Polymer mesogenic side chains

A phase diagram for a mesogenic side chain polymer and a related monomer is illustrated in Figure 7.3 (4,5). Note that the monomer by itself crystallizes. The nematic mixture clears in the temperature range of 360 to 370 K. 

The formation of mesogenic side-chain polymer complexes has been reported for pyridine-V-oxide/carboxylic acid [46] and trialkylamine/phenol [47]. Complex 30 exhibits smectic phases between 29 and 123 °C [46]. 

Fig. 18. Single steps of a write/read/erase cycle for an LC side-chain polymer (134). (—), mesogenic group (0), dyestuff. transition temperature...

The steric frustrations have also been detected in LC polymers [66-68]. For example, the smectic A phase with a local two-dimensional lattice was found by Endres et al. [67] for combined main chain/side chain polymers containing no terminal dipoles, but with repeating units of laterally branched mesogens. A frustrated bilayer smectic phase was observed by Watanabe et al. [68] in main-chain polymers with two odd numbered spacers sufficiently differing in their length (Fig. 7). 

Fig. 17a-c. Sketches of the molecular arrangements for the smectic structure with alternating layer-to-layer tilt a conventional and chevron smectic C layering in low molecular mass mesogens b ferroelectric hilayer chevron structures for achiral side-chain polymers c antiferroelectric hilayer chevron structures for achiral side-chain polymers. Arrows indicate the macroscopic polarization in the direction of the molecular tilt... 

Lastly, it was demonstrated with PPO substituted with a series of alkyl side-chains as we have here, that the glass transition temperature decreases with an increase in the side-chain length (28). At the same time, the Tg s of the more flexible side-chain liquid crystalline polymers investigated to date are always much higher than those of the corresponding polymers without the mesogenic side-chains (3). Therefore, it is quite likely that we may obtain side-chain liquid crystalline polymers of approximately the same Tg from PPO and PECH. 

In addition to their unusual rheological properties, the nematic phases of polymers, like those of simple compounds, can be oriented by the application of magnetic or electrical fields. These properties have been more fully examined for comb-type polymers with mesogenic side-chains than for polymers with the mesogenic groups in the main chain, since in the comb polymers it is possible to influence the side-chain orientation independently of the main-chain orientation. 

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... 

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. 

While for m-l.c. s the state of order is only determined by the anisotropic interactions of neighbouring molecules, for the polymers additionally a disturbing effect of the backbone via the flexible spacer on the anisotropic order of the mesogenic side chains is to be expected and vice versa. Therefore it is of interest to investigate whether... 

From these experiments we can conclude that the length of the flexible spacer has no measurable influence on the orientational long range order of the mesogenic side chains. If on the contrary l-l.c. s are attached to the polymer backbone, the state of order is reduced. 

With the I.c. side chain polymers a new class of I.c. material was realized and this poses the question, whether this new material also exhibits field effects in analogy to conventional I.c. s and whether the covalent bonding of the mesogenic groups to the polymer backbone changes the material parameters. In this chapter we therefore will compare the behavior of l-l.c. s and chemically very similar polymers in the electric and magnetic field. 

The realization of nematic side chain polymers implies the possibility of the existence of cholesteric side chain polymers, presuming the mesogenic molecules, which are linked to the backbone, are chiral. For these polymers it is of interest, whether the polymer fixation influences the helical twist and therefore the optical properties of the cholesteric phase. This will be discussed in 2.3.2.2. 

While for nematic polymers the statistical distribution of the centers of gravity of the mesogenic side chains is compatible with a more or less statistical main chain conformation, for smectic polymers a three dimensional coil conformation is no longer consistent with the layered structure of the mesogenic side chains. The backbone has to be restricted in its conformation, which will cause a more pronounced interaction between the main chain and the anisotropically ordered mesogenic side chains, compared to nematic and cholesteric polymers. 

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... 

In any case, both models have in common that owing to the positional ordering of the mesogenic side chains, the polymer backbone no longer exhibits a statistical three dimensional coil conformation. Therefore at the phase transformation isotropic to smectic or nematic (cholesteric) to smectic, in addition to the change of the anisotropic packing of the side chains, the main chain has to change its conformation, which must be consistent with the layered smectic structure. A direct interaction... 

These examples clearly indicate that the extent of the l.c. phase of the polymer is mainly influenced by the flexibility of the polymer main chain, determined by its chemical constitution. The flexibility of the backbone also influences the mobility of the mesogenic side chains. 

The thermodynamic investigations have indicated that the glass transition is a freezing-in process. Consequently the anisotropic liquid crystalline orientation of the mesogenic side chain should also freeze in, yielding an anisotropic glass having a liquid crystalline structure. This process is of interest in view of the applicability of l.c. polymers. 

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