The elastic constants of nematic liquid crystalline terminally substituted azoxybenzenes

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Abstract
The elastic constants K11, K22, and K33 of some members of two homologous series of nematic liquid crystals, the p,p′‐di‐n‐alkyl and p,p′‐di‐n‐alkoxy azoxybenzenes, are reported. Both K11 and K33 become greater with increasing alkyl chain length, while the ratio K33/K11 decreases along the series. For small alkyl chain length we find K33≳K11, for long chain length K33<K11, while for intermediate chain length K33?K11. In all cases K22 is the smallest of the three elastic constants, while K22/K11 is independent of temperature. The variation of K33/K11 with alkyl chain length can qualitatively be attributed to changes of correlation lengths parallel and perpendicular to the director. If the nematic distortion free energy is restricted to terms of the order S2, where S is the order parameter, theory predicts K33=K11. The experiments indicate that this equality holds approximately in some special cases, but in general higher order terms in S must be included to obtain a correct expression for the nematic distortion free energy.