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E. M. Aver'yanov

Author affiliations
Kirensky Institute of Physics, Federal Research Center KSC SB RAS,
50 Akademgorodok, Krasnoyarsk, 660036, Russia
E-mail: 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_pexp(–λ_pn). 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 = 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 = E_gt/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 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