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Zhidk. krist. ikh prakt. ispol'z. = Liq. Cryst. and their Appl., 2018, 18 (1), 37—46. DOI: 10.18083/LCAppl.2018.1.37 


Changes of the Nematic–Isotropic Liquid Phase Transition Temperature in Homologous Series of Calamitic Liquid Crystals and Conformational Mobility of Terminal Molecular Chains

E. M. Aver'yanov

Author affiliations Kirensky Institute of Physics, Federal Research Center KSC SB RAS,
50 Akademgorodok, Krasnoyarsk, 660036, Russia Email: aver@iph.krasn.ru

Abstract For calamitic nematics (cholecterics) of various chemical classes it was shown that monotonous increasing dependences t_{с}(n_{p}) of the nematic – isotropic liquid transition temperature on the number np of even (n_{e}) or odd (n_{o}) homologues are well approximated by functions F_{p}(n) = t_{lp} + D_{p}exp(–λ_{p}n). Two branches t_{с}(n_{o}) and t_{с}(n_{e}) are described by one function F(n) = F_{s}(n) + F_{a}(n) with smooth part F_{s}(n) = [F_{e}(n) + F_{o}(n)]/2 and alternating part F_{a}(n) =(–1)^{n}[F_{e}(n) – F_{o}(n)]/2. The dependence F_{s}(n) = t_{l} + Dexp(–λn) was derived, in which coefficient λ = –ln(q) is determined by the ratio q = S_{k}/S_{k–1} (k ≥ 2) of the orientational order parameters S_{k} = S_{1}exp[–(k–1)/ζ] for methylene fragments of aliphatic molecular chains in the nematic phase. The values of λ and ζ = 1/λ are related by the expression q = [4–exp(–ε)]/[4+8exp(–ε)] with effective difference ε = E_{gt}/RT between the energies of gosh(g^{±}) and trans(t)orientations of kth methylene fragment of the chain relative to (k–1)th one. A connection between the values λ_{p}, ζ_{p}, λ, ζ, q, ε with function F_{a}(n) and chemical structure of molecules was observed.
Keywords: nematic – isotropic liquid phase transition, homologous series of liquid crystals, conformational mobility of molecular chains

