Concentration Dependence of Nonlinear Rheological Properties of Hydrophobically Modified Ethoxylated Urethane Aqueous Solutions

Abstract

For aqueous solutions of a representative telechelic polymer, hydrophobically modified ethoxylated urethane (HEUR), a recent transient network model suggested that sparse and dense HEUR networks are formed at low and high HEUR concentrations c thereby exhibiting different rheological features [Uneyama, T.; Suzuki, S.; Watanabe, H. Phys. Rev. E2012, 86, 031802]. In this study, we examined those differences for nonlinear rheological behavior of HEUR solutions (with MHEUR = 4.6 × 104 and c = 1–10 wt %) under steady shear at 25 °C to test the model. For rather dilute solutions (c ≤ 3 wt %), the steady state viscosity η exhibited crossover from the linear (zero-shear) behavior to thickening and further to thinning with increasing the shear rate γ̇, whereas the first normal stress coefficient Ψ1 remained in the linear regime up to intermediate γ̇ (where η exhibited the thickening) and then decreased with γ̇ in the thinning regime of η. In contrast, for concentrated solutions (c ≥ 4 wt %), η exhibited no thickening and a direct linear-to-thinning crossover was observed for both η and Ψ1. The critical HEUR concentration for the disappearance of the thickening of η, c* ≅ 4 wt %, was in agreement with that noted for the linear viscoelastic data, suggesting that the HEUR network structure changed from the sparse state to the dense state at c*, as considered in the above transient network model: At low c < c*, linear sequences of HEUR chains (superbridges) connected at the HEUR micellar cores would have served as effective strands to form the sparse network. Those superbridge strands dissociate and reassociate under steady shear, and the orientation of the reassociated strands should be quite sensitive to anisotropy of the spatial distribution of the cores (intra-strand dissociation sites) so that the thickening of η at intermediate γ̇ is accompanied by linear Ψ1, as deduced from the model. In contrast, at large c > c*, the dense network mainly sustained by individual HEUR chains would have been formed. For this case, the anisotropy of core distribution should less significantly affect the orientation of the created strands, which possibly erased the thickening of η at intermediate γ̇, as suggested by the model. Thus, the changes of the nonlinear behavior with c observed in this study were in harmony with the expectation from the model, lending qualitative support to the structures and dynamics of the networks considered in the model. In addition, the flow visualization using tracer particles confirmed that the flow was uniform up to the onset of thinning of η and Ψ1 (which is again in harmony with the model) and that the flow was destabilized to form shear bands in the thinning regime as similar to the behavior of a wide variety of softmatters.