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Is Ortho-Terphenyl a Rigid Glass Former?

Johanna Kölbel1, Michael T Ruggiero2, Shachar Keren3

  • 1School of Engineering, Brown University, Providence, Rhode Island 02912, United States.

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Ortho-terphenyl (OTP) is not a rigid molecule, challenging its use as a simple model for the glass transition. Low-frequency terahertz vibrations reveal complex atomic motions crucial for understanding elasticity in materials.

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Area of Science:

  • Condensed Matter Physics
  • Materials Science
  • Spectroscopy

Background:

  • Ortho-terphenyl (OTP) is a widely used model compound for studying the glass transition phenomenon.
  • A prevailing assumption in previous research is that OTP is a rigid molecular structure.

Purpose of the Study:

  • To investigate the molecular rigidity of ortho-terphenyl (OTP) using advanced spectroscopic techniques.
  • To challenge the long-held assumption of OTP's rigidity and its implications for glass transition studies.

Main Methods:

  • Employed terahertz time-domain spectroscopy to probe low-frequency vibrational dynamics.
  • Utilized low-frequency Raman spectroscopy for complementary vibrational analysis.
  • Performed density functional theory and ab initio molecular dynamics simulations for theoretical validation.

Main Results:

  • Direct observation of low-frequency vibrational dynamics revealed complex, large-amplitude atomic motions in OTP.
  • Terahertz phonons in OTP involve a mixture of intramolecular and intermolecular atomic displacements.
  • Simulations confirmed that the assumption of OTP rigidity is an oversimplification.

Conclusions:

  • The rigidity assumption for ortho-terphenyl (OTP) must be revisited, impacting its utility as a simple glass transition model.
  • Terahertz vibrational modes significantly influence material elasticity, a factor critical for complex systems like biomolecules.