Mesogen linkage

A further variant on the spacer-mesogen linkage follows from the lath shape of typical mesogens. In most comb LCPs the linkage is to the end of the long axis of the mesogen (longitudinal attachment), but connection... 

Investigations in the past years have proved that applying the concept of flexible spacer, polymers can be synthesized systematically, which exhibit the l.c. state. Owing to the flexible linkage of the mesogenic molecules to the polymer main chain, very similar relations can be expected with respect to 1-l.c., like chemical constitution and phase behavior, or dielectric properties and field effects for the l.c. side chain polymers. This will be in contrast to main chain polymers, where the entire macromolecule, or in case of semiflexible polymers parts of the macromolecules, form the l.c. structure. The introduction of a flexible spacer between backbone and mesogenic group can be performed in a broad variety of chemical reactions. Some arbitrarily... 

Gangadhara et al. have linked the cyanobiphenyl mesogen via a dicarbox-imide-group to an oxanorbornene ring system Vl-n, n=2-8 (see Fig. 4). Polymerization was carried out with Schrock type initiator 4. The dicarboximide linkage probably hindered the formation of LC phases even the introduction of relatively long spacers between the polymer backbone and the mesogen did not lead to liquid crystalline monomers or polymers [41]. 

More recently, a systematic study of the properties of side-chain LC dendrimers has been undertaken on these PPI dendrimers and also on PAMAM systems by Serrano et al. [211]. In these dendrimers, the mesogenic group is connected to the dendritic scaffold by an imine linkage. In all cases, it was found that the enthalpic gain of the mesogenic units arranged as in a classical liquid crystalline mesophase dominates over the entropic tendency of the dendrimer core to adopt a globular isotropic conformation. The flexibility of the dendritic PAMAM and PPI cores allows the macromolecule to adopt... 

Polymers with methyl substitution on the central linkage in the stilbene mesogenic group have been prepared, Polymers 4 and 5, and show strictly nematic mesophases . The methyl group seems to act mostly to inhibit the existence of smectic order, but otherwise does not influence the mesophase formation or thermal stability very much. 

The terphenyl unit is a very rigid, extended mesogenic structure because of the direct linkage between phenylene rings. Polymers 17 and 18 of Table 1. A series of polymers... 

The synthesis described in this paper renders possible the preparation of block copolymers of uniform molecular weight composed of amorphous and LC side chain blocks. Beside the specific cholesterol mesogen introduced by carbonate linkages leading to a smectic system other mesogens with various spacer lengths can be introduced, e.g. by esterification. 

A very recent synthesis of a thermotropic copolyimide contains an AT-(car-boxyphenyl) trimellitimide ester unit linked to a p-aminobenzoic acid via an amide linkage, which in turn is linked by an alkane diol unit back to the trimellitimide unit [86]. This is probably the first example of a copoly(ester-amide-im-ide) with mesogenic properties identified. Its MI score is 9.6. 

The effect of a combination of photochromic azobenzene with mesogenic units within the same polymer on cooperative motion and anisotropic enhancement is exemplified by the CPI and K1 polymer systems of Zilker et al. [53] shown in Fig. 6.16. In the CPI-50 polymer, a clear distinction between chromophore and cooperative mesogen (CPI-50 polymer) existed. There was no apparent distinction between chromophore and mesogen in the Kl-40 polymer because mesogen also contained a photoactive azo linkage. The boundary between fast-chromophore and slow-mesogen reorientation is blurred and an effective enhancement of reorientation occurs. 

---