Smectic ordering

In the ordered smectic or nematic phase, the rigid rods are arranged in parallel arrays that allow for close packing. The nematic phase is the most common type found with synthetic polymer molecules. The molecules long axes are parallel, but there is no layering. Aromatic polymer chains that have stiff ester or amide linkages are ideal. 

X-ray measurements have shown that the positional and orientational ordering in highly ordered crystalline smectic phases is more comparable with the ordering in the solid state than the structures of the fluid smectic phases and the nematic phase. Therefore, a comparison of the structures of crystals and highly ordered smectic phases is possible because of their closer similarities. Up to now, only the crystal structures of few mesogenic compounds exhibiting highly ordered smectic phases have been reported in... 

Preliminary room temperature x-ray data of 0.65 Me4C00-PECH indicates that the sample presents a highly ordered smectic mesophase which was not yet completely assigned. The textures seen by polarized optical microscopy are also typical of smectic phases. Due to the very high molecular weights involved, textures specific to mesophase in thermodynamic equilibrium could not be developed within a reasonable amount of time by annealing. 

To determine if this phenomenon is isolated to amorphous monomers, a liquid crystalline diacrylate (C6M) was polymerized in W7,W82 at temperatures corresponding to the two smectic phases as well as the isotropic phase. The polymerization rate for C6M is plotted as a function of time for representative temperatures in Figure 6. Again, the polymerization shows marked acceleration in the ordered smectic C phase and occurs much faster than the isotropic polymerization. As seen in the HDDA polymerizations, the smectic A rate also lies between the rates of the other two polymerization temperatures. 

The mesophases of LC diols la-lg were also observed directly in polarized optical micrographs taken immediately after melting the sample. Textures were identified only by comparison with published micrographs (2 ), and are therefore tentative. A nematic texture is observed for If (Fig. 2), while more highly ordered smectic textures are observed for Ib-le and Ig (Fig. 3). 

It was first reported in the early 1970s that these melt processible polymers could best be described as thermotropic systems which usually display an nematic texture in the melt phase [5]. Subsequently, a number of additional phases have been reported ranging from discotic structures to highly ordered smectic E G systems with three dimensional order. In the last several years an IUPAC sponsored study on nomenclature on thermotropic LPCs has been underway. A more complete set of definitions will be available shortly as a result of Recommendation No. 199 IUPAC [6]. 

This assertion is borne out by the small deviations from fluid-isotropiclike values of the photoproduct ratios found in the Norrish II reactions of the two alkyl perfluoroalkyl ketones 76 (with m, n = 7,8 and 9,10) in the (macroscopically) highly ordered smectic phases of F10H10 [273,274]. As... 

That is why here we describe two groups of polymers- those. forming an ordered smectic phase (of SB type) and polymers with mesogenic groups, for which the formation of crystalline structure is proposed (Table 5). Sometimes it is difficult to distinguish between crystalline and LC states of ordered smectic phases. Thus, the interpretation of data on crystalline phases of some polymers, listed in Table 5, is also possible from the viewpoint of smectic polymorphism. 

The second group involves polymers with three-dimensional ordering of side branches (e.g., those forming Mj-phaseXTable 5). On X-ray patterns of these polymers 3-4 narrow reflexes at wide angles are observed. As a rule, the authors define this type of structure as crystalline, or ascribe a smectic type of structure, characteristic for ordered smectics in SE or SH phases. The heats of transition from anisotropic state to isotropic melt are usually small and do not exceed the heats of transition smectic liquid crystal — isotropic melt . The similarity of structural parameters of three-dimensionally ordered smectics and that of crystalline polymers of the type here considered, make their correct identification quite a difficult task. 

Thus, polymers with mesogenic groups in side chains form structural mesophases of the same types as low-molecular liquid crystals. This makes it possible to apply traditional mesophase classification for the description of the structure of LC polymers. At the same time, the structure of some of comb-like polymers (see Table 5) considered as crystalline, may probably be treated as one of highly-ordered smectic mesophases (SH or Sj), whose study is only started74). 

If p-methacrylylhydroxybenzoic acid is mixed with p-cetylhydroxy-benzoic acid, a smectic form of liquid crystals results, but if it is mixed with p-nonylhydroxybenzoic acid, the resulting form is nematic above 104°C. and is smectic below this. This makes it possible to compare the polymerization behavior of the same monomer in solution, where the mutual ordering of its molecules is minimal, in a liquid crystal state with only orientation order (nematic form), and in the liquid-crystal state involving both orientation and coordination order (smectic form). 

Brackets represent an extrapolated transition temperature several ordered smectic phases present. 

The data collated in the Tables 3.14-3.16 reveals that the presence of a trans carbon-carbon double bond in a limited number of specific positions gives rise to a higher clearing point and a higher 33/ 11 ratio than those of the corresponding material without a double bond in the terminal chain. Smectic phases, especially the ordered smectic B phase, are strongly suppressed. This is... 

One example of a 1 1 AB block copolymer with poly-(IX-8) and poly-(X-S) blocks showed a smectic A phase the mesophase of the homopolymer of X-8. The more ordered smectic A phase dominated the nematic phase of the polynorbornene block. 

On passing from the least ordered smectic A phase down to the more ordered crystal H and K phases the layer planes tend to sharpen up. In the smectic A and C phases the layers are therefore very diffuse, and can be thought of as one-dimensional density waves relative to the director. Thus, locally these two phases are very similar in structure to the nematic phase. 

Figure 10.37 Apparent moduli G (closed symbols) and G" (open symbols) at a strain amplitude of yo = 0.1 as functions of frequency for 8CB in the smectic A state. The curves labeled MD are for the macroscopically disordered smectic formed by a temperature quench into the smectic A state. The curves labeled MO are for macroscopically ordered smectic formed by large-amplitude (yo = 1) oscillatory shearing at cu = 10 sec for 20 min. (From Larson et al. 1993, reprinted with permission from Steinkopff Publishers.)...

Some liquid crystals form one or more smectic phases. These display a variety of microscopic structures that are indicated by the letters A, B, C, and so forth. Figure 23.9c shows one of them, the smectic A structure the molecules continue to display net orientational ordering, but now, unlike in the nematic phase, the centers of the molecules also tend to lie in layers. Within each layer, however, these centers are distributed at random as in an ordinary liquid. TBBA enters the smectic A phase at 200°C, before undergoing transitions to two other more ordered smectic phases at lower temperatures. 

Some examples of pyrrolidinium-based ILCs have been reported in recent years, in all cases forming smectic phases, from low ordered SmA phases of de Vries type to highly ordered smectic phases of T type. These will be discussed in the following paragraphs. 

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