Unifying the temperature dependent dynamics of glass formers

Area of Science:

• Materials Science
• Chemical Physics
• Condensed Matter Physics

Background:

• Amorphous materials exhibit significant changes in bulk properties like modulus and viscosity near their glass transition temperature (Tg).
• The microscopic origins of these macroscopic property changes have been a long-standing challenge in materials science.
• Understanding these transitions is crucial for designing materials with specific thermal and mechanical properties.

Purpose of the Study:

• To elucidate the atomic/molecular mechanisms underlying the evolution of localized relaxations into macroscopic structural relaxations above Tg.
• To develop a systematic classification of glass formers based on the number of rearranging units.
• To provide a unified theoretical framework applicable to diverse glassy systems.

Main Methods:

• Application of transition state theory (TST) to analyze atomic/molecular motion in amorphous materials.
• Classification of unit motion into two populations: simultaneous rearrangement of nα units and non-simultaneous rearrangement within the primitive lifetime τ1.
• Analysis of literature data from a wide variety of glassy materials and materials with glassy responses.

Main Results:

• Identified a critical number of rearranging units (nα = 1-4) that systematically classifies glass formers and correlates with fragility.
• Demonstrated that Johari-Goldstein β-relaxations originate from the rattling of nα units.
• Developed a four-parameter equation that accurately fits both strong and weak glass formers across a wide temperature range.
• Extended the applicability of the model to ion-transporting polymers and ferroelectric relaxors.

Conclusions:

• The TST approach successfully bridges the gap between microscopic unit relaxations and macroscopic property changes in amorphous materials.
• The proposed classification and equation offer a unified understanding of glass transition phenomena in diverse materials.
• The study highlights the significant role of activation entropy in apparent pre-exponential factors and illustrates enthalpy-entropy compensation.