Theoretical analysis of texture dependent extensional viscosity of discotic mesophases

Abstract

Rheological functions for uniaxial extensional flows predicted by a previously selected and validated constitutive equation (CE) for discotic mesophases are presented. The predicted relations between extensional viscosities, flow-induced microstructure, processing conditions, and material parameters of discotic mesophases are characterized and discussed. It is found that, in contrast to rod-like nematics, two distinct uniaxial extensional viscosities need to be defined to characterize the extensional rheological functions of discotic mesophases completely. The model predicts non-Troutonian extensional viscosities of discotic nematics, such as strain thinning and strain thickening, depending on the process temperature, and the ratio of viscous to elastic stress contributions. The uniaxial extensional viscosities are also found to depend strongly on the flow-induced microstructure. The rheological analysis is then used to characterize the relations between extensional flow viscosities and the classical microstructures that arise during the industrial fiber spinning of discotic mesophase pitches.