Dimers elastic constants

For the CB0n0.fSj2MB series with n - 7 and 9 a blue phase was observed but not for n = 6 and 8 thus, the chiral properties of these materials do indeed exhibit an odd-even effect as expected. This was rationalised in terms of the smaller pitch for the odd relative to the even membered dimers which arises from the smaller twist elastic constant of odd dimers and is related to their lower orientational order. Surprisingly, the helical twisting power of the dimers in a common monomeric nematic solvent appears to depend solely on the nature of the chiral group, the 2-methylbutyl chiral centre, and not on its environment. Thus similar helical twisting powers are observed for both odd and even membered dimers. We will return to the nature of the phases exhibited by some of these chiral dimers in Sect. 4.4. 

TABLE 9 Structures and elastic constants for dimers and trimers. 

The data presented here have excluded nematic polymer liquid crystals. These present particular difficulties for the conventional methods of measurement, which require well-aligned samples. A magnetic resonance technique has been developed which is particularly suited to the measurement of the elastic and viscous properties of liquid crystalline polyma s, and this technique and specimen results are described in Datarevicw 8.3 in this volume [40]. Thore have been some measurements on the precursors to polymers i.e. dimers and trimers (see Section C3), and these limited results do indicate the importance of molecular shape in determining the elastic properties. In particular, the extended even dimer has elastic constants almost twice those of the bent odd dimer. Thus in polymer liquid crystals, it is clear that the chain configuration will have a dramatic influence on the elastic properties. 

We can now estimate the collisional rates for Yb- Li dimers. On the basis of the results in Figure 10.12 we find that for a = 20 nm the upper bound of the inelastic rate constant is amax 4 x 10" cm /sec. The elastic rate constant for a thermal gas with A 20 nm and T 100 nK equals... 

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