Normal-to-Supercooled Liquid Transition in Molecular Glass-Formers: A Hidden Structural Transformation Fuelled by...

Area of Science:

• Materials Science
• Physical Chemistry
• Condensed Matter Physics

Background:

• Glass-forming systems exhibit significant changes in molecular dynamics and transport properties near the Arrhenius crossover.
• Understanding these transitions is crucial for characterizing the behavior of supercooled liquids.

Purpose of the Study:

• To investigate the dynamic processes and structural transformations in the glass-forming macrocyclic crown thiaether MeBzSO above its glass transition temperature.
• To identify and characterize the Arrhenius crossover and its relationship with the normal-to-supercooled liquid transition.

Main Methods:

• Broadband dielectric spectroscopy (BDS)
• X-ray diffraction
• Fourier transform infrared (FTIR) spectroscopy
• Molecular dynamics (MD) simulations
• Density functional theory (DFT) calculations

Main Results:

• Two crossover temperatures were identified: T_B at 309 K and T_A at 333 K, with T_A identified as the Arrhenius crossover.
• The Arrhenius crossover is associated with a change in the temperature dependence of structural relaxation times from activation-like to super-Arrhenius.
• A twofold local organization of nearest-neighbor molecules was observed, driven by conformational changes of MeBzSO, favoring the lowest-energy conformation in the supercooled state and promoting vitrification.

Conclusions:

• The Arrhenius transition in MeBzSO is primarily driven by conformational changes, a novel finding compared to existing theories.
• These conformational dynamics significantly influence short-range intermolecular order and contribute to the vitrification process.
• The study highlights the complex interplay between molecular conformation, dynamics, and structure in glass-forming materials.