Macromolecules with Side-chain Mesogenic Groups

3 Macromolecules with Side-Chain Mesogenic Groups 

Earlier studies of macromolecules with flexible, non-mesogenic side-chains had revealed that the transition properties change with increasing side-chain length from typical polymeric behavior to that of the small molecule which corresponds to the side chain 72,73). With short side-chains and proper backbone geometry (tacticity), 

Side-chain Main-chain Rigid Chain 

The transition behavior of a number of liquid crystals with side-chain mesogens is summarized in Table 5. The most obvious feature of macromolecular liquid crystals is the frequent absence of fully ordered crystals at low temperatures. If fully ordered crystals are observed, crystallization is incomplete, i.e. the observed phase states are to be described by an area on the right side of Fig. 3. Glass transitions, which were hard to find in low molecular weight liquid crystals (see Table 3), are now prominent. 

Information on the crystal to liquid crystal transitions is scarce and is to be treated with caution since partial crystallization is prominent and polymorphism of the smectic phase is frequent. Only the data on poly(acryloyloxybenzoic acid) (entry 2 of Table 5) have been extrapolated to 100% crystallinity. As with the low molecular weight liquid crystals, the total heat of transition is lower than expected for fully ordered crystals. Various combinations of two phase structures as suggested by Fig. 3 could be produced for the poly(acryloyloxybenzoic acid)21 . 

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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.

Figure 1. Structure of macromolecules with side meso-genic groups, (a) Mesogenic groups are directly linked to the main chain (b) meso genic groups are linked to the branches of comb-like polymers.

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. 

This can be explained by the fact that in a polymer molecule (Fig. 78b) the longitudinal components of monomer unit dipoles mh are mutually compensated and the main part in the observed EB is played by normal components of monomer unit dipoles, mi, which can be parallel to the main chain of the macromolecule owing to its comb-like structure. In other words, in molecules of comb-like polymers containing mesogenic side chains, the orientations of the Mi components of the side group dipoles are correlated with each other. As a result, the macromolecule as a whole or part of it can exhibit a considerable dipole moment m in the direction of the main chain L (Fig, 78b). The existence of this dipole accounts for the orientation of the main chain in the field direction leading to negative EB. 

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).

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

The possibility of synthesizing a large number of comb-shaped thermotropic polymers with a different structure of the main chain [31-35], different mesogenic fragments [36-41], and a different type of connection to the main chain provides broad opportunities for their investigation [1, 2, 19, 31, 43-50]. The presence of chemical groups which cause the formation of the liquid-crystalline phase in their side chains, while the main chain of the molecule can be flexible, is a basic structural feature of these comb-sh )ed mesogenic macromolecules. 

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