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Abstract

Phase diagrams of blends of poly(methylphenylsiloxane) in short PMPS and two low molecular weight liquid crystals (4-cyano-4'- n-pentyl-biphenyl and an eutectic mixture of paraphenylenes) are reported. Two polymers with very different weight-average molar masses are considered in an evaluation of the loss of miscibility resulting from a known increase in the weight- average molar mass. The experimental diagrams have been constructed via polarized optical microscopy and are rationalized in terms of the Flory-Huggins theory of isotropic mixtures and the Maier-Saupe theory of nematic order. The results show a good agreement between the theory and experiments and reveal a remarkable enhancement of miscibility with respect to similar systems involving poly(dimethylsiloxane). Variations of the interaction parameter with the temperature are compared for different systems of polysiloxanes. The effects of the nature of the liquid crystal and the polymer molar mass on the X parameter are evaluated. (C) 2002 Wiley Periodicals, Inc.