Macromolecules with Main-chain Mesogenic Groups

4 Macromolecules with Main-chain Mesogenic Groups 

The crystal to liquid crystal transition has been measured frequently, but not studied in detail. From the generally poor crystallization of the polyester types which make up the liquid crystals3) one expects typical samples to have low crystal- 

Shortening the flexible spacer must ultimately lead to rigid chain macromolecules. Many examples of such macromolecules have been reviewed by Preston99. An interesting example is the series of polyfethylene terephthalate-co-oxybenzoate)s 

Thermotropic Mesophases and Mesophase Transitions of Linear, Flexible Macromolecules 

C14H12 = 3,3 -dimethyl-4,4 -biphenyldiyl j) Mol- % ratio ethylene terephthalate to oxybenzoate 

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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.

The differences between standard thermotropic LCs and macromolecular condis crystals are summarized in Fig. 8. The first three and the last two points make it easy to experimentally identify low molecular mass LCs. For macromolecules, however, the viscosity may be suflBciently large to lose the obvious liquid character the birefringence does not always show the well-known LC texture (55) the small ASj of LCs may be confused with partial crystallinity of the condis crystals and in polymers, some larger main-chain rigid groups are not always easily identifiable as mesogens. This leaves points four and eight for differentiation between the two mesophases. Points five and six are more difficult to establish, and solid state NMR and detailed X-ray structure-determinations may be necessary for full characterization. Furthermore, borderline structures may be possible between thermotropic LCs, amphiphilic LCs, and condis crystals. A few examples and the resolution of their structures are discussed next, to illustrate the resolution of some of these problems. 

In examining the role of the main chain in the formation of the mesophase, the significant effect of the kinetic flexibility of the macromolecule on the temperature range of the existence of the mesophase cannot be overlooked. The values of Tg and T i for two series of polymers with the same mesogenic groups, the same spacer length, but different flexibility of the polymer main chain are... 

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... 

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).

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. 

MACROMOLECULES WITH MESOGENIC GROUPS IN THE MAIN CHAIN... 

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. 6.1. Schematic illustration of the maciomolecules of polymers with direct addition of mesogenic groups to the main chain (a), with binding groups (c), and the comb-shaped macromolecule (b) 1) main chain 2) mesogenic group 3) spacer 4) linkage.

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 ... 

With respect to the role of the configuration of the main chain in the formation of the LC phase, the situation here is close to what takes place in comb-shaped polymers without mesogenic groups a difference in the configurational structure of the macromolecules has little effect on the properties of the mesophase. A comparison of the properties of two samples of LC polymers with the general formula... 

Another model of the formation of the macromolecular chain in the LC phase is proposed in [66] based on the experimental data in [48] and [49]. The basic idea behind this model is the concept of the anisotropy of the local rigidity of the macromolecule and considering the smectic mesophase as a state with microphase separation of the mesogenic groups (which form the LC phase) and main chains (partially with aliphatic spacers perhaps), which are ejected from the smectic layer and form an intermediate layer [66]. In contrast to the model proposed in [48], the polymer backbone is located between the smectic layers (Fig. 6.19b), and passage of the chain through the smectic layer, although pos-... 

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... 

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