Mesogenic moieties

Note See Fig. 35. The mesogenic side-groups can be attached either as lateral substituents to the backbone mesogenic moieties that are connected to each other either (a) directly or (b) by spacers or (c) they can be attached to the spacer incorporated into the main-chain. 

Regarding the chemical constitution of these monomers, the polymerizable, functional group was always directly linked to the rigid, voluminous mesogenic moieties. As space filling models indicate, polymers result, having a rigid main chain. This... 

A broad variety of l.c. polymers is conceivable because of the wide range of well known mesogenic molecules, e.g. tabulated in the book of Dcmus27), and the different types of polymers. Further variations are possible by copolymers or systems, where each monomer unit carries more than one mesogenic moiety ( en bloc systems28)). Furthermore the synthesis of linear, branched and crosslinked systems has to be mentioned. Because of this broad variety a manifold influence on the phase behavior of the systems via the chemical constitution is feasible. In the following chapter we will discuss some basic considerations on the phase behavior of l.c.-side chain polymers. 

In comparison to a conventional polymer and l.c. mentioned above, we will now discuss the PVT behavior of a l.c. side chain polymer, which has linked mesogenic moieties as side chains, and is very similar to the previous monomer. The experimental results are shown in Fig. 5. It is obvious, that the phase behavior of the l.c. polymer differs from that of a 1-l.c. and amorphous polymer. At high temperature we observe a transformation from the isotropic polymer melt into the l.c. phase, indicated by the jump in the V(T) curve. At low temperatures no crystallisation is observed but the bend in the curves signifies a glass transition. Obviously the phase behaviour is determined by the combination of l.c. and polymer properties. 

While in case of main chain polymers S relates to the backbone or rigid segments of the backbone, for side chain polymers only the rigid mesogenic moieties of the side chains will be covered neglecting any possible anisotropic orientations of the backbone. 

With these three different examples it has been demonstrated that the systematics observed for the polymorphism of m-l.c. s is also valid for the side chain polymers, provided that a flexible spacer connects the rigid mesogenic moieties to the polymer main chain. Deviations from this behavior are observed, when the mesogenic moieties are directly linked to the backbone. Under these conditions, normally no liquid crystalline behavior is to be expected, according to the model considerations mentioned in Chap. 2.1. Some examples, however, proved l.c. properties for such systems, which are characterized by two striking properties Very high glass transition temperatures and only smectic structures even in case of short substituents... 

An example for a synthesis of a poly(siloxane) network is shown in Fig. 37b. In a one-step reaction the mesogenic moieties as well as the crosslinking agent are coupled via an addition reaction to the reactive linear poly(methylhydrogensiloxane) backbone 92). Because of similar reactivity of the crosslinking agent and mesogenic molecules, a statistical, disordered addition to the backbone has to be expected. 

Dynamic mechanical experiments, where the material is periodically strained, are common methods to characterize the visco-elastic behavior of elastomers by measuring the storage modulus G and loss modulus G". G is a measure for the maximal, reversibly stored energy for a periodical deformation and G" is proportional to the dissipated energy for the oscillation cycle. It is obvious to investigate, whether the l.c. state of the l.c. elastomers influences the dynamic mechanical properties and whether different modes of linking the mesogenic moieties to the backbone can be detected. 

Fig. Z36 Representation of the energy-minimized conformation of a rod-coil diblock prepared by Radzilowski ef al. (1993). The rod is a mesogenic moiety 60 A long, the coil is low molecular weight PI. The volume fraction of rod is about 0.2.

Willow-like cascade construction was facilitated by the preparation of the AB2-type building blocks, e.g., 6-bromo-l-(4-hydroxy-4 -biphenylyl)-2-(4-hydroxyphenyl)hexane (40 Scheme 6.12), 13-bromo-l-(4-hydroxyphenyl)-2-[4-(6-hydroxy-2-naphthalenylyl)-phenyljtridecane (41), and 13-bromo-l-(4-hydroxyphenyl)-2-(4-hydroxy-4"-p-terpheny-lyl)tridecane (42). Scheme 6.12 illustrates the monomer preparative strategy for the incorporation of the monomeric mesogenic moieties predicated on conformational (gauche versus anti) isomerism. Thus, one equivalent of 4-hydroxybiphenyl was allowed to react with 1,4-dibromobutane to yield the monobromide 43, which was converted under Finkelstein conditions to the corresponding iodide 44. Treatment of the latter with ketone 45 under phase transfer conditions afforded the a-substituted ketone 46. Reduc-... 

Since their discovery in the nineteenth century [1], hquid crystals have fascinated scientists due to their unusual properties and their wide range of potential apphcations, especially in optoelectronics. LC systems can be divided into two categories thermotropic LC phases and lyotropic LC phases. Thermotropic LC systems result from anisotropic molecules or molecular parts (so called mesogens or mesogenic moieties, respectively), and form hquid crystalline phases between the soHd state and the isotropic hquid state, where they flow like liquids but possess some of the characteristic physical properties of crys-... 

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 degree of polymerization was related to the minimum field needed to induce a significant effect. For DP=20 the minimum field was 6 T, whereas for DP=30 it was at least 10 T, rising to 19 T for DP=40. For a shorter spacer length (n=3), no induced effect was observed up to 20 T, which can be explained by the reduced mobility of the mesogenic moieties, resulting in reduced rotation towards the field director. 

Koltzenburg et al. report the synthesis of AB block copolymers of acetylene and norbornene derivatives bearing mesogenic moieties [68]. A norbornene... 

Complementary dendritic hexamers based on a central scaffold made up of linked pentaerythritol and tri(hydroxymethyl)amino methane units have been found to also exhibit liquid-crystalline properties. This star-shaped scaffold was used to create supermolecules containing two different hemispheres, referred to thereafter as Janus supermolecular Hquid crystals (Fig. 76) [323,324]. One of the hemisphere contains three cyanobiphenyl end-groups, whereas the other lobe consists of three chiral phenyl benzoate mesogenic moieties laterally attached. The type of mesophase observed (N ... 

Liquid-Crystalline Character of Novel Main-Chain Oligophosphates Bearing Lipophilic and/or Mesogenic Moieties... 

In a recent article we have established the liquid crystalline behavior exhibited by a, to diphosphate amphiphiles (20) Prompted by this investigation we decided to study the liquid crystalline character of their polymeric counterparts, i.e. of polyphosphates or rather oligophosphates ( Scheme I) prepared by the reaction of phosphorous oxychloride with 1,12 dodecanediol and 1,16 hexadecanediol respectively. In addition, the mesomorphic behavior of oligophosphates bearing the biphenyl mesogenic moiety was compared with the liquid crystalline behavior of the purely aliphatic oligophosphates. 

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