CHANGES OF THE NEMATIC-ISOTROPIC LIQUID PHASE TRANSITION TEMPERATURE IN HOMOLOGOUS SERIES OF CALAMITIC LIQUID CRYSTALS AND CONFORMATIONAL MOBILITY OF TERMINAL MOLECULAR CHAINS

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.