Phase Transition and Structural Studies of 70.8 and TBAA12 Liquid Crystals By Several Experimental Techniques

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Abstract
The present investigation concerns the phase transitions, the temperature dependence of the layer thickness in all the mesophases, and the determination of some structural parameters of the N(p-n-heptyloxy benzylidene) p′-n-octylaniline (70.8) and of the terephthalylidene-bis-p-n-dodecylaniline (TBAA12) compounds. DSC, optical microscopy and x-ray diffraction techniques were used. In particular x-ray diffraction clearly confirms in 70.8 compound the presence of the SG and SC mesophases and shows a phase coexistence at SB Sc phase transition. The x-ray diffraction data obtained on the puzzling S1 mesophases are discussed in comparison with the results of previous investigations on similar compounds. Regarding the TBAA12, which presents an uncommon and quite rich phase sequence (solid, SG, SF, S1, SC, I), x-ray diffraction shows, in particular, that the layer thickness decreases gradually when the temperature increases until the phase Sc is reached, after which it remains constant until the isotropic liquid phase. Whereas its behaviour is similar to that of the compounds with n = 9 and n = 10 for the ordered phases, the behaviour in the smectic C phase in TBAA12 is quite different, as compared to the other homologous, perhaps owing to the absence of SA , phase. In spite of this different behaviour, the jump observed in the layer thickness at the SrSc phase transition of TBAA12 is coherent with the values previously obtained for homologous with lower n.