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

Abstract

Nonlinear flow behavior and the corresponding fluorescent behavior were examined for a 3.0 wt% aqueous solution of hydrophobically modified ethoxylated urethane (HEUR; M w = 1.1×10 5 ) 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 h (g 4 ) and first normal stress coefficient Y 1 (g 4 ) at shear rates g 4 above the equilibrium relaxation frequency 1/t . Fluorescent emission intensity I E (g 4 ) from excimers formed by Py groups associated in a hydrophobic environment of the micellar cores and the intensity I M (g 4 ) from dissociated Py groups (referred to as monomers) isolated in the aqueous phase were measured under flow simultaneously with h and Y 1 . The I E /I M ratio was found to decrease only slightly (by a factor of ~4 %) on an increase of g 4 up to 6/t well in the thinning regime. (For g 4 = 6/t , h and Y 1 decreased from respective zero-shear values by factors of ~50 % and ~75 %, respectively) Thus, the significant thinning at those g 4 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.

Relationships between Diffusion and Viscoelasticity of Associative Polymer Networks

“…where w is the radius of the bleached area (w = 50 μm). the fluorescent-modification was also reported by Suzuki et al 27) Thus, we cannot compare the absolute values with the conventional data, but we believe that the comparison of the FRAP and viscoelastic data using the same sample is permittable.…”

“…It is seen that β value decreases from around 0.6 to 0.4 with increasing C HEUR , which is also observed in the previous study by Rao et al 25) This result suggests that the heterogeneity increases with C HEUR . It is known that the first-and second-order average relaxation times, <τ KWW > [1] and <τ KWW > [2] , of the KWW function can be determined by the following equations 27) .…”

From Architectures to Cutting-Edge Properties, the Blooming World of Hydrophobically Modified Ethoxylated Urethanes (HEURs)

“…The focus on the process of association/dissociation of a polymer chain in solution of this model is well adapted to explain the single-Maxwellian relaxation. Overall, the nonlinear shear thinning behavior of HEUR solutions is explained by the shear-induced fragmentation of the transient network, conversion of superbridges to superloops, leading to the drop of the solution viscosity. However, the shear thickening behavior is not explained by the Tanaka–Edwards model.…”

“…33,34,35,36,37 On the other hand, Watanabe and co-workers related the thickening to a new approach considering the anisotropy of the transient network. 32,38 Indeed, their model showed that the thickening could arise from the shear enhanced re-association of HEUR strands in the shear gradient direction. This effect was balanced with the dissociation of the network and probably combined to FENE effect and shear induced strand formation.…”

Molecular Understanding of Viscoelasticity in Transient Polymer Networks Based on Multiple Methods

“…Following their work, various models have considered the interplay between polymer-and crosslink dynamics as stickers, including the sticky Rouse model 20,21) , sticky reptation model [21][22][23][24] , and reversible gelation model 25,26) . Alternatively, considerable experimental studies have been conducted to elucidate the viscoelasticity of the transient networks by using thermoresponsive ABA block copolymers [27][28][29][30] , hydrophobically end-capped polymers [31][32][33][34][35][36][37] , ionomers 25,[31][32][33][34][35][36][37][38][39][40] , "slime" [41][42][43] , etc. In addition, some studies reported the use of wormlike (threadlike) micelles, which are also strongly relevant [44][45][46][47][48] .…”