Smectic optical textures

Note 4 At one time, a number of mesophases were identified as smectic on the basis of their optical textures, but they are in fact soft crystals characterised by very low yield stresses. Hence, these three-dimensionally ordered phases should no longer be called smectic mesophases. They are akin to plastic crystals with some elementary long-range order and are referred to by the letters E, J, G, H, and K. 

As in the case of low-molecular liquid crystals the majority of information about the structure of LC polymers is obtained from their optical textures and X-ray diffraction data. Because of high viscosity of polymer melts, which results in retardation of all structural and relaxation processes it is quite difficult to obtain characteristic textures for LC polymers. As is noted by the majority of investigators smectic LC polymers form strongly birefringent films as well from solutions, as from melts11 27-... 

Smectic phases described up to date in the literature, are restricted mainly to A, B and C phases (SA, SB and Sc — respectively) for low-molecular liquid crystals at the same time, there are already reported around ten smectic phases. Identification of polymeric smectics according to their optical textures is, with rare exceptions, impossible, as their textures are quite alike. 

All polymers showed semicrystalline behavior, as demonstrated by WAXD measurements. DSC thermograms were presented, but LC properties were found to be difficult to determine due to the concomitant thermally-induced crosslinking of the unsaturated polymers upon melting. In some cases smectic B phases were identified by POM. The melting temperatures of the saturated analogues were lower than for the corresponding unsaturated derivatives. Optical textures of the saturated polymers showed lancets in the background, typical of a solid-like smectic phase [89]. 

Because of the additional translational order, the dislocations can exist in the cholesteric and smectic liquid crystals, which makes the texture of these liquid crystals even more complicated. Each liquid crystal phase shows characteristic textures and thus the optical texture becomes an important means to differentiate the phase of the liquid crystals. Liquid crystalline polymers have the same topologically stable defects as small molecular mass liquid crystals do, but the textures may be different due to the difference in the energetic stability of the same topological defects in both low molecular mass and polymeric liquid crystals (Kleman, 1991). In Chapter 3 we will discuss the textures in detail. 

The optical microscopy of these polymer liquid crystals identifies both polymers as having smectic A phases only. However unlike monomer materials the evolution of a particular well defined texture may take several hours. As is shown in plates l(a-c) the smectic fan texture can be seen to grow gradually over 15 hours on annealing just below All of the photomicrographs are for PG296 which... 

The simplest PPEs are the dialkyl-PPEs 4 made by either alkyne metathesis of dialkyldipropynylbenzenes or by Pd-catalyzed couplings. The materials are thermally and photochemically very stable and form thermotropic liquid crystalline phases of the smectic variety [55], Optical textures under crossed polarizers resemble those of nematic phases due to the homeotropic alignment of the chains on glass... 

The formation of a nematic texture [9] in case of the homopolymer (6b) and of a smectic A texture [9] (s a) in case of the homopolymer (6c) was ascertained by Polarized Optical Light Microscopy (POM) in combination with a heating stage as shown in the POM-image in chart 7. POM was performed on a microscope of the company OLYMPUS in combination with a heating stage FP 82HAT of the company Mettler-Toledo. 

The formation of a smectic C texture [9] (s c) was ascertained by Polarized Optical Light Microscopy (POM) in combination with a heating stage as shown in the POM-image in chart 10 and by X-ray diffraction. 

The optical polarizing microscopy analysis of these copolymers showed in all cases schlieren nematic textures, in contrast to the corresponding polyethers which present smectic type textures. All these copolyethers are soluble in aromatic and halogenated solvents. 

Fig. 4. Typical polysiloxane side-chain liquid crystal optical textures for (a) nematic, (b) cholesteric and (c) smectic A phases.

Fig. 13. Biphasic optical texture showing the coexistence of the smectic A (Sa) and isotropic (I) phases.

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. 

Smectics F and I are tilted analogues of the hexatic Bh phase. They differ from each other by direction of the molecular tilt with respect to the vectors of the in-plane lattice. Their optical textures remind us of the smectic C phase. 

Short-chain derivatives exhibit two meso-phases unambiguously identified by their optical textures as a lamellar phase of smectic C type and a nematic phase (e.g., = 7 ... 

The uniaxial nematic (Ny) phase usually exhibits a schlieren texture with two and four point singularities (two and four brushes). However, the texture exhibited by complex 2, shown in Fig. 4, often consists of [S] = l/2 (two-brush) disclinations similar to that exhibited by a smectic C phase. Also, this complex showed zig-zag disclinations [24] occasionally, and since this type of disclination has been observed in N phases [25] it cannot be regarded as conclusive evidence for biaxiality. It is also possible that an entanglement of disclination lines may lead to topological rigidity, as discussed by Toulouse [26] and others [27-29]. These possibilities have not been examined experimentally. Hence optical textures that... 

The banana molecules are too interesting to say now since the ferroelectric phase has been demonstrated, let s move on. Their bizarre behavior sometimes leads to ferroelectric and sometimes to antiferroelectric phases. Their optical texture is also not uniform, but they exhibit a polydomain liquid crystal phase, as if they would contain a large amount of impurities (of course the compounds were as pure as possible). The reason for this erratic behavior was proposed at a Gordon conference in 1997 the complexity of the behavior stems from the tilting of the molecules with respect to the smectic layer. Initially, we considered the molecular tilting in the B2 phase because the measured thickness of the smectic layers was far less than the calculated molecular length [131]. This fact had always been in the back of my head, but I could not find a reason why the molecular axis must tilt. 

Figure 9-10(a) shows polarizing micrographs of the Mo-based PPCH503A polymer. It is evident that an optical texture of batonet, characteristic of a smectic liquid crystal, is oriented along the direction of the shear stress. From these results, we can confirm that the side chain liquid crystalline conjugated polymers are macroscopically aligned by the shear stress as an external force. 

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. 

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). 

With respect to the higher temperature transition at 445 °C, there are two conflicting views of this transition, namely that the phase above 445 °C is a smectic C and the other that it is nematic. Based on high temperature X-ray diffraction studies, Yoon et al. have concluded that it is a smectic C (see Fig. 4) [28], Thus, in Fig. 4, the disappearance of the 211 peak indicates that the nematic E structure is converting to a nematic C. In our work, using polarizing optical microscopy, we have observed a nematic texture for high molar mass specimens heated rapidly to 480 °C, sheared, and then quenched. In the case of a... 

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