Nematic polymer

Fig. 2.8.16 Director orientation, 0, as a function of shear rate for both flow aligning (solid squares) and tumbling (open squares 325 K, solid circles 328 K and open circles 333 K) nematic polymers. (From Siebert et al. [10].)...

The systematic synthesis of non amphiphilic l.c.-side chain polymers and detailed physico-chemical investigations are discussed. The phase behavior and structure ofnematic, cholesteric and smectic polymers are described. Their optical properties and the state of order of cholesteric and nematic polymers are analysed in comparison to conventional low molar mass liquid crystals. The phase transition into the glassy state and optical characterization of the anisotropic glasses having liquid crystalline structures are examined. 

Monomeric l.c. s show in the polarizing microscope under crossed polarizers characteristic textures, owing to their optical anisotropy 51). Examining a nematic phase, which is sandwiched between untreated glass plates, typical interferences are observed, because of the variations of the optical axis with respect to the incident of light. The nematic polymers exhibit a similar bevahior. In Fig. 10a a typical picture of the texture of a polymer is shown. While for 1-l.c, s the texture can be observed immediately after preparation because of their low viscosity, in most cases the polymers samples... 

Fig. 10 a and b. Textures of a nematic polymer (crossed polarizers, magnification 40) a after annealing of the sample b after fresh preparation... 

Fig. 12. Birefringence of nematic polymers 1) normal nematic polymer (An > 0)...

As already mentioned, in the simple case of cylindrical symmetry of the mesogenic molecules, the long range orientational order of the nematic polymers can be described by Eq. (3) ... 

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. 

Similarly with low-molecular nematics is manifested in that the nematic polymers may form equally well schlieren texture, typical for low-molecular nematics (Fig. 18a) (polymers B.3.3-B.3.4, Table 9). The enthalpy of transition from LC state to isotropic melt is also close to that for low-molecular nematics. At the same time, there also exist definite structural differences. X-ray patterns of the same polymers, even in unoriented state, display certain elements of structural ordering in the arrangement of side branches (a weak diffuse halo at small angles), which could indicate a sibotactic nematic type of ordering. These differences are most distinct for oriented polymer films. As an example Fig. 18b, c, present X-ray patterns of unoriented and oriented samples of one and the same nematic polymer 121 l24. In fact two sharp small angle... 

Analysis of flow curves of these polymers has shown that for a nematic polymer XII in a LC state steady flow is observed in a broad temperature interval up to the glass transition temperature. A smectic polymer XI flows only in a very narrow temperature interval (118-121 °C) close to the Tcl. The difference in rheological behaviour of these polymers is most nearly disclosed when considering temperature dependences of their melt viscosities at various shear rates (Fig. 20). 

For a nematic polymer in a transition region from LC to isotropic state, maximal viscosity is observed at low shear rates j. For a smectic polymer in the same temperature range only a break in the curve is observed on a lgq — 1/T plot. This difference is apparently determined by the same reasons that control the difference in rheological behaviour of low-molecular nematics and smectics 126). A polymeric character of liquid crystals is revealed in higher values of the activation energy (Ef) of viscous flow in a mesophase, e.g., Ef for a smectic polymer is 103 kJ/mole, for a nematic polymer3 80-140kJ/mole. 

The fusion of LC phases above Tcl causes a sharp change in the character of flow and the values of Ef for nematic and smectic polymers become closer. In an isotropic phase Ef for a polymeric smectic ( 140 kJ/mole) is only twice as large as Ef for a polymeric nematic (70—80 kJ/mole). In other words, the transition from LC phase to isotropic melt, accompanied by the liberation of mesogenic groups from the mesophase levels the differences in the character of flow of smectic and nematic polymers. The differences in Ef for isotropic phase are determined only by the differences in chemical nature of the main chain of smectic and nematic polymers. The values of Ef, in this case, are close to the Ef values for poly(butylmethacrylate) and poly(butylacrylate), respectively, which are structurally similar to polymers XI and XII except that they do not contain mesogenic groups. 

We turn to the relaxation processes observed in smectic polymers with different attachment of mesogenic groups to the macromolecular backbone and compare dielectric behaviour of smectic and nematic polymers having identical mesogenic groups but different main chain structure. 

A detailed comparative study of dielectric behaviour of smectic and nematic polymers was carried out for polymers of acrylic and methacrylic series, containing identical cyanbiphenyl groups (polymers XI and XII) 137 138>. The difference in structural organization of these polymers consists in a more perfect layer packing of smectic polymer XI (see Chaps. 4.1 and 4.2) with antiparallel orientation of CN-dipoles. This shifts the relaxation process of CN-dipole reorientation to a low frequency region compared to nematic polymer XII. Identification of Arrhenius plots for dielectric relaxation frequencies fR shows that for a smectic polymer the value of fR is a couple of orders lower than for a nematic polymer (Fig. 21). Though the values... 

For instance, for a nematic polymer with positive anisotropy of dielectric constant (Ae > 0) orientation of mesogenic groups along the applied field takes place (homeo-tropic orientation). The fact of orientation is illustrated in Fig. 25, which shows that under crossed polarizers the optical transmittance I of a film of nematic polymer with optically anisotropic texture (taken for 100%) falls practically to zero when a low-frequency field is switched on. 

Fig. 26a and b. Influence of the electric field frequency on the electro-optical behaviour of the nematic polymer XII and scheme of mesogenic groups orientation before (A) and after (B) the application of electric field (a) optical transmission as a function of time at different frequencies (U = 30 V T = 75 °C) (b) optical transmission as a function of time upon application of an electric field at U = 85 V (f = 50 Hz) (1) relaxation upon switching the electric field off (2), upon application of an electric field (U = 80 V) of different frequency f = I (3) 5 (4) 7 (5) and 20 kHz (6) during the relaxation process... 

In view of the effect of molecular mass on orientational phenomena the results of151) seem to be more explicable. In this work surprisingly low values for threshold voltage (U 8-40 V) and rise and decay times (x a 200 msec) were observed for an array of nematic polymers and copolymers. They are close to the corresponding values for low-molecular liquid crystals, which implies presumably that the polymers investigated were of low degrees of polymerization or had a very wide molecular mass distribution. 

In the case of LC polymers, the polymeric matrix performs as a host, while the guest is a dye, whose molecules are elongated in shape, and the absorption oscillator is parallel (or perpendicular) to the big axis of the molecule 65,163-165>. The experiments investigating guest-host effect in nematic polymers with dichroic dyes covalently attached to the polymer 163) (type I) and mechanically incorporated65) (type II) reveal the possibility to obtain regulated color indicators (see page 60). 

---