Smectic structures

Finally, whenever in matter there is no long range positional but still long-range orientational order, we have ordered liquids, instead of solids. The X-ray spectral features of an ordered liquid with a smectic structure is the occurrence of one or a few meridional sharp reflections in the fiber spectrum, plus polarized halos for an ordered liquid with nematic structure the occurrence of polarized haloes, only. 

Fig. 17a-c. Sketches of the molecular arrangements for the smectic structure with alternating layer-to-layer tilt a conventional and chevron smectic C layering in low molecular mass mesogens b ferroelectric hilayer chevron structures for achiral side-chain polymers c antiferroelectric hilayer chevron structures for achiral side-chain polymers. Arrows indicate the macroscopic polarization in the direction of the molecular tilt... 

Using this method, the M6R8/PM6R8 blend showed precisely the behavior expected for the achiral SmAPA structure. Specifically, the optical properties of the films were consistent with a biaxial smectic structure (i.e., two different refractive indices in the layer plane). The thickness of the films was quantized in units of one bilayer. Upon application of an electric field, it was seen that films with an even number of bilayers behaved in a nonpolar way, while films with an odd number of bilayers responded strongly to the field, showing that they must possess net spontaneous polarization. Note that the electric fields in this experiment are not strong enough to switch an antiferroelectric to a ferroelectric state. Reorientation of the polarization field (and director structure) of the polar film in the presence of a field can easily be seen, however. 

In a model study aimed at elucidating the behavior of the polymers, Watanabe found that the dimer polymethylene diol diesters of type 7 (Figure 8.18) formed smectic phases.38 When the spacer between the two mesogenic units in the dimer had an odd number of methylene groups (diester 7 has nine methylenes in the spacer), then an intercalated tilted smectic structure with all anticlinic layer interfaces was formed. This structure is often termed SmC2 in the literature. As for the B6 phase, all of the layer interfaces in this structure are equivalent, and the X-ray layer spacing is less than half the fully... 

Figure 8.18 Smectic dimer of Watanabe, possessing an odd number of methylene units in linking group. This material self-assembles into intercalated smectic structure very similar to B6 banana phase. As for B6 phase, this achiral phase is also neither ferroelectric nor antiferroelectric.

Decher et al. [420] have developed the idea of using freely suspended smectic liquid crystals and have used it to form thin hlms on solid substrates. A thermotropic smectic liquid crystal is drawn across an aperture in a solid support and is capable of bridging the aperture (which can be up to 15 mm in diameter) with a him which can be between two and several hundred layers thick. The him consists of a smectic structure with the layers lying in the plane of the him. The him is formed a short distance above a solid substrate and the apparatus is constructed so that a difference of pressure between the two sides of the him can be used to force the him down in contact with the substrate. These authors have thus formed good quality hlms up to an area of about 1 cm2. In the work described the material used was ethyl-4 -n-octyloxybiphenyl-4-carboxylate. 

Fig. 13. Schematic representation of macromolecular liquid crystals. A smectic structure is assumed for all cases...

With these three different examples it has been demonstrated that the systematics observed for the polymorphism of m-l.c. s is also valid for the side chain polymers, provided that a flexible spacer connects the rigid mesogenic moieties to the polymer main chain. Deviations from this behavior are observed, when the mesogenic moieties are directly linked to the backbone. Under these conditions, normally no liquid crystalline behavior is to be expected, according to the model considerations mentioned in Chap. 2.1. Some examples, however, proved l.c. properties for such systems, which are characterized by two striking properties Very high glass transition temperatures and only smectic structures even in case of short substituents... 

In any case, both models have in common that owing to the positional ordering of the mesogenic side chains, the polymer backbone no longer exhibits a statistical three dimensional coil conformation. Therefore at the phase transformation isotropic to smectic or nematic (cholesteric) to smectic, in addition to the change of the anisotropic packing of the side chains, the main chain has to change its conformation, which must be consistent with the layered smectic structure. A direct interaction... 

Fig. 31. Schematic model of the smectic structure of these liquid crystalline side chain polymers...

Layered Smectic Structure Mssogenic Units interacting... 

One of the main advantages of contact SFM is the high spatial resolution. Variations in height as small as 0.01 nm can be easily detected in the constant height mode. Figure 8a shows as an example the image of the smectic structure of per-... 

The poly(carbodiimide) 21 displays a specific rotation similar to the monomer. Solutions (20 %) of poly(N,N -di-n-hexylguanidine), using Cp-TiCl2NMe2 as catalyst in the polymerization reaction, form a layered smectic structure. ... 

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