Effect of molecular weight on the phase diagram and thermal properties of poly(styrene)/8CB mixtures

Equilibrium phase diagrams and thermophysical properties of mixtures of poly(styrene) (PS) and 4-cyano-4'-n-octyl-biphenyl (8CB) are investigated. Three systems with widely different molecular weights of the polymer are considered in an attempt to assess the effects of the polymer size on the miscibility of PS and 8CB. The experimental phase diagrams are established using polarized optical microscopy (POM), light scattering (LS), and differential scanning calorimetry (DSC), and the results were analyzed with the predictions of the Flory-Huggins theory of isotropic mixing and the Maier-Saupe theory of nematic order generalized by McMillan to include smectic-A order. Good agreement is observed between theory and experiments. The solubility properties of mixtures with different polymer sizes are analyzed using the method suggested by Smith. The solubility limit of 8CB in PS is deduced from enthalpy changes at the nematic-isotropic transition temperature (T-NI) as a function of polymer molecular weight. It is found that the decrease of the solubility limit with increasing molecular weight is not linear and reaches a plateau value for higher molecular weights. The results obtained for the systems investigated here and for three other systems reported in the literature fall on a single master curve representing the solubility limit at T-NI as a function of polymer molecular weight.