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<p>Nonlinear stress relaxation under step strain <i>γ</i> was examined for aqueous solutions of an end-associative telechelic polymer, hydrophobically modified ethoxylated urethane (HEUR) having hexadecyl groups at the two ends of the chain. At 20 °C where the end hexadecyl groups were in the molten liquid state, the solutions with the HEUR concentrations <i>c</i>=1 wt% and 5 wt% commonly exhibited strain-hardening at short time <i>t</i> and the strain-softening (damping) at long <i>t</i>. In the terminal relaxation zone at sufficiently long <i>t</i>, the nonlinear relaxation modulus <i>G</i>(<i>t,γ</i>) was found to obey the time-strain separability. These nonlinear features were discussed in relation to strain-induced changes in the associative network structure. In the aqueous HEUR solutions, aggregates of the precipitated end-groups should be stabilized by loops of dissolved HEUR backbones to form so-called flower micelles. At low <i>c</i>, most of those micelles would connect HEUR chains into a long linear sequence referred to as superbridge, thereby forming a sparse network. At higher <i>c</i>, those superbridges would become shorter to densify the network accordingly. Immediately after imposition of the large step strain, the flower micelles in the superbridge backbone would fuse each other to form a denser network thereby exhibiting the hardening. This micelle fusion would be more significant for longer superbridges to enhance the strain hardening at lower <i>c</i>. After this fusion, the micelles having liquid cores would be opened up because of the enhanced tension of the deformed superbridge, and then split to disconnect the superbridge if this tension overwhelms the micelle strength. This opening/splitting process possibly resulted in the partial breakage of the network and the corresponding softening. Finally, the surviving part of the network would relax on thermal dissociation of the remaining micelles thereby exhibiting the time-strain separable damping at long <i>t</i>. These features were semi-quantitatively described by a simple model that considered the strain-induced fusion followed by mechanical opening/breakage of the transient crosslinks.</p>

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Nonlinear Stress Relaxation of End-associative Hydrophobically Modified Ethoxylated Urethane (HEUR) Solutions under Step Strain

  • 期刊论文

  • 发表日期：

    2026年3月20日

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  • Yu-Xuan Pei,
  • Quan Chen,
  • Hiroshi Watanabe

摘要

Nonlinear stress relaxation under step strain γ was examined for aqueous solutions of an end-associative telechelic polymer, hydrophobically modified ethoxylated urethane (HEUR) having hexadecyl groups at the two ends of the chain. At 20 °C where the end hexadecyl groups were in the molten liquid state, the solutions with the HEUR concentrations c=1 wt% and 5 wt% commonly exhibited strain-hardening at short time t and the strain-softening (damping) at long t. In the terminal relaxation zone at sufficiently long t, the nonlinear relaxation modulus G(t,γ) was found to obey the time-strain separability. These nonlinear features were discussed in relation to strain-induced changes in the associative network structure. In the aqueous HEUR solutions, aggregates of the precipitated end-groups should be stabilized by loops of dissolved HEUR backbones to form so-called flower micelles. At low c, most of those micelles would connect HEUR chains into a long linear sequence referred to as superbridge, thereby forming a sparse network. At higher c, those superbridges would become shorter to densify the network accordingly. Immediately after imposition of the large step strain, the flower micelles in the superbridge backbone would fuse each other to form a denser network thereby exhibiting the hardening. This micelle fusion would be more significant for longer superbridges to enhance the strain hardening at lower c. After this fusion, the micelles having liquid cores would be opened up because of the enhanced tension of the deformed superbridge, and then split to disconnect the superbridge if this tension overwhelms the micelle strength. This opening/splitting process possibly resulted in the partial breakage of the network and the corresponding softening. Finally, the surviving part of the network would relax on thermal dissociation of the remaining micelles thereby exhibiting the time-strain separable damping at long t. These features were semi-quantitatively described by a simple model that considered the strain-induced fusion followed by mechanical opening/breakage of the transient crosslinks.