Dielectric spectroscopy of liquid crystal polymers

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). 

Because of the structural similarity between semi-rigid thermotropic polymers and LCPs, i.e. the presence of rigid moieties separated by flexible spacers, all polymers of this kind are commonly referred to as the main chain liquid crystal polymers (mcLCP). 

The mesogenic units can also be attached as pendants to a flexible backbone through flexible spacers (see Fig. 4.13), resulting in a completely new class of polymers with unique properties, so-called the side chain liquid crystal polymers (scLPC). scLCP manifest a unique competition between the tendency of the mesogenic units towards anisotropic liquid-crystalline order and the tendency of the backbone chain towards isotropic chain conformation. The liquid-crystalline order influences the polymer chain conformations and vice versa. The flexible 

For example, the interplay between the backbone chain and the mesogenic side chains can make the nematic phase of scLCPs more complex than a normal nematic (cf. Section 4.3). The nature and magnitude of such an interlay may be expected to depend, to a large extent, on the length, stiffness and shape anisotropy of the otherwise weakly interacting spacer group linking the side chains to the main chain. In the 

In the smectic phase of scLCPs, interactions between the liquid-crystalline and the backbone chain moieties takes a much more dramatic form. The side chains form the strongly interacting part of the smectic layers, while the weakly interacting backbone chain completes the layer and is basically excluded from the strongly interacting part. In a well-developed smectic phase the main chain thus finds itself very nearly confined to a two-dimensional plane within the smectic layers, with occasional interlayer hopping (see Fig. 4.13(c)). 

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