Flexible Chain Polymers with Mesogenic Groups

2 Flexible Chain Polymers with Mesogenic Groups 

These are divided into two broad classes main-chain LCPs and side-chain LCPs. Because in the latter the mesogenic side-chains are usually rather short and of uniform length, we prefer to name the side-chain LCPs as pendant LCPs. Both main-chain and pendant LCP families have been extensively described in the literature [59,60, 92 - 96] so that they need not be described below in great detail. However, examples will be given of main-chain and pendant LCPs demonstrating various structural features and chemical families, in order for the reader to appreciate the variety of such LCPs and the synthetic ingenuity invested in their creation. 

Typical main-chain LCPs are described by the general structure 

Combinations of various lengths of m and n together with multiple X,Y and Z groups create a rich menu of main-chain liquid crystal polymers. 

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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).

Figure 2. Schematic of liquid crystalline order formation in polymers with mesogenic side groups decoupled from the main chain by a flexible spacer...

The conformational properties of polymer molecules with mesogenic groups in the main chain were studied in the series of alkylene-aromatic thermotropic polyesters in which the length of flexible methylene parts was varied polydeca-, penta- and tetramethylene-tere-phthaloyl-di-para oxybenzoate (P-IOMTB, P-5MTB, P-4MTB). 

Fig. 11 (a) Structure of the side-chain polymer with two different mesogenic groups (X, Y)=(R1, R2). The corresponding elastomer is obtained by crosslinking with R3, which can be either VI (flexible) or V8 (rigid), (b) 2D X-ray picture of the non-oriented elastomer... 

The first group includes liquid-crystalline systems based on polymers containing mesogenic groups in the main chain wh they alternate with flexible firagments, or in the side chains where they are joined to the main chain by flexible spacers. Phase equilibrium between these systems is due to thermal transitions from the crystalline state to some type of liquid-crystalline phase and subsequently to an isotropic melt. 

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]. 

Aguilera C, Ringsdorf H (1984) Thermotropic polyesters with mesogenic groups based on substituted hydroquinone units and highly flexible siloxan spacer in the main chain. Polym Bull 12 93-98... 

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... 

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