Smectic phases order/disorder

The liquid crystal phases of calamitic mesogens fall into two types - nematic (N) and smectic (Sm). The nematic phase is the most disordered of the liquid crystal phases and possesses only orientational order, so that the long axes of the molecules are correlated in one direction (known as the director, n) while being positioned randomly (Fig. 2A). There are several smectic phases and these differ from the nematic phase in possessing partial posi-... 

A number of systems which in polymer literature are normally referred to as mesophases are obtained under kinetic control. Examples are the smectic phase of isotactic polypropylene [18,19], mesomorphic syndiotac-tic polypropylene [20-22], mesomorphic PET [23,24], and other instances where intermediate degrees of order result after quenching polymers from the melt to temperatures often close to Tg. In these cases disorder is plausibly more static than in bundles close to T0 and these phases usually crystallize upon heating to an appropriate temperature in the stable crystal phases. 

Depending on temperature, transitions between distinct types of LC phases can occur.3 All transitions between various liquid crystal phases with 0D, ID, or 2D periodicity (nematic, smectic, and columnar phases) and between these liquid crystal phases and the isotropic liquid state are reversible with nearly no hysteresis. However, due to the kinetic nature of crystallization, strong hysteresis can occur for the transition to solid crystalline phases (overcooling), which allows liquid crystal phases to be observed below the melting point, and these phases are termed monotropic (monotropic phases are shown in parenthesis). Some overcooling could also be found for mesophases with 3D order, namely cubic phases. The order-disorder transition from the liquid crystalline phases to the isotropic liquid state (assigned as clearing temperature) is used as a measure of the stability of the LC phase considered.4... 

Calamitic phases are essentially defined as a set of mesophases that occur between the breakdown of the long-range periodic, translational ordering of the crystal and the loss of long range orientational order at the transition to the isotropic liquid. In this context six mesophases exist the nematic phase and five smectic phases labelled A, B, C, F and I. Other closely related soft crystal phases, in which the molecules have long-range periodic order but are themselves rotationally disordered, also exist in concert with liquid crystal phases. This second set of mesophases are labelled E, J, G, H, and K, and are essentially the crystal versions of the... 

Thermotropic liquid crystals can then be furflier subdivided into high molecular mass, main and side-chain polymers [10] and low molecular mass, the latter class of compounds being one of the areas of this review. The phases exhibited by the low molecular mass molecules are then properly described with reference to the symmetry and/or supramolecular geometry of the phases, which are briefly introduced here and are discussed in more detail further below. Thus, the most disordered mesophase is the nematic (N), which is found for calamitic molecules (N), discoidal molecules (Nq) and columnar aggregates (Nc), among others. The more ordered lamellar or smectic phases (S) [11, 12] are commonly shown by calamitic molecules, and there exists a variety of such phases distinguished by a subscripted letter (e. g. Sa, Sb)- Columnar phases (often, if incorrectly, referred to as discotic phases) may be formed from stacks of disc-like molecules, or from... 

The final group of lamellar phases to be introduced here are the so-called crystal smectic phases. These are more ordered than the previous smectic examples and are characterized by the appearance of interlayer correlations and, in some instances, by the removal of freedom of molecular rotation. Consequently the (crystal) B, G and J phases are derived from the Sb, Sf and S] phases, respectively, but with the presence of interlayer correlations. Further the (crystal) E, H and K phases are B, G and J phases which have lost rotational freedom. These phases are still disordered and hence, still intermediate between the solid and liquid states. 

The liquid crystalline phase is called a mesopliase and is intermediate between solid and liquid. In this mesopliase the molecules show liquid-like long-range behaviour, i.e. are essentially disordered, but also some crystal-like aspects of short-range order. The type of long-range order in the mesopliase may vary. If the molecules align themselves as layers this is described as a smectic phase if the alignment is as parallel threads it is described as a nematic phase. 

Whereas monophilic liquid crystals can show a high diversity of smectic phases (SmA-SmQ), the amphotropic liquid crystals normally exhibit only the SmA phase. Tilted smectic phases are only observed in a few cases. The first indication of possibly tilted phases was given in 1933 for thallium stearate [ 170]. A disordered SmC phase was also clearly deseribed for mesogens containing a classical calamitic core aside to their amphiphilic structure [171]. Monophilic liquid crystals can show various ordered tilted smectic phases, for example, smectic I, F, G, J, H, and K. In the case of lipids only one mesophase, the j8 phase,... 

Figure 2.108 illustrates the chemical stmcture and a thermal analysis curve of a typical small molecule with hquid-crystal and condis-crystal phases, OOBPD. The mesogen is the rigid bisoxybenzalphenylenediamine. Two flexible octyl groups enable conformational disorder by rotation about the C- C and O - C bonds. The letter N represents the nematic phase, letters C, I, G , and H the increasingly better ordered smectic phases, and K designates condis phases. Note that phase Kj has still not... 

Starting from the isotropic phase, where the molecules have all three degrees of freedom, cooling will increase the density and rotation about the long axis becomes restricted. Series of models have been developed that consider the density of liquid in terms of the restriction of the order.These theories identify a critical density at which the isotropic to nematic transition would be predicted. Constraint of the molecule in terms of its rotation about the long axis defines the nematic phase. If now the translational freedom is restricted and layered alignment is imposed on the molecules, then smectic order is created. The smectic phase can still retain disorder in rotational freedom about the short axis. Loss of this final degree of freedom will lead to the creation of a erystalline ordered structure. This simple approach provides a description for the isotropic nematic smectic crystalline transitions. 

Liquid crystals themselves can exist in several states. These range from a well-ordered crystal state to a disordered liquid state. In-between states are known as the smectic phase, which have layering, and the nematic phase, in which the separate layers no longer exist but the molecules can still be ordered. 

Smectics exhibit positional disorder in two dimensions but have a center-of-mass correlation along the third dimension. Each layer can be likened to a two-dimensional fluid. There are various smectic phases. The smectic A and C phases are represented in Fig. Ic and d. The direction of molecular alignment is perpendicular to the layer planes in the smectic-A and is at a tilt to the layer normal in the smectic C phase. Numerous other smectic phases have been identified and are distinguished by various in-plane order and correlations between the layers. 

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