Mesogenic polymers, mesomorphic state

Chien and Cada [42] have prepared optically active and photoactive SCLC copolymers, 15, with the 4-alkoxyphenyl-4 -alkoxycinnamate chromophore, with the intention of creating LC polysiloxane networks that could be used to prepare macroscopically oriented organic ferroelectric polymers for electro-optical devices. Optical activity was introduced into the polymer by the use of a chiral spacer. Those copolymers which were mesogenic exhibited properties characteristic of a Sc. phase. UV-irradiation of thin films of the polymers in their mesomorphic states at 90°C, led to a loss of the IR absorption at 1635 cm-1 that is due to the cinnamate double bond, and to cross-linking. Long-term irradiation led to... 

In between the amorphous phase and the crystalline phase, there sometimes occurs an important intermediate phase for polymers carrying anisotropic groups (called mesogen groups) either on the chain backbone or on the chain branches, which is referred to mesophase. The mesogen groups become orientational-ordered under suitable thermodynamic conditions. This ordered state could be the well-known liquid crystal (LC) state, as a typical mesomorphic state between the amorphous state and the crystalline-ordered state for LC polymers. 

The mesomorphic state may be realized in two ways, namely, the two ways in which ordinary fluid phases are formed from solids dissolution and fusion. These two categories of mesomorphism are called lyotropism (liquid-crystalline solutions) and thermotropism (liquid-crystalline melts), respectively. The latter category consists of single-component substances and encompasses the large variety of low molecular weight mesogens used in LCDs. More recently, thermotropic specialty polymers... 

Brown GH, Shaw WG (1957) The mesomorphic state— liquid crystals. Chem Rev 57 1049-1157 Cardinaels T, Ramaekers J, Guillon D, Donnio B, Binnemans KA (2005) Propeller-like uranyl metallomesogen. J Am Chem Soc 127 17602-17603 Chai CP, Zhu XQ, Wang P, Ren MQ, Chen XF, Xu YD, Fan XH, Ye C, Chen EQ, Zhou QF (2007) Synthesis and phase structures of mesogen-jacketed liquid crystalline polymers containing 1,3,4-oxadiazole based side chains. Macromolecules 40 9361-9370 Chandrasekhar S, Sadashiva BK, Suresh KA (1977) Liquid crystals of disc-Uke molecules. Pramana J Phys 9 471 80... 

The mesogenic structure of a benzoic acid dimer has been introduced as a noncovalent cross-linker for polysiloxanes [79]. Polymer 57 exhibits a smectic C phase due to the dynamics of H-bonding. In contrast, mesomorphic order is locked in the solid state of poly[(4-acryloyl)benzoic acid] by polymerization in its mesophase [128]. No liquid-crystalline state is observed for this material because of the lack of flexibility of the structures. Main-chain-type polymeric liquid-crystal associates are formed from carboxyl-bifunctionalized aromatic compounds [129]. 

As pointed out by Finkelmann in his recent review, the essential features of the phase behaviour of liquid crystalline side-chain polymers are relatively well established. Usually, the DSC trace exhibits a glass transition, characteristic of the polymer backbone, and a first-order transformation from the mesophase to the isotropic phase due to the mesogenic side chains. Complementary studies with a polarizing microscope reveal that the texture observed in the liquid crystalline state can be frozen without change in the glassy state. A given polymer may have several mesomorphous phases and, so far as can be gathered from observations, these are then separated from one another by first-order transition points. 

The first liquid crystal polymers (LCPs), in which structural moieties known to lead to mesomorphic behavior in small-molecule liquid crystals have been incorporated into the main chain, were reported in early 1980s. The presence of the mesogenic units in the polymer chain enhances the tendency for the material to form the liquid crystalline state, and these polymers also show better thermal stability. Both the nematic and smectic phases can be observed in these materials, (see Fig. 4.12). 

Dynamics of the mesogenic unit and of the backbone in the mesomorphic and isotropic states. Above the static glass transition two new intramolecular motions become active in side-chain liquid crystal polymers. Firstly, as in flexible-chain polymers, the micro-Brownian segmental motions of the backbone chain are successively liberated. The increased mobility of the backbone chain enables in turn large-scale reorientations of the long axis of the pendant mesogenic units. As a result, new DR relaxation processes appear. ... 

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