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We found that local stiffness strongly influences the mobility gradient in polymer films, impacting activation energy. This effect is crucial for understanding polymer film relaxation, even above the glass transition temperature.

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Area of Science:

  • Polymer Physics
  • Materials Science
  • Computational Chemistry

Background:

  • Understanding the mobility gradient in polymer films is crucial for predicting their behavior.
  • The string model of glass-formation offers a framework for interpreting relaxation dynamics.

Purpose of the Study:

  • To investigate the mobility gradient in the interfacial region of substrate-supported polymer films.
  • To quantitatively assess the applicability of the string model to these gradients.
  • To identify key material properties influencing the mobility gradient.

Main Methods:

  • Molecular dynamics simulations were employed to model polymer films.
  • Layer-by-layer analysis was used to examine the mobility gradient.
  • Correlations between material properties (density, energy, stiffness) and activation parameters were investigated.

Main Results:

  • No significant gradients in collective motion were observed.
  • The string model quantitatively describes the relaxation time gradient.
  • Local stiffness showed the strongest correlation with the activation enthalpy gradient (ΔH₀(z)).
  • Entropy (ΔS₀(z)) also significantly contributes to the interfacial mobility gradient.

Conclusions:

  • The interfacial mobility gradient in polymer films is primarily driven by gradients in activation enthalpy and entropy.
  • Local stiffness is a key factor influencing activation enthalpy.
  • The string model provides a valid framework for understanding these interfacial dynamics.