Liquid crystalline polymers monotropic

Another interesting phenomenon found by Stevens et al. is the monotropic mesophase formed by the dimer and trimer of the polymer with m = 6. The dimer has a monotropic nematic phase, the trimer has a monotropic smectic phase. These metastable monotropic phases become stable enantiotropic phases with the increase of n by 1. At about the same time, Blumstein et al. (1984) found the low mass model compound of a main-chain type liquid crystalline polymer was monotropic while the mesophase of the polymer was enantiotropic. 

Zhou et al. (1986) studied the transformation of monotropic to enantiotropic mesophase of the main-chain type liquid crystalline polymer, (3.15), in terms of the molecular weight effect. 

At about the same time, Percec and Nave (1987) also found that the transformation of the monotropic liquid crystalline polymers to the enantiotropic ones could be achieved by increasing molecular weight. The polymer studied by these authors has the following structural formula... 

It should be mentioned that crystallization from a liquid crystalline phase may present its own peculiarities. The orientational order associated with mesophase may act as a precursor for further crystal growth, especially in monotropic liquid crystalline polymers where the metastability of mesophase generally leads to the formation of a more stable crystal phase. 

The thermal stability of liquid crystalline polymers can also be studied using X-ray scattering method. The powder X-ray diffraction data for the above polymer system annealed at different temperatures are shown in Figures 5.16 and 5.17. Although the polymer is a monotropic liquid crystalline polymer, at 145°C, the liquid crystalline phase is fairly stable, while at lower temperature, the polymer crystallizes and loses its mesophase character within a few minutes of annealing. 

Case 2. The structural unit displays a virtual or a monotropic mesophase the polymer displays a monotropic or an enantiotropic mesophase. The slope of the Tic-c versus molar mass (M) is steeper than that for Tc i, and as a consequence there will exist a critical value of M below which a mesophase is not observed. This effect has been observed in main chain and side chain liquid crystalline polymers. 

Let us first consider very briefly the influence of various parameters (i.e., nature of flexible spacer and its length, nature and flexibility of the polymer backbone and its degree of polymerization) on the phase behavior of a side chain liquid crystalline polymer. According to some thermodynamic schemes which were described elsewhere, the increase of the degree of polymerization decreases the entropy of the system and therefore, if the monomeric structural unit exhibits a virtual or monotropic mesophase, the resulting polymer should most probably exhibit a monotropic or enantiotropic mesophase. Alternatively, if the monomeric structural unit displays an enantiotropic mesophase, the polymer should display an enantiotropic mesophase which is broader. It is also possible that the structural unit of the polymer exhibits more than one virtual mesophase and therefore, at high molecular weights the polymer will increase the number of its mesophases. All these effects were observed with various polymer systems. ... 

Polymers formed between a and c, d and e all failed to show any liquid-crystalline behaviour. However, for all a examined (m = 2,4,6 and 8), nematic phases were observed with b-4 (all monotropic) - a further monotropic nematic material was the copolymer of a-6 and b-3. Unidentified crystal smectic mesophases were reported for a further three examples. 

A Cu2+ containing linear polymer [77] exhibiting smectogenic behaviour is obtained by a polycondensation technique (Fig. 27). A homologous set of polycondensates of 4,4 -[l,12 dodecanediyl bisfoxy)] bis benzoic acid with bis[AT-[[2,4 dihydrophenyl]methylene]-alkylamino] Cu(II) exhibits monotropic liquid crystalline behaviour with 4—13 carbon atoms in the alkylamino group [78]. 

The study by Percec, Tomazos and Willingham (15) looked at the influence of polymer backbone flexibility on the phase transition temperatures of side chain liquid crystalline polymethacrylate, polyacrylate, polymethylsiloxane and polyphosphazene containing a stilbene side chain. Upon cooling from the isotropic state, golymer IV displays a monotropic nematic mesophase between 106 and 64 C. In this study, the polymers with the more rigid backbones displayed enantiotropic liquid crystalline behavior, whereas the polymers with the flexible backbones, including the siloxane and the polyphosphazene, displayed monotropic nematic mesophases. The examples in this study demonstrated how kinetically controlled side chain crystallization influences the thermodynamically controlled mesomorphic phase through the flexibility of the polymer backbone. 

Doubly hydrogen-bonded supramolecular polymers 35 and 36 consisting of carboxylic acid and diaminopyridine moieties exhibit monotropic liquid-crystalline behavior [63a, 83]. A texture of dendritic growth characteristic of columnar phases is seen for the complex [63a]. 

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