Mesogenic units, SCLCP

In general terms, high molar mass liquid crystals are classified according to the location of the mesogenic unit in the polymer. Thus, they are either incorporated into the main chain (main-chain liquid crystal polymers - MCLCP Fig. 5A) or they are pendant from the main chain (side-chain liquid crystal polymers - SCLCP Fig. 5B). 

All Liquid Crystal Polymers are characterised by the fact that they contain stiff meso-genic groups, often inserted in flexible chain systems (so called "spacers") and connected to them by linking functional groups the mesogenic unit is inserted either in the main chain or in the side chains or (in exceptional cases) in both. We shall discuss MCLCPs and SCLCPs. A schematic representation of common structures of LCPs is displayed in Fig. 6.14 (Jansen, 1996). An example of a SCLCP with disc-like mesogens is displayed in Fig. 6.15 (Franse et al., 2002, 2004). 

Ring opening metathesis polymerization (ROMP) can be used to build up SCLCPs using various mesogenic units. ROMP-derived SCLCPs exhibit a num-... 

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]. 

Cho et al. described the synthesis and polymerization of 4,8-cyclododeca-dien-l-yl-(4 -methoxy-4-biphenyl) terephthalate VIII [54,55]. Polymerization was carried out with WCl4(OAr)2/PbEt4. The double bonds in the polymer backbone were subsequently hydrogenated with H2/Pd(C), leading to a SCLCP with a fully saturated hydrocarbon backbone. This polymer system had a very flexible polymer backbone but a stiff connection between the main chain and the mesogenic unit. The distance between two adjacent side chains was about 12 methylene units. This very flexible main chain allowed the polymer to organize into a LC mesophase. Both polymers - the unsaturated and the saturated -showed smectic liquid crystalline mesophases with almost the same transition temperatures (see Table 5). 

A block copolymer consisting of a SCLCP-block of monomer XXVII with a laterally-attached mesogenic unit, and butyl-acrylate, was synthesized using a combination of ROMP and atom transfer radical polymerization (ATRP) (Fig. 16) [81]. 

Discotic SCLCPs were synthesized by the group of Grubbs [82]. For this purpose, norbornenes XXVIII-n (n=5, 10) and cyclobutenes XXIX-n (n=5, 10) with alkoxy-substituted triphenylenes as mesogenic units (see Fig. 18) were prepared. Polymerization was carried out with initiator 6. The resulting polymers had a narrow PDI between 1.09 and 1.17. Physico-chemical data for poly-XXVIII and poly-XXIX are listed in Table 18. 

Many reviews [1-3] of SCLCPs describe the structure of the backbone (main chain), the spacer (flexible linkage), and the side group (mesogenic unit) of the SCLCP. For example, the most widely used backbones include polyacrylates or polymethacrylates, polysiloxanes, and polyphosphazenes po-ly-ct-chloroacrylates, itaconates, and ethylene oxides have also been reported. 

Side chain liquid crystal polymers (SCLCPs) consist of mesogenic structural moieties appended from a polymer backbone (Figure 5.2). The mesogenic units that have been used parallel those previously used for low molar mass liquid crystals, and the stmctural nature of the polymer backbone is widely variable. The mesogenic units (usually calamitic but many discotic types exist) are invariably separated from the polymer backbone by fairly long spacer units which are usually several methylene (-CH -) units,... 

Clearly the mesogenic unit will have a great influence on the liquid crystal phases generated and the transition temperatures. Just as was seen in Chapter 3, there is an enormous number of mesogenic units and each can easily be adopted as a side chain mesogenic unit for an SCLCP. Figure 5.6 shows a typical template for some possible mesogenic units commonly employed in SCLCPs (m and n are usually one or two). 

LCPs combine the mechanical properties of polymers with the order of LCs. The low molar mass mesogenic units, which form a LC phase, consist of a rigid core. If this core is extended, so-called main chain liquid crystalline polymers (MCLCPs) are obtained. A second method to obtain LCPs is to connect low molecular weight LC (LMWLC) units via flexible spacers. This approach allows the preparation of side chain liquid crystalline polymers (SCLCPs) (Box 1). 

There are different polymer structures that exhibit LC phases. The extension of the mesogen s rigid aromatic core leads to main chain polper systems. Other approaches are based on the spacer concept. Here rigid cores that are common in LMWLCs are decoupled by flexible alkyl chains. The mesogenic units can be placed either in the main or in the side chain. Also combined MCLCP/SCLCPs have been prepared (figure 3). 

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... 

We must also be aware that the dynamics of the mesogenic unit in each of the angular coordinates, i.e. a, jS, and y (cf. Fig. 4.4), can be significantly different, depending on the local environment, even on the small angular scale. For example, rotation about the long axis (y) in scLCPs can be much faster than two small-scale motions of the long axis, i.e. precession... 

The slowdown of the reorientation rate, increase of the activation energy, and the distribution of relaxation times in PA-scLCPs reflect the presence of the anchoring, the different environments and the dynamics that a mesogenic unit experiences in scLCPs in the isotropic and liquid crystalline states. This can be nicely demonstrated by variation of the polymer architecture. 

Broadening of the distribution of relaxation times is also found to be caused by interactions between the mesogenic units. Haase et al and Parneix et al compared the dielectric properties of two poly acrylic scLCPs with slightly different mesogenic units PA/6/-/CN and PA/6/COO/CN. Both materials demonstrate a profound difference in the quasi-static dielectric permittivity and dielectric anisotropy gJ (PA/6/ -/CN) 12 versus 8 (PA/6/COO/CH) 19, and (a — i) 65 for the former versus 12 for the latter. Consequently, the strength of 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. 

In this chapter we want to discuss the correlation of the mesophase behavior of a cyanobiphenyl-based SCLCP with its backbone structure. As shown before, the backbone structure, the spacer lengths, and the mesogen density per repeat unit have great influence on the LC mesophase evolved. Ligure 8 shows some examples of backbone structures bearing the cyanobiphenyl-moiety that have been reported in literature. The above-mentioned ROMP-derived polymers poly-(II-n) [39],poly-(IV-n) [42,47],poly-(VI-n) [41],andpoly-(VII-n) [53] will be compared with each other and with acrylate-based [56-59], siloxane-based [60] and vinylcyclopropane-based systems [61]. The detected mesophases and their transition temperatures are summarized in Table 6. 

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