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Rheo-optical and micromechanical analysis of viscoelastic properties in inhomogeneous systems: component contributions in a water-swellable microgel/hydrophobically modified ethoxylated urethane mixture

Polymer Journal (2025)Cite this article

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Abstract

We investigated the rheological behavior of mixtures of microgel particle (Hereinafter referred to as MGP) dispersions and aqueous solutions of hydrophobically modified ethoxylated urethane (Hereinafter referred to as HEUR), which are high-performance polymeric thickeners. Despite their intrinsic immiscibility and phase separation, the combination of MGP and HEUR produced a synergistic increase in the complex modulus and notable changes in relaxation behavior. Dynamic viscoelastic and rheo-optical (dynamic birefringence) measurements revealed that the addition of a small amount of HEUR to MGP dispersions significantly increased the modulus values and shifted the maximum relaxation frequency to lower values. The application of the modified stress optical rule (MSOR) enabled separation of the elastic moduli of the MGP and HEUR components, confirming that HEUR preferentially localizes between MGPs, counterbalances MGP-specific concentration fluctuations and induces cooperative relaxation in the MGP phase. Micromechanical modeling, combined with a parallel model incorporating connected HEUR domains, reproduced the slow HEUR relaxation and MGP deformation behavior, highlighting stress coupling between the two phases as the origin of the retarded relaxation—without requiring specific interactions such as chain entanglement. This study reports that combined rheological and rheo-optical analysis, supported by micromechanical modeling, provides insight into the viscoelastic behavior and structural organization of immiscible thickener mixtures, with implications for designing high-performance formulations such as cosmetics.

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Authors and Affiliations

  1. Shiseido Co., Ltd., MIRAI Technology Institute, Yokohama, Kanagawa, Japan

    Ayano Nakamura

  2. Department of Macromolecular Science, Graduate School of Science, The University of Osaka, Toyonaka, Osaka, Japan

    Ayano Nakamura, Kana Yamazaki & Tadashi Inoue

  3. Department of Bioengineering, Graduate School of Engineering, The University of Tokyo, Bunkyo-ku, Tokyo, Japan

    Takuya Katashima

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  1. Ayano Nakamura
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Correspondence to Tadashi Inoue.

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Nakamura, A., Yamazaki, K., Katashima, T. et al. Rheo-optical and micromechanical analysis of viscoelastic properties in inhomogeneous systems: component contributions in a water-swellable microgel/hydrophobically modified ethoxylated urethane mixture. Polym J (2025). https://doi.org/10.1038/s41428-025-01113-w

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