Mesophases types

Figure 9.1 Examples of texture of the materials formed on a short range from amorphous Si02 (a) silica gel (b) hydrothermally treated silica gel (c) porous glass (d) mesoporous mesophases type of MCM-41 and (e) opal.

The study of blends of polymeric liquid crystals with low-molecular liquid crystals of known mesophase types, aiming at identification of polymeric mesophases, is at its very beginning there are only a few works concerning polymers with mesogenic groups in the main chain 67 69) and in the side chains as well 70 74). in view of the importance of such investigations, note that the principle of miscibility is thoroughly developed for low-molecular liquid crystals, whose molecules are similar in their sizes the justifiability of its application to the blends of polymers with low-molecular liquid crystals is not equally evident, as the molecular sizes of the components differ substantially. 

As is seen from Table 5, there are quite a few polymers, that demonstrate an SB mesophase type. X-ray patterns of such polymers, along with a series of small angle reflexes, display, as a rule, a sharp and intensive reflex at wide scattering angles (Fig. 12). For LC polymers of this type there exist two possibilities of packing with... 

A specific feature of all polymeric smectics, independently of the mesophase type, is that the regions of LC ordering are limited, while low-molecular liquid crystals display macroscopic ordering. This follows from the analysis of absolute intensities of small angle X-ray diffraction performed recently108-l09) (see polymers C. 1.1-C. 1.5 Table 4). 

The case is quite different for copolymers 5-5. Here, the transformation from a nematic mesophase to a smectic one is a result of gradual decrease in flexibility of the main chain. As seen from Fig. 22a, b, the plots of Tcl and AHC, on composition have a minimum. This could be the consequence of a more defective packing of side groups due to incorporation within the backbone of units of different chemical nature. A sharp rise in AHC, indicating transformation to a smectic mesophase type, begins only at... 

Summing up, to the present time a substantial amount of data concerning the creation of thermotropic LC polymers has already been accumulated, methods for their synthesis were developed, certain relationships between the structure of the polymer and the mesophase type were established, the structure and some properties of such systems were studied. 

Segregation of the incompatible molecular segments takes place with formation of distinct nano-compartments organized on a one-dimensional (ID), two-dimensional (2D), or three-dimensional (3D) periodic lattice, separated by interfaces. These interfaces tend to be minimal in order to reduce the interfacial energy stored in the system. For amphiphilic molecules without anisometric segments (flexible amphiphiles) the mesophase type is mainly determined by the relative volume of the two incompatible segments, as shown in Fig. 2. 

Fig. 51 Examples of T-shaped bolaamphiphiles with non-polar lateral RF/RH-chains showing the effect of fluorination on mesophase types and transition temperatures (77° C) [316-318, Lehmann A, Prehm M, Tschierske C, unpublished results, 319]...

The technique is strictly complementary to optical microscopy, as all changes in optical texture do not necessarily correspond to a change in mesophase type and similarly all phase changes do not always lead to an easily identifiable change in texture. Therefore DSC traces should always be compared with the results of the optical microscopy to be sure of proper correspondence. 

Similar conditions, with two or more different mesophases, were encountered in several other ternary systems. Special attention was devoted to ascertaining whether there was coexistence between hexagonal mesophase type E, linear type D, and their interjacent two-phase region. 

For a review of viscometric studies on the different mesophase types see... 

The type of phase formed in a polymer liquid crystal can also be identified often by examining the manner in which it mixes with a small molecule mesogen of known mesophase type. If these textures are the same, then a mixed liquid crystal phase is formed with no observable transition between the two types of molecule. 

Mesophase Type strongly dependent on the ratio of main-chain and side-group Spacer Length ... 

Differential scanning calorimetry (DSC) is a valuable aid by which phase transition temperatures, transition heats, and transition entropies can be conveniently measured or calculated. This technique offers a direct and complimentary (to microscopy) evaluation of thermal behavior. Figure 4 shows the DSC curve for 4-octyloxybenzyli-dene-4 -chloroaniline in which can be seen Kj -K2, K2 Sg, Sg-S, and S -I transitions. All transitions are enantiotropic and all are reversible. The most extensive supercooling occurs for the meso-phase-solid transition, in this case the Sg-K2 transition. Optical microscopy and/or x-ray diffraction is required to assign the specific mesophase type. 

Chen, W. (1996) in Liquid-Crystalline Polymer Systems (ed. A.I. Isayev, T. Kyu and S.Z.D. Cheng), American Chemical Society, Washington, chapter 15], and liquid crystals (that is, condensed phases in which molecules exhibit orientational order and varying degrees of positional order [2]). The distinctions among these three mesophase types will be maintained throughout this chapter liquid crystalline systems which are thermotropic (that is, induced by changes in the temperature or pressure of a sample) rather than lyotropic (that is, induced by addition of an isotropic liquid as solvent ) will be emphasized. 

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