Macromolecules with mesogenic groups

J. MACROMOLECULES WITH MESOGENIC GROUPS IN THE SIDE CHAINS... 

The conformational properties of macromolecules with mesogenic groups in the main chain were studied in a series of studies by VJ. Tsvetkov et al. for alkylene aromatic polyethers [21, 100-103] with a different structure and length of the flexible spacers [101-103] and for some copolymers containing mesogenic fragments in the main chain [100]. The synthesis and features of formation of the mesophase in these polymers are described in [104, 105] and in Chapter 5 of the present book. The orientational order was also studied by IR spectroscopy for alkylene aromatic polyethers in [106, 107]. 

Hydrodynamic Properties of Macromolecules with Mesogenic Groups in the Main Chain... 

Analysis of the complete dependence of the Kerr constant on M permitted establishing [21] a characteristic property of macromolecules with mesogenic groups in the main chain correlation of the longitudinal components of the dipole moment starts to become manifest with an increase in the molecular... 

Fig. 36. Liquid-crystal dendrimers (a) with mesogenic groups in the whole volume of a macromolecule (b) with terminal mesogenic groups.

Fig. 78a,b. Explanation of the sign of EB for a monomer (a) and a polymer (b) in solutions of comb-like macromolecules with mesogenic side groups... 

Penner T. L., Schildkraut, J. S., Ringsdorf, H., and Schustei A. Oriented Films from Polymeric Amphiphiles with Mesogenic Groups Langmuir Blodgett Liquid Crystals Macromolecules 24,1041 (1991). 

Penner T L, Schiidkraut J S, Ringsdorf H and Schuster A 1991 Oriented films from polymeric amphiphiles with mesogenic groups—Langmuir-Blodgett liquid-crystals Macromolecules 24 1041-9... 

Figure 4 Schematic representation of macromolecules of different rigidity (a) flexible chains (b) rigid-rod LC polymers (c) main-chain LC polymers with mesogenic groups (d) side-chain LC polymers with mesogenic groups (comb-shaped LC polymers).

TABLE 3.8. Optical Properties of Comb-Shaped Macromolecules with Mesogenic Side Groups [49]... 

Fig. 63 Schematic illustration of the structure of macromolecules of LC polymers with mesogenic groups in the side brandies.

Polymer LC systems of more complex structure which are int mediate between comb-shaped LC polymers and LC polymers with mesogenic groups in the main chains have recently been synthesized using polycondensation meAods [29-31]. A schematic illustration of their macromolecules is shown in Fig. 6.3g, h, and some examples of the synthesis of such polymers are given below ... 

The information on the structure of the cholesteric mesophase of polymers is currently limited to data on cholesteric polymers of the comb-shaped type. The comb-shaped structure of macromolecules with mesogenic side groups determines their tendency to form layered structures. In this respect, the question arises of how the helical supermolecular structure is formed in such a system and what its features are in comparison to the cholesteric structure of low-molecular-weight liquid crystals. The answer to this question is given in [81, 82], where the structure of homopolymers and copolymers forming the cholesteric mesophase was studied. 

The focus of studies of the processes of ordering of linear LC polymers with mesogenic groups in the main chains of the macromolecules has up to now been concentrated on the study of their orientational behaviw undo the effect of a magnetic field. Blumstein et al. were able to obtain the x-ray fiber pattran of the nematic azoxy derivative of the polyester... 

Macromolecules with Side-Chain Mesogenic Groups... 

Schematically the arrangement of a macromolecule with side-chain mesogenic groups in the liquid crystalline state is shown in Fig. 13. Flexible spacers give the mesogenic group its mobility. It is of interest to note that in such liquid crystals all positional mobility of the mesogen is based on conformational motion of the flexible spacer and backbone. Restricting this mobility either prohibits ordering, or freezes the order into the glassy state.

Investigations in the past years have proved that applying the concept of flexible spacer, polymers can be synthesized systematically, which exhibit the l.c. state. Owing to the flexible linkage of the mesogenic molecules to the polymer main chain, very similar relations can be expected with respect to 1-l.c., like chemical constitution and phase behavior, or dielectric properties and field effects for the l.c. side chain polymers. This will be in contrast to main chain polymers, where the entire macromolecule, or in case of semiflexible polymers parts of the macromolecules, form the l.c. structure. The introduction of a flexible spacer between backbone and mesogenic group can be performed in a broad variety of chemical reactions. Some arbitrarily... 

This review deals with LC polymers containing mesogenic groups in the side chains of macromolecules. Having no pretence to cover the abundant literature related to thermotropic LC polymers, it seemed reasonable to deal with the most important topics associated with synthesis of nematic, smectic and cholesteric liquid crystals, the peculiarities of their structure and properties, and to discuss structural-optical transformations induced in these systems by electric and magnetic fields. Some aspects of this topic are also discussed in the reviews by Rehage and Finkelmann 27), and Hardy 28). Here we shall pay relatively more attention to the results of Soviet researchers working in the field. 

Thus, the remoteness of mesogenic groups from the backbone provided by a polymethylene spacer secures them sufficient autonomy from the main chain. On the other hand, the fact that mesogenic groups are chemically linked with the main chain of the macromolecule assists their cooperative interaction. This is why comblike polymers have come to be accepted as convenient matrices for constructing LC polymers. Already a few hundred liquid-crystalline polymers with various mesogenic side groups have been synthesized. 

These recent theoretical results are of evident interest as they permit to a certain extent, to construct macromolecules of LC polymers with predetermined molecular parameters x1( x2 and x3. The above model probably lacks perfection because already cases are known when a spacer group partially takes part in mesophase formation (in contrast to the postulate of the work cited). It is, besides, well known that LC polymers are characterized with anomalously high relaxation times and a tendency towards the frozen glassy state. This implies that kinetic considerations should necessarily be involved to analyze the possibilities of formation of various meso-phases. This approach should be further expanded to polymers with mesogenic... 

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