Structure of the Smectic F Phase

In addition to the homopolyesters 80a-i, 81a-i and 82a-i, three classes of co PEIs were studied 83a-g [24], 84a-d and 85a-f [84]. The combination of two different alkane spacers did not significantly change the properties of the co PEIs 83a-d, when compared to 82i. However, when the difference on their length increased, a destabilization of the smectic layer-structures became evident, with the consequence that a nematic phase was formed on top of the smectic-C phase... 

This region has been divided into two subphases, L and S. The L phase differs from the L2 phase in the direction of tilt. Molecules tilt toward their nearest neighbors in L2 and toward next nearest neighbors in L (a smectic F phase). The S phase comprises the higher-ir and lower-T part of L2. This phase is characterized by smectic H or a tilted herringbone structure and there are two molecules (of different orientation) in the unit cell. Another phase having a different tilt direction, L, can appear between the L2 and L 2 phases. A new phase has been identified in the L 2 domain. It is probably a smectic L structure of different azimuthal tilt than L2 [185]. 

Our BAE measurements indicate that the molecules in the first layer lie nearly fiat on the surface, while the molecules in the second layer are pointing more toward the surface normal. The structure of the second layer is, however, very diffeient from a homeotropically oriented bilayer of the bulk smectic phase which would give 2,bulk f °- Assuming that the molecular angular distribution /(0) is constant during the entire deposition of the... 

By a proper treatment of the electrodes, one can obtain a texture with a uniform orientation of the smectic normal in one direction within the cell plane. Between the crossed polarizers such a cell will be black if a polarizer is installed parallel to the smectic normal. Upon application of the electric field, the antiferroelectric structure becomes distorted. At low voltages of any polarity, the electrooptic response is proportional to E the bottom part of the curves has symmetric parabolic form [35] shown in Fig. 13.24b. Above the AF-F transition, the director acquires one of the two symmetric angular positions ( 9 on the conical surface) typical of the SmC phase. At these two extreme positions the transmission is maximum. With increasing temperature from T toTi the AF-F threshold decreases due to a decrease of the potential barrier separating structures with alternating and uniform tilt. It is natural because within the SmC A phase T1 is closer to the range of the SmC phase than T2 or T3. 

Three methods are used in the identiflcation of different smectic phases. The most direct is X-ray analysis [3, 4], which gives information on the nature of the packing of the molecules in the crystal. However, the physicochemical method based on the immiscibility of different phases is also widely used. In this method, any phase which does not form a homogeneous mixture with any of the earlier identified phases is regarded as a new phase. The physico-chemical method is usually used simultaneously, with observation of the nature of the corresponding optical texture under a microscope (the optical method). Each phase has its own characteristic microscopic appearance, its texture [5]. The study of the polymorphism of smectic phases by these three methods has clearly estabhshed the existence of modifications A, B, C, E, F, G, and H whose structures are more or less understood. Moreover, phases I, J, and K have recently been identified. In this book smectic phases will be denoted by 5a, 5b, 5c, etc. 

The symmetry arguments for achiral or chiral smectic C phases can also be applied in similar ways to the other smectic and soft crystal smectic phases. For example, all of the tilted phases (smectics I and F, and crystal phases J, G, H, and K ) would have broken symmetries leading to polar noncentrosymmetric structures resulting in ferroelectric properties. Even orthogonal phases, such as the smectic A phase would have different symmetries for the chiral versus the achiral forms. For instance, the smectic A phase has symmetry, where-... 

The structure of the chiral smectic F phase is similar to that of the chiral smectic I phase, except that the tilt direction is towards the side of the local hexagonal packing net, rather than towards the apex. 

The absorption of ultrasound in smectic phases is significantly more anisotropic than that in nematics, and even the velocity has a measurable anisotropy of about 5%. Details of the behaviour of SmA, SmB, SmC and SmE phases can be found in the literature [14—16, 18, 86, 94-97]. The usual approach to the analysis of smectic phases, based on the linear theory of elasticity and hydrodynamics, results in the relationship a—f, which does not agree with the experimental data. In the low-frequency range the coefficients a, o, 4 and c% demonstrate singularity, induced by nonlinear effects, in the form of oT. This results in a linear frequency dependence of the ultrasound absorption. The corrections for the coefficients of elasticity B and K, taking into account the nonlinear fluctuation effects in smectic phases, depend on the wavevector of the smectic phase layer structure B=(ln9,)- [96, 97]. In... 

Figure 24. (a) Structures, and (b) layer stackings of the ordered smectic phases. A, B and C denote the highest symmetry positions of molecules in adjacent layers, whilst the open squares and triangles show the lower symmetry positions (ortho-F and mono-C) which have been observed in the smectic B phase. (From [15]). 

As a result of the layered nature of the chiral nematic structure, like the smectic A, it can also exhibit focal-conic textures [79] and both phases exhibit screw and edge dislocations. A dislocation corresponds to a displacement of the layered structure in a plane orthogonal to the layer and may be formed by the pairing of two disclinations of opposite sign. A screw dislocation has a singular line along the screw axis and is equivalent to a f-screw disclination in a chiral nematic. An edge dislocation corre-... 

Introduction of up to 37 mol% of the chitt) monomer into the macramolecule of the smcctk F polymer does not destroy the tilled smectic structure. At lower (8%) and higher (37%) percentages of chiral monomer, a mooolropic Sc phase occurs. The spontaneous polarization which was calculated from the temperature dependence of the pyroelectric coefficient reached a maximum at 25 wt% chiral mononier (P > 3 nC/cm ) in the Sr phase. 

By our definition, the tilt plane is normal to the polarization in the ferroelectric state in the illustration in Figure 8.13 this is a vertical plane normal to the plane of the page. Since there is no tilt of the director projected onto this plane, the phase should be considered a type of SmA. We name this structure SmAPp (an untilted polar smectic the subscript F referring to a ferroelectric structure, in this case a ferroelectric state of an antiferroelectric phase). The antiferroelectric phase is therefore also an SmA denoted SmAPA (the subscript A for antiferroelectric). While this idea is certainly intriguing, no such antiferroelectric has yet been discovered. 

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