Cyanobiphenyls shear viscosity

If there is a phase transition from a nematic to a smectic phase, pretransitional effects are observed in the neighbourhood of the transition [9, 44, 49-51]. Figure 10 [9] shows this behaviour for 4-/7-octyloxy-4 -cyanobiphenyl (80CBP) with a nematic-smectic phase transition at 340.3 K. Pretransitional effects cause a divergence of the shear viscosity coefficient 772 and the rotational viscosity /j. 

Different rings in the central core and the insertion of lateral groups change the molecular packing density, which exerts an essential influence on the shear and the rotational viscosity (Fig. 19) [55, 85], Obviously, the reason for the large differences in the rotational viscosities of alkyl- and alkyloxy-cyanobiphenyls is the different packing density. Other explanations are not very convincing [82]. 

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