Smectic molecular models

McMillan W L 1971 Simple molecular model for the smectic A phase of liquid crystals Phys.Rev A 4 1238-46... 

We start with some elementary information about anisotropic intermolec-ular interactions in liquid crystals and molecular factors that influence the smectic behaviour. The various types of molecular models and commonly accepted concepts reproducing the smectic behaviour are evaluated. Then we discuss in more detail the breaking of head-to-tail inversion symmetry in smectic layers formed by polar and (or) sterically asymmetric molecules and formation of particular phases with one and two dimensional periodicity. We then proceed with the description of the structure and phase behaviour of terminally fluorinated and polyphilic mesogens and specific polar properties of the achiral chevron structures. Finally, different possibilities for bridging the gap between smectic and columnar phases are considered. 

Fig. 30 Schematic representation of the molecular model for dendrimers with one-terminal-chain mesogenic units. Model for the Smectic A supramolecular organization (here G3-Li dendrimer)...

Listed are the molecular model, the type of phases in coexistence, the plane in which the integration was conducted, the known coexistence datum used to initiate the series, and the literature citation. Abbreviations for the various phases in the Coexistence Phenomenon column are as follows vapor (V), liquid (L), solid (S), fluid (F), isotropic (I), nematic (N), rotator (R) and smectic (Sm). Abbreviations for the Integration Plane column are as defined in the text. 

Fisher space filling molecular models), (b) (right) Proposed smectic organization of PChAB, PChMA.and PChA. 

Finally note that the flexoelectric effect is also important in the smectic phase although the corresponding molecular theory is at a rudimentary stage. Recently a molecular model for the conventional and the so-called discrete flexoelectric effect in tilted smectic phases has been proposed... 

Lee, K.-J. Hsiue, G.-H. Wu, J.-L. Sha, Y.-A., Molecular Structure of Side-Chain Liquid Crystalline Polysiloxane in the Smectic C Phase from X-Ray Diffraction and Molecular Modeling Polymer 2007,48,5161-5173. 

Fig. 9.11. (a) Molecular model of a smectic C phase. Molecules are tilted at a constant angle relative to the normal to the layers, and liquid-type order is conserved in layers, (b) In certain regions, molecules may be tilted in the opposite direction... 

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. 

Different molecular models for the SmC phase can be separated into two main classes that actually correspond to different molecular mechanisms of the smectic A-smec-tic C transition. Some models (for example, those of McMillan and Meyer [81 ] and Wulf [80]) imply that the molecular rotation about the long axis is frozen out in the smectic C phase. It seems to be even more important that in these models the smectic A-smec-tic C transition is governed by the ordering of molecular short axes while the tilt of the long axes occurs as a consequence. By contrast, in other models [82] the transition is directly related to the tilt of the long axes and the biaxiality of the smectic C phase is neglected. 

We note that the more recent molecular models for the smectic C phase [79,83,84]... 

The molecular theory of Van der Meer and Vertogen is based on a specific molecular model that is not in contradiction with experiment. At the same time Barbero and Durand [89] have shown that the molecular tilt is an intrinsic property of any layered quadrupolar structure. This idea has been used by Poniwierski and Sluckin in their model [84] that presents a rather general mechanism for the stabilization of the smectic C phase. It is interesting to note that the mathematical form of the interaction potential in the Poniwierski-Sluckin theory is similar to the potential (Eq. 85). The energy of electrostatic interaction between two axial quadrupoles, employed in [84], can be written as... 

De Jeu et al. [70] have discussed the influence of molecular geometry on the elastic constant ratio. From simple geometrical considerations they stated that the splay/ bend ratio should correlate with the molecular length/width ratio for rigid molecules, i.e. Kj, Kii=L W with Kj, >Kn- (Pre-transitional effects of smectic-like ordering are discussed below.) This view has been confirmed by some experimental data [66]. If long, flexible alkyl chains are incorporated in the molecular model structure, the trend is reversed ( r33< Tii). In practice, both cases are observed. For example, the 4 -n-alkyl-4-cyanobiphenyls (nCB) are characterized by the bend/splay... 

W. L. McMillan, Simple Molecular Model for the Smectic-A Phase of Liquid Crystals, Phys. Rev., A4, p. 1238(1971). 

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