Alternating smectic phases

A semi-flexible main chain liquid crystal polymer is composed of mesogenic units separated by flexible spacers, normally alkyl chains [ 7]. These polymers are not only of interest for their application potential [8] they are also of major fundamental interest because of their unusual liquid crystalline properties. It is well known, for example, that the transitional behaviour of a semi-flexible main chain liquid crystal polymer shows a dramatic dependence on the length and parity of the flexible spacer linking the mesogenic units [9]. Other fascinating behaviour includes the observation of a nematic-nematic transition [ 10] and the occurrence of alternating smectic phases [11-15]. 

The classical scheme for dichroism measurements implies measuring absorbances (optical densities) for light electric vector parallel and perpendicular to the orientation of director of a planarly oriented nematic or smectic sample. This approach requires high quality polarizers and planarly oriented samples. The alternative technique [50, 53] utilizes a comparison of the absorbance in the isotropic phase (Dj) with that of a homeotropically oriented smectic phase (Dh). In this case, the apparent order parameter for each vibrational oscillator of interest S (related to a certain molecular fragment) may be calculated as S = l-(Dh/Di) (l/f), where / is the thermal correction factor. The angles of orientation of vibrational oscillators (0) with respect to the normal to the smectic layers may be determined according to the equation... 

Now suppose a smectic phase similar to the SmCA phase indicated in Figure 8.12 were an actual bilayer smectic with alternating synclinic and anticlinic layer interfaces, as illustrated in Figure 8.14a. It was already well... 

Chirality (or a lack of mirror symmetry) plays an important role in the LC field. Molecular chirality, due to one or more chiral carbon site(s), can lead to a reduction in the phase symmetry, and yield a large variety of novel mesophases that possess unique structures and optical properties. One important consequence of chirality is polar order when molecules contain lateral electric dipoles. Electric polarization is obtained in tilted smectic phases. The reduced symmetry in the phase yields an in-layer polarization and the tilt sense of each layer can change synclinically (chiral SmC ) or anticlinically (SmC)) to form a helical superstructure perpendicular to the layer planes. Hence helical distributions of the molecules in the superstructure can result in a ferro- (SmC ), antiferro- (SmC)), and ferri-electric phases. Other chiral subphases (e.g., Q) can also exist. In the SmC) phase, the directions of the tilt alternate from one layer to the next, and the in-plane spontaneous polarization reverses by 180° between two neighbouring layers. The structures of the C a and C phases are less certain. The ferrielectric C shows two interdigitated helices as in the SmC) phase, but here the molecules are rotated by an angle different from 180° w.r.t. the helix axis between two neighbouring layers. 

Preceding the discussion of orientational effects in LC polymers, it is worth mentioning that for a nematic and a smectic phase A of LC polymers only the S-effect was discovered and investigated. This started with works U9,124, 137 138), that demonstrated the ability of LC polymers to orient in permanent and alternating electric fields. The structural formulas of some of the polymers and copolymers investigated are given below ... 

Independently, a highly interesting problem recently revived is that of the alignment of molecules in liquid crystals due to externally applied static and alternating electric fidds. The problem was first approached by Jezewski and Kast, and was developed by Carr, Helfrich, Wysocki et al. and many others, studying various aspects, such as dielectric loss in electric fields and anomalous alignment in the smectic phase and domains, the effect of an electric field on the temperature of mesomorphic-isomorphic phase tranritions of liquid crystals, electric... 

P8 M is not the only polymer forming the isotropic smectic phase. To date, we have observed formation of that phase for a half-dozen chiral polymethacrylates and polysiloxanes. Table 5.1 summarizes the chemical structure and phase behavior of synthesized side-chain homopolymers, which carry chirally substituted side chains derived from asymmetric esters of terephthalic acid and hydroquinone. Such a structure with alternating orientation of carboxylic link groups seems to favour the formation of the IsoSm phase, whereas isomeric derivatives of p-hydroxybenzoic acid, where all carboxylic links have the same orientation, form only conventional Sm A and Sm C phases. Molar mass of all the synthesized homo- and copoly(meth)acrylates is within the range of 1 to 2-10 g mol the poiysilox-anes have the average degree of polymerization, p 35. 

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... 

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...

The even members of this series exhibit the intercalated smectic A phase while the odd members show only the intercalated alternating smectic C phase. In this respect the dimers are behaving in an identical fashion to the analogous polymers for which even membered spacers give rise to smectic A phases while... 

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