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<p>All-atom molecular dynamics simulations were performed to investigate the early-stage creep response and structural evolution of amorphous isotactic polypropylene under hydrostatic pressures of 1–500 atm at 240, 260, and 280&#xa0;K. The temperature-dependent creep-compliance curves could be approximately superimposed onto master curves at each hydrostatic pressure, indicating that time–temperature superposition can describe the early-stage creep response within the examined simulation window. The shift factors were represented by an Arrhenius-type relation as an empirical description over the limited temperature range. The apparent activation energy decreased with increasing hydrostatic pressure, suggesting that the shift behavior becomes less sensitive to temperature under stronger molecular packing constraints. Free-volume evolution exhibited analogous shift behavior and showed a systematic correlation with creep compliance, indicating that free-volume change is an important molecular descriptor linking pressure-dependent packing and creep deformation. Chain-level analyses further revealed changes in geometrically defined kink and entanglement-related descriptors, bond-angle distributions, and dihedral conformations, including an increase in <i>trans</i> conformations and a decrease in <i>gauche</i> conformations during creep. Although hydrostatic pressure reduced molecular mobility and available free volume, the local deformation pathway remained qualitatively similar across the examined pressure range. These findings support an atomistic interpretation of the approximate applicability of time–temperature superposition to early-stage creep under hydrostatic pressure.</p>

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Structural basis for the validity of time-temperature superposition in polypropylene under hydrostatic pressure by MD simulation

  • 期刊论文

  • 出版模式：

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  • 发表日期：

    2026年7月29日

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  • Shihong Yuan,
  • Takenobu Sakai

摘要

All-atom molecular dynamics simulations were performed to investigate the early-stage creep response and structural evolution of amorphous isotactic polypropylene under hydrostatic pressures of 1–500 atm at 240, 260, and 280 K. The temperature-dependent creep-compliance curves could be approximately superimposed onto master curves at each hydrostatic pressure, indicating that time–temperature superposition can describe the early-stage creep response within the examined simulation window. The shift factors were represented by an Arrhenius-type relation as an empirical description over the limited temperature range. The apparent activation energy decreased with increasing hydrostatic pressure, suggesting that the shift behavior becomes less sensitive to temperature under stronger molecular packing constraints. Free-volume evolution exhibited analogous shift behavior and showed a systematic correlation with creep compliance, indicating that free-volume change is an important molecular descriptor linking pressure-dependent packing and creep deformation. Chain-level analyses further revealed changes in geometrically defined kink and entanglement-related descriptors, bond-angle distributions, and dihedral conformations, including an increase in trans conformations and a decrease in gauche conformations during creep. Although hydrostatic pressure reduced molecular mobility and available free volume, the local deformation pathway remained qualitatively similar across the examined pressure range. These findings support an atomistic interpretation of the approximate applicability of time–temperature superposition to early-stage creep under hydrostatic pressure.