is said to be commensurate , or alternatively where the number of... 

The presence, and to some degree the discovery, of TGB phases in liquid-crystalline systems stems from theoretical studies by de Gennes [18], Through modeling of the N-A transition, de Gennes predicted that, for a second order nematic to smectic A phase transition, a defect stabilized phase could occur... 

When the terminal ester functionality in the phenyl propiolates is replaced with a carbonyl group, i.e., a keto function, TGB phases disappear altogether indicating that the increased size of the lateral dipole at terminus to the core has detrimental effects on the stability of TGB phases [35]. These small variations in structure show how delicate the stability of the TGB phase is, nevertheless, most chiral nematic to smectic phase transitions are accompanied by a TGB phase no matter how transient it is. 

Here, f, Fx and are local field factors, a is a molecular parameter including the dipole moment, second order and third order hyperpolarizabilities. Under condition F = Fx = the formula does not agree with the experiment for 8CB. Indeed, it follows that rji should increase at the transition from the nematic to smectic A phase according to the increase in (F2) However, in the experiment, Fn markedly decreases. The similar temperature behavior was earlier observed for dielectric permittivity of 8CB. In the latter case the decrease in is due to the antiparallel correlation of molecular dipoles in the smectic A phase, which results in a decrease in the effective dipole moment /x Thus, the decrease... 

In a recent work, isotropic-nematic-smectic A phase transitions in thermotropic liquid crystals were also induced by applying an electric field [140]. The liquid crystal investigated (a mixture of 8CB and lOCB) showed a first order isotropic to smectic A transition. When in the isotropic phase and near the spontaneous transition temperature, a field-induced first order transition was observed from a paranematic to a nonspontaneous nematic phase. For higher values of the applied electric field, another first order transition occurred from the nonspontaneous nematic to a phase exhibiting the same order as a smectic A phase. A phenomenological Landau-de Gennes model has been developed to describe these transitions [141],... 

M. Delaye, R. Ribotta, G. Durand, Rayleigh scattering at a second order Nematic to Smectic-A phase transition, Phys. Rev. Lett. 1973, 31 443-445. 

RECENT EXPERIMENTAL DEVELOPMENTS AT THE NEMATIC TO SMECTIC-A LIQUID CRYSTAL PHASE TRANSITION... 

Figure 10.4 Normalized force versus distance curve measured in 8CB between a glass sphere (radius 8.5-10 pm) and a glass plate with an AFM [1134], The force curve was recorded in the nematic phase at 34.2 °C that is, 0.7 K above the nematic to smectic phase transition. It was...

An exception to the mle that lowering the temperature causes transitions to phases with iacreased order sometimes occurs for polar compounds which form the smectic phase. Decreasiag the temperature causes a transition from nematic to smectic but a further lowering of the temperature produces a transition back to the nematic phase (called the reentrant nematic phase) (22). The reason for this is the unfavorable packing of the molecules ia the smectic phase due to overlap of the molecules ia the center of the layers. As the temperature is lowered, the steric iateractions overpower the attractive forces, causiag the molecules to pack much more favorably ia the nematic phase. The reentrant nematic phase can also be produced from the smectic phase by iacreasiag the pressure (23). 

With increasing temperature, phase transitions occur, including ciystalline to smectic C to smectic A to nematic to isotropic, or crystalline to nematic to isotropic. These examples demonstrate that not all possible transitions neeessarily oceur. Depending on the number of mesophases occurring, thermotropic mono-, di-, tri-, or tetramorphism may be distinguished. 

It may be asserted that the fundamental reason arises from the fact that, while parallel arrangements of anisotropic objects lead to a decrease in orientational entropy, there is an increase in positional entropy. Thus, in some cases, greater positional order will be entropically favorable. This theory therefore predicts that a solution of rod-shaped objects will undergo a phase transition at sufficient concentration into a nematic phase. Recently, this theory has been used to observe the phase transition between nematic and smectic-A at very high concentration (Hanif et al.). Although this model is conceptually helpful, its mathematical formulation makes several assumptions that limit its applicability to real systems. 

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