Thermodynamic modelling of carbonaceous mesophase mixtures

Abstract

The Maier–Saupe model is extended to the binary mixtures of uniaxial discotic nematogens to compute the equilibrium phase diagrams of carbonaceous mesophases. The ordering in the mixture is affected by temperature (thermotropic) and concentration (lyotropic). The magnitude of molecular weight asymmetry of two nematogens and the strength of molecular interaction control the type of mixture: (a) non‐ideal mixtures arise under sufficiently weak interaction and lower molecular weight differences; and (b) ideal mixtures arise under stronger molecular interaction and higher molecular weight asymmetries. Ideal mixtures have clearing temperatures that change monotonically with concentration and the higher molecular weight component has higher‐order parameter. Non‐ideal mixtures have a minimum in the clearing temperature at a critical concentration at which the binary mixture behaves like a pure nematogen and the ordering of the two species are identical; for non‐ideal mixtures the relative magnitude in the species' ordering depends on the concentration; the species with lower molecular weight can have a higher order if its concentration is high enough. Characterisation protocols based on X‐ray computations and direct methods are proposed to detect the type of mixture and the magnitude of molecular interaction. The results provide new tools to design carbon fibres based on molecular properties.

Keywords:

• discotic nematic liquid crystals
• carbonaceous mesophase
• mixture
• molecular weight
• concentration
• lytropic
• thermotropic
• X‐ray

Acknowledgements

Support was provided by the National Science Foundation under Award Number EEC‐9731689 and the Petroleum Research Fund of the American Chemical Society.