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Determination of Cooperativity Length in a Glass-Forming Polymer.

Yeong Zen Chua1,2, Reiner Zorn3, Jürn W P Schmelzer1,2

  • 1Institute of Physics, University of Rostock, Albert-Einstein-Str. 23-24, 18051Rostock, Germany.

ACS Physical Chemistry Au
|March 27, 2023
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Summary

This study determines the cooperativity length in glass-forming liquids using AC calorimetry and quasi-elastic neutron scattering (QENS). Accounting for temperature fluctuations provides consistent results, suggesting their crucial role in understanding liquid properties.

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Area of Science:

  • Materials Science
  • Physical Chemistry
  • Condensed Matter Physics

Background:

  • Cooperativity length and the size of cooperatively rearranging regions are key concepts for understanding glass-forming liquids.
  • Knowledge of these properties is crucial for predicting thermodynamic, kinetic, and crystallization behaviors.

Purpose of the Study:

  • To experimentally determine the cooperativity length in glass-forming liquids.
  • To compare results from AC calorimetry and quasi-elastic neutron scattering (QENS).
  • To investigate the impact of temperature fluctuations on cooperativity length determination.

Main Methods:

  • Utilized AC calorimetry and quasi-elastic neutron scattering (QENS) for experimental measurements.
  • Focused on comparable timescales for both methods.
  • Analyzed poly(ethyl methacrylate) (PEMA) as a model system.

Main Results:

  • Determined a cooperativity length of approximately 1 nm at 400 K and a characteristic time of ~2 μs from QENS.
  • Observed that results differ based on whether temperature fluctuations are accounted for in theoretical models.
  • Found consistent results between AC calorimetry and QENS when temperature fluctuations are incorporated.

Conclusions:

  • Accounting for temperature fluctuations in nanoscale subsystems is essential for accurate cooperativity length determination.
  • The study supports the inclusion of temperature fluctuations in theoretical treatments.
  • This approach allows deriving characteristic lengths from thermodynamic considerations at the glass transition.