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Liquid Crystals and their Application
Russian Journal

Zhidkie kristally i ikh prakticheskoe ispol'zovanie

Жидкие кристаллы и их практическое использование

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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

For calamitic nematics (cholecterics) of various chemical classes it was shown that monotonous increasing dependences tс(np) of the nematic – isotropic liquid transition temperature on the number np of even (ne) or odd (no) homologues are well approximated by functions Fp(n) = tlp + Dpexp(–λpn). Two branches tс(no) and tс(ne) are described by one function F(n) = Fs(n) + Fa(n) with smooth part Fs(n) = [Fe(n) + Fo(n)]/2 and alternating part Fa(n) =(–1)n[Fe(n) – Fo(n)]/2. The dependence Fs(n) = tl + Dexp(–λn) was derived, in which coefficient λ = –ln(q) is determined by the ratio q = Sk/Sk–1 (k ≥ 2) of the orientational order parameters Sk = S1exp[–(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 = Egt/RT between the energies of gosh(g±)- and trans(t)-orientations of k-th methylene fragment of the chain relative to (k–1)-th one. A connection between the values λp, ζp, λ, ζ, q, ε with function Fa(n) and chemical structure of molecules was observed.

Keywords: nematic – isotropic liquid phase transition, homologous series of liquid crystals, conformational mobility of molecular chains

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