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

Aver'yanov, E. M. Source: LIQUID CRYSTALS AND THEIR APPLICATION, 18 (1):37-47; 10.18083/LCAppl.2018.1.37 JAN-MAR 2018

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.