Fingerprinting the nonlinear rheology of a liquid crystalline polyelectrolyte
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ORIGINAL CONTRIBUTION
Fingerprinting the nonlinear rheology of a liquid crystalline polyelectrolyte Ryan J. Fox 1 & Wei-Ren Chen 2 & Changwoo Do 2 & Stephen J. Picken 3 & M. Gregory Forest 1,4 & Theo J. Dingemans 1 Received: 18 May 2020 / Revised: 28 July 2020 / Accepted: 3 August 2020 # Springer-Verlag GmbH Germany, part of Springer Nature 2020
Abstract We report on the rheology of isotropic and nematic aqueous solutions of a sulfonated all-aromatic polyamide, poly(2,2′disulfonyl-4,4′-benzidine terephthalamide) (PBDT), that forms high-aspect-ratio rod-like assemblies. For quiescently isotropic solutions, the concentration dependence of the zero-shear viscosity, longest relaxation time, and terminal modulus shows deviations in comparison to the Doi-Edwards theory for hard rods. For quiescently nematic solutions, we characterize the flow behavior through steady-state and transient nonlinear rheological measurements in conjunction with small-angle neutron scattering under shear. The steady-state flow curve is characterized by two anomalous shear thickening responses, one at moderate shear rates and the other immediately prior to flow alignment at high shear rates. We assign the origin of these shear thickening response to director “kayaking” and “out-of-plane steady” states, using predictions from prior high-resolution numerical simulations of sheared nematic rods. Utilizing transient shear flow reversals and step-down experiments, we characterize the oscillatory response of the nematic director through these flow regimes. When the first normal stress difference is plotted versus the shear stress during a transient step-down, the so-called dynamic stress path, the counterclockwise versus clockwise rotation has previously been shown to reveal the relative dominance of viscous versus elastic contributions to the stress tensor, respectively. Our measurements strongly suggest that the anomalous shear thickening behavior in nematic PBDT solutions arises from viscous stresses developed as the ensemble of rods undergoes periodic oscillatory motion under shear, rather than elastic stresses due to broadening of the orientational distribution function. Keywords Nonlinear rheology . Liquid crystal polymer . Polyelectrolyte . Small-angle neutron scattering . Rheo-SANS
Introduction
Electronic supplementary material The online version of this article (https://doi.org/10.1007/s00397-020-01234-4) contains supplementary material, which is available to authorized users. * M. Gregory Forest [email protected] * Theo J. Dingemans [email protected] 1
Department of Applied Physical Sciences, University of North Carolina at Chapel Hill, Chapel Hill, NC 27599-3050, USA
2
Neutron Scattering Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA
3
Faculty of Applied Sciences, Delft University of Technology, Van der Maasweg 9, 2629 HZ Delft, The Netherlands
4
Department of Mathematics, University of North Carolina at Chapel Hill, Chapel Hill, NC 27599-3250, USA
All-aromatic polyamides (aramids) are a class of rigid, thermally stab
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