Reorientation side chain mesogens

Fig. 15 a The structure of the side chain SLC block copolymer 30-31 used by Ober and Thomas to access temperature-dependant photonic band gap materials b A Schematic of the proposed model for free mesogen formation believed to be important for the reorientation of blends heated below ODT in an applied AC field... 

Liquid Crystalline Polymers are an important class of polymeric materials because they may exhibit optical properties similar to low-molar-mass liquid crystals and high mechanical properties of polymers. These polymers are broadly classified based on their molecular architecture, i.e. attachment of the mesogen to the polymeric backbone, as main-chain liquid crystal polymers (i) or side-chain liquid crystal polymers (2). In main-chain liquid crystal polymers, mesogens are incorporated into the backbone. The mesogens may be of different shapes and sizes, and are usually rodlike or disklike. Such polymers have not been used for optoelectronic applications because it is very difficult to reorient these materials by electric field. Instead, these materials find applications that use their exceptional mechanical properties. Even side-chain liquid crystal polymers, whose mesogen is attached to the polymer backbone through a flexible spacer switch too slowly for... 

Dielectric relaxation spectroscopy is widely used to study molecular dynamics of conventional and liquid crystal polymers. Since the mesogenic groups of a side-chain liquid crystal polymer contain strong permanent dipoles, the technique may be utilized to study the reorientation and, as Attard et have shown, the level of... 

If the main chain takes part in the reorientation, it is expected that its motion will be, at least partially, shifted in phase with respect to the side chain (spacer and mesogen). A detailed 2D analysis [30] in the region of the v(CH ) bands (2926 and 2863 cm ) does not reveal any significant asynchronicity. Hence, we may conclude that the main chain does not take part in the reorientation, or it moves only slightly. 

Interestingly, IR dichroism studies on liquid crystalline polymers can reveal some insight into the mechanism of mechanical stress-induced orientation processes. For example, in studies with polyurethanes carrying mesogenic side chains (Chart 2.7), it was found that an initially stress-induced orientation system would be broken up and reoriented when the strain surpassed a critical value [77]. 

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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