Concentration effect on the nonlinear measures of dense polyethylene oxide solutions under large amplitude oscillatory shear

In this paper, we study the nonlinear viscoelastic response of concentrated polyethylene oxide solutions under large amplitude oscillatory shear (LAOS). We investigate the concentration effect as a function of strain and shear rate by using nondimensional elastic and viscous Lissajous-Bowditch plots. We also employ the MITLAOS software to calculate the intracycle nonlinear viscoelasticity parameters. We find that the transition to nonlinear behavior occurs at lower strain values as the polymer concentration increases. For the higher concentration and at large strain amplitudes, the intracycle elastic stress component is almost zero to a large extent of the oscillation cycle and thus strain-independent. The corresponding stress decomposition in the shear rate domain shows a significant shear rate dependence, with a self-intersection of the total stress curve. By increasing the oscillation frequency and, correspondingly, the shear rate, by one order of magnitude, the intercycle nonlinear behavior does not qualitatively change. The spatiotemporal velocity profiles are obtained using a custom-made particle image velocimetry system. Interestingly, we record nonlinear velocity profiles at approximately 300% strain along with significant wall slip for both oscillation frequencies. The intracycle velocity data close to the stationary plate reveal strong nonlinear velocity waveforms. The results of this study suggest that the number of entanglements per chain is responsible for the strain thinning or thickening behavior in LAOS. (C) 2018 The Society of Rheology.