Viscosity of Nematics

The nematic phases of both prolate and oblate Gay-Beme ellipsoids seem to be flow stable. The prolate system becomes flow unstable near the nematic-Smectic A transition because the smectic layer structure is incommensurate with the Couette strain rate field. The effective viscosity of nematic phases of... 

V. V. Belyaev, S. Ivanov, and M. F. Grebenkin, Temperature dependence of rotational viscosity of nematic liquid crystals, Sov. Phys. Crystallogr. 30, 674 (1985). 

The phenomenological aspects of LCP rheology has been reviewed by the author [2]. More recently [14, 15], further data were obtained to compare the viscosity of the same polymer in the isotropic and liquid crystalline states. As with small molecule LC s, the viscosity of nematic LCP s was found to be lower than that of the isotropic forms. There are other aspects of the LCP rheology which require discussion, but first the evidence from theoretical considerations Is described. 

The transverse pressure gradient passes through a maximum at approximately =45°. A transverse pressure for this case and an angle dependence according to Eq. (57) has been experimentally confirmed [41]. In principle this experiment can be used for the determination of viscosity coefficient ratios. Because of experimental difficulties it should only be used to demonstrate the tensor property of the viscosity of nematic liquid crystals. 

Fig. 9.24. Temperature dependences of the viscosity of nematic (1) and smectic (2) comb-shaped polymers.

Furtlier details can be found elsewhere [20, 78, 82 and 84]. An approach to tire dynamics of nematics based on analysis of microscopic correlation fimctions has also been presented [85]. Various combinations of elements of tire viscosity tensor of a nematic define tire so-called Leslie coefficients [20, 84]. 

Molecules of nematic Hquid crystals also are aligned in flow fields which results in a viscosity that is lower than that of the isotropic Hquid the rod-shaped molecules easily stream past one another when oriented. Flow may be impeded if an electric or magnetic field is appHed to counter the flow orientation the viscosity then becomes an anisotropic property. 

The formation of ECC is not only an extension of a portion of the macromolecule but also a mutual orientational ordering of these portions belonging to different molecules (intermolecular crystallization), as a result of which the structure of ECC is similar to that of a nematic liquid crystal. After the melt is supercooled below the melting temperature, the processes of mutual orientation related to the displacement of molecules virtually cannot occur because the viscosity of the system drastically increases and the chain mobility decreases. Hence, the state of one-dimensional orientational order should be already attained in the melt. During crystallization this ordering ensures the aggregation of extended portions to crystals of the ECC type fixed by intermolecular interactons on cooling. 

The viscosity of thermotropic liquid crystals increases following the sequenee nematic< smectic A < smectic C. 

The discotic phases can show also a complex polymorphism. Nematic and cholesteric-like, low viscosity phases have been reported recently. In these, the director vector is perpendicular to the plane of alignment of the flat molecules56) in contrast to the normal nematics and cholesterics where it is parallel to the molecular axis. Most frequently, however, discotics form columnar arrangements as shown in Fig. 10. The order within the columns may change from liquid to quasi-crystalline. The columns are then packed in hexagonal or tetragonal coordination, but are free to slide in the direction parallel to their axes S7). The viscosity of these more ordered discotics is considerably higher than the nematic discotics. 

The fall in viscosity due to the onset of nematic behaviour has been estimated in one case, by comparing polymers of similar molecular weight but different chemical composition, to be over three orders of magnitude 101... 

What attracts attention in the first place, is a big absolute value for the viscosity of polymeric liquid crystals. For instance, already at Tcl (Fig. 20) the viscosity of polymers (103 Pa s) is 1-2 orders higher than that of SA phase and 4-5 orders higher than that of a nematic phase of low-molecular liquid crystals. 

An important dimensionless relationship between viscosity and concentration was found by Papkov et al. (1974) and reproduced in Fig. 16.30, where the variation of viscosity with polymer concentration for different molecular weights, expressed as intrinsic viscosities, is shown (left). The reduced viscosity t]/if vs. the reduced concentration c/c is shown on the right. The viscosity of the solution jumps down rapidly above the critical concentration as the nematic mesophase forms. The dimensionless relationship is remarkable. The relationship between the viscosity at the maximum and the intrinsic viscosity (see inset) appears to be r/max = 5.5b/]1 5, where rj is expressed in Ns/m2 and [77] in m3/kg. 

The flow viscosity of a nematic phase also determines the spatial and temporal response of the director to an applied field. The bulk viscosity of a nematic phase depends on the direction of flow of each molecule with respect to the director, averaged out over the whole of the sample. Therefore, bulk viscosity is... 

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