ROTATIONAL VISCOSITY (GAMMA(1)) STUDY OF NEMATIC PHASES IN BINARY-LIQUID CRYSTAL MIXTURES OF 6OCB AND 8OCB

Rotational viscosity coefficient is measured in different nematic phas es of a binary liquid crystal system 6OCB-8OCB, as a function of relat ive concentration of the constituents, and of temperature at each conc entration, using a transient NMR technique. Experiments are performed on four such mixtures with relative concentrations chosen such that fo r two of them there is a re-entrant nematic phase with an intervening smectic A phase (high temperature (HN) and re-entrant (RN) nematic pha ses), whereas for the other two there is only a single nematic phase ( SN). Variation of gamma(1) with concentration and temperature is disti nct in the HN and RN phases, while the corresponding behaviour in the SN phase is qualitatively the same as that in the RN phase. The variat ion of activation energies in all the phases as a function of the rela tive concentration further confirms this observation. It is suggested that the dynamics in HN phase is influenced by the smectic order fluct uations remnant in the system due to the underlying smectic phase, and hence is dynamically different from the SN and RN phases, which are o btained by direct melting of the solid.