Properties of Combined LC Polymers

Due to the interesting LC properties of combined LC polymers (i.e. broad LC phases, and the occurrence of different smectic phases and a nematic phases at different temperatures) and their intermediate nature between that of side-chain and that of main-chain polymers, a lot of research has been undertaken on these materials. Most of the research has been directed towards the preparation of cross-linkable polymers and LC elastomers [3-11] and of chiral combined LC polymers [4, 6, 7, 9, 12-16]. 

The great potential of side-chain LC polymers has not yet been fully realized on a large scale. Nevertheless, their anisotropy nature and easy orientation in shear, magnetic, or electric fields, combined with the ability to preserve the specific oriented structure in the solid (glassy) state of the polymer, make them at the present time very attractive in a variety of application areas, such as optics and electro-optics, information and display technologies, photonics, and sensorits for the aeation of a new neration of field-responsive smart materials. Some of the most important properties of the LC polymers associated with their use in the above-mentioned areas of application are presented below (Chapters 8.06 and 8.09). 

On the other hand, the interest towards this field is accounted for by the possibility to create polymeric systems, combining the unique properties of low-molecular liquid crystals and high molecular compounds, making it feasible to produce films, fibers and coatings with extraordinary features. It is well-known that the utilization of low-molecular thermotropic liquid crystals requirs special hermetic protective shells (electrooptical cells, microcapsules etc.), which maintain their shape and protect LC compounds from external influences. In the case of thermotropic LC polymers there is no need for such sandwich-like constructions, because the properties of low-molecular liquid crystals and of polymeric body are combined in a single individual material. This reveals essentially new perspectives for their application. 

Chiral lc-polymers can be prepared by a proper functionalization of lc-polymers with chiral and reactive groups. These elastomers are interesting, because they combine the mechanical orientability of achiral lc-elastomers with the properties of chiral lc-phases, e.g. the ferroelectric properties of the chiral smectic C phase. The synthesis of these elastomers was very complicated so far, but the use of lc-polymers, which are functionalized with hydroxyl-groups, has opened an easy access to these systems. Also photocrosslinkable chiral lc-polymers can be prepared via this route. 

Completely new a ects came into this field with the development of LCPs. Generally, conventional non-mesomorphic polymers play an important role in many technological areas, as deduced from their countless applications [17]. In particular, by exploiting the variety in chemical structures polymeric materials with quite different macroscopic properties can be produced. Here, the realization of LCPs offers new and interesting scopes of tedmolo cal applications, based on the unique combination of specific polymer features with the anisotropic properties of the LC state [18-21]. 

The starting point for the preparation of combined liquid crystalline (LC) main-chain/side-chain polymers ( combined LC polymers) was merely a curiosity about what would happen if the structural properties of main-chain and side-chain polymers were combined (see Figure 1) [1]. As a result of these syntheses a variety of new LC polymers were obtained which showed a surprisingly broad range of LC phases. Further interest in combining main-chain and side-chain structures arose from the... 

The investigation of combined FLCPs was initiated by Zentel et al. [91-93] as a part of their approach to ferroelectric LC elastomers [94]. Figure 12 shows typical structures of combined FLCPs and cross-linkable chiral combined LC polymers. Poths et al. [67] used that approach to synthesize combined polymers with axially chiral mesogenic side groups (similar to the acrylic side-chain polymer above). The smectic C structure of polymers has been identified by optical microscopy and x-ray data, but no ferroelectric properties of the polymers have been reported yet. 

It is evident from these results that the interactive properties of the investigated SEC PS/DVB or DVB gels are very different. Because polar electroneutral macromolecules of PMMA were more retained from a nonpolar solvent (toluene) than from polar ones (THF, chloroform), we conclude that the dipol-dipol interactions were operative. Columns No. 1 and No. 2 were very interactive and can be applied successfully to LC techniques that combine exclusion and interaction (adsorption) mechanisms. These emerging techniques are LC at the critical adsorption point (18), the already mentioned LC under limiting conditions of adsorption (15,18), and LC under limiting conditions of desorption (16). In these cases, the adsorptivity of the SEC columns may even be advantageous. In most conventional SEC applications, however, the interactive properties of columns may cause important problems. In any case, interactive properties of SEC columns should be considered when applying the universal calibration, especially for medium polar and polar polymers. It is therefore advisable to check the elution properties of SEC columns before use with the... 

Liquid crystallinity can be attained in polymers of various polymer architectures, allowing the chemist to combine properties of macromolecules with the anisotropic properties of LC-phases. Mesogenic imits can be introduced into a polymer chain in different ways, as outhned in Fig. 1. For thermotropic LC systems, the LC-active units can be connected directly to each other in a condensation-type polymer to form the main chain ( main chain liquid crystalline polymers , MCLCPs) or they can be attached to the main chain as side chains ( side chain liquid crystalline polymers , SCLCPs). Calamitic (rod-Uke) as well as discotic mesogens have successfully been incorporated into polymers. Lyotropic LC-systems can also be formed by macromolecides. Amphiphihc block copolymers show this behavior when they have well-defined block structures with narrow molecular weight distributions. 

SCLCPs combine liquid crystalline properties and polymeric behavior in one material. If the mesogenic unit is fixed directly to the polymer main chain, the motion of the liquid crystalline side chain is coupled with the motion of the polymer backbone, preventing the formation of a LC mesophase. Therefore, Finkelmann and Ringsdorf proposed that the introduction of a flexible spacer between the main chain and the mesogenic unit would decouple their motions, allowing the mesogenic moiety to build up an orientational order [29,30]. 

The most intensively developed materials, however, are the azobenzene-containing side chain LC polymers, which show a unique combination of liquid crystallinity and photochromic behavior in a single macromolecule, whereas copolymerization of comonomers with different functionality allows fine tuning of phase behavior, photo-optical properties, and other parameters to the requirements of a particular application. Starting from the first publications of Eich et work in that area has been done by several research... 

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