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<p>Surface-induced mobility enhancement propagates into polymer glasses over distances from nanometers to micrometers. The discovery of a mobile surface “bilayer” structure—comprising a nanoscale outer layer and a microscale sublayer—provides insight into this significant variability in propagation length. However, the origin of the extensive microscale sublayer (<InlineEquation ID="IEq1"><EquationSource Format="TEX">\({h}_{{\rm{t}}}\)</EquationSource><EquationSource Format="MATHML"><math><msub><mrow><mi>h</mi></mrow><mrow><mi mathvariant="normal">t</mi></mrow></msub></math></EquationSource></InlineEquation>) remains elusive. Using dynamic mechanical analysis (DMA), we establish a strong correspondence between nanolayer mobility enhancement and <InlineEquation ID="IEq2"><EquationSource Format="TEX">\({h}_{{\rm{t}}}\)</EquationSource><EquationSource Format="MATHML"><math><msub><mrow><mi>h</mi></mrow><mrow><mi mathvariant="normal">t</mi></mrow></msub></math></EquationSource></InlineEquation>, both of which increase with the intrinsic fragility of polymer glass. Coarse-grained simulations confirm our experimental findings, revealing that enhanced molecular velocity at the surface initiates long-range shear-like excitation modes that penetrate deeper into the bulk as polymer fragility increases. This combination of experiment and simulation provides a unified framework that reconciles historical discrepancies regarding the free surface effect, establishing fragility as a critical parameter for predicting the relative sizes of surface-enhanced mobility and its propagation length across different polymers.</p>

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Linking long-range surface-induced mobility enhancement and intrinsic fragility of polymer glasses

  • 期刊论文

  • 出版模式：

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  • 发表日期：

    2026年8月4日

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  • Haoran Nie,
  • Xiwen Chen,
  • Xiyue Li,
  • Zongyi Ma,
  • Rui Zhang,
  • Ophelia K. C. Tsui

摘要

Surface-induced mobility enhancement propagates into polymer glasses over distances from nanometers to micrometers. The discovery of a mobile surface “bilayer” structure—comprising a nanoscale outer layer and a microscale sublayer—provides insight into this significant variability in propagation length. However, the origin of the extensive microscale sublayer (\({h}_{{\rm{t}}}\)ht) remains elusive. Using dynamic mechanical analysis (DMA), we establish a strong correspondence between nanolayer mobility enhancement and \({h}_{{\rm{t}}}\)ht, both of which increase with the intrinsic fragility of polymer glass. Coarse-grained simulations confirm our experimental findings, revealing that enhanced molecular velocity at the surface initiates long-range shear-like excitation modes that penetrate deeper into the bulk as polymer fragility increases. This combination of experiment and simulation provides a unified framework that reconciles historical discrepancies regarding the free surface effect, establishing fragility as a critical parameter for predicting the relative sizes of surface-enhanced mobility and its propagation length across different polymers.