Neat and Aqueous Polyelectrolytes under a Steady-Shear Flow

Abstract

Materials enabling impact-energy absorption of high-velocity projectiles are of great interest for applications like aerospace. In such a frame, shear thickening fluids were found very useful. Here, we investigated nanorheological properties of neat and aqueous polyelectrolytes of low molecular weights containing poly­([2-(methacryloyloxy) ethyl] trimethyl ammonium) as polycations and poly­(acrylamide-co-acrylic acid) as polyanions. Results were compared with pure water. We employed nonequilibrium molecular dynamics with the SLLOD algorithm to compute the viscosity at various shear rates. Systems containing polyelectrolytes exhibit shear thickening. The analysis of molecular configurations revealed a strong disruption of the ionic structure and more clusters with smaller sizes on increasing the shear rate. Potential energies showed that shear thickening originates from an increase in intramolecular and van der Waals interactions resulting from the increasing difficultly of polyelectrolyte-based systems to relax at high shear rates. Our method and findings underscore the importance of accounting for the molecular scale in the design of materials absorbing the impact energy efficiently.

Viscoelasticity of Low-Molecular-Weight Polyelectrolytes

“…Here, molecular models considered are those built and equilibrated in ref , which include bulk water, neat PE, and an aqueous mixture of PE with PC and PA representing 20 wt % of the total mixture. We set a PC/PA mole ratio of 1:1 and used 50 ionic pairs.…”

“…Our aim is to provide a molecular-scale understanding of rheological properties observed in their experiments using molecular simulations. In a previous work, we investigated the shear-rate dependence of the viscosity under a steady flow (i.e., constant shear rates, ) for molecular systems having low-molecular-weight PEs with chemical compositions similar to those considered by Takeda et al , We showed that PE-based systems, that is, the neat phase and the aqueous mixture, exhibit shear thickening for the highest . Whereas neat PE showed only shear thinning at low and intermediate , water and the aqueous mixture exhibited first a plateau (Newtonian regime) followed by shear thinning .…”

“…In a previous work, we investigated the shear-rate dependence of the viscosity under a steady flow (i.e., constant shear rates, ) for molecular systems having low-molecular-weight PEs with chemical compositions similar to those considered by Takeda et al , We showed that PE-based systems, that is, the neat phase and the aqueous mixture, exhibit shear thickening for the highest . Whereas neat PE showed only shear thinning at low and intermediate , water and the aqueous mixture exhibited first a plateau (Newtonian regime) followed by shear thinning . Shear thickening originates from an increase in intramolecular and van der Waals interactions resulting from the increasing difficulty of PE-based systems to relax at high .…”

“…They are clearly and uniquely defined and are independent of the deformation and the deformation rate. , The linear viscoelastic regime is in the range γ 0 ∼ 1–10% for homopolymer melts and polymer solutions and γ 0 < 1% for dispersed systems or block copolymer solutions . σ xy (ω, t ,γ 0 ) deviates from a single-harmonic-sinusoid response at large γ 0 (i.e., the nonlinear viscoelastic regime). , Even if molecular simulations looked feasible in the latter regime, care must be taken because large γ 0 and ω may imply beyond the validity limit of the simulation technique. , Therefore, we restrained our study to mainly the linear viscoelastic regime.…”

Microstructure of the fluid particles around the rigid body at the shear-thickening state toward understanding of the fluid mechanics

“…In a previous study, we performed non-equilibrium molecular dynamics (NEMD) simulations to analyze the shear-thickening state of a polymer in an aqueous solution 13 . This type of NEMD simulation is useful for studying fluid particles as explicit atoms or molecules to reveal the characteristic properties of the hydrodynamic forces.…”