Mesophases and Their Transitions

Table I. Cholesteric mesophases and their transition temperatures.

In this chapter we want to discuss the correlation of the mesophase behavior of a cyanobiphenyl-based SCLCP with its backbone structure. As shown before, the backbone structure, the spacer lengths, and the mesogen density per repeat unit have great influence on the LC mesophase evolved. Ligure 8 shows some examples of backbone structures bearing the cyanobiphenyl-moiety that have been reported in literature. The above-mentioned ROMP-derived polymers poly-(II-n) [39],poly-(IV-n) [42,47],poly-(VI-n) [41],andpoly-(VII-n) [53] will be compared with each other and with acrylate-based [56-59], siloxane-based [60] and vinylcyclopropane-based systems [61]. The detected mesophases and their transition temperatures are summarized in Table 6. 

Sect. 5.5. For general literature on the discussions of mesophases and their transitions see also Sect 2.5. Many of the here discussed examples are reviewed in Gordon M, ed (18983/84) Liquid Crystal Polymers I-III. Springer, Berlin (Adv Polymer Sci, Vols 59-61). 

A concise review of the relative order, mobility, density, and possible types of phase transitions of one-component systems is presented by the schematic of Fig. 2.115, along with the dictionary definition of the word transition. This schematic is discussed in Sect. 2.5 in connection with an initial description of phases and their transitions. More details of the structure and properties of crystals, mesophases, and amorphous phases are given in Chap. 5. Some characteristics of the three types of mesophases are given in Fig. 2.107. Quantitative information on the thermodynamic parameters of the transitions between the condensed phases is summarized in Fig. 2.103 and described in more detail in Sect. 5.5. The dilute phases in Fig. 2.115, the gases, are of lesser interest for the present description, although the ideal gas law in Figs. 2.8 and... 

Further specific volume measurements with chiral mesogens, cholesteric, and blue phases have been made [26-28, 159-161]. The influence of chiral dopants on C/A and C/N phase transitions has been studied [37, 162]. Earlier studies, particularly on cholesteric mesophases and their mixtures, have been published, for example, for cholesteryl acetate [163], cholesteryl myristate [53], cholesteryl nonanoate [54], cholesteryl... 

In 1978, Bryan [11] reported on crystal structure precursors of liquid crystalline phases and their implications for the molecular arrangement in the mesophase. In this work he presented classical nematogenic precursors, where the molecules in the crystalline state form imbricated packing, and non-classical ones with cross-sheet structures. The crystalline-nematic phase transition was called displacive. The displacive type of transition involves comparatively limited displacements of the molecules from the positions which they occupy with respect to their nearest neighbours in the crystal. In most cases, smectic precursors form layered structures. The crystalline-smectic phase transition was called reconstitutive because the molecular arrangement in the crystalline state must alter in a more pronounced fashion in order to achieve the mesophase arrangement [12]. 

Several liquid-crystalline mixtures have been employed In this work. They and several of their physical characteristics are collected In Table I. As expected, the bulky BN molecules disturb mesophase order causing transition temperatures to be lowered. Monotropic c+K transitions are approximate since they depend upon the rate of cooling and other factors. 

For a specific polymer, critical concentrations and temperatures depend on the solvent. In Fig. 15.42b the concentration condition has already been illustrated on the basis of solution viscosity. Much work has been reported on PpPTA in sulphuric acid and of PpPBA in dimethylacetamide/lithium chloride. Besides, Boerstoel (1998), Boerstoel et al. (2001) and Northolt et al. (2001) studied liquid crystalline solutions of cellulose in phosphoric acid. In Fig. 16.27 a simple example of the phase behaviour of PpPTA in sulphuric acid (see also Chap. 19) is shown (Dobb, 1985). In this figure it is indicated that a direct transition from mesophase to isotropic liquid may exist. This is not necessarily true, however, as it has been found that in some solutions the nematic mesophase and isotropic phase coexist in equilibrium (Collyer, 1996). Such behaviour was found by Aharoni (1980) for a 50/50 copolymer of //-hexyl and n-propylisocyanate in toluene and shown in Fig. 16.28. Clearing temperatures for PpPTA (Twaron or Kevlar , PIPD (or M5), PABI and cellulose in their respective solvents are illustrated in Fig. 16.29. The rigidity of the polymer chains increases in the order of cellulose, PpPTA, PIPD. The very rigid PIPD has a LC phase already at very low concentrations. Even cellulose, which, in principle, is able to freely rotate around the ether bond, forms a LC phase at relatively low concentrations. 

As a result of their low Tg values and lack of crystallinity, many of these polymers showed liquid crystallinity at room temperature. The liquid crystalline mesophases of the monomers were identified as nematic but the polymeric mesophases were not identified, although they possessed a very broad thermal stability between their Tg and their clearing transitions. A mesophase temperature stability of up to 170 °C was observed for the polymers with bicyclohexane central mesogenic units. These polymers showed decreases in Tg and Tj with increased spacer lengths. 

A few series of azo and azoxy group containing liquid crystalline copolyesters were prepared by limura and coworkers and their phase transition temperatures were examined. No unusual phenomena were found, although monotropic mesophases were observed for the following copolyesters, depending on the combinations and fractions of alkylene groups ... 

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