Equilibrium phase diagram of polystyrene and 8CB

The experimental equilibrium phase diagram of a mixture of linear polystyre ne of molecular weight M-omega = 44,000 g/mol and 4-cyano-4'-n-octyl-biphen yl (8CB) is established. The three transitions smectic A-nematic, nematic-i sotropic, and isotropic-isotropic are observed. The first two are observed both by optical microscopy and differential scanning calorimetry (DSC) whil e the isotropic-isotropic transition could be seen only via optical microsc opy. Two series of samples with the same compositions were independently pr epared and yielded consistent results both by microscopy and DSC. Measureme nts of sample compositions with less than 50 weight % of 8CB were influence d by the vicinity of the glass transition temperature (T-g) of the polymer in the mixture. This quantity is also determined by DSC as a function of co mposition. A single T-g is observed, which decreases with composition of th e LC. Other thermodynamic quantities such as the enthalpy variations of LC in the nematic-isotropic transition and the fraction of LC contained in the droplets are also considered. (C) 1999 John Wiley & Sons, Inc.