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 pol y(styrene) (PS) and 4-cyano-4'-n-octyl-biphenyl (8CB) are investigated. Thr ee systems with widely different molecular weights of the polymer are consi dered in an attempt to assess the effects of the polymer size on the miscib ility 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 predict ions of the Flory-Huggins theory of isotropic mixing and the Maier-Saupe th eory of nematic order generalized by McMillan to include smectic-A order. G ood agreement is observed between theory and experiments. The solubility pr operties of mixtures with different polymer sizes are analyzed using the me thod 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 th e solubility limit with increasing molecular weight is not linear and reach es a plateau value for higher molecular weights. The results obtained for t he systems investigated here and for three other systems reported in the li terature fall on a single master curve representing the solubility limit at T-NI as a function of polymer molecular weight.