Rheology of Aqueous Solution of Hydrophobically Modified Ethoxylated Urethane (HEUR) with Fluorescent Probes at Chain Ends: Thinning Mechanism

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Nonlinear flow behavior and the corresponding fluorescent behavior were examined for a 3.0 wt% aqueous solution of hydrophobically modified ethoxylated urethane (HEUR; Mw = 1.1×105) having fluorescent pyrenyl (Py) groups at the chain ends. These end groups associated with each other to serve as cores of HEUR flower micelles, and these micelles were further connected into super-bridges to form a transient network. Correspondingly, single-Maxwellian relaxation reflecting the thermal reorganization of this network was observed in the linear viscoelastic regime. Under steady shear flow, the HEUR solution exhibited thinning of both viscosity η(γ) and first normal stress coefficient Ψ1(γ) at shear rates γ above the equilibrium relaxation frequency 1/τ. Fluorescent emission intensity IE(γ) from excimers formed by Py groups associated in a hydrophobic environment of the micellar cores and the intensity IM(γ) from dissociated Py groups (referred to as monomers) isolated in the aqueous phase were measured under flow simultaneously with η and Ψ1. The IE/IM ratio was found to decrease only slightly (by a factor of ∼4 %) on an increase of γ up to 6/τ well in the thinning regime. (For γ = 6/τ, η and Ψ1 decreased from respective zero-shear values by factors of ∼50 % and ∼75 %, respectively) Thus, the significant thinning at those γ was accompanied by a negligible change in the HEUR core structure that corresponds to just a slight shift of the association/dissociation balance of the Py groups to the dissociated state. From this result, the thinning is quite possibly attributed to flow-induced disruption of the network connectivity through conversion of the super-bridges into super-loops, both having the same core structure.

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